te Rh
portion of the total radioactivity.
from the same samples, however,
Gamma spectra obtained
indicated that non-fission-
product radioisotopes contributed more of the total radio-
activity than was observed in the chemical separations,
which were designed primarily for determination of fission
products.
Consequently a detailed study was made to de-
termine the radioisotopic content of several plankton
samples collected during the September survey.
In addition,
gamma spectra were determined for these and most other
plankton samples collected in the latter survey.
The re-
sults are reported in this paper.
Materials and Methcds
The equipment and methods used for collecting the
samples at sea have been described in previous reports
(6, 21).
The initial counting for beta activity was done aboard
ship soon after collection of the plankton samples, and the
remainder of the plankton from each station was preserved in
lO per cent neutralized formalin in sea water and returned
to the Seattle Laboratory.
The concentration of plankton in the tropical seas and
the levels of radioactivity in the plankton are both limited,
thus
plankton samples from adjacent stations were combined
to provide sufficient material for analysis.
Figure 1 shows
the track of the September survey and the groups of stations