APEX heavy ion counters

Nuclear Instruments and Methods in Physics Research A 348 (1994) 252-255 North-Holland

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A

A P E X heavy ion counters D.J. M e r c e r a D. Mikolas a,1, j. Y u r k o n a, S.M. Austin a, D. Bazin a,2 S. Gaff ~, E. Kashy ~, D. Kataria ~,3, J.S. Winfield a,., R. Betts b, D. H e n d e r s o n b, A. Hallin c, M. Liu c a National Superconducting Cyclotron Laboratory, Michigan State Unit,ersity, East Lansing, Michigan 48824, USA b Argonne National Laboratory, Argonne, IL 60439, USA ~ Queen's Uniuersity, Kingston, Ontario K7L 3N6, Canada

A large solid-angle array of low pressure multiwire proportional counters has been constructed as part of the Atlas Positron EXperiment (APEX). Eight three-element trapezoidal counters provide 360° coverage in ~ and 20°-68 ° coverage in 0. Angle sensitivity in 0 is provided by a transmission-line delay cathode, while the segmentation provides angle sensitivity in ~. Sample data are presented from the 1450 MeV 23Su + lSlTa reaction.

1. Introduction

A large solid-angle array of Low Pressure MultiWire Proportional Counters (LPMWPCs) has been constructed as part of the Atlas Positron E X p e r i m e n t (APEX). The A P E X experiment [1-3], which is in progress at Argonne National Laboratory, seeks to investigate the puzzling peaks which have been observed in the spectra of positrons and electrons emitted in heavy-ion collisions near the Coulomb barrier [4-11]. The experiment is a collaborative effort involving researchers from Argonne National Laboratory, the University of California, Florida State University, Lawrence Berkeley Laboratory, the National Superconducting Cyclotron Laboratory, Princeton University, Queen's University, the University of Rochester, the University of Washington, and Yale University. In the A P E X spectrometer, a 238U beam strikes a high-Z target (such as 181Ta) which is centered in a large solenoid capable of producing a 300 G magnetic field transverse to the beam. Positrons and electrons emitted in the reaction are transported in helical paths along the field towards Si PIN diode arrays and NaI(TI) annihilation radiation detectors which determine the emission angle, time-of-flight, and species of the leptons. These components of the apparatus have been described in refs. [12-14]. The array of LPMWPCs,

* Corresponding author. 1 Present address: nano-Optics, Inc., P.O. Box 98, Ithaca, NY 14851, USA. Present address: GANIL, BP 5027, 14201 Caen Cedex, France. 3 Present address: Nuclear Science Centre, New Delhi, 110 067 India.

which will be described here, detects heavy ions from the same reaction and determines their scattering angle and time-of-flight. This information assists in complete kinematic reconstruction of the reaction.

2. Design and construction

The heavy ion array consists of eight trapezoidal counters assembled into a truncated right octagonal pyramid, as shown in Fig. 1. The array is positioned so that any unscattered 238U beam passes through its center, and so that it does not interfere with the trajectories of positrons and electrons emitted from the target. The active area spans polar angle 0 from 20 ° to 68 ° measured with respect to the beam direction, and 360 ° in azimuth angle ~. The flight path from the target to the array varies from 56 cm to 65 cm, with the minimum occurring at 0 = 50 °. Each counter is divided into three electrically isolated elements, giving a total segmentation of 24. This geometry was chosen because it closely approximates a cone and it keeps the ~b segmentation adequately high, while also keeping the number of parts and independently sealed gas volumes low. The operating principle of the counters is similar to that of smaller low-pressure multiwire proportional counters, and has been described in the literature [15]. A volume of low pressure isobutane gas is enclosed by a pair of cathode planes 6.4 mm apart, and a series of parallel wires midway between the cathodes forms an anode plane, to which positive bias voltage is applied. When high-Z particles pass through the counter, the gas is ionized. Freed electrons drift toward the anode wires and experience further ionizing collisions, giving

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D.J. Mercer et aL / NucL Instr. and Meth. in Phys. Res. A 348 (1994) 252-255

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Fig. 1. Assembly of the LPMWPC array, which fits inside the APEX main scattering chamber downstream from the target.

A P E X scattering c h a m b e r and to the gas handling system. 2.) Delay-line cathode. The back c a t h o d e consists of 500 curved traces, 0.5-ram wide with a 1-mm pitch, e t c h e d on a 0.4-mm thick FR-4 pc board with 80-~m c o p p e r cladding. The traces are c o n n e c t e d e n d - t o - e n d ,

1 rise to a multiplicative avalanche. Electrons liberated in the avalanche are collected by the a n o d e wires, and image charges are induced on the cathodes. Our counters employ a variation of the delay line r e a d o u t m e t h o d for the d o w n s t r e a m cathode, with a single m e a n d e r i n g transmission line serving as both a c a t h o d e plane and as a delay line. The low pressure of the gas allows for high drift velocities, and makes n a n o s e c o n d time resolution possible. A diagram of an individual c o u n t e r ( 1 / 8 of the array) is shown in Fig. 2. The t h r e e active s e g m e n t s in each c o u n t e r are about 48 cm long with width varying from 4.5 cm to 13 cm. T h e r e are d e a d strips 0.8 cm wide b e t w e e n each of the segments, which are necessary so the e l e m e n t s may be electrically isolated from each other, and d e a d strips 1.1 cm wide along the sides. The c o m p o n e n t s are described in g r e a t e r detail below: 1.) Back plate. This is m a d e of 6.35-mm aluminum alloy, and includes h a r d w a r e so that each c o u n t e r can be attached to an octagonal m o u n t i n g frame in the

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D.J. Mercer et al. /Nucl. Instr. and Meth. in Phys. Res. A 348 (1994) 252-255

making o n e long delay line, as shown in Fig. 3. T h e copper-clad back of the pc b o a r d serves as a g r o u n d plane. T h e total delay along this line is 200 _+ 4 ns, a n d the characteristic i m p e d a n c e is 51 _+ 2 1~. Differential nonlinearities are small a n d periodic. Signals are detected from b o t h the forward-angle (F) a n d backwardangle (B) ends of the delay line, a n d the angle 0 of the incident ion is d e d u c e d from a comparison b e t w e e n the arrival time of amplified F a n d B signals. 3.) A n o d e Wire Plane. T h e a n o d e wires are 12-txm gold-plated tungsten, s t r e t c h e d parallel to the forward a n d b a c k w a r d edges of t h e c o u n t e r with a spacing of 1 mm. T h e s e wires are c o n n e c t e d in parallel to a c o p p e r strip which runs a r o u n d the edge of each s e g m e n t on a 3.18-mm FR-4 support frame. Time-of-flight ( T O F ) information is extracted from the time difference between the amplified a n o d e signal and an acceleratorg e n e r a t e d R F pulse. 4.) F r o n t c a t h o d e plane. A g r o u n d e d front c a t h o d e foil is used to t e r m i n a t e the electric field lines extending from the anode. It is m a d e from 1.5-txm aluminized Mylar epoxied to a 3.18-mm thick FR-4 frame. 5.) Pressure window. T h e pressure window serves to m a i n t a i n the gas volume. A s e p a r a t e pressure window insures that no differential pressure occurs across the front c a t h o d e foil. T h e window is m a d e from 12-p,m aluminized K a p t o n epoxied to a 3.18-mm FR-4 s u p p o r t frame. 6.) M a s k a n d support wire frame. T h e front surface of each c o u n t e r is m a d e from 3.18-mm aluminum, a n d serves as a mask to define the acceptance of each of the t h r e e elements. Wires m a d e from 76-1xm CuBe, spaced at 12.7 mm, are c o n n e c t e d to the u n d e r s i d e of this f r a m e to p r e v e n t excessive bowing of the K a p t o n pressure window. Masks used for angle calibrations attach to the front of this frame. T e n of the trapezoidal c o u n t e r s have b e e n constructed. They are i n t e r c h a n g e a b l e a n d d a m a g e d counters may b e r e p l a c e d by spare ones. A single prototype parallel plate avalanche c o u n t e r ( P P A C ) has also b e c n c o n s t r u c t e d with an a n o d e foil a n d a delay-line cathode p l a n e s e p a r a t e d by a b o u t 3 mm, b u t otherwise having similar dimensions as the L P M W P C s . It will be tested in the n e a r future a n d its p e r f o r m a n c e comp a r e d to t h a t of the L P M W P C s .

3. Performance T h e counters have b e e n tested with a variety of b e a m / t a r g e t conditions. W e report h e r e results from a 1450 M e V 238U b e a m scattering on a lSlTa target. T h e data were collected in August 1993 at A r g o n n e National Laboratory. T h e c o u n t e r s were o p e r a t e d at a bias of 530 V a n d were filled with isobutane gas at 5.0 Torr. E v e n t rates

Fig. 4. Two-dimensional histogram of heavy ion time-of-flight (from anode time) and scattering angle 0 (from cathode time difference), which may be used for particle identification.

for each of the 24 s e g m e n t s were typically 20 kHz. A t this rate, each c o u n t e r ( t h r e e s e g m e n t s sharing a voltage supply) drew a b o u t 1.6 ~ A of current. It is conject u r e d that ~ rays or light c h a r g e d particles may contribute to this abnormally high current. Typical rise times are 5 ns for a n o d e a n d c a t h o d e signals. Typical a n o d e pulse heights were 10 mV; this c o r r e s p o n d s to a b o u t 10 ~ e l e c t r o n s / a v a l a n c h e , close to the empirical limit of ref. [16]. C a t h o d e pulse heights were variable and smaller. Fig. 4 shows a two-dimensional histogram of timeof-flight a n d scattering angle data from one of the 24 segments. T h e T O F resolution is 1.1 ns F W H M for ions at small 0, which includes a 0.5 ns time resolution from the beam. T h e angular resolution in 0 is a b o u t 0.25 ° F W H M , with possible systematic errors up to 1°. Particle identification is possible from a histogram such as this: two s e p a r a t e branches, c o r r e s p o n d i n g to U a n d Ta, are visible. U is strongly forward-peaked, and has a m a x i m u m 0 of 49.5°; Ta is visible to 68 °, and has smaller T O F t h a n U at any given angle. T h e efficiency of the c o u n t e r s can be d e t e r m i n e d from back-to-back heavy ion coincidences. If o n e assumes t h a t the scattering is elastic or nearly so, t h e n detection of one heavy ion (within a certain r a n g e of 0) implies that a second heavy ion should b e seen in the opposing c o u n t e r 180 ° different in (b. It was f o u n d using this a s s u m p t i o n that the intrinsic efficiency of the counters is a b o u t 85%. T h e efficiency for Ta ions scattered at 0 > 60 ° was s o m e w h a t worse, mostly because of smaller kinetic energy of the ions a n d the possibility of stopping in the K a p t o n pressure window. A m o r e detailed p a p e r describing the o p e r a t i o n a n d p e r f o r m a n c e of the counters will be s u b m i t t e d to Nuclear I n s t r u m e n t s a n d M e t h o d s .

D.J. Mercer et al. /Nucl. Instr. and Meth. in Phys. Res. A 348 (1994) 252-255

Acknowledgements W e are g r a t e f u l to all o f t h e m e m b e r s o f t h e A P E X c o l l a b o r a t i o n . This w o r k was s u p p o r t e d in p a r t by grants from the National Science Foundation and the United States Department of Energy.

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I. GASEOUS DETECTORS