Photon Detector Performance and Radiator Scintillation in the HADES RICH

Transcription

1 Photon Detector Performance and Radiator Scintillation in the HADES RICH R. Gernhäuser,B.Bauer,J.Friese,J.Homolka,A.Kastenmüller, P. Kienle, H.-J. Körner, P. Maier-Komor, M. Münch, R. Schneider, K. Zeitelhack. For the HADES collaboration Physik-Department E, TU München, 8578 Garching, Germany We report the performance of a VUV-sensitive MWPC with a pad cathode covered by a solid CsI photon converter. Cherenkov photons produced by relativistic C-ions in a MgF radiator have been detected with 95% single photo electron efficiency. In the HADES design a resulting figure of merit N > 66 promises proper operation of the RICH as a fast hadron blind trigger device. In addition, we have investigated the scintillation properties of several fluoro-carbon radiator gases in the VUV wavelength region. A significant emission around λ 7 nm is found for CF. Introduction In the dilepton spectrometer HADES [], presently under construction, e + e -pairs produced in heavy ion collisions will be investigated for projectile energies up to E = A GeV at the heavy ion accelerator facility GSI (Darmstadt). A fast Ring Imaging CHerenkov (RICH) detector surrounding the target is designed to act as a hadron blind trigger device. Leptons emitted from central collisions will produce Cherenkov radiation when passing a gaseous C F radiator, while hadrons have Supported by BMBF and GSI. velocities far below the threshold. As shown in Fig. the emitted radiation is reflected by a spherical mirror and focused to rings onto a position sensitive photon detector with CaF entrance window. To match the expected collision rates of up to 6 s, this device is presently designed as a sixfold segmented, fast Multi Wire Proportional Chamber (MWPC) with a solid photon converter evaporated onto the cathode pad plane. Reflective layers of CsI have proven to allow for efficient photon conversion in the VUV wavelength region even in the presence of standard detector gases [][3][]. A detailed description of the whole HADES project and its detector components Preprint submitted to Elsevier Preprint 8 August 995

2 6 3 6 isgivenin[]. photondetector support e VUV mirror CaF -window support beam segmented target gas radiator (C F or mixture with other fluoro-carbons) Fig.. Schematic cross section of the HADES - RICH. The spherical mirror reflects Cherenkov photons from the radiator to a segmented CsI- based photon detector upstream the target (the size is given in [mm]). To estimate the photon background in the RICH, we have investigated the scintillation properties of several fluoro-carbon based radiator gases. The results are presented in chap.. In chap.3 we describe a.5m large photon detector with CsI photo cathode. Measurements of Cherenkov rings from C-ions induced radiation in MgF are presented in chap.. From these we deduce a figure of merit N and discuss the resulting perspectives for HADES. Scintillation of fluoro-carbon gases. GEANT [5] calculations simulating central 97 Au + 97 Au collisions at E kin = A GeV show that in one event up to charged particles (c.p.) are produced and give rise to a total energy deposition of E c.p. 35MeV in the radiator gas. Radiator scintillation might contribute significantly to the total radiation output and could severely shadow the identification of primary Cherenkov rings in the photon detector. We have therefore measured the scintillation light output of several purified fluoro carbons ( CF,C F 6, C F )andofch and Ar for reference purposes [6][7]. Scintillation was stimulated using proton and 6 O beams (E p =MeV, E O =8MeV) from the Munich Tandem-van-de- Graaff accelerator, and α particles from a Am source (E α =5.85 MeV). The projectiles were stopped in a gas target cell (p 5mbar) and the spectral distribution of the emitted light was analysed with a VUV-monochromator setup [8]. Within the experimental uncertainties, the observed light yield was proportional to the total energy deposition in the corresponding radiator volume. No significant influence on the emission pattern was found for the different projectile species. Experimental details and the absolute normalization method are described in [9]. As an example, we have plotted in Fig. normalized spectral distributions obtained for 6 O projectiles. Whereas CH shows no significant scintillation yield, the emission pattern of Ar reveals the well known second continuum [6] around λ 8nm. A similar peak of comparable intensity (5% with respect to Ar) is found for CF at λ 7nm, in addition to intense continua for λ>nm. Thispro- hibits the use of CF as a Cherenkov radiator, when CaF windows or

3 photons / (GeV nm) Ar CF C F 6 C F 3 The photon detector We have assembled a photon detector with quartz entrance window and A=8 8 cm active area, which, although rectangular, is similar in size to one segment of the HADES RICH. The device consists of a MWPC [] with a d = 3.8 mm wide field gap and a pad cathode (see Fig. 3). The anode wires (µm quartz UV-photon CH.5mm 3mm e.6mm.mm CsI U = V C U A = +3kV wavelength [nm] Fig.. Photon yield for various scintillating gases excited by 6 OionsofE kin = 8MeV. Intensities are normalized with respect to constant energy loss. A systematic error of about 5% has to be attributed to the absolute yield. even windowless systems are considered. The other fluoro-carbons show no strong emission in the wavelength region accessible by the CsI photo cathode. For C F 6 resp. C F as radiator gas in the HADES RICH, N det < photons/collision are expected from scintillation. The resulting number of hits is neglible with respect to the total of. cathode pads if one assumes a homogeneous illumination of the whole photon detector. pad-electrode U = V pad 6mm pad-readout AMPLEX PCB-substrate Fig. 3. Schematic view of the photo sensitive Multi Wire Proportional Chamber (MWPC) with a pad cathode covered by CsI. gold plated tungsten, 3 mm spacing) are fixed at h =. mm above the cathode pad plane. The asymmetric field configuration results in an enhanced charge fraction induced onto the cathode, which was measured to ε coup =.77. The cathode plane, segmented into 6 6 mm pads, is realized as a printed circuit board with additional chemical deposition of thin Ni/Au layers. On top of those a thin CsI layer (d=5nm) is evaporated applying the technique described in ref.[]. The 6 cathode pads are individually connected to the read out electronics on the rear side of the circuit board. Signal amplification was performed by 38 low noise 6 channel shaping amplifiers AMPLEX []. The multiplexed out- 3

4 - - - put of 3 amplifier chips was sequentially directed to the CAMAC based digitalization module DRAMS [3]. In laboratory measurements applying VUV-lamps and x-ray sources a satisfactory performance of this detector could be achieved with pure CH at p = mbar []. From the pulse height distribution obtained for single photo electrons a mean visible gain g = 3 5 and a single electron efficiency ε se > 95% could be deduced. Measurements with C- beams Photon detector pad plane C (3 A. MeV) Si-strip-counter x, y - position MgF radiator VUV mirror Si-strip-counter x, y - position Fig.. Experimental setup to detect Cherenkov rings produced by Cparticles passing a mm MgF radiator. Silicon strip detectors serve for ion tracking and ring center determination. The device was installed in a setup at GSI, Darmstadt, to detect Cherenkov photons produced by E kin 3 A MeV C-ions passing a l rad = mm thick MgF radiator. The parameters were chosen such, that intensities and radii of the Cherenkov rings were similar to those expected in the HADES RICH. The whole setup, schematically shown in Fig., was contained in a light tight aluminum box continuously flushed by purified nitrogen. The detector was operated at U A =55V with pure CH at normal conditions. A variation of the beam position revealed a homogeneous efficiency across the whole detector area. In Fig.5 four typical Cherenkov rings produced by single C-ions are displayed in an expanded view (5 5 pads) of the pad plane. With the threshold of the pad readout set to s 6e,the ring pattern is not severely affected by random hits due to noisy electron- Y-direction Y-direction X-direction X-direction Fig. 5. Sample Cherenkov rings of single incident carbon projectiles (grey scale indicates pulse height). ics and charged particle background. With respect to the low photo electron density expected, the cluster structure of the hit pattern indicates a charge spread over several pads even for single incident photons. To discriminate multiple photon hits, we attribute only those clusters to single photons, which exhibit one isolated maximum of pulse heights in an area of 5 5 pads. The pad multiplicity

5 for these events agrees reasonably well with previous measurements of single photons from a VUV-light source as shown in Fig.6. In addition, the summed pulse heights of these clusters yield a charge distribution typical for single electrons (see Fig.7). A mean visible gain g =. 5 is obtained by a Polya fit [5] to this distribution and corresponds to a single photo electron detection efficiency of ε se =95%. VUV-lamp Chernenkov photons were obtained from a center of gravity analysis around every local pulse height maximum including the multiple hit clusters. The expected radial distribution is shown for comparison. The calculation took into account the optical properties of all components either measured or given by the supplier, and the CsI quantum efficiency QE(λ) as reported in [6] [] as RD6 average. Energy loss and multiple scattering of the projectiles were included but show no significant influence. While the mean ring counts 5 5 pad multiplicity / cluster Fig. 6. Pad multiplicity for single photon hits on Cherenkov rings (open bars) and for single photons from a VUV-lamp (hatched bars). N / (ring mm) det simulation.5 measurement radius [mm] counts 3 CH : bar 5 polya fit : g =. Fig. 8. Radial photon distribution obtained for 7 rings (hatched area) in comparison to a calculation x 3 pulse height [ e ] Fig. 7. Charge distribution for single photon hits obtained by summing pulse heights of all contributing pads. The hatched curve in Fig.8 displays a normalized radial distribution of photon hits with the ring centers deduced from the tracks of the impinging ions. Photon hit positions radius and the shape of the measured distribution is reproduced by the calculation, the integrated photon yield N det 8. photons is a factor of.9 ring lower than expected. We attribute this discrepancy to a reduced effective quantum efficiency of the CsI photon converter within the accessible wavelength region (λ >65nm). For the setup in this measurement we obtain from a standard analysis [7] a figure of merit of N 3. 5

6 5 Conclusion The stable operation of the described photon detector and the favourable scintillation properties of the fluorocarbon based radiator gases C F 6 and C F give experimental evidence for a successful realization of the RICH concept in the HADES project. When using a C F radiator and assuming a reduced CsI - quantum efficiency as deduced above, a figure of merit N 66 will be achieved. At the present design status of the spectrometer an average number of 9 photons per lepton induced Cherenkov ring will be detected at small emission angles (l rad N det 6 photons ring angles (l rad = cm), and up to for larger polar 75cm). However, it has to be stressed, that due to possible ageing of the photon converter caused by not yet optimized handling procedures, no final conclusion can be drawn with respect to the HADES RICH. An average number of N det 7 photons even for small ring emission angles may be obtained when the CsI quantum efficiencies reported for a photon detector of similar size by [][8] are achieved. Therefore, further investigations and improvements in preparation and handling of the photo cathode seem inevitable. supported by BMBF References [] Proposal for a High-Acceptance Di-Electron Spectrometer (HADES Collaboration); GSI-Darmstadt, INTERNAL - REPORT (99). [] J. Seguinot et al.; Nucl. Instr. Meth. A 97 (99) 33. [3] A. Breskin et al.; Nucl. Instr. Meth. A 33 (99) [] H. Berger et al.; Nucl. Instr. Meth. A 36 (995) [5] GEANT-detector description and simulation tool, CERN program library long writeup W53 (993) [6] Ch. K. Rhodes. ;Excimer lasers Volume 3 of topics in applied physics. (98) second edition. [7] A. Pansky et al.; Nucl. Instr. Meth. A 35 (995) 6-69 [8] W. Krötz, A.Ulrich,B.Busch,G.Ribitzki,; Phys. Rev.A 3(99) [9] J.Friese et al.; TU/LMU München, Annual Report of the Accelerator Laboratory (99) 558. [] F. Piuz et al.; Nucl. Instr. Meth. A 333 (993) - [] P. Maier-Komor et al.; Nucl. Instr. Meth. A in press (995). [] E. Beuville et al.; Nucl. Instr. Meth. A 88 (99) 57 [3] E. Chesi et al.;drams, A Digital Readout of Analog Multiplexed Signals CERN PS note, (989) [] J.Friese et al.; GSI Scientific Report [5] H. Genz ; Nucl. Instr. Meth. (973) 83 6

7 [6] Status report of the CsI-RICH collaboration 99 CERN/DRDC 9-9 [7] T. Ekelöf; The experimental method of RICH counters. CERN - EP/8-68. [8] F. Piuz et al.; Nucl. Instr. Meth. A 33 (99)