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3.3
DETECTOR
The detector consists of an RCA 5819 photomultiplier and a phosphor glued to the cathode
end of the tube. The phosphor is madeup of 5-mil transtilbene crystal glued on top of a %-in.
lucite disk. The disk {s formed to fit the contour of the tube face.
The phosphor, being far moreeffective in attenuating beta than gamma radiation, is therefore more sensitive to beta than to gamma radiation. The phosphor waseffective in increasing
detector output to such an extent that it was possible to eliminate the d-c amplifier (i.e., to
feed the detector output directly to the oscilloscope) and also to double the distance between
source and detector. Doubling the beta path meant doubling the material sample whose density
was measured,
The detector circuit is shown In Fig. 3.2. The power supply is made up from the 30-volt
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3.4
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cells of 300-volt Eveready No. 493 batteries. Each power suply consists of duplicate 1260volt banks of batteries, paralleled for greater reliability. Construction of the power supply in
this manner resulted in very short leads, which is an essential requirement for minimizing or
eliminating spurious signals.
The voltages applied to the various pins are as follows:
1. Cathode to dynode No. 1: 180 volts.
2. Between dynodes (No. 1 to No. 10): 90 volts each.
3. Dynode No. 10 to plate (across load): 270 volts.
The plate load was variable between 200,000 and 400,000 ohms with the center of the load
grounded. This point was selected as ground to permit a balanced-line output to the recording
unit,
.
The photomultiplier power was turned off by relays which opened to remove voltage from
dynodes No. 2 and No. 4. This method was effective. When the power was turned off in this
manner, there was no photomultiplier output nor fatigue. This method was particularly
coavenient in that no high-voltage circuits had to be turned on oroff.
The photomultiplier was made light tight by housing it in a steel tube (Fig. 3.3), which
also afforded magnetic shielding. The cathode end was sealed with a 1.5-mil aluminum foil.
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RECORDING UNIT
The recording unit consisted of a dual-trace cathode-ray tube, a strip camera, timing
lights and oscillators, control and delay circuits, and associated power supplies (Fig. 2.2).
The entire assembly was housed in a watertight aluminum cylinder (Fis. 2.1).
The output (current) of the photomultiplier was fed to a variable (resistance) load,
balanced about ground and located in the recording unit (Fig. 3.4). The voltage drop across
this load was displayed directly on the screen of a dual-gun cathode-ray tube.
Any spurious noise picked up by the beta-signal leads in cable D would also be picked up
by an additional pair of leads terminated across a 600,000-ohm load in the battery box (Fig.
3.2) between terminals d and e. In this way spurious signals that might be mistaken for betasignal variations woud be displayed in greater amplitude by the second or noise-monitoring
trace of the cathode-ray tube.
The cathode-ray tube was a 3U2P11 manufactured by the Electronic Tube Corporation.
The power supply was made up of dry batteries as follows:
Ground: anodes No. 2 and No. 3, unused deflection plates, and center taps of signal leads.
—600 to —900 volts: focusing or znode No.1.
—1170 to -1206 volts: control grids (30-voit batteries).
6 volts: across filaments (2t —1200 volts).
180 and 120 volts: positioning voltages.
15
RESTRICTED DATARS SECURITY INFORMATION