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A portable device for detecting radio-active ores Smith, Ronald 1933

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U.B.C. LIBRARY OAT. m . ^ A i ^ i m . d & ^ g i A PORTABLE DEVICE FOR DETECTING l%hm6r>mTXm»JMmJMiL by Ronald Smith. A Thesis submitted f o r the Degree of MASTER OF ARTS i n the Department of PHYSICS. THE UNIVERSITY OF BRITISH COLUMBIA APRIL 1933. A PORTABLE DEVICE FOR DETECTING RADIO-ACTIVE ORES* -Page 1 o TRIBQ-ELEGTRIG EFFECT BETWEEN GLASS AND MERCDRT. Page 1 3 < A SIMPLE APPARATUS FOR SPUTTERING METALLIC FILMS, Page 17, A SENSITIVE PHOTO-ELECTRIC QUANTUM COUNTER FOR ULTRA-VIOLET LIGHT. Page 19* (1) A PORTABLE DEVICE FOR DETEGTING RADIO*ACTIVE ORES. In t r o d u c t i o n : Tremendous i n t e r e s t has been aroused i n Canada over the recent discovery of r i c h radium ore d e p o s i t s i n the Great Bear Lake d i s t r i c t . The p r o b a b i l i t y of the l o c a t i o n of s t i l l f u r t h e r d e p o s i t s In the neighborhood makes t h i s an opportune time f o r the development of a p r a c t i c a l Instrument capable of i n d i c a t i n g the presence o f radium ore* P r o s p e c t i n g f o r r a d i o - a c t i v e ore d e p o s i t s d i r e c t l y by the d e t e c t i o n of the gamma r a d i a t i o n s from them has i n the past been confined to the use of the electroscope« S a t i s f a c t o r y operation of t h i s instrument i n the open i s handicapped by s e v e r a l p r a c t i c a l d i f f i c u l t i e s , such as the f r a g i l i t y of i t s working p a r t s and the n e c e s s i t y of keeping i t a b s o l u t e l y dry. (1) H. Geiger and W, M u l l e r , Phys. Z e l t s . 29, 839, (1928) Some subsequent work on the Geiger-Muller tube i s described i n the f o l l o w i n g l i t e r a t u r e , (2) H. Geiger and W. M u l l e r , Phys. Z e i t s . 30, 4-89, (1929) (3) H. Knlepkamp, Phys. Z e i t s . 30, 237, (1929) (4) J.A. Van den Akker, Rev. S c i . I n s t . 1, 672, (1930) (5) L.F. G u r t i s s , Bur. of Stand. Jour, of Res. 5, 115, (1930) (2) An instrument i s here described that a t t a i n s a l l the s e n s i t i v i t y of the electroscope to gamma ra y s and i s moreover conveniently p o r t a b l e and f o o l - p r o o f i n use, r e q u i r i n g no t e c h n i c i a n to operate* The G-eiger-MuIler Gounting Tube; Advantage was taken of the E l e c t r o n (1) Counting tube developed by Geiger and M u l l e r i n 1928 f o r the purpose of d e t e c t i n g gamma and s i m i l a r r a d i a t i o n s . I t was necessary to modify these tubes so tha t they c o u l d be operated on a voltage low enough t o be p r a c t i c a l f o r a por t a b l e o u t f i t s R G-M. V A W — — -16 'w B 10'Q TL. L.S. F i g . I.' G.H.- Geiger-nailer t u b e ; A - A m p / i / i ' e r ; flattery - B ; L . J . - L o u d -sp ea.ker. Such a tube c o n s i s t s of a hollow metal c y l i n d e r and a wire arranged' c o n c e n t r i c a l l y , the space between these two el e c t r o d e s being f i l l e d w i t h a gas at a pressure of 7 cm. of mercury or l e s s . A high p o t e n t i a l i s app l i e d across the two electrodes through a r e s i s t a n c e of the order of 1C-9 ohms. A two or three stage r a d i o a m p l i f i e r (3) enables current surges In t h e . c i r c u i t to be d e t e c t e d . ( F i g . 1 ) . When the p o t e n t i a l across the Geiger-M t i l l e r tube Is r a i s e d s u f f i c i e n t l y c l i c k s are heard i n the loud-speaker at i r r e g u l a r I n t e r v a l s . The frequency of these c l i c k s w i l l be g r e a t l y increased i f a small q u a n t i t y of radium I s placed c l o s e to the tube. The r e s i d u a l count, when a l l r a d i o - a c t i v e m a t e r i a l s are removed from the neigh-borhood, i s due t o cosmic rays and s t r a y gamma rays from the ear t h , a c l i c k being heard when one of these passes through the tube. There Is a d e f i n i t e minimum operating p o t e n t i a l which depends on the dimensions of the wire and surrounding metal c y l i n d e r , and on the gas between the two. Above t h i s t h r e s h o l d there i s a voltage range, the breadth of which depends on var i o u s v a r i a b l e c o n d i t i o n s i n the gas or on the in n e r metal s u r f a c e s , i n which the count per minute i s f a i r l y constant; t h i s count being assumed to correspond to the number of cosmic and gamma rays passing through the tube. Above t h i s range, spurious c l i c k s are produced, the number Increasing w i t h the a p p l i e d p o t e n t i a l . A Low Operating; P o t e n t i a l ; The m a j o r i t y of Geiger-Muller tubes used f o r l a b o r a t o r y purposes contain a i r at from 3 to 7 cm. pressure, r e q u i r i n g from 1000 to 1500 v o l t s to operate them. As t h i s i s nearly p r o h i b i t i v e f o r f i e l d work, i t was necessary to develop a tube working on a much lower v o l t a g e . (4) There are three main f a c t o r s which determine the t h r e s h o l d p o t e n t i a l . 1. Diameter of outside metal c y l i n d e r . 2. Pressure of the gas i n the tube,. 3« Nature of the gas. The v a r i a t i o n of the t h r e s h o l d V w i t h the diameter b of the metal c y l i n d e r i s given by the (6) formula & l o g b/a where X, the e l e c t r i c f i e l d at the surface of the wire remains constant f o r a constant wire diameter a; that i s , the l e s s b i s , the lower w i l l be V. However the lowering of b Is accompanied by a corresponding decrease of the e f f e c t i v e area of the tube, thereby decreasing the s e n s i t i v i t y . On t h i s account a diameter of 1/2 t o 5/8 inches was chosen as the most s a t i s f a c t o r y s i z e . The voltage v a r i e d very l i t t l e w i t h the r a d i u s of the w i r e , f i n e tungsten or manganin wire being used throughout. (6) I t has been shown that the t h r e s h o l d p o t e n t i a l of a G-eiger-Muller tube changes w i t h gas pressure i n the same way as does the sparking p o t e n t i a l of the gas used.. Consequently a considerable decrease should be obtained by lowering the pressure of the gas. In t h i s way the voltage was reduced to s i x or seven hundred v o l t s f o r a i r at one to ten m i l l i m e t r e s pressure; but spurious c l i c k s (6) D. Cooksey and MoC. Henderson, B u l l . Am. Phy. S o c , June 9S 1932. (5) produced at v o l t a g e s l i t t l e above the t h r e s h o l d i n t h i s r e gion made the tube u n r e l i a b l e f o r measurement purposes. In a d d i t i o n the s e n s i t i v i t y (count per minute) was very appreciably decreased at these pressures, as Geiger and (2) . M u l l e r have already pointed out. The t h i r d v a r i a b l e , the nature of the gas held the g r e a t e s t promise of g i v i n g r e s u l t s . A number of workers have used gases other than a i r i n these tubes s u c c e s s f u l l y , I t being found t h a t each gas r e q u i r e s In general a d i f f e r e n t operating v o l t a g e . The lowest p o t e n t i a l reported bv Geiger and M u l l e r f o r any gas I n v e s t i g a t e d by them (2) (7) (8) was t h a t f o r argon. Bosch and Klumb and Schulze have shown however that i n e r t gases have c e r t a i n d i s t i n g u i s h i n g p r o p e r t i e s when used i n these tubes» Using pure neon and helium they obtained a continuous discharge on r a i s i n g the voltage above the t h r e s h o l d , as against the r a p i d l y e x t i n g u i s h e d discharge c h a r a c t e r i s t i c of base gases. When a tra c e of a i r or other base gas was added, a normal c l i c k was produced. In view of these r e s u l t s a tube was baked out i n a good vacuum and pure argon admitted. This gas e x h i b i t e d the same p r o p e r t i e s as r e p o r t e d f o r pure neon and helium. Upon heating the tube again w i t h the argon i n i t , i t was found t h a t , provided the brass had not been thoroughly baked out, enough occluded gas could be d r i v e n o f f (7) C. Bosch and H. Klumb, Die Naturwlss. 18, 1098. (1930) (8) W. Schulze, Z e i t . f u r P h y s i k 89, 92, (1932) (6) to cause the tube t o c l i c k normally. The t h r e s h o l d p o t e n t i a l was now about 360 v o l t s at one centimetre pressure and moreover the r e l i a b l e v oltage range compared f a v o r a b l y w i t h that f o r a i r at 5 cm,, p r e s s u r e . I t was.found t h a t i f one of these argon tubes was sealed o f f , i t s c h a r a c t e r i s t i c s sometimes changed over a p e r i o d of t i m e . I f the brass had been w e l l baked out before a d m i t t i n g the pure gas, the t r a c e of gas d r i v e n o f f by r e - h e a t i n g as above seemed to be re-absorbed by the metal, the tube developing a tendency to discharge con t i n u o u s l y . On the other hand i f no de-gassing was done the t h r e s h o l d voltage would sometimes r i s e a f t e r a month or so, apparently owing to gases d i f f u s i n g out of the brass i n t o the argon. To obtain a c o n d i t i o n under which the gas mixture would remain constant I n d e f i n i t e l y , a p a r t i a l de-gassing was f i r s t g i v e n the assembled tube, which was then f i l l e d to the proper pressure w i t h argon c o n t a i n i n g a small percentage of a i r . Frc seven to ten m i l l i m e t r e s pressure was found most s a t i s f a c t o r y , an operating p o t e n t i a l of from 350 to 450 v o l t s being then r e q u i r e d . Neon gas, when t r i e d i n the same way, showed s i m i l a r p r o p e r t i e s , r e q u i r i n g about 50 v o l t s l e s s than argon under the same c o n d i t i o n s . The Oomplete P o r t a b l e O u t f i t ? Having developed a Geiger-Muller Counting tube which would operate on a reasonably low v o l t a g e , i t was now necessary to c o n s t r u c t a complete (7) portable o u t f i t comprising a tube and a l l i t s a u x i l i a r y equipment. A s u i t a b l e design of the tube i s shown i n F i g . 2. F i 9 . H. The outer e l e c t r o d e B, a l e n g t h of brass tubing 5/8 inches i n diameter and from one to s i x inches long, i s h e l d i n place by a copper wire W soldered t o i t and clamped onto the tungsten wire T sealed i n t o one end of the pyrex g l a s s covering D. The c e n t r a l e l e c t r o d e M, a f i n e tungsten or manganin wire, i s suspended at one end from a g l a s s hook G-, fused onto the g l a s s w a l l , and at the other by a second tungsten wire sealed i n t o the outer case. The f i n a l s e a l o f f , at S, Is made at one end. In t h i s way a l l side p r o j e c t i o n s are e l i m i n a t e d , a l l o w i n g the tube to be conveniently handled or packed. One end ( T 1 i n F i g . 2) i s then immersed i n p a r a f f i n i n s i d e a g l a s s container j u s t wider than the diameter of the tube i t s e l f , the s e r i e s r e s i s t a n c e (R, F i g . 1) going i n w i t h i t . This outer covering acts not only as a (8) p r o t e c t i o n but a l s o prevents any e l e c t r i c a l leakage a t the t e r m i n a l T 1, an important f a c t o r because of the high r e s i s t a n c e (lO^ohms) of R. The whole i s now i n c l o s e d i n a hollow V ; Wood c a s e j I: Insulated w i r e s . c y l i n d r i c a l wooden case, a l e n g t h of i n s u l a t e d wire connecting t h i s u n i t to the r e s t of the apparatus* The a u x i l i a r y equipment c o n s i s t s of a set of ten 45 v o l t b a t t e r i e s , a two-stage audio a m p l i f i e r and a p a i r of ear-phones. The e l e c t r i c a l arrangement i s shown i n F i g . 4 , the p l a t e voltage f o r the a m p l i f i e r being tapped from the 450 ¥olts supplying the G-eiger-Muller tube. Small r a d i o " B " b a t t e r i e s were obtained and they together w i t h the a m p l i f i e r were mounted i n a water-proof case on a standard Pack-board, the whole weighing about 55 pounds. The ear-phones are plugged i n , the two leads from the tube connected to t h e i r corresponding terminals and the complete apparatus c a r r i e d as shown i n F i g . 5» The tube i s mounted on the end of a l i g h t handle so t h a t i t can (9) be kept close to the ground. Fig. V S e n s i t i v i t y Tests. As w i t h a l l small Geiger-Muller tubes, the number of c l i c k s per minute when a l l r a d i o - a c t i v e m a t e r i a l i s removed from the neighborhood v a r i e s c o n s i d e r a b l y from minute to minute. One tube gave the f o l l o w i n g readings f o r twenty consecutive minutes: 10, 12, 1 1 , 8 , 1 0 , 1 3 , 1 3 , 1 0 , 9 , 13, 6 , 1 1 , 14, 9 , 9 , 1 3 , 7 , 1 0 , 1 0 , 1 0 each f i g u r e being the number of cosmic and stra y gamma rays passing through the tube during that minute. For f i v e minute counts, however, the percentage v a r i a t i o n s are much smaller, as the f o l l o w i n g readings obtained from the same tube show: 5 1 , 5 8 , 4 9 , 5 0 Thus i t may be seen t h a t by ta k i n g f i v e minute readings only, an Increase of three gamma rays per minute through the tube could be d e f i n i t e l y detected. In case of doubt as to whether a h i g h count i s due to a d d i t i o n a l r a d i o - a c t i v e m a t e r i a l i n (10) the neighborhood or to s t a t i s t i c a l f l u c t u a t i o n of the normal count, a longer-reading w i l l s e t t l e the que s t i o n . Tests were made w i t h radium and radium ore, A 53 gram piece of pitchblende from Great Bear Lake c o n t a i n i n g 60% uranium oxide and about 9 thousandths of a m i l l i g r a m of radium gave a count of 62 f o r f i v e minutes when s i x f e e t from the d e t e c t i n g tube and 88 when three f e e t away. The zero count (no radium i n the neighborhood) was 51» E i g h t y m i l l i g r a m s of radium, k i n d l y loaned by Dr. G.W. Prowd of S t , Paul's H o s p i t a l , Vancouver, could be detected a t a distance of 350 f e e t t a k i n g only f i v e minute readings. R e s u l t s of a t e s t w i t h t h i s standard are shown i n Table 1 and the graph i n F i g , 6 . Table 1. Distance ( f e e t ) 100 150 200 250. 300 350 Count (5 minutes)• 265 139 99 82 64 55 Zero Count; 45 (11) d = d i s t a n c e i n f t . i F i g . E L I .. . i . • • l. • •. • • - : j I t may be seen from the graph t h a t the inverse square law i s obeyed w i t h i n the l i m i t s of s t a t i s t i c a l e r r o r . I t i s of i n t e r e s t to note that only about f i v e out of every ten thousand gamma rays passing through the-tube are"'detected. This r a t i o was a r r i v e d at from the a v a i l a b l e data on gamma rays from radium i n co n j u n c t i o n w i t h the r e s u l t s of the afore-mentioned t e s t w i t h 80 m i l l i g r a m s of radiurn. Other uses f o r t h i s instrument, besides that of prosp e c t i n g f o r radium ore may suggest themselves; f o r exaraple a the l o c a t i n g of radium needles m i s l a i d i n h o s p i t a l s . •Dr. G.M. Shrum of the U n i v e r s i t y of B r i t i s h Columbia i s r e s p o n s i b l e f o r the o r i g i n a l idea of adapting a Geiger-Muller tube to the purpose o u t l i n e d In t h i s (12) paper and has a l s o taken an a c t i v e p a r t In the development of the complete instrument. (13) TRIBO-EXEGTRIG EFFECT BETWEEN GLASS AND MERCURY. I n t r o d u c t i o n ; When a p a r t i a l l y evacuated g l a s s tube, c o n t a i n i n g s e v e r a l grams of mercury, i s v i o l e n t l y a g i t a t e d , l i g h t i s given o f f which i s e a s i l y observ/ed In a dark room* The explanation of the production of t h i s l i g h t was the pur-pose of t h i s i n v e s t i g a t i o n . Two p o s s i b l e a l t e r n a t i v e s were considered; 1. tribo-lumlnesoence between the mercury and g l a s s ; 2. discharges i n the gas contained i n the tube between opposite t r i b o - e l e c t r l c charges generated on the mercury and g l a s s surfaces .• Trlbo-luminesoenee.. Most s o l i d s e x h i b i t tribo-luminescenee to some degree, the e f f e c t being produced by rubbing or r u p t u r i n g the s u r f a c e . L i g h t w i t h a continuous spectrum i s emitted, the i n t e n s i t y d i s t r i b u t i o n v a r y i n g f o r d i f f e r e n t ' (2) .; m a t e r i a l s . No l i q u i d s have been reported as showing t h i s phenomenon or as being s u c c e s s f u l l y used as '• exciters-. 1. K a r l : Comptes Rendu 146, P. 1104; 1907 W. Kluge: Ann. der Physik 1, 1 P. 1, Jan. 2, 1929. 2. D.M. Nelson. J.O.S.A. and R . S . c l . , 12, P. 207 March 1926. CoS. Beals: Roy. Soc. Canada Trans. 17, Sect. 3, P. 125 1923• & others. (14) T r l b o - e l e c t r l c l t y . The t r i b o - e l e c t r i c e f f e c t i s the pro-d u c t i o n of opposite e l e c t r i c a l charges on two surfaces of d i f f e r e n t m a t e r i a l when they "break contact or are rubbed (3) together. Daws on has i n v e s t i g a t e d the case f o r mercury and q u a r t z , o b t a i n i n g a charge of 1 e.s.u./cm. 2 on a quartz p l a t e which had been q u i c k l y separated from a clean mercury . s u r f a c e . A p o t e n t i a l d i f f e r e n c e of 3 5 0 v o l t s e x i s t e d between the two when 1 » 5 m i l l i m e t r e s a p a r t . , Thus a spectroscopic a n a l y s i s of the l i g h t i n question should decide whether a tribo«luminescent or t r i b 6 -e l e c t r i c e f f e c t i s t a k i n g p l a c e . I f the former i s c o r r e c t , a continuous spectrum should be observed; i f the l a t t e r , the c h a r a c t e r i s t i c spectrum of the gas present i n the g l a s s tube should appear. Experimental Procedure. P r e l i m i n a r y experiments w i t h v a r i o u s gases and gas pressures r e s u l t e d i n argon at about .4 m i l l i -metres pressure being used f o r the f i r s t t r i a l s , t h i s gas g i v i n g the g r e a t e s t luminescence of any t r i e d . The k i n d of g l a s s or p u r i t y of the mercury used, w i t h i n c e r t a i n l i m i t s , had no appreciable e f f e c t on t h i s . A s m all spectroscope w i t h a long c o l l i m a t o r and short focus camera lens f o r good l i g h t 3» l i . H . Dawson. J.6.S.A, and R.S.- I . , 18, P. 344, A p r i l 1929 ( 1 5 ) g a t h e r i n g power was used. A mechanical device f o r shaking the tubes w i t h a n e a r l y v e r t i c a l motion enabled the s l i t to be continuously i l l u m i n a t e d . Even so the feebleness of the l i g h t made exposures of from 12 hours to 3 or 4 days necessary. Experimental R e s u l t s ; A p l a t e taken w i t h argon at .4 m i l l i -metres pressure In a pyrex tube showed 2 6 l i n e s on i t , 6 of which v/ere i d e n t i f i e d as strong l i n e s of the mercury spectrum from 4048 t o 5769 R*, 7 as l i n e s In the argon red spectrum, 12 from the argon blue spectrum and one as the l i n e of water vapouro A pyrex tube c o n t a i n i n g a i r at .5 m i l l i -metres pressure gave two strong l i n e s which corresponded to two of the f i r s t negative bands of nitrogen*. 3914 and 4278. Three very weak l i n e s were a l s o present. A f a i n t l i n e spectrum was obtained from a quartz tube c o n t a i n i n g an unknown gas and producing a r e l a t i v e l y feeble luminescence« Gonclusion; These r e s u l t s i n d i c a t e d e f i n i t e l y that the l i g h t emitted by these tubes i s coming from the gases present i n them, that I s , from small discharges between opposite charges produced on the mercury and g l a s s w a l l s •> No evidence of a continuous spectrum being observed at any time, i t l a concluded that t h i s phenomenon i s pu r e l y a t r i b e -e l e c t r i c e f f e c t . The presence of only the f i r s t negative bands of nit r o g e n on the p l a t e from the a i r tube was explained, when p r a c t i c a l l y the same d i s t r i b u t i o n o f i n t e n s i t i e s was found i n the spectrum of a weak r i n g discharge through t h i s tube; t h a t i s w i t h the low p o t e n t i a l g r a d i e n t s obtained i n both cases the f i r s t negative bands predominate. The exi s t e n c e of only small voltage d i f f e r e n c e s a l s o e x p l a i n s the g r e a t e r l u m i n o s i t y of the argon tubes, since argon has a lower sparking p o t e n t i a l than a i r . (17) A_SfflPLE APPARATUS FOR SPUTTERING METALLIC FILMS. A convenient form of apparatus f o r cathode s p u t t e r i n g of t h i n metal f i l m s has been c o n s t r u c t e d , A l a r g e h e l l j a r 10 cm. i n diameter and 20 cm. t a l l , w i t h a ground opening (G., F i g . 1) at the top and a ground l i p at the bottom r e s t s f l a t on a p l a t e g l a s s base ( B ) , the j o i n t being made a i r - t i g h t w i t h stop-cock grease. The top s e c t i o n , T, which i s ground to f i t the opening G acts as a support f o r the demountable cathode, C, and a l s o as a connection to the pumping system. An aluminium anode (L, F i g • 2) i s b o l t e d to the base B through a hole d r i l l e d i n i t , the gaskets S w i t h t h i n l a y e r s of stop-cock grease s e r v i n g to make the connection leak-proof and to ease any s t r a i n s on the g l a s s p l a t e . As i t i s e s s e n t i a l t h a t no metal be exposed i n the discharge chamber other than that a c t u a l l y being sputtered, (the aluminium anode excepted since i t does V: Water to cool anode, A. (18) not s p u t t e r appreciably) the tungsten hook supporting the cathode i s coated w i t h g l a s s , a small p o i n t being l e f t exposed to make e l e c t r i c a l contact w i t h the cathode hook* For the same reason there i s only a small opening ( J , F i g . 1) from the. chamber to the pumping system, The cathode i s constructed t o s u i t the requirements of the work to be done. I t must have a f l a t surface of diameter greater than the width of the object to r e c e i v e the m e t a l l i c f i l m and h e l d from two to four centimetres above i t . Copper cathodes can be e l e c t r o - p l a t e d w i t h many metals w i t h which I t i s inconvenient or expensive to make a complete cathode. In any case only the one metal being sputtered should be exposed during the discharge. From 500 to 1000 v o l t s D.C. are r e q u i r e d to operate the discharge, although s a t i s f a c t o r y work has been done using A,G. v o l t a g e . The pressure i s adjusted so tha t the surface to be coated i s tangent to the edge of the cathode dark space, i . e . about .04 m i l l i m e t r e s of mercury. G-ood r e s u l t s have been obtained w i t h t h i s apparatus i n s i l v e r i n g , a h a l f - s i l v e r e d surface on a Micnelson i n t e r f e r o m e t e r m i r r o r proving superior t o those p r e v i o u s l y prepare d by chemical means. Best o p t i c a l g l a s s may be sputtered i n t h i s way, there being no appreciable heating e f f e c t i f the discharge i s run s u f f i c i e n t l y s l o w l y . (19) A SENSITIVE PHOTO-ELECTRIC QUANTUM COUNTER FOR ULTRA-VIOLET . , ;.. .  -., . •. • L I G H T* , ; ; . \ . v . ; A Gelger-Mtiller quantum counting tube has been developed that i s notable f o r i t s great s e n s i t i v i t y t o u l t r a - v i o l e t l i g h t . I t s c o n s t r u c t i o n d i f f e r s i n no way from the standard design of G-elger-Miiller tube, c o n s i s t i n g of a p o l i s h e d tungsten wire ,07 m i l l i m e t r e s i n diameter suspended i n the centre of a len g t h of 5/8 i n c h brass tubing 9 cms. long, the whole enclosed i n pyrex g l a s s as shown i n F i g . l e Fi9. I. : T-B rass tube; W-Tunqsteri u/ire ; Soldered C o n n e c t i o n at S . A s e n s i t i v e cathode surface was pre-pared by heating the whole tube In vacuum t i l l some of the z i n c i n the lump of solder at S evaporated; then when the brass was allowed t o c o o l , the z i n c deposited i n a t h i n l a y e r over the Inside w a l l at one end. The tube was now f i l l -ed w i t h neon at 9 m i l l i m e t r e s pressure and sealed o f f , operating i n the dark s i m i l a r l y to other Geiger-Muller tubes. (20) The s p e c i a l l y t r e a t e d end was s e n s i t i v e to u l t r a - v i o l e t l i g h t of wave-length not more than 3100 1* and although pyrex g l a s s t r a n s m i t s very l i t t l e r a d i a t i o n below 2900 A., the s e n s i t i v i t y seems to be greater than t h a t of the best surfaces I n v e s t i g a t e d by G.L. Locher r e c e n t l y , using a quartz window. Counts from two d i f f e r e n t l i g h t sources are shown: Count/min/sq.cm. Cathode Area Count/min cathode at 1 Source Distance Exposed metre distance * Non-luminous 7 metres .04 sq. cm. 48 60^000 Bunsen Flame 50 Watt Lamp 7 " .015 11 18 45 150,000 liocher g i v e s a value of 188 i n the l a s t column f o r a Bunsen flame, using a t i n cathode, although a r e c a l c u l a t i o n from h i s readings i n d i c a t e s that t h i s f i g u r e should have been 1880 i n s t e a d of 188. His most s e n s i t i v e surface t o u l t r a - v i o l e t l i g h t was 15 times as s e n s i t i v e as the t i n cathode, but even t h i s Is l e s s e f f i c i e n t than the cathode surface here d e s c r i b e d . # G.L. Locher, Phys. Rev. 42, 525, (1932). 

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