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2,
RESULTS OF RECENT STUDIES
Theoretical data which provided more detail than previously available
on the output and spectra of neutrons from Little Boy and Fat Man were
circulated in a 1976 letter by W. E. Preeg of LANL (7). In the case of
Fat Man, there were no experimental neutron-output data that could be
compared directly with Preeg's theoretical data. However, excellent
agreement was found between Preeg's theoretical data and experimental
neutron-output data from measurements using the Ichiban Critical Assembly
which had nuclear characteristics similar to Little Boy (8).
Included in the 1976 letter by Preeg (7) were results of additional
calculations of neutron penetration in an infinite air medium that
predicted significantly less neutron exposure per unit energy yield of
Little Boy than the T65D estimates. This was also found to be the case
in more realistic air-over-ground calculations in 1977 by J. V. Pace (9)
of the Oak Ridge National Laboratory (ORNL) and by D. C. Kaul and R. Jarka
(10) of Science Applications, Inc. (SAI). Some troublesome discrepancies
existed in the air-over-ground calculations, however. It was shown in
1979 by Pace (11) that the moisture content of the air was an extremely
important parameter (see Fig. 1). Kaul and Jarka had used a dry NTS-
type air in their 1977 calculations, while Pace in his 1977 calculations
had used a moist air composition derived from data on atmospheric conditions
existing in Hiroshima ATE.
The LANL neutron-output and ORNL air-transport calculations were
further substantiated in a 1980 study by Kerr (12) using sulfur acti-
vation data from early tests of Fat Man devices and from Japanese studies
made immediately after the bombings in 1945. The Japanese data came
from measurements of fast-neutron induced 32P radioactivity in pure
sulfur used as an adhesive material in binding the metallic holders to
the porcelain insulators on utility poles (13). These various experimental
data on sulfur activation were found to be in good agreement with theo-
retical values based on results of Pace's 1977 calculations except at
smaller ground distances in Hiroshima (see Fig. 2). It was later
determined that this was probably due to the spherical mockup of Little
Boy used by Preeg and his calculated one-dimensional neutron output
(i.e., 1-D neutron output)(14). The 1-D neutron-output approximation
becomes less important at the larger ground distances since neutron
scattering in air will tend to mask any initial anisotropy in the actual
neutron output of the cylindrically symmetrical Little Boy device.
The JNIRS estimates of neutron exposure were based on the fast-
neutron induced 60Co radioactivity in a steel sample embedded at a depth
of 8 cm in a concrete wall facing ground zero. To convert the measured
6OCo activation to tissue kerma from fast neutrons, Hashizume et al. (5)
used the HPRR output spectrum ghich provides a mean tissue kerma of
about 2.5 x 10 Gy per n cm from fast neutrons having energies
greater than 1 keV. Since Pace's 1977 calculations indicated a mean
tissue kerma in Hiroshima of about 1.0 x 1011 Gy per n cm-2 from fast
neutrons having energies greater than 1 keV, the JNIRS estimates of
neutron exposure in Hiroshima should be high by a factor of two and one-
half or more (13). It was discovered before a more detailed study of
the JNIRS 60Co-activation data was started at ORNL that a study of these
data was also being conducted at the Lawrence Livermore National Laboratory
(LLNL). A revised JNIRS value of about 0.11 Gy at a ground distance of
1180 m in Hiroshima compared to the original JNIRS value of 0.51 Gy (see
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Kerr, G. D. Review of dosimetry for the atomic bomb survivors, article, January 1, 1981; Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc1206720/m1/3/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.