Friese, J.

Kunz, T.

Schmidt-Sommerfeld, K.

GSI Repository

Feasibility study for a MAPMT readout of the HADES RICH* K. Schmidt-Sommerfeld1, J. Friese†1, T. Kunz1,  1Phys. Dept. E12, TU München, Garching, GermanyThe HADES RICH is equipped with a MWPC type photon detector and a solar blind CsI photon converter. Operational since 1999, the device starts to suffer from ageing effects in gas amplification, quantum efficiency and hence e+/e- - identification power. In view of future experiments with - beam and HI induced collisions at SIS and FAIR energies [1] we have studied the feasibility of a MWPC replacement by multi anode photo multiplier tubes (MAPMTs). Driving perspective is an enhancement of the e+/e- - identification efficiency and count rate capa-bility. Since commercially available MAPMTs offer pixel sizes similar to the pad dimensions of the present detector, geometrical and granularity effects on ring recognition efficiency are expected to be negligible. Hence, the exist-ing radiator-mirror system may stay unchanged.  We have studied basic RICH performance aspects [2] utilizing the properties (QE and geometry) of Hamamatsu H8500C tubes foreseen also in the CBM RICH [3]. The bi-alkali photo cathodes shift the spectral sensitivity of the present detector from the VUV to the 200 nm <  < 600 nm region and may lead to additional background effects. In a first step, we have investigated the scintillation light yields of several purified radiator gases [2]. The gas samples were excited with 32S8+ beams (E = 90 MeV) and 241Am -particles. The spectral distributions (Figure 1) show very little emission for C4H10 but a pronounced structure around 300 nm for C4F10. The total light yield in the MAPMT sensitive region has been estimated for these gases to YC4H10 < 2 ± 1 and YC4F10 ~ 160 ± 60 photons per MeV energy deposit. As known for other cases, a few percent admixture of other gases (O2, etc.) can quench the scintillation light significantly without affecting the Che-renkov yield.  Figure 1: Spectral distributions of scintillation light emit-ted from purified C4F10 and C4H10  gas samples. Simulation results for a pure electron sample with ho-mogeneous phase space coverage show a strong increase of recognized rings preferably at lower polar angles and thereby short effective radiator lengths (see Figure 2, left), due to the high MAPMT quantum efficiency. Since scin-tillation background is negligible for reactions with low track multiplicities, the gain in e+e- - pair detection effi-ciency allows considerable signal enhancement and beam time saving for -beam experiments. In Au + Au colli-sions, however, the average energy deposit E ~ 130 MeV/event in the standard radiator gas C4F10 leads to ~ 20.000 scintillation photon hits on the detector plane. Predominantly in the area close to the beam axis they give rise to a substantial amount of fake rings. True Cherenkov rings may, however, still be discriminated to a large extent due to their significantly better ring quality parameters (pattern matrix PM QA, see right panel of Figure 2). Hence, more refined ring finder algorithms and/or selec-tion of different radiator gas mixtures are feasible options easy to realize.   Figure 2: Simulated angular distribution of Cherenkov rings for pure electrons (left) and ring qualities PM of all rings in Au + Au collisions (right) for MAPMT readout.  The MAPMT readout of the HADES RICH has very promising perspectives for the future HADES experi-mental programme. We have therefore started a joint ef-fort with the CBM RICH collaboration to elaborate on and realize this upgrade scheme within the upcoming two years.  [1] P. Finocchiaro et al., TDR HADES-100, GSI 2013 [2] K. Schmidt-Sommerfeld, Mast. Thes., TUM 2014  [3] C. Höhne et al., TDR CBM-RICH, GSI-2014-00528  ___________________________________________  * Work supported by BMBF 05P12WOGHH, GSI TMLFRG1316 †  juergen.friese@ph.tum.de GSI SCIENTIFIC REPORT 2014 MU-NQM-HADES-29DOI:10.15120/GR-2015-1-MU-NQM-HADES-29 33

Feasibility study for a MAPMT readout of the HADES RICH

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Feasibility study for a MAPMT readout of the HADES RICH

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