Abstract or Summary 
 The gamma contribution to reactor afterheat
depends on gamma energy since highenergy commas are
more likely to escape from tilt. fuel. The objective of
this work is to investigate the gammaenergy spectrum
of a pure U235 fission reactor.
Fissionproduct nuclides are first listed in order'
of importance as contributors to total gamma energy at
various times after shutdown. In particular, lists are
drawn up using LNDF/BIV data for 1, 10, 100, 1,000,
and 10,000 seconds after a fission burst and after an
operating time of 24 hours. Luclides from these lists
are then compared with nuclides from a magnetic tape
entitled "radioactive Nuclide Decay Data FileCWR
1774" by Charles W. Reich of the Aerojet Nuclear
Corporation. The Reich tape provides little data about important gamma contributors in the early stages after
short irradiation times.
A simrlified approach is chosen to estimate the
gamma spectra of nuclides without Reich data. That is 1
nuclides in the Reich. tape are sorted into "boxes"
(collections of nuclides) on the basis of simple nuclear
systematics and the "box" averages are assumed to
represent the nuclides without Reich data. Isomeric
states, nuclides with less than ten gamma spectral
lines, and nuclides with atomic mass numbers above 200
are generally excluded from sorting.
Each sorting into boxes is done in two ways: (1)
by the "absoluteenergy" method, where gamma spectra
are split into "energy groups" of fixed intervals, such
as 250 keV, and (2) by the "adjustedenergy" method,
where the gamma spectrum of each nuclide is split into
"energy groups" found by dividing that nuclide's Q
value by a fixed number, such as ten. The second
method is preferred.
In a "zeroeth" sorting, all nuclides are put into
just one box. In the first sorting, nuclides are put
into four boxes (eveneven, evenodd, oddeven, and
oddodd) based on the oddness or evenness of proton
number and of neutron number. In the second sorting,
nuclides are put into four boxes (1 Z magic, N0 magic,
0Z magic, and 00 magic) on the basis of distance from the magic numbers 28, 50, 82, and 126. A nuclide is
deemed "Nmagic" (for its neutron number) or "Zmagic"
(for its proton number) according to its "closeness" to
a magic number, where "closeness" is defined by a
bandwidth around each magic number. In the third
sorting, nuclides are put into one of four boxes
according to distance from beta stability, which is
equal to the number of consecutive betaminus decays
required to reach a stable nuclide (or a very longlived
nuclide). In the fourth sorting, nuclides are
put into two boxes (positive and negative) on the basis
of the parity of the ground states.
For each box, an average spectrum (normalized to
100%) is computed to indicate the percentage of gammas
contributed by each energy group. Also calculated for
each energy group is its standard deviation from the
average spectrum. Since the standard deviations are
usually too large to be of value, "composite" spectra
are created by weighting the average spectra from
various individual sortings. In the "tricomposite"
case, each of three factors (oddevenness, distance from
magic number, and distance from beta stability) are used
in several combinations of weighting. In the "optimum"
combination (where the sum of the squares of the
standard deviations is minimized), oddevenness has
about twice the weight of distance from magic number, while distance from beta stability is insignificant.
Finally, the optimized composite spectra are used
to provide estimates of actual gamma spectra at various
instants after reactor shutdown. several operating
histories prior to shutdo,cm are considered for purposes
of comparison.

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