Neutron- and neutrino-induced reactions : their physical description and influence on r-process calculations

When I started this thesis my supervisors F. K. Thielemann and T. Rauscher
spoke to me enthusiastically about the exciting and interesting field of nuclear astrophysics
at our very first meeting. A field actually consisting of several branches
of physics: thermodynamics, quantum mechanics, hydrodynamics, astrophysics,
nuclear physics and numerics, just to mention some. During my thesis I had
the opportunity to get deeper insights in nuclear physics, and astrophysics. Nuclear
astrophysics and nuclear physics are inseparably connected. Astrophysical
simulations, such as the modelling of supernova explosions or nucleosynthesis calculations,
depend directly on nuclear physics information, like nuclear masses,
half-lives, or nuclear reaction rates which can impose some constraints on the
astrophysical parameter space. On the other side astrophysical simulations can
provide constraints on nuclear models, such as the prediction of nuclear masses.
Current accelerator facilities are able to provide this information for stable nuclei
and for a broad range of neutron-rich nuclei. However, extremely neutron-rich
nuclei still cannot be investigated due to their extremely small half-lives. Experimental
information for these nuclei will be available at the earliest from the
next generation of accelerators, which are currently under construction and will
provide experimental information for such neutron-rich nuclei starting in a couple
of years.
The first part of this thesis is dedicated to nuclear physics. The calculation
of reaction rates which enter nucleosynthesis calculations in astrophysical simulations
is an important ingredient. The way these rates are calculated depends,
among other things, on the projectile energy and the mass region where the reaction
takes place. For intermediate and heavy masses the reaction rate can
be described by the so-called compound nucleus picture. In this model the the
projectile and target nucleus form a compound state which de-excites by various
particle evaporation modes. The compound nucleus picture describes the
reaction mechanisms well if there are enough levels at the energy at which the
compound nucleus is formed so that an average over individual resonances can
be performed. This model is called the Hauser Feshbach or statistical model. In
the case of absence of the described levels at the formation energy, other reaction
mechanisms have to be utilized to calculate the reaction rates, like direct reactions
for example. The nuclear level density (NLD) is an important ingredient
in the calculation of nuclear cross sections. In Chapter 2 the derivation of the
NLD is reviewed. It will be shown that the NLD can be decomposed into three
parts: a spin-, an excitation-energy-, and a parity dependent part. It will be
assumed that the odd and even parity states are equally distributed. Chapter 3
is devoted to the calculation of the NLD - but this time without the assumption
of equally distributed parities. An energy dependent parity-distribution function
will be derived.
Nuclei heavier than iron are predominantly made by neutron-capture processes.
The solar system abundance pattern of heavy nuclei indicates that two
distinct neutron-capture processes occur in nature - one at low neutron density,
called s-process, and one at high neutron density, called r-process. The r-process,
or rapid neutron-capture process will be discussed in detail in the second part
of my thesis. General aspects of nucleosynthesis calculations are reviewed in
Chapter 4 including a discussion of the mathematical framework. A discussion of
possible r-process sites, observational informations and astrophysical parameters
are presented in Chapter 5. In Chapter 6 the focus will be set on the neutrino
driven wind as a possible site for the production of elements heavier than iron. For
the first time, neutrino-induced, neutron-induced, and beta-delayed fission are included
simultaneously in a r-process nucleosynthesis calculation. Possible effects
of all fission channels on the final abundance distribution and the discussion of
the relevant nuclear physics are presented in Chapter 6, too. The thesis concludes
with a short discussion of the results and an outlook on future improvements and
investigations.