Single photon detection with H8500 MAPMTs for the CBM RICH detector*

Single photon detection with H8500 MAPMTs for the CBM RICH detector* Christian Pauly for the CBM collaboration Introduction The CBM experiment The CBM RICH detector Single Photon detection with H8500 MAPMTs Tests on few individual tubes Series test on 12x H8500C H8500C in magnetic field Summary + outlook * supported by BMBF 06WU9195I Slide 1

The CBM experiment at FAIR Compressed Baryonic Matter The CBM experiment gold-ion beam, 2 15 AGeV (SIS100) 2 90 AGeV (SIS 300) Fixed target (gold) Beam intensity: up to 109 ions/sec first beam 2018 GSI Darmstadt, Germany Slide 2

CBM physics CBM goal: Explore the landmarks of the QCD phase diagram at high net-baryon densities in Au-Au collisions from 2-45 AGeV First order phase transition Chiral restoration at high baryon densities QCD critical endpoint CBM approach: focus on dilepton probes originating from early, high density phase of collision ρ, ω, Φ J/Ψ, Ψ' e + e e+ e- Experimental challenges: rare probes! Suppressed by 4 orders mag. partonic matter hadronic matter High luminosity Up to 10MHz reaction rate Good particle ID / pion suppression CBM RICH detector Slide 3

The CBM RICH detector Aim: clean electron identification below 8 GeV/c pion suppression 500-1000 from RICH alone, 104 together with TRD concept: CO2 focusing gas RICH detector with PMT readout behind magnetic field, tracking detector Photodetector: Hamamatsu H8500 MAPMT (present design assumtion) 2.4 m2, ca 900 pc, 55k channels UV-transparent window CBM RICH Dipol Mirror: 11.8 m2 spherical mirror,f0=3m segmented mirror, ~100 rect. tiles Al+MgF2 reflective cover on glas, <6mm thick Radiator: CO2: γthr = 33, p π-thr = 4.65 GeV/c 2mbar overpresssure, dryed and cleaned MVD+STS 21 Np.e. / ring (electrons) N0 = 130-140 γ ca 10% X0 Slide 4 TRD

The test candidates: H8500C-03: 12 stage, 64 channels, UV-window size: 52x52 mm^2, 89% effective area typ. gain: 1.5*10^6, 0.25 pc per photon time res.: better 1ns, risetime 0.8ns only available in standard- Bialcali, ca 26% max q.e. for technical reasons (Hamamatsu representative) not explicitly recommended for single photon counting due to larger gain-inhomogeneity within pixels, broader SPE-spectrum H8500C H8500D H10966A H8500C-100 mod8: (new name: H10966A) 8 stage modification, similar to H8500C typ. gain: 0.33x10^6, 0.05pC per photon available with Super-Bialcali cathode, ca 35% max q.e. for comparison: R8900U-100: R8900-100 10 stage single channel, available also as 16x MAPMT, similar pixel size as H8500 size (16ch): 26x26mm², 80% effective area typ. gain: 1.0x10^6, 0.16 pc per photon Super-Bialcali cathode recommended alternative for Single-photon counting by Hamamatsu Slide 5

Photon detection with H8500C-03 Slide 6

Photon detection with H8500CMOD8-100 Slide 7

Photon counting R8900 H8500C (12 / 8 stage) illumination with identical photon intensity from pulsed LED source (470nm): R8900 single channel active area: 23.5x23.5 mm² R8900 SBA cathode quantum eff.: 28% (470nm, measured) 19.9 photons / mm² Geometric coverage: 80% H8500C Sum of 8 diagonal pixel active area: 49x49mm² * 8/64 H8500C 8-stage SBA cathode quantum eff: 25% (470nm, measured) 14.6 photons / mm² H8500C 12-stage normal cathode quantum eff: 20% (470nm, measured) 14.1 photons / mm² Geometric coverage: 89% Slide 8

Series test: 12x H8500C Comparison of single-photon spectra from 12 different H8500C MAPMT (12-stage) single photons from pulsed LED source (470nm), photon registered per pixel in 5% of events constant light intensity for all PMTs confirmed using reference PMT Su 7 d m spe iag ona ctra f l pi or xel Slide 9

Series test: 12x H8500C Comparison of single-photon spectra from 12 different H8500C MAPMT (12-stage) single photons from pulsed LED source (470nm), photon registered per pixel in 5% of events constant light intensity for all PMTs confirmed using reference PMT all individual pixel spectra gainnormalized Su 7 d m spe iag ona ctra f l pi or xel Slide 10

Photon detection efficiency Comparison of photon detection for 12x H8500C Equal illumination of all 12 PMTs with pulsed LED source (470nm) Three different counting thresholds: 30%, 50% and 70% of single photon amplitude Low threshold necessary for good efficiency! smooth variation of quantum efficiency by 20% with Serial-Nr!!! (PMTs were measured in different, random order!) Slide 11

individ. efficiency for diagonal pixel Lower efficiency partly caused by reduced efficiency in center pixels some difference also seen by eye, probably connected to Bialcali coating (maybe thickness variation?) Slide 12

Setup for testing magnetic field effects Two operation modi: Max 15 mt (5 A per coil) (limited by powersupply, 300W) MAPMT Free Ø: 30cm Pulsed LED fixed to PMbox Large pulses, ~500 ph-e- : study effect on overall gain Small pulses, single ph-e- : study detection efficiency calibration using Hall-probe -> very uniform B-field at PM pinhole mask Slide 13

Effect of magnetic field H8500C gain and single-photon efficiency Open symbols: signal charge (for ~1000photons) full symbols: single photon efficiency (lower right plot) all courves normalized to 100% at B=0mT significant effect already at few mt field strong effect on edge pixels very strong effect on corner pixels AP M M T Y X Z similar effect on gain and efficiency magnetic field effects the low energetic photo-electrons between cathode and 1st dynode. Slide 14

Summary and outlook According to present design we plan to use Hamamatsu H8500C (12-stage) Multi-anode PMTs for CBM-RICH photo detector Tests of few samples look promising, single-photon detection well possible with good performance. Only major drawback : reduced quantum efficiency due to non-available SBA-cathode Small series test (of 12 pieces) show some performance variations which need further investigation Operation in magnetic fields >>few mt will need extensive shielding efforts Outlook: Full scale prototype beamtest with gas-radiator in Nov. 2011 at CERN Using 16 MAPMTs in the photo detector 4x4 MAPMTs will be mounted on PCB board, readout electronics on backside Slide 15

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spares Slide 17

The CBM detector electron setup Slide 18

1000V 1100V (max) ge 900 V sta 800 V H8500Cmod8 8stage 12 H8500C 12stage How critical is the HV value / gain? age t s 8 Slide 19

Always same light intensity Photons / dark counts for given threshold 1 channel active area: 23.5x23.5 mm² SBA-cathode q.e.@470nm : 28% < 0.9 dark counts /s /mm² Sum of 8 diagonal pixel active area: 49x49mm² *1/8 8-stage SBA-cathode q.e.@470nm : 25% 12-stage normal cathode q.e.@470nm: 20% < 2.7 dark counts /s /mm² < 100 dark counts /s /pixel Slide 20

Effect of 0.2 mm μ-metal shielding Ø 8cm Two B-field directions: - along mumetal shield (black) - perp. to μ-metal shield (red) open symbols : with shield filled symbols : no shield B shield = 2.5 d cyl H =8.5 T 2 t shield Ca 10 x saturation value for tμ-shield= 0.2 mm, H=15mT Slide 21

Dependence on signal amplitude Signal Amplitude : 20% 50% 100% ca 200 500 1000 photo-e- minor charge dependance Slide 22

Comparison H8500C 8 vs 12 stage Two graphs per color: H8500C 12-stage : open symbols H8500Cmod8 : closed symb. similar behaviour 8-stage 12-stage Y X Z Slide 23