Fission Product Yield Data for the Transmutation of Minor Actinide ...

4.5. MODAL APPROACH TO THE DESCRIPTION OF FRAGMENT MASS YIELDS
IN NEUTRON AND PROTON INDUCED FISSION OF ACTINIDES
AT INCIDENT PARTICLE ENERGIES FROM 5 TO 200 MeV
D.M. Gorodisskiy, S.I. Mulgin, A.Ya. Rusanov, S.V. Zhdanov
National Nuclear Centre, Kazakhstan
Experimental and analytical work has been
performed to obtain information on the relative
yields, mean values and variances of masses and
kinetic energies of fragments from the fission of
actinide nuclei through independent fission modes.
Regularities in the behaviour of these characteristics
with respect to their dependence on the
nucleonic composition of the fissile actinides and
incident particle (proton and neutron) kinetic
energies have been used to develop new systematics
for fragment mass yields from the fission of target
nuclei from Th to Bk at incident particle energies
between 5 and 200 MeV.
4.5.1. Introduction
Numerous experimental investigations of the
mass and energy distributions of fragments in the
fission of nuclei from Pb to No [4.5.1] have
confirmed the validity of a hypothesis concerning
the existence of independent fission modes, as first
stated by Turkevich and Niday [4.5.2]. This
hypothesis has received physical substantiation in
theoretical works by Pashkevich [4.5.3] and Brosa et
al. [4.5.4]. These studies have shown that multimodality
of the mass and energy distributions of fission
fragments is caused by the valley structure of the
deformation potential energy surface of a fissioning
nucleus.
The experimental mass and energy distributions
from the fission of actinide nuclei are usually
assumed to consist of different mass and energy
distributions for three independent fission modes:
one symmetric (S) and two asymmetric — standard
1 (S1) and standard 2 (S2). Mode S is mainly
conditioned by the liquid drop properties of nuclear
matter, and therefore the most probable values of
fragment masses M are close to A CN /2, where A CN is
the compound nucleus mass. The asymmetric mode
S1 with average masses of the heavy fragments M H
≈ 134 and high kinetic energies is due to the
formation of spherical heavy fragments with Z FH
and N FH (proton and neutron numbers of a heavy
fragment) close to the magic numbers 50 and 82,
respectively. The predominant asymmetric mode S2
is characterized by average masses of the heavy
fragments M H ≈ 140, and kinetic energies are 10–12
MeV less than those of the S1 mode. Usually the
origin of mode S2 is attributed to the deformed
neutron shell closure N ≈ 88 in the heavy fragments
(see, e.g., Ref. [4.5.5]). Recently, the existence of an
additional asymmetric mode S3, conditioned by the
close to sphere shells formed in light fragments, has
been experimentally revealed [4.5.6].
One of the most promising procedures for
elaborating the mass and energy distribution
calculation technique consists in obtaining quantitative
information on the dependences of the basic
characteristics of distinct fission modes (relative
mass yields, average fragment masses and charges,
kinetic energies of fragments, variations of masses,
charges and energies, etc.) on the nucleon
composition and the excitation energy of a
fissioning nucleus. This method reveals the main
regularities of their behaviour and permits reconstruction
of the observed mass and energy distributions
as a superposition of the mass and energy
distributions of the distinct modes. In order to be
successful in the realization of this approach, a
rather large set of experimental data usable for a
comparative quantitative analysis is needed, and
an appropriate method to extract the characteristics
of the modes from the experimental mass
and energy distributions, since the distinct modes
are rarely observed directly in the fission of
actinides.
An analysis of literature data shows that
investigators in different countries concentrated
basically on the studies of spontaneous fission and
low energy neutron induced fission of the actinides.
In particular, there is only one study in which the
same experimental technique has been employed to
measure the mass and energy distributions of
fragments from neutron induced fission of 238 U at
neutron energies from 1 to 450 MeV [4.5.7].
Mass and energy distributions measured in
reactions with protons can be included in the
analysis to compensate partially for the lack of
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