-39-
decline to which a significant regression line can be fitted with a slope
of -2.02 (dotted line).
In conclusion, gross beta levels of fish at Rongelap Atoll in the fall
of 1958 were only slightly higher at Kabelle Island than in the vicinity of
Rongelap Island.
For the areas of the lagoon sampled, these levels
were (expressed in uc/kg wet tissue): muscle, 0.005-0. 008; bone, 0. 0080.016, and liver, 0.10-0.13.
Rates of decline ranged from -1.2 for
muscle to -2.0 for liver.
In the absence of further contamination from fallout, and at the rates
of decline shown in Figure 11, fish muscle at Rongelap Atoll may be expected to decline from its present low level of less than 0.01 uc/kg wet
to about 0.002 uc/kg wet, within 20 years.
The latter figure is approxi-
mately the level due to naturally occurring K40 in tuna muscle.
Due to its predominant gamma content and low beta energy the disintegrations from zn65 maybe detected by beta counting to only a slight
extent--anywhere from about 1 to 10 per cent of their actual intensity.
However, since its 245-day half life allows Zn®5 to decay to about 1.5
per cent in 4 years, and to 0.024 per cent or 1/4000 in 8 years,
zn65
could be ignored after a few years.
The almost complete absence of Cs!37 or other long-lived isotopes
in fish muscle as reported by Welander (UWFL-55:15) suggests that flattening of these decline curves probably will not occur before the background levels due to K49 are reached.