The HERA-B RICH

Nuclear Instruments and Methods in Physics Research A 368 (1995) 234-238

ELSEVIER

NUCLEAR INSTRUMENTS

f”ZEZ RESEARCH

Section A

The HERA-B RICH

Thomas Hamacher * University of Texas, Austin, 7X, USA

For the HERA-B Collaboration

Abstract The HERA-B RICH system is described. The system will be made by a C4Fra gaseous radiator, a 11.4 m radius spherical

mirror and either TMAE- or CsI-based photon detectors. The overall resolution of the chamber will be good enough to separate pions from kaons on a single-track basis up to 80 GeVIc.

1. Introduction

The HERA-B [ I] experiment at DESY will study CP violation in the B-system by the decay B” --) J/ul Kf -+ l+l-a+~-. The experiment will use internal targets in the halo of the 820 GeV/c HERA proton beam and will run in parallel with the e-p collision experiments. An interaction rate of about 40 MHz is required to detect a few thousand of the B” decays per year. This interaction rate will be achieved with 4 interactions per bunch crossing every 96 ns.

For CP studies it is important to know the initial flavor of the B-meson under investigation. This is done by tagging the flavor of the other b quark produced. The most powerful tag in HERA-B is the kaon tag. Fig. 1 shows the momentum of the tag kaons. The only way to identify kaons in that momentum range is with the help of a RICH counter.

* E-mail [email protected]

B + tag

Fig. 1. The momenhlm of the tag kaons.

2. The overall detector design

A schematic view of the HERA-B RICH is shown in Fig. 2. The three principal parts of the HERA-B RICH detector are the gaseous radiator, an optical focusing system and photon detectors. As mentioned before, the HERA-B RICH needs to be capable of separating isolated kaons from pions up to 80 GeV/c at a three-standard-deviation level.

2.1. The radiator

C4Fto gas was chosen for the radiator. The criteria for the radiator in order of importance are the expected number of photons, the chromatic aberration and the difficulties in han- dling the gas. Gases like argon and nitrogen would be easier to handle but would not produce enough photons. A gas like CsFr2 would produce more photons but would have a shorter

hoton detector

Fig. 2. A schematic drawing of the HERA-B RICH.

Elsevier Science B.V. SSDlO168-9002(95)00990-6

1 Hamacher/Nucl. Insfr. and Meth. in Phys. Res. A 368 (1995) 234-238 235

absorption length and would need to be heated, which is too complicated for as large a system as the HERA-B RICH will be. The volume of the radiator tank will be 100 m3. The chromatic aberration of nitrogen and argon is smaller, but this does not compensate for the smaller number of photons and higher radiation thresholds.

A gas system for CdFia is being successfully operated at CERN in the DELPHI [2] experiment. The system for HERA-B will be designed in a similar way.

2.2. The optical focusing system

The optical focusing system is performed by a 24 m2 large spherical mirror which is divided in an upper and lower part and again subdivided into approximately a hundred hexagons with a diagonal of 70 cm. The radius-of-curvature of the mirror will be 11.4 m. The two parts are tilted with respect to the beam axis to allow the photon detector to be outside the spectrometer aperture, in order to avoid an unacceptable amount of background. Tilting the mirrors in- troduces an additional error due to spherical aberration. To keep this error small the tilt angle should not exceed 10”. The RICH environment makes it necessary to introduce a second planar mirror to place the focal plane at a position which is not occupied by other detector components.

The hexagonal mirror segments will be produced by in- dustry. Two prototypes have already been delivered and are being tested. The angular resolution of the minors is expected to be in the range of 0.1 mrad. The alignment of the individual mirror modules should be equally good.

The incident angle of the photons on the focal plane is on average twice the tilt angle of the mirror. The optimal focal plane was found with the help of ray tracing programs. Fig. 3 shows a picture of this plane.

Fig. 3. Tbe surface of least-spherical aberration: the distance (in mm) of the best focus from the center-of-curvature of the mirror as a function of the equatorial and azimuthal angles.

t-gmi - I

8mm

Fig. 4. Schematic view of the TMAE chamber.

2.3. The photon detectors

TWO types of photon detectors are under discussion. A multiwire proportional chamber (MWPC) with a CsI cathode and and a TMAE-based tube chamber (see Fig. 4).

The CsI detector is an asymmetric multiwire proportional chamber of 24 x 24 cm* dimension. Optimal operation was achieved with a high voltage of -560 V on the cathode wire plane, +1650 V on the anode wires with the CsI cathode grounded. The cathode plane is divided in pads of cell sizes of 7.5 x 7.5 mm’. The chamber gas will be pure methane at room temperature. A sketch of the detector is shown in Fig. 5.

The TMAE detector is composed of cells with cross section 8 x 8 mm* and 100 mm depth. The design is similar to the photon detectors for the JETSET [ 31 experiment at CERN. The chamber gas will be methane with a small admixture of TMAE. The long absorption path allows the TMAE concentration to be low, which is expected to de- crease aging effects. The walls of the cells are made of gold- coated bronze sheets. Investigations are under way to im- prove the wall reflectivity. By evaporating aluminum on the chamber wall, the JETSET experiment at CERN reached re- Rectivities in the 50% range and higher. Pure aluminum loses its reflectivity by oxidation in less than two weeks when ex- posed to air. Therefore the JETSET group kept the chamber under a special atmosphere. In the case of HERA-B, this will

i

-24 l

cathode wires

Fig. 5. Schematic view of the CsI chamber.

V. DETECTOR AND TRiGGER PROJECTS

236 I: Hamacher/Nucl. lnstr. and Meth. in Phys. Rex A 368 (1995) 234-238

0.6 -

0.4 -

0.2 -

0 ILIIII,,IIIIIIIII,,,I,,I,I,,

53 5.15 6 6.25 6.5 6.75 1 'I.25 75 7.15 8

photon energy (eV)

Fig. 6. Transmission curves for C4Ftt-, (7 m) [2], fused quartz (3 mm)

[ 6 ] and the reflectivity of the mirrors [ 7 1. Also shown is the quantum

efficiency of the CsI photocathode [ 81 and an optically thick TMAE gas

volume [ 91.

be difficult because wiring modules with a thousand channels or more might take longer than two weeks. Therefore we will try to use gold instead of aluminum and attempt to achieve similar results.

The photon flux will be 2 MHz/cm* in the hottest regions of the detector. Investigations are under way to test the life- time of the chambers under these conditions.

The readout will be either done with the ASD-8 [4] or with the RAL-110 [ 51 chip. Both chips are under investigation now. Final tests on the chambers will tell which chip we choose for the final design.

2.4. Detector resolution

The detector resolution is determined by the angular resolution for single photons and the square root of the number of detected photons.

The number of detected photons is given by the number of produced photons in the radiator gas multiplied with the gas transmission factor, the mirror reflectivity, the quartz window transmission factor and the chamber efficiency. Fig. 6 shows curves for the gas and window transmission and re- alistic mirror reflectivities. The expected measured photon numbers are listed in Table 1. It is important to emphasize that the numbers in Table 1 are based on the quantum effi- ciencies given in Fig. 6 and not on the measurements dis-

Table I Expected photon numbers

Photon detector

CSI TMAE

Single electron counting efficiency 0.9 0.9

Detector active area 0.75 0.75

Absorption in chamber gas screening 0.92 0.6 Number of photons (2.7 m) 40 35

Table 2

Table of overall issues of detector resolution

Track parameters

Chromatic aberration

Mirror imperfection

Minor misalignment

Spherical aberration

Chamber cell size

crfi = 0.29 x 1O-5

op = 2.15 x 10-5

crp = 0.73 x 10-5

np = 0.93 x 10-5

rrp = 1.60 x 1O-5

up = 2.10 x 10-s

cussed later on in the text. In Table 2, the factors of me angular single-photon reso-

lution are summarized. The overall single-photon resolution is then

gP = 3.6 x 10-5. (1)

The estimated final resolution is given by the single-photon resolution divided by the square root of the number of detected photons, which are 35 in the TMAE case and 40 in the CsI case.

2.5. Test beam results

To make a decision between the two proposed photon detector types, a small RICH was set up at the DESY syn- chrotron test beam (see Fig. 7). A 5-m long aluminum cylin- der flushed with argon was used as the radiator. At one end of the 80 cm diameter pipe a mirror with S-m focal length was placed. At the other end, two photon detector of the differ- ent types with a thousand channels each were installed. This setup allows a direct comparison between the two chamber types, without knowing the absolute number of photons.

The photon detectors were constructed as described above. Both chambers were flushed with methane. In the TMAE case, 40% of the gas was bubbled through liquid TMAE at room temperature. The ARGUS pVDC readout system [lo] was used for both detectors. The CsI detector used specially designed preamplifiers [ 111 (conversion gain 8.3 mV/fC, rise time 18 ns, fall time 24 ns, equivalent noise charge 710 electrons and 40 dB common mode RF suppression).

TMAE chamber

sohericol r-

_I

\, Csl c amber

length Sm

Fig. 7. Drawing of the test setup at the DESY testbeam T24 b.

‘II Hamacher/Nucl. Instr. and Meth. in Phys. Res. A 368 (1995) 234-238 231

Fig. 8. Sum of Cherenkov rings in the CsI and TMAE chamber.

2.5. I. Number of photoelectrons per ring The average number of detected Cherenkov photons per

incident beam electron was 6.2 photons for the CsI chamber and 10.2 photons for the TMAE chamber always cor- rected to the whole ring. Fig. 8 shows a sum of events for both chambers. The setup is not yet understood to the point where we can predict the expected number of observed photons and from that extract the efficiency of the chambers; the reflectivity of the mirror was not measured nor is the transmittance of the gas known to sufficient precision. Work to understand these factors is under way. A new mirror with known reflectivity is being purchased and the transmission measurement apparatus will be upgraded to allow measurement in the 1% range.

2.5.2. Time spread The chambers are required to have a response time shorter

than the 96 ns duty cycle of the experiment. Fig. 9 shows the drift time distribution for both chambers which fulfill this requirement. The drift time distribution tells us in addition that there is practically no photon conversions on the walls of

140 160 180 200 220 240 260 280 &

lime(m)

6000 t

5000 -

4000 -

3000 -

2000 -

1000 -

140 I60 IS0 200 220 240 260 280 300

time(m)

Fig. 9. Drift time distribution in the TMAE and C.4 chamber. Both chamber

were flushed with methane.

the chamber. Therefore, we conclude that no TMAE vapour condensed on the chamber walls.

2.53. Angular resolution The angular resolution of the chamber is determined by

the cell size. The pad size is 7.5 x 7.5 mm* in the CsI case and 8 x 8 mm* in the TMAE case. Fig. 10 shows the measured distribution of Cherenkov radii for the TMAE chamber. The circle center was found with the help of the tracking chambers. The width of the distribution is found to be u = 0.82 mrad which is close to the quadratic sum of the chamber cell size (~~11 = 0.46 mrad), multiple scattering of particles in the radiator( rm”lti = 0.62), resolution of particle track direction ( Utik = 0.2 mrad) and mirror imperfections

( urniron = 0.2 mrad).

2.5.4. Clusters of hits To gain information on possible cross-talk between cells,

clusters of neighbouring hits were analyzed. The cell with the shortest drift time was chosen as the cluster center. Fig. 11 shows a distribution of clusters for both chambers. It can be seen that only a small fraction of events have clus-

V. DETECIIOR AND TRIGGER PROJECTS

238 I: Hamacher/Nucl. Inar. and Meth. in Phys. Res. A 368 (1995) 234-238

200

0 25 50 75 100 125 I50 175 200 225

r@W Fig. 10. Distribution of the Cherenkov ring radius as measured by single photons in the TMAE chamber.

ters with two or more hits; these are roughly consistent with the expected number of neighbouring single-photon hits.

3. Conclusion and outlook

The resolution that will be achieved with the HERA-B RICH is sufficient to separate single kaons from pions on a three-standard-deviation level up to 80 GeV/c. ‘TLvo photon detectors types are operational and are under further investigation, especially concerning their aging properties. The other detector components are well understood and are within reach of today’s engineering capabilities.

References

[ I ] T. Lohse et al., Proposal for the HERA-B, DESY-PRC 94102 (May 1994);

Fig. Il. Cluster distribution. Two-dimensional diseibution of hits in a cluster relative to the hit with the shortest drift time.

See also HERA-B papers in the Beauty ‘94 and Beauty ‘95 Workshop Roceedings (Nucl. Insu. and Meth. A 333( 1) (1993) and Nucl. Insu. and Meth. A 351(l) (1994)).

[2] G. Lenzen et al., Nucl. Insu. and Meth. A 343 (1994) 268. [ 31 B. Rrth et al., Nucl. Insa. and Meth. A 3 ll(1992) 484. 141 F. M. Newcomeret al., IEEETrans. Nucl. Sci. NS-40(4) (1993) 630. [5] M. French et al., Nucl. Insa. and Meth. A 324 (1993) 511. [6] R. Arnold et al., Nucl. Instr. and Meth. A 270 (1988) 255. [7] R. 1. Apsimon et al., Nucl. Instr. and Meth. A 241 (1985) 339. [ 81 J. Scguinot et al., Nucl. Instr. and Meth. A 297 (1990) 133. [9] R. Holroyd et al., paper 6408, submitted to the 23th Int. Conf. High

Energy Physics, Berkeley. CA, July 1986. [IO] E. Michel et al., Nucl. Instr. and Meth. A 283 (1989) 544. [ 111 M. Zavrtanik. M.Sc. thesis, University of Ljubljana (1994);

R Krizan et al., NSS, 1994 IEEE Conf. Record. Vol. 2, p. 985.

The HERA-B RICH

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