magnetic-field spark chambers

NUCLLAR

INSIRUMENTS

AND

METHODS

20

(1963)

176-179,

NORTH-HOLLAND

PUBLISHING

Co

MAGNETIC-FIELD SPARK CHAMBERS G

K

O'NEILL, F

V

MURPHY,

K

WRIGHT

and D

YOUNT

Princeton Umvevs~ty, Prtnceton, New Jersey Presented b y G

The work r e p o r t e d here was b e g u n early In 1960, when the d e v e l o p m e n t s m a d e b y F u k u l a n d Mlyam o t o h a d been r e p e a t e d a n d e x t e n d e d b y Cork a n d Cionln Our a u n was to l n v e s t t g a t e the decay propertles of the s h o r t - h v e d K°'s, a n d for this purpose

K

O'Neill

in t h a t report T h e s c a t t e r of spark positrons in the 50-gap c h a m b e r (fig 1) h a d a full-width at h a l f - m a x of 0 40 m m , while wath no clearing tleld, sparks m a l t e r n a t e gaps were displaced b y 0 4 m m as a result of the E × B force (The c h a m b e r was

5lu()n of ~ 0 35 G e \ / c m o m e n t u m 1 lg

1

Frack of,t cosmm-ra}/~-mcson m a 50-gap ncon-fllled ~park c h a m b e r o p e r a t i n g m 10 kgauss l h c gap spacing 11 3 m m

filled with neon at 1 3 a t m absolute pressure, and a pulse ribe-ttme of 7 nb was used ) In thl> chamber, 30 cm long, the m a x t m u m detectable m o m e n t u m was 15 GeV/c After testing the 2-gap a n d 50-gap chambers, we built a 128-gap c h a m b e r with thin plates a n d 3 m m spacang, h a v i n g a volume of 30 cm × 30 cm × 60 cm The construction (fig 2) differs from the n o w - p o p u l a r Meyer a n d Terwtlllger m e t h o d Its a d v a n t a g e s are (a) the (considerable) stored energy in the high-capacity gaps c a n n o t be dissipated m the form of a dlscolonng s p a r k across a lucite

we were i n t e r e s t e d tn developing magnetxc field s p a r k c h a m b e r s of high resolution Initial tests were m a d e with a 2-gap, v a l l a b l e spactng, variable pressure s p a r k c h a m b e r filled successtvely with neon, argon, h e h u m a n d h y d r o gen The c h a m b e r was o p e r a t e d with spactngs as small as 2 2 m m a n d prebsures (in helium) as high as 5 a t m o s p h e r e s This work was r e p o r t e d m the Proceedings of the 1961 Argonne S y m p o s i u m on S p a r k Chambers, R e v Scl I n s t r , Vol a, p 528 A plcttn e t a k e n in a 50-gap, 3 m m spacing c h a m b e r o p e r a t e d in a field of 10 kgauss is also included 176

177

1,1g 2

Assembl} of 30 cm X 30 cm x b0 cm t h i n - p l a t e s p a r k c h a m b e r , w i t h s t r e t c h e d p l a t e s r[~,AN A HA PLUG

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END FLANQE

ELECTRODE

S G 7 e 9 ~o - CM

S p a r k gap, n o r m a l l y o p e r a t e d a t 6 2 5 / 2 , w i t h L/R ~<~ 10 ns T h e g a p c a n o p e r a t e a t 50 k V w i t h 60 lbs [sq in pressed a tr T h e t h i r d e l e c t r o d e (needle) r e c e i v e s a 30 k V t r i g g e r p u ls e f r o m a v a c u u m t u b e

V SPARK

of c o m -

CHAMBERS

178

G K O ' N E I L L et al

surface t h r o u g h which one is viewing, (b) operation over a range of pressures up to a b o u t three a t m o s p h e r e s is possible, a n d (c) a t r a c k leaving t h e c h a m b e r at a large angle need n o t go t h r o u g h m u c h

E a c h plate consists of two 0 025 m m sheets of a l u m i n u m s u p p o r t e d on 1 5 m m t h i c k frames of epoxy fiberglass (NEMA Grade G-10) The plates are m a i n t a i n e d u n d e r continuous tension b y dacron

6 TURNS F'ERRITE IN 5 0 .t-~.

IO0 C M

ON CORE

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I0o CM _~IOUT 12 S ....P,..

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CM '~"6 T U R N S FERRITE

Fig

ON CORE

4 C a b l e - a d d i n g transformer, of nse41nle < 3 ns, used t o t r a n s f o r m 23 kV a t 50 Q i n t o 12 5 k V a t 12 5 Q

m a t e r i a l before reaching counters d o w n s t r e a m Possibly none of these characteristics will t u r n o u t to be I m p o r t a n t , b u t we h a v e n o t yet explored all the posslbihtles of t h i s m e t h o d o f c o n s t r u c t l o n

strings a t t a c h e d to each corner Spacers, used only at t h e corners, are m a d e of G-10, 3 m m thick The c h a m b e r is viewed in 90 ° stereo, a n d is operated m a 40-ton m a g n e t designed for 18 kgauss at 1 5

MAGNETIC-FIELD

MW The m a g n e t design leaves a large solid angle open in t h e d o w n s t r e a m direction, so t h a t a threshold C h e r e n k o v c o u n t e r located in this position m a y be used to select l e p t o m c decays The c h a m b e r drive system consists of a hardt u b e trigger ( E F P - 6 0 ' s driving a 3E29, d l i v m g a 4PR250A) which fires a 3-electrode pressurized-air apark-gap Thi~ gap t h e n drlve~ eight other ldentl-

SPARK CHAMBERS

179

formers ) W h e n we first assembled a n d t e s t e d the large c h a m b e r , we found t h a t the contact~ to its plates (~htch were typically 1 or 2 ohms) arched u p o n the arrival of the pulse, a n d t h a t this triggered edge spark~ which k e p t the c h a m b e r from ~howmg tracks This design error can easily be corrected upon reassembly of the c h a m b e r , b u t fig 5 IS a

l ' l g ,5 l~lectron-palr in 60 cm chamber, o p e r a t i n g a t 7 kV pulbe v o l t a g e in 7 kgauss

cal gaps, each of which h a s eight (RG 9/U) o u t p u t cables These go to simple ferrlte-core cable-adding transformers, so t h a t as m a n y as 256 final o u t p u t cables (small R G 58/U) are available, each of which can carry a 12 kV pulse This n u m b e r is sufficient to drive each " h o t " plate of the 128-gap c h a m b e r at all four corners, b u t in practice we expect to use only a fraction (one q u a r t e r to three quarters) of t h e available cables for this c h a m b e r O t h e r s can be used as needed for a n y subsidiary c h a m b e r s we m a y a d d later (Figs 3 a n d 4 show the spark-gap design a n d one of the cable-adding trans-

picture showing an electron-positron pair in the c h a m b e r before the error was t h o r o u g h l y corrected The pulse voltage ,sas 7 kV a n d only one cable was feeding each hot plate The m a g n e t i c held ~ a s a b o u t 7 kgauss, at 120 k W m a g n e t power We h a v e found t h a t it is possible to establish a clearing time in a magnetic-field c h a m b e r w i t h o u t a clearing field, b y adding a b o u t 1/4 ° 5 SO 2 to the neon This work will be r e p o r t e d elsewhere By establishing the clearing time in this way, one of the sources ot s p a r k drift w h e n a c h a m b e r is oper a t e d in a m a g n e t i c field is eliminated V SPARK

CHAMBERS