eeSR
CHAPTER3
INSTRUMENTATION
3.1
GENERAL
The densitometer used at Ivy Mike shot was the d-c type. Asice from having the desirable
features of previous models, it had en added simplification in tnat it did not use a second
photomultiplier for balancing the background gamma signal. This simplification was made
possible by the special sitvation expected to prevail at Mike shot. Calculations by F. B. Porzel
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predicted negligible background radiation at the instrument stations at the times of blast-wave
arrival.
With only one photomultiplier to receive both the beta and the background signal, background radiation (mainly gamma) would increase the total signal and could, if sufficiently
strong, drive the electron beam, which is supposed to respond to the beta signal only, off the
face of the oscilloscope. Any thermal* or preshock dust, on the other hand, would diminish the
signal. Miscaiculation of the gamma-radiation rate could lead to confusion in the interpretation
of the beta trace, even if the beta trace between zero time and time of blast arrival appears
comparable to the preshot trace. It could mean that there was no detectable gammanor preshock dust, It could also mean that the effects of gamma background and preshock dust
neutralized each other. Tumbler records showed that the first possibility is more likely than
the second, especially if the trace returns to its preshot level rapidly. Preshock dust raises
the beta trace to a fairly steady level. Decaying gamma, on the other hand, brings the trace
down continuously. As it turned out, calculations of gamma-radiation rate and prediction on
the behavior of the beta trace were correct.
°
$3.2
BETA SOURCES
Three t-curie Sr beta sources were used at each station, except at Station 690.02,
where two l-curie sources were used. Sources were 100 cm from the 2-mil aluminum foil
which covered the detector.
Each l-curie source of Sr’* was contained in a cylindrical stainless steel shell, 1.0 in. in
diameter and 0.3 in. deep. The active face was sealed with a 1-mil stainless steel foil. Details
of the source holder are shown in Fig. 3.1.
*When a nuclear bomb is detonated over a dry ground surface, dust is literally exploded from
the surface shortly after detonation and usually before the arriva. of the blast wave. The exact
mechanism of this phenomenon is complex and is believed to be due to the intense burst of
thermal energy released by the bomb, hence the term thermal dust.
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