ation
a
Gorpon M. DUNNING
the Pacific Islands, the winds were light
and the first rainfall did not occur until
about two weeks later. Graph 2 shows
the gamina dose rates taken at 3 feet
above the ground on the island of Rongelap >
over a period of nearly a year. In the
first ten days the decrease in activity, or
disintegrations per unit time, is roughly
consistent with the kuown radiological de-
.
*
-
.
20
DOSE (PER CENT) »
a
4
oti
vet
consideration is the energy spectrum of the
radiation. The gamma spectrum emanating from fallout material is complex. In
Graph 1 is shown the gamma spectrum for
fallout after the detonation of March 1,
1954, at the Pacific Proving Ground (+4),
with the estimated percentage contribu“tions of the gamma quanta of differing
energies (million electron volts). it is
Ajcil 1956
ad. apy ay 4
586
0.6
"0.8
i.0
t.2
GAMMA ENERGY (MEV}
Graph 1.
Percentage of total dose contributed by gamma quanta energies shown (million electron volts).
recognized that such spectra may vary and
that any single value may conceal important features, but an estimate of 0.7 Mev
mean energy has been quoted as a first
approximation (5).
WEATHERING AND SHIELDING
The variable nature of the two parameters of weathering and shielding makes
establishment of a precise rule, covering
allsituations, impossible; yet these factors
are operative in determining the total exposure received from fallout.
One example will be used here to give
some perspective as to weathering effects.
After the detonation on March 1, 1934, in
cay rate for fallout material, #.e., a slope
of minus 1.2. The break between the
tenth and twenty-fifth day, therefore, undoubtedly represents the effects of rain
(and possibly winds), which was known to
have occurred.
The rest of the points
fall roughly on a line of (time)~!’, reflecting principally the effects of weathering and possibly, to a smaller degree, the
fact that the number of gamma quanta re“leased per disintegration decreases after
the first thirty to forty days. In employing these data, however, one is faced with
the problem of translating the effects from
a Pacific island to larger land areas with
different climatic conditions.