-26-
axis apply only to each curve separately, and do not permit comparison
of absolute radioactivity between curves.
as March 1,
The date of reference is taken
i954, because the detonation of this date is believed to have
contributed many times as much radioactivity to the atoll as did sucCurves 1-3 (Fig. 9) represent the radio-decayof
samples collected 10-11 months after March 1,
1954, while curves 4-6
pertain to samples collected shortly after the Redwing series.
groups differ in both
steepness and direction of curvature.
The two
The first
group, curves 1-3, decayed with a log-log slope of -1.3 to -1.4 which
agrees with the slopes of the remarkably straight decay curves over almost the same period of time for the plankton samples from Eniwetok
Atoll (UWFL-53:21).
However, the decay curves for plankton from
Rongelap Atoll differ from those of Eniwetok Atoll in having a downward flexure.
In fact, curve 2 displays up to the 1, 350th day almost a
uniform half life of about 310 days, and from days 1, 350 to 1, 740, a
430-day half life.
Curves 1 and 3 deviate only slightly from this pat-
tern, being steeper in the early sections.
Gamma spectrometry of the
sample of curve 2 on November 6 and December 9,
1957 showed Cel44,
of 285-day half life, to be the primary constituent, which is presumably
accompanied by small, undetected amounts of longer-lived isotopes,
contributing to the 430-day half life after 1,350 days.
The second group
curves 4-6, decayed rapidly (slopes -4.1 to -10.8) because of the recent
se ee
ceeding series.