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 2 Stony Brook University, Stony Brook, NY N IEEE Nuclear Science Symposium and Medical Imaging Conference Dresden, Germany October 20, 2008

C.Woody, NSS-N02-2, October 20, The PHENIX Hadron Blind Detector Cherenkov blobs e+e+ e-e-  pair opening angle ~ 1 m Gas volume filled with pure CF 4 radiator Radiator gas = Working gas for triple GEM detectors Mesh CsI layer Triple GEM Readout Pads e-e- Primary ionization g HV PA 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, Single and Double Electron Separation in the HBD Separation depends on the primary number of photoelectrons collected by the GEMs = 25 Photoelectrons Cut = 15 Single electron Double electron Combined spectrum Photoelectrons Cut Z.Citron, SUNY Stony Brook

C.Woody, NSS-N02-2, October 20, Photoelectron Production and Collection In the HBD, Cherenkov light produced in radiator  N  Amount of light reaching the photocathode is limited by the transmission of the gas (intrinsic UV cutoff, impurities) N pe produced = N  x QE of CsI N pe collected = number of primary p.e. entering the gain region of the GEM and contributing to the final charge collected Total Photoelectron Collection Efficiency :  C = N pe collected / N pe produced =  ext x  trans Extraction efficiency  ext (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  trans 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, Total Collection Efficiency Use a calibrated light source (“Scintillation Cube”) to produce a know flux of UV light on the CsI photocathode  N pe produced Measure the number of photoelectrons collected that contribute to final signal from the GEM  N pe collected Measure the extraction efficiency  ext 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  trans

C.Woody, NSS-N02-2, October 20, Photoelectron Extraction Efficiency Monte Carlo Simulation  ext (,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 J.Escada et.al., Conf. Rec, 2007 IEEE NSS/MIC 160 nm and 5 kV/cm We do observed a wavelength dependence, although not as much as predicted B.Azmoun, BNL 1700 Å1240 Å

C.Woody, NSS-N02-2, October 20, Scintillation Cube Lucite with Al/MgF 2 coating CF 4 has a strong scintillation emission at 160 nm Use this as a calibrated light source  particles from an 241 Am source traverse ~ 1 cm of CF 4 gas depositing several MeV Energy of the  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) 55 Fe 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, 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 = 160 nm, G ~ 4x10 5 ) N  = 9.6 ± 0.5  /MeV Place this cube on top of a CsI photocathode GEM and measure the number of photoelectrons collected N pe produced = N  incident x T mesh x T GEM x QE GEM (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, Photoelectrons Collected by the GEM Measure N pe collected using 2 methods: 1.Fitting method Fit the shape of the measured spectrum to a convolution of a Poisson (primary N pe ), gain fluctuation of the GEM (Polya distribution), and measured Gaussian pedestal 2.Gain method Use total the measured charge and gas gain using 55 Fe to determine N pe N pe = Q tot (electrons)/G

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

C.Woody, NSS-N02-2, October 20, Photoelectrons Lost to the Mesh Transport efficiency depends strongly on voltage between mesh and GEM 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 T.Hemmick, SUNY Stony Brook

C.Woody, NSS-N02-2, October 20, D Maxwell Simulation of the Electric Field at the GEM Field at the GEM surface  5 KV/cm Collection region for photoelectrons is within ~ 100  m 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, Possible Losses of Photoelectrons During Transport More electron recombination at the photocathode due to additional scattering/diffusion in CF 4 in the 100  m drift region ? Resonance in electron capture cross section for CF 4 at ~ 7eV Measurements at different drift lengths indicate no observable loss due to capture mfp ~ 40  m J.Escadea et.al., Conf. Rec, 2007 IEEE NSS/MIC B.Azmoun, BNL

C.Woody, NSS-N02-2, October 20, Scintillation Light Yield in CF 4 As a byproduct of our measurements of the photoelectron collection efficiency, we have measured the absolute scintillation light yield of CF 4 using a CsI photocathode GEM Variable distance between Am source and SBD Variable distance between light source and GEM Scintillation Yield = 314 ± 15  / MeV Preliminary results reported last year: B.Azmoun et.al., Conf. Rec IEEE NSS/MIC A.Pansky et.al., Nucl. Inst. Meth. A354 (1995) Y= 250 ± 50  /MeV with PMT

C.Woody, NSS-N02-2, October 20, Summary  We observe a lower photoelectron collection efficiency for GEMs operating in gain mode compared with parallel plate mode in CF 4, even with an optimized forward bias collection field  The extraction efficiency in CF 4 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 CF 4 using a CsI photocathode GEM which gives a yield of 314  15  /MeV

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

C.Woody, NSS-N02-2, October 20, Quantum Efficiency 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 QE measured in vacuum in parallel plate collection mode (GEM or planar PC) VUV Beam from Spectrometer Beam SplitterMesh-CsI plane PC Monitor PMT Vessel: Vac or Gas

C.Woody, NSS-N02-2, October 20, Production and Collection of Cherenkov Light Cherenkov ~ 1/ 2 Intrinsic wavelength cutoff in CF 4 Absorption in gas due to impuritiesLoss of primary photoelectrons due to O 2 and H 2 O absorption ~ 110 nm  HBD requires very high purity gas system (typically < 5 ppm O 2, < 10 ppm H 2 O)

C.Woody, NSS-N02-2, October 20, Photoelectron Yield for the HBD Yield = convolution of:  N  produced in Cherenkov radiator (50 cm CF 4, N  /d ~ 1/ 2 )  Absorption in gas (cutoff ~ 108 nm, ppms of O 2 and H 2 O)  Transparency of mesh (0.9) and GEM (0.80)  GEM QE (~ 1/ from 200 nm  108 nm)   C =  ext (,E) x  trans (E)  Pad threshold (readout electronics and cluster reconstruction) Measured 14 p.e. per single electron in RHIC Run 7 ~ 40 ppm H 2 O, ~ 5 ppm O 2 -30V negative bias Expect ~ p.e. in upcoming Run 9 < 10 ppm H 2 O with higher gas flow, < 5 ppm O 2 ~ -10V negative bias Better clustering algorithm