The detection of a particle as hot depends unon the number of
silver grains exposed in the film, and the determining factor in such
&@ measuremant is the total flux (radiation per unit area of observation
per unit tine), This is a function of the activity concentration and
of the particle size, shape, i.e,
Flux = KA,(R)V (R)G(R)E
Kk
= constant of proportionality
Am(R)= activity concentration which may vary with
size
V(R) = volume (4/3913)
G(R) = geometry factor (inverse square law for
spherical particles and isotropic radia-
BE
tion)
= correction factor used to express the
portion of the energy distribution of
beta particles to which the film is
sensitive
As the limit of detection is reached, the percentage of hot
Yen
particles detected will decrease from 100 to 0.
This variation is to
ve expected since the experimental methods of detecting, the process of
radioactive decay, and the exposure of developing centers of a filn
are all statistical in nature. This effect will be sudden or gradual
(with variation in flux), depending on the statisticaly spread, which
may be quite large,
It is then necessary to make a correction for the
detection efficiency (percentages of hot particles detected) in
the range where this effectis present.
Because of the delay before the processing of the semples under
consideration, the activity concentration may be low enough that this
effect will be observed. At the time the autoradiographs were made,
the activity concentration was estimated to be about 0.15 po/mg.
It
was found with artificial hot Sr?°clg particles that the point of 50
per cent detection efficiency was at about 10 microns for a specific
activity of 0.005 po/mg. Two considerations make these figures cornarae
ble. The fission products have a considerably higher mean energy than
sr 6 and therefore proportionately less will be registered by this fiin.
The sr70Cl2 crystals wera very flat and would have a much smaller
geometry correction than spherical particles. The flux will then in.
crease much faster with particle size, so that a smaller particle -1i11
be detected as hot.
The fallout particles in the range under consi ‘era-
tion are essentially spherical. Therefore, the effect under consi teration may result in a variation in detection efficiency from 109 per cant
as the particle size decreases, which would have to be corrected for.
39
SECRET — RESTRICTED DATA