Published in Physical Review C
91 044316 (2015)
LA-UR-14-24795
Calculated Fission-Fragment Yield Systematics
in the Region 74 ≤ Z ≤ 94 and 90 ≤ N ≤ 150
Peter Möller
Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
Jørgen Randrup
Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
Background:
In the seminal experiment by Schmidt et al. [Nucl. Phys. A 665, 221 (2000)]
in which fission-fragment charge distributions were obtained for; 70 nuclides,
asymmetric distributions were seen above nucleon number A ≈ 226
and symmetric below. Because asymmetric fission had often loosely been explained
as a preference for the nucleus to always exploit the extra binding of fragments near
132Sn it was assumed that all systems below A ≈ 226
would fission symmetrically
because available isotopes do not have a proton-to-neutron Z/N ratio that
allows division into fragments near 132Sn. But the finding by Andreyev
et al. [Phys. Rev. Lett. 105, 252502 (2010)] did not conform to
this expectation because the compound system 180Hg
was shown to fission asymmetrically. It was
suggested that this was a new type of asymmetric fission, because no strong shell effects
occur for any possible fragment division.
Purpose:
We calculate a reference data base for fission-fragment mass yields for
a large region of the nuclear chart comprising 987 nuclides.
A particular aim
is to establish whether 180Hg is part of a contiguous region of asymmetric
fission and if so its extent,
or if, in contrast to the actinides
there are scattered smaller groups of nuclei that
fission asymmetrically in
this area of the nuclear chart.
Methods:
We use the by now well benchmarked Brownian shape-motion method and
perform random walks on the previously calculated
five-dimensional potential-energy surfaces. The calculated shell corrections
are damped out with energy according to a prescription developed earlier.
Results:
We have obtained a theoretical reference data base of fission-fragment
mass yields for 987 nuclides.
These results show an extended region of asymmetric fission with approximate
extension 74 ≤ Z ≤ 85 and 100 ≤ N ≤ 120.
The calculated yields are highly variable.
We show 20 representative plots of these variable features and summarize the main
aspects of our results in terms of "nuclear-chart" plots showing calculated
degrees of asymmetry versus N and Z.
Conclusions:
Experimental data in this region are rare: only ten or so yield distributions
have been measured, some with very limited statistics. We agree with
several measurements with higher statistics. Regions where there might
be differences between our calculated results and measurements
lie near the calculated transition line between symmetric and asymmetric fission.
To draw more definite conclusions about the accuracy of the
present implementation of the Brownian shape-motion
approach
in this region experimental data, with reliable statistics, for a fair number of
suitably located additional nuclides are clearly needed.
Because the nuclear potential-energy structure is so different in
this region compared to the actinide region additional experimental data together
with fission theory studies that incorporate
additional, dynamical aspects should provide much new insight.
The complete manuscript in color
as a .pdf file
is
available for download.
We provide the 4 figures as individual .eps.gz and .pdf files files.
Please note that the .eps files do not convert properly
to .pdf by use of the ps2pdf utility. Therefore
please use the .pdf files provided. The .eps files do have
the correct bouding box to allow correct incoporation into,
for example,
.tex manuscripts.
Figure
Fig01-moller-syst.eps.gz
in format .eps.gz and
Fig01-moller-syst.pdf
in .pdf format is
available for download.
Figure
Fig02-moller-syst.eps.gz
in format .eps.gz and
Fig02-moller-syst.pdf
in .pdf format is
available for download.
Figure
Fig03-moller-syst.eps.gz
in format .eps.gz and
Fig03-moller-syst.pdf
in .pdf format is
available for download.
Figure
Fig04-moller-syst.eps.gz
in format .eps.gz and
Fig04-moller-syst.pdf
in .pdf format is
available for download.
Page by
Peter Möller
Created: Monday Aug 31 2015 -->
Last modified: Monday Aug 31 2015