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The sensitive surface of Geiger Muller counter tubes Makepeace, Ronald A. 1933

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U.B,G. LIBRARY 1 | CAT. m.LMAj^MhjVzs^ J Ace. R o : « « ^ M ; i ^ s s ^ B f THE SENSITIVE SURFACE OF GEIG-SR MULLER COUITLR TUBES. by ROHA1D A. MAKEPEACE A Thesis submitted for the Degree . c of MASTER OF ARTS May, 1953. at TEE UIIYERSITY OF BRITISH COLUMBIA. THE SENS I f IYEysURPACE OP / GEIGEB WIIulRv GOTJHTER TUBES. IITROPUGTIOlsfc """ Since Geiger l i i l l e r tubes 'are used as a /.standard method l o r detecting cosmic rays i t i s important to know' the ^ correct"e^ a Geiger tube of given s i z e . The agreement between various observers on t h i s point i s not very good. It i s commonly accepted that the number o l c l i c k s per minute for a counter depends d i r e c t l y on the surface area of the metal, tube forming the cathode so that the count obtained "by various observers for Geiger tubes i s given here i n t h i s form. *J» A* Van den Akker 1.4 per cm. per min. Kniepkamp 7 per cm. per min. Geiger and Miiller ' .51 per cm! per min. 1.1 per cm! per min* if. * Present A r t i c l e .S'Z. per cm I per min. Writers marked with a star give more complete data on conditions which would affect the r e s u l t s . According -to : t h i s data the re s u l t s should have been s i m i l a r . A l l results ; l i s t e d are ; for; a •''bailer, count except 'perhaps that of Kniepkarnp which i s from :quite ah early a r t i c l e . Much- of the discrepancy may be a t t r i b u t e d to di f f e r e n t conditions of shielding due to the ^ surrounding^building and also : to the unknown presence of .Review If 30 p<j. & : s l i g h t l y : 1 ' FVAyvs-I 'K;A :r-j 5-.c fee- . ^ " . e : .v. t " s G i ' H ^ f ; ..,3o- ;nvf J>f z 3 7 -H' Z e , f 5 c h r i f t f u r P h y s / K &\n<f J% S<zbr-$> mi-radioactive material. However, the writer found that two Geiger tubes made from the same length of brass tubing, , polished and then cut i n h a l f ...'and using, the :same hind of wire electrode given approximately the same treatment showed quite a di f f e r e n t p a l i e r count when f i l l e d with dry a i r to the same pressure. With- these premises, i t was thought not inadvisable to attempt to fi n d out what influences caused .the rate of the counting to vary. i I. F. Curtisshas already given results on t h i s subject suggesting -that- the tube walls have a predominant eff e c t on the count. He also gives a p a r t i a l explanation of the operation of the counter. The explanation, however, does not cover' the r e s u l t s of Schulze and others who state that the inert gases when used pure i n the counter, maintain a continuous discharge. This a r t i c l e i s written i n an attempt to show that the presence of adsorbed gases on the walls of the tube together, perhaps, with an oxidized layer r on this:surface control the operation'of the tube to a large extent. A theory similar to that of Curtis i s also suggested .on the function of the gas layer i n the Geiger counter based on experimental results given here.and also on re s u l t s of gas adsorption experiments of G-. I. Finch and J . C. Stimson. APPARATUS:' For most of the work, a demountable form of . Geiger counter was used. Instead of the usual ground glass bureau o f Standards •• Jo-ur.ndl of R e s e a r c h Hav I^SO j?cj CO I Pro c e e j i nc£S of J^ c^ 3 I S o c i e t y oi London S e r i e s f\ Vol- ne y 0 ( . izo mg.3 Vol- Vol- i n 3 j o i n t , a "brass head was threaded to screw into a brass socket which was fixed on to the glass tube with a Dekhotinsky j o i n t . The d e t a i l s are shown c l e a r l y i n Figure I . When the threads were coated with stopcock grease, a seal was formed quite s u f f i c i e n t for the purpose. One end of the wire for the counter was fastened to the centre of the brass head, the other was embedded i n a small glass cylinder containing s u f f i c i e n t lead weight to hold the wire taut when the head was i n place and the tube was held v e r t i c a l . The brass and aluminum tubes used i n the counter f i t t e d the inside of the glass tubing c l o s e l y and rested on a narrow brass ring which was i n turn supported by a brass s p i r a l . A connection to the outside was made with a tungsten s e a l . A glass tube f i t t i n g inside, the brass s p i r a l prevented,any e l e c t r i c a l disturbance being produced.except through the aid of the tube used as counter. The assembly was clamped"in a v e r t i c a l p o s i t i o n quite r i g i d l y .so; that the wire remained f a i r l y well centred* The diagram of the other form of Geiger counter used i s s e l f explanatory (see Figure I I ) . During the early part of the work the source of high p o t e n t i a l was a transformer fed by 110 v o l t s A . C . A vacuum tube r e c t i f i e r i n series with condensers and a high resistance was s u f f i c i e n t to convert the high A * C i voltage into a reasonably constant D*G. supply, l a t e r i t was found more convenient to use a D . C . high voltage generator, motor driven. The motor was operated by storage batteries. A variable resistance i n series with the batteries made i t possible to regulate the speed of the motor and therefore the p o t e n t i a l at the generator terminals.' The voltage r i p p l e was reduced by means of a choke c o i l and condensers. The e l e c t r i c a l Geiger tube c i r c u i t used i s shown i n Figure III * The resistance W was 1 0 ohms and was measured quite accurately by determining the rate of discharge of a 1 condenser through W. It was made by marking an India ink line•.. on a piece of cardboard which was then dipped i n p a r a f f i n and surrounded by glass. An e l e c t r o s t a t i c voltmeter measured the p o t e n t i a l across the Geiger tube. It was calibr a t e d against batteries ,'of known E.M.ff. EXPSRIMEET: av Graphs I and II give the curves for two tubes • . of apparently i d e n t i c a l construction. It w i l l ' be noticed that one tube gives a p a l i e r count twice that of the other ; and that the tube 'with' the lower curve has .a longer palier.. This w i l l be found to agree with l a t e r experiments. b. In.order to try the effect of various surfaces .on.the count,, keeping everything .else; constant as far as possible, four brass tubes were, prepared; the f i r s t had a polished brass surface which was l e f t to age for several days, the second was given a l i g h t oxide coat by heating with a gas flame, the t h i r d was coated with a l a y e r of soot from a gas flame, the fourth was coated with mercury. Before use i n • t h i s experiment, the; mercury coated tube was aged by standing , i n a i r for a few, days, since, as w i l l be explained l a t e r , a ,. fresh, surface behaves abnormally. A l l , surfaces re f e r to the inside of the brass tube only. Ihese prepared tuhes were placed successively i n the demountable tube holder. The counter was cleaned with ether at desirable points to prevent possible insulation leaks. Before each curve was taken.,the Geiger counter was pumped but for three hours with an o i l pump after which dry ,, a i r : was admitted to a pressure of fi v e centimetres of mercury. As much precaution was taken to avoid contamination with mercury vapor as was possible without the use of l i q u i d a i r . Graphs I I I , IV, V,,and VI indicate the re s u l t s of threb minute readings taken at Intervals of twenty-five v o l t s which was as close as : could be reasonably estimated with the . e l e c t r o s t a t i c voltmeter* Graphs ¥ 1 1 and VIII are curves for a .polished aluminum"tube which had been aged and for an : aluminum tube coated with soot. Graphs IX and X indicate v the results- of readings taken for the oxide and soot coated .brass tubes-four days a f t e r the f i r s t set of readings for these tubes* Sothing had been changed between two sets of readings for the .same tube* V c* Figure IV sho^ -fs the arrangement for t h i s experiment* Sliminatlon of a l l gases except mercury vapor i n the Geiger tube was obtained "by keeping oven I at a temperature of 350°C. for three hours and at the same time pumping out the.tube Iwith a mercury d i f f u s i o n pump. The lower heater was then operated so that the vapour pressure of merbury" increased. The Geiger tube was kept at a higher leraperatiire than the mercury below so that there would be no tendency for merctify to condense on the tube walls. The heaters were then taken away and the mercury removed from the lo?;er tube. The tube Yv-as now f i l l e d with dry a i r at fi v e centimetres pressure. On application of the necessary p o t e n t i a l for normal operation, i t was found that the tube ;went. • into a Continual clieking; discharge. The tube was then heated to 200°0. and evacuated for h a l f an hour. Afte r t h i s the counter operated normally. A Similar .experiment, was performed i n which only a brass tube i t s e l f was heated with mercury vapor for several hours. The tube was then inserted in the demountable Counter and showed i d e n t i c a l r e s u l t s with the above tube immediately a f t e r i n s e r t i o n . In t h i s case the counter was l e i t untouched and aft e r several hours returned to normal : operation. The a i r i n the counter was not changed during ;thls::tim:e. I f a:,tube:>;i,s\':;baked.,v'6ut-" and, evacuated without the use of mercury,no i n t e r e s t i n g r e s u l t i s obtained. .&» A brass tube was polished with fine emery ..paper td,:remove the., old surface. 'When placed i n the demount-.-Vabie;'-t.ub.:e.fhpld.e,r ,and used as the. .negative , electrode for a-;Geiger" counter,,a C l i c k i n g ^discharge took place, as i n the previous experiment, Ho difference i n the effect could be detected with nitrogen or oxygen used to operate the counter although the nitrogen i n the counter was not absolutely pure. ;;This ; was -'.because 1theeffect l a s t s r o n l y t h i r t y minutes and; the usual methods of baking out and evacuating a vessel to drive:out gases require four hours. Aluminum and magnesium tubes act s i m i l a r l y but the, emery paper i n these, cases does not remove the old surface so well. 7 i'- There was a 'possibility that the phenomenon was a photoeleetric elfeot since, the oxide film.had been removed from the metal surface* It was found t h a t ' c l i c k s due to l i g h t f a l l i n g on the surface did increase greatly i n the case,; of- aluminum and magnesium :?/hen the surface Was: pOlished but that:, i n complete darkness vthe number of c l i c k s per, minute was; s t i l l much too -great and that with .--a :fresh brass^surface, the change i n the discharge when the l i g h t s were switched . o f f v was not, noticeable. A change i n the type of sandpaper used (not the grade) made no difference -'to the r e s u l t . A similar effect :was -noticed:; mercury was used but In t h i s case there was some p o s s i b i l i t y that water vapor , ;on:;the:;;surface was, causing,.;the phenomenon* ; e. As a r e s u l t of conclusions drawn from previous , experiments i t was :thought possible: t h a i :pure nitrogen used \for:::a^&elger counter might not prove-satisfactory. Iltrogen was.obtained by heating sodium azote, and alumina, and passing the gas produced through/a phosphorus pentoxide: drying tube-.', The usual procedure of baking out and evacuating the Geiger tube was carried out before admitting the nitrogen. A f t e r one: flushing out with nitrogen, the cl i c k s , appeared.rather, ^ prolonged but- no: further -change .in the -quality of the c l i c k s could be obtained. Gn admitting a l i t t l e a i r into the tube, immediately the same type of,,. c l i c k i n g discharge was obtained that; existed i n experiments already described. The tube was l e f t untouched t i l l the next morning. It was then working normally. 8, 1 ^-;v^-:>-;fv;- I t was found by R. I* Smith that a Geiger counter using a heavily oxidized brass tube would produce only a;v;do;.scont.ihuous.vdischarge.-..-'-. 'This-..was not due to a mechanical defect i n the counter since a l i t t l e water washed ^oy.er the surface of the -tube and then removed, restored the counter to normal operation. -•g.. •''..•Using;-former• work 'as--a guide i t was decided to perform the following experiment. Some powdered, activated charcoal was attached to the inside wall of a brass tube by '•"mean's : of a very thlh.layer of lekhofinsky cement, f fie/tube ^ was then plae«d i n the tube holder,and the Geiger counter 'thus/produced was Immersed, as far as possible, 'In'.' l i q u i d a i r . The pressure was kept f a i r l y constant at five centimetres d.ue to the r e l a t i v e l y large reservoir of dry a i r connected,to'the tube. The number of c l i c k s per minute d i d ,. not : change/ hot ice ably while the ;temperature 'decreased but '/after- •the. l i q u i d a i r was removed while; the temperature of the tube :was r i s i n g , a'discbntinuous discharge took place s i m i l a r to those before mentioned. This procedure was repeated twice* The third- t r i a l was not successful but on repeating the experiment some hours l a t e r , the.phenomenon was "again evident;. A l s o , the first''-ahd- l a s t times the experiment was ca r r i e d out, when the tube had almost recovered room • temperature, the -counting stopping completely. This 'was- not due (as thought at f i r s t ) ' to leakage of the current along E. I. 'Smith .- graduate -.student-'.in\PhysiesV University of B r i t i s h Golumbia. 9. the outside of the tube owing to the formation of moisture. *. "7ThU:"t*ube/will'-resUm'e- normal -.operation af t e r approximately V; V f l v e fflihute-s.; In addition to these experiments may be mentioned two r e l a t e d facts already known..,/. The f i r s t i s thab a tube goes Into a discontinuous, discharge .when, heated; the other that' a tube f i l l e d with pure argon discharges. , Continuouslyas Soon as the f i r s t c l i c k takes place : •':-: v The p r e c i s i o n necessary In certain : : :meehamoa'l.details" o l : c o n s t r u c t i o n ofVthe counter Should be /-.pointed";outvC'-VThe design of the demountable- tube makes i t •, ;;,y'ery^ from the centre of the tube... by t i l t i n g the counter s l i g h t l y . Investigation showed that the centering of the wire was not c r i t i c a l . Roughness of . V-\. the walls of/the* sJetal tube used as cathode did not have. any- pronounced e f f e c t . P a r t i c l e s of the size of phosphorus. . yr::' vpentoxide :.dust• .-pr-.; sbb'tv'paroleles'---.::bn- the wire-*, however, cause : sllentfcontinuous-SISQUSSIDN OP THE RESULTS: . A s b:e.for.'e ;,st-ate.d',' tile: .resultfi v ' bear a r e l a t i o n to those obtained by Pinch and Stimson for the adsorption of gases on hot metallic surfaces. When a m e t a l l i c surface i s heated i n vacuo or In contact, with a gas i t , becomes charged to -a p o t e n t i a l dependent on. the gas, the temperature and the metal. The potential attained i s always negative except i n the case of oxygen between 200° and 300° G i n which case i t i s p o s i t i v e . This Is explained by assuming the adsorption of both p o s i t i v e l y and negatively charged moli* 10. ecules oh.the m e t a l l i c surface* i n t h i s instance 'the p o s i t i v e l y charged,particles predominate. • Suppose that due to the negative p o t e n t i a l of the tube with respect to the wire some, of the - a d s o r b e d , molecules on, the surface of the tube, are charged negatively. The operation of the counter i s then explained.as follows. . Electrons created by the i n i t i a l i o n i z a t i o n i n the tube move towards the wire, producing additional i o n i z a t i o n i n the intense f i e l d near the wire. P o s i t i v e l y charged molecules created .'-'by.;'-'the o r i g i n a l fast p a r t i c l e i n the tube and by the resultant electrons move towards the tube walls. By e l e c t r o s t a t i c a t t r a c t i o n electrons are released from the negatively charged molecules u n t i l over the small l o c a l area i n which the action takes place the supply- of electrons i s exhausted. .The discharge,, then, stops. This description may be a sl i g h t Q l a r i f i c a t I o n over that given by. Gurtiss. . The .graphs f o r - d i f f e r e n t , surfaces were plotted"' i n an e f f o r t to .uh'de'r.st.aad' 'the-, widely, ' - . d i f f e r i n g -re"sult;s. of Curtisson various surfaces. Curves for four d i f f e r e n t surfaces 'bh, a ; brass tube (Graphs 111, .'IY, Y and YI) are given i n which gaseous and other conditions were kept as constant as possible.. Using the p a l i e r count for the surfaces immediately a f t e r i n s t a l l a t i o n i n the counter, i t cannot be said that the number of c l i c k s per minute varies more than 10$ except i n the case of the mercury surface, which apparently has,only a very s l i g h t i n d i c a t i o n of a p a l i e r at f i f t y counts per minute.' i i v The two surfaces on the aluminum tube also showed quite-s i m i l a r r e s u l t s (Graphs ¥11 and. YTIT): although the count, per Square,centimeter was higher because of the aluminum. Graph I I I for the soot'coat on the brass tube has two p a l i e r s The lower one i s assumed correct since i t occurs at the same s;.p-Otrehti.al' 'as..- th:ps:S';.r;eprse'S'eitting' -the.' other surfaces. l o explanation can'he offered for the second p a l i e r . low consider the two curves shown for counters which have heeh untouched f o r a few days.. The count i s -d i s t i n c t l y lower and the p a l i e r f l a t t e r than before (Graphs 'iXv-.-ahd X) * Also the percentage increase with a sample of '-'radI--OB.c'tl-v-e at the same distance a w a y 'is greater (Graphs SII and X I I I ) . ' ' ' : -.'-:;, . - : I t - . i$ .sugges-t'ed.:- that - the-., concentration, of . molecules on the surface of: the tube i s greater at atmospheric pressure' than at f i v e centimetres. To restore equilibrium when the pressure : i s reduced, molecules' leave .the surface.. I f thei.'molecule i s charged on leaving the surface, i t w i l l , producea c l i c k . These extra c l i c k s should diminish i n ah exponential manner. The two lower curves approximate more, closely, to the corrBCt nomher of e d i c t s per minute as determined by "a separate very s a t i s f a c t o r y tube giving the curve shown i n Graph XI and also according to results, of Bi,Hamilton^Working i n the same room. 1R. Hamilton - graduate student i n Physics, University ....... of B r i t i s h Columbia. 12. A theory of charged molecules leaving the surface could also he adduced to explain r e s u l t s detailed i n parts (cl (e) and (g| . In part .(e), evaporating mercury molecules may be. assumed charged. In part (e), the small quantity of oxygen admitted displaces the nitwgeh on the -surface of the tube, the nitrogen coming o f f charged* f h l s ^agreeswitli resuli;s on hot surfaces i n Vvhich i t i s found that 2^ Oxygen w i l l cause the nitrogen charge to be displaced by the oxygen charge on the met a l l i c surface. A further comparison with the hot surface suggested that . nitrogen might behave l i k e argon i n a Geiger tube. The •result of this; experiment described in;(e) was: not satisfactory although i t did appear that pure nitrogen gives prolonged clicks*;. .11 Is; possible that the nitrogen use not e n t i r e l y pure* -Referring to part (g), i t i s known that charcoal adsorbs gases when cooled to l i q u i d a i r temperatures and l i b e r a t e s them when the temperature i s raised. A spongy copper surface .should.; give similar r e s u l t s . ; The, fact that a Geiger tube f i l l e d with a :very; pure .rare gas Is unstable suggests that the f i r s t , p o s i t i v e ions which approach the cathode attract the charged ';g£a:iH"qleculesbthems-eiVeB'--;frpm the Surface, making i t possible for future electrons to be attracted from the surface of the tube. I t s e l f and thus maintaining a discharge. I f a small quantity of oxygen i s admitted, i t w i l l at once displace the argon on the surface and form a stable layer. / 13. The discharges caused by a fresh surface seemed to have some r e l a t i o n to contact p o t e n t i a l s . A l l metals become mere elec t r o p o s i t i v e when the surface f i l m i s removed. Electrons are more e a s i l y emitted from a f i l m free surface,. This film,may be an oxide coat or adsorbed gas but u n t i l i t i s formed apparently a supply of electrons from the .tube walls keeps up a discontinuous discharge. A s i m i l a r effect due to a heavy oxide coat on a brass: tube i s s t i l l without adequate explanation*. It was found i n hot surface experiments also that a heavy oxide :Coat .on; the surface of a copper sheet prevented any "charge • ;frbm/appearing. \ i n i c k e l sheet which had been oxidized, hbwever, preserved the same.characteristics as the pure nickel"surface. It would be in t e r e s t i n g therefore to ascertain the .result of using a n i c k e l oxide coat for a Geiger tube. -Examination o f • the-'graphs for a counter with 'a.--/clean; brass;^surface and also;:for the| checking counter which, had a polished brass tube Shows that at" voltages higher than the p a l i e r Voltage, the curve i s f l a t t e r than for other t u b e s . T h e aluminum tube which was polished and aged does not ;:sh.ow t h i s effect So markedly: due to more complete oxidation. /-For /curves;;;haying t h i s abnormal flatness,/ the / c l i c k s were prolonged over that of the curve 4, and sounded-'like three or four c l i c k s i n quick succession. • . M: H e . Ph 14 •'Ap^ar eht'lyva'"'thiii-'oxide f i l m on a m e t a l l i c s u r f a c e or a n o n - m e t a l l i c s u r f a c e serves the - ipurpose of p r e v e n t i n g e l e c t r o n s ; from; ^ emerging from the su and. thus extending the c l i c k s . . As a Geiger:counter i s heated* the a c t i v a t i o n . o f - t h e molecules on the tube s u r f a c e due t o temperature begins;./ v I f . a c c o r d i n g to theory, both p o s i t i v e l y and • ;&egat :iY0lytcharged,•• mbieeuies :- are adsorbed on the s u r f a c e then • "begatively charged i o n s " w i l l be r e l e a s e d w i t h i n c r e a s i n g frequency as the temperature .Is r a i s e d . T h i s . f o l l o w s from the; f a charging up of the s u r f a c e i n c r e a s e s Cwlth t.emperat.nre and t h a t an I n c r e a s i n g c u r r e n t may be drawn frbrh the * s u r f a c e as the temperature r i s e s . While the e x p l a n a t i o n of tbe r e s u l t d e t a i l e d i n t h i s paper/may •'•hot be a l t o g e t h e r c o r r e c t , I t i s safe to •«'oi£"eiMe>'that-,the '"operation o f the Geiger counter i s very much dependent o n s i i r f a c e f i l m s which form a coat over tbe i n s i d e o f the hathode* A l s o c e r t a i n r e l a t i o n s which e x i s t between the o p e r a t i o n of G e i g e r tubes and other f i e l d s of work are i n d i c a t e d . • ' " ' • ' .' U s i n g r e s u l t s from a l l sources mentioned above one; might g i v e c e r t a i n p r e c a u t i o n s to be regarded i n making a good counter. One should use a smooth r a t h e r f i n e wire f r e e from dust p a r t i c l e s . The f i n e r the wire used, the less c r i t i c a l w i l l .be the c e n t e r i n g o f the wire i n the tube. The tube, should have a t h i n oxide coat i f a metal s u r f a c e i s used. A s m a l l percentage o f oxygen or hydrogen should be mixedvWith any other gas to be used i n the counter due to 15. t h e h i g h a f f i n i t y o f these two gases f o r the s u r f a c e . Water vapor and mercury vapor are t o be avoided. There should be no leakage due to poor i n s u l a t i o n . The a c t u a l m a t e r i a l composing the s u r f a c e w i l l not a f f e c t the count p r o v i d e d ( l ) t h a t i t i s a conductor, ( S ) t h a t i t i s not.an e l e c t r o n e m i t t e r under c o n d i t i o n s found i n Geiger counters, (3) that i t has no adsorbed gases which w i l l be g i v e n o f f , (4) that t h e r e are no sharp p o i n t s although a f a i r l y rough surface i s not d e t r i m e n t a l . I wish to express my indebtedness to Dr. G. M. Shrum for. many h e l p f u l suggestions and c r i t i c i s m s i n t h i s work. 1 . .j _ .Soo'f Coe s Tul>g n I 1 1 ) •go 60 < © ( • - -© < Y GO ) © - " " • 1 ( JV 10 ( < © } • I .1 w ) 0 10 00 II. fo /* Soot COA1 : ™-I V f—>, ' , : 1,,-,,,.•„. 50 /Si j Tute > 0 /J. 0 OO /¥ > ^ 2 - - — -Grdpk 0 5 - 0 visit5 — — © 1 W I T m < 1 © J in. c o 0 1 :e/ii'*te ly -v se r t W r v y t e r . > / Qn «C / T a ken. cif f e w c f . ' ^y s , Lo 0 > > 1/. . . . >0 MO III X \ SO 1300 i3Z: '0 ffr-C >o /yiro Volts ! :-7/5 --• SO HQ M e r c U r-\/ Cod .fed Tub e. '"/ -C r A p K J T 30 1 • -• II ro iu so 13* OO /i/ i 3e to -, © « © < G i 1 C K e c k i r . 1 -10 =>0 /OffO ll'pO l/ii 'O IZ]P0 12 . /3C3 0 /J. ro Vo/ts 

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