Precision reconstruction of charged tracks with simultaneous electron identification in a gaseous detector using transition radiation

Nuclear Instruments and Methods in Physics Research A31(I (It~i) 535-539 North-Holland

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Precision reconstruction of charged tracks with simultaneous electron identification in a gaseous detector using transition radiation H. Gr~issler, M. H o h l m a n n , G. Kemmerling, S. Masson, W. Pilgram and W. Struczinski IlL Physikalisches hlstimt B, RWTH Aachen, D-S100 Aachen, GermmLr G.A. Beck, J.B. Dainton, E. Gabathuler, D. Gillespie, P. Mason, S.J. Maxfield, J.M. Morton, D.G. N u n n , G.D. Patei, D.P.C. Sankey and L.A. Womersley Department of Physics. Olir'er Lodge La~Jratort.'. Unirersity of Lit't'rpool, Lirerpt~ol. UK

Results are presented from tests of a radial wire drift chamber, the design of which is optimised fl)r both accurate spatial reconstruction of charged tracks and efficient detection of incident X-rays. With flash digilised readout, we demonstrate that analysis of pulse profile can yield good spatial accuracy (O'dnlt ~ 1511 ttm, ~rch,h~ ~ 1~/, wire length) together with u~ful hadron/ electron discrimination (~r/e ~ 8% at 60 GeV/c) using a transition radiator immediately preceding the chamber. The exploitation of this technique at high energy proton-proton and electron-proton collider storage rings is briefly discussed.

1. Introduction

secondary" particles due to the large asymmetry in beam m o m e n t u m between protons at 820 G e V / c and electrons at 30 G e V / c . The cylindrical symmetry of the F T D and uniform solenoidal magnetic field is matched by the radial wire design. At forward angles high m o m e n t u m particle tracks are measured most effectively by transverse wires, with ionisation drift in the R@ plane. For radial wires, even in the presence of substantial l_x,.cntz angle, the drift time always measures track sagitta. Thus the most accurate coordinate measurement optimises m o m e n t u m precision. In addition, charged parti-

Simultaneous track reconstruction and electron identification have been achieved in a radial wire drift c h a m b e r combined with a transition radiator. A set of three such chambers forms an essential component of the forward track detector ( F T D ) of the H I experiment at the H E R A e l e c t r o n - p r o t o n collider. The F T D (fig. 1) covers laboratory angles 5 ° to 30 ° with respect to the forward proton direction, and is designed to measure charged-track m o m e n t u m p with a precision ap/pz o f 0.3% G e V - i The F T D sees a high flux o f

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VII. PARTICLE PHYSICS

536

II, (;riissler et al, / Precision reconstruction t~f charged tracks

cle tracks are straight lines in the (h-: plane, which greatly eases pattern recognition. Space points arc formed by combining the drift coordinate with the radial coordinate found from charge division along the wires. The radial wedges have the further advantage of smaller acceptance close to the beam axis where the track density and accumulation of charge on the wires are greatest. The radial wire arrangement covers the detector volume optimally for a given number of wires and readout channels. An important addition to the capability of the detector is the discrimination of electrons from pions at a useful level by detecting transition radiation.

2, Chamber design The drift chamber gcomctry is shown in fig. 2. Planes of wircs and scgmentcd cathodes extend radiFront fiR) Window

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:dly from the beam [1]. The cathode potential varies linearly with radius and is adjusted so that the drift electric field is uniform and perpendicular to the wires. The field near the back and the outer circumference of the chamber is maintained by strips which run parallel to the wire planes. A similar array of strips at the front of the chamber is mounted upstream of the gas window. This structure is transparent to X-rays generated by fast electrons in the transition radiator. Each wire plane has 12 sense wires spaced I cm along the beam direction and alternating with I 1 grid wires. The grid wires are 125 pm diameter beryllium/ copper and arc electrically grounded. The sense wires are 50 p,m diameter and are staggered alternately by + 300 I,tm out of the wire plane to resolve the left-right track ambiguity. The material of the sense wires is Stablohm-800 (resistance 645 [ l / m ) [2] which enables determination of the radial coordinate by charge division. The sensitive volume of the chamber extends from 150 to 760 mm in radius and is divided in • by the cathode planes into 48 wedges. Further description of the chamber construction, electrostatics and operation is given in rcf. [3]. The chambers will be filled initially with a mixture of 50% argon and 50% emane at atmospheric pressure. For optimum X-ray detection the argon will be replaced by 30% xenon and the balance with helium. Tests have been made with both gases with a drift field of 1.2 kV/cm and a gas gain between 1 x 104 and 3 × 10 4, which allows good tracking precision in both coordinates whilst operating in the proportional mode for ionisation measurements.

3. Precision of manufacture Fieldforming strips

After construction of each of the three chambers the positions of the wires were surveyed in three axes [4]. The measured wire coordinates were extrapolated to define the mean centre of the chamber. The "impact parameters" of the measured sense wires relative to this centre are shown in fig. 3a. The two peaks correspond to the left and right stagger of the sense wires. The measured error in azimuthal angle of the wires is plotted in fig. 3b.

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Fig. 2. Schematic exploded view of a radial wire drift chamber for Hl. The wires are strung radially between the inner Nory[ cylinder and the outer composite shell. The cathode planes are rectangular semi-rigid paper pcbs supported along all four edges, and are voltage graded, lonisation drift is normal to the wire planes and measures the Rqb coordinate. Charge division along pairs of sense wires determines the R coordinate.

4. Drift coordinate precision Pulses on the sense wires are amplified close to the chamber and recorded by 100 MHz flash ADCs [5]. The maximum difference of consecutive ADC samples determines the time of the leading edge of a pulse to one 10 ns bin. This precision is refined by weighting the adjacent samples. The weights are adjusted so that over a large number of pulso~ the 3istribution of recon-

H. Griisder et al. / Precision n'constnwtion of charged tracks 5. R a d i a l p r e c i s i o n : Sta~er = +_285 lain R M S = 38 I.Im

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equipped with 6-bit flash A D C s has b e e n measured with a collimated X-ray source as a function of position along the wire. T h e results arc shown in fig. 5a together with those from a Monte Carlo simulation. The simulation was repeated, substituting the pulse height

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The radial coordinate is fl,und from charge division along each sense wire. The need to have readout electronics close to the beam is avoided by connecling each wire at the inner radius to a n o t h e r in a wedge at 105 ° r o u n d the chamber. T h u s the charge divides between two lengths of sense wire (900 [! in total) and the input resistance of the preamplifier (200 fl) at each o u t e r end. T h e inner ends of the wire lie inside crimp tubes a n d serve as a resistor ( ~ 20 [1) separating hits on a particular wire from those on its 1050 partHer.

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structed times within the 10 ns bin is uniform [6]. T h e drift coordinate R@ then follows from the electron drift velocity in the gas. Its precision is found to be a moderate function of drift distance (fig. 4) and varies b e t w e e n 150 and 200 ixm.

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Radial coordinate ( cm ) Fig. 5. (a) Charge division precision measured with 6-bit flash ADCs and an X-ray source (55Fe) at known positions along a pair of sense wires. Also shown are results from a Monte Carlo simulation which included the observed pulse height distribution, electronic noise, and flash ADC digitisation. (b) The Monte Carlo prediction for minimum ionising tracks using 8-bit flash ADCs. for three levels of electronic noise (plotted for one half of the wire pair). VII. PARTICLE PHYSICS

H. Griissh'r et al. / Pn'ciskm reconstruction of charged tracks

538

distribution observed for minimum ionising tracks and the planned 8-bit A D C digitisation (fig. 5h).

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The variation of pulse integral with position in thc chamber has been investigated with cosmic ray particles. The mean value for each of the 12 sense wires in a wedge is shown in fig, 6a. The fall in mean charge on wires 1 and 2 is associated with the behaviour of the front field former strips sited outside the gas envelope, and can be compensated by adjusting the strip voltages. The uniformity of response along the radial coordinate depends on the variation of potential along the cathode plane, Potentials for the inner and outer radii were predicted from a simple electrostatic model [3] and produced an adequately uniform response (fig, 6b).

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The transition radiator (TR) immcdiately upstream of each drift chamber consists of 400 foils of polypropy-

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lone, each 19 v,m thick, spaced rcgularly within a total thickness of 96 mm. An incident electron gives rise to an average of 1.4 X-rays detected in each chamber. Test data were taken with 5 G e V electrons, a polypropylene fibre radiator and a prototype chamber filled with xenon : ethane : helium (30: 40: 30) [7]. The majority of T R X-rays were detected on the front six wires of the chamber. The distribution of charge on these wires, averaged over three electron events to simulate three modules, is shown in fig. 7a. The distribution for pions at high momentum ( ~ 60 G e V / c ) is simulated by data taken without the radiator (fig. 7b). From these two distributions the high momentum pion contamination for 90% electron efficiency is predicted to be 8%. An estimate for the pion distribution at 5 G e V / c (fig. 7c) was obtained by scaling by the relativistic rise of the mean pion d E / d x . In this case the predicted pion contamination is 0.5%. Further studies are foreseen, including experimental pion data and a more refined trcatment to account for the variation of X-ray intensity with depth in the chamber gas. An important background in T R detectors with a small number of wires arises from upstream interactions when ionisation from two or more unresolved

!t. Grii.~ter et al. / Precision reconstn¢crion of ckarged tracks

tracks is misinterpreted as an X-ray. This background should bc easily recognised in the radial chambers from the twelve consecutive ionisation samples.

8. Conclusions The results reported here show the practicality of the concept of radial wire drift chambers in terms of the uniformity of pulse integral, precision of coordinate measurement, and the simultaneous identification of electron tracks within the same detector volume. The reconstruction of an event with multiple tracks together with identification of the electron tracks is a very important combination of qualities for detectors at LHC/SSC-type supercolliders, where some part of new physics (for example Higgs production) can be extracted from Icptonic decays. The present design of chamber is a strong candidate for development for end-cap track detectors. A disadvantage for work with short bunch-crossing times is the length of drift time in the gas. Further development will include work on gases with higher drift velocities, and on the engineering to increase the density of cells while maintaining a low material density, especially close to the beam.

539

Acknowledgements We thank the HI and survc.¢ groups at RAL flw their help with the design, survey and cosmic ray tc~,ls. Mechanical design and construction of the chamber~, were due largely to A. Amery, P. Jando, D. King, A. Muir, F. Robinson, K. Woolfall and M. Wormald with help from S. Holt (Lancaster University).

References [i] M. Alac et al., IEEE Trans. Nucl. Sci. NS-33 (I) (1986} 189. [2] Manufaclured by California Fine Wire Co., Gr~r,'er Cit.v. CA, USA. [3] G.A. Beck el al.. Nucl. Inslr. and Melh, A283 (1989) 471. [4] LKg0 granite table and coordinatograph at RAL. manufactured by Mondo Lid.. Leicester. UK. [5] 6-bit nonlinear Heidelberg-Slruck DI_'4(XI FADC System. or 8-bit nonlinear DESY-Struek F-1000 FADC System. [6] D.P.C. Sankey. Ph.D. thesis. Unive~ity of Liverpool. UK 0989}. [7] H. Grassier et al., Nucl. instr, and Meth. A283 (1989} 622.

VII. PARTICLE PHYSICS