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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. Craig Woody Brookhaven National Lab B.Azmoun 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 Craig Woody Brookhaven National Lab B.Azmoun1, 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 N03-2 2008 IEEE Nuclear Science Symposium and Medical Imaging Conference Dresden, Germany October 20, 2008

  2. The PHENIX Hadron Blind Detector Mesh g Primary ionization HV e- CsI layer Cherenkov blobs e- Triple GEM e+ qpair opening angle Readout Pads PA ~ 1 m Gas volume filled with pure CF4 radiator Radiator gas = Working gas for triple GEM detectors • UV photons produce photoelectrons • on a CsI photocathode and are • collected in the holes of the top GEM • Operated in “Reverse Bias” where • primary ionization is drifted away from • the GEM and collected by a mesh, • photoelectrons are collected into GEM • holes and amplified C.Woody et.al., Conf. Rec., 2006 IEEE NSS/MIC C.Woody, NSS-N02-2, October 20, 2008

  3. 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 Separation depends on the primary number of photoelectrons collected by the GEMs C.Woody, NSS-N02-2, October 20, 2008

  4. 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 (not counted in the QE measured in vacuum) • 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 C.Woody, NSS-N02-2, October 20, 2008 4

  5. 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 C.Woody, NSS-N02-2, October 20, 2008 5

  6. Photoelectron Extraction Efficiency 0.82 @ 160 nm and 5 kV/cm We do observed a wavelength dependence, although not as much as predicted Monte Carlo Simulation 1700 Å 1240 Å J.Escada et.al., Conf. Rec, 2007 IEEE NSS/MIC B.Azmoun, BNL • 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 C.Woody, NSS-N02-2, October 20, 2008 6

  7. 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 • Energy of the a particle is measured with • a silicon surface barrier detector • Light is collected by a reflecting cavity • (for a “black cube”, only light collected • by geometrical acceptance) • 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 C.Woody, NSS-N02-2, October 20, 2008

  8. 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 = 9.6 ± 0.5 g/MeV 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) = 9.6 x 4.3 MeV x 0.8 x 0.83 x 0.23 = 6.3 ± 0.3 (measured in vaccum) C.Woody, NSS-N02-2, October 20, 2008 8

  9. 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 total the measured charge and gas gain • using 55Fe to determine Npe Npe = Qtot(electrons)/G C.Woody, NSS-N02-2, October 20, 2008

  10. GEM Photoelectron Yield Npe Collected Fitting Method Gain Method 4.0 ± 0.05 4.4 ± 0.03 (stat) Avg = 4.2 ± 0.2 (stat + sys) Total Collection Efficiency eC = Npe collected / Npe produced = 4.2 / 6.3 = 0.66 ± 0.06 eext = 0.82 ± 0.03 @ 160 nm, 5 kV/cm extraction field Transport Efficiency etrans = 0.66 / 0.82 = 0.80 ± 0.08 Scint .Signal [arb. Units] B.Azmoun & A.Caccavano, BNL C.Woody, NSS-N02-2, October 20, 2008

  11. 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 C.Woody, NSS-N02-2, October 20, 2008 11

  12. 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) • Some field lines go to mesh • Forward Bias (+120V) • Almost all field lines go holes J.Kamin, SUNY Stony Brook C.Woody, NSS-N02-2, October 20, 2008

  13. Possible Losses of Photoelectrons During Transport J.Escadea et.al., Conf. Rec, 2007 IEEE NSS/MIC mfp ~ 40 mm B.Azmoun, BNL 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 ? C.Woody, NSS-N02-2, October 20, 2008

  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 = 314 ± 15 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 C.Woody, NSS-N02-2, October 20, 2008

  15. Summary • We observe a lower photoelectron collection efficiency for GEMs operating in gain mode compared with parallel plate mode in CF4, even with an optimized forward bias collection field • The extraction efficiency in CF4 exhibits a wavelength dependence which decreases at shorted wavelengths • If one assumes that the extraction efficiency is the same in parallel plate and gain mode, then there are additional losses due to the transport of photoelelectrons to the gain region of the GEM • We have measured the scintillation light yield in CF4 using a CsI photocathode GEM which gives a yield of 314  15 g/MeV C.Woody, NSS-N02-2, October 20, 2008

  16. Backup Slides C.Woody, NSS-N02-2, October 20, 2008

  17. Quantum Efficiency Vessel: Vac or Gas Monitor PMT VUV Beam from Spectrometer QE measured in vacuum in parallel plate collection mode (GEM or planar PC) Beam Splitter Mesh-CsI plane PC Photocathodes are manufactured in a Clean Tent at Stony Brook University • CsI Evaporator • QE measurement inside evaporator • Large glove box for installing GEMs in HBD C.Woody, NSS-N02-2, October 20, 2008

  18. Production and Collection of Cherenkov Light Cherenkov ~ 1/ l2 Intrinsic wavelength cutoff in CF4 ~ 110 nm Absorption in gas due to impurities Loss of primary photoelectrons due to O2 and H2O absorption • HBD requires very high purity gas system (typically < 5 ppm O2, < 10 ppm H2O) C.Woody, NSS-N02-2, October 20, 2008

  19. 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 C.Woody, NSS-N02-2, October 20, 2008

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