578
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I. I S T R O D r C T I O S
The Geiger counter tube ha, hy thiy time become ;1 nidely known instrument Many investigators have used Geiger counter tubes a5 a tool in some research of nuclear, physical, chemical, or perhaps biological nature. It is safe to say that, whenever they have, their inteiezt wa? concentrated on the experiment for which the counter served as :L meaturing instrument, and that they did not care about the various physical and chemical phenomena which took place i n d v the Geiger counter every time it produced an electrical impulse. Thi5 paper will not deal with the Geiger counter a~ the familiar research tool a t all hut will describe some of the phenomena occiirring during the discharge proc~s, in the interior of the tube. The mechanism of this particular type oi self-yu*taining corona di5charge has been studied by many investigators, and much detailed knowledge has been accumulated, although this is not always apparent because of the vattered nature of the literature. This paper will review in a somewhat more integrated 1 Pieseiited at tho S ~ n i ~ ) o s i u ioii ii Itadint io11 ( li( i n i s t i 1 , i r i t l 1'1ioti)clic i i i i z t I 1 held at t h c I-nivcisit> of \-otic I),tinc Y o t i c L),iiiie I n d i n n n , .Juiir 24-27 1047
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11
GEIGER-JITELLER
COUSTEK DISCII.IKGI:
579
form what might he called the present concept oi that part oi the discharge mechanism which is called “quenching” and ii, responsible for the self-estinguishing of each discharge pulse. I t \\-ill try t o indicate what radiation-chemical problems ale involved, and iurthei.more try to show that the study of some aspects of the Geiger counter discharge n q - herve t o investigate problems of ionicr and radiation chemistry itself. 11. T H E QUES(’HISCt JIlX’H.ih*ISlI
Some fundamental niagnitudes will aid in introducing the subject : typical Geiger counter tube consists of two concentric cylindrical electrodes (see figure 1): a cylindrical metal cathode of radius, say, b = 1 cni.; axially suspended within t l i k csthode :in anode of m t i c h smallel. tli:imrt:li.. such a s w m e t d wire with a ANODE WIRE A = . 0 1 CM.
C A T H O D E CYLINDER B = I CM.
SHEATH
TO C A T H O D E
1 I ( , 1 ( ‘ l O b ~ - b t ! L l t l O l l O f L O l l l l t t ‘ l tUt)t’ h r ) ~ ) l U \ I I l l h t < l \ 10 “ bC’C’ d i t ? l ~ > l O t i U C t l ( J I lOf a d l S vhatgc Thc. clct I i o i i s p t o t l u c ~ dsiniultitricouslv n i t l i thc p o s i t i r t i o n b h a l e all leached t h c R I I O ( I ( ’ \\ I I ( ’
radius u = 0.01 em. There is a gas 01 gas mixture present in such a seli-quenching counter a t a total pressure of around 10 cm of mercury, about 90 per cent of this gas being an inert monatomic gas such as argon, and 10 per cent being a polyatomic molecule such as ethyl alcohol, ethyl ether, amyl acetate, butane, or others. constant potential of about 1000 volts is applied between the tn70 electrodes. Theie typical hgures give us additional basic information, such as the fact thut the gradient of the electric field is highest near the wire, and high enough t o produce electron multiplication (build-up of *,avalanche”) only within a distance of a few wire-diameters from the wire, that ions produced do not gain appreciable energies between oollisions t o produce secondary ionization in the gas, and that an ion traversing the tube radius must make about lo5 collisions with gas molecules. By way of introduction, it should be said that the condition for setting off a caounter discharge is that of production oi a t least one tree low-energy electron within the volume of the cathode cylindei. Only radiation which accomplisheq this either directly or indirectly will initiate a measurable Geiger-counter pulce On the other hand, the condition for the production of a short electrical
580
PAUL B. WEIS%
discharge rather than it wntnluou5 hcalidown oi the c.lectrodc interspace IS that no more free electrons shall be liberated or exist in thi. interspace after a certain characteristic time interval from any sourr~.- other than radiation originally intended to be registerecl- has elaphed ..ince initiation oi the previous discharge pulse. This condition, involving a time interval of some 60 to 100 microsec., means that a thorough clean-up oi all elecltron5 and agents capable of prodiic.ing electrons must havc been accomplished within such a time interval. I n general the only two agents which h a r ~t o be ronsidered are ( 1 ) the positively charged, gaheous ions n-hich arc formed in the primary ionization arid electron-multiplication process and ( 2 ) the photons which are emitted during the multiplication procesq as :L by-product of the electron bombardment of moleculec.. This bring, to :L discussion of the nature of the quenching mcchanism, which must be one’ designed to suppr(w the production of any new rlectrons by tlic positivc ions or by photons u p to ant1 including the time of the final collection of the ion-, bj. ihc fIwtrode3. Suppo-c that a single gay n erc present in thc tulic. Ion3 of this gas ~vouldbc produced, having an ionization encigy of 1 e.v. Fiu thermore, we can expect to find photons produced wit11 energieh ranging perhaps neiirly :is high in energy as the ionization energy I. Then the Iollon-ing pr0ceb.s of detrimental electronproduction may be poh.ihlt. : (1) Produetioil of secondary electrons by rI1r. action of ion5 or ( w i t e d nioiwule. :it the cathode surface; ( 2 ) prodwtion of photoelectrons on the cathode surface, since in general the work iunction, @, of -,iich surfaces is niucli smaller than tlw quantum energies available. Quenching, therefore, reqiiires that i f ) no excited or ionized molecules capable of producing a secondary clectron from the cathode surface shall reach the latter, and ( 2 ) :L mininiiim of high-energy photons ( E = h v 2 a) shall reach the cathode surface. It is known that neutralization will take place by the act oi drawing an electron from the metal (7- 8),when the positive ion approaches the cathode surface, at an espenditure of energy equal to, a t least, the work iunction *, leaving the neutralized molecule rvith n potential energy of 1 - a, or less. It is now important that this neutral but excited niolecule cannot eject another electron through further interaction with the metal, which is another well-known phenomenon ( 2 , 6, 11). The probabilitie, of’ electron emission as a function of distance from the metal surface have been calculated for the e 01 the ion (neutralization process) (8’1 arid for thr cahc of escited niolec s ( 5 , 8) (elrctron ej ect ion) . These calculations indi e that the critical distance foi tlic neutralization process is appreciably greater (of the order of 5 to 10 X cm. for @J =4.5 e.17.) than the critical distance for the electron emission due to an excited atom or molecule, the latter being of the order of 2 X cm. The first process is one of field emission due to the strong local electrical field set up by the positive ionic charge when it approaches the metal. The latter process can be visualized as requiring a close and direct interaction between the escited molecule and the electron atmosphere of the metal,-classically speaking, a collision of the second
GEIGER-MUELLER
C O C S T E R DISCH1RGI:
L8 1
kind. Thui a certain a\-ernge interval of time will e l a p e between chc IWO processes, which for the nearly thermal ion velocitieq in the countvr is of the order of to 10.'' sec. Thus one of the principal pioblenis of discharge quenching becomes a problem of deesciting a molecule during that time interval of to sec. to avoid electron emis\ion subsequent t o neutralization. There are essentially three mechanisms that could be hoped for t o obtain the loss of potential energy. Each has, if for a given species of molecule and escitation level it is a t all possible, a certain order of magnitude of probability of occurrence in ;Llo-'? see. time interval. Table 1 summarizes these po3sibilities. Only the labt mechanism provides deescitation fast enough to haze value as a quenching mechnni,m. The absorption spectra of molecules, when available, yield the best available information on the behavior of the molecule5 with respect to dissociation, and continuous absorption in the band region whc.re TABLE I Z C H i N I S U OF DEEXCITATION
-__
Collision with other iiiolecule Katlintioii of q u a n t a *
ORDER O F Y I G I I T U D E O F TIME REQUIRED FOR PR0CES.I
______ 10-9 sec. (average time bctxvceri collisioiiq 10-8 scc'. ilif(,tiiiie of qu:~iit:t-etnittiii::
states) 10-11 to 10-18 sec. [lifetime of states 1e:itliiig t o spontaneous dissociation) * licference 12.
vibrational structure should be found indicates dissociable stnteh having lifetimes near sec., which is about the time of a single interatomic vibration. The probability for dissociation of electronically excited states generally ribes so rapidly with atomic complexity of the molecule that it is almost a certainty that a polyatomic molecule having four or more atomic constituents will dihsociate under the conditions here dealt with. Starting out with the condition that no new electrons shall be liberated during the process of ion collection, a necessary requirement has been derived, namely, that the neutralized ion species must deescite within some lo-'? sec. or less; ions of most polyatomic gases will satisfy this condition. For a number of reasons (low operating voltage, favorable condition5 for pulse-equalization, etc.) it is usually desirable to have an inert gas such as argon present as the major constituent in the gas phase. Ions of argon, if permitted to reach the 1 ~ 1 1 would , not satisfy the above conditions. However, if a misturc of argon and a polyatomic gas is used, a transfer of ionization from the argon ion3 initially produced t o ions of the polyatomic gas can be effected, since the number of collisions before reaching the wall is high. One requirement, however, is that the ionization potential of the quenching gas be loiver than that of argon. This happens to be a condition which is easily satisfied, since as a rule the ionization potential decreases n-ith increased complexity of the molecule and thus
is usually higher for the monatomic inert gases (\Tit11 the possible exception of xenon). Xs was mentioned earlier, photons emitted (luring the initial electron multiplication process must be prevented from reaching the cathode surfare also. Since photons of energy larger than the cathode-mctal work functions are harmful, they involve the same order of magnitude :is electronic excitation levels involved in the molecular deexcitation by disociation. Since absorption bands in that energy region \\-oultl l x cxpcctecl t u exist to satisfy the deescitation condition, it is usually safe t o :issumr that the po1y:itornic constituent will also serve as an effective absorber of such photons as ~~--ould otherwise reach the cathode and produce photoelectrons’. 111. lI.~DI.ITIOS-CH~:JIIC.II, (‘OSSEQUE
The fact that dissociation of molecules is necessarily involved brings chemistry directly into the pict,ure. The dissociation phenonicna are naturally linked to the structure of the molecule, interatomic, binding forces, and their electronic strncture. There are three phases of interest, and in each, specific problems might be answered by st d i e s on the dischargequenching mechanism: 1, Phenomena regarding the neutralization mechanism at metal surfaces, such as lifetime against dissociation and critical distance of neutralization. 2. Phenomena concerned with the travel of polyatoinic ions through a gas, such as ion mobility, change of mohility due to cluster formation, or possibly dissociation during collisions or spontaneous dissociation. 3. Phenomena concerning the kinetics of the dissociation products, such as studies on the ne\y products formed hy them.
Kinctics of iou neutralizatioib
OH
metal surfaces
The disappearance of initial polyatomic gas and the re-formation of new molecular material are closely related, of course, to the lifetime of the counter tubes. I n connection with a fen- types of gases, t8helifet’ime has been found t o be limited to the number of discharges after which an appreciable percentage of the quenching gas has been dissociated. This was found t’o be t’rue for ethyl alcohol (9) and for ethyl acetate (3). The lifetime of a methane-filled tube was reported much smaller (9), and more recent measurements (1) indicated that proper operation of the methane tube is not so much limited by the consumpt,ion of quenching gas as it is by formation of deposits on the cathode cylinder. The author has found this to be true for other hydrocarbons, such as propane Strictly speaking. t lic estcsiit t o which photoclrctroris relt~asedat thc: cathode nay be harmful must be tied i n with t h e phciiomcnon of rlectron capture. -1ny photoelectron would arrive in t h e high-ficltl region during t h r “clcaci-tiliic,” of t h c tube. because of t h e relatively high mohility of t h e electrons coniparcd t o t h a t of t h c ion sheath. However, any electrons captured t o form negative ions mity warti t h a t region after t h e “dead-time” period a n d release t h e tlischargc mechanism ancw. With the product anA\- (CY= effective electron attachment coefficient, / I = avet’agia numtier of c l e c t r ~ ~ n - n ~ o l e c ~ u loel l i s i o n sfor a p a t h equal t o t h e t u b e radius. A\- = nuniher of photoelectrons rc~leaseclfrom t h e cathode) sufficientl?. sniall--nanic~ly. much snmller t h a n unity--such phot oils ~ v o u l dactually be unable t o interfere with t h c cluewhiny mechanism.
and butnnc. It appeal5 necebmy t o assume the deposition oi clielcctric polymers on m a l l preierred areas of the cathode surface. Formation of such polymers i: knon-n t o occur easily in elcctricul dischai (4) in the pre.enct’ of even smiill quantitie.: of hydrocarbon material. \\T:tt,on has mentioned a t this Symposium the observation of polymer deposits on metal surfacer in the electric fields of electron microscopel. I t seems worth pointing out that the abovedescribed surface mechanism provides an intense 5ource of molecular radicals near metal hurfaces, so that the metal surface act5 2s a catalyst t o chemical action by providing radical production from whole molecules in excited statesentially meawres recovery oi the electrical field due t o the motion of the ion sheath as a function of time after an individual pulse has commenced. Figure 2 show* the type of characteristic obtained. This curve is obtained directly on a cathode-ray tube and reflects the position of the ion sheath a t every instant, figiired from thc time of its formation. -1side from the value of such an experiment for obtaining knowledge as t o the practical recovery time of the counters, this type of measurement happens to yield the position of the ions on a continuous basis, and therefore upon recalculation will yield a continuous record of mobility along a path of travel some sec. in duration, or over somr 10j moleciilar collisions. Since the present s(*ope of knon-ledge on the mobilities of large ions is contradictory and somewhat chaotic, such measurements may be very valuable. The shape of the “recovery time curve” can be calculated and an analytical expression obtained with the mobility as a variable. It is then posqible t o determine: ( 1 ) whether the mobility of the ions formed is essentially a conqtant over their entire path, i.e., during and after about lo5 kinetic impacts; ( 2 ) what the mobility i. tinder the particular pressure and field conditions. 3 This n a s iecently pointed out t o iiic hv S C Hrov,11 of t h c AIassachusetts I n s t i l u t c o f Trrhnology.
58'4
PAUL B. IVEISZ
.I stiihirig fact which such work has brought forth so tLtr1- that the observed mobilities, I;, of the various organic gases do not nccebsarily relate to the molecular mass J1 of the parent molecule (e.g., by the relation k = const. (MI-') as might be expected. This conchision is not 5urpri.ing in view ot nia.;s-spectroniett~r re,iearch data which show very conch-ively that, in the electron bombardineat of a gas of complex molecular >triiciiire, ion- are iormed which are not necessnrily the ions of the parent molecule, but that di>-ociation is taking place along with ionization, leading t o the ion- of qm:dler ma\>.. The relati\,e abundance is often definitely in favor of fragment ion>; in tact in bonw molecule., such as tetramethyllead, no measurable parent ion intmsity ut d l i- iound, W
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T l U E E L A P S E D A F T E R DISCHARGE
1 L G 2 The variativn oi t h c c,tfecti\e clectiical fitld neai tliv n i l e a i t L 1 d cilscllaige n a s iintiatcci. Eo = field hen no discharge t a k c s place E , = tlueshold field, o i minimum field in \\hich P full cllcctloll ,L\alanchr can b(3 ioiInt7tl I ) i a n n - o u t c u r h e IS observed e\pr'i inientallJ
I\Iinetacs of dissociatiojL prodrids
Molecular fragmentation not only into fragment ions, but also into electrically neutral radicals, Takes place not only during the neutralization act at the cathode but also because of direct electron bombardrneiit in the initial multiplication process. -1salready ineritioned, there 1- yome evidence tor the iormation ot polymers on the cathode when normal hydrocarbon, supply thc bulk of thc effects. But aside from this effect there i> considerable re-tormation of gaseous products, and the modes of reaction and the type of products are of considerable interest. Conditions are similar t o those which have been studied in connection with chemical effects in T-arious gaseous electrical discharges (4). However, there is a difference in that there is not a continuous flow of bombarding electrons in the counter discharge, but only n short period of electron bombardment of less than
REACTIOXY OF C-ARUOS TJ3TRLACHLORlDEIVIrlH BROlIIKE -\t’TIV-iTED BY ISOMERIC SUCLEAR TK.ISSITIOX X S D BIT T H E SEUTROS-C;rlbli\lA RE-ICTIOS’ , J U H A I < \YII,I,.ilU Depaifiiierif
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This paper will discuss investigations of the reactioii of carbon tetrachloride with bromine activated by isomeric nuclear transition and by neutron capture (2, 14, 15). These studies have yielded evidence pertinent t o the following topics : (1) tlw physical and chemical (Szilard-( ’halnierh process) phenomena associated with isomeric transition and neutron-gamnin processes; ( 2 ) the possibility of multiple-bond rupture in a molecule which act5 a4 a “third body” in the process of neutralization of an ion; (3) the Fianck-Rabino~~-itch “cage” hypothesis. They are part of a more general program of investigation of reactionq of halogens with halogenated methanes being curried on in our laboratories. TTPES O F ACTIY.\TIOS
Saturally ocniriiiig bromine consiats oi approximatrl? equal miounts of two isotopes, Rr79and RrS1. Each of these isotopes is able to capture slow neutrons 1 l’ie,riited at t h e Synip~isiuniuii Rndi:itioii (’lit inisti \ ‘ i ~ i i lP h o t ocheniistiy I r c ~ l t l $it t h e Umveisit\ of Tot i c 1)ariit. Ynt I c 1):iiiic~ Indiaiix ,Tunc 21-27 I W i
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