148
HAINES AND MUSGRAVE
From the available data several requirements appear necessary
for a source of tritiated methane: (1) a source in the northern troposphere, (2) a continuous injection over some period of time, and (3)
an injection that has probably ceased or a rate of injection that is not
large compared to the present total. It is clear that release of tritium
by production laboratories would mostly have occurred in the northern
hemisphere and that these production systems would have continuously
injected tritium during the time of production. The rate of release of
tritium from this source would have decreased when sufficient tritium
was stored. Continued release of tritium and tritiated methane will
occur each time the large amounts of tritium in storage are processed
to recover the accumulated “He.
Helium-3 is recovered by condensing tritium onto charcoal and
pumping off the 3He. Charcoal adsorbs about 20 ml of hydrogen per
gram at a vapor pressure of 1.3 x 107° atm. This is equivalentto 1
curie per liter of vacuum-line volume; therefore a significant amount
of tritium may be pumped away during any processing of the stored
material.
The fact that organic molecules exposed to tritium gas undergo
exchange because of the ionization effects of the tritium betas was first
reported! in 1956. Since then numerous studies that have extended this
observation indicate that an important product in the self-labeling of
any organic molecule containing a methyl group withtritium is tritiated
methane. The preparation system at Los Alamos must have included
mechanical pumping as a primary stage. Therefore all the tritium released was passed through hot pumpoil on the way to the atmosphere.
Under conditions of steady operation, the mechanical-pump oil would
be saturated with T,.
If oil diffusion pumps were used as boosters, additional exposure
of hot organic molecules would have occurred, Unfortunately, there are
no applicable data available to allow reasonable estimates of the rate
of methane production from such sources. Wilzbach?? reports incor-
poration of up to 1% of tritium gas per day whenit is exposedto solid
materials. The incorporation of T, dissolved in hot organic oils should
occur at a rate sufficient to produce the observed tritiated-methane
activity.
The rate of exchange of most gases between the northern and
southern hemispheres!® is about 0.5 per year. Although the data are
not accurate enough to allow precise calculation, an estimate of the
T/H ratio in methane can be made for dates earlier than those measured. For a rate as high as 0.5 per year, the T/H ratios in northern-
hemisphere methane may be estimated as 2 to 4 x 10° in 1953. Methane
is present® in the atmosphere at about 1 ppm; therefore thetotal tri-
tium in methane in 1953 would have been of the order of 0.1 g. This is