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4.4 Nuclear Astrophysics
neutron emission. At some point in an isotopic chain,
proton and/or α emission will become faster and deflect
the reaction path. After the shockwave has passed, the
proton-rich, unstable nuclei can decay back to stability.
Current models cannot consistently explain the formation
of all p-nuclei. Nuclear data is largely missing in the relevant
energy range to compute the astrophysical reaction
rates for the capture and photodisintegration reactions,
even closer to stability where only few measurements are
available close to the relevant energy. As the nuclear level
density is high, Hauser-Feshbach models can be used
to predict cross sections, but the required inputs still
need improvement, in particular optical potentials and
nuclear levels. The experimental determination of cross
sections at the relevant energies would be preferable, for
unstable targets with, e.g., decelerated beams at FAIR, or
directly in inverse kinematics at SPIRAL2 using a recoil
separator like FULIS. It has to be mentioned that there
is a general shortcoming in the production of 92,94 Mo
and 96,98 Ru which may not be solved by improving the
nuclear input alone. Alternative production mechanisms
and sites have to be explored, such as the νp-process
in ccSN or explosive burning in SNIa.
Type Ia Supernovae result from the disruption of
a White Dwarf in a binary system (or the merging of
two White Dwarfs as a sub-class). The infall of material
from the companion star pushes the WD over the
Chandrasekhar limit and leads to a collapse and explosion.
They are the main Fe factories in the Galaxy and
have become important recently as distance indicators
for cosmology. Much of the nuclear physics in these
sites mirrors that of explosive C and O burning in ccSN.
In terms of modelling, the main nuclear uncertainties
include 12 C+ 12 C, 16 O+ 12 C and electron captures on nuclei
in the iron region necessary to determine the yield of 56 Ni.
However, the largest uncertainties are in the astrophysical
modelling of the White Dwarf disruption, in particular
the speed of the burning flame and the likely transition
from deflagration to detonation – subsonic to supersonic
burning front.
Neutron Stars
Born from catastrophic gravitational core-collapse
supernovae, neutron stars are the largest nuclear systems
found in the universe, with ∼10 57 baryons confined
inside a radius of about 10 km. The density in the central
cores of neutron stars can exceed several times
that found inside heavy atomic nuclei. The properties
of such matter remain largely unknown and its theoretical
description is one of the most challenging issues of
nuclear and particle physics.
About two thousand neutron stars have been detected
but many orders of magnitude more are expected to exist
Figure 5. The basic structure of a neutron star
(F. Weber, SDSU,2010).
in our Galaxy. Most of them are radio pulsars but various
other kinds of neutron stars have been found. Binary pulsars
are extremely interesting since neutron star masses
can be very precisely measured and various effects
predicted by General Relativity can be tested. Several
decades of intensive observations from ground-based
and space-based instruments have lead to the discovery
of remarkable phenomena such as quasi-periodic oscillations
in low mass X-ray binaries, bursting millisecond
pulsars, X-ray superbursts, quasi-periodic oscillations in
giant flares from soft-γ repeaters and the thermal relaxation
of soft X-ray transients. More refined observations
are expected to come with the advent of new instruments,
such as the International X-ray Observatory (IXO). The
development of atomic stellar spectroscopy of neutron
stars, together with the improvement in the observational
techniques, will allow more accurate measurements of
their mass and radius. Neutron stars are also powerful
accelerators of high-energy particles as discussed in
the section about supernovae. The last two decades
have seen the construction of several European (VIRGO,
GEO600) and other (LIGO, TAMA300) gravitational-wave
interferometers which are now collecting data. More
advanced detectors are already under development.
Although the interpretation of all these observations is a
difficult task, it can ultimately shed light on the intimate
properties of matter.
The outermost solid layers of a neutron star represent
only a few percent of the star’s mass but are directly
related to many observable phenomena. The outer
crust is formed of a crystal lattice of neutron-rich nuclei
immersed in a dense electron gas. Its composition is
completely determined by the masses of exotic nuclei
with Z/A reaching ∼0.3. Below ∼10 11 g/cm 3 , the masses
of the nuclei present in the crust have been precisely
measured. However at higher densities nuclear masses
138 | Perspectives of Nuclear Physics in Europe – NuPECC Long Range Plan 2010