c momentum region

Nuclear Instruments and Methods 176 (1980) 325-331 © North-Holland Publishing Company

Part FIlL Particle identification

A TRANSITION RADIATION DETECTOR F O R PION IDENTIFICATION IN THE 100 GeV/c MOMENTUM REGION V. COMMICHAU, M. DEUTSCHMANN, H. GODDEKE, K. HANGARTER, U. PUTZHOFEN, R. SCHULTE and W. STRUCZINSKI lit. Physikalisches Institut der Rheiniseh-WestfSlischen Technischen Hochschule, Aachen, Germany

A large area transition radiation detector has been proposed for the European Hybrid Spectrometer at CERN. Its construction is based on experience with a smaller size model detector which has been tested in hadron beams between 40 and 140 GeV/c momentum. The detector allows identification of pions versus kaons and protons at momenta beyond ~90 GeV/c.

radiators (mylar or carbon) coupled to xenon wire chambers.

1. Introduction Particle identification has become an essential requirement in any modern high energy experiment. In the very high energy region above 100 GeV where conventional methods like the Cherenkov effect or ionization sampling are less or no more practical, transition radiation (TR) offers an attractive alternative choice. Although the technique of TR detection is still being refined [1 ] it appears timely to consider practical applications in high energy hadron experiments. We therefore have proposed [2] a large area TR detector (TRD) for the European Hybrid Spectrometer (EHS) to identify pions against kaons and protons at momenta in excess o f ~ 9 0 GeV/c. Fig. 1 shows the EHS set up. The TRD is positioned in the so-called second lever arm for high energy particles (behind the bending magnet M2). The TRD will have an area o f 1 X 2 m z and will be 3.5 m long. It will consist of 20 sampling units of

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With no sufficient experience to build such a large scale detector from scratch, we have decided to set up a model detector first in order to demonstrate the efficiency o f such a device in hadronic beams at CERN. Apart from its smaller size, all essential features, as for instance the number o f sampling units, type of radiators, wire chambers and the read-out electronics, were intended to be as similar as possible to those o f the final TRD. In the following part we give a short description o f the model detector and o f the beam test results updating those given in an earlier report [3]. More detailed information can be found in two recent thesis [4,5].

D4

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Fig. 1. The layout of the European Hybrid Spectrometer (EHS) showing the position of TRD ~35 m downstream of the vertex detector RCBC. 325

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Fig. 2. A photograph of the model TRD showing the foil stacks, wire chambers and attached electronics. The first foil stack has been removed, therefore the first wire chamber is seen at the left.

As radiators to produce the TR we have chosen mylar, due to the fact that this material is easily available in optimum foil thicknesses and in any desired foil area. There are, o f course, chances that in the future better low-Z radiators become available. This means that the test results, to be given below, will represent a lower level of efficiency which hopefully can be surpassed. A photograph of the detector with its 20 sampling units is shown in fig. 2. The active area is 10 cm high and 32 cm wide. The foil stacks were produced by winding around metal frames mylar 5/am thick, interspersed at the edges with 140/2m thick paper strips (wound at 90 degree to the foils) to keep the foils at the desired distances. As TR X-rays from a stack are not fully absorbed b y the subsequent xenon chamber but are partly added to the TR produced in the following stack, it is practical to start with a large number of foils in the first radiator and then gradually decrease the foil numbers until an equilibrium is reached. Therefore in our TRD the number o f foils decreases from 960 in the first stack to 420 in the

fifth and then stays constant at 400 in the following 15 stacks. The resultant total material layer in the detector amounts to 44 mm mylar, which corresponds to ~15% of a radiation length or ~10% of a proton collision length (the rest o f other materials, e.g. xenon, being negligible). The 1 cm thick xenon riffled wire chambers (95% xenon + 5% methane) have 64 (vertical) sense wires (20/~m diameter) at 5 mm distance. Two neighbouring wires are coupled to one amplifier giving 32 cells per chamber at 1 cm horizontal resolution. 32 preamplifiers, peak sense amplifiers and 8-bit ADCs are directly plugged to each chamber, as can be seen from fig. 2. The xenon gas is continuously circulated through a closed loop system containing a gas purifier. The chamber windows, which also serve as cathodes, are made from 12 ktm thick aluminized mylar. To keep changes of the gas gain (due to bending of foils) small, the gas presure is carefully regulated to (+1 + 0.5) Pa with respect to the atmosphere. The data read-out of the 32 × 20 channels o f the detector is done by a microprocessor TM990 transfer-

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ring data via CAMAC to a PDP 11/10, which controls a dual magtape unit. To check the stability of the detector system additional calibration data are written on tape for later off-line corrections: The gas amplification and energy scale is continuously monitored by SSFe point sources fixed near wire pair 32 on each chamber. F r o m time to time a large area SSFe source is placed in front of each chamber to measure the gas gain of all wires. The linearity (slope and offset) of all 640 electronic channels is regularly measured by means of electric test pulses; no time variation larger than 1% has been observed. Applying all off-line corrections a maximum systematic error on individual pulse heights of +10% for X-rays and o f +14% for dE/dx has been found. By sampling over pulse heights o f all 20 chambers these errors are correspondingly reduced. For a larger sample of many tracks a residual error of-+0.2 keV (pulse height independent) is approached due to principal uncertainties o f the energy calibration.

3. Test in a hadron beam The model TRD has been exposed at CERN to hadrons in the momentum range from 40 to 140 GeV/c in the rf separated $3 beam (hadron beam line to BEBC) [6]. The rf separation is supplemented by two Cherenkov counters to provide tagging o f pions, kaons and protons up to a maximum momentum of ~ 1 1 0 GeV/c. Two hodoscopes measure horizontal positions of beam tracks to +5 mm at the position of the TRD. On the average a beam rate o f 2 particles per spill (duration 4 - 5 /~s) was chosen at all momenta. Table 1 summarizes the number o f recorded spills, number of particles per spill and beam purity as indi-

cated by the Cherenkov counters. Of the total sample o f some 260 000 beam particles 8% are discarded for too high multiplicity (~>6) and 22% for too small horizontal distance between neighbouring tracks (~<2 cm). 11% of the remaining sample show interactions or high energy delta rays in the radiators (from visual inspection of the hit pattern on a display). This percentage compares well with expectation for a radiator mass o f 10% o f 'an interaction length. Another 6% fail correlation between TRD and hodoscope hits. Finally, to avoid ambiguities in the data sample below 110 GeV/c momentum, only those beam particles are accepted for further analysis for which the Cherenkov tagging agrees with the rf selected particle type. Above 110 GeV/c Cherenkov information is disregarded.

4. Results Figs. 3a, b and c show pulse height spectra for protons and pions at 40, 90 and 140 GeV/c respectivley. For better comparison proton and pion samples have been normalized to equal numbers of entries on each plot. One notes that protons show typical Landau curves, whereas pion spectra become broader with increasing momentum, due to additional transition radiation. In figs. 4a, b and c (arithmetic) mean pulse heights - averaged over all 20 sampling units of the d e t e c t o r - are plotted for 40, 90 and 140 GeV/c, respectively. Again corresponding proton and pion histograms are normalized to equal particle numbers. The effect o f TR is clearly visible, e.g. at 140 GeV/c protons generate an average pulse height of 7 keV (ionization) whereas pions produce additional TR of 5 keV.

Table 1 Number of recorded beam particles and purity of the beam as tagged by the Cherenkov counters. Momentum (GeV/c)

40 70 90 110 125 140

Pions

Kaons

Spills

Particles per spill

Purity (%)

8159 9022 10485 26595 5849 14792

1.4 2.2 1.7 2.0 2.1 2.3

93.8 96.7 93.0 70.0 .

Spills

6819 1700 .

.

Protons Particles per spill

Purity (%)

Spills

Particles per spill

Purity (%)

2.3 1.4 .

90 43.7 -

16128 8751 9103 3272

2.2 1.5 1.8 1.6

95.6 98.8 90.4 97.4

10707

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Fig. 4. Pulse heights, averaged over the 20 sampling units ot the detector, for pions and protons at 40, 90 and 140 GeV/c.

The average TR produced by pions, kaons and protons in the investigated momentum range can be found in fig. 5 where the mean pulse height per chamber is plotted as a function o f the Lorentz factor T. Errors of the data points, being all equal to -+0.2 keV, have been indicated on some representative points

only. The figure clearly shows that TR is produced by pions above 40 GeV/c momentum. Kaons and protons, on the other hand, have too low T values to produce TR at the momenta studied here. The pulse heights produced by these latter particles are consistent with the expected ionization in xenon. To prove

V. Commichau et al. / Transition radiation detector

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that the large pulses by pions are really due to TR the mylar radiators have been removed from the detector during part of the runs at 90 and at 110 GeV/c. As a result, the pulse heights dropped down to the level of ionization as can be seen from fig. 5. It may be noted in fig. 5 that the 140 GeV/c proton point somewhat deviates from its expected position. Similarly in fig. 4c the same proton sample has a long tail at large pulse heights, possibly due to wrongly classified pions (or unresolved double tracks) in the beam. Before proceeding further with our TR analysis, we first have checked the purity of our data sample by means of a likelihood method [7]. In case of the 140 GeV/c protons the likelihood method provided us with the distribution of probabilities shown in fig. 6: Most of the beam particles have probabilities (of being protons) larger than 70%. Probabilities between 2% and 70% are very rare, but below 2% a narrow spike appears, containing 3.7% of the beam. We have checked that these latter events are indeed those o f the tail in fig. 4c and consequently have discarded them for being ambiguous events. The same procedure has been applied to all other beam data with the result that a few percent impurities had to be eliminated. With the purified data sample confidence levels for correct identification can be determined in the fol-

lowing way. The likelihood method produces two probability distributions at fixed momentum for two types of particles to be identified one against the other. Making different cuts on the probability axis one finds the acceptance of one kind of particle versus the contamination by the other. In this way fig. 7 has been obtained. It shows the contamination N . by pions as a function of the acceptance N p of protons and NK of kaons. (In fig. 7 the 140 GeV/c kaons have been simulated by 40 GeV/c pions having the same 7 value.) The full lines serve to guide the %

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Fig. 6. The probability distribution for the 140 GeV/c proton sample as obtained by a likelihood method. VIII. PARTICLE IDENTIFICATION

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V. Commichau et al. / Transition radiation detector i

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eye; the broken ones indicate 100% total acceptance [Np(NI0 + NTr = 100%]. It has to be noted that fig. 7 is valid for equal numbers of pions and of ka9ns or protons * Another way of presenting the information of fig. 7 has been chosen in fig. 8. Here the probability of correct identification (again on the basis of equal numbers) is shown as a function of the particle too-

mentum. The upper half-figure 8a is valid for 100% total acceptance. Lowering the acceptance to only 90% (fig. 8b) improves, of course, the identification. From fig. 8a we conclude that the 90% confidence level is reached for nip identification at ~ 8 0 GeV/c and for rr/K identification at ~ 9 5 GeV/c.

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Based on the experience obtained with the model TRD construction of the large TRD has now started. Fig. 9 shows a drawing of the assembly of the 20 radiators and wire chambers. For easy access to each unit all elements are mounted on rails. It has been found that the big radiators (free area 1 × 2 m 2) can be produced in the same way as the small ones by winding the foils around metal frames. Recent tests in an electron beam at the Bonn electron synchrotron do not show any significant difference in the TR produced by the big and by the small foil stacks ** * This equality is justified in the following cases. In hadronic events initiated by K, p or ~- beams the high momentum phase space is almost equally populated by pions and by beam-like particles (in contrast to the central region where pions do minate). ** Note added in proof: In the meantime we have tested radiators composed of loosely packed carbon fibers yielding 10% higher TR signals than the mylar foil stacks. They will therefore replace the latter ones.

V. Commichau et al. / Transition radiation detector

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0 t"-

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The wire chambers are 4 cm thick and have 96 sense wires at 19 m m distance (3 wires coupled to one pre-amplifier). To avoid losses of TR quanta in dead counter gas volumes the chamber windows (metallized mylar) act as cathodes too. To avoid bending of the windows and consequently local variations of the gas g a i n - t h e heavy xenon is diluted by helium (plus quenching gas) to arrive at a density equal to that of the outside gas. Careful control of the xenon content is required. For the same reason the chamber windows are strongly strechted (by ~ 8 0 0 N/m) and the gas pressure is regulated to

(0-+ 1) Pa. Tests on two prototype chambers haw shown promising results.

References [ 1] See e.g.W. Willis,these proceedings. [2] M. Benot et al., CERN/SPSC/79-117, SPSC/P42/Add. 6. [31 V. Commichau et al., CERN/EP/EHS/PH 79-3. [41 H. G6ddekc, Thesis (RWTH Aachen, Dec. 1979). [51 U. Ptitzhofcn, Thesis (RWTH Aachen, Dec. 1979. [6] I. Lehraus, C E R N / E F / B E A M 76-3. [7] C. Camps et al., Nucl. Instr. and Meth. 131 (1975) 411.

VIII. PARTICLE IDENTIFICATION