480
LOCKHART, PATTERSON, SAUNDERS, AND BLACK
to 10,000 or more counts was recorded, depending on the sample activity and the time available. No corrections were made for radioactive
decay.
These_ashed samples were later combined on a monthly or bimonthly basis and subjected to sequential radiochemical analysis for
such radionuclides as "sr, "sr(®y), “ty, '3'cs, “Ce, “4ce(4pr),
47pm, '85w and 2!°pp(RaD) by conventional procedures.’
Since transfer of this operating network to HASL, some changes
have been instituted by that agency, namely, the use of polystyrenefil-
ters instead of cellulose—asbestos filters, gamma counting instead of
beta counting, and filter changes on a four per month schedule rather
than on a weekly schedule.’ Radiochemical analyses for a selected list
of nuclides, including many of the preceding and some additional ones,
are also being undertaken; thus there should be reasonable continuity
in this program.
GROSS FISSION PRODUCTS IN THE AIR
The monthly average gross fission-product concentrations in the
air along the 80th meridian (west) during the period from 1957 through
1962 are reported in Figs, 1 and 2 for representative low-altitude sites
in the northern and southern hemispheres. Since no corrections have
been made for radioactive decay during the period between collection
and measurement, the actual air concentrations existing during periods
of active nuclear testing must have been considerably higher (perhaps
as much as a factor of 2 in some cases).
Maximums in fission-product levels can be observed at the northern hemisphere sites (Fig. 1) during the spring of each year regardless
of the extent of testing during the previous season. The presence of
such peaks during the moratorium on testing proved conclusively that
there was a seasonal factor involved in the rate of stratospheric deposition of radioactive debris. Other peaks can be associated with
direct tropospheric contamination by devices detonated immediately
preceding the measurement.
In the southern hemisphere the seasonal variation in radioactivity
levels is not at all obvious, and the major peaks of activity may be
readily correlated with prior nuclear tests in the tropics (Pacific
Proving Grounds or Christmas Island area) or subtropics (Sahara
Desert),
These data can also be presented in the form of tropospheric burdens of gross fission-product activity, a method of display that ac-
centuates the major trends taking place in radioactivity levels and gives
a rough idea of the total quantity of radioactivity in the air at any one
time. The activity burdens calculated for the northern and southern