Cylindrical wide gap and streamer chambers

NUCLEAR

INSTRUMENTS

AND

METHODS

CYLINDRICAL

WIDE

5o9-5t4; ©

IO0 (I972)

NORTH-HOLLAND

GAP AND STREAMER

PUBLISHING

CO.

CHAMBERS

W. S. RISK

College of Arts and Sciences, Department of Physica and Astronomy, University of Maryland, College Park, Maryland 20742, U.S.A. Received 14 December 1971 Cylindrical chambers as long as 96 in. with a 3 in. gap turned on a 6 in. radius have been built and operated successfully as wide gap chambers. Made from 3.0 mil thick Mylar toroidal gas cells surrounded with appropriate conducting electrodes, these chambers are completely self-supporting upon inflation with helium or spark chamber grade neon gas. Requiring no massive internal or external supports, their large solid angle acceptance, high visibility and low mass make them the ultimate in simplicity. Due to the non-linear field of the cylindrical geometry, however

attempts to operate the chamber in the streamer made have failed to produce streamers useful for precision measurements of particle trajectories. Details of construction, operation and response of the chamber are included in the text along with photographs of particle tracks observed in the chambers. Additional photographs are available in W. S. Risk, University of Maryland Technical Report No. 72-030. Development work on these chambers was terminated in February of 1970.

I. Introduction

l i n e a r i t y o f the electric field o f a c y l i n d r i c a l geometry1). H o w e v e r , all o f t h e s e c h a m b e r s p e r f o r m e d e x c e e d i n g l y well as w i d e g a p c h a m b e r s . H e n c e we p r e s e n t h e r e o u r results o n b o t h the c h a m b e r s successful o p e r a t i o n in the wide gap mode and their unsatisfactory perform a n c e in the s t r e a m e r m o d e . W e m e n t i o n at this p o i n t t h a t s t r e a m e r s w e r e o b s e r v e d in the c h a m b e r s b u t t h a t t h e i r o v e r a l l q u a l i t y was u n s u i t a b l e for precision m e a s u r e m e n t s o f p a r t i c l e trajectories.

D e s i g n c o n s i d e r a t i o n s f o r a n e x p e r i m e n t to m e a sure t h e t h r e s h o l d c r o s s - s e c t i o n s f o r t h e r e a c t i o n s 7t ± p ~ 7r ± rc+n s u g g e s t e d t h a t a l o w m a s s c y l i n d r i c a l s t r e a m e r c h a m b e r t h e o r d e r o f 80 in. l o n g s u r r o u n d i n g a o n e a t m o s p h e r i c h y d r o g e n gas t a r g e t is a h i g h l y efficient w a y o f c o l l e c t i n g the final state pions. U n f o r t u n a t e l y , several a t t e m p t s to o b t a i n s a t i s f a c t o r y perf o r m a n c e o f c y l i n d r i c a l c h a m b e r s o p e r a t e d in the s t r e a m e r m o d e w e r e u n s u c c e s s f u l d u e to t h e n o n -

Because

MYLAR TAPE SEALS

f

MYLAR FLANGE (O.OIO in.) GAS FILL TUBE

~~RTV

(OOlO

cylindrical

geometry,

WIRE MESH OUTER ELECTRODE

MYLAR FLANGE SEALJl

WIRE MESHINNERELECTRODE--~ MYLAR FLAN

o f the

,i

~.. ~

J

.TV SEAL--

LUCITE WINDOW(0.030 in.)

f "

lll

~

MYLAR WALL (O.OO2 in.)

MYLAR FLANGE(0.010 ia.) LUCITE SUPPORTRING (0.030 in.)

GAS TARGET WINDOW ( 0.002: in.}

Fig. 1. Details of construction of clear acrylic plastic end pieces and attachment to Mylar cylinder. 509

of

special

510

w.s.

interest is the fact that the chambers are virtually massless, yet entirely self-supporting, being inflatable like large balloons. Details of construction are presented below. 2. Details o f construction

The basic structural element of the cylindrical chambers we have built and tested is a toroidal shaped gas cell. These cells have been made from both 2.0 mil and 3.0 rail clear Mylar [type D Mylar for thicknesses of 3.0 mil or more, type S for thicknesses below 3.0 rail]. Thicknesses down to 1.0 mil could be used before gas leakage through pinholes becomes a serious problem. The fabrication technique for the gas cells is as follows. A sheet of Mylar is joined along its length to itself to form a cylinder. The joint is secured inside and out with a strip of ¼ in. or 1.0 in. wide Mylar pressure

Fig. 2. Photograph ofa typical cylindrical chamber with stainless steel crossed wire mesh electrodes discussed in text. Temporary mirrors at right were used to obtain 90° stereo view.

RISK

sensitive tape. For the inside seal, the cylinder is suspended above a flat table with the joint resting on the table so the tape can be stretched inside the cylinder and smoothed with a long cloth-tipped pole. Two such cylinders are made with diameters chosen for the desired size of the inner and outer walls of the chamber. The two sizes we have used have been a 2.0 in. gap chamber with an 8.0 in. diameter inner electrode and a 3.0 in. gap chamber with a 12.0 in. diameter inner electrode. The two Mylar cylinders are then fabricated into a torus by joining them to clear acrylic plastic end pieces 0.030 in. thick. The end pieces are fitted with 0.010 in. thick by 2.0 in. wide Mylar flanges sealed with R.T.V. 2) adhesive for gas tightness. The Mylar cylinders are then taped onto the flanges with Mylar tape. Since the overpressures in the completed torus needed to keep it rigid are quite low, there is no danger of the taped joints coming apart. Fig. 1 illustrates the details of the clear acrylic plastic end pieces. A third cylinder, fabricated by the same techniques with a diameter slightly ( ~ 1/32 in.) smaller than the inside diameter of the torus, is used as a central supporting column for the inside wall of the torus. It is also used as the base for the inner (high voltage) electrode of the chamber and as the gas target. Hence, it extends at least one gap width beyond the acrylic endpieces. A flange is attached to the outside wall of the torus to extend its length also one gap width beyond the acrylic ends. The outside wall of the torus thus serves with its flanges as the base for the outer (ground) electrode. Fig. 2 is a photograph of a completed chamber. Gas outlets and inlets are built into the two acrylic ends of the chambers (fig. 1). Orifices 3/8 in. in diameter were found to be satisfactory for this purpose allowing adequate flow ( ~ 1.0 scfh spark chamber grade neon) of the chamber gas to keep it uncontaminated without excessive overpessure in the chamber. Two styles of electrodes were tried with satisfactory results for wide gap operation. Style one involved stretching # 14 gauge tinned copper wire lengthwise along the chamber with ¼ in. spacing between wires. For this style electrode an external frame supporting two acrylic plastic pieces ½ in. thick was needed. Holes wire drilled in the acrylic and the wires were threaded through the holes and pulled tight. Tests on this chamber were made without shaping the Marx generator high voltage pulse and hence only wide gap operation was achieved. Style two electrodes involved covering the central supporting column (gas target) referred to above and

CYLINDRICAL

WIDE

GAP

outer wall of the Mylar torus with stainless steel crossed wire mesh. The mesh size used was 50 x 50 wires per inch using 2.0 mil wire. Hence, a normal view through the side of the chamber was 80% transparent. A third type of electrode which we did not use on the cylindrical chambers but did use on parallel plate chambers would permit eliminating the non-uniformity

AND STREAMER

CHAMBERS

511

of the wire mesh mass distribution by replacing the inner electrode with ½ mil aluminum foil. This of course would necessitate viewing the two opposite halves of the chamber from opposite sides rather than viewing both halves from one side as can be done with the mesh electrode. Aluminized Mylar was also used in parallel plate

Fig. 3. Tracks obtained in cylindrical c h a m b e r with style two electrodes a n d M a r x generator connected directly to the c h a m b e r . Shorter tracks at right are 90 ° stereo view obtained by mirrors s h o w n in fig. 2. Crescent shaped image is stray r o o m light a n d is n o t associated with the tracks themselves.

512

w . s . RISK

chambers but it did not hold up under long term operation in wide gap mode since the heavy sparking in the wide gap mode burned away the aluminum in the vicinity of the spark. 3. Performance of chambers with style two electrodes

The first tests on the chambers with style two electrodes also involved connecting the Marx generator 3) directly to the chamber. As with the style one chamber excellent wide gap operation was obtained with no difficulty. Fig. 3 shows typical tracks obtained in these

tests. The chamber has a 2 in. gap and an 8 in. diameter inner electrode. Like most wide gap chambers, the track quality becomes very poor for track angles greater than 45 ° relative to the electric field direction. Like the style one chamber, tests on cosmic rays gave no indication of visible streamers in the chamber. The operating conditions for fig. 3 were 15 kV/cm electric field strength, chamber terminated in twice its characteristic impedance, high voltage pulse length 50 nsec, K o d a k S.O. 265 film, camera focused at 10 ft with f5.64).

Fig. 4. Photograph of 45 ° angle track with energetic knock-on electron parallel to high voltage electrode. 45 c~ track shows nonuniform streamer formation due to the radial electric field. K n o c k - o n track shows uniform streamers in constant field region.

CYLINDRICAL

WIDE

GAP

The next tests on this type of chamber involved testing it in a n - beam at the Princeton-Pennsylvania Accelerator in the fall of 1969. During these tests, in which the Marx generator was still connected directly to the chamber, stray beam tracks, almost parallel to the chamber axis, gave the first indications that visible streamers could be obtained in such chambers. While the streamers discharged across the full 2 in. gap of the chamber, the striations characteristic of streamers were very much in evidence. On the basis of the character of the stray beam tracks it was decided that pulse shaping of the Marx generator pulse was necessary. After studying the work done at the Stanford Linear Accelerator 5) and after making some tests of our own we developed a compact high speed glycerine loaded Blumlein 6) to produce, at the time, 8 nsec wide pulses of 200 kV amplitude. Using a larger chamber (12 in. inner electrode diameter and 3 in. gap) for tests with the Blumlein line we obtain results that demonstrated that cylindrical streamer chambers are not capable of producing good quality tracks except in highly limited circumstances.

Fig. 5. Enlargement of a 45 ° track similar to the one shown in fig. 4.

AND

STREAMER

CHAMBERS

513

Figs. 4 and 5 show some of the results obtained. Fig. 4 shows a cosmic ray traveling at roughly 45 ° to the chamber axis. The non-uniform development of the streamer discharge is obvious. In addition to the cosmic ray track, on the left a rather energetic knock-on electron is seen traveling parallel to the chamber axis. The fact that the chamber is operating in the streamer mode is quite evident. Fig. 5 is an enlarged view of streamer formation for a 45 ° angle track similar to the one in fig. 4. The fact that only the central portion of the track is anywhere close to being measurable severely limits the accuracy that can be expected for the particle trajectory. Additional photographs have been included in W.S. Risk, University of Maryland Technical Report No. 72-030.

4. Summary and conclusions A number of cylindrical chambers have been tested for streamer operation with little success other than demonstrating that streamers are found in the chamber. The variation of the rate of streamer formation in radial electric fields results in variations of streamer length and brightness along a single track that at best makes the average track quite difficult to measure. Preliminary tests (not discussed above) in which pulse length was varied from 20 nsec down to 8 nsec were made to determine if higher electric fields of shorter duration would improve the response. These tests indicated that while the tracks became brighter with the shorter pulse, the higher field caused somewhat greater variation of streamer development along a fixed length of track. Since a drastic increase in brightness occurs when the streamers extend across the full gap of the chamber, our present results indicate that only if the high voltage pulse were shorter than the time it takes a streamer to avalanche across the gap might it be possible to get tracks of satisfactory quality along their full length. However, at the electric field strengths involved here, avalanche velocities are approaching 101° cm/sec which means high voltage pulses in the range of 100 psec wide would be necessary. Such short pulses are currently well outside the limits of present technology. On the other hand, for detection of tracks making angles less than 45 ° to the chamber axis, the chamber works extremely well in the wide gap mode and is particularly attractive by virtue of its low mass, large solid angle acceptance and high visibility. I would like to acknowledge the assistance of J.H. Crouch throughout the course of this work.

514

w.s.

References 1) The experiment itself has been performed using a bank o f four parallel plate streamer chambers forming a square cylinder around the gas target. J. H. Crouch and W. S. Risk, Performance o f a bank o f four 7 ft streamer chambers, University o f Maryland Report No. 72-032 (1971).; Nucl. Instr. and Meth. 100 (1972) 525. 2) R.T.V. silicon rubber adhesive, General Electric Co., Silicon Products Department, Waterford, N.Y. ~) The Marx generators used in these tests will be described in a separate report.

RISK 4) The lens used for all tests was a Cannon lens w i t h f s t o p down to 0.95. 5) F. Bulos, A. Odian, F. Villa and D. Yount, SLAC Report No. 74 (1967). 6) j. H. Crouch and W . S . Risk, A compact high speed low impedance Blumlein line for high voltage pulse shaping, University o f Maryland Technical Report No. 72-029 (1971). The Blumlein line used on the parallel plate streamer chambers (ref. l) has a minimum pulse width o f ~ 5 nsec. Ref. 6 includes a description o f our pulse measuring techniques.