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Collection of Photoelectrons from a CsI Photocathode in Triple GEM Detectors

Collection of Photoelectrons from a CsI Photocathode in Triple GEM Detectors. Talk at the 2008 NSS/MIC in Dresden. C. Woody B.Azmuon 1 , A Caccavano 1 , Z.Citron 2 , M.Durham 2 , T.Hemmick 2 , J.Kamin 2 , M.Rumore 1 1 Brookhaven National Lab, Upton NY

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Collection of Photoelectrons from a CsI Photocathode in Triple GEM Detectors

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  1. Collection of Photoelectrons from a CsI Photocathode in Triple GEM Detectors Talk at the 2008 NSS/MIC in Dresden C. Woody B.Azmuon1, A Caccavano1, Z.Citron2, M.Durham2, T.Hemmick2, J.Kamin2, M.Rumore1 1 Brookhaven National Lab, Upton NY 2 Stony Brook University, Stony Brook, NY (Many thanks to the guys who did all the work !)

  2. Photoelectron Production and Collection • In the HBD, Cherenkov light produced in radiator  Ng • Amount of light reaching the photocathode is limited by the • transmission of the gas (intrinsic UV cutoff, impurities) • Npe produced = Ng x QE of CsI • Npe collected = number of primary p.e. entering the gain region • of the GEM and contributing to the final charge collected • Total Photoelectron Collection Efficiency : • eC = Npe collected / Npe produced = eext x etrans • Extraction efficiency eext • Backscatter to the photocathode by the gas • Occurs very close (few mfp) to the photocathode • Transport efficiency etrans • Loss of photoelectrons (after the first few mfp) while traveling • to the holes of the GEM where amplification occurs

  3. Total Collection Efficiency • Use a calibrated light source (“Scintillation Cube”) to produce • a know flux of UV light on the CsI photocathode •  Npe produced • Measure the number of photoelectrons collected that contribute • to final signal from the GEM •  Npe collected • Measure the extraction efficiency eext with a CsI coated • GEM in a UV spectrometer in parallel plate collection mode • where we can verify that we are collecting all of the charge • Assume the extraction efficiency is the same for a GEM operating • in parallel plate mode and normal gain mode to determine etrans

  4. Photoelectron Extraction Efficiency Monte Carlo Simulation Plane PC - Ar J.Escada et.al., Conf. Rec, 2007 IEEE NSS/MIC • eext(l,E) : • Depends strongly on the extraction field • Quickly rises to 100% in vacuum • Slower rise to lower efficiency in gas • due to backscatter of photoelectrons • off of gas molecules • Plateau value depends on gas 0.82 @ 160 nm and 5 kV/cm We do observed a wavelength dependence, although not as much as predicted

  5. Scintillation Cube Lucite with Al/MgF2 coating CF4 has a strong scintillation emission at 160 nm Use this as a calibrated light source • a particles from an 241Am source traverse • ~ 1 cm of CF4 gas depositing several MeV • Light is collected by a reflecting cavity • (or not, for a “black cube”, which then • gives only the geometrical acceptance) • Energy of the a particle is measured with • a silicon surface barrier detector • 55Fe source mounted to base of cube allows • simultaneous measurement of the gas gain • One of these devices is installed in • each of half of the HBD to monitor the • QE of the photocathodes • Also use scintillation produced by • MIPs to measure gain of each pad

  6. Photoelectrons Produced at the GEM Measure the absolute photon flux from the cube using a calibrated CsI photocathode PMT with known gain and QE (Hamamatsu R6835, QE = 8.2% @ 160 nm, G ~ 4x105) Ng = 9.62 ± 0.45 g/MeV (avg. of mean and zeros method) Place this cube on top of a CsI photocathode GEM and measure the number of photoelectrons collected Npe produced = Ng incident x Tmesh x TGEM x QEGEM(160 nm) = 9.62 x 4.32 MeV x 0.8 x 0.83 x 0.23 = 6.35 pe

  7. Photoelectrons Collected by the GEM • Measure Npe collected using 2 methods: • Fitting method • Fit the shape of the measured spectrum to a • convolution of a Poisson (primary Npe), gain • fluctuation of the GEM (Polya distribution), and • measured Gaussian pedestal • Gain method • Use the total measured charge and gas gain • using 55Fe to determine Npe Npe = Qtot(electrons)/G

  8. GEM Photoelectron Yield Npe Collected Fitting Method Gain Method 4.0 ± 0.3 4.4 ± 0.3 Total Collection Efficiency eC = Npe collected / Npe produced = 4.2 / 6.35 = 0.66 ± 0.04 eext = 0.82 @ 160 nm, 5 kV/cm extraction field Transport Efficiency etrans = 0.66 / 0.82 = 0.80

  9. Photoelectrons Lost to the Mesh Transport efficiency depends strongly on voltage between mesh and GEM T.Hemmick, SUNY Stony Brook For our efficiency measurements, the field was always optimized for maximum collection (~ +100 V/cm) For the HBD, we operate at a slight negative bias (~ -200 V/cm) which reduces the transport efficiency

  10. 3D Maxwell Simulation of the Electric Field at the GEM • Field at the GEM surface  5 KV/cm • Collection region for photoelectrons • is within ~ 100 mm of the surface • Reverse Bias (-30V) • ~ 3% of the field lines go to mesh • ~ 6% of field lines go to bottom GEM • Forward Bias (+120V) • Negligible number of field lines go to mesh • ~ 6% go to bottom of GEM J.Kamin, SUNY Stony Brook

  11. Possible Losses of Photoelectrons During Transport J.Escada et.al., Conf. Rec, 2007 IEEE NSS/MIC mfp ~ 40 mm Measurements at different drift lengths Indicate no observable loss due to capture Resonance in electron capture cross section for CF4 at ~ 7eV More electron recombination at the photocathode due to additional scattering/diffusion in CF4 in the 100 mm drift region ?

  12. Photoelectron Yield for the HBD • Yield = convolution of: • Ng produced in Cherenkov radiator (50 cm CF4, Ng/dl ~ 1/l2) • Absorption in gas (cutoff ~ 108 nm, ppms of O2 and H2O) • Transparency of mesh (0.9) and GEM (0.80) • GEM QE (~ 1/l from 200 nm  108 nm) • eC = eext(l,E) x etrans(E) • Pad threshold (readout electronics and cluster reconstruction) • Measured 14 p.e. per single electron in RHIC Run 7 • ~ 40 ppm H2O, ~ 5 ppm O2 • -30V negative bias • Expect ~ 20-25 p.e. in upcoming Run 9 • < 10 ppm H2O with higher gas flow, < 5 ppm O2 • ~ -10V negative bias • Better clustering algorithm

  13. Single and Double Electron Separation in the HBD <Npe> = 15 <Npe> = 25 Single electron Double electron Combined spectrum Cut Cut Photoelectrons Photoelectrons Z.Citron, SUNY Stony Brook

  14. Scintillation Light Yield in CF4 As a byproduct of our measurements of the photoelectron collection efficiency, we have measured the absolute scintillation light yield of CF4 using a CsI photocathode GEM Scintillation Yield = 320.8 ± 9.8 g / MeV Preliminary results reported last year: B.Azmoun et.al., Conf. Rec. 2007 IEEE NSS/MIC A.Pansky et.al., Nucl. Inst. Meth. A354 (1995) 262-269 Y= 250 ± 50 g/MeV with PMT • Variable distance between Am source and SBD • Variable distance between light source and GEM

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