The CERN neutrino spark chamber

NUCLEAR

INSTRUMENTS

AND

METHODS

20

(1963)

215-219,

NORTH-HOLLAND

PUBLISHING

CO

T H E CERN N E U T R I N O S P A R K C H A M B E R H

FAISSNER, F

FERRERO*, T

A

GHANI**, E

HEERt,

F

13 N O V E Y It, M R E I N H A R Z * * t and R

KRIENEN, A

G

MURATOR1,

SALMERON

C E R N , Geneva Presented b y H

Fatssner

T h e use of a large s p a r k c h a m b e r m t h e p l a n n e d C E R N n e u t n n o e x p e r i m e n t was suggested b y G B e r n a r d l m as early as spring 1960 Discussions a b o u t t h e detailed design w e n t on all t h r o u g h 1961 The first tests w i t h large area t h r e e - p l a t e c h a m b e r s were m a d e a b o u t a y e a r ago The a c t u a l construction s t a r t e d late in D e c e m b e r lq61 The design of t h e c h a m b e r was m a i n l y determ i n e d b y t h e p o s t u l a t e of m a x i m u m flexlblhty The set-up was n o t m e a n t to be m a t c h e d to one specific problem, say the o n e - o r t w o - n e u t n n o question, t h e i n t e r m e d i a t e boson, or t h e a n g u l a r a n d energy d i s t r i b u t i o n of n e u t r i n o - p r o d u c e d m u o n s R a t h e r , it should meet, w i t h m i n o r modifications, t h e r e q u i r e m e n t s of t h e t h r e e experiments, and, if possible, of some o t h e r s which are n o t yet foreseen Therefore, r i g h t from t h e outset the plann i n g c o n c e n t r a t e d on t h e assembly of m o d u l a r sub-units, which could be c o m b i n e d w i t h counters and, If necessary, w i t h absorbers in a n y suitable way T h e a d v a n t a g e s of this a p p r o a c h are obvious, a n d y e s t e r d a y t h e y were described v e r y a d e q u a t e l y b y D r P a r k e r l) As a consequence of t h e principle of m a x i m u m flexibility, t h e m o d u l a r s u b - u n i t s were m a d e t h e m i n i m u m p r a c t i c a l size two outside plates w i t h one h i g h - v o l t a g e leading plate in between The dimensions are a compromise b e t w e e n the desire

for large size a n d high weight, a n d the limitations imposed b y technology a n d available space I n partlcular, the plate thickness was fixed to 5 m m , which was considered to be close to the m i n i m u m thickness for which the required flatness of 0 5 m m over a large surface could be achieved F o r the same reason, the gap w i d t h was fixed to 10 ram, a l t h o u g h a smaller w i d t h would h a v e given more b i t s per u n i t length The cross-section of t h e c h a m b e r s was chosen to be 160 × 100 cm e, a n d it is p l a n n e d to h a v e each two of these u n i t s s t a n d ing side b y side, t h u s realizing a d e t e c t o r of 2 m width and 1 6 m height Two different types of three-plate u n i t s were c o n s t r u c t e d t y p e A, m a d e of a l u m m l u m , m order to see details of the interaction, a n d t y p e B, m a d e of brass, w h e n high average d e n s i t y a n d short r a d i a t i o n l e n g t h was required I n particular, the l a t t e r p o i n t was considered I m p o r t a n t for the conversion of electrons, a r o u n d the typical neutrino energy of 500 MeV, a l u m l n m m is notoriously bad, brass at least decidedly b e t t e r * Some properties of the two s p a r k c h a m b e r types are listed m t a b l e 1 A t h i r d column C is added, referring to the triggering plastic counters There are al~o antlcomcldence h q u l d scintillation 2) a n d directional Cherenkov counters However, I h a v e no time to discuss the counters t o d a y

* C E R N Fellow, on leave from I s U t u t o dt Fxslca, U m v e r slt~, dl Tormo, I t a l y ** F O R D Fellow, on leave from the P a k i s t a n A t o m i c E n e r g y Commlssmn, K a r a c h l * U n i v e r s i t y of G e n e v a t t Argonne N a t m n a l L a b o r a t o r y , L e m o n t , Ilhnols, U S ))t C E R N Fellow, on leave from Centre de P h y s i q u e Nucl6alre, Bruxelles, B e l g l u m

* F r o m the vxew-pomt of ph?sxcs, iron ,~ould ha~e done as well B u t we never got an iron c h a m b e r to work satisfactorll 3 , the efflexenc~ ~ a s low and there ~ e r e man'~ spurious sparks q_he reasons for this failure are n o t c o m p l e t e l \ clear, b u t one of t h e m was certainly the roughness of the surface of the commercially a v a i l a b l e xron (or stainless steel) plates 1) C R e y a n d S Parker, Nucl I n s t r and Meth 20 (1963) 173 2) H Fatssner et al Nucl I n s t r and Meth 20 (1963) 289

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CHAMBLRS

214

H FAISSNER

et al

I'ABLE 1 Some properties of the modules Type

A.

Material

A1

Thickness in cm Thickness in g/cm * Thickness in MeV

3x05 38 63

Vve i g h t (kg)

60

No of urals per ton No of u m t s per r a d i a t i o n length No of u n i t s per i n t e r a c t i o n length

/ ~ n s u l a t o r supports 7h votlage lead as inlet

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165 65 20

B

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The construction of the three-plate chambers is shown schemahcally in fig 1 The two outside plates and some metalhc spacers are glued together with araldlte, so as to form a mechamcally stable box The central plate has carefully rounded edges and it is suspended by httle plexlglas insulators which are introduced through the holes left by the spacers The front face is closed by an optically pohshed plexlglas window All removable parts (1 e insulators andwmdow) are sealedoff by a coldsetting silicon rubber compound The chambers are mounted, together with the triggering counters (and If wanted, some additional absorbers) on chariots Fig 2 shows an assembly of brass chambers and plastic counters In its final mounting It is planned to build 80 alummlum and 100 brass chambers, amounting to a total weight of 25 tons About 10 tons have been assembled at present The electronics is conventional, and has been described already yesterday by Dr Ferrero ~) Maybe one should emphasize agaln that each hot plate is supphed by a separate condenser, which we consider essential for obtaining a good efficiency for mulhple events Also the spark gaps deserve mentlomng t h e y are of the open-air type, three stareless steel electrodes mounted on a plexlglas frame, and they are extremely simple and rehable The centre electrode is hollow and contains a needle supplying a corona discharge No readlustment of the needle's position was found necessary d u n n g test runs lasting for several 10~ pulses a) H

Falssner

et

al

,

Nucl I n s t r and Meth 2 0 (1963) 161

THE

CERN

NEUTRINO

T h e optical s y s t e m is n o t y e t m its from s h a p e W e h a v e to p h o t o g r a p h from b o t h sides and, because of the stereoscopy, we h a v e to use two cameras on each side Possibly we shall use career-

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CHAMBER

to t h e s~des, angles up to 45 ° are m v o l v e d F o r these angles the simple m e t h o d of refracting prisms, ubed so successfully b y the C o l u m b m - B r o o k h a v e n group4), work~ no longer I n t h e beginning we

F i g 2 T w o three-plate c h a m b e r s a n d t w o (carl)) fl~e-plate c h a m b e r s m o u n t e d t(>gcthcr ~ t l l three plastic c o u n t e r s on a m o d u l a r c h a r i o t T h e b o x m o u n t e d on t o p houses t h e h t -condensers a n d th~ s p a r k g a p X()te tile s h i n y bra~s surface

as with two lenses of different apertures in order to o b t a i n on the same film one n o r m a l l y exposed a n d one over-exposed view The big problem is how one gets all t h e h g h t rays parallel to the c h a m b e r plates over the entire l e n g t h of the a p p a r a t u s of a b o u t 5 m Because of t h e h m l t e d apace available

tried out refractmg a n d al>o reflecting c 5 h n d r m a l rods B o t h of t h e m give a very desirable ~queeztng of the image, b u t u n f o r t u n a t e l y al~o a n unde~lrable 4) G 1)anbx, J \ I Galliard, 1"~ Gouhanos, I. _XI L ¢ d c r m a n , N Xhstrx,\I Sch~artzandJ Stelnberger, P h x s Rex I e t t e r s 9 (1962) 36, and p r l ~ a t e c o m r n u n m a t l o n "V S P A R K

CHAMBERS

216

H

FAISSNER

b l u r r i n g u p L a t e r on we changed to reflecting prisms of a s o m e w h a t peculiar shape (in tact t h e y are simply e q u i v a l e n t to pairs of mirrors) This m e t h o d worked, b u t the a m o u n t of m a c h i n i n g a n d polishing is excessive The m o s t likely solution will be to use a pair of s m t a b l y tilted flat mirrors m f r o n t of each three-plate c h a m b e r W i t h this fairly

et aI

The particle selection was m a i n l y done b y tlme-off h g h t For t h e selection of electrons a gas Cherenkov counter was used 5) which registered particles in a 1 per mille velocity b a n d a r o u n d fl = 1 N e u t r a l particles produced in a CH 2 t a r g e t ~ 1 m in front of the s p a r k c h a m b e r s could be observed b y dem a n d i n g an antlcolncidence between the " b e a m "

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F i g 3 An c- of 1 GeV/c in a test set-up consisting o5 19 alun n m u m and 4 brass chambers The particles t r a v e l from r i g h t t o left The brass u n i t s are on the e x t r e m e left o5 the assembl}

l~lg 4 A ~ - of 400 MeV/c m a k i n g a ~0 t r a c k is a reflection )

s t r a i g h t f o r w a r d optics we hope to achieve the same resolution as o b t a i n e d in the test-runs, described yesterda} 3) R a t h e r t h a n to elaborate f u r t h e r on technical details I should like to d e m o n s t r a t e the possibilities a n d l i m i t a t i o n s of our c h a m b e r s b y showing a few pictures which h a v e been t a k e n in the course of the p a s t few weeks I n these t e s t - r u n s an assembly of 19 a l u m m l u m c h a m b e r s followed b y four brass c h a m b e r s was placed into a m o m e n t u m - s e l e c t e d particle b e a m at the C E R N P r o t o n S y n c h r o t r o n

counter covering the front face of the c h a m b e r setup The spark c h a m b e r assembly was 85 cm long, a n d it was p h o t o g r a p h e d with a very provisional optics t h e plates were m a d e parallel to t h e light rays simply b y tilting the c h a m b e r s The stereoscopy was achieved b y two 45 ° mirrors, as already described yesterday 3) The lens aperture was [/8,

coincidence

signal

and

a

1 0 x

(Above the mare

1 6

m 2

plastic

5) ~R Mermod, K W i n t e r , G W e b e r and G yon Dardel, Proc I n t Conf I n s t r H E P h y s , Berkeley (Intersclence Pubhshers, N Y , 1961) p 172

THE

CERN

NEUTRINO

a n d b n g h t s p a r k s gave a d o t r a t h e r t h a n a line on the film, t h e r e b y m a k i n g some of o u r p h o t o s look h k e emulsion pictures W e h a v e t a k e n some 104 p h o t o g r a p h s of almost all k n o w n e l e m e n t a r y particles in the m o m e n t u m range b e t w e e n 300 a n d 4000 MeV/c As our chamber assembly was several r a d i a t i o n lengths a n d

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CHAMBER

217

t r o n of 1 GeV/c One notices t h e slow m u l t l p h c a t l o n in t h e t h r e e r a d i a t i o n lengths of a l u m m m m a n d t h e q m t e efficient c o n v e r s m n of b r e m s s t r a h l u n g q u a n t a in t h e s u b s e q u e n t brass sections T h e shower d e v e l o p m e n t is, of course, even poorer a t lower energies A t 300 MeV/c for instance, an elect r o n in a l u m m m m looks typically like t h e first

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more t h a n one i n t e r a c t i o n length t h i c k (see table 1 ) one could hope to see a fair fraction of the particles' energy dissipated in the c h a m b e r s The aim is of course to get a m e a s u r e m e n t of th]s energy, m o t h e r words to use t h e c h a m b e r a r r a y as a t o t a l a b s o r p t i o n s p e c t r o m e t e r F o r g d m m a s a n d elect r o n s one c o u n t s simply t h e t o t a l n u m b e r of sparks, as h a s been done already b y Cronm 6) F o r strongly i n t e r a c t i n g particles s o m e w h a t more sophlstmated m e t h o d s seem to be m d m a t e d Fig 3 show's a typical shower reduced b y an elec6) j

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2 0 ( 1 9 6 3 ) 143

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k s s o c m t e d p r o d u c t i o n of A ° a n d IC-° b y a p r o t o n of 4 GeV/c (reside th e l e f t - h a n d V a reflect]on)

half of t h e shower in the present pmture, which m a k e s Its ldentiflcat]on r a t h e r difficult We h a v e also studied the shower d e v e l o p m e n t in a pure brass set-up The average n l u l t l p h c l t y there is a b o u t twice as h]gh as t h a t in a l u m m l u m , as one would expect from the ratio of the critical energies, at least if one beheves in shower t h e o r y approxlmat]on B T h e n e x t picture (fig 4) shows a 400 MeV/c a producing a ~0 l n t h e c h a m b e r One of the decay g a m m a s of the n e u t r a l p]ons goes down a n d m a k e s a race little electron pa]r The shower initiated b y V

SPARK

CHAMBERS

218

H

et al

FAISSNER

t h e o t h e r g a m m a is less clear I n fact, it is quite possible t h a t the track, which goes lightly up, Is In p a r t due to the original pion or to a n o t h e r charged particle elected m the reaction However, the double sparks appearing In its more d i s t a n t p a r t s a n d t h e isolated spark b e y o n d Its end suggest t h a t the t r a c k is, at least in part, caused b y the second pair

A t higher energies a n e u t r a l pion looks m u c h more Impressive One of 4 GeV/c can be seen in fig 5 I t was p r o d u c e d b y a negative pion of the same m o m e n t u m in an elastic charge exchange scattering in the CH 2 target in front of the c h a m b e r s The two pairs are initiated in the second a n d third c h a m b e r respectively Because of the small open-

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Incidentally, the p h o t o shows one of the trivial >hort-comlngs of our first b a t c h of c h a m b e r s there are reflections from the m e t a l slabs, closing the far end of the c h a m b e r E v e r y b r i g h t spark is accompanied b y a fainter satelhte which appears above the real s p a l k in the top view a n d below it in the b o t t o m view A l t h o u g h these reflections m i g h t In principle be used for stereoscopy, we decided to suppress t h e m in the future, as t h e y get confusing for multiple events This was easily done b y p a i n t i n g the m e t a l surfaces black

•

number

at 4 Gt Vic with an unusuall) o f -r°'s

m g angle they a p p e a r mmall~ as single tracks (which, b y the way, is good for the m e a s u r e m e n t ) I n the l a t e r stages of the shower d e v e l o p m e n t t h e y t e n d to merge together On top of the first two c h a m b e r s one sees the discharges across the supportm g insulators m e n t i o n e d yesterday b y Ferreroa), which occur quite frequently when there is no charged partmle t r a n s v e r s m g The n e x t p h o t o (fig 6) is mainly shown in order to Illustrate one of the f u n d a m e n t a l limitations of p r e s e n t - d a y spark c h a m b e r techniques I t is pre-

THE

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s u m a b l y a n associated p r o d u c t i o n of A ° a n d K ° b y a n incoming p r o t o n of 4 GeV/c T h e lower decay t r a c k of t h e A ° leaves t h e c h a m b e r a t the b a c k (as can easily b e seen from t h e reflections), t h e o t h e r tracks e n d inside t h e c h a m b e r The u p p e r decay plon of t h e K ° h a s a v e r y small s p a r k density as long as t h e slower second plon is present, a n d it l u m p s i m m e d i a t e l y to 100 % as soon as t h e seco n d plon h a s stopped (The five sparks parallel to its e n d are again reflections ) This " r o b b i n g effect" m a y become a serious h a n d i c a p in t h e analysis of c o m p l i c a t e d e v e n t s However, we h a v e evidence t h a t it can be avoided to a large e x t e n t This is d e m o n s t r a t e d b y t h e last two pictures Fig 7 is a typical a n t l p r o t o n annihilation at 4 GeV/c I t shows four charged prongs a n d possibly a n ° travelling d o w n w a r d T h e r o b b i n g effect is serious only in the second t r a c k from the top, a n d even t h e r e one recognizes on the film a few v e r y faint s p a r k s going right b a c k to the origin Finally, fig 8 is a very spectacular a n t m u c l e o n a n n i h i l a t i o n One recognizes at least three charged particles in t h e presence of a large n u m b e r of elect r o n showers from n°'s Note, in particular, the pair e m i t t e d a p p r o x i m a t e l y in b e a m direction, which undergoes r a p i d m u l t i p l i c a t i o n as soon as it crosses the borderline b e t w e e n a l u m m m m a n d brass I t is this t y p e of picture which gives us hope to come close to t h e aim of efficiently detecting

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m i n i m u m ionizing particles in the presence of a n y n u m b e r of o t h e r particles, even if t h e y h a v e higher ionization a n d a r b i t r a r y angles a n d distances The d i s t u r b i n g t h i n g is t h a t the efficiency v a n e d greatly during the run, a l t h o u g h the "official" working conditions r e m a i n e d the same I t should also be m e n t i o n e d t h a t t h e average efficiency was worse t h a n in the electron shower tests 3) This h a s p r e s u m a b l y a trivial e x p l a n a t i o n we used a cheap 60 % Ne plus ~ 40 % He mixture, which m i g h t h a v e c o n t a i n e d some unspecified impurities, we suspect argon a n d / o r nitrogen \Ve i n t e n d to use a purer n e o n - h e h u m m i x t u r e in the future, a n d we shall systematically v a r y the size a n d the shape of the high tension pulse, in order to find which are t h e critical p a r a m e t e r s One final r e m a r k t h e c h a m b e r s were designed with high-energy n e u t r i n o physics in m i n d However, it is obvious t h a t t h e y h a v e interesting properties also for o t h e r applications An assembly, as the one used in our recent test runs, is in good approxim a t i o n a t o t a l a b s o r p t i o n d e t e c t o r for all nonweakly Interacting particles, a n d it h a s good spatial resolution a n d almost 2 n solid angle as well I t Is therefore a n almost ideal tool for the s t u d y of n e u t r a l reaction p r o d u c t s I n fact, selecting clear cases of single n ° production, we were able to get a first rough m e a s u r e m e n t of the elastic .ncharge exchange cross-section at 4 GeV/c

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