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Factors of merit for radiation detectors Unwin, Alexander Matthew 1953

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FACTORS OF MERIT FOR RADIATION DETECTORS  by ALEXANDER MATTHEW UNWIN  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department o f PHYSICS  We accept t h i s t h e s i s as conforming t o the standard required from candidates f o r the degree o f MASTER  OF  SCIENCE  Members of the Department of Physics  THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1953  V  ABSTRACT  A d i s c u s s i o n i s given o f the many uses o f photoconductive c e l l s , e s p e c i a l l y of those of the lead sulphide type. A Factor of M e r i t f o r r a d i a t i o n detectors as proposed by C l a r k Jones i s presented, which i s intended to cover a l l types of d e t e c t o r s , and which i s applied to the lead s u l phide c e l l s s t u d i e d . mentioned.  Other Factors o f M e r i t are a l s o  From information obtained the Factors of M e r i t  are evaluated f o r the c e l l s .  These Factors of Merit are  found to vary w i t h the temperature o f the c e l l l a y e r .  It i s  found that l i m i t i n g noise i s not due t o Johnson n o i s e , but rather to r a d i a t i o n f l u c t u a t i o n s ;  and that the ultimate sen-  s i t i v i t y has been reached i n some c e l l s .  The c e l l s are  assumed to be type I I detectors according t o C l a r k Jones's classification.  I t I s found that the engineering l i m i t pro-  posed by R. J . Havens does not apply here.  P a r t i c u l a r l y good  agreement between various expressions f o r the F a c t o r o f M e r i t i s shown, assuming a type I I d e t e c t o r . A d e s c r i p t i o n o f the apparatus i s given i n some detail.  A black body r a d i a t o r and associated temperature con-  t r o l , a 900 cycles per second tuned a m p l i f i e r , a wide band p r e a m p l i f i e r and a m u l t i v i b r a t o r used i n measuring time constants o f such c e l l s are d e s c r i b e d .  vi  The methods of measurement of r e s p o n s i v i t y to noise r a t i o , of n o i s e , of time constants, frequency response curves and s p e c t r a l response of a detector are o u t l i n e d . that the black body i s o p t i c a l l y a l i g n e d ;  I t i s found  t e s t s show that the  response o f a c e l l i s d i r e c t l y p r o p o r t i o n a l to the i n t e n s i t y of the i l l u m i n a t i o n .  ACKNOWLEDGEMENTS  The author wishes t o thank Dr. A. M. Crooker f o r h i s help and encouragement i n d i r e c t i n g t h i s research.  Thanks are a l s o due t o Mr. P. A. Lee  f o r much assistance and f o r many i n v a l u a b l e suggestions.  The author wishes to acknowledge h i s  indebtedness t o the Defence Research Board f o r use of the 900 cycles per second tuned a m p l i f i e r , and f o r permission to use some o f the c e l l s s t u d i e d .  iii  CONTENTS  Page ABSTRACT I II III IV V VI  INTRODUCTION  1  FACTORS OF MERIT  7  THE APPARATUS  19  THE EXPERIMENTAL PROCEDURE  33  THE RESULTS  54  DISCUSSION  67  BIBLIOGRAPHY ACKNOWLEDGEMENT  70  ILLUSTRATIONS  FIGURE 1  The Black Body Radiator  2  O p t i c a l System of the Black Body  3  V i r t u a l Source Theory  4  The 900 c/s A m p l i f i e r  5  Response v. Frequency of 900 c/s A m p l i f i e r  6  Response v. Input of 900 c/s A m p l i f i e r  7  The Wide Band P r e a m p l i f i e r  8  Response v. Frequency of P r e a m p l i f i e r  9  C e l l Mounting  10  The Measurement of Time Constants  11  S i g n a l v. I n t e n s i t y of I l l u m i n a t i o n  12  Block Schematic of Time Constants  13  C e l l 293 S p e c t r a l Response  14  Response v. Modulation H a l f Width  Chapter I  INTRODUCTION Much a t t e n t i o n has been given t o photoconductive c e l l s as i n f r a r e d detectors i n the past few years, e s p e c i a l l y to those operating i n the region between one and s i x microns. Such c e l l s , which were developed mainly during the war years, now f i n d many i n t e r e s t i n g a p p l i c a t i o n s i n pure and applied research. A b r i e f summary of such research w i l l now be given, f o r i t i s the purpose o f t h i s research to study the p r o p e r t i e s of several such detectors w i t h a view to evaluating t h e i r r e l a t i v e merits and demerits i n a s i n g l e comprehensive "Factor of M e r i t " . Many a p p l i c a t i o n s , together w i t h a summary of performance: o f such c e l l s , are given i n a review a r t i c l e by Simpson and Sutherland  (1).  above one micron;  Detectors to be considered are s e n s i t i v e those w i t h a c u t - o f f i n t h i s region are  already w e l l known and include t h a l l i u m sulphide c e l l s , whereas such detectors as are studied here extend our present knowledge i n many f i e l d s . Three substances, photoconductive  beyond one micron,  are of greatest i n t e r e s t and form the basis o f the most research;  2  lead sulphide, lead selenide and lead t e l l u r i d e .  Such mater-  i a l s are normally deposited i n t h i n l a y e r s from IO" " to 10~^cm. 4  , •  i*  thickness on glass or some other non-conductor, by evaporation or a chemical process. 4 f t are  from 10  to 10  Dark resistances so f a r encountered  ohms, and response times are found to be  between 10"^ and 10"^ seconds.  I t w i l l be seen i n Chapter V  that a l l measurable properties of the c e l l s encountered i n t h i s research f a l l w i t h i n the ranges given.  I t i s a l s o immed-  i a t e l y apparent that one great a t t r i b u t e of such d e t e c t o r s , compared to bolometers, thermocouples and other heat d e t e c t i n g devices, i s t h e i r r e l a t i v e l y short time of response. An e x c e l l e n t a r t i c l e by Sosnowski, Starkiewicz and Simpson (2) describes the preparation of lead sulphide c e l l s and also the main aspects of the theory.  Many such c e l l s  have been made, which operate at room temperature and are posed to the atmosphere. vacuo;  ex-  However, most layers' are kept i n  a f u r t h e r betterment i n t h e i r r e s p o n s i v i t y being  obtained by c o o l i n g the l a y e r , e i t h e r to s o l i d carbon d i o x i d e or l i q u i d oxygen temperatures.  This i s a l s o known to extend  the long wave l i m i t of such c e l l s from about 3.3 microns to about 3-6 microns.  On the other hand, lowering the tempera-  ture also r a i s e s the r e s i s t a n c e and increases the time constant; consequently a cooled c e l l i s not always advantageous.  Selenide  and t e l l u r i d e c e l l s must be cooled to the neighbourhood of dry i c e temperatures before photoconductivity takes e f f e c t . At present, only lead sulphide c e l l s are commercially  3  available.  Each of the semiconductors h i t h e r t o mentioned  has i t s own response versus wavelength curve, c h a r a c t e r i s t i c of the m a t e r i a l i n the l a y e r and also of the method of preparation.  Such a curve i s shown i n f i g u r e (13), f o r three  d i f f e r e n t temperatures of the same d e t e c t o r .  For lead s u l -  phide c e l l s , the/peak i n the curve g e n e r a l l y occurs at two microns, f o r lead selenide between three and four microns, f o r lead t e l l u r i d e a l i t t l e beyond four microns. I t i s i n t e r e s t i n g to observe how approach/their  such detectors  t h e o r e t i c a l ultimate s e n s i t i v i t i e s .  This  l i m i t , determined by the r e s p o n s i v i t y and the noise of  any  d e t e c t o r , i s discussed i n s e v e r a l of the papers quoted, the most important of which i s that by C l a r k Jones ( 3 ) ;  this  paper i s discussed at some length i n Chapter IV, s e c t i o n ( 1 ) . F e l l g e t t (4) c a l c u l a t e s the l i m i t i n g s e n s i t i v i t y imposed by r a d i a t i o n f l u c t u a t i o n s on a lead sulphide c e l l , from i t s measured r e s p o n s i v i t y wavelength c h a r a c t e r i s t i c s , assuming the c e l l s to have been cooled and exposed to a s o l i d angle 2TT of r a d i a t i o n (from surroundings taken to be at 15° C). ultimate s e n s i t i v i t y was  The  then c a l c u l a t e d to be 2.1 x l O " ^ 1  watt at the optimum wavelength, compared w i t h an a c t u a l meas-13 ured noise equivalent power per u n i t bandwidth of 4.9 x 1G watt.  These r e s u l t s i n d i c a t e that the t h e o r e t i c a l l i m i t f o r  the lead sulphide c e l l had been reached;  the t o t a l noise  being only twice as great as that due to the r a d i a t i o n f l u c t u a t i o n s alone.  4 Moss (5) has a l s o studied the u l t i m a t e s e n s i t i v i t y and i t s dependence on the s p e c t r a l response curve.  For photo-  conductive d e t e c t o r s , the background r a d i a t i o n i s normally of greater wavelength than the wavelength at maximum response. Upon cooling a c e l l , Moss found the l i m i t i n g s e n s i t i v i t y to d e t e r i o r a t e from 5.2 x 1 0 " at 90°K at 2.3 microns.  1 4  watt at 273°K to 17 x 1 0 "  1 4  watt  This i s because of the p r e f e r e n t i a l  increase of the response at longer wavelengths upon c o o l i n g . On the other hand, at 3«5 microns the s i t u a t i o n i s reversed. The l i m i t i n g s e n s i t i v i t y f o r the same d e t e c t o r improved from -11 -14 2 x 10  watt to 15 x 10  watt.  I t i s then of p a r t i c u l a r importance, i n choosing a detector f o r some s p e c i f i c purpose, to define a s u i t a b l e F a c t o r of Merit f o r detectors i n general.  Without such a simple  c r i t e r i o n , i t would be necessary to compare the performance of a l l a v a i l a b l e types of detector under the proposed experimental conditions.  This long standing need f o r a F a c t o r of M e r i t has  been met by many workers, notably C l a r k Jones (6) and Daly and Sutherland ( 7 ) .  These references are discussed i n the next  chapter. Some of the i n t e r e s t i n g a p p l i c a t i o n s f o r photoconduct i v e c e l l s i n pure physics are i n the f i e l d of i n f r a r e d spectroscopy.  By using photoconductive c e l l s , notable progress has  been made i n r e s o l v i n g power and scanning speed;  the point  having been reached, where the i n f r a r e d spectrometer i s becoming l i m i t e d by o p t i c a l considerations and not by the d e t e c t i n g  5  element as heretofore.'  Sutherland, B l a c k w e l l and F e l l g e t t  (8) reported that they achieved  a r e s o l v i n g power of 30,000  i n the water vapour spectrum near 2.5 microns, whereas prev i o u s l y only a r e s o l v i n g power of 7,000 had been a t t a i n e d i n this region.  Advances have a l s o been made, using photocon-  ductive c e l l s , i n the observation of spectra of e x t r a t e r r e s t r i a l objects. Rapid changes i n spectra o c c u r r i n g i n explosions or i n the e a r l y stages o f a chemical r e a c t i o n are o f t e n items o f I n t e r e s t i n g study.  With a f a s t bolometer, the scanning time  f o r the range of a few microns i s o f the order of s e v e r a l seconds i f r e s o l v i n g power i s to be maintained ( 9 ) . Silverman  B u l l o c k and  ( 1 0 ) , by using photoconductors, are able to scan a  range of two microns between one and f i v e microns i n some thousandths o f a second and w i t h a r e s o l v i n g power of 100 near three microns.  I n t h i s way they have been able to study the  f i r s t stages o f the explosive r e a c t i o n between oxygen and carbon monoxide. I n f r a r e d spectroscopy  has already made a l a r g e conv  t r i b u t i o n t o the study of the atmospheres of p l a n e t s .  Kuiper  ( 1 1 ) , using photoconductive c e l l s , has shown that the p o l a r caps on Mars do not c o n s i s t of s o l i d carbon d i o x i d e but are almost c e r t a i n l y composed of i c e . I n i n d u s t r y , lead sulphide c e l l s open up new p o s s i b i l i t i e s i n r a d i a t i o n pyrometry.  Lee and Parker (12) have  shown that temperatures as low as 100°C can be measured t h i s  6  way w i t h f a i r accuracy, and those of  500°  C, sometimes encountered  i n the rapid braking of locomotive wheels, can be followed cont i n u o u s l y and measured to an accuracy of one per cent. The purpose of t h i s research w i l l be to study the prope r t i e s of s e v e r a l photoconductive c e l l s , and evaluate s e v e r a l Factors of M e r i t f o r them.  Any i n c o n s i s t e n c i e s i n these w i l l  be noted, the whole w i t h a view to enabling f u t u r e workers i n the f i e l d to choose the best of these detectors on merits of s i z e of s e n s i t i v e l a y e r and time constant.  Chapter I I  FACTORS OF MERIT 1. The C l a r k Jones Factor of M e r i t  (6)  In h i s paper cn" Factors of M e r i t , C l a r k Jones  (6)  points out the need f o r " ... a s i n g l e , q u a n t i t a t i v e Factor of Merit f o r use i n comparing the s e n s i t i v i t y of various radiation detectors."  A c r i t e r i o n i s needed f o r comparison  of s i m i l a r d e t e c t o r s , such as an evaporated thermocouple and a wire thermocouple, as a l s o f o r the comparison of d i s s i m i l a r d e t e c t o r s , such as a Golay pneumatic heat detector and a lead sulphide photoconductive c e l l . This Factor of M e r i t must be capable of comparing detectors w i t h g r e a t l y d i f f e r e n t s e n s i t i v e areas, response times and s p e c t r a l response curves, when measured w i t h an amplif i e r having any given frequency-response curve. I n part I . of the second of. three papers d e a l i n g w i t h r a d i a t i o n d e t e c t o r s , (3> 13?  6) C l a r k Jones defines a type n  detector as one whose noise equivalent power (defined i n 13) P  m  i n the reference c o n d i t i o n A depends upon the reference time  constant t: and the s e n s i t i v e Area A according to  where k  n  i s a parameter independent of A and.T, but which has  8 d i f f e r e n t values f o r d i f f e r e n t d e t e c t o r s . poses a s u i t a b l e m u l t i p l e of k  C l a r k Jones (6) pro-  as a numerical Factor of M e r i t .  n  For type 1 detectors C l a r k Jones uses equation of paper I ( 3 ) .  (3.8)  F o r a d e t e c t o r whose quantum e f f i c i e n c y i s  u n i t y at every r a d i a t i o n wavelength,  equation (3.8)  s t a t e s that  the minimum value of the noise equivalent power i s given by  w  ^ - -  •  <  '  2)  where k i s Boltzmann's constant, cr i s the Stefan Boltzmann r a d i a t i o n constant, and where T i s the absolute temperature o f the detector and of the surrounding r a d i a t i o n f i e l d .  At the  temperature T = 300 K . the l a s t equation may be w r i t t e n a  where P  m  seconds.  i s i n watts, A i s i n square m i l l i m e t e r s , and "C i s i n I f a detector s a t i s f y i n g the above i s considered to  have a F a c t o r of Merit equal t o u n i t y then the Factor of M e r i t f o r any other type I d e t e c t o r may be w r i t t e n  =  where P  m  VO _V2  i s i n watts, A i s i n square m i l l i m e t e r s , t- i s i n  seconds, k i s I n the u n i t s r e s u l t i n g from using the u n i t s j u s t mentioned i n equation (6) and R  Q  i s i n watt  9 Equation (4) i s C l a r k Jones's proposed Factor o f Merit f o r a type I d e t e c t o r . For type I I d e t e c t o r s , C l a r k Jones (6) bases h i s m u l t i p l e , chosen f o r k , on Havens's L i m i t . 2  This l i m i t  —  based on an estimate of the minimum value o f the noise equival e n t power which could be obtained w i t h thermocouples and bolometers w i t h c u r r e n t l y a v a i l a b l e m a t e r i a l s and techniques  —  was made i n 1946, by R. J . Havens (4) f o r a detector at room temperature.  This l i m i t , which i s of an o p t i m i s t i c engineer-  ing and not of a fundamental nature, has been very w e l l confirmed . Havens * s L i m i t i s  ?^,s>.io-"-(K*/t.)  <»  where P i s i n watts, A i s i n square m i l l i m e t e r s , and ~c i s i n m seconds.  I f a d e t e c t o r which s a t i s f i e s equation (5) be consid-  ered t o have a F a c t o r of M e r i t equal t o u n i t y , then the Factor of Merit f o r any other type I I d e t e c t o r may be w r i t t e n  =3 x l o - ^ ( K  where P  m  k  2  / ^ V  i s i n watts, A i s i n square m i l l i m e t e r s , X  seconds, k  2  ( 6 )  is i n  i s i n the u n i t s r e s u l t i n g from using the u n i t s  10  j u s t mentioned i n equation (1) and R Equation  o  i s , i n watt  (6) i s the proposed Factor of Merit f o r a  type I I d e t e c t o r . In d i s c u s s i n g the s i g n i f i c a n c e of the Factors of M e r i t , C l a r k Jones (6) then goes on to show that two  detectors  w i t h the same Factors of Merit and of the same type ( e.g. both bolometers) but w i t h d i f f e r e n t s e n s i t i v e areas and d i f f e r e n t time constants w i l l not i n general y i e l d the same r e s u l t s i n any p a r t i c u l a r a p p l i c a t i o n .  However, i f the  two  detectors are so reconstructed that they each have the o p t i mum  s e n s i t i v e area arid the optimum time constant f o r the  p a r t i c u l a r a p p l i c a t i o n , then the performance of the two  will  be the same. This point cannot be stressed too g r e a t l y , since t h i s i s the great s i g n i f i c a n c e —  and explains i n the best f a s h i o n  the importance and usefulness -- of Clark Jones's proposed Factor of M e r i t .  Thus w i t h a knowledge of the Factors of  Merit of d i f f e r e n t types of r a d i a t i o n d e t e c t o r s , one may,  by  knowing the optimum value of s e n s i t i v e area and of time cons t a n t , choose a detector type of the appropriate s i z e and response time which has a Factor of Merit close to u n i t y , r e gardless of whether the d e t e c t o r be a bolometer, thermocouple, or photoconduetive c e l l .  Thus a knowledge of the Factor of  Merit of many d i f f e r e n t detectors of known area and time constant seems h i g h l y d e s i r a b l e .  This has been attempted f o r  11 several photoconductive  c e l l s i n t h i s research.  C l a r k Jones (6) c a r r i e s the argument f u r t h e r , i n that i f the two detectors have d i f f e r e n t Factors of Merit and are constructed so that each has the optimum s e n s i t i v e area and time constant f o r a p a r t i c u l a r a p p l i c a t i o n , the s i g n a l to noise r a t i o s so obtained w i l l be d i r e c t l y i n prop o r t i o n to t h e i r Factors of M e r i t . For detectors such as bolometers  and thermocouples,  where i t i s assumed that the only source of noise i s the Johnson Noise associated w i t h the r e s i s t a n c e of the d e t e c t o r , M  2  may be w r i t t e n (6)  KA-2. 0.O4-<£>* where S  D  (7)  K/W)^  s  i s the e f f e c t i v e zero frequency r e s p o n s i v i t y , meas-  ured i n v o l t s per watt, A the s e n s i t i v e area i n square m i l l i meters, R the r e s i s t a n c e i n ohms, and t i s the reference time a  constant i n seconds. I n the case of pbjotoconductive c e l l s , the noise i s not l i m i t e d by Johnson noise o n l y , and where the noise i s a c t u a l l y measurable, the f o l l o w i n g Factor of Merit i s obtained. Let P_ be the steady i n c i d e n t power which produces a steady output voltage equal to the noise voltage under the a c t u a l conditions of measurement.  Where the measurement i s made w i t h  a squarely modulated s i g n a l , P  i s obtained by reducing the o  measured r e s u l t to zero frequency by making use of the measured square wave frequency response curve of the d e t e c t o r .  12  The q u a n t i t y  A f i s the noise equivalent band width i n the  a c t u a l measurement, and A and T~ denote the s e n s i t i v e area and the reference time constant. M  2  C l a r k Jones then shows (6), that  may be w r i t t e n  where P  Q  i s i n watts, A i s i n square m i l l i m e t e r s ,  is in  cycles per second, and "X i s i n seconds. A l l the Factors of Merit have been stated f o r detectors  operating at room temperature.  The values f o r detectors  operating a t the d r y i c e and l i q u i d oxygen temperatures, 201°K. and 90°K., are given i n Chapter V, r;; i . . . { /„ I n h i s d i s c u s s i o n , C l a r k Jones (6) notes that no thermopile or bolometer operating at room temperature has a Factor of M e r i t as l a r g e as u n i t y . . Only the super-conducting bolometers and the Golay pneumatic heat d e t e c t o r have Factors of Merit greater than u n i t y ( v i z . , from 1.29 to 13.9» and 4.69 respectively).  The Golay d e t e c t o r operates at room temperature,  but since i t i s a type I d e t e c t o r (3) i t i s not p e r m i s s i b l e t o state the F a c t o r of Merit M of Merit  2  f o r this detector.  The F a c t o r  f o r the Golay heat detector i s 0.30. C l a r k Jones a l s o notes that the maximum a t t a i n a b l e  Factors of M e r i t at room temperature are s u b s t a n t i a l l y the same for thermocouples and bolometers. The s u i t a b i l i t y of h i s proposed F a c t o r of M e r i t i s  13  a l s o borne out, according to C l a r k Jones (6) by the r e s u l t s obtained f o r three t h e r m i s t o r bolometers, which use the same type of s e n s i t i v e element, but employ g r e a t l y d i f f e r e n t c o e f f i c i e n t s of thermal c o n d u c t i v i t y between the s e n s i t i v e element and i t s surroundings.  The time constant f o r a bolo-  meter i s defined as the r a t i o of the heat capacity per u n i t area to the thermal c o n d u c t i v i t y .  In s p i t e of the hundred  to one range of time constant, the l a r g e s t F a c t o r of M e r i t f o r these t h e r m i s t o r bolometers i s only 15 per cent greater than the s m a l l e s t . C l a r k Jones a l s o states the need f o r the proper s p e c i f i c a t i o n of the relevant properties of r a d i a t i o n d e t e c t o r s . He states what he considers to be the necessary i n f o r m a t i o n about d e t e c t o r s :  a) whose o n l y source of noise i s the Johnson  noise associated w i t h t h e i r r e s i s t a n c e as 1.  The e l e c t r i c a l r e s i s t a n c e  2.  The s e n s i t i v e area  3.  The curve of r e l a t i v e r e s p o n s i v i t y versus frequency.  If  the frequency response may be characterized by a s i n g l e time constant, a statement of i t s value i s s u f f i c i e n t . 4.  . The r e l a t i v e response to d i f f e r e n t r a d i a t i o n wave lengths  5.  The r e s p o n s i v i t y ( i n v o l t s per watt) at a s i n g l e frequency w i t h a s p e c i f i e d s p e c t r a l energy d i s t r i b u t i o n .  and b) f o r detectors other than the above as 1.  The s e n s i t i v e area  14  2.  A curve of r e l a t i v e r e s p o n s i v i t y versus frequency  3.  A curve of the r e l a t i v e noise power per u n i t band width versus frequency, under the same conditions used to determine item 2  4.  ,The r e l a t i v e response to d i f f e r e n t r a d i a t i o n wavelengths  5.  A s i n g l e measurement of the s i g n a l to noise r a t i o under f u l l y defined c o n d i t i o n s *  I n the case of photoconductive c e l l s , the n e c e s s i t y of 4 i n both a) and b) i s due to the l a r g e v a r i a t i o n of s e n s i t i v i t y as a f u n c t i o n of wavelength.  I t i s worthwhile to consider f u r t h e r the c l a s s i f i c a t i o n system proposed by C l a r k Jones (3). A d e t e c t o r i s defined to be a type n d e t e c t o r f o r a given range of reference time constants i f over that range of reference time constants the zero frequency r e s p o n s i v i t y to noise r a t i o n R  Q  depends upon the A and the reference time  constant "C i n accordance w i t h  (9)  where k  n  i s a constant which i s independent of A and X. C l a r k Jones then l i s t s eight d i f f e r e n t kinds of rad-  i a t i o n d e t e c t o r s (3) which he can c l a s s i f y as e i t h e r type I or  14 type I I detectors on the basis of t h e o r e t i c a l c o n s i d e r a t i o n s . The l i s t i s as f o l l o w s : Range of x In seconds Detector - Type I Golay pneumatic heat d e t e c t o r Vacuum phototubes l i m i t e d by shot noise Gas phototubes l i m i t e d by shot noise P h o t o m u l t i p l i e r tubes l i m i t e d by shot noise Dipole antenna l i m i t e d by temperature noise = shot noise  10~2 - co 10" - oo 2.5 x 10" - oo 10"9 - co J  - Type I I Bolometers Thermocouples and Thermopiles Photographic p l a t e s l i m i t e d by g r a i n structure  10-3 - l 10 -1 R e c i p r o c i t y law  This paper w i l l t r y to assess the type number of the various types of photoconductive the b a s i s of experimental  c e l l s under c o n s i d e r a t i o n purely on evidence.  The a c t u a l reference c o n d i t i o n of measurement proposed by Glark Jones (3) i s the f o l l o w i n g .  I t s a t i s f i e s these condi-  t i o n s :-  1.  The noise equivalent power of the d e t e c t o r i s measured i n  the presence of the noise i n a manner such that the band width of the noise i s approximately equal to the band width of the detector. 2.  The band width of the d e t e c t o r i s measured a f t e r the  a m p l i f i e r gain has been equalized so that the noise spectrum is  flat.  15  Then follows a p r e s c r i p t i o n f o r the adjustment o f the a m p l i f i e r and f o r the measurement of the noise equivalent power.  However, t h i s has not been c a r r i e d out i n the present  work, but i t i s f e l t that time and m a t e r i a l do not j u s t i f y such a step as the measurements c a r r i e d out a l l o w enough l e e way f o r e f f e c t i v e values to be obtained at the expense o f only s l i g h t l o s s i n p r e c i s i o n and g e n e r a l i t y .  The above i s men-  tioned as a f u r t h e r step necessary only to the absolute f u l f i l l m e n t of a l l the c o n d i t i o n s l a i d down by C l a r k Jones (3) f o r the assignment of Factors of M e r i t .  However, since C l a r k  Jones (6) has assigned Factors of Merit to various detectors on the b a s i s of previous published reports on t h e i r c h a r a c t e r i s t i c s which do not f o l l o w h i s unique s p e c i f i c a t i o n s , and since he has assumed c e r t a i n evidence to be able to convert published f i g u r e s to h i s own s p e c i f i c a t i o n s " , i t i s thought to be s u f f i c i e n t merely to do the same.  Further, C l a r k Jones uses such  above mentioned evaluations o f Factors of Merit i n s t r a i g h t forward comparisons between d e t e c t o r s , which i s e s s e n t i a l l y the purpose of t h i s paper. I n summary, C l a r k Jones (6) has devised a thoroughly p r a c t i c a l , and widely a p p l i c a b l e method f o r assigning to c e r t a i n types of r a d i a t i o n detectors a number, c a l l e d t h e i r F a c t o r of M e r i t , which when properly i n t e r p r e t e d allows complete comparison of a l l detectors covered, and which a l s o i n d i c a t e s the best detector a v a i l a b l e f o r a thoroughly s p e c i f i e d purpose.  To a  large extent, t h i s paper seeks t o extend h i s d e f i n i t i o n to the large number of photoconductive  c e l l s now being developed or  16  already i n use, i n order to a i d workers i n f i e l d s associated w i t h t h e i r use i n choosing s a t i s f a c t o r y d e t e c t o r s . 2 . The Daly and Sutherland F a c t o r of Merit (7)  One may express (7) the mean square f l u c t u a t i o n v o l tage at the detector output as  V  (10)  where Jx i s c h a r a c t e r i s t i c o f the detector alone and ^ f i s c h a r a c t e r i s t i c of the a m p l i f i e r (and d i s p l a y ) alone.  Further,  d e f i n i n g O" as the r e s p o n s i v i t y i n microvolts per microwatt and (7)  "C as the time constant of the d e t e c t o r , Daly and Sutherland propose t a k i n g  (11)  as the Figure of Merit of a d e t e c t o r , when s e n s i t i v i t y , speed, and noise l e v e l have a l l been taken i n t o account. To compare the Daly and Sutherland Figure of M e r i t w i t h that proposed by C l a r k Jones, the inverse of the former must be considered, since i t i s i t s e l f p r o p o r t i o n a l to the minimum detectable power, whereas the number proposed by C l a r k Jones i s i n v e r s e l y p r o p o r t i o n a l to the minimum detectable power. A l s o , Daly and Sutherland omit reference to the s e n s i t i v e area,  17  which, being combined w i t h the above remarks, gives as a s u i t able Factor of Merit as proposed by Daly and  Sutherland  (12)  A ^ <T/^-O 3. Other Factors of M e r i t . The Hornig and O'Keefe F a c t o r of Merit  (15)  Hornig and O'Keefe propose a f a c t o r of Merit  (15)  f o r thermal detectors which employ t h e r m o e l e c t r i c p r o p e r t i e s i n thermocouples of various m a t e r i a l s .  They propose a f i g u r e  (13)  where Q i s the thermoelectric power of a t h e r m o e l e c t r i c junc t i o n attached to the r e c e i v e r , k i s the thermal c o n d u c t i v i t y of the wire m a t e r i a l , and material.  f the r e s i s t i v i t y of the wire  Denoting the two wire materials by the s u b s c r i p t s  1 and 2, a p r e c i s e Factor of Merit i s e s t a b l i s h e d , of the form  (14)  Q i s measured i n microvolts per degree Centigrade, k i n watts per centimeter degree Centigrade, and ^ i n ohm  centimeters.  18  I t i s f e l t that the Factor of M e r i t given by C l a r k Jones c a r r i e s the greatest weight since i t i s u n i v e r s a l , applying to a l l types of d e t e c t o r , whereas both the above are s p e c i a l i s e d cases.  I t a l s o considers the s e n s i t i v e area,  which i s a matter merely of adjustment to the above f i g u r e s . However, C l a r k Jones's Factor of Merit i s chosen on the basis of t h e o r e t i c a l p r e d i c t i o n , and i s not merely a good guess, and embodies considerations such as minimum detectable power, proper time constant and c l a s s of d e t e c t o r .  Chapter I I I  THE APPARATUS  The apparatus must be able to measure r e s p o n s i v i t y to noise r a t i o , time constant, and also the s e n s i t i v e area. The l a t t e r may be measured by the usual means of t r a v e l l i n g microscope, since f o r most detectors i t i s impossible f o r a d e t a i l e d examination of the s e n s i t i v e area to be made, as the detectors are i n vacuum. The r e s p o n s i v i t y to noise r a t i o i s measured by using a 900 cycles per second tuned a m p l i f i e r w i t h properties described i n s e c t i o n (3) of t h i s chapter.  The measurement of  time constants i s performed by using a neon f l a s h bulb as a source of v a r i a b l e modulation frequency (square wave modulated) radiant energy, a wide band p r e a m p l i f i e r a f t e r the d e t e c t o r whose time constant i s being measured, and an o s c i l l o s c o p e , a l l of known frequency response, a p l o t then being made of response versus frequency from which the time constant i s determined. A d e t a i l e d d e s c r i p t i o n of the above-mentioned i n s t r u ments f o l l o w s .  1.  The Black Body Radiator. The r a d i a t o r i s shown i n f i g u r e ( 1 ) .  A i s a steel  20 c y l i n d e r 10 centimeters long and 6 centimeters i n diameter. A c o n i c a l hole subtending an angle of f i f t e e n degrees i s bored i n one face (the c y l i n d e r then being thoroughly baked f o r o x i d i s a t i o n purposes) and i s surrounded by a d i s c of a i r of 6 centimeters diameter and 1 centimeter t h i c k n e s s .  It i s  covered w i t h a c i r c u l a r aperture of 1 centimeter d r i l l e d i n a brass d i s c B .  Another brass d i s c i s placed i n a symmetrical  p o s i t i o n , 1 centimeter from the l a t t e r face of the s t e e l cylinder.  The whole i s enclosed i n an alundum c y l i n d e r C  which i s .wound n o n - i n d u c t i v e l y w i t h a nichrome heater E.  A  r e s i s t a n c e thermometer F i s then wound over the heater, shorts being avoided by having a l l wires E and F embedded i n alundum cement.  This c y l i n d r i c a l r o l l i s then covered by a r o l l o f  several thicknesses of asbestos sheeting D, surrounded by rock wool, which c o n s t i t u t e s a rectangular f i l l i n g of a wooden box, whose inner dimensions are 12 cm. by 12 cm. by 20 cm.  The  f r o n t end of t h i s box i s covered by a thickness o f aluminum J w i t h a 1 cm. aperture.  A hole was bored through the back  brass d i s c B and the s t e e l c y l i n d e r A to the apex of the c o n i c a l hole as shown i n f i g u r e ( 1 ) .  This hole contains.: the thermo-  couple, which i s of i r o n constantan.  The thermocouple  cali-  b r a t i o n curve was p l o t t e d , i n d i c a t i n g close agreement w i t h the c a l i b r a t i o n curve f o r such a couple found i n the Wheelco t a b l e s . The r e s i s t a n c e thermometer has a temperature c o e f f i c i e n t of .0045 per degree Centigrade.  The wire used has  a length of 20 f e e t , r e s i s t a n c e at 0° C. being 24.6 ohms, and at 200° C. 25 ohms.  21  In figure (2), r a d i a t o r i s shown.  the o p t i c a l system of the b l a c k body  A l l p l a t e s of aluminum are h i g h l y p o l i s h e d  on the side f a c i n g the source, and painted w i t h a mixture of lamp-black and methyl hydrate on the side away from the source. O r i g i n a l l y , the b a f f l e system consisted of two b a f f l e s of type A (figure (2)  ), and two b a f f l e s made of s i n g l e aluminum p l a t e s ,  of the dimensions i l l u s t r a t e d and of square area.  The present  system, which n e c e s s i t a t e d removal of a l l b a f f l e s except that shown at A, was due to s t a n d a r d i z a t i o n of the l e n g t h of separat i o n between the s e n s i t i v e area of the d e t e c t o r and the v i r t u a l source to 20 centimeters. of the sheet type.  The asbestos used i n s h i e l d A i s  F i s a s l i d i n g s h u t t e r of aluminum, a l s o  o r i g i n a l l y of s e v e r a l a i r separated aluminum p l a t e s w i t h p o l i s h e d and blackened s i d e s , now c o n s i s t i n g of a s i n g l e p l a t e of aluminum. The chopper d i s c B ( f i g u r e (2)  ) may be used e i t h e r "  f o r s i n u s o i d a l modulation or square wave modulation of the incoming r a d i a t i o n , depending on the d e s i r e d a p p l i c a t i o n .  This i s  effected by having holes and spaces of equal widths f o r square wave modulation, or i n a s p e c i f i e d r a t i o f o r s i n u s o i d a l modulation. 10.2  There are 30 holes i n the chopper wheel at a radius of centimeters.  The chopper motor D i s an 1800 r.p.m.  synchronous motor, and i s mounted below a brass tube J , blackened on the i n s i d e . C i s an aperture d i s c , w i t h eight holes d r i l l e d i n i t . The diameters of these h o l e s , as measured by a t r a v e l l i n g microscope, are .798,  .631,  .577,  .482,  .369,  .3,  .189 and  .109  22 centimeters r e s p e c t i v e l y .  The aperture standard  normally  used.is 3 m i l l i m e t e r s i n diameter. The detector i s held by means of clamps w i t h i t s s e n s i t i v e area at K.  Great care was exercised i n designing  t h i s o p t i c a l system, so that a l l c e l l s would be uniformly . radiated from the black body, f o r a l l s e n s i t i v e areas encountered.  The maximum radius f o r the s e n s i t i v e area of a detector  c e n t r a l l y mounted on the a x i s of the o p t i c a l system i s o f 1 centimeter radius f o r an aperture of 3 m i l l i m e t e r s required to a c t as a v i r t u a l  source.  A b r i e f d i g r e s s i o n i s i n order here to e x p l a i n how the c i r c u l a r 3 m i l l i m e t e r aperture acts as a v i r t u a l  source.  A complete treatment i s given by Roberts (16) on page 390 and i s as f o l l o w s w i t h reference to f i g u r e ( 3 ) . The small hole cd i s the 3 m i l l i m e t e r aperture i n question, the r e c e i v e r i s ab, and the r a d i a t o r i s a'b . 1  The  e s s e n t i a l feature i s that the l i n e s ad and be produced i n t e r s e c t the r a d i a t o r , taken as the 1 centimeter c o n i c a l hole.  This  c o n d i t i o n ensures that the r a d i a t i o n received by any and every point on the r e c e i v e r i s the same as i f the r a d i a t o r were of the same area as the aperture cd i n the screen and were s i t u a t e d i n the plane of the aperture.  For proof of t h i s , consider the.  r a d i a t i o n received by an i n f i n i t e s i m a l area at the point P on the r e c e i v e r .  As f a r as the point P i s concerned, the only  part of the r a d i a t o r : which i s e f f e c t i v e i s represented I f A , i s the area of cd, and A J that of c ' d , 1  by c'd'.  23  K  M  -  C ^ / C o - " ) *  ( 1 5 )  that i s , the areas are p r o p o r t i o n a l to the squares o f t h e i r distances from the point P.  But the i n t e n s i t y of r a d i a t i o n  f a l l s o f f i n v e r s e l y as the square o f the distance from the source.  Since t h i s i n t e n s i t y i s also p r o p o r t i o n a l to the area  of the source, the increase i n area compensates  f o r the increased  d i s t a n c e , and the r a d i a t i o n at P from the surface c'd i s the 1  same as the r a d i a t i o n from a surface o f the s i z e of ed s i t u a t e d i n the plane of the screen would be. Under experimental procedure i n Chapter IV, s e c t i o n (6) i t w i l l be seem.that an experiment was performed to t e s t whether the source was a c t u a l l y a v i r t u a l one. The whole framework containing the black body may be mounted e i t h e r v e r t i c a l l y or h o r i z o n t a l l y , depending on whether c e l l s which have to be cooled have side or end windows, since the  c e l l s must always be mounted v e r t i c a l l y to allow t h e i r being  cooled.  2.  Temperature Control f o r the Black Body Radiator.  The standard s e t t i n g of the black body i s  500°  K.,  and to ensure that t h i s temperature i s produced w i t h no change over long durations of measurements, i t was found necessary to use a temperature c o n t r o l of the type d e s c r i b e d .  24  A bridge network i s made up, of which two arms are the r e s i s t a n c e thermometer already described i n the preceding s e c t i o n , and a c o n t r o l , R  ttl  and R^. r e s p e c t i v e l y .  The c o n t r o l  i s made up of two wire-wound potentiometers i n s e r i e s ;  one  has a value o f 2G0 ohms and acts as the coarse c o n t r o l , the other being a 5 ohm r e s i s t a n c e i n s e r i e s w i t h i t which may be used as the f i n e c o n t r o l . E s s e n t i a l l y , the f u n c t i o n o f the c i r c u i t i s the mixing of two a.c. v o l t a g e s , one o f which has a constant  amplitude  . and a constant phase, the second having v a r y i n g amplitude and approximately constant phase.  When the amplitude of the  second voltage i s v a r i e d , the output s i g n a l v a r i e s i n phase and i t s amplitude i n c r e a s e s .  This s i g n a l i s applied to the g r i d  of an F.G. 57 t h y r a t r o n . The f i r s t s i g n a l i s provided by an a.c. network across the 6.3 v o l t heater, supply, and can be adjusted t o have a phase v a r y i n g w i t h i n plus o r minus o f 120° w i t h respect t o the a.c. voltage supplied to the t h y r a t r o n anode.  This phase  i s adjusted by means o f a 5G* kiloohm potentiometer so t h a t , when the bridge i s i n balance, the s i g n a l which i s applied t o the g r i d of the t h y r a t r o n gives e x a c t l y the current through the heater winding of the black body needed to maintain the black body r a d i a t o r at i t s balance I f the R temperature,  v  temperature.  s e t t i n g i s above the present b l a c k body  the bridge goes o f f balance and t h i s a d d i t i o n a l  s i g n a l produced across the temperature bridge i s added t o the  25  network s i g n a l , i n c r e a s i n g the g r i d voltage i n amplitude and changing i t s phase.  This phase change i s such that the on  period of the t h y r a t r o n i s i n c r e a s e d , which i n t u r n gives an increase i n the heater current.  The heater i s i n the anode  load of the t h y r a t r o n . I f the black body i s too hot, however, the phase angle v a r i e s such that the on period decreases. A 6K6 tube i s l i n k e d i n the c i r c u i t as a cathode f o l l o w e r to provide a low impedance s i g n a l to the g r i d w i t h l i t t l e or no d i s t o r t i o n . The o v e r a l l c i r c u i t gain i s about 3 0 , 0 0 0 , and regula- . :  O  ^  o  t i o n has been found b e t t e r than 0 . 1 K. f o r 500 K. o p e r a t i o n , by measurements made w i t h a thermocouple. The thermocouple which measures the e f f e c t i v e b l a c k body temperature may be read on a small m i l l i v o l t m e t e r marked "Centigrade" on the apparatus.  For higher accuracy than t h i s ,  an e x t e r n a l potentiometer must be used f o r the temperature measurements. The advantage of t h i s type of c o n t r o l u n i t over other types i s that the black body can be brought from room temperature to anywhere i n the (400-600)° K. r e g i o n i n a matter o f one and one-half hours without having to b r i n g the r a d i a t o r to w i t h i n a few degrees f i r s t , as i s necessary w i t h most other conv e n t i o n a l types.  26  3.  The 900  cycles per second tuned a m p l i f i e r . The a m p l i f i e r was b u i l t a f t e r a design by Brown ( 1 7 )  and i s i l l u s t r a t e d i n f i g u r e ( 4 ) .  The merits of the ampli-  f i e r are that i t has a f l a t peak of s e v e r a l cycles to compensate mains voltage changes a f f e c t i n g the chopping  frequency,  a high gain and a l i n e a r response. There are three main stages of a m p l i f i c a t i o n i n the o r i g i n a l c i r c u i t , each c o n s i s t i n g of three tubes.  The  cathodes  of the f i r s t and t h i r d tubes are connected to give negative feedback.  An attenuator i s placed between the f i r s t  and  second group of three tube stages, to step up the a m p l i f i c a t i o n by convenient  f a c t o r s of t h r e e .  A tuned anode load i s placed'  on the t h i r d tube i n each of the f i r s t two stages, one tuned at 880 cycles per second and the other at 920 c y c l e s per second, to give a f l a t - t o p p e d frequency response curve at 900 second.  c y c l e s per  The c o i l s are t o r o i d a l , and are mounted i n mumetal  cases to give magnetic and e l e c t r o s t a t i c screening. The output i s fed through a transformer w i t h a 1 : 1 r a t i o from the primary to each h a l f of the secondary.. The present a m p l i f i e r i s e s s e n t i a l l y the same as the above with the f o l l o w i n g m o d i f i c a t i o n s .  F i r s t l y , a low noise  twin t r i o d e 12AY7 replaces the f i r s t two pentodes i n each of the three groupings.  Secondly, some p o s i t i v e feedback i s employed  between the cathodes of the second and t h i r d tubes of each r i n g of three.  T h i r d l y , the output twin diode has been replaced  by two 1N34  rectifiers.  The heater voltage i s adjusted to give  27  l e a s t hum by means of a 150 ohm potentiometer  across the  heater winding connected to some point on the power supply. The output i s shown on a 0-200 microammeter whose f u l l - s c a l e d e f l e c t i o n corresponds to 10 m i l l i v o l t s  output.  The data f o r the a m p l i f i e r i s as f o l l o w s . The gain = (50)  + (50) + (24)  = (124) db = 1.5 x 1 0  7  With input terminals shorted the noise l e v e l i s equivalent to ( . 2 2 ) j>V.  No noticeable change on the output meter i s noted  f o r a mains voltage range 90 - 135 v o l t s on a m p l i f i e r s c a l e 11. On open c i r c u i t - n o i s e l e v e l i s equivalent to ( 1 . 5 3 )  jA v o l t s .  Figure (5) shows the frequency-response curves f o r inputs o f 2.5 m i l l i v o l t s and of 5»01 microvolts r e s p e c t i v e l y .  Figure  (6) shows the l i n e a r i t y of response f o r a gain s e t t i n g of  (11).  The peak of the frequency-response curve occurs at a frequency of 895 cycles per second.  The bandwidth i s 44 cycles per  second at minus 3 db. Considering R  2  as the dynamic impedance of the tuned  load at the operating frequency, the gain per r i n g of three stage i s R /R]_, where R-^ i s the cathode load of the t h i r d 2  tube.  With desired values of the tuning c o i l and the "Q" of the tuned l o a d , Brown (18) has shown that R  2  i s approximately  40 kiloohms at the required operating frequency.  equal to To o b t a i n  maximum g a i n , one must make R^ s m a l l , which c a l l s f o r a tube of short g r i d base and high g  m  i n both the t h i r d and f i r s t  tubes.  Brown (18) shows that a f r a c t i o n a l change of loop gain dA/A and the f r a c t i o n a l change of gain dG/G are r e l a t e d by  28  Thus, f o r A = 999 > a 10$ change i n A gives a 0.1% change i n G . The a m p l i f i e r gives long-term s t a b i l i t y of g a i n , low noise l e v e l , and l i n e a r i t y of response, and i s therefore i d e a l l y s u i t a b l e i n the work attempted here.  4. Time Constants Apparatus.  The time constants are measured by a method explained under Experimental Procedure, chapter IV, s e c t i o n ( 2 ) . A neon 30 tube i s used whose time of response i s characterised by a time constant of 'v/lO  sec.  I t s response  curve was obtained using the method o u t l i n e d i n chapter IV, s e c t i o n (5).  Neon, as the gas f o r d i s c h a r g e , was chosen  because although the time constants of such tubes may be somewhat longer than f o r other gases, notably hydrogen, hydrogen sources lead to other complications due to the s c a r c i t y o f i n f r a red l i n e s obtainable from them. The l i m i t i n g f a c t o r to be s c r u t i n i s e d i n choosing a s u i t a b l e source of i n f r a - r e d r a d i a t i o n , i s the i o n i s a t i o n time of the gas, which f o r the tube used was found to be<lo~ sec.. Q  29  The M u l t i v i b r a t o r used c o n s i s t s of s i x tubes, and employs a p l a t e - t o - g r i d c o u p l i n g .  The s i g n a l i s taken from  a cathode f o l l o w e r to give a low impedance source f o r switching the neon.  The range of the m u l t i v i b r a t o r extends f o r  square waves w i t h half-widths from 5 microseconds to 10  milli-  seconds, and the edges of the square waves are l e s s than 0 . 5 microseconds long;  a f i g u r e good i n comparison to that quoted  by Elmore and Sands (19)  on page 81, who give a steepness of  0 . 1 microseconds as a t h e o r e t i c a l l y a t t a i n a b l e l i m i t , f o r a m u l t i v i b r a t o r using two pentodes and having a l a r g e consumption of power;  a s i m i l a r scheme to that used i n t h i s m u l t i v i b r a t o r  as mentioned i n t h i s paragraph. The frequency of o s c i l l a t i o n can be v a r i e d f a i r l y c o n v e n i e n t l y . i n steps, each step having a continuous range overlapping those of other steps, and covering the t o t a l range' of 5 to 10,000 microseconds mentioned  above.  Voltages are supplied by an e l e c t r o n i c a l l y regulated power pack.  S p e c i a l a t t e n t i o n has been given to the b u i l d i n g  and design of t h i s power pack, to enable use of various neon tubes w i t h a l a r g e v a r i a t i o n of s t r i k i n g v o l t a g e s . A wide band p r e a m p l i f i e r was designed f o r use w i t h the apparatus f o r measurement of time constants.  Originally,  a s i n g l e stage p r e a m p l i f i e r was b u i l t w i t h a gain of 50;  this  was not of high enough gain f o r measuring time constants o f some of the lead selenide and lead t e l l u r i d e c e l l s , whose output i s smaller than that of most lead sulphide c e l l s , f o r which  30  l a t t e r t h i s a m p l i f i e r was o r i g i n a l l y intended.  Therefore a  new wide band p r e a m p l i f i e r was b u i l t w i t h a gain of 2000, which could be used to measure time constants of most of the detectors studied i n t h i s research.  The c i r c u i t i s shown i n  f i g u r e (7)> and c o n s i s t s of two high g  m  sharp c u t - o f f  miniature pentodes and a cathode f o l l o w e r . put e i t h e r d i r e c t l y on the  6AG5  The output i s  plates of a double-beam o s c i l l o -  scope, or through the low gain a m p l i f i e r on the o s c i l l o s c o p e , which has a f l a t response out to seven megacycles per second. The frequency range of the p r e a m p l i f i e r i s governed at the low end by the c o n d i t i o n that 1/RC be very much l e s s than 50 where R and C afe as i n f i g u r e 7'•  The values o f R  (3.3 Megohms) and C ( 0 . 5 microfarads) s a t i s f y t h i s c o n d i t i o n . At high frequencies, the value of R must be much l a r g e r than 1/Cw, where C i s the input capacitance of the 6AG5 tube, and w i s the highest angular frequency used. also f u l f i l l e d .  This c o n d i t i o n i s  Decoupling between stages has been used to  eliminate p o s i t i v e feedback, and the screens of the two pentodes are decoupled by 8 microfarad e l e c t r o l y t i c condensers as shown.  Use of negative feedback has not been found necessary  (by the removal of the 250 microfarad cathode s e l f - b i a s condensers).  The g r i d o f the cathode f o l l o w e r has been put at  150 v o l t s to a l l o w output pulses of t h i s height to be seen on the o s c i l l o s c o p e . scope i s 22 v o l t s .  The a c t u a l maximum s i g n a l i n t o the o s c i l l o -  31  Figure (8)  shows the frequency-response curve f o r the  wide band p r e a m p l i f i e r , which covers the range of square wave s i g n a l s from the m u l t i v i b r a t o r f a i r l y adequately and w i t h no distortion,  as has been found by using a frequency adjusted  attenuator f o r using the 120 v o l t m u l t i v i b r a t o r output as an input s i g n a l to the p r e a m p l i f i e r , and comparing the input  and  the ouput by means of the double-beam o s c i l l o s c o p e .  5.  C e l l Mountings  Most of the c e l l s studied were e i t h e r lead or lead t e l l u r i d e .  sulphide  The lead sulphide Admiralty c e l l s were  used at room temperature, and mounted i n brass s h i e l d i n g ( f i g u r e ( 9 ) ) w i t h c o a x i a l cable connectors.  The other lead  sulphide, lead t e l l u r i d e and lead selenide c e l l s were e i t h e r commercial ones and already mounted w i t h a tube base (e.g. the B.T.H. lead sulphide and lead t e l l u r i d e c e l l s ) and thus r e quired a metal screen w i t h the tube socket i n one side and  a  c o a x i a l connector i n the other w i t h a screened lead c a r r y i n g the s i g n a l i n s i d e the can, or the c e l l s were sometimes of glass with two tungsten electrode leads at one end and a window at the other, i n which case a screened lead w i t h brass connectors was used, and the whole screened i n a r o l l of grounded t i n f o i l .  6.  S p e c t r a l Response Measuring Equipment The equipment consisted of a Brown recording p o t e n t i o -  32  meter, whose f u l l s c a l e reading corresponds to 10  millivolts,  a P e r k i n and Elmer model 12C i n f r a - r e d monochromator w i t h a r o c k s a l t prism, a globar f o r a source of b l a c k body r a d i a t i o n , a standard P e r k i n Elmer thermocouple d e t e c t o r and associated d.c. a m p l i f i e r , and a 900 c y c l e s per second tuned a m p l i f i e r , a chopping wheel and motor. The globar operates w i t h a current of 2.25 a temperature  amps, at  of 1400° K. and w i t h an e m i s s i v i t y of about .9-  More d e t a i l s are given i n s e c t i o n (4) of chapter IV d e a l i n g w i t h the experimental  procedure.  Chapter IV  THE EXPERIMENTAL PROCEDURE  The Factor o f Merit M> 2  as defined i n C l a r k Jones's  paper (6) uses the f o l l o w i n g measureable p r o p e r t i e s of a d e t e c t o r , namely: r e s p o n s i v i t y , minimum detectable energy, time constant, the e l e c t r i c a l r e s i s t a n c e and the s e n s i t i v e area. Dealing w i t h each o f the above i n order, i t may be said that the measurement o f the e l e c t r i c a l r e s i s t a n c e and the s e n s i t i v e area poses no great d i f f i c u l t y , only i n c e r t a i n cases o f high r e s i s t a n c e o r o f unusual envelopes and surfaces of detectors may d i f f i c u l t i e s be encountered. 1. Responsivity and Responsivity to Noise Ratio  For the measurement o f the r e s p o n s i v i t y , defined as the r a t i o o f the output voltage of a detector w i t h an e l e c t r i c a l output t o the power i n c i d e n t upon the d e t e c t o r (13), and w r i t t e n as S, a standard r a d i a t i o n s i g n a l i s the f i r s t r e q u i r e ^ ment.  To o b t a i n t h i s , use i s made o f Stefan's Law of r a d i a t i o n  from a p e r f e c t l y black body, that the t o t a l emissive power o f such a body i s p r o p o r t i o n a l t o the f o u r t h power o f i t s absolute temperature, £he constant o f p r o p o r t i o n a l i t y , v T , being constant, and numerically equal t o  5*672 x 10"^ ergs  Stefan's  cm"  2  34  degrees  sec v  The amount o f r a d i a t i o n i n the form o f  - J  energy received per second by a p e r f e c t l y black r e c e i v e r o f area A  (the chopper wheel being at a temperature o f T-L ° K . )  2  from a r a d i a t i o n o f p e r f e c t l y black body type a t temperature T ° K., where A-^ i s the area o f the v i r t u a l aperture mentioned 2  i n chapter I I I , s e c t i o n ( 1 ) i s an amount  V  Q  L  T  S  -  T  ^  K  K  ^  (17)  where D i s the distance o f the r e c e i v e r from the aperture. From ( 1 7 ) ? usingCr= 5 . 6 7 x 1 0 ~ T  2  = 5G0  0  K., T  x  = 3 0 0 ° K., A  X  1 2  joules  = CTV4)  1  cm.^degree'^sec." , 1  (.3)  2  cm.  2  and D =  20 cm., Q / A may be obtained, and w i l l be i n u n i t s o f watts 2  per  square centimeter. A number o f standard values are seen t o have been  used. the  The black body i s heated t o a temperature o f  500°  K.,  c i r c u l a r aperture i s made to be 3 m i l l i m e t e r s i n diameter,  and the d i s t a n c e used from the aperture t o the s e n s i t i v e l a y e r of the c e l l i s made equal t o 2 0 centimeters. (The c e l l s are worked at the temperatures of l i q u i d oxygen ( 9 0 ° K . ) , a mixture of d r y i c e and methyl hydrate ( 2 0 1 ° K.) and a t room temperature  ( 3 0 0 ° K.) ) .  The exact amount of r a d i a t i o n received  by any detector placed at the standard d i s t a n c e and centered on the o p t i c a l a x i s o f the black body system i s thus known, making the f o l l o w i n g assumptions.  The source must be a v i r -  t u a l one, and that t h i s i s so may be seen by reference t o the  35 experiment mentioned i n s e c t i o n (6) o f t h i s chapter.  Second-  l y , the o p t i c a l - s y s t e m must be p e r f e c t l y a l i g n e d ;  thirdly,  the black body must be black and a t a temperature  o f 500° K.  Tests have not been performed as t o the blackness o f the black body, an assumption of 0.9 t o t a l black body emission as a m i n i mum conservative estimate seems to be q u i t e j u s t i f i a b l e , and c e r t a i n l y not an unreasonable  figure.  The temperature  control  i s accurate t o 0.1° K. i n 500° K. (see chapter I I I , s e c t i o n (2) ), and the temperature  s c a l e i s c a l i b r a t e d by means o f an  accurate Leeds and Northrup potentiometer, using the c a l i b r a t e d i r o n constantan thermocouple o f the black body.  The values  of the detector's s e n s i t i v e area ( A i n equation  (17) ) and  2  of A i are the l e a s t w e l l known, A i s known to approximately 1  one per cent, A  2  to w i t h i n as l i t t l e as ten per cent.  The i n c i d e n t power must be corrected f o r modulation; since t h i s i s square wave then the root mean square value o f the i n c i d e n t power f a l l i n g on the r a d i a t o r i s equal to one h a l f of the value given i n equation (17) •  The photoconductive  cell  i s also a poor black body r e c e i v e r f o r wavelengths greater than the neighbourhood of r3 t o 5 microns, which corresponds to a very small percentage o f the t o t a l power from the black body, o f the order o f one per cent.  Therefore equation (17)  must also be corrected by m u l t i p l y i n g i t by a f a c t o r o f the order o f 1/100.  These c o r r e c t i o n s are a p p l i e d i n the next  chapter. The detector i s connected  to the 900 cycles per  36 second tuned low noise a m p l i f i e r by means o f a short c o a x i a l cable going d i r e c t l y t o the input condenser o f the a m p l i f i e r , bypassing any input connectors, and i s s h i e l d e d , as shown i n f i g u r e (9) by a brass c y l i n d e r .  The s e n s i t i v e l a y e r l i e s  i n s i d e the tube J of f i g u r e ( 2 ) , which i s blackened  i n s i d e by  a mixture o f lamp-black, a l c o h o l and a small trace o f s h e l l a c . The a m p l i f i e r passes a current through the detector (a photoconductive  c e l l ) producing a voltage across i t o f some  number o f m i c r o v o l t s .  The detectors have r e s i s t a n c e s o f the  order o f kiloohms, and the current used i s o f the order o f microamps.  The detector "sees" the black body through a suc-  cession o f b a f f l e s and the aperture, which l a t t e r are a l l at room temperature.  A chopper wheel, painted b l a c k , cuts the  r a d i a t i o n i n t o approximately square bursts at a frequency o f 900 cycles per second;  each burst of r a d i a t i o n changes the  c o n d u c t i v i t y of the c e l l by some f r a c t i o n o f the zero  frequency  change of c o n d u c t i v i t y from 300° K. t o 500° K. (or from the operating c e l l temperature t o 500°K.).  This f r a c t i o n may be  taken as u n i t y , and i s a c t u a l l y given i n chapter V.  Thus a  s i g n a l i s generated, a t a frequency o f 900 cycles per second, and i s a m p l i f i e d and r e c t i f i e d by the a m p l i f i e r to give a reading equivalent t o the root mean square value o f v o l t s i n put (or v o l t s output from the d e t e c t o r ) . The reading taken i s on a microammeter c a l i b r a t e d f o r each scale t o give root mean square v o l t s input from the detector.  37  The a m p l i f i e r has the f o l l o w i n g c h a r a c t e r i s t i c s 1. A bandwidth o f 44 cycles per second at minus 3 db. 2. A peak centered at 895 c y c l e s per second on a l l ranges (see f i g u r e (10) ) 3. A f l a t response from 890 t o 900 cycles per second on a l l ranges (see f i g u r e (10) ) 4. L i n e a r response on a l l ranges (see f i g u r e (11) ) 7  5. A gain o f (124) db = 1.5 x 10  on scale 11  6. An open c i r c u i t s i g n a l o f (1.53) microvolts 7. A shorted input s i g n a l of (.22) m i c r o v o l t s 8. No detectable change i n output on the top o f scale 11 f o r a v a r i a t i o n i n mains voltage from 90 to 135 v o l t s 9. Good r e p r o d u c i b i l i t y at a l l times These considerations make the a m p l i f i e r i d e a l f o r measuring s i g n a l s from the detectors under study, and mean that the a m p l i f i e r can be used f o r detectors w i t h short time constants and of r e s i s t a n c e ranges from 50,000 to 1,000,000 ohms, and which have noise appreciable i n comparison w i t h the noise value of the a m p l i f i e r ( i . e . with (.22) m i c r o v o l t s ) . A change i n mains frequency w i l l change the chopper frequency but w i l l not change the output s i g n a l because of the f l a t a m p l i f i e r response.  A change i n mains voltage i s a l s o  seen to have n e g l i g i b l e e f f e c t on the output. i s eliminated by having a tuned a m p l i f i e r .  Stray r a d i a t i o n Linearity of  response ensures single-valued-ness of output as a f u n c t i o n o f  38  the i n p u t , and speaks f o r i t s e l f .  S t a b i l i t y , which i s  achieved by the use of a great amount of negative feedback, cannot be overemphasised  i n importance.  The f a c t that a  tuned a m p l i f i e r (or a.c. a m p l i f i e r ) i s used e l i m i n a t e s the p o s s i b i l i t y of d r i f t encountered i n d.c. a m p l i f i e r s , and enables the achievement of a very high g a i n . The f l a t top of the frequency-response  curve f o r  the a m p l i f i e r has been achieved by the use of two tuned stages, each of three tubes, and having equal gain but centered at d i f f e r e n t frequencies. The disadvantages are seen to be that the a m p l i f i e r i s a very serious p r o p o s i t i o n i n b u i l d i n g , and i n achieving low noise.  This means great care i n s o l d e r i n g j o i n t s , d i f f i -  c u l t y i n r e p l a c i n g low noise tubes, c a r e f u l grounding, and only at one p o i n t i n the c i r c u i t , c a r e f u l screening between stages and use of screened i n t e r s t a g e l e a d s , and the use of expensive and bulky paper and o i l condensers  r a t h e r than e l e c -  t r o l y t i c ones (except f o r screen decoupling purposes).  It  i s a l s o necessary to use wire-wound r e s i s t o r s i n the f i r s t stage, which have very low noise of the order of the Johnson noise of the r e s i s t o r .  Avoidance of 60 c y c l e per second  pickup i s a l s o of the utmost importance, not only i n the amplif i e r i t s e l f , but on the d e t e c t o r too.  I n the case of the  d e t e c t o r , t h i s i s accomplished by very c a r e f u l screening, and care i s taken to ensure w e l l - s o l d e r e d j o i n t s on the d e t e c t o r . On the a m p l i f i e r , the c o l l s used as p l a t e loads are a l s o  39 screened, as are input lead and output f o r o s c i l l o s c o p e t e s t ing  and the d.c. output.  The bottom o f the chassis i s ' t i g h t l y  covered, and the power supply i s kept as remote as p o s s i b l e from the a m p l i f i e r i t s e l f .  Any audio-frequency a m p l i f i e r ,  such as t h i s one, i s also very prone t o hum and motorboating, besides which any 900 cycles per second harmonics o f the main v o l t a g e , unless very c a r e f u l l y kept removed and i s o l a t e d , w i l l cause o s c i l l a t i o n s to develop.  O s c i l l a t i o n s between stages  must also be avoided at a l l c o s t s , by keeping inputs and outputs o f stages w e l l removed from each other.  The heater v o l t -  age windings o f the transformer are t h e r e f o r e c a r e f u l l y biased by means o f a 150 ohms potentiometer ( f i g u r e (4) ) t o give l e a s t noise f o r the a m p l i f i e r output under shorted input condition of operation.  Another problem i s microphonics, which  at audio-frequencies f r e q u e n t l y cause havoc i n the readings. These too must be avoided, or measurements made under c o n d i t i o n s of l e a s t l o c a l i n t e r r u p t i o n ( p r e f e r a b l y a t n i g h t ) . The attenuator i t s e l f must be c a r e f u l l y designed so that the g r i d input of the second stage of a m p l i f i c a t i o n i s never l e f t f l o a t i n g .  For t h i s i t i s u s e f u l to o b t a i n an a t t e n -  uator which has s h o r t i n g spacers such that when the a m p l i f i e r i s on no range (between ranges) the g r i d o f the f i r s t tube o f the second stage ( r i n g o f three) i s shorted to ground. For c a l c u l a t i n g ' r e s p o n s i v i t y to noise r a t i o (R, an accurate determination o f the noise voltage needs to be made.  C l a r k Jones (13)  defines R as  U  s/M-  3  (18)  where N denotes the noise power per u n i t band width.  The  noise power may be defined as the mean, square value of the noise v o l t a g e . In d i s c u s s i n g the n o i s e , r e f e r e n c e must be made to the  f o l l o w i n g sources of n o i s e .  C l a r k Jones (3) describes  these a p p r o p r i a t e l y as;  a) The R a d i a t i o n Background This i s due to the black body r a d i a t i o n f i e l d the  detector i t s e l f and from the surroundings.  from  The noise due  to t h i s background i s c a l l e d temperature noise (3, 13)> and i s due to the f l u c t u a t i o n s i n temperature of the heat d e t e c t i n g element.  Since photoconductive c e l l s are not temperature  d e t e c t o r s , i . e . do not operate by changes i n t h e i r temperature caused by the i n c i d e n t r a d i a t i o n , t h i s aspect need not be considered.  The photoconductive c e l l does not operate by changes  of i t s temperature, exchanging energy w i t h i t s surroundings only by r a d i a t i o n .  Thus f l u c t u a t i o n s i n the output of such a  photoconductive c e l l are due e n t i r e l y to f l u c t u a t i o n s i n the r a d i a t i o n , and thus the r e s u l t s quoted by C l a r k Jones (3) w i l l h o l d , since he derives these on j u s t such an assumption of  f l u c t u a t i o n s i n the r a d i a t i o n .  The r e s u l t s obtained by him  hold f o r detectors whose s i z e i s l a r g e compared to the wavelengths of the r a d i a t i o n being detected, and which obey Lambert's Law.  This r a d i a t i o n background i s seen to give r i s e  to a minimum detectable power of the d e t e c t o r , H , m  equal to  i t s noise equivalent power, where f o r a d e t e c t o r w i t h an e m i s s i v i t y of u n i t y at a l l frequencies  '  ^ 4 ' L o - T * K T / t ^ / ^  where ^  s  ( 1 9 )  i s the absorption c o e f f i c i e n t , taken as u n i t y , A i s  the area of the d e t e c t o r , T i s i t s temperature, ~C- i s i t s time constant, and c r and k are Stefan's constant and Boltzmann's constant, r e s p e c t i v e l y .  The f a c t o r (^u./^s") has been taken  equal to u n i t y .  b) The I n t e r n a l Background This type of background i s introduced w i t h i n the detector i t s e l f , and c o n s i s t s of Johnson n o i s e , Current noise and perhaps Semiconductor n o i s e .  c) S i g n a l Noise Whereas the r a d i a t i o n background i s due to the b l a c k body noise of the detector i t s e l f and of i t s surroundings, i n d e t e c t i n g s m a l l changes i n steady s i g n a l s , the s t a t i s t i c a l v a r i a t i o n i n the s i g n a l may,  i f l a r g e , determine the smallest  42 detectable change.  This i s of l i t t l e concern i n t h i s paper.  The a m p l i f i e r i s used to measure the noise when a black s h u t t e r ( f i g u r e (2) ) i s placed across the black body source.  The a m p l i f i e r noise i s subtracted from t h i s n o i s e ,  but i s u s u a l l y too small w i t h the a m p l i f i e r used to be considered at a l l compared to the detector noise..  The  shutter  must be cooled or kept c o o l , and the chopper need not be operated since only the noise i n the a m p l i f i e r bandwidth w i l l appear at the output, and the chopper f r e q u e n t l y causes severe microphonics  due to v i b r a t i o n s i n the system containing the  chopper motor and the d e t e c t o r . A scheme e x i s t s t h e r e f o r e , f o r the  experimental  determination of the r e s p o n s i v i t y , the n o i s e , and thus of the r e s p o n s i v i t y to noise r a t i o .  2. The Measurement of Time Constants  Figure (12) shows i n block schematic form the arrangement of apparatus used to measure time constants of the range which these photoconductive  c e l l s under c o n s i d e r a t i o n  possess.. The m u l t i v i b r a t o r mentioned i n chapter I I I , s e c t i o n (4) has two outputs. ( I I I , (3)  One i s applied to the neon 30 tube  ), the other d i r e c t l y to the  beam o s c i l l o s c o p e .  p l a t e s of a double  The square wave 120 v o l t s on-off voltage  applied to the neon causes i n f r a - r e d r a d i a t i o n to f a l l on the photoconductive  c e l l , which i s e l e c t r o s t a t i c a l l y  screened  43  from the neon by a f i n e wire mesh.  The  speed of response  of the neon i s demonstrated by a procedure o u t l i n e d i n sect i o n (5) of t h i s chapter. The photoconductive c e l l i s attached to the  input  of the wide band p r e a m p l i f i e r , whose output i s then displayed on the Y  2  p l a t e s of the o s c i l l o s c o p e .  The p r e a m p l i f i e r i s  provided w i t h a v a r i a b l e gain c o n t r o l , and has a frequencyresponse curve covering the range of m u l t i v i b r a t o r frequencies adequately, as shown i n f i g u r e (10). The  theory of the measurement of time constants by  t h i s method may  be o u t l i n e d as f o l l o w s .  With reference to  f i g u r e (12), consider a detector r e c e i v i n g a square wave r a d i a t i o n s i g n a l of modulation frequency f cycles per second. h a l f - w i d t h t i s then seen to be equal to l / 2 f .  The  Considering  f i r s t l y a r i s e i n response of the detector to the s i g n a l , f i g u r e (10b),  (20)  But from f i g u r e  (10a)  3  _ k - c  (21) a  Therefore  a - L  —  c  —  c  )  44  which gives _-t/-t  (22)  S i m i l a r l y , f o r a f a l l i n response, w i t h reference to f i g u r e (10c)  (  2  3  )  Combining equations (22) and (23) C  a-\=>  which holds f o r b - c.  >  (24)  Thus the response exponential wave  i s symmetrically placed between the zero and peak amplitudes of the i n c i d e n t  square wave, or of the t h e o r e t i c a l l y a t t a i n -  able output wave f o r "C approaching  zero.  A, the observed amplitude, i s given by  /U(a-W)(l-eSince  14- e  and our f i n a l r e s u l t i s  W  ,  .  (  2  5  )  45  Q  \  (26)  —-c  V 4-  ^  There are two important l i m i t i n g eases.  For low  frequencies, where t i s very l a r g e compared to "C, A becomes equal to a.  At high f r e q u e n c i e s , "C i s very l a r g e compared  to t , the exponential approaches 1, and A may be w r i t t e n  (27)  When a p l o t of the response A versus the h a l f - w i d t h t i s made, a frequency-response  type curve r e s u l t s , which approaches the  value a at the high t end, and has the form of a s t r a i g h t l i n e given by equation (27) at the low t end.  These l i n e s meet  when  (28)  where t = t , the value at the i n t e r s e c t i o n o f these l i n e s , o and f i s the corresponding value of the frequency f . 0  Thus the time constants of d e t e c t o r s may be c a l c u l a ted from the value t o / z .  (  2  9  )  or -C  =  1/4-Sro  (30)  46 I n p r a c t i c e , the p r e a m p l i f i e r gain i s set so that at low frequencies the Y^ and Y have the same amplitude.  2  traces ( s i g n a l and response)  A graph w i t h s e v e r a l points a t low  t values of A versus t i s then p l o t t e d , t  0  i s found from the  i n t e r s e c t i o n of the best low t l i n e w i t h the l i n e A = a, from which "C i s c a l c u l a t e d . The method o u t l i n e d i s l i m i t e d only by the range of the m u l t i v i b r a t o r , the range o f the wide band p r e a m p l i f i e r , and the range oyer which the o s c i l l o s c o p e may be t r i g g e r e d . A l s o , the neon i o n i s a t i o n time constant, i f l a r g e , may enter i n the l i m i t a t i o n s of t h i s method.  I n chapter I I I , s e c t i o n  (4) , the range of m u l t i v i b r a t o r half-widths i s given as being from 5 microseconds to 10 m i l l i s e c o n d s , which means that the t h e o r e t i c a l l i m i t s over which time constants may be measured i s f o r X between 2.5 microseconds and 5 m i l l i s e c o n d s .  In  p r a c t i c e , most of the detectors encountered had time constants w e l l w i t h i n t h i s range. The neon has a very short time constant (see s e c t i o n (5) belo\*) and does not enter i n f o r purposes o f range considerations f o r the measurements of time constants. The p r e a m p l i f i e r has a range from 0 cycles per second to 1 megacycle per second.  The range of the m u l t i v i b r a t o r ,  expressed i n cycles per second f o r square waves, i s from 50 cycles per second to 100 k i l o c y c l e s per second, and the range of the p r e a m p l i f i e r i s , f o r s i n u s o i d a l frequencies from 0 cycles per second to 1 megacycle per second.  This gives a  47 f a c t o r 1G at e i t h e r end of the range f o r conversion between square waves and s i n u s o i d a l waves, that i s , the p r e a m p l i f i e r responds to s i g n a l s of 5 cycles per second s i n waves, and w i l l therefore respond to square waves of 50 cycles per second f r e quency with l i t t l e or no d i s t o r t i o n ;  s i m i l a r l y a square wave  s i g n a l of 100 k i l o c y c l e s per second, when going i n t o the prea m p l i f i e r w i t h a f l a t response out to 1 megacycle per second, w i l l show l i t t l e or no d i s t o r t i o n . The p r e a m p l i f i e r has been designed f o r photoconduct i v e c e l l s , and supplies them w i t h current, i t may a l s o be used i n the measurement of bolometer time constants, and, by removing the  1 megohm input to h i g h t e n s i o n l i n e r e s i s t o r , and adding  more stages i f r e q u i r e d , the a m p l i f i e r may be used w i t h h i g h speed thermocouples and other detectors too.  Thus a s a t i s f a c -  t o r y method f o r the measurement of time constants has been described, and the procedure o u t l i n e d .  3.  The Frequency-Response  Curves of a Detector  The above method, s e c t i o n ( 2 ) , f o r the measurement of time constants conveniently y i e l d s curves of r e l a t i v e r e s ponse versus frequency.  I f any sources of noise and otherrTad-  i a t i o n e x i s t , small i n comparison w i t h the s i g n a l achieved, then the curves w i l l i n t e r s e c t the response a x i s at some p o s i t i v e value f o r half-widths t equal to zero.  This does not  a l t e r the time constant when the extraneous r a d i a t i o n or s i g n a l  48  i s small and steady, a l s o the r e l a t i v e response versus f r e quency curve may be corrected f o r t h i s d e f e c t , knowing the appropriate time constant.  This i s so because the slope of  the low t s t r a i g h t l i n e i s known (and equal to  and  since X i s known and a more accurate determination of a  may  be made using a shutter between the neon and the d e t e c t o r , and s u b t r a c t i n g such a s i g n a l from a  at low frequencies g i v -  ing a . 1  Thus, w i t h these curves, the r e s p o n s i v i t y to noise r a t i o CR may be converted to i t s zero frequency value ^RQ, since the neon output i n watts i n the s e n s i t i v e r e g i o n of microns of the detector i s not presumed to change w i t h i t s modu l a t i o n frequency, and the noise of the c e l l s i s constant  over  a l l chopping frequencies due to the narrow band width of the amplifier.  4.  The Measurement of S p e c t r a l Response of a Detector.  The detector i s placed at the focus of the model 12C P e r k i n Elmer monochromator ( I I I , (6)  ), and i t s output, which  i s from a high gain 900 cycles per second tuned a m p l i f i e r , i s displayed on a Brown recording potentiometer.  The source of  r a d i a t i o n i s a 1400° K. 900 cycles per second chopped globar source.  The wavedrive i s set f o r a c e r t a i n speed, the Brown  recorder i s attached, and a s p e c t r a l curve f o r each d e t e c t o r  49 i s obtained, f o r any p r e v i o u s l y chosen s l i t width. The same procedure, using the same monochromator s e t t i n g s o f s l i t width, wave d r i v e speed, and a P e r k i n Elmer thermocouple and i t s associated d.c. a m p l i f i e r ( I I I , repeated.  (6) ) i s  The graphs are marked at convenient wave d r i v e  numbers, which may be converted to wavelengths from a r o c k s a l t c a l i b r a t i o n curve of wavelength versus wave d r i v e s e t t i n g obtained f o r the r o c k s a l t prism used i n the monochromator. The procedure then i s to take the r a t i o s of the detector  and thermocouple curves at many points (corresponding t o  many wavelengths), and to draw a graph of r e l a t i v e response o f detector to that o f the thermocouple, p l o t t e d against wavel e n g t h i n microns;  t a k i n g the point a t long wavelengths at  which t h i s r e l a t i v e response has f a l l e n to h a l f o f i t s maximum value as the c u t - o f f p o i n t .  Such a curve may then be replaced  by one having uniform response, equal to maximum response f o r wavelengths up to the c u t - o f f wavelength, at which the detector wavelength may be conisdered to f a l l a b r u p t l y to zero. The assumption has been made that the P e r k i n Elmer thermocouple i s a perfect b l a c k body r a d i a t i o n d e t e c t o r .  That  t h i s i s so f o r long wavelengths of the order o f 4 microns and higher i s known, but f o r l e s s e r wavelengths c o r r e c t i o n s may be necessary. satisfactory.  Therefore the procedure followed i s not a l t o g e t h e r However, l a r g e errors are not foreseen i n t h i s  method, and i t may be considered as being w i t h i n the l i m i t s o f  50  other experimental e r r o r s , sueh as the measurement of s e n s i t i v e areas and a l s o of time  5.  constants.  Measurement of the Neon Response  The procedure followed here was to put the neon and a p h o t o m u l t i p l i e r tube of known short time constant i n s i d e a l i g h t - t i g h t enclosure (to minimise p h o t o m u l t i p l i e r n o i s e ) . The neon was covered by a brass d i s c containing a f i n e p i n hole (to prevent f l o o d i n g of the p h o t o m u l t i p l i e r tube) and was connected to the m u l t i v i b r a t o r output.  I t was placed at a  v a r i a b l e distance from the photocathode o f the p h o t o m u l t i p l i e r , and the p h o t o m u l t i p l i e r output was connected to the input of the p r e a m p l i f i e r . the Y  2  The p r e a m p l i f i e r output was then put on  p l a t e s o f the o s c i l l o s c o p e , the other output o f the  m u l t i v i b r a t o r output was connected to the Y^ p l a t e s o f the o s c i l loscope, as before, the only d i f f e r e n c e from f i g u r e (12) being that the photoconductive c e l l was replaced by the photomultip l i e r tube. The r e s u l t s obtained  were, that upon varying the  photomultiplier tube to neon tube distance to give a l a r g e enough output s i g n a l on the o s c i l l o s c o p e to be seen e a s i l y , i t was found when the two o s c i l l o s c o p e traces were matched i n amplitude at the lowest frequencies, they remained so matched to the highest frequency, and the Y  2  (neon tube) trace was seen  to remain square to the very highest frequency.  51  These r e s u l t s c l e a r l y i n d i c a t e the s u i t a b i l i t y o f the neon tube used.  They do not i n d i c a t e what might be the  s i t u a t i o n at higher frequencies, since a m u l t i v i b r a t o r going to much higher frequencies (with 120 v o l t s output) than the one used here was not a v a i l a b l e .  6.  Measurement of S i g n a l versus Area o f V i r t u a l Source ( I n t e n s i t y o f I l l u m i n a t i o n versus S i g n a l )  The d i s c w i t h the v a r i a b l e aperture, used w i t h the black body, and l a b e l l e d C I n f i g u r e ( 2 ) , was used here i n various p o s i t i o n s , and the r e s u l t i n g s i g n a l s f o r various detectors were measured.  F o r each d e t e c t o r , a p l o t of area  of source versus output from a c e r t a i n detector was made f o r many d e t e c t o r s .  These p l o t s were o r i g i n a l l y p a r a b o l i c i n  shape, the s i g n a l f a l l i n g o f f r a p i d l y with l a r g e source The black body was consequently  areas.  reassembled to give a true  v i r t u a l source, even when the aperture diameter was 8 m i l l i metres, f o r the c e l l s i z e s i n question (see the requirement i n chapter I I I , s e c t i o n (1) f o r v i r t u a l sources).  The r e s u l t s  showed a s t r a i g h t l i n e graph ( f i g u r e (in.) ) f o r a l l c e l l s tested and f o r v i r t u a l sources of up to 8 m i l l i m e t e r s i n diameter. The conclusions to be drawn from t h i s , n e g l e c t i n g the p o s s i b i l i t y of pure coincidence, are the f o l l o w i n g : 1. The black body system i s o p t i c a l l y balanced f o r v i r t u a l sources of up to 8 m i l l i m e t e r s i n diameter, which covers  52  the standard s e t t i n g of 3 m i l l i m e t e r s diameter, a l l a t 20 cm* 2. The detectors give a s i g n a l p r o p o r t i o n a l to i n c i d e n t watts;  i . e . v o l t s per watt r a t i o i s independent of power  i n c i d e n t on the detector 3.  The r a d i a t i o n i s black body, since according to Lam-  bert ts law a black body i s e q u a l l y b r i g h t i n a l l d i r e c t i o n s ( i . e . f o r a l l s i z e s of v i r t u a l source), as was found to be the case here.  7.  The Measurement of the S e n s i t i v e Area and o f the E l e c trical  Resistance-  This has already been discussed above at the beginning of t h i s chapter.  The s e n s i t i v e area i s defined as the  average l e n g t h of the two outside electrodes times t h e i r average separation, expressed  i n m i l l i m e t e r s squared.  Since the  s e n s i t i v e l a y e r s of most.of the c e l l s used are f l a t t h i s i s the s e n s i t i v e area.  For other d e f i n i t i o n s of area, where  the s e n s i t i v e area i s not f l a t , other p r e s c r i p t i o n s must be given f o r the d e f i n i t i o n of t h i s q u a n t i t y ( 3 ) .  The area i s  assumed independent of the temperature of the c e l l . The e l e c t r i c a l r e s i s t a n c e i s measured at the temperature of the c e l l coolant, f o r coolants such as dry i c e o r l i q u i d oxygen and a l s o at room temperature.  With some detec-  t o r s , i t i s found that on c o o l i n g , the s t a t i c temperature e q u i l i b r i u m i s reached only a f t e r s e v e r a l seconds or even  53 s e v e r a l minutes, and not immediately.  This has been taken  i n t o c o n s i d e r a t i o n both i n the measurement of the r e s i s t a n c e , and i n the measurement of the s i g n a l to noise r a t i o . Both the r e s i s t a n c e and the area have p o s s i b l e errors as great as 10 per cent, the area because of the vague d e f i n i t i o n o f the electrode boundaries, the r e s i s t a n c e because of the d i f f i c u l t y of accurate measurement of high r e s i s t a n c e s , as w i t h an o r d i n a r y ohmmeter.  Chapter V  THE RESULTS  Nine photoconductive lead sulphide c e l l s were the main items under study.  Four o f these were from the Admir-  a l t y Research Laboratory (A.R.L.), numbers 7 3 , 1 1 9 , 1 2 3 and 131.  F i v e were B r i t i s h Thomson-Houston Company (B.T.H.)  cells.  The chemically deposited one which i s not i n vacuo  but i s open to the atmosphere i s l a b e l l e d "chemical"; the other four are i n vacuo and may be cooled; 292,  293> 3 6 7  and  these are numbered  The A.R.L. c e l l s cannot be cooled,  395.  although they are evacuated. The measurement of the time constants as described i n Chapter IV, s e c t i o n ( 2 ) , was c a r r i e d out and y i e l d e d the r e s u l t s shown i n Table I .  TABLE I Time Constants of Various Lead Sulphide C e l l s a t Three Temperatures ( i n microseconds)  Cell No. 73 119 123 131  Ch.  (300° 165 135 151 82 79  K)  Cell No.  (300°  292 293 367 395  93 79 72 125  —  —  K)  (200° 1000 292 775 1800  K)  (90° 1050 1750 980 1800  K)  55 A t y p i c a l p l o t f o r e v a l u a t i n g X i s shown ( f o r c e l l 131) i n f i g u r e (14).  Most of the measurements, when repeated,  showed very s i m i l a r r e s u l t s ; to reach a temperature  time was allowed f o r cooled c e l l s  equilibrium.  I n chapter IV, s e c t i o n  (3)> mention was made of graphs which showed p o s i t i v e  response  when extrapolated t o zero h a l f width ( i . e . to i n f i n i t e frequency). I t has already been pointed out that t h i s w i l l not a l t e r the value of T?.  I n cases where such a s i t u a t i o n e x i s t e d , i t was  found on subsequent measurement that normal curves were obtained y i e l d i n g the same order of value of time constant. In h i s second paper, C l a r k Jones (13) p o i n t s out that where the response-frequency  curve corresponds to the existence  of a s i n g l e time constant, the f o l l o w i n g r e l a t i o n between f r e quency and r e s p o n s i v i t y t o noise r a t i o w i l l h o l d , namely  (3D  Assuming that the value o f the noise power per u n i t band width i s constant w i t h the frequency f , t h i s r e l a t i o n w i l l hold f o r the response p l o t t e d against frequency.  This p r o p o s i t i o n was  checked f o r s e v e r a l detectors by r e p l a c i n g f i n ( 3 D by l / 2 h , where h i s the h a l f w i d t h as i n f i g u r e (14). a very small e r r o r was found.  I n a l l cases only  This i s a good i n d i c a t i o n that  the detectors studied may be characterised by a s i n g l e time constant.  56  A very i n t e r e s t i n g measurement was made of the time constant of a lead selenide c e l l produced here.  This was  found to have a time constant of 225 microseconds, and showed the v e r s a t i l i t y of the time constants apparatus. The response-frequency  curves as mentioned i n chap-  t e r IV, s e c t i o n (3) are obtained from those used to measure X by r e p l a c i n g h the h a l f - w i d t h by l / 2 f (see f i g u r e ( 1 4 ) J . f a c t o r (1 + ( 2 - f t f - c ) ) ^ 2  was i n v e s t i g a t e d , being close to u n i t y  f o r some c e l l s w i t h short time constants, and as high as for others.  The  10.25  The values obtained are shown i n Table I I .  TABLE I I Frequency C o r r e c t i o n F a c t o r f o r Various Lead Sulphide C e l l s at Three Temperatures  Cell No.  73  ^  (300° K)  119  1.367 1.257  131 Ch.  1.095  123  1.314 1.102  Cell  ^  No.  (300° K)  292 293 367 395  1.130  ^  (200° K) 5.736  1.930 4.492 10.25  1.095 1.080  1.224  (90  'J  K)  6.024 9.940 ' 5.619 10.25  —  The s p e c t r a l response curves, obtained using the Perkin-Elmer thermocouple as a standard, and described i n chapter IV, s e c t i o n ( 4 ) , y i e l d e d curves such as the ones obtained f o r c e l l 293 ( f i g u r e (13)  ).  The f o u r a d m i r a l t y c e l l s gave curves i n d e n t i e a l i n  57  shape, w i t h a strong peak at approximately 2 . 5 microns.  The  c u t - o f f (taken at h a l f the maximum) v a r i e d from 2.8 to 2 . 9 5 -5  microns;  the absolute (10" ) c u t - o f f wavelength v a r i e d from  3 . 1 to 3.4 microns. The B.T.H. evacuated c e l l s have a r a t h e r d i f f e r e n t c h a r a c t e r i s t i c shape o f s p e c t r a l response curve.  I n chapter  I , reference was made to the f a c t that d i f f e r e n t methods o f preparation y i e l d e d d i f f e r e n t s p e c t r a l curves.  Whereas the  admiralty c e l l s had a strong maximum at 2 . 5 microns, the B.T.H. c e l l s and the chemical button c e l l , a l l o f which are prepared chemically, have two strong peaks;  one i s a t 2.15 microns and  the other at from 1.2 to 1.5 microns.  I t i s noted that  decreasing the temperature o f the l a y e r increases the c u t - o f f f o r the B.T.H. c e l l s , as shown i n f i g u r e ( 1 3 ) .  This e f f e c t i s  not very great, however. P u r e l y as a matter o f i n t e r e s t , two A.R.L. lead t e l l u r i d e c e l l s were a l s o s t u d i e d .  Changing t h e i r temperature  from 200° K down to 90° K extended the c u t - o f f i n one case from 4.65 to 5.4-5 microns.  The other c e l l gave apparently negative  results.  The peak o f the good c e l l s h i f t e d from 3 . 8 t o 4.6  microns.  The i n e f f e c t i v e c e l l had a very l a r g e (100$) absorp-  t i o n at 3 microns, and i t may have been r e c e i v i n g s t r a y r a d i a t i o n i n one case.  These r e s u l t s are quoted as a matter o f  interest only. Using the 900 cycles per second a m p l i f i e r (chapter I I I , s e c t i o n (3) ) , a s e r i e s o f measurements was performed on a l l  58  the c e l l s , p l o t t i n g the v a r i a t i o n of the r e s p o n s i v i t y to noise r a t i o R ( f ) as a f u n c t i o n o f the c e l l c u r r e n t .  R ( f ) was found  to be very constant w i t h the c e l l c u r r e n t , which was measured by means of a very accurate microammeter.  The s i g n a l was  always found to be l i n e a r w i t h the c e l l c u r r e n t ;  the noise was  a l s o o f t e n l i n e a r , e x p l a i n i n g the constancy of R ( f ) . However, i n some cases the noise was p a r a b o l i c , causing the value o f R ( f ) to be f l a t only i n the middle range of c e l l c u r r e n t s . A f a i r l y wide range of c e l l currents may t h e r e f o r e be used. T y p i c a l values are from 20 to 100 microamps. R e p r o d u c i b i l i t y of s i g n a l to noise r a t i o s was found to be good, being a f f e c t e d by the c e l l - t o - s o u r c e d i s t a n c e , and by the centering of the c e l l l a y e r about the o p t i c a l a x i s of the black body system.  Care must be taken to avoid heating  the s h u t t e r , as t h i s increases the noise considerably. Q, the power i n c i d e n t upon the c e l l , i s obtained from equation ( 1 7 ) . cm .  The value so obtained i s 17.4 microwatts/  This value must be halved to give an r.m.s. value f o r  square wave modulation of the power.  Another c o r r e c t i o n must  be applied f o r the s p e c t r a l response curve of each d e t e c t o r . The c u t - o f f i s taken at h a l f the maximum, and the response i s assumed to be u n i t y up to t h i s wavelength.  The t o t a l percent-  age of Q/A which corresponds t o the c u t - o f f wavelength i s then calculated. microns. of Q/A.  A 500° K black body curve has i t s peak at 5 . 8 The r a d i a t i o n up to t h i s value corresponds to 25$  C u t - o f f s encountered  are of the order of 3 microns  59 corresponding t o approximately 1% o f Q/A. are applied i n Table V as shown.  These c o r r e c t i o n s  Q i s a l s o shown.  The s i g -  n a l voltage d i v i d e d by the corrected Q i s then shown and gives the estimated value of the r e s p o n s i v i t y i n v o l t s / w a t t ( S ) , values ranging from 200 to 30,000 v o l t s / w a t t being S  Q  encountered.  i s obtained by using the c o r r e c t i o n f a c t o r shown i n equation  (3D. The noise equivalent power P the noise voltage by S .  m  i s obtained by d i v i d i n g  To get the noise equivalent power  Q  per u n i t bandwidth, t h i s value must be d i v i d e d by AJA^- where = 44 cycles/second, and i s the bandwidth of the a m p l i f i e r used.  Table I I I shows that c e l l s 293 and 367 have a value of  t h i s q u a n t i t y equal to 1.69-and 1-75  ( x l O ^ watt) r e s p e c t i v e l y , - 1  a value c l o s e t o the t h e o r e t i c a l l i m i t f o r such c e l l s ( 4 , 5 ) .  TABLE I I I Noise Equivalent Power per Unit Band Width as a Function of the C e l l Temperature (watt x l O - ^ )  Cell No.  292 293 367 395  (300°  446 573 844 1270  K)  (200° K )  (909 K)  10.8 45.5 11.4 52.6  18.3 1.69 1.75 22.8  *  60  The optimum wavelengths o occur when the c e l l s are at 9 0 cell  292,  where t h i s occurs at  ( 5 ) f o r these B.T.H. c e l l s  K. w i t h the exception of 200°  K. .  I t i s suggested  that the A.R.L. c e l l s would have b e t t e r c h a r a c t e r i s t i c s and p r o p e r t i e s , i f they were b u i l t l i k e the B.T.H. c e l l s , which provide f o r c o o l i n g . Table IV shows the evaluated Factors of M e r i t . C l a r k Jones ( 6 ) proposes a value of  f o r type I d e t e c t o r s , (where the numerical f a c t o r depends on the temperature as shown) and  proposes  (6)  f o r type I I d e t e c t o r s ; no c o r r e c t i o n i s made to equation ( 6 ) Equation ( 6 ) may a l s o be w r i t t e n  f o r the l a y e r temperature.  -  ^  I  O  J  l  (kV"P«.t\  as i n chapter I I , s e c t i o n ( 1 ) . C l a r k Jones ( 1 3 ) .  (6a)  A d e r i v a t i o n i s given by  There I s a d i f f e r e n c e between these two -2  expressions of a f a c t o r 1 0  , as seen i n Table IV.  Since  the low frequency c u t - o f f of the detectors has not been c o n s i d ered, t h i s might be thought to be the cause.  However, the  61  percentage of the b l a c k body r a d i a t i o n below 1 micron i s only 0.0001$ of the whole.  The d i f f e r e n c e between t h i s value and  1% (corresponding to the long wavelength c u t - o f f at 2.9 microns) i s n e g l i g i b l e compared to 1%. The proposed reason i s as f o l l o w s .  (13)  C l a r k Jones  prescribes a d j u s t i n g the frequency response curve of the amplif i e r so that the noise power per u n i t band width i s constant at a l l frequencies, whereas i n t h i s research t h i s has not been done (chapter I I , s e c t i o n (1)  ).  From t h i s equalised ampli-  f i e r , a frequency response curve f o r the d e t e c t o r i s to be drawn from which the time constant i s to be  determined.  The noise equivalent power f o r the d e t e c t o r i s to be determined w i t h the a m p l i f i e r now governed by a high frequency RC c u t - o f f which i s added, where RC i s equal to the time constant . Since t h i s has not been done, a constant f a c t o r of 2  approximately 10 (6) and (6a)  has been neglected throughout, and  d i f f e r by t h i s amount.  as predicted by C l a r k Jones (13)  equations  The values of t and  w i l l also have t h e i r  P  m  product  d i f f e r i n g from the product obtained by t h i s f a c t o r . Equation (7)» assuming that the only noise i s Johnson noise, gives a F a c t o r of M e r i t  62  Equation (8) given i n chapter I I , s e c t i o n (1)  may e a s i l y be shown to be i d e n t i c a l w i t h equation ( 6 ) . I n chapter I I , s e c t i o n ( 2 ) , Daly and Sutherland's proposed F i g u r e of Merit has been modified by the author to give  ^  (12)  This may a l s o be shown to be equivalent to equations ( 6 ) and (8).  These r e s u l t s are shown i n Table IV.  the values o f Q, P , R e t c . . '  in '  Table V shows  63  TABLE IV  The Factors of M e r i t of Lead Sulphide Photoconductive  300°  K. M  M  2  2  Cell No.  (6a)  73 119 123 131 292 293 367 395  2.26 421 323 1.73 1.68 316 382 2.34 .346 59.3. .326 52.9 36.4 .231 .0849 15.5 43.6 .271  10  Ch.  2Q0° 292 293 367 395  90° 292 293 367 395  (6)  2 (8)  M  M  2  (12)  n  2  (7)  (4)  4  420  7  322 318 384 59.2 52.6 36.4 15.4 44  5.3 5.25 6.38 .986 .877 .606 .256 .733  678 525 545 825 83 121 67 25.3 60.3  0.0293 0.0590  146  2.43 3.26 3.3 .195  257 170 230 28.6  1.35 .328 1.26 .259  212 276 550 128  .109 1.8 1.11 .0815  1.03 0.65 0.706 0.533 0.0895 0.071  0.046  K. 1.33 1.11 1.59  .141  146 195 199 11.7  196 198 11.7  K. .747 7.46 8.2 .326  80.8  624  912 26.9  80.6 1.34 626 10.4 912 15.2 .451 27.1  Cells  TABLE V Showing Corrected Values Needed to C a l c u l a t e M  K  •c 300°  K. 73  165  400  Conredr  2  f o r the lead Sulphide C e l l s  ^  S C00AU0'Comi.  Cootrt/c(nn  24  550  2.9  0.95  8.26  O.87  1.98  480  3.32  2.63  0.396  1.367  119  135  130  24  440  2.95  1.1  9.58  0.5  2.30  220  1.20  4.17  0.629  1.257  123  151  540  24  312  2.8  0.75  6.53  1.08  1.57  337  2.82  3.82  0.576  1.314  131  82  560  24  335  2.86  0.9  7.83  1.37  1.88  458  2.70  5.09  0.767  1.102  292  93  190  22.8  23.8  2.58  0.4  3.48  0.52  0.794  124  0.176  29.6  4.46  1.130  293  79  155  24  17.5  2.55  0.35  3.04  0.77  0.730  13.5  0.202  38.0  5.73  1.095  367  72  212  22  10.1  2.52  0.32  2.78  0.72  0.612  7.3  0.128  56.0  8.44  1.080  395  125  25  20  7.6  2.62  0.45  3.92  0.22  0.784  1.68  0.0261  84.1  12.7  1.224  Ch  79  380  2.53  0.33  2.87  0.72  0.717  10.1  0.154  46.6  2 0 0 ° K. 1000 292  25  3500  22.8  202  2.6  0.42  3.65  2.28  0.832  460  31.8  293  292  3500  24  150  2.6  0.42  3.65  1.13  0.876  170  3.74  367  775  1550  22  198  2.55  0.35  3.04  1.01  0.669  200  395  1800  80  20  20.4  2.6  0.42  3.65  0.48  0.730  9.8  14  13.4  1.38  7.02  1.095  O.718  0.108  5-736  3.02  0.455  1.930  0.755  0.114  4.492  3.49  0.526  10.25  TABLE V continued  90°  K.  292  1050  3500  22.8  123  2.62  0.45  3.92  2.28  0.895  280  18.9  1.21  0.183  6.024  293  1750  10000  24  210  2.8  0.75  6.53  0.65  0.157  137  86.6  0.074-9  O.OI69  9.94  367  980  5000  22  220  2.8  0.75  6.53  0.65  0.144  143  55.8  0.116  0.0175  5.619  395  1800  270  20  50.3  2.62  0.45  3.92  1.15  0.784  58  7.6  1.51  0.228  10.25  Chapter VI  DISCUSSION  Table IV leads to the conclusion that the best detect o r i s c e l l 367 at 90° K.  Other u s e f u l d e t e c t o r s are c e l l 293  at 90° K., the A.R.L. c e l l s , and c e l l s 292, 293 and 367 at 200° K..  The chemical c e l l and c e l l 395 (at a l l three temperatures),  and the r e s t of the B.T.H. c e l l s at room temperature show the poorest Factors of M e r i t .  Such conclusions are drawn from a l l  columns except those marked (4) and (7)» which do not show the close agreement of the other columns. Both M  2  and Mi are shown f o r purposes of comparison.  Column ( 7 ) 5 by i t s disagreement, shows that the l i m i t i n g noise f o r photoconductive c e l l s i s not Johnson n o i s e .  C l a r k Jones,  i n h i s f i r s t paper (3)> points out that f o r d e t e c t o r s which are cooled by r a d i a t i o n , as these a r e , the temperature f l u c t u a t i o n s would be zero i f there were no f l u c t u a t i o n s i n the t r a n s f e r of heat by the r a d i a t i o n .  He concludes that output f l u c t u a t i o n s  i n t h i s case are due to r a d i a t i o n f l u c t u a t i o n s .  He gives as  the mean square f l u c t u a t i o n i n power per u n i t frequency bandwidth (32)  68  the s i z e of d e t e c t o r used here t h i s gives 7*36  For  x l O " ^ watt, which i s of the order obtained i n many cases 1  (see  Table I V ) .  See a l s o chapter I , where i t was quoted  ( 4 , 5 ) that the measured noise approaches the l i m i t imposed by radiation fluctuations. I t i s proposed that the detectors studied be c l a s s i f i e d as type I I detectors ( 1 3 ) .  This i s because g.^  i s  independent o f T , as f o r type I d e t e c t o r s . d e f i n i t e statement on c l a s s i f i c a t i o n of equation (1) corresponding to M temperature.  2  n  °t  However, a  cannot be made, since k  2  i s found to vary w i t h  I f i t , remained constant w i t h temperature f o r  the B.T.H. c e l l s ,  could be p l o t t e d againstJC*, a s t r a i g h t  l i n e e s t a b l i s h i n g that the type number i s I I .  This f o l l o w s  from equation (33) from the second paper by C l a r k Jones (13) (33) Since k i s not a constant, and i s derived from 2 equation (33)? t h i s equation can give no clue as to the proper value of n.  Another reason f o r taking n as 2 i s the good  agreement between the f i r s t four columns of Table IV. M]_ i s seen to behave p r o p e r l y , never g r e a t l y exceeding u n i t y .  M  2  on the other hand i s much greater than u n i t y ,  showing that Havens's l i m i t w i l l have to be r e v i s e d .  It i s  not a fundamental l i m i t , and i s seen to hold f o r other types of detector (chapter I I , s e c t i o n (1)  ).  69 The B.T.H. c e l l s are i d e a l i n t h i s work since they can be used at three d i f f e r e n t temperatures, and since t h e i r areas and other p h y s i c a l p r o p e r t i e s are unchanged by c o o l i n g . For t h i s reason, i t had been hoped a d e f i n i t e statement on type number could have been made. The A.R.L. c e l l s could c e r t a i n l y be much improved i f they could be cooled.  This i s i n d i c a t e d because of the  improvement c o o l i n g makes on the B.T.H. c e l l s (Tables I I I and IV). Photoconductive c e l l s have the great advantage over most temperature detectors that they have a s e l e c t i v e s p e c t r a l response, so that used as d e t e c t o r s i n the one to f i v e micron region, t h e i r ultimate s e n s i t i v i t y i s attainable.  I n the  i n f r a red above f i v e microns, they are no b e t t e r than thermocouples or bolometers f o r the same reason.  70 BIBLIOGRAPHY  1.  Simpson, 0 . , and Sutherland, G.B.B.M., Science. 1 1 5 , 1 (1952).  2.  Sosnowski, L., S t a r k i e w i c z , J . , and Simpson, 0 . Nature, 159, 818 (1947).  3.  Jones, R. C., J . O p t i c a l Soc. Am., 3 7 , 879 (194-7).  4.  F e l l g e t t , P. B., J . O p t i c a l Soc. Am., 3 9 , 970 (194-9).  5.  Moss, T. S., J . O p t i c a l Soc. Am.. 40, 603 ( 1 9 5 0 ) .  6.  Jones, R. C., J . O p t i c a l Soc. Am., 3 9 , 344 ( 1 9 4 9 ) .  7.  Daly, E. F., and Sutherland, G.B.B.M., Proc. Phys. S o c London, A, 6 2 , 205 (1949). 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G., and Sands, M., E l e c t r o n i c s , New York, McGraw H i l l (1949).  THE  BLACK  BODY  H  0 ° o  0  o  0  Q  ° r ,  -D-  H  FIGURE  I  0  0  ° r , ° r , o ^ ° ^ ° o  0  o  0  o  0  o  0  Q  0  o  0  Q  0  o  0  o ° Q  RADIATOR  A  STEEL  CYLINDER  B  BRASS  C  ALUNDUM  D  ASBESTOS  SHEETING  E  N I CH R O M E  HEATER  F  RESISTANCE  G  ROCK  H  WOOD  J  ALUMINUM  K  TH ER M O C O U P L E  DISCS CYLINDER  THERMOMETER  WOOL CONTAINER  THE THE  OPTICAL BLACK  SYSTEM BODY  OF  RADIATOR  A  ASBESTOS ALUMINUM  B  CHOPPER  C  APERTURE  DISC  D  CHOPPER  MOTOR  E  BLACK  F  SLIDING  G  BRASS  K  SENSITIVE  H  STEEL SCALE  SHIELD BETWEEN PLATES  BODY  SHUTTER  DISC  LAYER  OF  CELL  CYLINDER I  INCH  EQUALS  IO  CMS-  O S C I L L O S C O P E  J20  J _ 2 C 2 0 0 K  2 0 _ ] _  £  4 (  P O I N T  ®  ™  I LQO(  W W  T E S T  20  I  20  I  20]^  • 4 7 K IW  I O K  4 7 K  I N 3 4  -HI O O K >  K  IOOKSS6K<[3L' ^ , f .Ol f - T -  5  N34 7 2 0  K  > 5 Q O  ^ 1 2 0 0 0 :  ^  ZJ200ZIM7IOOW  II I  2 A Y 7  6 A U 6  2 A Y 7  2  9  0V|.20 ^f 26 ^| 20 2  ,5,  V  -,  ^  3  .  oov  V  I 2 A Y 7  h  lh H I 5 5  . •  5 Y 3 I  I O  2 7 0 K ^  I  2  t  o c  J  9 8 0  > 2 7 K F I G U R E  ISO  ?  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