SESSION II DISCUSSION
325
assumed. During the U.S.S.R. tests in the far northern atmosphere,
the fallout was considerably faster than what it would be based on this
assumption. Could it be that this is due to the fact that the debris itself was injected at a fairly low, about 65,000-ft, altitude instead of
at the higher altitudes of the shots that gave rise to this five-year halfresidence time.
FEELY: I think the five-year half-residence time is an overestimate. For debris from all past tests—and I know that many others
who have worked with fallout data would agree— the data indicate that
a one-year half-residence time would be a better estimate. For debris
from the 1959 U.S.S.R. tests, a half-residence time of much less than
one year was indicated.
If there is a one-year half-residence time in 1963 to 1964, this
would be in general agreement with past observations. The only debris
that might approach a five-year half-residence time, I believe, would
be debris from rocket shots such as the 1958 Orange shot. The '?Rh
data indicate a long residence time. Hopefully, measurements of the
109Cd from the 1962 Starfish Prime shot will help us to estimate the
residence time of the debris from it.
BENSON:I have another question that concerns the increase in
the activity belt at 75,000 ft. It appeared from your slides that this
belt was rather constant regardless of the height of the tropopause
and the latitude at which the sample was taken. Is this true? Do you
have any explanation for why the activity belt should be that high
above the tropopause, in terms of equilibrium particle content, for
example ?
FEELY: We do find most of the debris from low-altitude bursts
stabilizes within, say, 30,000 ft above the tropopause for very high
yield shots. We don’t have as much evidence on tropical nuclear bursts
aS we would like, because most of our conclusions are based on data
from WU-2 aircraft sampling. Balloon sampling is not being performed
in the equatorial area where injections have been made.I do think this
belt increases with altitude toward the equator and decreases with alti-
tude toward the poles, more or less subparalleling the tropopause and
the isentropic surfaces. The belt is not exactly horizontal. |Editor’s
note: The AEC Balloon Sampling Program began high-altitude sampling
at a tropical site in October 1964,]
FROM
THE FLOOR: Dr. Whitby, what is the purpose of RaD
[?°pp] in your equipment, and what is the charge on the aerosol you
use?
WHITBY: I am sorry that I did not mention in my condensed presentation here that the ions we use are -positive. The complete paper
has all of these details.