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

available high energy neutron induced fission data
on the basis of the following considerations. The
heavy nucleus fission reaction passes through a
stage of compound nucleus formation in which the
mass and energy distributions of fragments from the
fission of the nucleus with given nucleon composition,
excitation energy and angular momentum do
not depend on the mode of formation of the
compound nucleus. Protons as well as neutrons
introduce rather small angular momentum into the
fissioning nucleus. Of course there are some
features of direct and pre-equilibrium reactions
with neutrons and protons, but they have no
decisive influence on the majority of the basic
characteristics of the distinct fission modes, as a
preliminary analysis of the data has shown. These
circumstances allow the expansion of the experimental
database with respect to both (a) an
increasing incident particle energy range and (b)
possibilities to study the mass and energy distributions
for important fissioning minor actinides from
the Np and Am isotopes, where experiments with
Np and Am target nuclei in neutron induced fission
are rather difficult to perform due to the radiation
safety requirements in handling highly radiotoxic
target materials.
The mass and energy distributions in the
proton induced fission have been spectroscopically
measured in terms of the coincident fission
fragments by means of surface barrier detectors for
the following compound nuclei:
234,236,237,239 239,240,241,243 245
Np, Am, Bk at proton energy
Ep = 10.3 MeV;
234,236,237,239 240,241,243
Np, Am at proton energy Ep =
22.0 MeV;
233 Pa and 236 Np at proton energy Ep = 7.4–30.0 MeV.
Another problem to be solved in the
multimodal approach is connected with the deconvolution
of the experimental mass and energy distributions
into the mass and energy distributions of
distinct fission modes. For this purpose, a new
method of multicomponent analysis has been
developed free from any assumptions about the
shapes of the mass distributions of distinct modes.
This method was applied to the analysis of the
available experimental information.
This analysis revealed regularities in the
behaviour of the fission fragment mass yields that
184
proved to be particularly useful in developing the
predictive systematics of the yields. The systematics
have been realized in the form of the PYF computer
code (Program for Yields from Fission), which allows
calculation of the pre- and post-neutron emission
fragment mass yields in the proton and neutron
induced fission of target nuclei ranging from Th to
Bk for incident particle energies of 5–200 MeV.
4.5.2. Experiments
4.5.2.1. Experimental technique
Measurements have been carried out with
external beams from the Almaty isochronous
cyclotron U-150, which enables the protons to be
accelerated in the energy range from 7.4 to 30.0
MeV within energy steps of about 2–3 MeV. A pair
of Si–Au surface barrier detectors and eight targets
are located in the vacuum area of a fission chamber.
The targets of fissile isotopes were in the form of
layers of thickness 20–40 mg/cm 2 , made from
chlorides of the corresponding elements by vacuum
evaporation on 60 mg/cm 2 backings of Al 2O 3. The
investigated isotope was enriched to ≥99.98% in all
cases.
Mass and energy distributions (MEDs) have
been measured by the fast spectrometry of
coincident fragments, where background events have
been identified by their characteristic flight time
difference [4.5.8]. This version of the experimental
technique differs from the commonly used method
in that the short current signals with a pulse length
of about 7 × 10 –8 s and a front of about 5 × 10 –9 s are
used in the time and spectrometric channels. Thus
the probability of ‘false’ events, such as random
coincidences of pulses from fragments belonging to
different fission events, and random overlapping of
the pulses from a fragment and a beam particle
scattered in the target, decreases by a factor of
almost 10 in comparison with the traditional
technique.
During the experiment, the following
information has been recorded for every fission
event:
Q 1 , Q 2 — values of the charges induced by
fragments in both detectors;
t 1 , t 2 — rates of pulse rise;
dT exp — difference in flight times of two fragments.