ne ee
- Another serious source of difficulty in subtracting the gamma signal with the amplifier
used at Greenhouse was the fact that the 5819 photomultipliers were turned on and were
fatiguing several hours prior to shot time. The gain of two photomiultipliers, carefully selected
as they may be, could vary considerably after several hours of fatigue. Of the two records
obtained at Greenhouse, one was a pure gamma (background) record without superimposed beta
signal. The other was the net output of the balancing system.
2.2.2
Buster Model
In the Buster model a mechanical chopper cutting off the beta flux 400 times per second
was used with a single 1P21 photomultiplier. A band-passfilter and amplifer displayed the
400-cps variation of the beta signal and the superimposed noise. This was a sound design that
could have worked, except for the high spurious noise picked up that made the record unreadable.
The amplifier was noisy when disturbed by shock, but otherwise it operated satisfactorily.
The excessively long leads, about 20-ft loops, between successive dynodes and photomultiplierpower supply picked up and amplified spurious signals of magnitude comparable to that of the
detected beta signal.
As in Greenhouse no system of field calibration was incorporated in the instrument.
Laboratory calibration with a pressure tank was relied upon to relate detected beta signal to
material density along the beta path.
2.2.3
Tumbler Modei
Both the Greenhouse d-c model and the Buster a-c model with completely redesigned
electronic systems were used at Tumbler with the idea of:
1, Eliminating deficiencies from both systems.
2. Testing a field calibration method using removable [oils in the beta path.
3. Comparing the systems for best results.
4. Determining feasibility of use at Ivy.
5. Obtaining density-time measurement for Tumbler.
Both the d-c and the a-c systems, described in the Tumbler report, worked satisfactorily. The
d-c system is favored over the a-c system for the following reasons:
1. It smoothes out statistics and therefore produces a cleaner record.
2. The d-e system requires simpler instrumentation, eliminating the mechanical chopper
and band-passcircuits.
The amplifiur was used in the linear and nonlinear range; this required accurate knowledge
of the amplifier characteristic in the interpretation of results. A completely linear reproduction
of the beta signal would have been preferable, especially because the variation of beta signal
with material density was logarithmic. An amplifier with a larger voltage variation could be
used, but this would require more batteries and more circuitry. A more attractive solution,
achieving both linear response and simplified circuitry, is to use a high-sensitivity fastresponse phosphorwith the photomultiplier.
REFERENCES
1. F. B. Porzel and J-7 Blast Measurement Group, Measurement of Air Density in a Shock
Wave, Greenhouse Preoperational Report, Project 6.4.1.
2. P. R. FlorCruz, C. G. Young, Jr., and A. T. Brousseau, Beta-Densitometer Feasibility
Test, Tumbler-Snapper Project 19.2c Report, Part I, WT-556.
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