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Single Photoelectron Detection Efficiency

Single Photoelectron Detection Efficiency. Itzhak Tserruya HBD meeting, June 12, 2007. Definitions.  det : single photoelectron detection efficiency.

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Single Photoelectron Detection Efficiency

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  1. Single Photoelectron Detection Efficiency Itzhak Tserruya HBD meeting, June 12, 2007

  2. Definitions det : single photoelectron detection efficiency. extr: due to collisions with the gas molecules, a fraction of the produced photoelectrons can backscatter to the photocathode and be reabsorbed. hole: probability that the extracted photoelectron drifts into the GEM holes. trans: probability that at least one electron is transferred to the next amplification stage (trans = 0 or 1). In our case where ED = 0, det depends on VGEM and Etrans. det = extr hole trans • A measurement of the absolute single photoelectron detection efficiency requires a measurement of the number of detected photoelectrons as well as the number of produced photoelectrons. • This was not easily possible in our set-up and we therefore decided to measure the relative efficiency rather than the absolute one.

  3. Experimental set-up • 3x3 cm2 GEMs produced at CERN • All gap distances = 1.5 mm • Hg lamp positioned above a CaF2 window on the cover of the detector box with an absorber to reduce the photon rate to about 10KHz. • Mesh and top face of GEM1 connected to the same PS (to ensure ED = 0) that defines VGEM1-top • Bottom face of GEM1 connected to an independent PS that defines VGEM1-bottom and Etrans1 • Control the overall gain by adjusting the voltage in the triple stack GEM2,3,4. • One single pad 3x3cm2 connected to a charge sensitive PA

  4. The method • Measure the rate of detected photoelectrons as function of VGEM and Etrans • Since the pulse height of a single photoelectron has an exponential shape small changes in the gain can cause large fluctuations on the counting rate. • Keep the gain constant by adjusting the HV in the triple GEM2,3,4 stack and the discriminator threshold constant so as to count always the same fraction of photoelectrons. • The slope of the pulse height distribution can be adjusted to better than 10%. The corresponding difference in the counting rate for the two case shown in the figure is ~4%. Sensitivity to gain adjustment

  5. Photocathode current as function of the GEM1-mesh voltage GEM1 gain GEM1 gain • Gain measurement of GEM1: • mesh and top of GEM1 connected together • current I measured at the interconnected bottom GEM1 and top GEM2. • gain determined as the ratio of I to the maximum current measured at the photocathode in vacuum.

  6. Single photoelectron detection efficiency • The detection efficiency as expected increases with VGEM1 and with Etrans, reaching saturation at VGEM1 > 480V and Etrans > 2.7 kV/cm. In regular operation where Vtrans = VGEM≈ 500 V, the transfer field is 3.3 kV/cm. • The absolute detection efficiency in CH4 was measured to be ~1 at a gain of 20. The similar rates measured with CF4 provides a proof of full detection efficiency also in CF4.

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