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Studies of the Quantum Efficiency of Prepared CsI Photo-cathodes in the Presence of CF 4.

Studies of the Quantum Efficiency of Prepared CsI Photo-cathodes in the Presence of CF 4. Bob Azmoun/SUNY Stony Brook/BNL; 12/3/02. Schematic of VUV Spectrometer Apparatus. 1. The absolute flux of the VUV source is measured using a calibrated PMT.

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Studies of the Quantum Efficiency of Prepared CsI Photo-cathodes in the Presence of CF 4.

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  1. Studies of the Quantum Efficiency of Prepared CsI Photo-cathodes in the Presence of CF4. Bob Azmoun/SUNY Stony Brook/BNL; 12/3/02

  2. Schematic of VUV Spectrometer Apparatus 1. The absolute flux of the VUV source is measured using a calibrated PMT. 2. A 2cm x 2cm collecting mesh and confronting photo-cathode are separated by 3mm. 3. High purity CsI deposited onto a metal photo-cathode substrate is ~.5 microns in thickness. 4. + 300V is applied to the mesh, either in vacuum, or when the gas pressure is ~760 Torr.

  3. QE Measurements from 7/11/02 1.The following measurements were taken the same day the photo-cathodes were produced. 2. Exposure to air was kept below 5 min.

  4. Initial Measurements of CsI Photo-cathode QE in the Presence of CF4 The following measurements conflict with similar measurements done by other groups.

  5. Measurement of the Absolute QE of Recently Produced CsI Photo-cathode Samples 1. QE measured on same day as cathode production; air exposure < 5 min. 2. Measurement made before exposure to any gas. 3. Anomalous behavior between 110-130 nm???

  6. Photo-cathode QE Response to Gas Exposure (I) 1. The following plot depicts the ratio of the measured CsI QE in vacuum: before any exposure to gas and after exposure, for a given duration. The process is iterated three times for each gas. 2. The decreasing QE is speculated to be the result of a photo-cathode/mesh electric discharge during a gas fill. 3. The orange curve depicts the QE ratio of a healthy photo-cathode before and after electric discharge in Ar at low pressure.

  7. The Explicit Effect of Gas Exposure on the Photo-cathode QE 1. The following plot is generated using the data from the plot in slide #6. 2. The following curves depict the QE ratio of the measured QE from a given scan and the scan immediately prior to it. 3. Thus, each curve represents the absolute effect in [%] of each trial exposure to gas. 4. The inconstancy of values may be attributed to the uncertainty in the photo-current and the small fluctuations in VUV source intensity, in time.

  8. Photo-cathode QE Response to Gas Exposure (II) The following plot is similar to that displayed on slide #7. The only exception is that the exposure time to each gas is increased with each iteration.

  9. QE Measured in the Presence of CF4 1. Unlike the previous plots, this plot depicts the ratio of the measured QE of a healthy photo-cathode before exposure to gas and the same photo-cathode immersed in each gas at a pressure of 760 Torr. 2. The QE ratio is also convoluted with the effects of: a) the photo-current dependence upon the extraction voltage, b) out-gassing and contamination from the sample chamber, and c) the absorbance of each gas. 3. In addition, this plot contains two curves depicting the ratio of the QE measured in vacuum before and after gas exposure. The “after” measurement was performed after ~30 min. exposure time.

  10. Conclusions • For the exposure times considered, the QE of the prepared CsI photo-cathodes was unharmed as a result of exposure to high purity CF4. Although the ratios of each curve within each plot reveal about a 10% (max.) oscillation in their respective values, there is no discernible trend in any set of curves to conclude that CF4 is in fact leading to the degradation of each photo-cathode. • Any electric discharge between the collecting mesh and the photo-cathode (at < 100 Torr) can significantly decrease the quantum efficiency of the CsI photo-cathode. This harmful effect is wavelength dependent.

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