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Magnetic multiparticle spectrometer using digitized cylindrical spark chambers
NUCLEAR
INSTRUMENTS
AND
METHODS
II 3
(I973)
535-540;
©
MAGNETIC MULTIPARTICLE SPECTROMETER
NORTH-HOLLAND
PUBLISHING
CO.
USING DIGITIZED
CYLINDRICAL SPARK CHAMBERS* J. R. FICENEC and B. C. STR1NGFELLOWt Physics Department, Virginia Polytechnic Institute and State University +, Blacksburg, Va. 24061, U.S.A. G. B. COLLINS§, A. R A M A N A U S K A S * * , P. SCH1]BELIN and F. T U R K O T
Physics Department, Brookhaven National Laboratory, Upton, N. Y. 11973, U.S.A. Received 21 May 1973
We describe the construction and performance of the vertex spectrometer in the Multiparticle Argo Spectrometer System (MASS)used at Brookhaven. Nine digitized cylindrical wire spark
chambers, surrounding a hydrogen target and operating in a l0 kG magnetic field, detected most charged particles with high efficiencyup to multiplicities of 12.
1. Introduction
2. Chamber construction
The need for definitive studies of high multiplicity, high m o m e n t u m transfer reactions in the 10-30 GeV energy range 1) prompted us to design and build a w~rtex spectrometer which combined the large solid angle advantages of a bubble chamber with the triggerability and digitized readout capabilities of spark chambers. Other devices met some but not all of our requirements, e.g., bubble chambers 2) are not triggerable, streamer chaml;ers 3) do not have digitized readout, and conventional spark chamber spectrometers cover a limited solid angle. Our solution to these limitations is the vertex spectrometer of MASS4). It consists of a nested set of 9 cylindrical wire spark chambers surrounding a 20 cm long hydrogen target located in a 10 kG magnetic field of the A r g o magnetS). The field volume measures 1.5 x 1 x I m 3. The chambers have radii varying from 15;.25 cm (6") to 47.02 cm (183") with an active height of 73.5 cm. The spark gap o f each chamber is 0.95 cm (0.375"). High multiparticle efficiency is obtained by pulsing the chambers in a transmission line mode. Spark location is obtained by means of magnetostrictive lines located inside the magnet and shielded from the 10 kG field.
The first phase of chamber construction consisted of preparing the electrodes making up the chamber's spark gap. These electrodes were prepared from commercially available 50/tin Kapton (H-film) 6) with a bonded 25/~m aluminum backing. This foil was stretched over a wooden cylindrical drum with Kapton side up and a thin layer of epoxy 7) was applied. The 130/~m gold plated copper wire was then wound continuously on the foil with a pitch of 12.5 wires/cm. This procedure provided a strong b o n d between the wires and the backing foil, but left the top surface of the wires exposed so as not to impede spark formation. The wires and aluminum backing foil were electrically connected, half the wires at the top and half at the bottom of the cylinder, by bonding a copper strip to the aluminum with conducting epoxy s) and soldering the wires to the copper. The free ends of the wires were cut back and eventually buried inside the chamber end structure to prevent edge sparking. q-he r~ext phase of chamber construction was to build a structure to maintain two of these electrodes in a cylindrical shape with uniform separation. Each chamlzer was formed from two " h a l f chaml;ers" which were assembled on separate aluminum cylindrical drums. The " h a l f c h a m b e r s " were joined by slipping the outer one over the inner one. Each " h a l f c h a m b e r " was nearly identical, but the order of assembling the individual layers was inverted. The inner " h a l f c h a m b e r " was assembled in the following manner: A 76/~m mylar sheet was stretched over the assembly drum. A metal chamber skeleton consisting of steel (type 1010)headers at the top and bottom, stainless steel vertical support bars at _+90 ° from the forward
* Work performed under the auspices of the U.S. Atomic Energy Commission. + Portions of this work are in partial fulfillment of a Ph.D. Degree in Physics. Present address: C E R N , Geneva, Switzerland. + Work supported in part by the Research Corporation. § Present address: Physics Department, Virginia Polytechnic Institute and State University. ** Deceased.
535
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J.R. FICENEC et al.
direction, and vertical aluminum support bars at the edges of the chamber was assembled on the drum and epoxied to the mylar. A sheet of nylon honeycomb cells 9) (cell size 0.95 cm, height 0.95 cm, wall thickness 25/~m) was fitted into the area outlined by the metal skeleton and epoxied to it and the mylar. An electrode foil was stretched over the skeleton and honeycomb, and epoxied to them with wire side up. The high voltage electrode was isolated from the steel by a 2 mm lucite separator and the ground electrode by 50/;m of Kapton tape. Steel spacer rings, similarly insulated, were placed at the top and bottom of the active
chamber area, as shown in fig. 1. These rings, together with vertical lucite spacer bars, located at the edges of the active area, maintained the 0.95 cm gap spacing. This completed the assembly of the inner "half chamber'. After the two "half chambers" were joined together, removable steel T-caps were fastened to the top and bottom steel headers to enclose the cavity which contains the magnetostrictive lines. When these end structures were imbedded in a steel plate the effective field reduction was measured to be ~ 10: 1. A finished chamber (fig. 2) is capable of withstanding large forces, . TEFLON
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M A G N E T I C M U L T I P A R T I C L E SPECTROMETER
but contains a minimum of material in the active area, namely 0.25 g/cm 2. The unique mylar-honeycomb-foil sandwich, besides providing exceptional structural strength maintains the gap spacing to -I-0.02 cm~°). Each chamber has a high-voltage electrode with vertical wires and a ground electrode with wires at either +26.5 ° or - 2 6 . 5 ° with respect to the vertical. Since half the wires of each electrode are soldered at the bottom of the foil and half at the top, we were able to detect sparks from all regions of the chamber on both the high voltage and ground electrodes (fig. 1). 3. Chamber readout
The magnetostrictive lines and their pickup coils were housed in the cavity formed by the steel headers at the top and bottom of the cylindrical chambers.
Fig. 2. Spark chamber assembly with top lid removed (bottom) and individual chamber (top); in the background the Argo magnet.
537
Each cavity contained two magnetostrictive lines [0.3 mm x 0.13 mm Remendur 11) ribbon], whose velocity was premeasured. Pickup coils, 40 turns of # 40 copper wire, are located at both ends of each line inside cylindrical biasing magnets 12). The readouts were built as complete units and placed into the cavity after removing the T-caps. Each unit consisted of two lucite strips formed to the correct radius with grooved notches to house the magnetostrictive lines (see fig. l). Curved copper boxes, which contained pickup coils, biasing magnets, electrical standoffs and clay to damp out signal reflections, were epoxied onto the ends of the lucite. The magnetostrictive line was inserted into a teflon tube (diameter 0.8 ram), around which was wound a solenoidal biasing coil giving us the capability of polarizing the magnetostrictive line remotely. The teflon tube was then placed into the groove in the lucite strip. The lucite carriers were fitted to the chambers and shimmed tightly against the wires of the electrodes (the distance between the chamber wires and the magnetostrictive line was approximately 1 mm). The pickup coil and solenoid leads were soldered to cable connectors mounted on another copper box which was fastened to the chamber. From here, shielded twinex cables carried the signals to the amplifiers13), which were located 3 m away outside the magnet. The sparks from the chambers were digitized by a commercial M I D A S 14) system located in the electronics trailer ~ 3 0 m from the amplifiers. The minimum separation distance between sparks resolvable by this system is 3 ram. Each pickup coil in the six full-round chambers was assigned six scalers, while the three downstream chambers had four scalers per coil. Since each magnetostrictive line had two coils and each line read half of an electrode, each full round chamber was capable of recording up to 24 sparks. If the number of sparks was less than this value, the double coil readout provided for consistency checks. Because fiducial wires were not mounted on the chambers, the scalers were started by a fast logic signal. Since the chambers and readouts were located in the 10 k G magnetic field, we were confronted with the well-known readout problem of deadspots and signal inversion. Tests performed prior to the experiment indicated that the magnetostrictive wand was unaffected by magnetic fields of 1.5 k G normal to the wire and 3 0 4 0 G parallel to the wire15). The magnetostrictive lines were oriented perpendicular to the main field component. The normal component seen by the readout wire was approximately 1.0 kG, while a 10%
538
J.R. FICENEC et al.
non-uniformity of the field caused the longitudinal component to be as high as 100 G close to the Argo magnet coils. A few readout lines were affected, causing inversion of the magnetostrictive signals and short dead spots. Since the inverted signal digitizings have a distinctive double-spark pattern and occurred mainly at large angles away from the forward direction, the double signals can lze averaged into one digitizing by software without loss of resolution.
4. Chamber operation To obtain a high multiparticle efficiency, the chambers were operated in a transmission line mode ~6) by making the high voltage and ground connections to the aluminum backing sheet instead of to the wires directly. In addition, each chamt;er was terminated into a resistor-diode combination which matched the inherent chamber impendence (approx. 5Q). This resulted in a uniform high voltage pulse with a rapid rise time over the entire chamber area, and reduced the voltage reflections from the end structure. The chambers were operated with standard spark chamber gas (90% neon - 10% helium) bubbled through isopropyl alcohol at 0°C to provide a quenching agent. This mode of operation preduced sparks with relatively uniform signal height and reduced spark robbing effects. The chambers had an operating voltage of 5 kV and were pulsed with an HY-13 hydrogen thyratron ~7) which produced a negative high voltage pulse with a 20 ns risetime and 400 ns R C decay time. A regulated high voltage power supply tS) and recharge circuit TM) was capable of operating at rates of 100 pulses/s; the chambers were normally operated with a 10 ms dead time. A +80 V dc and a +500 V pulsed (2 ms width) clearing field were used to keep the chamber free of electrons. This dc clearing field value resulted in a single particle memory time (efficiency down to 30%) of approximately 1.5 Its. Delay time between particle passage and the high voltage pulse was 550 ns.
5. Spectrometer assembly The completed spark chamber system consisted of six full round chambers (active area 270 °, radii 15.25, 22.88, 30.50, 38.13, 43.84, 47.02 cm), and three half round chambers (active area 180 °, radii 34.95, 38.13, 47.02 cm) (fig. 2). The steel header of each chamber was placed in a groove cut into the steel base plate of the support structure. After the chambers were aligned and the high voltage and readout cables were attached, the structure was closed by lowering another grooved
plate over the chamber headers and bolting it in place. Since the additional steel (grooved plate and chamber headers) formed a continuous plate-like extension of the pole pieces, we were able to reduce the critical field components sufficiently without changing the homogeneity of the A R G O magnetic field. The entire assembly (fig. 2) was lowered into the A R G O magnet through a hole left by removing a portion of the top pole piece. This pole piece was put back when the assembly was in place. The liquid hydrogen target was then inserted into the upstream median spacer gap of the magnet and positioned at the center of curvature of the six full round chambers. Both the structure housing the chambers and the A R G O magnet have clearance about the median plane to allow the external spectrometers to "see" the hydrogen target (fig. 2). The support structure had to be designed to prevent structural damage to the chambers, since the chamber headers were made of steel to shield the magnetostrictive lines from the magnetic field and hence experienced a force from that field. This was done by adding an additional 1 cm of steel to the base and top plates (see fig. 1) to obtain magnetic forces which would stretch the chambers rather than compress them. The amount of stretching was controlled by set screws tapped through the top plate. The 10 kG magnetic field applied a stretching force on the chambers of about ¼ t, with no resulting structural damage.
6. Spectrometer performance The vertex spectrometer of MASS was first used in a survey study of high multiplicity, high inelasticity and large momentum tranfer events from protonproton interactions at 28.5 GeV/c. The proton beam was capable of instantaneous rates up to 107 protons/s. We therefore deadened the portion of the chambers through which the incident proton beam travelled by epoxying 0.25 m m Kapton to each electrode. In spite of this precaution, the number of sparks from beam halo, knock-on's, 6-rays and unwanted interactions in and around the target proved to be too large, i.e., it prevented us from sorting out tracks in the forward direction belonging to the event of interest. We therefore had to reduce the beam intensity to ~ 6 x 105 protons/s. Very high multiparticle efficiency was a cardinal feature of our vertex spectrometer. To determine the efficiency we have reconstructed 2°) a sample of events, calculated the number of sparks expected for each track and compared it with the number of sparks actually found. We then broke up our sample into one
MAGNETIC
MULTIPARTICLE
and two prongs, three and four prongs, etc. The result of this analysis is shown in fig. 3. These efficiencies could possibly be biased by the fact that the tracks had to be f o u n d first before the efficiency was calculated. We have therefore checked our efficiency in two ways i n d e p e n d e n t of reconstructing the tracks. We have tracked back from one of our external spectrometers into the vertex spectrometer and asked the question how often did we find a spark in the predicted location. The answer is 96.5% for events with an average multiplicity of a b o u t 5. We have also t a k e n elastic events where we can predict from the i n f o r m a t i o n of an external spectrometer where the recoil p r o t o n has gone. This 2-prong efficiency - i n d e p e n d e n t of finding the track - turns out to be 99.7%. The fact that this efficiency is slightly higher than the one in fig. 3 can be attributed to the n o n - m i n i m u m ionization of the particle. The spatial resolution of the c h a m b e r was measured both for magnetic field on a n d off. F o r zero field runs, a straight line fit was made to the particle trajectory a n d then the distance "fit m i n u s m e a s u r e m e n t " was f o u n d to be _+0.5 m m at half m a x i m u m . Field-on runs, requiring a circle fit to the particle trajectory a n d corrections to a c c o u n t for the electron drift resulting from the crossed electric and magnetic fields, yielded a value of _+0.8 mm. It is clear that for tracks m a k i n g a large angle with respect to the n o r m a l to the electrode the resolution will broaden. The electron drift has been measured at 9.35 a n d 10.4 k G and found to be 3.3 m m at both field settings21). ARGO C H A M B E R E F F I C I E N C Y (INTEGRATED OVER THE 6 F U L L ROUND CHAMBERS) I
I
I
T
I
I
I
1
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04 >_LLI LL LL W 95
I 2
4 6 8 NUMBER OF PRONGS
I
I0
Fig. 3. Spark chamber efficiency vs multiplicity.
12
SPECTROMETER
539
We wish to t h a n k J. F u h r m a n n , T. Tuttle, R. H o d o r a n d their crew (George Tucker, Bob Shirley, John Hessinger, Eugene Hartchuck, Ted Klowswitz, a n d T o m Connizzo) at Brookhaven National L a b o r a t o r y for the excellent help a n d w o r k m a n s h i p in m a k i n g the chambers and c h a m b e r assembly a reality. We are grateful to Joe Fischer and his group (S. Shibata, j. Gatz, F. Merritt a n d S. MacCormick) in the Instrum e n t a t i o n Division at B N L for much valuable advice and help with the chambers and associated electronics. We t h a n k Ed Bihn and Dick Rothe for their valuable help. Finally we want to t h a n k all our collaborators, who contributed in so m a n y ways and helped us to succeed in our difficult u n d e r t a k i n g .
References 1) Study of multiparticle production in pp interactions at 28 GeV/c, BNL A.G.S. Proposal no. 396 (1968). 2) R. P. Shutt, Bubble and spark chambers (Academic Press, London, 1967). 3) G. E. Chikovani, V. A. Mikhailov and V. V. Roinishvili, Phys. Letters 6 (1963) 264. 4) j. R. Ficenec, T. S. Clifford, W. N. Schreiner, B. C. Stringfellow, W. P. Trower, E. W. Anderson, G. B. Collins, N. C. Hien, K. M. Moy, A. Ramanauskas, P. Schiibelin, A. Thorndike, F. Turkot and L. yon Lindern, Experimental meson spectroscopy (eds. C. Baltay and A. Rosenfeld; Columbia University Press, New York, 1970) p. 581. A full description of the design and performance of MASS is in preparation. ~) On loan from Cambridge Electron Accelerator. See also A. Buffington, D. H. Frisch S. Smith and C. E. W. Ward. Nucl. Instr. and Meth. 67 ([969) 157. 6) Sun Chemical Company, Facile Div. 185 6th Ave., Paterson, N. J. 07355, U.S.A. 7) Scotch Cast Brand Epoxy, Resin #8 - Cure time, 70% in 24 h, 100% in 7 d. 3-M Company, St. Paul, Minn. 55101, U.S.A. 8) Bipak Conducting Epoxy, manufactured by Emerson and Cummings Company, Canton, Mass. 02121, U.S.A. 9) American Cyanamid Company, Havre De Grace, Maryland 21078, U.S.A. t0) Further details concerning the chamber construction may be obtained from J. Fuhrmann, Physics Design Group, Brookhaven National Laboratory. 11) "Remendur" is an alloy of 49% Fe, 49% Co and 2% V, manufactured by William B. Driver Company, Newark, N. J. 07102, U.S.A. re) V. Perez-Mendez and J. Pfab, Nucl. Instr. and Meth. 33 (1965) [41; S. Miyamoto, Nucl. Instr. and Meth. 35 (1965) 323. t3) Designed by J. Fischer and S. Shibata, Instrumentation Division, Brookhaven National Laboratory. 14) Science Accessories Corp., Southport, Conn. 06490, U.S.A. a5) L. Kaufman, V. Perez-Mendez and J. Pfab, Preprint UCRL16536 (March 1966); M. L. Marshak and S. M. Pruss, Nucl. Instr. and Meth. 62 (1968) 295; 1. A. Golutvin, Yu. V. Zanevsky, Yu. T. Kiryushin, V. D. Peshekhonov, V. D. Ryabtsov and 1. M. Sitnik, Nucl. Instr. and Meth. 67 (1969) 257.
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16) R. Van Tuyl, K. Lee and V. Perez-Mendez, LRL Report UCID 3099 (Sept. 1967); J. Fischer and S. Shibata, IEEE Trans. Nucl. Sci., NS-15, no. 3 (1968) 572; O.C. Al[kofer, C. Grupen and G. Maxion, Nucl. Instr. and Meth. 79 (1970) 181. tT) Thyratron manufactured by E. G. & G. Co., 160 Brookline Ave., Boston, Mass. 02215, U.S.A. The pulsers were designed and assembled by J. Fischer and his group, Instrumentation Div., Brookhaven National Laboratory. is) 10 kV unregulated power supply, manufactured by Universal Voltronics Corp., 27 Radio Circle Drive, Mt. Kisco, N. Y.
10549, U.S.A. 19) The power supply regulator and fast recharge circuit designed by J. Fischer, Instrumentation Div., Brookhaven National Laboratory. 2o) D. R. Gilbert, W. N. Schreiner, W. P. Trower and P. Schtibelin, submitted to Nucl. Instr. and Meth., Sept. 1973. 21) K. R. Bentley, J. D. Davies, J. D. Dowell, C. McLeod, T. J. McMahon, H. B. Van der Raay, T. G. Rhoades, F. J. Wickens, C. I. S. Damerell, R. J. Homer and M. J. Hotchkiss, Nucl. Instr. and Meth. 104 (1972) 299.
INSTRUMENTS
AND
METHODS
II 3
(I973)
535-540;
©
MAGNETIC MULTIPARTICLE SPECTROMETER
NORTH-HOLLAND
PUBLISHING
CO.
USING DIGITIZED
CYLINDRICAL SPARK CHAMBERS* J. R. FICENEC and B. C. STR1NGFELLOWt Physics Department, Virginia Polytechnic Institute and State University +, Blacksburg, Va. 24061, U.S.A. G. B. COLLINS§, A. R A M A N A U S K A S * * , P. SCH1]BELIN and F. T U R K O T
Physics Department, Brookhaven National Laboratory, Upton, N. Y. 11973, U.S.A. Received 21 May 1973
We describe the construction and performance of the vertex spectrometer in the Multiparticle Argo Spectrometer System (MASS)used at Brookhaven. Nine digitized cylindrical wire spark
chambers, surrounding a hydrogen target and operating in a l0 kG magnetic field, detected most charged particles with high efficiencyup to multiplicities of 12.
1. Introduction
2. Chamber construction
The need for definitive studies of high multiplicity, high m o m e n t u m transfer reactions in the 10-30 GeV energy range 1) prompted us to design and build a w~rtex spectrometer which combined the large solid angle advantages of a bubble chamber with the triggerability and digitized readout capabilities of spark chambers. Other devices met some but not all of our requirements, e.g., bubble chambers 2) are not triggerable, streamer chaml;ers 3) do not have digitized readout, and conventional spark chamber spectrometers cover a limited solid angle. Our solution to these limitations is the vertex spectrometer of MASS4). It consists of a nested set of 9 cylindrical wire spark chambers surrounding a 20 cm long hydrogen target located in a 10 kG magnetic field of the A r g o magnetS). The field volume measures 1.5 x 1 x I m 3. The chambers have radii varying from 15;.25 cm (6") to 47.02 cm (183") with an active height of 73.5 cm. The spark gap o f each chamber is 0.95 cm (0.375"). High multiparticle efficiency is obtained by pulsing the chambers in a transmission line mode. Spark location is obtained by means of magnetostrictive lines located inside the magnet and shielded from the 10 kG field.
The first phase of chamber construction consisted of preparing the electrodes making up the chamber's spark gap. These electrodes were prepared from commercially available 50/tin Kapton (H-film) 6) with a bonded 25/~m aluminum backing. This foil was stretched over a wooden cylindrical drum with Kapton side up and a thin layer of epoxy 7) was applied. The 130/~m gold plated copper wire was then wound continuously on the foil with a pitch of 12.5 wires/cm. This procedure provided a strong b o n d between the wires and the backing foil, but left the top surface of the wires exposed so as not to impede spark formation. The wires and aluminum backing foil were electrically connected, half the wires at the top and half at the bottom of the cylinder, by bonding a copper strip to the aluminum with conducting epoxy s) and soldering the wires to the copper. The free ends of the wires were cut back and eventually buried inside the chamber end structure to prevent edge sparking. q-he r~ext phase of chamber construction was to build a structure to maintain two of these electrodes in a cylindrical shape with uniform separation. Each chamlzer was formed from two " h a l f chaml;ers" which were assembled on separate aluminum cylindrical drums. The " h a l f c h a m b e r s " were joined by slipping the outer one over the inner one. Each " h a l f c h a m b e r " was nearly identical, but the order of assembling the individual layers was inverted. The inner " h a l f c h a m b e r " was assembled in the following manner: A 76/~m mylar sheet was stretched over the assembly drum. A metal chamber skeleton consisting of steel (type 1010)headers at the top and bottom, stainless steel vertical support bars at _+90 ° from the forward
* Work performed under the auspices of the U.S. Atomic Energy Commission. + Portions of this work are in partial fulfillment of a Ph.D. Degree in Physics. Present address: C E R N , Geneva, Switzerland. + Work supported in part by the Research Corporation. § Present address: Physics Department, Virginia Polytechnic Institute and State University. ** Deceased.
535
536
J.R. FICENEC et al.
direction, and vertical aluminum support bars at the edges of the chamber was assembled on the drum and epoxied to the mylar. A sheet of nylon honeycomb cells 9) (cell size 0.95 cm, height 0.95 cm, wall thickness 25/~m) was fitted into the area outlined by the metal skeleton and epoxied to it and the mylar. An electrode foil was stretched over the skeleton and honeycomb, and epoxied to them with wire side up. The high voltage electrode was isolated from the steel by a 2 mm lucite separator and the ground electrode by 50/;m of Kapton tape. Steel spacer rings, similarly insulated, were placed at the top and bottom of the active
chamber area, as shown in fig. 1. These rings, together with vertical lucite spacer bars, located at the edges of the active area, maintained the 0.95 cm gap spacing. This completed the assembly of the inner "half chamber'. After the two "half chambers" were joined together, removable steel T-caps were fastened to the top and bottom steel headers to enclose the cavity which contains the magnetostrictive lines. When these end structures were imbedded in a steel plate the effective field reduction was measured to be ~ 10: 1. A finished chamber (fig. 2) is capable of withstanding large forces, . TEFLON
TdBE
r-SOLE /
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,
D L Ib'&S 2LA'~
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[q,
M A G N E T I C M U L T I P A R T I C L E SPECTROMETER
but contains a minimum of material in the active area, namely 0.25 g/cm 2. The unique mylar-honeycomb-foil sandwich, besides providing exceptional structural strength maintains the gap spacing to -I-0.02 cm~°). Each chamber has a high-voltage electrode with vertical wires and a ground electrode with wires at either +26.5 ° or - 2 6 . 5 ° with respect to the vertical. Since half the wires of each electrode are soldered at the bottom of the foil and half at the top, we were able to detect sparks from all regions of the chamber on both the high voltage and ground electrodes (fig. 1). 3. Chamber readout
The magnetostrictive lines and their pickup coils were housed in the cavity formed by the steel headers at the top and bottom of the cylindrical chambers.
Fig. 2. Spark chamber assembly with top lid removed (bottom) and individual chamber (top); in the background the Argo magnet.
537
Each cavity contained two magnetostrictive lines [0.3 mm x 0.13 mm Remendur 11) ribbon], whose velocity was premeasured. Pickup coils, 40 turns of # 40 copper wire, are located at both ends of each line inside cylindrical biasing magnets 12). The readouts were built as complete units and placed into the cavity after removing the T-caps. Each unit consisted of two lucite strips formed to the correct radius with grooved notches to house the magnetostrictive lines (see fig. l). Curved copper boxes, which contained pickup coils, biasing magnets, electrical standoffs and clay to damp out signal reflections, were epoxied onto the ends of the lucite. The magnetostrictive line was inserted into a teflon tube (diameter 0.8 ram), around which was wound a solenoidal biasing coil giving us the capability of polarizing the magnetostrictive line remotely. The teflon tube was then placed into the groove in the lucite strip. The lucite carriers were fitted to the chambers and shimmed tightly against the wires of the electrodes (the distance between the chamber wires and the magnetostrictive line was approximately 1 mm). The pickup coil and solenoid leads were soldered to cable connectors mounted on another copper box which was fastened to the chamber. From here, shielded twinex cables carried the signals to the amplifiers13), which were located 3 m away outside the magnet. The sparks from the chambers were digitized by a commercial M I D A S 14) system located in the electronics trailer ~ 3 0 m from the amplifiers. The minimum separation distance between sparks resolvable by this system is 3 ram. Each pickup coil in the six full-round chambers was assigned six scalers, while the three downstream chambers had four scalers per coil. Since each magnetostrictive line had two coils and each line read half of an electrode, each full round chamber was capable of recording up to 24 sparks. If the number of sparks was less than this value, the double coil readout provided for consistency checks. Because fiducial wires were not mounted on the chambers, the scalers were started by a fast logic signal. Since the chambers and readouts were located in the 10 k G magnetic field, we were confronted with the well-known readout problem of deadspots and signal inversion. Tests performed prior to the experiment indicated that the magnetostrictive wand was unaffected by magnetic fields of 1.5 k G normal to the wire and 3 0 4 0 G parallel to the wire15). The magnetostrictive lines were oriented perpendicular to the main field component. The normal component seen by the readout wire was approximately 1.0 kG, while a 10%
538
J.R. FICENEC et al.
non-uniformity of the field caused the longitudinal component to be as high as 100 G close to the Argo magnet coils. A few readout lines were affected, causing inversion of the magnetostrictive signals and short dead spots. Since the inverted signal digitizings have a distinctive double-spark pattern and occurred mainly at large angles away from the forward direction, the double signals can lze averaged into one digitizing by software without loss of resolution.
4. Chamber operation To obtain a high multiparticle efficiency, the chambers were operated in a transmission line mode ~6) by making the high voltage and ground connections to the aluminum backing sheet instead of to the wires directly. In addition, each chamt;er was terminated into a resistor-diode combination which matched the inherent chamber impendence (approx. 5Q). This resulted in a uniform high voltage pulse with a rapid rise time over the entire chamber area, and reduced the voltage reflections from the end structure. The chambers were operated with standard spark chamber gas (90% neon - 10% helium) bubbled through isopropyl alcohol at 0°C to provide a quenching agent. This mode of operation preduced sparks with relatively uniform signal height and reduced spark robbing effects. The chambers had an operating voltage of 5 kV and were pulsed with an HY-13 hydrogen thyratron ~7) which produced a negative high voltage pulse with a 20 ns risetime and 400 ns R C decay time. A regulated high voltage power supply tS) and recharge circuit TM) was capable of operating at rates of 100 pulses/s; the chambers were normally operated with a 10 ms dead time. A +80 V dc and a +500 V pulsed (2 ms width) clearing field were used to keep the chamber free of electrons. This dc clearing field value resulted in a single particle memory time (efficiency down to 30%) of approximately 1.5 Its. Delay time between particle passage and the high voltage pulse was 550 ns.
5. Spectrometer assembly The completed spark chamber system consisted of six full round chambers (active area 270 °, radii 15.25, 22.88, 30.50, 38.13, 43.84, 47.02 cm), and three half round chambers (active area 180 °, radii 34.95, 38.13, 47.02 cm) (fig. 2). The steel header of each chamber was placed in a groove cut into the steel base plate of the support structure. After the chambers were aligned and the high voltage and readout cables were attached, the structure was closed by lowering another grooved
plate over the chamber headers and bolting it in place. Since the additional steel (grooved plate and chamber headers) formed a continuous plate-like extension of the pole pieces, we were able to reduce the critical field components sufficiently without changing the homogeneity of the A R G O magnetic field. The entire assembly (fig. 2) was lowered into the A R G O magnet through a hole left by removing a portion of the top pole piece. This pole piece was put back when the assembly was in place. The liquid hydrogen target was then inserted into the upstream median spacer gap of the magnet and positioned at the center of curvature of the six full round chambers. Both the structure housing the chambers and the A R G O magnet have clearance about the median plane to allow the external spectrometers to "see" the hydrogen target (fig. 2). The support structure had to be designed to prevent structural damage to the chambers, since the chamber headers were made of steel to shield the magnetostrictive lines from the magnetic field and hence experienced a force from that field. This was done by adding an additional 1 cm of steel to the base and top plates (see fig. 1) to obtain magnetic forces which would stretch the chambers rather than compress them. The amount of stretching was controlled by set screws tapped through the top plate. The 10 kG magnetic field applied a stretching force on the chambers of about ¼ t, with no resulting structural damage.
6. Spectrometer performance The vertex spectrometer of MASS was first used in a survey study of high multiplicity, high inelasticity and large momentum tranfer events from protonproton interactions at 28.5 GeV/c. The proton beam was capable of instantaneous rates up to 107 protons/s. We therefore deadened the portion of the chambers through which the incident proton beam travelled by epoxying 0.25 m m Kapton to each electrode. In spite of this precaution, the number of sparks from beam halo, knock-on's, 6-rays and unwanted interactions in and around the target proved to be too large, i.e., it prevented us from sorting out tracks in the forward direction belonging to the event of interest. We therefore had to reduce the beam intensity to ~ 6 x 105 protons/s. Very high multiparticle efficiency was a cardinal feature of our vertex spectrometer. To determine the efficiency we have reconstructed 2°) a sample of events, calculated the number of sparks expected for each track and compared it with the number of sparks actually found. We then broke up our sample into one
MAGNETIC
MULTIPARTICLE
and two prongs, three and four prongs, etc. The result of this analysis is shown in fig. 3. These efficiencies could possibly be biased by the fact that the tracks had to be f o u n d first before the efficiency was calculated. We have therefore checked our efficiency in two ways i n d e p e n d e n t of reconstructing the tracks. We have tracked back from one of our external spectrometers into the vertex spectrometer and asked the question how often did we find a spark in the predicted location. The answer is 96.5% for events with an average multiplicity of a b o u t 5. We have also t a k e n elastic events where we can predict from the i n f o r m a t i o n of an external spectrometer where the recoil p r o t o n has gone. This 2-prong efficiency - i n d e p e n d e n t of finding the track - turns out to be 99.7%. The fact that this efficiency is slightly higher than the one in fig. 3 can be attributed to the n o n - m i n i m u m ionization of the particle. The spatial resolution of the c h a m b e r was measured both for magnetic field on a n d off. F o r zero field runs, a straight line fit was made to the particle trajectory a n d then the distance "fit m i n u s m e a s u r e m e n t " was f o u n d to be _+0.5 m m at half m a x i m u m . Field-on runs, requiring a circle fit to the particle trajectory a n d corrections to a c c o u n t for the electron drift resulting from the crossed electric and magnetic fields, yielded a value of _+0.8 mm. It is clear that for tracks m a k i n g a large angle with respect to the n o r m a l to the electrode the resolution will broaden. The electron drift has been measured at 9.35 a n d 10.4 k G and found to be 3.3 m m at both field settings21). ARGO C H A M B E R E F F I C I E N C Y (INTEGRATED OVER THE 6 F U L L ROUND CHAMBERS) I
I
I
T
I
I
I
1
I
I00
04 >_LLI LL LL W 95
I 2
4 6 8 NUMBER OF PRONGS
I
I0
Fig. 3. Spark chamber efficiency vs multiplicity.
12
SPECTROMETER
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We wish to t h a n k J. F u h r m a n n , T. Tuttle, R. H o d o r a n d their crew (George Tucker, Bob Shirley, John Hessinger, Eugene Hartchuck, Ted Klowswitz, a n d T o m Connizzo) at Brookhaven National L a b o r a t o r y for the excellent help a n d w o r k m a n s h i p in m a k i n g the chambers and c h a m b e r assembly a reality. We are grateful to Joe Fischer and his group (S. Shibata, j. Gatz, F. Merritt a n d S. MacCormick) in the Instrum e n t a t i o n Division at B N L for much valuable advice and help with the chambers and associated electronics. We t h a n k Ed Bihn and Dick Rothe for their valuable help. Finally we want to t h a n k all our collaborators, who contributed in so m a n y ways and helped us to succeed in our difficult u n d e r t a k i n g .
References 1) Study of multiparticle production in pp interactions at 28 GeV/c, BNL A.G.S. Proposal no. 396 (1968). 2) R. P. Shutt, Bubble and spark chambers (Academic Press, London, 1967). 3) G. E. Chikovani, V. A. Mikhailov and V. V. Roinishvili, Phys. Letters 6 (1963) 264. 4) j. R. Ficenec, T. S. Clifford, W. N. Schreiner, B. C. Stringfellow, W. P. Trower, E. W. Anderson, G. B. Collins, N. C. Hien, K. M. Moy, A. Ramanauskas, P. Schiibelin, A. Thorndike, F. Turkot and L. yon Lindern, Experimental meson spectroscopy (eds. C. Baltay and A. Rosenfeld; Columbia University Press, New York, 1970) p. 581. A full description of the design and performance of MASS is in preparation. ~) On loan from Cambridge Electron Accelerator. See also A. Buffington, D. H. Frisch S. Smith and C. E. W. Ward. Nucl. Instr. and Meth. 67 ([969) 157. 6) Sun Chemical Company, Facile Div. 185 6th Ave., Paterson, N. J. 07355, U.S.A. 7) Scotch Cast Brand Epoxy, Resin #8 - Cure time, 70% in 24 h, 100% in 7 d. 3-M Company, St. Paul, Minn. 55101, U.S.A. 8) Bipak Conducting Epoxy, manufactured by Emerson and Cummings Company, Canton, Mass. 02121, U.S.A. 9) American Cyanamid Company, Havre De Grace, Maryland 21078, U.S.A. t0) Further details concerning the chamber construction may be obtained from J. Fuhrmann, Physics Design Group, Brookhaven National Laboratory. 11) "Remendur" is an alloy of 49% Fe, 49% Co and 2% V, manufactured by William B. Driver Company, Newark, N. J. 07102, U.S.A. re) V. Perez-Mendez and J. Pfab, Nucl. Instr. and Meth. 33 (1965) [41; S. Miyamoto, Nucl. Instr. and Meth. 35 (1965) 323. t3) Designed by J. Fischer and S. Shibata, Instrumentation Division, Brookhaven National Laboratory. 14) Science Accessories Corp., Southport, Conn. 06490, U.S.A. a5) L. Kaufman, V. Perez-Mendez and J. Pfab, Preprint UCRL16536 (March 1966); M. L. Marshak and S. M. Pruss, Nucl. Instr. and Meth. 62 (1968) 295; 1. A. Golutvin, Yu. V. Zanevsky, Yu. T. Kiryushin, V. D. Peshekhonov, V. D. Ryabtsov and 1. M. Sitnik, Nucl. Instr. and Meth. 67 (1969) 257.
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16) R. Van Tuyl, K. Lee and V. Perez-Mendez, LRL Report UCID 3099 (Sept. 1967); J. Fischer and S. Shibata, IEEE Trans. Nucl. Sci., NS-15, no. 3 (1968) 572; O.C. Al[kofer, C. Grupen and G. Maxion, Nucl. Instr. and Meth. 79 (1970) 181. tT) Thyratron manufactured by E. G. & G. Co., 160 Brookline Ave., Boston, Mass. 02215, U.S.A. The pulsers were designed and assembled by J. Fischer and his group, Instrumentation Div., Brookhaven National Laboratory. is) 10 kV unregulated power supply, manufactured by Universal Voltronics Corp., 27 Radio Circle Drive, Mt. Kisco, N. Y.
10549, U.S.A. 19) The power supply regulator and fast recharge circuit designed by J. Fischer, Instrumentation Div., Brookhaven National Laboratory. 2o) D. R. Gilbert, W. N. Schreiner, W. P. Trower and P. Schtibelin, submitted to Nucl. Instr. and Meth., Sept. 1973. 21) K. R. Bentley, J. D. Davies, J. D. Dowell, C. McLeod, T. J. McMahon, H. B. Van der Raay, T. G. Rhoades, F. J. Wickens, C. I. S. Damerell, R. J. Homer and M. J. Hotchkiss, Nucl. Instr. and Meth. 104 (1972) 299.