GLAS-PPE/97-14December 1997
RICH detectors meet the experimental criteria for particle identification in the required momentum range. It is proposed to use two RICH detectors in the LHC-B experiment, see fig 1. The upstream detector (RICH1), fig. 2, has a combined gas and aerogel radiator and is situated in front of the dipole magnet. The aerogel radiator is placed against the entrance window of the second gaseous (C4F10) radiator. A spherical mirror with a radius of curvature of 190 cm is tilted by » 250 mrad to reflect the Cherenkov light onto an array of photodetectors situated outside the experimental acceptance. The downstream RICH (RICH2), fig. 2, uses CF4 as its radiator with a spherical mirror with a radius of curvature of 820 cm tilted by 370 mrad. An additional flat mirror is tilted by 240 mrad to bring the Cherenkov photons out of the acceptance of the experiment.
Each RICH detector has two photodetector planes giving a total area of 2.9 m2. The chosen detector technology must have a high quantum efficiency, a spatial resolution of at least 2.5 ×2.5 mm2 and to have a fast readout consistent with 25 ns bunch crossing of the LHC. Two candidate technologies exists [3]: hybrid photodiodes (HPDs) and multianode photomultipliers. These detectors are commercially available but not in designs that meet the experimental needs of LHC-B. A program of R&D is currently underway on the development of a HPD with a large active area.
The SCT-128A [8] analogue chip which was developed for the ATLAS silicon tracker is being modified to achieve a noise level of ~ 600 e-. Additional modifications will be needed to meet the requirements of the pad HPD for LHC-B, in particular the multiplexing properties of the chip.
A prototype of the downstream RICH detector was tested in the T9 test beam at the CERN SPS during the Spring and Summer of 1997. A planar array of seven 61-pixel HPD's from DEP were used to detect the Cherenkov photons produced in aerogel, air and C4F10 radiators. In configuration-1, fig. 3, of the prototype the light is focussed by a 240 mm focal length mirror which corresponds to a 1/4 scale of the RICH1 detector. A full scale prototype (configuration 2) was also used which has a 1143 mm mirror to focus rings from C4F10 onto an array of six 61-pixel HPD's. This was achieved by adding extension arms to configuration 1.
The 61 pixel HPD has a silicon diode detector segmented as a hexagonal array with pad dimensions of 2 mm face-to-face. The HPD was operated at a high voltage of 12kV. Using a pulsed light emitting diode the complete readout and data acquisition chain was tested. The pedestal, the single, double and triple photoelectrons peaks were clearly visible with a signal/noise ratio of » 5.7. Most of this noise is associated with the input capacitance of the feedthrough and printed circuit boards.
The test beam provides charged particles of either polarity and the momentum can be tuned in the range 2-15.5 GeV/c. The particle type is identified by measuring the signal pulse height from a CO2 threshold Cherenkov counter installed 30 m upstream from the prototype. The prototype vessel was aligned with the beam axis. Charged particles which provide the trigger are selected using using scintillation counters, two upstream and two downstream of the vessel. A photoelectron hit is defined to be a HPD pixel with a signal pulse height 4s above the pedestal mean, where s is the rms width of the pedestal peak.
Using RICH configuration 1, data were taken with a 10 GeV/c negatively charged beam with 18 mm thickness of aerogel. Fig. 4 shows an arc of a ring on the central HPD, whose radius is compatible with that expected from C4F10. The outer HPD's clearly exhibit a ring which originates from the aerogel radiator.
The number of photoelectrons per triggered event was measured for all three radiators in the vessel. For this analysis a threshold of 3s was set for individual pixels and multiple photoelectrons were taken into account. The mean number of photoelectrons are shown in table 1. The partial geometrical coverage of the aerogel and gas rings was calculated from simulation with » 5% uncertainity. The expected photoelectron yields was calculated from simulation which included the properties of the aerogel, mirror and photocathode efficiencies. The overall precision in the expected yield is estimated to be 15%. The comparison between observed and expected yields are given in table 1. The numbers from this preliminary analysis are compatible within 30%.
| Radiator | Raw | Bkg. | Eff. | Ratio |
| hits | corr. | corr. | ||
| Air | 4.92 | 4.56 | 4.80 | 0.99 |
| C4H10 | 7.85 | 7.49 | 33.55 | 1.07 |
| Aerogel | 1.79 | 1.31 | 10.71 | 0.72 |
The full scale RICH1 prototype was studied using configuration-2. The longer focal length of the mirror means the C4F10 ring now spans the outer 6 HPDs. The event display shown in fig. 5 is obtained from negatively charged 15.5 GeV/c momentum beam. The K:p ratio of the triggering particles has been enhanced to 1:2 using the threshold Cherenkov counter. Fig. 5 shows segments of two rings; an inner ring from the incident kaons and an outer ring from the pions. It can be seen that the number of hits observed in HPD 3 is lower than in HPD 4. (Similarly HPD 5 has fewer hits than HPD 2.) This is because HPD's 3 and 5 have mylar windows in front of their photocathodes which absorb the UV photons.
The RICH prototype tests have been successful. Clear Cherenkov rings from gas and aerogel radiators have been observed for the first time using HPD's as photon detectors. The preliminary measured photon yields are compatible within 30% of expectations based on simulations. Further analysis of the data is investigating the reconstruction resolution of the Cherenkov angle for each recorded hit from both the gas and aerogel radiators. Further prototype testbeam runs are being planned to study RICH 2. It is also planned to use the RICH prototype to test the various photodetectors as they become available.