Photomultiplier window materials under electron irradiation: fluorescence and phosphorescence.
The fluorescence and phosphorescence of photomultiplier window materials under electron irradiation have been investigated using a Sr(90)-Y(90) beta emitter as the electron source. Spectral emission curves of uv-grade, optical-grade, and electron-irradiated samples of MgF(2) and LiF, and of CaF(2), BaF(2), sapphire, fused silica, and uv-transmitting glasses were obtained over the 200-650-nm spectral range. Fluorescence yields, expressed as the number of counts in a solid angle of 2pi sr/MeV of incident electron energy deposited [MeV(-1) (2pi sr)(-1)], were determined on these materials utilizing photomultiplier tubes with cesium telluride, bialkali, and trialkali (S-20) photocathodes, respectively. Typical yields observed with a uv/visible sensitive bialkali cathode range from 10 MeV(-1) (2pi sr)(-1) for uv-grade MgF(2) to approximately 200 MeV(-1) (2pi sr)(-1) for CaF(2). For comparison, sodium-activated cesium iodide, one of the most efficient scintillator materials, yields about 700 MeV(-1) (2pi sr)(-1). High-purity fused silica has the lowest yield, approximately 6 MeVW(-1) (2pi sr)(-1). Optical-grade MgF(2)and LiF, as well as electron-irradiated uv-grade samples of these two materials, show enhanced fluorescence due to color-center formation and associated emission bands in the blue and red wavelength regions. Large variations in fluorescence intensities were found in uv-grade sapphire samples of different origins, particularly in the red end of the spectrum, presumably due to various amounts of chromium-ion content. Phosphorescence decay with time is best described by a sum of exponential terms, with time constants ranging from a few minutes to several days. Phosphorescence intensity expressed as a fraction of the steady-state fluorescence intensity is an extremely sensitive measure of crystalline perfection and purity. This fraction ranges from a high of approximately 10(-2) for some fluoride samples to a low of </=2 x 10(-6) for fused silica. Application of the parameters obtained in this work to the analysis of recent flight observations on low light-level experiments yields good quantitative agreement with flight data from OAO-3, OSO-7, and Apollo 17.
Viehmann
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W
Eubanks
A G
AG
Pieper
G F
GF
Bredekamp
J H
JH