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UBC Theses and Dissertations

Fast timing using NaI crystals Maywood, David John 1969

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FAST TIMING USING N a l CRYSTALS by DAVID JON MAYWOOD B.A. Sc., U n i v e r s i t y of B r i t i s h Columbia, 1965 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n the Department of PHYSICS We accept t h i s t h e s i s as conforming to the req u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA 1969 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r an a d v a n c e d d e g r e e a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and S t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e Head o f my D e p a r t m e n t o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s . i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . D e p a r t m e n t o f P h y s i c s  The U n i v e r s i t y o f B r i t i s h C o l u m b i a V a n c o u v e r 8, Canada Date August 22, 1969 i ABSTRACT The s u i t a b i l i t y of using a Nal c r y s t a l as a f a s t coincidence gamma ray dete c t o r was i n v e s t i g a t e d u s i n g the multichannel " f a s t - s l o w " delayed 22 coincidence method to o b t a i n prompt time r e s o l u t i o n curves w i t h a Na gamma ray source at Nal temperatures of 170, 185, 200°K. A comparison of the time r e s o l u t i o n curve obtained u s i n g two NE 102 p l a s t i c s c i n t i l l a t o r s (FWHM: 640 psec) w i t h that using a cooled N a l s c i n t i l l a t o r and a NE 102 s c i n t i l l a t o r -(FWHM: 680-psee). showed that- Nal. -is & satisfactory-fast coincidence gamma ray det e c t o r a t l85°K. i i TABLE OF CONTENTS Page ABSTRACT 1 TABLE OF CONTENTS xx LIST OF TABLES xv LIST OF FIGURES v ACKNOWLEDGEMENTS vx CHAPTER I - INTRODUCTION 1 CHAPTER I I - SCINTILLATORS 3 (a) General (b) Conventional S c i n t i l l a t o r s (c) Figure of Merit (d) C h a r a c t e r i s t i c s of Nal S c i n t i l l a t o r s (e) S c i n t i l l a t o r Comparison (f) C r y s t a l Mountings (g) C r y s t a l Cooling Assembly CHAPTER I I I - PHOTOMULTIPLIER 8 (a) General (b) Photomultipliers f o r Experiment (c) Optimization of Photomultiplier Output (d) Photomultiplier E l e c t r i c a l Outputs (e) Photomultiplier Housings CHAPTER IV - LIMITER 12 (a) General (b) Tunnel Diode CHAPTER V - TIME SORTER (a) I n t r o d u c t i o n (b) Inputs (c) Shaper C i r c u i t (d) Time-to-Amplitude Converter (e) C a l i b r a t i o n of Time S o r t e r ( f ) Power Board CHAPTER VI - EXPERIMENTAL PROCEDURE.-; (a) General (b) Experimental Layout (c) "Slow" Coincidence C i r c u i t and Mu l t i c h a n n e l Analyzer (d) Source 22 (e) Na Spectra from S c i n t i l l a t i o n Counters ( f ) O p t i m i z a t i o n of Apparatus (g) P h o t o m u l t i p l i e r High Voltage (h) Zero-Crossover P o i n t ( i ) Tunnel Diode Quiescent P o i n t CHAPTER VI I - EXPERIMENTAL RESULTS AND CONCLUSIONS (a) General (b) R e s o l u t i o n of E l e c t r o n i c s (c) Published Time Resolutions (d) R e s o l u t i o n of NE 102 (e) R e s o l u t i o n of Nal ( f ) Time Variance (g) Conclusions APPENDIX A APPENDIX B. BIBLIOGRAPHY. i v LIST OF TABLES Page I Detector Comparison 5 II Photomultiplier Tube Data 9 I I I S o l i d State Components i n Limiter C i r c u i t 15 IV Limiter Output C h a r a c t e r i s t i c s 16 V S o l i d State Components i n TAC C i r c u i t 19 VI S o l i d State Components i n Stretcher C i r c u i t 19 VII . S o l i d State Components i n Recovery C i r c u i t 20 VIII S o l i d State Components i n Output C i r c u i t 20 IX S o l i d State Components i n Power Board 21 X Published Results 30 XI Nal Results at D i f f e r e n t Temperatures 32 LIST OF FIGURES To f o l l o w page 1. Low Temperature Nal C r y s t a l Mounting 6 2 . NE 1 0 2 C r y s t a l Mounting 6 3- P h o t o m u l t i p l i e r Housing f o r Nal 7 k. P h o t o m u l t i p l i e r C i r c u i t 9 5. P h o t o m u l t i p l i e r Housing f o r NE 1 0 2 1 1 6 . Tunnel Diode C h a r a c t e r i s t i c 1 3 7 . Back Diode C h a r a c t e r i s t i c 1 3 8 . L i m i t e r C i r c u i t . lk 9 . Time S o r t e r 1 7 1 0 . Time-to-Amplitude Converter (TAC) and T r i g g e r C i r c u i t 1 7 1 1 . S t r e t c h e r C i r c u i t 1 7 1 2 . Recovery C i r c u i t 1 7 13- Output C i r c u i t 1 7 1 4 . Power Board 21 15. Experimental Arrangement 2 2 1 6 . Nal Energy Spectrum 25 1 7 . NE 1 0 2 Energy Spectrum 23 1 8 . NE 1 0 2 - NE 1 0 2 Coincidence Curve 29 1 9 . Nal - NE 1 0 2 Coincidence Curve 31 ADKWOWLEDGEMENTS I wish t o thank Dr. G. Jones f o r h i s s u p e r v i s i o n during t h i s research p r o j e c t . I wish a l s o t o express my g r a t i t u d e t o my parents and my w i f e f o r t h e i r patience and encouragement over the course of t h i s t h e s i s work. CHAPTER I INTRODUCTION Time spectroscopy is the experimental measurement of time relations between nuclear events. In the simplest case there are only two events: the "i n i t i a l " and the "delayed" event. There are many cases where more than two events are present but they will not be considered here. Examples of time spectroscopy experiments are stripping reactions, nuclear decays, and mean lifetime determinations. The work described in this' thesis is primarily concerned with the evaluation of Nal" crystals as scintillators for the measurement of short nuclear mean lives. From the various measurement methods discussed by R.E. Bell (1965) the multichannel "fast-slow" delayed coincidence method was selected and appropriate equipment developed and constructed. In such a system, the equipment must detect the nuclear events and provide for energy selection as well as extract the time information for each event. The apparatus for the multichannel "fast-slow" delayed coincidence method consists of the following: scintillators, photomultipliers, limiters, linear amplifiers, single channel analyzers, a time sorter, a multichannel analyzer and a slow coincidence gate. The nuclear event is detected by the scintillators and photomultiplier. The time information is extracted from the photomultiplier output by the limiter. The relative time delay between the time information for the " i n i t i a l " event and the "final" event is converted by the time sorter into a pulse whose amplitude is proportional to the delay time. The multichannel analyzer records the pulse amplitudes, and the amplitude spectrum is accumulated in one measurement. (The amplitude spectrum can be converted Into an absolute 2 delay time spectrum by c a l i b r a t i n g the analyzer.) A t the same time another output of the p h o t o m u l t i p l i e r i s used t o provide a l i n e a r s i g n a l which, a f t e r a m p l i f i c a t i o n , i s s e l e c t e d as t o amplitude by the s i n g l e channel analyzers. The s i n g l e channel analyzer outputs operate the slow coincidence c i r c u i t . T his c i r c u i t c o n t r o l s the multi c h a n n e l analyzer i n p u t gate and allows only the delay times from events of the d e s i r e d energy range t o be recorded. The " f a s t " coincidence c i r c u i t produces the time delay r e s u l t s and the "slow" coincidence c i r c u i t makes the energy s e l e c t i o n . A d e t a i l e d d e s c r i p t i o n of the equipment i s provided i n the f o l l o w i n g chapters. 3 CHAPTER I I SCINTILLATORS General Detection of the events under study i s the f i r s t t a s k i n time spectroscopy. As the time of the event, r a t h e r than the energy a s s o c i a t e d w i t h the event i s the most important, parameter t o "be measured the detectors are chosen f o r t h i s o b j e c t i v e . To produce accurate f a s t time i n f o r m a t i o n a d e t e c t o r must c h a r a c t e r i s t i c a l l y have a r e p r o d u c i b l e output response of short d u r a t i o n and detectable amplitude. Conventional S c i n t i l l a t o r s S c i n t i l l a t i o n counters w i t h f a s t p l a s t i c s c i n t i l l a t o r s have been a p p l i e d s u c c e s s f u l l y t o f a s t time spectroscopy (W.E. Mott and R.B. Sutton i 9 6 0 ) . These s c i n t i l l a t o r s have a short d u r a t i o n output (fluorescence) when a gamma ray impinges on them but they e x h i b i t a low d e t e c t i o n e f f i c i e n c y and a s m a l l amplitude output. Low e f f i c i e n c y i s a disadvantage because i t increases the time needed t o accumulate the necessary r e s u l t s . The d u r a t i o n of the experiment may be too s h o r t , or the experiment or the apparatus may be too unstable f o r the time r e q u i r e d . A low output magnitude reduces the number of detectable events because the s i g n a l t o noise r a t i o w i l l be low and the output s i g n a l has a h i g h l y s t a t i s t i c a l s t r u c t u r e which can only be detected by a t h r e s h o l d device. A high amplitude output d e t e c t o r gives more dete c t a b l e events and w i l l reduce the time necessary to accumulate the experimental data. I t would thus be d e s i r a b l e t o have a l i g h t pulse whose width i s of the same order as t h a t of a p l a s t i c s c i n t i l l a t o r but w i t h a h i g h e f f i c i e n c y and a higher output magnitude. H a l ( T l ) c r y s t a l s have a l s o a p p l i e d s u c c e s s f u l l y t o f a s t time spectroscopy (V. Grabarl 19&7)' These s c i n t i l l a t o r s have a high d e t e c t i o n e f f i c i e n c y and l a r g e amplitude output when a gamma ray impinges on them hut they e x h i b i t a long d u r a t i o n output ( f l u o r e s c e n c e ) . Figure of M e r i t A Figure of M e r i t must be d e f i n e d i n order t o compare va r i o u s s c i n t i l l a t o r s . Post and S c h i f f (1950) and G a t t i and S v e l t o (1966) show t h a t the variance " of the Cth e l e c t r o n a r r i v a l time i s E 2 = ( t 2 / R 2 ) C (1 + 2/3(C + 1) + . . . j ' (1) f o r an i l l u m a t i o n of ex p o n e n t i a l shape: I(t) = ( R / t j exp (-t/t) (2) Where: 2 E = variance of the Cth e l e c t r o n s a r r i v a l time c t = time constant of exp o n e n t i a l i l l u m a t i o n f u n c t i o n l ( t ) R = average number of e l e c t r o n s emitted as a consequence of a s c i n t i l l a t i o n l ( t ) = photocathode i l l u m a t i o n f u n c t i o n 2 A re d u c t i o n i n E the variance of the Cth e l e c t r o n a r r i v a l time w i l l give c to a s c i n t i l l a t i o n response which has smaller s t a t i s t i c a l v a r i a t i o n s . From ~ 2 2 ( l ) a r e d u c t i o n i n (t/R) w i l l give a corresponding r e d u c t i o n i n E^. However, R i s a l i n e a r f u n c t i o n of H the amplitude of the s c i n t i l l a t o r response so t/R = f(t/H) (3) So E c . w i l l a l s o be reduced by a r e d u c t i o n i n (VH)' A s a r e s u l t (t/H) can be defined as the Fi g u r e of M e r i t f o r comparing s c i n t i l l a t o r s . A low F i g u r e of M e r i t i n d i c a t e s t h a t a s c i n t i l l a t o r w i l l have a response which has small s t a t i s t i c a l v a r i a n c e s . 5 A Figure of Merit comparison of various s c i n t i l l a t o r s i s given i n Table I below. TABLE IA Decay FWHM Constant ""..Relative _ Detector Resolution Figure of _ S c i n t i l l a t o r NSEC (t) Amplitude (H). E f f i c i e n c y j NSEC Merit Qt/H) Naton 136 1.6 26 .21 .06 WE 102 3.5 26 1-2 .6k .13 Nal (Tl) 250 100 13 I . 5 5 2 .5 TABLE IB(*) Nal (300°K) 15 20 26 .75 ""JTal (185°K) 33 65 11 .50 Nal (80°K) 60 100 2 .60 (*) Van Sciver (1956) Characteristics of Nal S c i n t i l l a t o r s An investigation of the l i t e r a t u r e revealed that Nal showed promise as a s c i n t i l l a t o r with higher e f f i c i e n c y and output magnitude than a p l a s t i c but having the same short duration output. (W. Van Sciver (1956)) . Nal i s a s a l t c r y s t a l i n which no thallium has been introduced. Nal (Tl) crystals (Nal with thallium) are the most commonly used Nal type detectors. Van Sciver studied the s c i n t i l l a t i o n spectra and decay times of Nal crystals as a function of activator concentration ( T l ) , temperature, and the sp e c i f i c energy loss (dE/dx) of the p a r t i c l e e x c i t i n g the c r y s t a l . As a resul t of t h i s work Van Sciver suggested that "the high luminescent e f f i c i e n c y rapid decay time, and r e l a t i v e high density of unactivated Nal at 83°K make i t a unique s c i n t i l l a t i o n material that may prove to be useful i n experimental applications". 6 Van Sciver's results show that a Nal c r y s t a l would have optimum fast time characteristics i f operated at a temperature of 185°K. At t h i s temperature the Figure of Merit i s a maximum (Table IB). This i s the temperature at which the experiment was done. S c i n t i l l a t o r s Comparison A comparison of the two s c i n t i l l a t o r s used i n the experiment follows: For the p l a s t i c s c i n t i l l a t o r NE 102, the s c i n t i l l a t i o n has a charac t e r i s t i c decay time of 3.5 nsec, a spectrum maximum at .H30 u, and an amplitude which i s 60% of anthracene's s c i n t i l l a t i o n amplitude (anthracene i s the standard i n organic phosphors). The Nal spectrum has a strong temperature dependent peak i n the v i c i n i t y of 0.31 p from room temperature to h°K. The s c i n t i l l a -t i o n decay time f o r Nal increases as the temperature i s lowered while the pulse height has a maximum i n the v i c i n i t y of l i q u i d nitrogen temperature. See Table I B for characteristics of Nal. The advantages of the Nal c r y s t a l are that the charac t e r i s t i c decay time i s short, the pulse height i s large and the energy conversion e f f i c i e n c y i s large. Crystal Mountings The Nal c r y s t a l was mounted i n a low temperature c r y s t a l mounting which was purchased from Harshaw Chemical Co* (Figure l ) . I t consists of an encapsulated Nal c r y s t a l held on the end of a quartz l i g h t pipe by a stainless s t e e l bellows. The mounting i s hermetically sealed. Quartz was * Harshaw Chemical Co, Crystal S o l i d State Division, 19A5 E 97th Street, Cleveland, Ohio,. USA: Special Assembly Low Temperature Crystal Mounting. FIG 1. LOW TEMPFIIATURE NA '€':^YS^^'^$^iSQ--FIG 2. NE 102 CRYSTAL MOUNTING 7 was chosen f o r the l i g h t pipe because of i t s good l i g h t t r a n s m i s s i o n i n the 0.31 Y r e g i o n . The NE 102 i s mounted i n an aluminum cap and can be mounted d i r e c t l y on the end of the p h o t o m u l t i p l i e r (Figure 2). C r y s t a l Cooling Assembly The c r y s t a l assembly was cooled by a c o l d f i n g e r which was dipped i n t o a l i q u i d n i t r o g e n bath. The c o l d f i n g e r was a l a r g e aluminum b l o c k and a small l/k" diameter aluminum rod connected the block t o the copper e.nd p l a t e of the p h o t o m u l t i p l i e r housing (Figure 3)' The e n t i r e p h o t o m u l t i p l i e r housing and c r y s t a l mounting are enclosed i n a p l a s t i c bag t o keep moisture from condensing on the cooled apparatus. Thermal i n s u l a t i o n was provided by a styrofoam container which encased the e n t i r e c o l d area ( p h o t o m u l t i p l i e r housing, c r y s t a l mounting, c o l d f i n g e r , and l i q u i d n i t r o g e n b a t h ) . The temperature was v a r i e d by changing the d i s t a n c e between the end p l a t e of the p h o t o m u l t i p l i e r housing and the c o l d f i n g e r . The temperature range a v a i l a b l e was 185°K + 15°K. A l a r g e r range could have been obtained by modifying the apparatus. A thermacouple (Chromel - Alumel) screwed t o the copper body of the low temperature Nal c r y s t a l mounting was used t o monitor the temperature of the c r y s t a l . Copper End Plate Aluminum Block FIG 3. PHOTO MULTIPLIER HOUSING FOR NA I 8 CHAPTER I I I PHOTOMULTIPLIER General A f t e r d e t e c t i n g a nuclear event by producing a s c i n t i l l a t i o n i n a c r y s t a l , the second task i s t o convert the l i g h t pulse i n t o an e l e c t r i c a l pulse and t o amplify t h i s p u l s e . This f u n c t i o n i s done by a p h o t o m u l t i p l i e r tube. The l i g h t from the s c i n t i l l a t o r produces photoelectrons from the tube's photocathode which i s the f i r s t stage of the tube. At each succeeding dynode stage, a m p l i f i c a t i o n of the pulse i s obtained by secondary emission. The shape of the p h o t o m u l t i p l i e r output i s normally a Gaussian or a c l i p p e d Gaussian curve. T h i s i s the response due t o a s i n g l e e l e c t r o n r e l e a s e d from the photocathode. The output wave form due t o a s c i n t i l l a t i o n i s the c o n v o l u t i o n of the l i g h t impulse and the s i n g l e e l e c t r o n response of the p h o t o m u l t i p l i e r . The pulse has s t a t i s t i c a l v a r i a t i o n s i n amplitude, width and time p o s i t i o n of i t s c e n t r o i d . The p h o t o m u l t i p l i e r tubes used i n time spectroscopy must have a s p e c t r a l response which w i l l accept the s c i n t i l l a t i o n spectrum of the detectors t o be used, a short t r a n s i t time w i t h smaller time f l u c t u a t i o n s , and a f a s t r i s e time f o r the output. P h o t o m u l t i p l i e r s f o r Experiment The three tubes ( 2 - 5 6 AVP's and I - 5 6 OVP from P h i l l i p s ) chosen f o r the experiment are f a s t coincidence, high g a i n , high r e s o l u t i o n , p h o t o m u l t i p l i e r tubes. The s p e c t r a l response curve of the 5 6 AVP which peaks at.k2 p makes i t s u i t a b l e f o r use w i t h the NE 1 0 2 p l a s t i c s c i n t i l l a t o r s . The s p e c t r a l response curve of the 5 6 UVP peaks at . 1 + 0 u and extends down t o 9 ,20 u, makes i t s u i t a b l e f o r use w i t h the Nal s c i n t i l l a t o r . The photo-m u l t i p l i e r tubes have the same p h y s i c a l s t r u c t u r e except f o r the end windows. The end window i s p o l i s h e d B ho g l a s s i n the 56 AVP and p o l i s h e d o p t i c a l quartz i n the 56 UVP thus p r o v i d i n g a response t h a t extends i n t o the u l t r a - v i o l e t r e g i o n . Data on the tubes used i n experiment: TABLE I I 1 2 3 Type 56 AVP 108 56 AVP 108 56 UVP 108 Gain V o l t s 2180 2220 2320 Operating V o l t s 2150 2200 2220 Detector NE 102 NE 102 Nal Window BUO Glass BkO Glass Quartz Opt i m i z a t i o n of P h o t o m u l t i p l i e r Output The output of the p h o t o m u l t i p l i e r was optimized by the f o l l o w i n g procedure: (See Figure k) (1) The p o t e n t i a l of the f o c u s i n g e l e c t r o d e (G^) t o the photocathode K was adjusted t o obt a i n the sm a l l e s t t r a n s i e n t time f l u c t u a t i o n s . The optimum p o t e n t i a l was obtained by v a r y i n g the p o t e n t i a l u n t i l the narrowest coincidence curve was obtained. (2) The supply voltage f o r the dynodes was d e r i v e d from a p o t e n t i a l d i v i d e r chosen i n accordance'-with the manufacturer's s p e c i f i c a t i o n s t o y i e l d the maximum gain and the f a s t e s t output p u l s e s . (3) The p o t e n t i a l of the d e f l e c t i o n e l e c t r o d e (Gg) t o the f i r s t dynode (S n ) was adjusted t o o b t a i n the sm a l l e s t t r a n s i t time f l u c t u a t i o n s . K H. O VvV-3 0 0 K acc g 2 S l S 2 -wv—4 -AAA/-3 . 9 M 100K 150K 1+7K 3 1 0 3 1 1 3 1 2 1 3 1+7K -AAA/ W V k7K - A A A r -HE . 0 0 1 Energy. Select ion Output, k7K -AAA/-)+7K A A A r 1 0 0 3 3 0 -L-1+7K -wv-J1 0 0 J. H-hf—$ . 1 0 0 z o = 5 0 0 0 1 1 5 0 K hi I FIG. Ij. PHOTOMULTIPLIER ELECTRONICS 10 P h o t o m u l t i p l i e r E l e c t r i c a l Outputs I t was necessary t o ob t a i n three outputs from the p h o t o m u l t i p l i e r : one f o r energy s e l e c t i o n and two f o r time i n f o r m a t i o n . The output f o r energy s e l e c t i o n was obtained separately from t h e outputs f o r t i m i n g i n f o r m a t i o n . The two t i m i n g outputs were subsequently added t o y i e l d a b i p o l a r p u l s e f o r the l i m i t e r . A b r i e f d e s c r i p t i o n of the op e r a t i o n of a p h o t o m u l t i p l i e r tube w i l l be given t o i l l u s t r a t e the nature of these p u l s e s . As s t a t e d before a p h o t o m u l t i p l i e r tube converts l i g h t i n t o e l e c t r o n s which are m u l t i p l i e d by secondary emission t o produce a detectable s i g n a l . At each dynode more e l e c t r o n s pass on t o the. next dynode than a r r i v e from the previous one. As a r e s u l t there i s a net f l o w of e l e c t r o n s t o each dynode from the p o t e n t i a l d i v i d e r c i r c u i t . At the 11th dynode ( S ^ ) t h i s f l o w produces a p o s i t i v e pulse which i s cap a c i t o r - c o u p l e d t o the energy s e l e c t i o n c i r c u i t . I f the secondary emission m u l t i p l i c a t i o n f a c t o r at the ikth dynode ( S ^ ) i s M, and i f the e l e c t r o n current t o the dynode i s i ^ , then the e l e c t r o n current emitted by the dynode i s Mi-^I).* The net p o s i t i v e current t o the 1^-th dynode i s ( M - l ) i ^ , and the negative c u r r e n t t o the anode (a) i s Mi-^» The p o s i t i v e pulse i s then (M-l)/M t h a t of the: negative anode p u l s e . For t y p i c a l values of M, t h i s f r a c t i o n i s about .75- The c i r c u i t r y which e x t r a c t s these pulses and combines them t o form a b i p o l a r zero c r o s s i n g pulse i s r e l a t i v e l y simple (Figure h and 8). The negative pulses e x t r a c t e d from the anode (a) are coupled t o the c o a x i a l cable by a c a p a c i t o r -r e s i s t a n c e chain (Figure k and 8). I t should be noted t h a t great care 11 must be taken i n t e r m i n a t i n g the c o a x i a l cable t o e l i m i n a t e r e f l e c t i o n s of the ex t r a c t e d p h o t o m u l t i p l i e r p u l s e s . I f r e f l e c t i o n s occur many-pulses w i l l r e s u l t and these can t r i g g e r the l i m i t e r c i r c u i t . P h o t o m u l t i p l i e r Housings The N al p h o t o m u l t i p l i e r housings (Figure 3 a n ( l 5) has f o u r purposes: t o hold the c r y s t a l mounting on t o the face of the p h o t o m u l t i p l i e r , t o i n s u l a t e the c r y s t a l mounting from outside heat, t o provide a l i g h t - t i g h t container f o r the p h o t o m u l t i p l i e r , and t o s h i e l d the p h o t o m u l t i p l i e r from e x t e r n a l magnetic f i e l d s . To f u l f i l l these requirements, the Nal photo-m u l t i p l i e r housing c o n s i s t e d of two p a r t s , a s t a i n l e s s s t e e l c y l i n d e r and a mumetal c y l i n d e r (Figure 3)• The low temperature c r y s t a l assembly was mounted on the s t a i n l e s s s t e e l end, while the p h o t o m u l t i p l i e r tube was mounted on the other. The s t a i n l e s s s t e e l was used as i t i s a poor heat conductor and as a r e s u l t , i n s u l a t e s the c r y s t a l mounting. The NE 102 p h o t o m u l t i p l i e r housing (Figure 5) was made of a mumetal c y l i n d e r . FIGURE 5 O i M U l _ T I P I — I M o u t s i ~r For NE 102 £ P 3 B R A S S -CVj . A L U M I K i t U M W A l _ l _ T H I C K K i E i S S O. 5 ' 5 ' H I <3rH-i QUAU'I TY B A K B U I T e , TUfSrslOL, E T C o p A a u e . ) . ^ — IS T H I C K N E S S J J . P H O T O M U L T I F > l _ l E R x i - J . " B o x . .... 2., . . 2 ; .,. . 12 CHAPTER IV  LIMITER General The third task i n time spectroscopy i s to extract the time information from the photomultiplier pulse output, a pulse which has variations i n amplitude, width, and i n time position of i t s centroid. The limiter c i r c u i t i s triggered by the photomultiplier output and produces a standardized output pulse which contains the time information of the original photo-multiplier pulse without i t s large variations. The time information can - be obtained using the f i r s t photoelectron, the leading edge, the centroid, a fraction of the total charge, etc. In this experiment the time information was extracted by using a fraction (about 50$) of the total charge. It i s not advisable to use the f i r s t electron arriving at the anode as this ignores the time information contained i n the rest of the pulse while use of the leading edge is undesirable because the slope of the leading edge of the photomultiplier output i s proportional to the pulse amplitude, and so must be compensated for by the limiter c i r c u i t to minimize time slewing. (Time slewing i s the time shift of the limiter output pulse as a function of the input pulse amplitude.) In this experiment, a fraction of the total charge was used for the timing information. The fraction was defined by the zero-crossover method (Jones (1968)). The basic technique in the zero-crossover method i s to use the photomultiplier output to produce a bipolar pulse i n which the zero-crossing point i s the time reference for the photomultiplier pulse. For a linear system, the zero-crossing point i s well defined in time and independent of pulse amplitude. 13 There are three major f u n c t i o n s t o be performed. These are the production of the zero-crossover p u l s e , the t r i g g e r i n g o f the l i m i t e r at the zero-c r o s s i n g point,'and the production of a standardized l i m i t e r output pulse having a f l a t top, a short w e l l defined r i s e and f a l l time, and a constant amplitude. The zero-crossover pulse i s produced by t a k i n g the two outputs from the p h o t o m u l t i p l i e r tube and combining them together (see Chapter I I I ) . The z e r o - c r o s s i n g p o i n t of the b i p o l a r pulse was detected by having the subsequent switch from one s t a t e t o another as the i n p u t s i g n a l crossed the zero reference. The s w i t c h i n g device employed was a t u n n e l diode. Tunnel diodes are q u i t e s u i t a b l e as d e t e c t i o n devices because they have f a s t s witching times, can t o l e r a t e l a r g e s i g n a l s i n e i t h e r d i r e c t i o n without damage or h y s t e r e s i s , e x h i b i t good s t a b i l i t y c h a r a c t e r i s t i c s and can be t r i g g e r e d from one s t a b l e DC s t a t e t o a second s t a b l e DC s t a t e by a zero-crossover a c t i o n . Tunnel Diode The operation of the t u n n e l diode zero c r o s s i n g d e t e c t o r i s s i m i l a r i n p r i n c i p l e t o t h a t d e s c r i b e d by Jones and Orth (1968). A t u n n e l diode has a c h a r a c t e r i s t i c curve (Figure 6) which has an unstable r e g i o n of operation of negative r e s i s t a n c e , w i t h two s t a b l e regions of p o s i t i v e r e s i s t a n c e on e i t h e r s i d e . The t u n n e l diode can be DC b i a s e d t o operate i n e i t h e r one or both of the p o s i t i v e r e s i s t a n c e r e g i o n s . With a p r o p e r l y chosen DC b i a s a t u n n e l diode can have two quiescent s t a t e s : a high c u r r e n t , low voltage s t a t e A and a low c u r r e n t , h i g h voltage B i p o l a r pulse and t r i g g e r p o i n t s f o r d i f f e r e n t l o a d l i n e s . CU 4> iH FIG 6. TUNNEL DIODE CHARACTERISTIC FIG 7. BACK DIODE CHARACTERISTIC 11+ s t a t e B. I n the l i m i t e r c i r c u i t (Figure 8) the t r i g g e r i n g t u n n e l diode (TDl) i s b i a s e d t o have two quiescence p o i n t s . TD1 normally operates i n the low current s t a t e B. The p o s i t i v e l e a d i n g edge of the b i p o l a r pulse reduces the current through diode and t h i s causes the tu n n e l diode t o switch t o the low voltage r e g i o n . T h i s t r a n s i t i o n occurs i n l e s s than 10 nsec. As the b i p o l a r pulse r e t u r n s t o zero, the tu n n e l diode recovers and reaches s t a b l e s t a t e A which i s very c l o s e t o the sharp knee of the tu n n e l diode c h a r a c t e r i s t i c . At s t a b l e s t a t e A, the s t a r t of the negative p a r t of the b i p o l a r pulse w i l l t r i g g e r the tu n n e l diode back t o the high voltage r e g i o n near the z e r o - c r o s s i n g p o i n t . T h i s second change of s t a t e which takes l e s s than 10 nsec i s used t o t r i g g e r the r e s t of the l i m i t e r c i r c u i t . When the b i p o l a r pulse i s over, the tu n n e l diode has returned t o the low current s t a b l e p o s i t i o n B and the l i m i t e r i s ready f o r another input p u l s e . Stable state A i s used as the zero-crossover t r i g g e r i n g p o i n t because the knee, of the curve i s much sharper there than a t s t a b l e s t a t e B and as a r e s u l t the t r a n s i t i o n from the low voltage r e g i o n t o the high voltage r e g i o n occurs at a much more sharply d e f i n e d time than the t r a n s i t i o n from the high voltage r e g i o n t o the low voltage r e g i o n . (NOTE: A C o a x i a l Delay L i n e and a Trombone Are I n s e r t e d Between J and J') 15 TABLE I I I Solid State components i n Limiter Circuit TD1 1H38^7 TD2 1N3716 BD1 BD6 T l BTL* S. PHP (f : 1200 MHz) i t T2 2W3702 T3 2K709 Tit- 2F709 Dl lNk59 D2 lNi)-59 ZD BZ100 * B e l l Telephone Laboratories experimental transistor Steps are produced by the tunnel diode each time i t switches from the high voltage region to the low voltage region or visa versa. Thus i n the limiter the TD1 produces a positive step at the start of the positive part of the bipolar pulse and a negative step at the zero-crossover of the bipolar pulse. The TD i s capacitively coupled to a back diode. (A back diode i s a tunnel diode which has a very shallow valley characteristic.) (Figure 7) The capacitor differentiates the steps. The back diode conducts when the positive step occurs and this step i s terminated, but when the negative step occurs the tunnel diode appears as a f i n i t e resistance and i t passes on. The back diode thus transfers only the negative tunnel diode step which was initia t e d by the zero-crossover 16 of the photomultiplier bipolar pulse. The negative step turns on t r a n s i s t o r T l which triggers tunnel diode TD2. The recovery time of TD2 i s controlled by the time constant of the L/R network which i s p a r a l l e l to i t . The pulse from TD2 i s amplified by transistor^T3 and Th • producing a pulse across the 5 0 ohm output resistance. The duration of the output pulse i s controlled by the current through the variable r e s i s t o r to TD2. The output amplifier (T3 and T 4 ) i s held i n the cutoff region by the DC biased tran s i s t o r T2. The diode D2 associated with t r a n s i s t o r T2 makes i t temperature compensating. The output from the l i m i t e r i s a flat-topped negative pulse of constant amplitude with a fa s t r i s e and f a l l time. I t s characteristics are i l l u s t r a t e d i n the following table: TABLE IV Limiter output pulse cha r a c t e r i s t i c s : Rise Time: 5 nsec Decay Time: 5 nsec Duration: variable (50-600 ns) Amplitude: 3 volts into 50 ohms The duration of the output defines the "dead time" of the apparatus and eliminates after pulses from the photomultiplier. The output must also be long enough to be shaped by the cl i p p i n g stubs at the input of the time sorter (Chapter V). Therefore, i t i s an advantage to be able to vary the duration of the output. 17 CHAPTER V  TIME SORTER I n t r o d u c t i o n The f o u r t h t a s k i n time spectroscopy i s t o e x t r a c t the r e l a t i v e delay time between the " i n i t i a l " event and the "delayed" event from the l i m i t e r outputs. The r e l a t i v e delay time t o be measured i s i n the sub-nanosecond (lO ^ second) r e g i o n so the heart of the time s o r t e r i s a time-to-amplitude converter (TAC). The TAC converts the r e l a t i v e delay time between the l i m i t e r pulses i n t o a pulse whose amplitude i s p r o p o r t i o n a l t o the delay time, by the overlap pulse p r i n c i p l e . The r e l a t i v e advantages and d i s -advantages of va r i o u s time spectroscopy methods and s p e c i f i c a l l y the overlap pulse p r i n c i p l e have been discussed by B o n i t z (1963) and B e l l (1965). A schematic diagram (Figure 9) i n d i c a t e s the b a s i c u n i t s of the time s o r t e r . These are: shaper c i r c u i t s (Figure 10) which detect the input p u l s e s ; a time-to-amplitude converter (Figure 10) which, as st a t e d above, converts the r e l a t i v e delay time between in p u t p u l s e s i n t o a pulse whose amplitude i s p r o p o r t i o n a l t o the delay time; a s t r e t c h e r c i r c u i t (Figure l l ) which increases the d u r a t i o n of the time-to-amplitude "'converter output; a recovery c i r c u i t (Figure 12) which r e s t o r e s the s t r e t c h e r c i r c u i t t o i t s quiescent s t a t e a f t e r 3 p sec; and the output c i r c u i t (Figure 13) which a m p l i f i e s the pulse at the output. Inputs The outputs from the l i m i t e r c i r c u i t s were t r a n s m i t t e d t o the time s o r t e r by 50 ohm c o a x i a l cable. At the inp u t t o the time s o r t e r a shor t -c i r c u i t e d c o a x i a l cable (Figure 9) c l i p s the l i m i t e r p u l s e , and the L i m i t e r L i m i t e r Clipping Stub Time to Amplitude Conventer C i r c u i t JL B' Clipping Stub Stretcher C i r c u i t Recovery C i r c u i t FIG 9. TIME SORTER Output C i r c u i t Time Sorter Output FIG. 10 TIME-TO-AMPLITUDE CONVERTER (TAC) AND TRIGGER CIRCUIT 30V 2.2K T l IK 10V ^ 3 0 ' 100K G 4 F -4- . 001 FIG. 11 STRETCHER CIRCUIT I .8 K 10 .015 T l • M f 330 - A W 12 OK A V v || l T2 ,006 820 F I G . 12 RECOVERY CIRCUIT .1 ZDl FIG. 13 OUTPUT CIRCUIT 18 front of a limiter pulse produces a short negative pulse with a well defined pulse width. The overall time stability of the time sorter depends on the time stability of the negative pulses because these are the pulses which carry the timing information to the TAC. The back of the limiter pulse produces a short positive noise which is not used. The length of the short negative pulses depends on the length of the clipping cable, (for this experiment the pulse length is 30 nsec: twice time taken for pulse to travel length of clipping cable). Shaper Circuit The shaper circuits consist of a tunnel diode and a resistor (TBI & Rl and TD2 & R2) in a series. They help to standardize the shapes of the negative input pulses, by switching on the front and back edges. Time-To-Amplitude Converter The time-to-amplitude converter (TAC) shown in Figure 10 is similar to one used by Falk (1965) Simms (1961), but does have basic differences. The power supply is negative 30 V, the shaper circuit is part of the TAC circuit, the recovery circuit is triggered differently and very fast NEN transistors were used for the switching transistors in the TAC. The change in the power supply polarity allows NPN transistors to replace PNP. The tunnel diode shaping circuit gives the TAC a faster response time to the limiter pulses. The trigger pulse for the recovery circuit is produced by tunnel diode TD3 whenever both switching transistors Tl and T2 in the TAC operate during a coincidence event. The switching transistors are two fast KPN transistors which have an f. = 1590 MHz. TABLE V So l i d State components i n TAC TD1 IN3861 TD2 IN3861 TD3 IN371U TDU 1W3714 TD5 1W3716 T l BTL* Ge KPN T2 BTL* Ge NPN T3 2N706A D l 1N914 1590 MHz) 1580 MHz) * B e l l Telephone Laborator ies experimental t r a n s i s t o r TABLE VI So l i d State components i n S t re tcher C i r c u i t T l 2 N 9 6 3 T2 2 N 7 9 7 T3 2 N 2 9 C 4 Tk 2N29C4 T5 2 N 2 2 1 8 ZD BZY63 20 TABLE VH Solid State Components i n Recovery Circuit T l 2 K 9 6 U T2 2N963 T3 2W1305 Dl 0A10 TD1 TD2 TABLE VIII Solid State Components in Output Circuit T l 2W290U T2 2W696 T3 AF116 Tk 2 N 6 9 6 ZD BZY57 Calibration of Time Sorter 22 A Na source i n an aluminum rod was placed between the two detectors. The outputs of the two limiters were fed into the TAC by coaxial cables of equal length and the output of the TAC was connected to the kicksorter. The spectrum was collected for k minutes, and then a short piece of calibrated coaxial delay line was inserted i n the cable from one of the limiters to the TAC. A second spectrum was accumulated and the number of channels between the two peaks indicated the time difference created by the cable. A complete time spectrum for the TAC i s produced by inserting several different delays i n the system and taking spectrums with these delays. The calibration of the delay lines i s discussed i n Appendix A. 2 1 P o w e r B o a r d T h e p o w e r s u p p l y c i r c u i t ( F i g u r e 1 ^ ) s u p p l i e d , a l l t h e v o l t a g e l e v e l s f o r t h e T i m e S o r t e r . T h e 30 v o l t l e v e l w a s s u p p l i e d b y a T e c h n i p o w e r * M o d e l M C 3i.5-O.i25 p o w e r s u p p l y . T A B L E I X S o l i d S t a t e C o m p o n e n t s i n P o w e r B o a r d T l B C Z 1 1 J 5 D 1 Bgl63 Z D 2 BSY57 ZD3 B2Y57 * T e c h n i p o w e r I n c , S u b s i d i a r y B e n r u s W a t c h C o I n c , B e n r u s C e n t e r , R i d g e f i e l d C o n n , .06877 U S A . (From 3 0 v Power Supply) 3 0 V - • • 3 0 V r. 5K 50 .05 T l • 05 3.3K ZD1 ZD2 •°5 == IK ZD3 20V 10 VS+ 10VS 10V • 5V FIG. Ill- POWER BOARD 22 CHAPTER VI  EXPERIMENTAL PROCEDURE General The purpose of the experiment was t o i n v e s t i g a t e the property of Nal as a f a s t coincidence s c i n t i l l a t o r . The experiment was done i n two p a r t s . In the f i r s t p a r t NE 102 c r y s t a l s were used as detectors and a set of coincidence curves were obtained f o r the .51 MeV, p o s i t r o n a n n i h i l a t i o n 22 gamma rays from Na . In the second p a r t of the experiment one of the NE 102 c r y s t a l s was replaced by a Nal c r y s t a l and a second set of prompt coincidence curves were obtained. Experimental Layout The arrangement of the experimental apparatus i s shown i n F i g u r e 15. The source and the detectors have a c o l i n e a r geometry and the detectors are e q u i d i s t a n t from the source so t h a t the a n t i - p a r a l l e l photons from the source w i l l impinge on both detectors at the same time. The other apparatus i n F i g ure 15 i s described i n the appropriate previous chapters. "Slow" Coincidence C i r c u i t and M u l t i c h a n n e l Analyzer The apparatus f o r the "slow" coincidence c i r c u i t and the m u l t i c h a n n e l analyzer are a l l commercially a v a i l a b l e . The "slow" coincidence apparatus c o n s i s t s of a l i n e a r a m p l i f i e r and a s i n g l e channel analyzer f o r each p h o t o m u l t i p l i e r and a "slow" coincidence gate. The a m p l i f i e r and s i n g l e channel analyzer were manufactured by Cosmic R a d i a t i o n Labs, I n c * The a m p l i f i e r i s Cosmic's Model 901 d o u b l e - d e l a y - l i n e A m p l i f i e r and the s i n g l e channel a n a l y z e r i s Cosmic's Model 901-SCA * Cosmic R a d i a t i o n Labs. Inc 16^5 Montaulc Highway B e l l p o r t , NY. USA FIG 15.. EXPERIMENTAL ARRANGEMENT 23 S i n g l e Channel Analyzer, This a m p l i f i e r and Si n g l e Channel Analyzer are designed toiwork together. The "slow" coincidence gate i s a pulse generator manufactured by Datapulse.** This pulse generator has a coincidence g a t i n g f e a t u r e . In t h i s mode of operation an output pulse occurs only where e x t e r n a l g a t i n g and e x t e r n a l t r i g g e r i n g c o n t r o l pulses are c o i n c i d e n t . The e x t e r n a l g a t i n g and e x t e r n a l t r i g g e r i n g c o n t r o l pulses are generated by the s i n g l e channel analyzer. Then.when .'the outputs of the p h o t o m u l t i p l i e r s are i n the c o r r e c t energy range, the outputs of the s i n g l e channel analyzers w i l l t r i g g e r . t h e "slow" coincidence gate which w i l l i n turn, c l o s e s the input gate of the mul t i c h a n n e l analyzer so t h a t the de l a y time i n f o r m a t i o n f o r the coincidence event can be recorded. The m u l t i c h a n n e l analyzer, manufactured by V i c t o r e e n Instrument Company***, i s t h e i r Portable Instrument Package, PIP-UOO, Pulse Height Analyzer. ** Datapulse, D i v i s i o n of Datapulse Incorporated 509 Hindry Avenue, Inglewood, C a l i f o r n i a 90306, USA *** V i c t o r e e n Instrument Company Tullamore D i v i s i o n 5857 West 95th S t r e e t Oak Lawn, I l l i n o i s , 60^53, USA 2^ Source 22 A common source of simultaneous gamma rays i n Na was encapsulated i n an 22 22 aluminum holder. The Na decays to the f i r s t excited state of Ne 10$ v i a e l e c t r o n capture, and .90$ v i a p o s i t r o n emission (Nuclear Data Tables, Part k, US Atomic Energy Commission, i960). The p o s i t r o n emitted i s slowed down by i o n i z a t i o n and i n e l a s t i c c o l l i s i o n s i n the aluminum to an energy (lOOev) at which i t can ann i h i l a t e d i r e c t l y with an electron from the aluminum. The p a i r of p a r t i c l e s , which are e s s e n t i a l l y at r e s t , w i l l a nnihilate emitting two quanta at l80°, each with an energy of 0.51 MeV. Jauch and Rohrlich (1955) and DeBenedetti (196M describe t h i s process i n d e t a i l . The t r a n s i t i o n from the excited state to the ground state of 22 -11 - " Ne occurs i n l e s s than 10 sec (Alkhazov et a l - (1959)) v i a a 1.28 MeV gamma ray. Sources were prepared by evaporating a small amount of active Na C l solut i o n (from "The Radiochemical Centre", Amersham, England with a s p e c i f i c a c t i v i t y of I.65 mc/mgm of Na Cl) placed on- an aluminum f o i l . The f o i l was then encapsulated i n the t i p of an aluminum rod having a w a l l thickness of .1 inches. From a consideration of the range of- positrons i n aluminum i t was estimated that about 90$ of the positrons annihilated i n the source. The strength of the source used was 10 u Curie (10 Nov 66) and the f l u x of gamma rays at the detectors was co n t r o l l e d by moving the detectors closer to or further away from the source. 22 Na Spectra from S c i n t i l l a t i o n Counters Since the experiment required that the photons detected be those corresponding to 2 - quantum a n n i h i l a t i o n , the sing l e channel analyzers were set to s e l e c t the desired p o r t i o n of the" .51 MeV a n n i h i l a t i o n spectrum. The spectra produced by the NE 102 and Nal detectors are d i f f e r e n t . 25 The complete spectrum consists of a Compton edge and a photopeak. The cross section for Compton scattering depends to the f i r s t order on Z the atomic number of the atoms of the detector. The photo production cross section is a function to the fourth order of Z. The NE 102 detectors (low Z) give a clear Compton distribution, but negligible photopeak whereas the Nal detector gives both a Compton edge and a photopeak. The spectrum from each scintillation counter showing the portion of the spectrum selected by each Single Channel Analyzer is shown in Figures 16 and 17• The photopeak is selected from the Nal produced spectrum and the. top portion of the Compton edge is selected from the NE 102 produced spectrum. Optimization of Apparatus In order to obtain the best results for the experiment several apparatus performance parameters were optimized. These are as follows: Photomultiplier High Voltage Due to the high gain of photomultipliers used i t was found that the voltage had to be chosen so as to reduce the number and amplitude of noise pulses. The operating voltage for each tube were, 56AVP #1 ^  '2150, 56AVP #2 ,@ 2200 56UVP #3 @' 2220. These were the maximum voltages that could be applied without triggering the associated limiter on noise pulses. (The value for the 56UVP tube was set when the Nal crystal was cooled.) Zero-Crossover Point The zero-crossover point has to allow about 50$ of the photomultiplier pulse to trigger the limiter circuit for best time spectroscopy results. A direct measure of the optimum point of zero-crossover is the slope of the sides of the prompt coincidence curve. The steepest slope corresponds to the best zero-crossover point. For fine adjustments a variable length line (General Radio Type 87^-LTL Trombone Constant-Impedance Adjustable CQ w o 01 > CQ a 8 CJ CU H CD CD 43 » ,4 o CO o H u CO C M CO C J O rt CO !4 o b 0 cd , A * i 9J o S3 •r) •H •2. H o o o o o o o o o o o o o SPECTRUM SELECTED Bl SINGLE CHANNEL ANALYZER GATING OF KICKSORTER o 0 26 Line) was used t o a d j u s t the zero-crossover p o i n t . (There must be no impedence mismatch i n the delay l i n e as r e f l e c t i o n s can cause the l i m i t e r c i r c u i t t o t r i g g e r more than once on any p h o t o m u l t i p l i e r pulse.) The b e s t s e t t i n g f o r the zero-crossover p o i n t allowed 57$ of the p h o t o m u l t i p l i e r pulse t o t r i g g e r the l i m i t e r f o r NE 102 and h2$ f o r N a l . Appendix B gives an approximate c a l c u l a t i o n of the $ of the p h o t o m u l t i p l i e r pulse used t o t r i g g e r the l i m i t e r . Tunnel Diode Quiescent P o i n t To minimize time slewing the low voltage quiescent p o i n t must be near the knee of the t u n n e l diode c h a r a c t e r i s t i c curve. The quiescent p o i n t was set at the optimum valve by v a r y i n g the current through the diode w i t h a v a r i a b l e r e s i s t o r . An e x p l a n a t i o n of how the quiescent p o i n t a f f e c t s the time slewing f o l l o w s . As described i n Chapter IV the b i p o l a r pulse from the p h o t o m u l t i p l i e r tube t r i g g e r s the l i m i t e r by causing the t u n n e l diode t o swtich from the low voltage r e g i o n t o the h i g h voltage r e g i o n . The p o s i t i o n of the l o a d l i n e w i t h respect t o the t u n n e l diode c h a r a c t e r i s t i c curve d e f i n e s the p o i n t on the b i p o l a r pulse (Figure 6) at which the t u n n e l diode i s t r i g g e r e d . For l o a d l i n e b (Figure 6) the t u n n e l diode switches at the zero-crossover p o i n t , w h ile f o r l o a d l i n e a the t u n n e l diode switches before the zero-crossover occurs and f o r l o a d l i n e c the t u n n e l diode switches a f t e r the zero-crossover occurs. Since the zero-crossover p o i n t does not change w i t h amplitude the l i m i t e r output would not e x h i b i t time slewing when zero-crossover t r i g g e r i n g occurred. When the t u n n e l diode i s i n c o r r e c t l y biased as w i t h l o a d l i n e a or c time slewing r e s u l t s . I n the case of l o a d l i n e a the smaller amplitude b i p o l a r pulses t r i g g e r the t u n n e l diode sooner 27 than the l a r g e b i p o l a r p ulses do, as a r e s u l t the l i m i t e r output w i l l occur soonest f o r s m a l l pulses and l a s t f o r l a r g e p u l s e s , but both w i l l t r i g g e r before a zero-crossover t r i g g e r i n g would occur. The e f f e c t of time slewing caused by lo a d l i n e a on the prompt coincidence curve depends on whether the l i m i t e r i s the "prompt" l i m i t e r or the "delayed" l i m i t e r . I f i t i s the "prompt" l i m i t e r then the e a r l y a r r i v a l of the l i m i t e r output w i l l cause the Time-to-Amplitude-Converter (TAC) t o produce an output which i s smaller than i t should be and as a r e s u l t the coincidence curves thus produced w i l l be l e s s steep on the lower s i d e . I f i t i s the "delayed" l i m i t e r then the e a r l y a r r i v a l of the l i m i t e r output w i l l cause the TAC t o produce an output which i s l a r g e r than i t should be and as a r e s u l t the coincidence curves thus produced w i l l have the side on the higher side of the time scale slewed. Conversely, time slewing caused by lo a d l i n e c w i l l a f f e c t the lower time side of the coincidence curve f o r the "delayed" l i m i t e r and the higher time side of the coincidence curve f o r the "prompt" l i m i t e r . By observing the coincidence curve and v a r y i n g the b i a s of the t u n n e l diode the l o a d l i n e was set so t h a t the zero-crossover w i l l t r i g g e r the l i m i t e r . 28 CHAPTER V I I EXPERIMENTAL RESULTS AMD CONCLUSIONS General The u l t i m a t e l i m i t i n the time r e s o l u t i o n of time spectra a r i s e s from s t a t i s t i c a l f l u c t u a t i o n s i n the s c i n t i l l a t i o n counters r a t h e r than the asso c i a t e d e l e c t r o n i c s . These time u n c e r t a i n t i e s r e s u l t i n a time spread t h a t may he comparable w i t h the mean l i f e t t o be measured or longer. The time r e s o l u t i o n curve of the apparatus f o r prompt coincidences ( i . e . coincidences where t i s n e g l i g i b l e ) has a f u l l width ¥ at h a l f maximum, which i s a measure of t h i s time spread (R.E. B e l l 1965). R e s o l u t i o n of E l e c t r o n i c s To measure the i n t r i n s i c time r e s o l u t i o n of the time-to-amplitude converter (TAC) the outputs of two pulse generators were f e d i n t o the TAC i n p u t s . The pulses from the pulse generators had the same amplitude and d u r a t i o n as the l i m i t e r outputs. The f i r s t pulse generator produced a "prompt" pulse and a " t r i g g e r " pulse which t r i g g e r e d a "delayed" pulse from the second pulse generator. The output of the TAC was recorded by the multichannel analyzer, which was c a l i b r a t e d w i t h respect t o time. The f u l l width W at h a l f maximum of the r e s o l u t i o n curve was l e s s than the width of one channel which i s 6 2 . 5 psec. Published Time R e s o l u t i o n s The r e s u l t s presented i n s e v e r a l p u b l i s h e d papers are summarized i n Table X. B e r t o l i n e e t . a l . (1966) r e p o r t s the best r e s o l u t i o n w i t h a FWHM of 60 22 l6k psec u s i n g Co as a source. Using Na as a source the best r e s o l u t i o n was reported by Miene e t . a l . (1966) (FWHM: 290 p s e c ) . The be s t crossover 29 measurements were reported by Williams (1967) with a FWHM of 730 psec 60 with Co . J . B e l l (1965) reported the best results f o r a system using WE 102 s c i n t i l l a t o r s and 56 AVP photomultiplier tubes (FWHM: 500 psec). The other results i n Table X are given f o r comparison. Resolution of WE 102 The prompt coincidence curve f o r two WE 102 detectors detecting 0.51 MeV positron annihilation gamma rays (Figure 18) has a FWHM of 6k0 psec. The slope of the sides of the curve plotted on semi-logarithmic paper corresponds to an instrumental mean l i f e of 60 psec f o r the l e f t side and 57 psec f o r the right side i f meaaxred between 0 . 1 and 0 . 0 1 of the peak. A FWHM of 61+0 psec f o r a NE 102 s c i n t i l l a t o r on a 56 AVP photomultiplier with a Na source using crossover t r i g g e r i n g compares favourably with J . B e l l ' s (1965) results using leading edge triggeri n g and Co^°. Considering the higher energy of the Co^° gamma, rays the Na 2 2 results compare quite favourably. Resolution of Nal The prompt coincidence curve f o r an NE 102 detector and an Nal "pure" detector (Figure 19) has a f u l l width at half maximum of 680 psec. The slope of the sides of the curve corresponds to an instrumental mean l i f e of 62 psec f o r the l e f t (Nal) -side and 58 psec for the right (NE 102) side i f measured between 0 .1 and 0.01 of the peak. The temperature of the Nal "pure" c r y s t a l for these measurements was. l85°K. Varying the temperature i n the temperature range from 170 to 200°K did not improve the resolution results (Table XI). FWHM 640 Psec FIG 18. NE 102 - NE 102 COINCIDENCE CURVE (K icksorter channel count ( log) vs time (nsec)) -1 nsec 0 nsec 1 nsec 30 TABLE X PUBLISHED RESULTS S c i n t i l l a t o r FWHM (psec) Photo-M u l t i p l i e r Source (a) Naton 136 164 XP1020 Co (*) Naton 136 185 C-70045 Co (b) Naton 136 290 C-70045 22 Na (c) Naton 136 470 56 AVP n 60 Co (a) Naton 136 270 XP1021 „ 60 Co Naton 136 Nal(Tl) 625 XP1021 8575 n 60 Co (d) Naton 136 (Note 1) 730 8575 58 AVP _ 60 Co (e) NAI(TI) 800 56 AVP n 60 Co (e) Nal(Tl) & Nal 80°K 1080 56 AVP « 2 2 Na (e) Nal 80°K (Note 2) 520 n 60 Co (e) Nal 170°K 1150 56 AVP __. 22 Na (f) NE102 500 56 AVP n 60 Co Note 1: These results were obtained by a zero-crossover timing system, a l l other results were obtained using leading edge triggeri n g or threshold timing. Note 2: Quoted by Ga t t i (1966). (a) B e r t o l i n i et. a l . (1966) (b) Miene et. a l . (1966) (c) Simms (1961) (d) Williams (1967) (e) Braunfurth (1965) (f) B e l l J . (1965) 31 A FWHM of 680 psec f o r a NE102 s c i n t i l l a t o r and a N a l s c i n t i l l a t o r o 22 at 185 K us i n g a Na source compares f a v o u r a b l y w i t h the r e s u l t s i n Table X. Time Variance Jones (1968) shows t h a t the FWHM (W l / 2 ) , of a time r e s o l u t i o n curve f o r two gamma rays detected i n counters 1 and 2, i s r e l a t e d t o the variance of the t i m i n g pulses by: W 1/2 - 2 (2 l n 2) ( E ^ + E g 2 ) 1 / 2 ( l ) 2 E^: variance of the t i m i n g pulse When the counters and phot o m u l t i p l e r s have the same c h a r a c t e r i s t i c s , then W 1/2 = 3.33 E (2) App l y i n g equation (2) t o the case where we have the two NE 102 s c i n t i l l a t o r s (W 1/2 = 640 psec). Then E (NE 102) * 192 psec With E (NE 102) = 192 psec and W l / 2 = 680 psec, then E (Nal 185°K) a 216 psec With two N a l s c i n t i l l a t o r s , a FWHM of 710 psec would be expected u s i n g the apparatus developed f o r t h i s experiment which i s i n the same range as Braunfurth's (1965) r e s u l t s w i t h : Nal a t 80°K (FWHM: 520 psec) and NaT a t 170°K (FWHM: 1150 psec) FWHM s 68G psec - 1 nsec FIG 19. NA I - NE 102 COINCIDENCE CURVE (Kicksorter channel count (log) vs time (nsec)) 0 nsec 1 nsec; TABLE XI Nal results at different temperatures: Mean L i f e (psec) F u l l Width @ Half Maximum Left Side (Nal) Right Side (NE 102) 170 66 62 680 185 62 680 200 70 66 730 Conclusions Nal crystals are suitable s c i n t i l l a t o r s f o r "fast " time spectroscopy. A "Figure of Merit" comparison of Nal and NE 102 shows that prompt time resolution curves obtained with Nal at 185°K would be comparible to but not better than those obtained with NE 102. Prompt time resolution curves obtained with NE 102 had a FWHM of 64-0 psec showing that NE 102 has a time variance E of 192 psec. Curves, obtained with Nal @ 185°K had a FWHM of 680 psec showing that Nal (185°K) has a time variance of 192 psec. In a cursory comparison of Nal prompt resolution curves at differant temperatures the best resolution was observed at. 185°K though the resolution was not much worse at 170 and 200°K, as predicted by "Figure of Merit" calculations. 33 APPENDIX A  CALIBRATION OF DELAY LINES. For p a r t s of t h i s experiment the lengths of c o a x i a l delay l i n e s had t o be c a l i b r a t e d w i t h respect t o time. A Techtronic Type 6 6 l o s c i l l i s c o p e w i t h Type 5T1 t i m i n g u n i t and a Type 1+S1 (50 ohm) du a l - t r a c e sampling u n i t was used t o c a l i b r a t e the c o a x i a l delay l i n e s . F i r s t the Type 6 6 l o s c i l l i s c o p e w i t h p l u g - i n s was c a l i b r a t e d . The Type 6 6 l o s c i l l i s c o p e has a wave generator which produces a sine wave w i t h a frequency of 10 Hz. The p e r i o d of t h i s sine wave generator was measured u s i n g a Hewitt-Packard 5253B frequency converter and a Hewitt-Packard 52^5L frequency counter. The sine wave, w i t h a p e r i o d of 10.08 n' sec, was f e d i n t o the o s c i l l i s c o p e and the t r a c e was measured on the 1 nsec/cm' sca l e f o r both the t r a c e s from the sampling u n i t . - This time c a l i b r a t i o n showed t h a t the scope time was accurate t o 1$ on the 1 nsec/cm s c a l e . Next the o s c i l l i s c o p e was used t o c a l i b r a t e the c o a x i a l delay l i n e s . The T?ype 6 6 l o s c i l l i s c o p e has a pulse generator which produces pulses having r i s e times of l e s s than 1 nsec. This pulse was f e d i n t o a c o a x i a l T a t one of the o s c i l l l s c o p e s i n p u t s . The c o a x i a l delay l i n e t o be c a l i b r a t e d was connected between the other branch of the T and the other o s c i l l i s c o p e i n p u t . The t r a c e s , from the o s c i l l i s c o p e pulse a r r i v i n g a t both i n p u t s , were d i s p l a y e d on the screen and the delay of the c o a x i a l l i n e was view d i r e c t l y . Reversing the in p u t s t o the o s c i l l i s c o p e -did not change the delay time. The c o a x i a l delay l i n e s were c a l i b r a t e d , w i t h respect t o time, t o 1$. 34 APPENDIX B An illumination of the exponential type (from Chapter II) produced by NE 102 and Nal (assuming the i d e a l case) w i l l have the form: I (t) . (R/t) exp. (-t/t) where I (t) = photocathode illumination function R = average number of electrons emitted as a consequence of a s c i n t i l l a t i o n t = decay time of illumination function =3.5 nsec f o r NE 102 = 33 nsec for Nal (185°K) I f a second exponential illumination I * (t*) with amplitude 4/3 I (t) (from Chapter III) i s added to the f i r s t at time t then a bipolar pulse created with a zero-crossover point at approximately t . The $ amplitude of I* (t*) at the zero-crossover point i s then: $ " I (t) X 100$ I* (t*) r I (t) X 100$ 4/3 I ( 0 ) = 5 (R/E) exp (-t/t) 4 (R/E) 3/4 exp (-t/t) For NE 102 t = 3, t = 3.5 $ = 3/4 exp (-3/3.5) = 32$ For Nal t = 18, t : 33 $ = 3/4 exp (-18/33) $ = 41$ BIBLIOGRAPHY Alkhazov, O.K., G r i r i b e r t , A.P., Kh. Lemburg, I . , and Rozhdeswenskii, V.V. (1959). Soviet P h y s i c s . JETP 26, 222. B e l l , J . , TAO S.J. and Green J.H. (1965). N u c l . I n s t r . and Meth. 3J>, 213. B e l l , R.E. (1965). "Alpha -, Beta and Gamma - Ray Spectroscopy" (chap. 17). North - H o l l a n d P u b l i s h i n g Company, Amsterdam. B e r t o l i n i , G., Gocchi, M., Manal, V., and Rota, A. (1966). IEEE Trans. Nuclear S c i . NS-13_, No. 4 , 119. B o n i t z , M. (1963). N u c l . I n s t r . and Meth. 22, 238. Braunsburth, J . , and Korner, H.J. (1965). N u c l . I n s t r . and Meth. 3Jt, 202. De B e n e d e t t i , S. (1964). "Nuclear I n t e r a c t i o n s , " (chap. 6 ) . John Wiley and Sons, I n c . , New York. F a l k , W.R . '(1965). Ph.D. Thesis, U n i v e r s i t y of B r i t i s h Columbia. (Unpublished). G a t t i , E., and S u e l t o , V. (1959). N u c l . I n s t r . and Meth. 4., 189. G a t t i , E.,and S u e l t o , V. (1966). Nucl . I n s t r . and Meth. 4J3, 248. G r a l i a r i , V. (1967); N u c l . I n s t r . and Meth. J>3_, I 3 6 . Janch, J.M. and R o h r l i c k , F. (1955) "The Theory of Phptoni and E l e c t r o n s " , (chap. 12). Addison - Wesley P u b l i s h i n g Co. Jones, G. and Orth, P.H.R. (1968). N u c l . I n s t r . and Meth. j>2, 3 0 9 . Miene, J.A., Ostertag, E., and Coche, A. (1966). IEEE Trans. Nuclear S c i . NS-13., No. 4 , 127. Mott, W.E. and Sutton R.B. ( i 9 6 0 ). Hand der Phys. XXV Page 86. ( B e r l i n S p r i n g e r ) . P o s t , R.F. and S c h i f f , L . I . (1950). Phys. Rev. 80, 1113. Presjent,, G., Schwarzchild, A., S p i r n , I . , and Wetter spoon, N. ( 1964) . N u c l . I n s t r . and Meth. 3JL, 71 . Simms, P.C.(l96l). Rev S c i I n s t r . 12, 894. Van Server, ¥. and Hof s t a d t e r , R. (1955). Phys. Rev. ,9_7_, 1181. Van S c i v e r , ¥. (1956). Nucleonics 14,, No. 4, 50. Van S c i v e r , W. (1956). I.R.E. Trans. Nuclear S c i . NS-3_, No. 4, 39. W i l l i a m s , C.W. (1967). Ortec Corporation "Ortec News" March 1967. 

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