Collection of Photoelectrons from a CsI Photocathode in Triple GEM Detectors 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 2 Stony Brook University, Stony Brook, NY Talk at the 2008 NSS/MIC in Dresden (Many thanks to the guys who did all the work !)

2 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 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

3 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

4 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 160 nm and 5 kV/cm J.Escada et.al., Conf. Rec, 2007 IEEE NSS/MIC We do observed a wavelength dependence, although not as much as predicted Plane PC - Ar

5 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 Light is collected by a reflecting cavity (or not, for a “black cube”, which then gives only the geometrical acceptance) Energy of the  particle is measured with a silicon surface barrier detector 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

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 = 160 nm, G ~ 4x10 5 ) N  = 9.62 ± 0.45  /MeV (avg. of mean and zeros method) 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) = 9.62 x 4.32 MeV x 0.8 x 0.83 x 0.23 = 6.35 pe

7 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 the total measured charge and gas gain using 55 Fe to determine N pe N pe = Q tot (electrons)/G

8 GEM Photoelectron Yield N pe Collected Fitting Method Gain Method 4.0 ± ± 0.3 Total Collection Efficiency  C = N pe collected / N pe produced = 4.2 / 6.35 = 0.66 ± 0.04  ext = 160 nm, 5 kV/cm extraction field Transport Efficiency  trans = 0.66 / 0.82 = 0.80

9 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

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  m 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 More electron recombination at the photocathode due to additional scattering/diffusion in CF 4 in the 100  m drift region ? J.Escada et.al., Conf. Rec, 2007 IEEE NSS/MIC 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

12 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

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

14 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 = ± 9.8  / 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