Intermediate-mass-fragment Production in Spallation Reactions

1
M. Valentina Ricciardi Aleksandra Kelic
Karl-Heinz Schmidt
GSI Darmstadt Germany
Intermediate-mass-fragment Production in
Spallation Reactions
INTRODUCTION The precise knowledge of production
cross-sections of every nuclide produced in
proton- or neutron-induced reactions up to the
GeV regime is of extreme importance for the
design of accelerator-driven system (ADS), and
for many other technical applications. Nuclear
reaction codes based on the statistical model
have high predictive power thanks to the high
physics content. They are relatively fast and
represent the best tool to provide nuclear data.
During the last ten years a large body of
experimental data concerning spallation residues
was obtained in a devoted experimental campaign
realized at GSI, Darmstadt 1. Parallel to the
experimental campaign, the statistical-model code
ABLA, which describes the statistical
deexcitation of a heated nucleus, was developed.
Here we report on the progress in the development
of the ABLA code concerning the production of of
intermediate-mass fragments (IMFs). The range of
emitted fragments was extended above Z2. Two
mechanisms were implemented. At low excitation
energies IMFs are emitted as evaporated nuclei.
If the excitation energy of the system exceeds a
given threshold, IMFs are formed in the
simultaneous break-up of the system, modelled
according to a power-law distribution, which is
suggested by several theoretical models. The
improved code was benchmarked on a large amount
of experimental data, among which the
high-precision data measured at GSI.
EVAPORATION OF IMFs The evaporation of light
nuclei with Zgt2 and below the Businaro-Gallone
maximum of the mass-asymmetry dependent barrier
was implemented in ABLA. The statistical weight
for the emission of these fragments is calculated
on the basis of the detailed-balance
principle. The decay width as a function of the
excitation energy depends on the inverse cross
section, on the level densities of the two final
nuclei (the evaporated IMF and the remaining
daughter nucleus) and on the level density of the
compound nucleus above the ground state. The
barrier is calculated using the fusion nuclear
potential of Bass 2.
The figures show the comparison of our
calculation (lines) with the experimental data
(dots), taken during the GSI campaign, for the
reaction 1 GeV protons on 238U. The Z
distribution, the mean N/Z-ratio and width of the
isotopic distributions are shown. The results are
very good.
BREAK-UP PRODUCTION OF IMFs If the excitation
energy acquired during the short collision stage
overcomes the freeze-out temperature of 5.5 MeV,
the nucleus enters the region of spinodal
instabilities and breaks-up in pieces. The IMFs,
which are produced simultaneously, turn apart due
to the repulsive Coulomb force among them. In the
ABLA code the mass of the break-up IMF is sampled
from an exponential distribution (power law)
with a slope parameter fixed to 2, a value which
is found experimentally to be rather universal.
Its charge is sampled from a Gaussian
distribution, whose mean value is determined from
the A/Z-ratio and the width from the symmetry
term of the nuclear equation of state and from
the nuclear temperature 3. The figure show the
comparison of our calculation (lines) with the
measured isotopic distributions from the reaction
1 GeV p on 56Fe 1. The results are very good.
2 R. Bass, Nuclear Reactions with Heavy Ions
(Springer, Berlin, 1980) 3 A. R. Raduta and F.
Gulminelli, Phys. Rev. C 75 (2007) 024605.
REFERENCES 1 www.gsi.de/charms/data.htm