Quantum efficiency of opaque Csl photocathodes with channel electron multiplier arrays in the extreme and far ultraviolet

Christopher Martin and Stuart Bowyer University of California, Space Sciences Laboratory, Berkeley, California 94720. Received 30 July 1982. 0003-6935/82/234206-02$01.00/0. © 1982 Optical Society of America. We report measurements of the quantum efficiency (QE)

of channel electron multiplier arrays (CEMA) overcoated with a CsI photocathode in the VUV. These measurements, the

4206 APPLIED OPTICS / Vol. 21, No. 23 / 1 December 1982

first presented in the EUV for CsI, are part of the detector development program for the Extreme Ultraviolet Explorer (EUVE) satellite which will be launched in 1987.

Powdered UV grade CsI from Harshaw Chemical was evaporated onto the front surface of the input CEMA of a tandem pair incorporated in a RANICON detector,1 using electron beam evaporation. The ITT 25-mm CEMA, fully loaded into a detector housing, was placed ~30 cm directly above the evaporation crucible, and the evaporation direction was normal to the CEMA surface. The microchannels were biased at 8° and had a pore diameter of 12 μm. A quartz crystal microbalance monitored the evaporated Csl thickness, and a shutter was used to control the evaporation time. The evaporation rate was ~100 Å sec-1. The shutter was partially inserted at three successive times to generate three 8-mm strips of 1000-, 2300-, and 3500-A thick CsI. This allowed a measurement of the dependence of QE on photocathode thickness.

After evaporation the chamber was backfilled with dry N2, and the detector was transferred quickly into a small vacuum chamber with a motorized door. The exposure time to low humidity (~20%) air was ~100 sec. Hence little degradation of the photocathode is expected to have occurred.2 The sta­bility of CsI photocathodes evaporated in this manner will be presented in a subsequent report.3 The small chamber was evacuated immediately with cryogenic pumps, and the de­tector remained evacuated throughout the QE measurements. The entire small chamber was placed inside a large calibration chamber for QE measurements, and the motorized door was opened only when the ambient pressure was below 1 Torr.

Monochromatic light from a hollow cathode discharge source4 passing through a McPherson grazing incidence monochromator was used to illuminate the CsI photocathode. The beam diameter was kept small (~2 mm) to confine it within the individual thickness strips. A bias grid was used to produce a 50-V/mm electric field to guarantee collection of all photoelectrons emitted by the CEMA webbing.5 The CEMAs were operated with a gain of 2-3 X 106 and were moderately saturated (FWHM ~ 150%). A channeltron secondary transfer standard6 was used to determine the ab­solute QE in the EUV, while an NBS calibrated windowed photodiode was used to measure the FUV absolute QE.

Figure 1 shows the absolute QE measured for two of the thicknesses (1000 and 3500 A), with an illumination angle approximately normal to the CEMA. The QE of the inter­mediate thickness (2300 A) falls between these two curves.

Fig. 1. Quantum efficiency vs wavelength of uncoated and CsI-coated CEMAs.

The angular dependence, which will be presented elsewhere,6

indicates that the increase in thickness beyond 1000 A in­creases the yield inside the microchannels but not on the webbing. Also shown on the figure for comparison is the QE of a typical uncoated CEMA. We note that the CsI gives a factor of 3 increase in QE in the EUV and factor of 50-5000 in the FUV. Peak efficiencies are ~30% in the EUV and at 1400 A. The measured QE at Ly-α (1216 A) is lower than that found by Timothy et al.7 and Hartig et al.8 but exhibit a considerably slower falloff toward long wavelengths than exhibited by the data in Ref. 7. Although no other mea­surements of the QE of CEMAs overcoated with CsI in the EUV have been reported, we note that high photoyields in this band are commonly observed.2,9

This work was supported by NASA grant NASW-3636.

References 1. M. Lampton and F. Paresce, Rev. Sci. Instrum. 45, 1098 (1974). 2. E. B. Saloman, J. S. Pearlman, and B. L. Henke, Appl. Opt. 19, 749

(1980). 3. C. Martin, in preparation (1982). 4. F. Paresce, S. Kumar, and S. Bowyer, Appl. Opt. 10, 1904

(1971). 5. R. Taylor, M. Hettrick, and R. Malina, submitted for publication

(1982). 6. J. E. Mack, F. Paresce, and S. Bowyer, Appl. Opt. 15, 861

(1976). 7. J. G. Timothy, G. H. Mount, and R. L. Bybee, Proc. Soc. Photo-

Opt. Instrum. Eng. 172, 199 (1979). 8. G. Hartig, H. W. Moos, R. Pembroke, and C. Bowers, preprint

(1982). 9. P. Metzger, J. Phys. Chem. Solids 26, 1879 (1965).

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