1 / 6

Single Photoelectron Detection Efficiency

Learn about the definitions and measurements of single photoelectron detection efficiency, as well as details of the experimental set-up using GEMs and Hg lamp with CaF2 window. Understand the method of measuring efficiency and sensitivity to gain adjustments.

mickie
Download Presentation

Single Photoelectron Detection Efficiency

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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.

More Related