This is subsection lists most isotopes of interest for direct, indirect,
or theoretical nuclear weapon applications. Basic isotopes
characteristics, and summary tables of significant neutron reaction cross
sections are included.
The neutronic data is derived from the
authoritative ENDF-VI evaluated nuclear data base compiled and maintained
by the National Nuclear Data Center (NNDC) at the Brookhaven National
Laboratory (BNL). The data listings below were prepared with the
assistance of the Japan Atomic Energy Research Insititue (JAERI).
Notes:
- The Maxwellian average cross sections
are for a peak neutron energy distribution at 0.0253 eV (a room
temperature thermal distribution).
- Molar volume is the minimum volume per
mole for the densest phase at standard pressure and temperature (STP)
- SF = spontaneous fission
- Critical mass estimates for the fissile
isotopes are given for bare spheres at the densest STP phase. Where
available these estimates are from other sources and are based either
on actual experimental measurement, or reasonably sophisticated
numerical computations. All fissile isotopes also include for
comparison critical mass estimates made by me, using an exact
criticality solution with a one-group representation of neutronic
properties. The one-group parameters are fission spectrum averages
calculated from the ENDF-VI evaluated nuclear data base.
As can be seen where outside critical
mass estimates are also available, this one-group calculation method
consistently underestimates the true critical mass - primarily because
it does not take into account the effects of inelastic scattering in
softening the neutron spectrum. The one-group calculated critical mass
estimates are thus lower bounds on the true value. Comparison between
the one-group calculations and the actual values for the highly
fissile isotopes for which good experimental data is available (U-233,
U-235, Pu-239, and Pu-241) shows a consistent underestimate of 70-75%
of the true value. For less fissile isotopes, where critical mass
estimates have been offered by others (these are mostly calculated
estimates also, but with more sophisticated models), the
underestimates are more severe (at worst 22-29% of the 'true' value
for Pu-242). This too is to be expected because the effects of
inelastic scattering is relatively greater in less fissile materials.
On the other hand, the estimates for extremely fissile transuranics
like the californium isotopes should be quite good.
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