FISSION PRODUCTS IN THE ATMOSPHERE AND RAIN
609
response of the ®%Sr/Sr ratio data to the late December 1962 nuclear
explosions were slower than that of the ‘!Ce/*4Ce ratio data,
54Mn, 124Sb, and 125Sb in the Stratosphere
A few weeks after a nuclear explosion, the antimony fraction of
fission debris consists predominantly of 2.7-year '7°Sb. This nuclide
has previously been determined in ground air by Andersonetal. § and
by Edvarson and Léw.® Crooks et al.!° determined '7°Sb in air and rain-
water, and Fry and Karras'! observed it in rain. Antimony-125 data
from balloon samples
collected from the upper stratosphere (above
60,000 ft) have been reported by Salter’? from Project Ashcan.
Also 60-day ‘*4Sb, a shielded nucleus with a very low primary
fission yield, has recently been observed in the atmosphere; its concentration in some cases was extremely high, This nuclide was most
probably produced through the (n,y) reaction from '*°Sb, In 1962, '4Sb
was detected by Gustafson et al.!? between 60,000 and 100,000 ft above
Thule, Greenland, with a balloon-borne gamma spectrometer. Later,
high '4Sb concentrations in the upper stratosphere were reported by
Salter.!2 His report also indicates that the U.S.S.R. tests in late 1961
and again in late 1962 are the sourcesof the observed !“4Sb. Because
of its short half-life, the large amounts of !"4Sb that were observed
in 1963 were assumed to have been produced bytests only in the
year 1962.
Samples from high latitudes and altitudes collected in July and
September 1962 show an unusual composition (Fig. 5), The main
gamma peak is due to Mn that, according to Holland,'* was produced
predominantly in one single 55- to 60-Mt U.S.S.R. detonation in October 1961. The same samples also contain an exceptionally high
amount of '*°Sb, which deserves some discussion.
The thermal fission yield of mass chain 125 is very low (0.021%
according to Katcoff'5), but for fast neutrons it is considerably higher.
For thermonuclear explosions, Hallden et al.'® report a cumulative
yield of 0.29% for '°Sb, based on a half-life of 2.0 years. We have
used the somewhat longer half-life of 2.7 years, which has been re-
cently confirmed by two investigations: Klehr and Voigt!’ and Wyatt
et al.'® This value changes the yield of '*sb to 0.394%. Hallden’s yield
for Sr is 3.50%, which gives the activity ratio *sb/®Sr = 1.16 for
fresh thermonuclear debris (allowing for the decay of 9.4-day '5Sn),
Tables 1 and 2 give the experimental values for this activity ratio (the
data for 1963 have been corrected for decay to Jan. 1, 1963). In most
cases,
the
experimental data are very close to the calculated ratio.
However, the samples from the high northern atmosphere in July and
September 1962 show '°Sb/Sr activity ratios between 3 and 5 (Table 2). Salter’? reports '*5sb/"Sr ratios of up to about 8 for some
balloon samples collected at San Angelo, Tex., in July and August 1962.