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4.4 Nuclear Astrophysics
ceeds and becomes fully dynamical. Nuclei with proton
numbers Z40 are produced
for which GT+ transitions are blocked in an independent
particle model picture. However, many body correlations
and finite temperature excitations provide an unblocking
of the GT strength. Here, it will be necessary to develop
theoretical approaches that consistently treat both correlations
and finite temperature in the description of
weak transitions and include both contributions from GT
and forbidden transitions. From the experimental point
of view it will be necessary to extend charge exchange
experiments to unstable nuclei using radioactive ionbeams
and inverse kinematics techniques.
The series of electron captures stop once the density
becomes large enough (~ 10 12 g/cm 3 ) for neutrinos to
be trapped allowing neutrino absorption (the inverse
of electron capture) to take place. In addition, besides
neutrino-electron scattering, inelastic neutrino-nucleus
interactions become important for the thermalisation of
neutrinos. Reliable estimates of these cross sections
require the knowledge of the GT and first-forbidden
spin dipole response at finite temperature. Theoretical
calculations can be constrained using the similarities
between the weak and electromagnetic excitations of
the nucleus. In particular, M1 data can be used to constrain
the GT 0 response. However, a direct determination
of neutral current neutrino-nucleus cross sections is
certainly desirable.
Once the core reaches nuclear matter densities,
the collapse is suddenly stopped and a shock wave
is launched that completely dissociates the outer core
material. In this way, the shock wave becomes a standing
shock wave subject to several hydrodynamical instabilities.
Some of these instabilities will become accessible
experimentally to the NIF (USA) and PHELIX (GSI/FAIR,
Germany) facilities. A full understanding of the explosion
mechanism requires multidimensional radiation hydrodynamics
simulations with accurate neutrino transport and
state of the art nuclear physics input. Different simulations
have shown that the post bounce evolution is rather
sensitive to the Equation of State (EoS) and in particular
to the symmetry energy and compression modulus
(see discussion on neutron stars). The EoSs presently
used in supernova simulations are rather schematic.
More microscopic, self-consistent EoSs constrained by
the experimental data accumulated in the last decade
should be developed and implemented in simulations.
At the same time the recent suggestion that a transition
to quark matter can take place during the early postbounce
evolution should be further explored and the
observational consequences determined.
As the shock wave propagates out from the core,
explosive nucleosynthesis takes place in the higher lying
layers of the star. Different classes of nuclei are produced
in the different regions, providing a way to probe conditions
in the layers of a star. The yields of some the nuclei
(such as 44 Ti, 56 Ni) are sensitive to the actual explosion
mechanism. This offers the possibility to study details of
the explosion mechanism by combining multidimensional
models with improved nuclear input and observations
with current (e.g., INTEGRAL) and future (e.g., NASA’s
NuSTAR) γ-ray satellites.
The detection of neutrinos from SN1987 helped to
confirm the basic features of supernova physics and
in particular the important role of neutrinos. A future
supernova detection in all neutrino flavours with accurate
neutrino energy determination will provide valuable
information about the explosion mechanism. The nuclear
physics input enters in several ways. Firstly, neutrinos are
mainly emitted from regions with subnuclear densities
whose properties, equation of state and composition, are
not fully understood, particularly at the finite temperature
conditions relevant for supernova. Secondly, once,
they are emitted they travel through regions where their
interaction with nuclei can produce modifications in the
neutrino spectra. Finally, they are detected on earth via
the interaction of neutrinos with the nuclei present in the
detector material. Consequently, in order to fully exploit
the potential of a future neutrino detection it becomes
necessary to have reliable estimates of neutrino-nucleus
cross sections constrained by experimental measurements.
In this sense, the construction of a dedicated
detector for the measurement of neutrino-nucleus cross
sections at the future European Spallation Source could
be a very valuable tool. This is important not only for
improving our understanding of supernova physics
but also for disentangling purely nuclear effects from
oscillation effects due to the propagation of neutrinos
through the stellar mantle, enabling us to learn about
neutrino properties including the mass hierarchy and
mixing angle.
As a result of the explosion a neutron star is formed.
Initially, this protoneutron star is very hot and cools emitting
neutrinos of all flavours. These neutrinos interact
with the matter in the outer layers of the neutron star
producing an outflow of matter known as neutrino-driven
wind. Depending on the neutrino and antineutrino spectra
and luminosities the outflows can be either proton or
neutron-rich. Proton-rich outflows constitute the site of
the recently predicted νp-process. In this scenario the
cooling of the ejected matter results in the formation
of N=Z nuclei, mainly 56 Ni and 64 Ge, with a substantial
amount of free protons left. At this moment antineutrino
captures on free protons ensure a substantial supply
of neutrons that can be captured in the nuclei present,
mainly by (n,p) reactions, and allow for the matter flow
beyond 64 Ge in the short dynamical time scales of super-
136 | Perspectives of Nuclear Physics in Europe – NuPECC Long Range Plan 2010