‘uture reports from the Health
itural y-dose rates are in the
erage for all locations surveyed
contributions from potassium-
es are gonerally comparable,
roximately 80 per cent of the
uranium series contribution
than 25 per cent of the total
zh natural levels have been
New Hampshire, Denver and
o, and in an area of North
Jetgh. These high levels are
n and potassium content of the
ar.
1, the observed fall-out y-dose
n of the total. Between July
dose rates of 3-4 ur./h seemed
rm United States. Somewhat
ed in someareas in the western
ited to relatively less rainfall.
1ents in the San Francisco Bay
periods of very little rainfall.
igs were observed during the
, when levels of 4-6 ur./h were
sions scattered throughout the
an effort was made to monitor
‘els in the New York City area
; at several locations. Three
aster County, New York, were
it 5 miles from one another.
; indicated that the natural
cations were all approximately
al y-dose rates were generally
h until September. Our best
levels at these sites are shown
‘ide strong evidence that open
1e New York area remained
‘ovel (~ 5 pur./h) from at least
and thon decreased, roughly
went ‘half-life’ slightly longer
This is consistent with the
field spectra that **Zr—Nb
) per cont of the fall-out dose
measurement.
trip to the Pacific coast, a
m, to obtain measurements on
mpic peninsula (Clallam Co.),
ce of approximately 50 miles,
ries by a factor of nearly ten
Forks); under such conditions
itial variation in the fall-out
0
levels, since fall-out deposition is strongly influenced
by the quantity of rainfall. Alexandere¢ al.”5 have found
a clear correlation between strontium-90 deposition and
moan rainfall-levels at five sampling locations in Clallam
Co. Measurements of external y-levels at these and several
other sites in early October 1962 are summarized in
Table 4. The increase of the fall-out y-dose rates with
mean annual rainfall is noteworthy, and the degree of
correlation seems quite as good as that for the accumulated
strontium-90 soil content. But care must be exercised
here in coming to appropriate conclusions, since the
quantity of relatively short-lived y-emitters present de-
pends on recent rainfall to a much groator oxtent than
is the case with strontium-90.
The Forks locations are of particular interest since these
measurements were made during or between periods of
heavy rainfall. The spectra show a distorted potassium-40
peak at 1-46 MeV, whichis clearly the result of a considerable lanthanum-140 contribution at 1-6 MeV. This
implies substantial recent fall-out deposition; this fact is
also indicated by the lack of agreement between the two
methods of estimating fall-out dose rates. As mentioned
previously, recent deposition generally implies a more
nearly plane source than used in our model and thus
more peak counts per unit dose rate for the various radioisotopes. Therefore, using our depth source model and
the peak method, we would tend to overestimate the dose
rate. If the band method were used to estimate the natural
dose rate, the difference method would provide an under-
estimate of the fall-out dose rate at Forks, since the
lanthanum-140 peak would fall in the A and U bands
and produce an overestimate of the inferred natural dose
rate. Under such conditions, the actual dose rate at
Forks probably lies between those listed in the last two
columns of Table 4.
In addition to the conspicuous 0-5 and 0-75-MeV peaks
seen at all locations and the 1-6-MeV peak found in the
Washington spectra, we have observed a small fall-out
peak near 1-7 MeV in manyof our late 1962 spectra which
we have tentatively attributed to antimony-124.
This
has also been found in a few 1963 spectra. In no case
were there sufficient counts to indicate a significant doserate contribution.
The origin of this debris is probably
the late 196] Soviet test series, as balloon sampling at
high altitudes (60,000-100,000 ft.) in Texas and Australia
indicated the presence of antimony-124 throughout
1962 (ref. 26).
In general, the agreement between our two methods of
determining the fall-out y-dose rate indicates that the
assumptions involved in carrying out the peak method
of analysis are reasonable.
Thus, practically all the
fall-out y-radiation during late 1962 and 1963 derived from
*Zr—-Nb, @Ru—!6Rh, 1#Ru—-'Rh, and 4’Cs, with the
ll