For example, 240py/23%pPy ratios measured in environmental samples vary from
0.02 at NTS to 0.40 in the South Pacific Test Area (Efurd et al., 1976).
T
T
T
0.26
Weapons grade plutonium has 7"9%pu/239Pu ratio of 0.06 (Krey and Krajewski,
1972).
—- NORTHERN HEMISPHERE
—--SQUTHERN HEMISPHERE
The 249py/239pu isotope ratios may be used to identify and quantify plutonium
resulting from a specific release even in the presence of copious amounts of
fallout plutonium (Hardy et ai., 1972).
This separation procedure is based on
the assumption that fallout plutonium has a unique 240py/23Spy fsotope ratio.
Krey et al. (1976) derived a mean atmospheric value from the samples reported
in HASL-273. They pooled the samples collected before mid-1967 to calculate a
_
0.22
240pu/239py ratio of 0.179 that is in excellent agreement with che mean value
of 0.176 + 0.014 observed in soils containing only fallout plutonium.
The approach of Krey et al. (1976) is an excellent method for predicting the
buildup and isotopic composition of plutonium in soil samples where the debris
from the individual injections have been superimposed to form a composite
sample.
For soil samples, one may assume 25% of the atmospheric plutonium
burden was injected during 1956; most of the remaining plutonium was injected
during 1962.
The mean residence times of the stratosphere and troposphere are
short; therefore, the majority of debris from these atmospheric test series
were deposited on the earth's surface by mid-1967. The injections of 1967 and
subsequent years have been too small to change the 240py/239py isotope ratios
in the soil.
However, one may not assume that there has been a constant
The atmosphere is self-cleansing:
therefore, its 2" Pu/233py isotope ratio tends to reflect the more recent
injections. Although the injections after mid-1967 did not alter soil concentrations, they were large enough to change the 24%py/239pu isotope ratios of
the atmosphere.
The variations in the 7"%py/239py isotope ratios in the northern and southern
hemispheres on a yearly basis as calculated from the data in HASL-273 are
shown in Fig. 2.
It is interesting to note that the plutonium isotope ratios
in both hemispheres track fairly well until 1968.
From 1968-1970, the ratios
in the southern hemisphere became significantly lower than those in the northern
hemisphere.
Because of the small number of analyses reported for the southern
hemisphere between 1968-1970, the estimations of the average annual ratios
should be considered only approximations.
However, they do definitely indicate
that the atmospheric ?4°Pu/?#39py isotope ratios have changed with time.
trends in the northern hemisphere are less obvious.
The
240 Pu/ 239 Pu
240py/239py isotope in the atmosphere.
wd
0.18
4
0.14
0.10
=
F
The purpose of this report is to document the plutonium analysis performed by
the McClellan Central Laboratory that complement the 1959-1970 study.
In addition, this report documents the americium measurements which were
obtained from 1963-1970.
0.06
FIG.2
252
l
1960
PLUTONIUM
{
J
1965
1970
ISOTOPIC
253
RATIOS