~4o-
isotope is linear on a semi-log plot, exemplified by bird
thyroid containing predominantly t131, mentioned in the section
on birds.
Among the 28 plates counted most often (10 - 73 times) and
presumed to be counted frequently enough to detect the existence
of a linear semi-log relationship, only one other sample was
more nearly linear by semi-log than by log-log plot.
This was
the gastric mill of a crab, Grapsus grapsus, taken March 26,
1954, at Kabelle.
The graph (Figure 11) was sufficiently curved
to indicate the presence of more than one Lsotope.
The early
portion 50 - 300 days gave a half-life of 78 days, and the
portion 300 - 430 days gave a half-life of 107 days.
‘
A settion of
the curve of another sample, muscle of sea cucumber (Figure 11),
was typical of semi-log linearity.
The radioactivity of this
sample decayed cver the period from 50 to nearly 200 days with
a half-life of about 75 days, but more slowly later.
Although a single isotope displays a downwardly concave
curvilinear plot by log-log presentation,
a mixture of as few
as two isotopes with half-lives of similar orders of magnitude,
such as Celttt and Cel4+ of 30- and 280- day half-lives, may
appear almost linear on a log-log plot over the period of
79 to 500 days.
Most decays were best suited to log-log plotting as seen
in the seven examples in Figures 12 and 13.
slightly curved,
Although some appear
straight lines were fitted and slopes were
sealed graphically.
Definition of the curves requires evaluation not only of
"EN ES
4
Yar
[Nat.
52