242
LOWDER, BECK, AND CONDON
close. The decline in late 1963 is quite striking. It should be noted that
the differences in the various natural-gamma readings are at least
partly real. For example, the low reading in April 1964 was obtained
when the ground was quite wet. This would tend to reduce all components to some extent.
Rainfall can have a significant effect on both the total observed
fallout dose rates and on how well they can be determined. This is exemplified by our October 1962 measurements on the Olympic Peninsula
in the state of Washington, given in Table 4. Over a distance of 50
Table 4 MEASUREMENTS ON THE OLYMPIC PENINSULA, WASHINGTON,
OCTOBER 1 AND 2, 1962
Mean
annual
rainfall,
in.*
Town
Sequim
Sequim
Port Angeles
Port Angeles
Joyce
14
14
24
24
54
Clailam Bay
81
Forks
118
Forks
Forks
118
118
-
Gamma dose rates, ur/hr
Sr,
me/sq milet
42.0
65.3
84.1
133
153
%57r—®5Nb
Natural
Total
Fallout 1t
Fallout 2§
1.2
1.4
1.0
0.9
0.8
0.6
0.8
1.1
1,0
1.8
3,3
3.8
3.5
3.2
3.7
0.8
1.1
1.4
1.3
2.4
0.7
14
1.7
1.5
1.3
“K
"87 282yh
1.2
1.5
1.6
1.2
1.7
0.9
0,9
0.9
1.1
1.2
1.9
17
2.2
1.3
0.4
2.2
2.7
5.2
3.4
2.8
2.6
1.5
1.1
1,7
1.1
1.2
0.9
2.5
2.3
4.4
3.1
3,7
3.4
2.8
2.8
1.3
2.6
2.7
*1960—1962 value.
+Oct. 1-2, 1962, soil determinations (L. T. Alexander).
tFrom photopeak calibrations.
,
§ From total terrestrial] dose-rate measurements with natural component subtracted.
miles from east (Sequim) to west (Forks), the mean annual rainfall increases by a factor of nearly 10. This was strikingly illustrated for us
at Forks by the soggy condition of the ground, the mist in which we
made our measurements, and the two sudden downpours that nearly
put a permanent end to our survey. The strong increase in the falloutgamma levels with mean annual rainfall is clear from the table, as is
the poorer agreement between our two methods for estimating these
levels at Forks than at the other locations. The latter phenomenon can
be explained as the effect of recent deposition of significant quantities
of fission products, particularly *Zr—*Nb, forming a more nearly
plane source than our theory assumes and thus resulting in an overestimate of the dose rate from the peak method. The presence of the .
short-lived °Ba—'°La peaks in our Forks spectra provides further
evidence of recent deposition.
Our survey results taken as a whole indicate that, until late in
1963, *Zr—*Nb contributed 60 to 80% of the observed fallout-gamma
dose rate in the environment and that these dose rates were of the
order of 50% of the natural levels at typicallocations over open ground.
By one year after the last nuclear weapons tests in December 1962,
these levels had fallen to 10 to 20% of the average natural-gamma dose