W. Viehmann, A. G. Eubanks, G. F. Pieper, and J. H. Bredekamp, "Photomultiplier window materials under electron irradiation: fluorescence and phosphorescence," Appl. Opt. 14, 2104-2115 (1975)
The fluorescence and phosphorescence of photomultiplier window materials under electron irradiation have been investigated using a Sr90–Y90 beta emitter as the electron source. Spectral emission curves of uv-grade, optical-grade, and electron-irradiated samples of MgF2 and LiF, and of CaF2, BaF2, 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 2π sr/MeV of incident electron energy deposited [MeV−1 (2π 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 (2π sr)−1 for uv-grade MgF2 to ≃200 MeV−1 (2π sr)−1 for CaF2. For comparison, sodium-activated cesium iodide, one of the most efficient scintillator materials, yields about 700 MeV−1 (2π sr)−1. High-purity fused silica has the lowest yield, approximately 6 MeV−1 (2π sr)−1. Optical-grade MgF2 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 ≃ 10−2 for some fluoride samples to a low of ≤2 × 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.
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N-09 photocathode.
After β excitation of 9.5 × 1010 cm−2 (0.25-MeV average energy).
Sample 2 after β excitation of 9.5 × 109 cm−2 (0.25 MeV).
Not determined.
Not significant within accuracy of measurement.
Table V
Intensity Ratios of Phosphorescence at 1 min After Removal of β-Excitation and Steady State Fluorescence Under 0.4 mCi Sr90 – Y90; Time of Exposure: 30 min.
Material
ζ
Photomultiplier: 541 N-09
MgF2
Ultraviolet grade
0.25 × 10−3
0.2 × 10−3
Optical grade
10 × 10−3
5 × 10−3
LiF
Ultraviolet grade
1 × 10−3
Optical grade
3 × 10−3
CaF2
0.4 × 10−3
to 4 × 10−3
BaF2
16 × 10−3
Sapphire
1
2.5 × 10−3
2
Not measured
3
0.05 × 10−3
4
0.01 × 10−3
5
0.005 × 10−3
Spectrosil
<0.002 × 10−3
Suprasil
0.02 × 10−3
Ultraviolet glasses
0.5 × 10−3
1 × 10−3
Table VI
Dark Count Rates of OAO-3 (Copernicus) PEP Tubes and of OSO-7 Star Scanner at Peak of SAA and Pertinent Parameters of PMT's on which they were observed
Although V1 and V2 are nominally identical, the dark count rate on V1 is about twice that of V2; we assign a high value of ∊ = 20 counts MeV−1 to V1 and a more typical value of ∊ = 10 counts MeV−1 to V2.
Measured value, not included in Table II.
Table VII
Summary of Dark Count Data for OAO-3 (PEP)a Outside the SAA (Ref. 5) and for Apollo 17 UVS (Translunar Orbit) (Ref. 6)b
Corrected to orbit zero.
Comparison of observed and calculated data suggest that protons are responsible for enhanced dark counts in free space (Apollo 17) and He nuclei and heavier cosmic rays in near-earth orbits (OAO-3).
Estimated value.
Tables (7)
Table I
Photomultiplier Window Materials
Material
Ultraviolet cutoff (nm) (10% transmission)
50% Transmission point (nm) 2 mm thick
Ultraviolet grade LiF
105
122
Ultraviolet grade MgF2
110
125
Ultraviolet grade CaF2
110
126
Ultraviolet grade BaF2
135
150
Ultraviolet grade sapphire
145
160
Fused silica
∼160
∼175
Corning 9741 glass
200
230
Corning 7056 glass
270
310
Table II
Luminiscence Efficiencies of PMT Window Materials and Some Scintillator Materials for Ultraviolet, Ultraviolet-Visible, and Visible Photocathodes
N-09 photocathode.
After β excitation of 9.5 × 1010 cm−2 (0.25-MeV average energy).
Sample 2 after β excitation of 9.5 × 109 cm−2 (0.25 MeV).
Not determined.
Not significant within accuracy of measurement.
Table V
Intensity Ratios of Phosphorescence at 1 min After Removal of β-Excitation and Steady State Fluorescence Under 0.4 mCi Sr90 – Y90; Time of Exposure: 30 min.
Material
ζ
Photomultiplier: 541 N-09
MgF2
Ultraviolet grade
0.25 × 10−3
0.2 × 10−3
Optical grade
10 × 10−3
5 × 10−3
LiF
Ultraviolet grade
1 × 10−3
Optical grade
3 × 10−3
CaF2
0.4 × 10−3
to 4 × 10−3
BaF2
16 × 10−3
Sapphire
1
2.5 × 10−3
2
Not measured
3
0.05 × 10−3
4
0.01 × 10−3
5
0.005 × 10−3
Spectrosil
<0.002 × 10−3
Suprasil
0.02 × 10−3
Ultraviolet glasses
0.5 × 10−3
1 × 10−3
Table VI
Dark Count Rates of OAO-3 (Copernicus) PEP Tubes and of OSO-7 Star Scanner at Peak of SAA and Pertinent Parameters of PMT's on which they were observed
Although V1 and V2 are nominally identical, the dark count rate on V1 is about twice that of V2; we assign a high value of ∊ = 20 counts MeV−1 to V1 and a more typical value of ∊ = 10 counts MeV−1 to V2.
Measured value, not included in Table II.
Table VII
Summary of Dark Count Data for OAO-3 (PEP)a Outside the SAA (Ref. 5) and for Apollo 17 UVS (Translunar Orbit) (Ref. 6)b
Corrected to orbit zero.
Comparison of observed and calculated data suggest that protons are responsible for enhanced dark counts in free space (Apollo 17) and He nuclei and heavier cosmic rays in near-earth orbits (OAO-3).
Estimated value.
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