ELECTRONIC IMAGING OF IONIZING RADIATION WITH LIMITED AVALANCHES IN GASES

Nobel Lecture, December 8, 1992

G EORGES C H A R P A K

CERN, Geneva, Switzerland

Detecting and localizing radiation is the very basis of physicists’ work in a variety of fields, especially nuclear or subnuclear physics. Certain instruments have assumed special importance in the understand- ing of fundamental phenomena and have been milestones in the building up of modern theories. They make up a long list: the ionization chamber, the cloud chamber, Geiger-Müller counters, proportional counters, scintilla- tion counters, semiconductor detectors, nuclear emulsions, bubble cham- bers, spark and streamer chambers, multiwire and drift chambers, various calorimeters designed for total absorption and then measurement of parti- cle energy, Cherenkov or transition radiation counters designed to identify or select particles, and many other detectors, some very important examples of which are still being developed. However, some of this equipment has become obsolete as physicists’ requirements have changed. Wire and drift- chambers, introduced in 1968, met the then requirements of physicists, whereas the properties of the most productive detectors available at the time, mainly bubble and spark chambers, were no longer capable of meet- ing those needs. Multiwire chambers gave rise to further developments in the art of detectors, of which some are highly innovative. Most high-energy physics experiments make use of these methods, but their application has extended to widely differing fields such as biology, medicine, and industrial radiology. Our study of multiwire proportional chambers, which began in 1967, was triggered by the problems with spark chambers which then faced us. The latter, introduced in 1959 by Fukui and Myamoto, beautifully supplement- ed the bubble chamber. Whereas the latter was still peerless in the quality of the information which it provided and from which one single exposure could on its own lead to an interesting discovery, the spark chamber gave a repetition rate more than 100 times higher. Moreover, as it had a memory of almost 1 µ s, the instrument could be triggered only for events selected by faster auxiliary counters, making it possible to address the study of phenom- ena which occurred much more rarely in very high-energy interactions. Nevertheless, the need to store the information on photographic films led 2 4 Physics 1992 to a bottle neck: beyo n d a fe w millio n p hotogra p hs per year or per ex peri- ment, the ex pos ure analysis eq ui p ment was sat urate d. P hysicists t herefore ha d to i n ve nt met ho ds of rea di ng t he s parks w hic h bypassed photographs. We introduced t wo ne w methods: one was based on t he meas urable delay of t he sig nal pro d uce d by a s park i n reac hi ng t he e n d of a n electro de; t he seco n d, base d o n t he divisio n of t he c urre nt pro d uce d b y a s par k i n pla ne or wire electr o des, at t he e n ds of w hic h t he c urre nt p ulses are meas ure d, ma de it possible to obtai n t he coor di nates of t he s park a n d he nce of t he particle by p urely electro nic mea ns. T his latter met ho d was develo pe d i n several laboratories for t he focal pla nes of s pectro meters a n d we have o urselves th us perfor me d ex peri ments on the n uclear reactions in d uce d by pions. Other a p proaches, so me of the m better than o urs, were develo pe d si m ul- ta neo usly: so nic s park c ha mbers a n d wire s park c ha mbers w hic h gave rise to very i m porta nt develo p me nts. Nevert heless, t he fact t hat it is i m possible to trigger s park cha mbers at rates above abo ut 100 ti mes per secon d re d uce d t heir sco pe. I n t he mi n ds of so me p hysicists arose t he i dea of li miti ng t he disc harge pro d uce d fro m electro ns release d i n a gas to a m uc h lo wer level t ha n t hat attai ne d b y t he s par k s o as n ot t o disc har ge t he ca pacita nce for me d by t he electro des; t he a d ditio nal gai n t h us nee de d was to be ob- tai ne d by mea ns of electro nic circ uits. I n 1967, I u n dertook t his ste p, ar me d wit h so me ex perie nce acq uire d at t he College de Fra nce fro m 1948, so me te n years before I joi ne d C E R N. I b uilt cyli n drical si ngle- wire pro portio nal c ha mbers a n d also de mo nstrate d t he possibility of maki ng use of t he lig ht p he no me na pro d uce d by a n a vala nc he of electro ns i n a gas. T his a p proac h le d to no practical met ho d. It was greatly exte n de d later o n d uri ng work do ne at t he U niversity of Coi m- bra i n Port ugal by E. Policar po. T his work, fro m w hic h I dre w a great deal of i ns piratio n, prove d very val uable for t he ex perie nce I gai ne d a n d, more partic ularly, t he u n dersta n di ng of t he pri nci ples go ver ni ng t he m ulti plica- tio n of electro ns i n gases. It le d me to a n atte m pt to b uil d c ha mbers wit h a vala nc hes ma de visi ble b y s hort electric p ulses. It res ulte d, i n 1956, i n t he first detector wit h s parks follo wi ng t he trajectory of particles. It playe d no role i n t he i ntro d uctio n of s park c ha mbers i n particle p hysics. Fi g ure 1 s ho ws t he desi g n selecte d i n 1967 to st u d y pro portio nal m ulti wire str uct ures. A st u d y of t h e el e ctri c fi el ds s h o ws t h at, i n t h e r e gi o n n e ar t h e wir e t a k e n to a positi ve pote ntial, w here a li mite d-scale a vala nc he is to be pro d uce d, t h e el e ctri c fi el d i s t h e s a m e a s t h at pr e v aili n g n e ar a wir e t e n si o n e d i n t h e a xis of a c yli n dri c al t u b e, as c a n b e s e e n i n fi gs. 2 a n d 3. Wit h t h e p ar a m- eters we c hose, i n a gas i n co m mo n use i n pro portio nal co u nters, t he a vera ge mea n free pat h of a n io nizi n g collisio n s ho ul d, at at mos p heric press ure, be abo ut 1 µ m (fig. 4). We mig ht t herefore ex pect a gai n of abo ut l 0 5 for a n avala nc he exte n di ng over a dista nce close to t he dia meter of t he wir e, i. e. 2 0 µ m. Georges Charpak 2 5

1 5

G u ar d stri p

Fi g. 1: A fe w construction details of multi wire cha mbers. The scnsitives anode wires are separated by 2 m m fro m each othcr; their dia meter is 20 µ m. They are stretched bet ween t wo cathode meshes, in a gas at at mospheric pressure. The edges of the planes are potted in Ar al dit e, all o wi n g o nl y t h e hi g h v olt a g e t o e nt er a n d o nl y t h e p uls es t o l e a v e t o g o t o a 1 0 a m plifi er.

Fi g. 2: E q ui p ot c nti als a n d el e ctri c fi el d li n es i n a Fi g. 3: D ct ail of fi g. 2 s h o wi n g t h e el e ctri c fi el d multi wire proportional cba mbcr. The effect of around a wire ( wire spacing 2 m m, dia meter 20 t he sli g ht s hifti n g of o ne of t he wires ca n bc see n. µ m). It has n o effect o n t he fiel d cl ose t o t he wire. 2 6 Physics 1992

1

0. 0 0 1 0. 0 1 0. 1 1 Di st a n c e fr o m c e ntr e of wir e ( c m)

T here still re mai ne d o ne c halle ngi ng proble m, t hat of t he ca paciti ve co u- pli n g bet wee n t he wires. T he closer t he y were, t he more li kel y it was t hat a p ulse i n d uce d i n o ne wire wo ul d be pro pagate d o n its neig hbo urs. T his was tr ue for p ulses pro d uce d by a n exter nal electric ge nerator b ut u ntr ue for the internal generator for me d by the positive an d negative ions se parating u n der t he effect of t he electric fiel d. T here ha d, i n t he past, bee n exa m ples of wire co u nters, es pecially i n cos mic-ray ex peri me nts, w here t his fear of c o u pli n g le d t o t he i ns ulati o n of eac h of t he p ositi ve a m plif yi n g wires b y partitio ns or t hick i nter me diate wires. We merely ha ve to exa mi ne t he p ulse- ge neratio n mec ha nis m i n a pro portio nal co u nter to see t hat, w hatever t he dista nce bet wee n t he wires, t he o ne w hic h is t he seat of a n a vala nc he will develo p a negative sig nal, w hereas t he neig hbo uri ng wires a n d, i n ge neral, all t he neig hbo uri ng electro des, develo p a positive sig nal w hic h is t herefore eas y to disti n g uis h fro m t he ot her. M ost of t he electr o ns pr o d uce d i n t he first micr o ns i n fr o nt of t he wire pass t hro ug h a very s mall pro portio n, ∆ V, of t he p ote nti al V a p plie d bet wee n t he wire a n d t he cat ho de o n t heir trajectory. T he collecte d c harge will pr o d u c e o n t h e wir e t a k e n t o t h e p ot e nti al V o nl y a p ulse of c har ge s o t h at Wit h o ur selecte d para meters, t he ti me nee de d to collect t he electro n c harge was a fractio n of a na noseco n d. Ho wever, t he positi ve io ns ha ve to pass t hro u g h t he w hole of t he pote ntial dro p V a n d t h us i n d uce al most all of t he c harge p ulse w hic h develo ps as a f u nctio n of ti me accor di n g to a la w w hic h reflects t he co nsi dera ble fiel d close to t he wir e a n d t h e d e cr e asi n g fi el d f ar fr o m it. T h e i niti al v er y f ast i n cr e as e of t h e Georges Charpak 2 7 p ulse gave so me peo ple t he ill usio n t hat t he i nitial p ulse observe d was ca use d by t he collectio n of t he electro ns of t he a vala nc he. For a pro portio n- al c o u nter wit h a ra di us of 1 c m, a n d a wire wit h a dia meter of 20 µ m, t h e electro n co ntrib utio n is o nly 1 %. Fig ure 5 s ho ws t he c haracteristic s ha pe of the develo p ment of a p ulse in a pro portional cha mber o wing to the motion of t h e i o ns. Alt h o u g h t h e t ot al ti m e t a k e n f or i o n c oll e cti o n is cl os e t o 5 0 0 µs i n t he exa m ple c hose n, al most half of t he sig nal develo ps wit hi n a ti me cl ose t o a t h o usa n dt h of t his val ue, w hic h is ver y usef ul f or fast detecti o n.

2 0 0 3 0 0

T he key p he no me no n is t his: w hereas t he i n d uce d sig nal is negative o n t he wire fro m w hic h t he positi ve io ns are goi n g a wa y, it is positi ve o n t he neig hbo uri ng wires or electro des. T his has t wo i m porta nt co nseq ue nces: - It is a si m pl e m att er t o l o c at e t h e wir e w hi c h is t h e s e at of t h e a v al a n c h e, w hatever t he dista nce bet wee n t he wires. - T he p ositi o n of t his a vala nc he al o n g t he wire ca n be o btai ne d if t he cat ho de is ma de of wires or stri ps per pe n dic ular to t he a no de wire. T he distri b utio n of t he i n d uce d positi ve si g nals is t he n ce ntre d o n t he avala nc he. Ex perie nce was to s ho w t hat t his observatio n was of pri me i m porta nce to t he i magi ng of ne utral ra diatio n, p hoto ns, or ne utro ns. T he ti me resol utio n of t he detector de pe n ds o n t he dista nce bet wee n t he wires. Pro portio nal cyli n drical c ha mbers ha d bee n aba n do ne d i n particle- p h ysics ex peri me nts as t his resol utio n was poor: t he ti me ta ke n b y t he el e ctr o ns r el e as e d i n t h e g as t o r e a c h t h e m ulti pli c ati o n r e gi o n n e ar t h e wir e is act uall y varia ble. It i m me diatel y a p peare d t hat t he dela y was easil y meas ure d i n a m ulti wire c ha mber a n d t hat it gave precisely t he dista nce of t h e i o ni zi n g p arti cl e fr o m t h e wir e. 2 8 Physics 1992

T his cleare d t he way for a class of detectors derive d fro m wire c ha mbers i n w hic h t he detecti ng wires are very far a part a n d w here t he coor di nate is calc ulate d by meas uri ng t he drift-ti me of t he electro ns i n t he gas. Fro m 1968 we s ho we d t hat it was possi ble to o btai n acc uracies of t he or der of 100 µ m wit h str uct ures s uc h as t hat of fig. 6, w hic h pro vi de a co nsta nt electric fiel d o ver a l o n g dista nce. I n 1969, a gr o u p at Sacla y be ga n t o b uil d drift c ha mbers wit h a migratio n le ngt h of 20 c m, w hile, i n Hei delberg, st u dies were un dertaken on cha mbers constr ucte d, si milarly to or dinary wire cha m- bers, wit h a fiel d wire fitte d bet wee n t he a n o de wires t o re pel t he electr o ns. T he drift c ha mber see me d to be t he i deal i nstr u me nt for large detectors; ma ny were t he gro u ps w hic h t he n bega n to fit co nsi derable areas wit h t hese detect ors, reac hi n g, f or i nsta nce, 5 5 m, maki ng possible a precisio n of a fe w h un dre d microns, with electronics co m prising a li mite d n u mber of c ha n nels.

Fi g. 6: Operating principle of a detector ba wd on the drift-ti me of the electrons in a constant el e ctri c fi el d ( 1 9 6 8).

Starti n g i n 1969, we stresse d t hat t he ex pl oitati o n of electr o n drift i n large vol u mes, co mbi ne d wit h t he meas ure me nt of t he ce ntroi d of t he avalanches in d uce d in a wire cha mber, was the way lea ding to three di men- sio nal detectors. It was, ho wever, D. Nygre n w ho, by co mbi ni ng t he effects of parallel mag netic a n d electric fiel ds a n d solvi ng for mi dable data acq uisi- tio n proble ms, s uccee de d i n creati ng a n i nstr u me nt w hic h provi des t he fi nest i ma ges of t he most co m plex co nfi g uratio ns obtai ne d i n colli ders; for this p ur pose the re petition rate m ust be lo w eno ugh to acco m mo date an electr o n drift o ver l o n g dista nces. I n fact it was t he de ma n ds of p h ysics, differi ng very wi dely de pe n di ng o n t he nat ure of ex peri me nts a n d accelera- t ors, w hic h dictate d t he str uct ures of gas detect ors, ma ki n g use of t he pro perties that we de monstrate d in 1968. T he de velo p me nt of tra nsistor electro nics, ho we ver, ma de it possible to Georges Charymk 2 9 desig n syste ms req uiri ng te ns of t ho usa n ds of c ha n nels. T he a dva ntage of wire over drift c ha mbers was t heir ca pacity of acce pti ng very hig h co u nti ng rates. T he resol utio n ti me, of abo ut 30 ns, a n d t he possible co u nti n g rate of l 0 5 p ulses per seco n d, ma de it feasible to tackle t he st u dy of rare p he no m- e na w hic h were beyo n d t he reac h of s park c ha mbers as t hey nee de d very hig h co u nti ng rates. T he desig n st u dy of a gia nt detector, t he S plit-Fiel d Mag net (SF M) was la u nc he d at C E R N i n 1970 u n der t he directio n of A. Mi nte n for a n ex peri me nt at t he I ntersecti ng Storage Ri ngs (IS R). T his detector co m- prise d 70,000 wires, so me of t he m 2 m lo ng. A not her gro u p u n der J. Stei nberger u n dertook t he co nstr uctio n of a detector desig ne d to collect a lar ge n u mber of e ve nts violati n g c har ge parit y co nj u gatio n i n t he st u d y of kao n decay, A large n u mber of diffic ult proble ms ha d to be sol ve d to go fr o m t h e 1 0 10 c m c ha mber to t hese large areas. T he i nte nsity of t he p ulses collecte d o n t he wires of a c ha mber was pro portio nal to t he e nergy d e p osit e d i n t h e v ol u m e d efi n e d b y t h e el e ctri c fi el d li n es e n di n g at a wir e. I n a cyli n drical co u nter, si m ple co nsi deratio ns s ho we d t hat t he logarit h m of t he gai n was pro portio nal to a factor off = w here V i s t he a p plie d voltage a n d t he t hres hol d voltage. T he be havio ur of t he wire c ha m ber is exactl y t he sa me as t hat of a c yli n drical co u nter. My gro u p was t he n rei nforce d by F. Sa uli, w ho, toget her wit h collabo- rators joi ni ng t he vario us projects la u nc he d by us, co ntrib ute d greatly to t he s uccess of ma ny ne w detectors. We u n dertook a syste matic st u dy of t he factors co ntrolli ng t he acc uracy i n drift c ha mbers. We i n vestigate d t he ulti mate accuracy which can be reache d in multi wire cha mbers by measur- i ng t he sig nals i n d uce d o n t he cat ho des. We s ho we d t hat t he avala nc hes co ul d exte n d to o nly a very li mite d exte nt aro u n d t he wire a n d t hat by meas uri ng t he ce ntroi d of t he i n d uce d sig nals it was possible to deter mi ne t he azi m ut h of a n avala nc he. O ur res ults a n d t hose of a fe w ot hers, w hic h starte d a s yste matic st u d y of t he m ulti wire str uct ures, le d to a ge neralizatio n of t he use of wire c ha mbers a n d drift c ha mbers wit h a co nsi derable di versifi- cati o n of t he detect or’s str uct ure best a da pte d t o t he variet y of sit uati o ns e nco u ntere d i n particle p h ysics. Mixt ures of u p t o f o ur gases were f o u n d t o re d uce t he c ost of t he necessary electro nics, maki ng it possible to obtai n hig h sat urate d p ulses i n de pe n de nt of t he e nergy de posite d i n t he gas, a n d req uire d less se nsitive a n d ex pe nsive electro nics t ha n wit h t he pro portio nal syste m, as well as bei n g s ufficie ntl y resista nt to a gei n g effects. It was de mo nstrate d t hat t hese p ulses were pro d uce d by a series of avala nc hes w hic h sto p pe d at t he u nifor m lo w fiel d far fro m t he wire. A syste matic st u dy of c ha mbers fille d at very lo w press ures, co n d ucte d i n Israel by A. Breski n, was to s ho w t hat t he c ha mbers o perate d at press ures as lo w as 1 Torr wit h asto nis hi ng ti me resol utio ns: t he ra nge of a p plicatio n of wire cha mbers was wi dening. W hereas wire a n d drift c ha mbers were esse ntial i n all t he particle p h ysics ex peri me nts, ra pi dly re placi ng s park c ha mbers a n d, i n so me i nsta nces, 3 0 Physics 1992 reinforcing bubble cha mbers with structures outsi de the m, many groups were maki ng use of t he ne w o p port u nities offere d for i magi ng vario us ty pes of io nizi ng ra diatio n. T he mai n a p plicatio ns a p peare d i n t he fiel d of X-rays wit h a n e nergy close to 10 ke V. U n der t he i ns piratio n of V. Perez- Me n dez, c ha mbers were b uilt to st u dy t he str uct ure of protei ns by t he i magi ng of X-rays diffracte d by t heir crystals. Gas detectors ha d t he dra wback of bei ng hig hly tra ns pare nt to X-rays a n d t he met ho ds use d to overco me t his re- quire d co mpresse d xenon. W e, o n o ur si d e, tri e d t o s ol v e t his pr o bl e m b y b uil di n g a s p h eri c al drift c ha mber ce ntre d o n t he macro molec ule crystal. T he ra dial li nes of t he electric fiel d eli mi nate a n y parallax a n d t he electr o ns drifti n g o ver 15 c m are tra nsferre d i nt o a 50 50 c m m ulti wire c ha mber i n w hic h t he a vala nc hes are meas ure d to a precisio n of 0.5 m m. It is also fo u n d t hat t he res po nse is co nti n uo us i n bot h di me nsio ns, as t he diff usio n e ns ures a n ex pansion of the clo u d of ionization electrons which al ways covers t wo wires, ma ki n g it possi ble to i nter polate t he positio n of t he a vala nc he be- t w e e n t h e m. The a p parat us has consi derable a dvantages over photogra phy in ter ms of data-ac q uisitio n rate a n d t he si g nal-to- noise ratio. Use d ro uti nel y wit h t he X-ray bea ms pro d uce d by the synchrotron ra diation fro m an electron stora ge ri n g at Orsa y it gi ves a goo d cro p of i m porta nt res ults. T he i magi natio n s ho w n by vario us gro u ps has ma de it possible also to exte n d t he fiel d of a p plicatio n of wire c ha mbers for hig her X- or γ -r a y e ner gies. T h us, a Novosibirsk gro u p has develo pe d a c ha mber maki ng it possible to X-ray t he h u ma n bo dy wit h a decrease i n dosage over t he most po werf ul e q ui p me nt a vaila ble o n t he mar ket of at least a factor of 10. A gro u p at Schl u mberger has pro d uce d a syste m for ra diogra phing giant co ntai ners usi ng X-rays of u p to 5 Me V. A. Jea vo ns has co nstr ucte d posi- tron ca meras ca pable of detecting 0.511 Me V γ -ra ys wit h a precisio n of abo ut 1 m m. These ca meras, which are not efficient eno ugh for the metho d to be a p plie d to n uclear me dicine, ma de possible a re markable a dvance in t h e fi el d of s oli d-st at e p h ysi cs. Fi n all y, a fir m is n o w m ar k eti n g a c a m er a f or γ -rays, co m peti ng wit h t he A nger ca mera, w hic h is s pecially s uitable for use o n c hil dre n. Its pri nci ple is base d o n a wire c ha mber fille d wit h co m presse d xe no n. It gives a co nsi derable re d uctio n i n t he doses a d mi nistere d a n d also i ncrease d precisio n. T hese fe w exa m ples are e no ug h to s ho w t hat we may be o n t he t hres hol d of t he ge neral use of ra diatio n detectors origi nally i n ve nte d for particle p h ysi cs. It will d e v el o p i n li n e wit h t h e pr o gr ess m a d e i n p arti cl e p h ysi cs researc h laboratories. A n i m porta nt sta ge i n t he wi de ni n g of t he fiel d of a p plicatio n of gas detectors was ma de wit h t he i ntro d uctio n, by J. Seg ui not a n d T. Y psila ntis, of p hotose nsiti ve va po urs. T hey make it possible to locate p hoto ns i n t he far ultra violet of a n e ner g y a bo ve a bo ut 5.3 e V wit h a precisio n of less t ha n 1 m m. Major instr u ments designe d to i dentify particles thro ugh Cherenkov Georges Charpak 3 1 ra diatio n are no w i n use i n so me gia nt colli der detectors. D. A n derso n ex plore d t he o p port u nities provi de d by t he detectio n of t he p hoto ns e mit- te d by sci ntillators. Fig ure 7 s ho ws a 9 Ge V p hoto n s pectr u m obtai ne d wit h a cha mber containing a tetrakis(di methyla mine)ethylene (T M AE) vapour. S ubseq ue nt researc h also s ho ws t hat it is possible to use co n de nse d p hoto- catho des co m patible with gas a m plification.

8 0 0

0 4 6 8 1 0 E(GeV)

Fi g. 7: Energy spectru m of 9 Ge V particles (electrons and pions) with Ba F 2 cr yst als c o u pl e d t o wir e c h a m b ers fill e d wit h a p h ot os e nsiti v e g as ( T M A E, s e e t e xt)

As t he i nte nsity of accelerators i ncrease d, it was fo u n d t hat t he wire c ha m- bers t he mselves co ul d not co pe wit h t he ex pecte d co u nti ng rates. Beyo n d particle fl u xes fr o m l0 4 t o l 0 5 hits per seco n d a n d per milli metre of wire, t he s pace c harge pro d uce d by t he acc u m ulatio n of positi ve io ns offset t he a p pli e d fi el d a n d a n n ull e d t h e g ai n. To overco me t his proble m I t ho ug ht, toget her wit h F. Sa uli, of se parati ng t he gas a m plificatio n i nto t wo stages: a gai n i n a prea m plificatio n str uct ure f oll o we d b y a partial extracti o n of t he electr o ns i nt o a drift re gi o n fitte d wit h a co ntrol gri d, a n d t heir tra nsfer i nto a wire c ha mber w hic h wo ul d be re q uire d o nl y t o a m plif y t he electr o ns acce pte d b y t he gri d (fi g. 8). It is t he n p ossi ble t o acce pt or reject a n e ve nt wit h a ti me precisi o n of a b o ut 30 ns a n d a delay defi ne d by t he electro n drift-ti me. Toget her wit h S. Maje wski we f o u n d a str uct ure reac hi n g t his g oal. It ma de it p ossi ble easil y t o a m plif y si ngle p hotoelectro ns release d by a n ultra violet p hoto n i n a p hotose nsiti ve gas. T he drift re gi o n i n fact lar gel y eli mi nates t he effect of t he ra diati o ns e mitte d by excite d ato ms pro d uce d in an avalanche, which are res ponsible 3 2 Physics 1992 f or sec o n dar y effects b y ejecti n g electr o ns near t he i nitial p ositi o n of t he p hotoelectro ns. We ha ve b uilt lar ge detectors of t his ki n d i n colla boratio n wit h Saclay to obtai n i mages of C here nkov ri ngs i n a n ex peri me nt per- for me d at the Fer mi Laboratory Tevatron in the Unite d States. This devel- o p ment prove d most fr uitf ul, ho wever, i n a s pecial bi ol o gical fiel d of a p plicatio n. T here are se veral researc h fiel ds i n w hic h it is necessar y to obtai n t he i mage of t he distrib utio n of molec ules marke d wit h ra dioacti ve ele me nts. Several co m mercial fir ms have trie d to make use of wire c ha mbers for t his i magi ng p ur pose. T he mai n diffic ulty lies i n t he ge nerally co nsi der- able distance which the electrons fro m the ra dioactive bo dies may have to travel i n t he gases. We have observe d t hat a n a m plifyi ng str uct ure base d o n parallel gri ds di d not s uffer fro m t his defect. M ulti plyi ng a n electro n by a To w nse n d avala nc he, i n fact, ex po ne ntially hel ps t he io nizatio n electro ns release d i n t he gas near t he cat ho de, w hic h is t he e ntry wi n do w. Precisio ns of t he or der of 0.3 m m ha ve t h us bee n obtai ne d for ra diatio n e mitte d b y p hos p hor us-32, for i nsta nce.

C & D - - - - P A

a) b) c)

Fi g. 8: V ari o us m ultist a g e str u ct ur e\. ( a) A pr e a m plifi c ati o n ( P A) a n d tr a nsf er ( T) c o m p o n e nt is follo wed by a multi wire proportional cha mber ( M WP C); the conversion and drift space ( C and D) m a k es it p ossi bl e t o i nj e ct a c o nst a nt c h ar g e ( X-r a ys et c.) i nt o t h e a m plifi c ati o n r e gi o n. ( b) T wo parallel-plate units with charge transfer fro m P AI to P A2 The best ti me resolutions were o bt ai n e d wit h t his c o nfi g ur ati o n. ( c) A m ultist a g e c h a m b er wit h a g at e: t h e wir es of gri d G ar e at

alt er n ati n g p ot e nti als of 70 V. The electrons are trans mitted fro m co mponent P A + T 1 t o M W P C and they are transferred when the wires arc brought to the sa me potential by short p uls es of 3 0 ns, m a ki n g it p ossi bl e t o s el e ct r ar e e v e nts i n a hi g h r at e e n vir o n m e nt.

We t he n too k a not her ste p b y co u pli n g s uc h a m ulti plicatio n re gio n wit h a met ho d of rea di ng o ut t he positio n of t he a vala nc hes, maki ng use of t he lig ht e mitte d. We ha d de velo pe d t his met ho d i n or der to st u dy very rare Georges Charpak 3 3 pheno mena such as double decay or collisio ns bet wee n gas ato ms a n d certai n ty pes of hi d de n matter. We t ho ug ht t hat o nly t he re d u n da ncy provi de d by photogra phic metho ds co ul d characterize a rare event. A Britis h gro u p was also i n de pe n de ntly tackli ng t his met ho d for i magi ng Cherenkov photons. They, too, use d the m ultistage cha mbers which we ha d i ntro d uce d a n d reac he d t he sa me co ncl usio n: wit h s uitable va po urs it was possible to obtai n a s ufficie ntly ple ntif ul e missio n of p hoto ns to detect t he lig ht e mitte d by t he a vala nc hes wit h si m ple o ptics. T h e gr e at a d v a nt a g e of t his m et h o d li es i n t h e r e a d- o ut w hi c h it p er mits i n the use of h un dre ds of tho usan ds of channels no w making u p the C C D (c harge-co u pli ng device), w hic h is t he basic co m po ne nt of vi deo ca meras. Moreo ver, t he fact t hat t he i ma ge of a li g ht a vala nc he co vers se veral pixels ma kes it possi ble to i nter polate t he ce ntre of t he a vala nc he a n d m ulti pl y t he n u mber of real c ha n nels by al most 10. We were t h us able to make a n i nstr u me nt for vis ualizi ng t he distrib utio n of t he electro ns e mitte d by triti u m i n a slice of rat’s ki d ne y, ma ki n g it p ossi ble t o o bser ve details of a b o ut 100 µ m. T he fact t hat t he sa me data (fi g. 9) were o btai ne d i n a si n gle day i nstea d of t he t hree mo nt hs previo usly req uire d by t he p hotogra p hic metho d aro use d i m me diate interest a mong certain biologists an d gave rise to a de velo p me nt w hic h is still u n der wa y.

Fi g. 9: I mages of sa mples taken fro m a tritiu m- marked rat’s kidney. The four parts of diagra m (a) were obtained with a gas detector: the windo w, centred at three different levels, has a grey s c al e of 2. 5 6. T h e i nt e nsit y s p e ctr u m is t a k e n al o n g t h e o bli q u e li n e m ar k e d o n t h e first vi e w wit h 4 0 µ m pi x els i n t h e s a m pl e pl a n e. T h e g as mi xt ur e is x e n o n wit h 2. 5 % tri et h yl a mi n e. T h e d et ails of the renal channels, measuring 50 µ m, can be seen. This i mage may be obtained in 20 hours. (b) Autoradiograph sho wing the adjacent slice of rat’s kidney to which a photographic e mulsion w as a p pli e d f or 3 m o nt hs. 3 4 Physics 1992

O ur latest ex peri me nt wit h gas detectors co m prisi ng parallel-gri d str uc- t ures hel pe d co nvi nce us t hat t he co ntrolle d m ulti plicatio n of avala nc hes i n gases, i n str uct ures as di verse as wires or parallel s urfaces, b y selecti n g p h o- tocat ho des co m patible wit h gas a m plificatio n is still a so urce of fr uitf ul d e v el o p m e nts: i n m a n y fi el ds t his m ulti pli c ati o n m a y still r es ult i n pr o gr ess i n all fiel ds w here ra diatio n m ust be i mage d, fro m ultra violet p hoto ns to y-rays a n d t he hig hest-e nergy particles. At t he c urre nt stage of hig h-e nergy p hysics, ho wever, si m ply maki ng use of t he locatio n of free electro ns near t he wires of pro portio nal c ha mbers a n d t he drift-ti me of t he electro ns provi des an i mage of config urations rivalling in co m plexity those provi de d b y b ubble c ha mbers. T his is s ho w n i n fi g. 10, t he i ma ge of a n e ve nt ge nerate d i n t he A L E P H detector i nstalle d at o ne of t he i ntersectio ns of L E P, t h e l ar g e colli der o perate d at C E R N.

Fi g. 1 0: I mage of an collision obtained at the A L E P H experi ment at L E P using an instru ment making use of the drift-ti me in a large volu me and the read-out of coordinates projected in a wire cha mber. Auxiliary outside detectors provide the infor mation on the energy of t h e p arti cl es, t h e tr aj e ct or y of w hi c h w as dis pl a y e d. Georges Charpak 3 5

BIBLI O GR AP HY

A very co m prehensive historical st u dy with most references corres pon ding to the peri o d c o vere d b y t his arti de is t o be f o u n d i n I. Ga mbaro, T he develo p me nt of electro n detectors at C E R N (1966 - late 1970s), C E R N History Study No. C HS-39, Ja n uary 1992.

O ur first article t hat describe d t he pro perties of m ulti wire str uct ures is G. C har pa k, R. B o uclier, T. Rressa ni, J. Fa vier a n d C. Z u pa ncic, T he use of m ulti wire pro portio nal c ha mbers to select a n d localize c harge d particles, N uclear Instru ments and Methods 62 (1968) 202 - 2 6.

It was follo we d by se veral articles, o ne of t he m gi vi ng a descri ptio n of t he progress we ma de d uri n g o ur first year of st u dies of t he wire c ha m bers a n d t he drift cha mbers: G. C har pak, D. Ra h m a n d H. Stei ner, So me develo p me nts i n t he o peratio n of m ulti wirc pro portio nal c ha mbers, N uclear I nstr u me nts a nd Methods 8 0 (1970) 13 - 3 5.

We refer t he rea der to hi g hl y detaile d wor ks s uc h as F. Sa uli, Pri nci ples of o peratio n of m ulti wire pro portio nal a n d drift c ha mbers, C E R N 7 7- 0 9 (1977). P. Rice- E va ns, Spark, strea mer, proportio nal a nd drift cha mbers ( Ric helie u, Lo n do n, 1 9 7 4), T. F er b el, Tech niq ues a nd co ncepts of high-e nergy physics (Plenu m Press, Ne w York, 1 9 8 7), a n d to a book givi ng a goo d a nalysis of t he develo p me nt of detectors i n relatio n to t he e vol utio n of particle p hysics: F. Cl os e, M. M art e n a n d C. S utt o n, T he particle e x plosio n ( Oxfor d U niversity Press, 1 9 8 7).