Friese, J.

Kunz, T.

Schmidt-Sommerfeld, K.

GSI Repository

Cherenkov photon detection with the HADES RICH in Au + Au collisions* T. Kunz†1, J. Friese1 , K. Schmidt-Sommerfeld1 1Phys. Dept. E12, TU München, Garching, GermanyIn the past years, the HADES experiment has under-gone a mayor upgrade of the detector readout and DAQ system [1] to enhance its performance stability and count rate capabilities also for collision systems like Au + Au. Within this enterprise, the GASSIPLEX based frontend electronics of the gaseous MWPC type Ring Imaging Cherenkov detector has been replaced by readout cards utilizing the APV25S1 amplifier chip [2]. The photon detector is operated with CH4 at normal pressure and equipped with a CsI photo cathode sensitive in the wave-length region 150 nm <  < 200 nm. Consequently, we have performed a dedicated study of the MWPC response to single photo electrons and updated the detector model-ling in the HADES analysis and simulation framework Hydra such as to allow for proper electron and positron identification in the recent Au + Au experimental runs. In an off-line experiment we have used 241Am α-particle induced xenon excimer emission around ~ 170 nm to illuminate the whole detector plane with a photon areal density 10ିଷ	cmିଶ <  < 10ିଵcmିଶ per emission event. Events triggered by correlated -particles were recorded with rates 0.1 kHz < R < 30 kHz for various MWPC an-ode voltages 2200 V < Uan < 2500 V. Up to ~10ସ photo electron induced signals have been recorded for each of the 28272 pads allowing for a systematic study of the detector response to single VUV photons. The zero bias pulse height distribution for each pad ex-hibits for nearly all voltages a clear single photo electron signal at higher amplitudes. A sample spectrum is shown in the left panel of Figure 1. Recorded photon signals with  Figure 1: Pulse height spectra with noise contribution (left) and for single photon signals after noise subtraction in comparison to detector simulations (right). noise suppressing 3 thresholds (Figure 1, right panel) show after noise subtraction for each photon a pad cluster size ranging from 1 to 4 pads with an average multiplicity 〈ܯ௣௔ௗ〉 = 1.3. Simulations with tuned detector model pa-rameters reproduce amplitudes and pad coupling in all aspects reasonably well. The photo electron detection efficiency varies in the range 0.88 < se < 0.94 across the whole pad plane. With a preliminary calibration of the emitted photon yield per trigger we extract a ~ 20% re-duction of the overall photon detection efficiency com-pared to values determined in experimental runs before. Cherenkov photons radiated by electrons and positrons from reconstructed ߨ଴ – Dalitz eାeି pair decays were then used to verify the photon detector response in Au + Au reactions at E = 1.23 AGeV.  In spite of the high charged particle multiplicity environment and significant background in the photon detector, Cherenkov rings could be identified and analysed. The observed signal distribu-tions of the ring and pad patterns show reasonably good agreement with the VUV lamp data and the detector mod-el simulations. Figure 2 shows as an example the pad multiplicity per ring (left panel) and the relative cluster type abundances attributed to single and multiple photon hits in the ring (right panel).  Figure 2: Pad multiplicity and cluster type abundance distributions in identified Cherenkov rings measured and simulated for reconstructed e+/e- tracks in Au + Au colli-sions.  After more than a decade of operation, the MWPC photon detector with the new frontend electronics is still capable to detect Cherenkov rings even in Au + Au colli-sions, albeit with a somewhat lower overall efficiency.  [1] J. Michel et al., IEEE Trans. Nucl. Sci. 58 (2011)  [2] L.L. Jones et al., CERN/LHCC/99-09 162   ___________________________________________  * Work supported by BMBF 05P12WOGHH, GSI TMLFRG316  † Tobias.Kunz@tum.de GSI SCIENTIFIC REPORT 2014 MU-NQM-HADES-14DOI:10.15120/GR-2015-1-MU-NQM-HADES-14 15

Cherenkov photon detection with the HADES RICH in Au + Au collisions

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Cherenkov photon detection with the HADES RICH in Au + Au collisions

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