36
RADIATION STANDARDS, INCLUDING FALLOUT
out at greater distances. This fractionation is most pronounced im
the case of low-yield, land-surface bursts and least in air bursts and
very high yield bursts.
Theparticle size distribution in the debris cloud from a land-surface
burst, and hence in the fallout, shifts with time toward the smaller
sizes which contain a larger proportion of fission products relative to
earth material. This is because the larger particles fal] out first. The
fraction of the nuclide, which is soluble and hence immediately available biologically, tends to increase with time and distance from the
burst. Actual measurements at the Nevadatest site show a low ratio
of biologically available to total Srwithin a few miles from the
burst point as compared with the ratio at greater distances. The ratio
of Sr * to total fission products in the closer in area is also low.
Irregularities and “hot spots” in the distribution pathway of the
fallout within the first few hundred miles have been observed unrelated to rainstorms which seem to be the principal immediate cause of
“hot spots” farther away. Irregularities of terrain and small-scale
irregularities of the wind field in time and space are probably responsible, however, present meteorological data are inadequate to
account for and hence precisely predict them.
On the other hand, upper wind observations closely spaced in time
have permitted substantial improvementin predicting the direction of
the major axis or “hot line” of the fallout pattern.
Stratospheric fallout has somewhat different characteristics and
distributions. It consists of particles which rise into the upperatmosphere and which are too small to fall out as local or tropospheric
fallout during the first month following their formation. Because of
their small size, 0.01 to 1 micron (one-millionth to one ten-thousandth
of a centimeter in diameter), they are removed from the atmosphere
slowly and their average time of suspension, depending on their initial
placement in the stratosphere, is a matter of several months or years.
The distribution of both artificial and natural radioactivity in the
stratosphere, and the time scales and mechanisms involved in their
transport and deposition on the surface of the earth, have received
considerable study, and during the past 3 years a numberofsignificant
advances have been made in understanding these phenomena.
It has become clear now that the principal mechanism of transport
of these particles from the lower stratosphere into the troposphere is
downward movementof air masses. The hypothesis put forward in
the 1959 hearings of a spring maximum anda fall minimumof stratospheric fallout in the Northern Hemisphere has been established by
observations made in the absence of recent testing during 1960 and
1961. A similar though less pronounced seasonal variation was observed in the Southern Hemisphere.
Now if I may havethe next chart (see fig. 2) it is now quite apparent
that tropospheric concentrations and deposition rates of stratospheric
debris are higher in the middle latitudes (20° to 60°) of both hemispheres than they are at the Equator, regardless of the latitudeor altitude of the stratospheric injections. You can see in this chart T have
indicated in the gray area the middle latitudes where most of the fal]out tends to accumulate regardless of our original point of injection
into the stratosphere. I also have indicated the general latitude of
various places around the world where testing has been carried ont.
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