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treated as if it were the result of a single mean excitation energy.
There-
fore, in an analysis of individual mass yields that does not involve the exci-
tation energy explicitly (e.g., plotting the relative fission yield of one mass
vs that of another mass), data obtained from bombardments carried out with
projectiles of a mixed energy spectrum can be used together with data obtained from bombardments with monoenergetic particles.
Ill
EXPERIMENTAL
Fission at low excitation energies is best effected by neutron bombardment because of the lack of a Coulomb barrier. Available sources of monoenergetic neutrons in this energy range do not have fluxes of high enough in-
tensity to permit many radiochemical mass-yield determinations.
However,
one can obtain neutron fluxes of mixed energies with sufficient intensities to
permit a broad range of mass yield determinations.
It was pointed out that
if linear relationships between individual mass yields do exist as predicted
' by the two-mode fission hypothesis, then these relationships hold regardless
of the energy spectrum of the particles causing fission.
Neutron fluxes with
broad energy spectra can be produced by the Crocker Laboratory 60-inch
cyclotron using charged particle bombardment of various light elements.
The
intensities of these neutrons in the forward direction are much higher than
can be obtained ‘rom monoenergetic sources.
For these experiments we
have used the following neutron spectra, listed by method of production, in
order of increasing mean neutron energy:
A
io!
Be’ + 12-Mev p: Cy365 + 24- Mev a’:
+ 48-Mev He’; Be’ + 48-Mev He'’; Be? + 24-Mev a’. and Lil’? + 24-Mev at.
No precise measurements ‘were made of the mean energies of these neutron
spectra, but roughly speaking they ranged from 2 or 3 Mev to about 10 Mev.
In some cases, bombardments were repeated using the same neutron source.
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