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I .... MDDC-1034
ATOMIC ENERGY COMMISSION
COSMIC-RAY INDUCED FISSION
Darol K. Froman
Bruno B. Rossi
i Published for use within the Atomic Energy Commission. Inquiries for additional. spies
-tand ay questions regarding reproduction by recipients of this document may be referred
S:to the Technical Information Division, Atomic Energy Commission, P. 0. Box E, Oak Ridge,
L.nasmuch as a declassified document may differ materially from the original classified
document by reason of deletions necessary to accomplish declassification, this copy does
not constitute authority for declassification of classified copies of a similar document which
Smay bea the same title and authors.
Date of Manuscript: Unknown
:. Jument Declassified June 20, 1947 I- i EPOl
.This document consists of 3 pages.
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S.* -- MDDC-1034
: COSMIC-RAY INDUCED FISSION
S.By Darol K. Froman, Louis Rosen, and Bruno B. Rossi
Fission produced by cosmic-ray neutrons is, of course, exactly like any other kind of
1inion. Fission fragments can be detected by the well-known photographic method. Special,
i'stman fission plates show tracks of fission fragments without troublesome background.
Ifi6 first slide shows microphotographs of tracks produced by the fission.of U-235 by cosmic-
Igf neutrons. An interesting case of a collision of a fission fragment with a heavy nucleus
in the emulsion, probably silver, is shown.
On examining a total of some thirty thousand fields of view on photographic plates placed
:Incontact with foils of U-235, U-238, and thorium, fission has been found to occur in each
:ease when the plates were exposed to the cosmic radiation at an altitude of 30,000 ft. In the
case of U-235, the fission rate is increased very appreciably by surrounding the foil with
.two inches of paraffin. This is to be expected from the slowing-down properties of paraffin
for neutrons and the increased fission cross section of U-235 for slow neutrons. Since both
U-238 and thorium exhibit fission, it is clear that there are at least a few neutrons of energy
above the fission thresholds of these elements.
Previous measurements of the cosmic-ray neutrons have been made at 30,000 ft, using
B-10 as a detector; The B-10 filled counters were shielded in various ways with neutron
-absorbing material. It is clear that a knowledge of the fission cross sections, the B-10
cross section, and the absorber cross sections, as functions of energy, enables one to make
up-an energy spectrum for the cosmic-ray neutrons. This can be done either by trial and
Error or by fitting an assumed spectrum containing several constants to the data. The abso-
lute value of the fission cross sections fixes the spectrum on an absolute basis. Using the
fission data from the photographic plates, and the ratios of the counting rates of B-10 coun-
ters shielded in three different ways, a spectrum has been deduced for the cosmic-ray neu-
trons at 30,000 ft. This spectrum does not change much with altitude up to 30,000 ft, since
the ratios of the B-10 counts remain constant from 10,000 to 30,000 ft.
Slide 2 shows the spectrum obtained in this way. The abscissae are the energies on a
logarithmic scale. Curve 1 shows the neutron flux vn(E) and curve 2 shows the density of
neutrons.. Curve 2 is plotted on a logarithmic scale. Because of the poor statistics in the
counts of fission tracks, no great precision is claimed for this spectrum. Also, it may not
be a unique solution but it does fit the experimental data of six different energy-dependent
detectors. This spectrum is probably right to a factor of two or three. In any case, it is
certain that the flux of thermal neutrons is very small as expected from the neutron absorp-
tion by nitrogen, and it is quite probable that the large decrease in flux as the neutrons are
slatedd down through the kilovolt region is real. The total density of neutrons of all energies
:at 30,000 ft comes out to be about 0.8 per cubic meter.
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