Perspectives of Nuclear Physics in Europe - European Science ...

Significant uncertainties remain for certain key reactions
involving radioactive nuclei, particularly, 18 F(p,α) 15 O,
25 Al(p,γ) 26 Si, and 30 P(p,γ) 31 S. The former reactions play
a role in the predicted abundances of the cosmic γ-rays
sought in satellite telescope missions, such as the 511
positron annihilation line. The latter reaction is key to
understanding the production of elements such as Cl
and Ar in the ejecta of ONe novae. Accompanying these
nuclear physics uncertainties, are uncertainties in novae
modelling regarding the mechanism responsible for
mixing at the core-envelope interface – this is required
to explain the large metallicity enhancements inferred
for the ejected nova shells. This and other aspects will
require further development of multidimensional hydrodynamic
codes – to date most of our knowledge on the
nature of classical nova outbursts relies on spherically
symmetric models.
Type I X-Ray Bursts
With a neutron star as the underlying compact object
site for the explosion, peak temperatures and densities
in the accreted envelope reach quite high values:
T peak > 10 9 K, and ρ ~ 10 6 g.cm -3 . These values are about
an order of magnitude larger than in a typical classical
nova outburst and consequently break-out from the hot
CNO cycles may occur by the onset of the 15 O(α,γ) 19 Ne
reaction, and (α,p) reactions on 14 O and 18 Ne.
The burst conditions depend on a delicate balance
between these uncertain nuclear burning rates and the
fuel supply from the in-falling envelope material. Following
breakout, the αp process ensues. Here, detailed features
of X-ray burster light curves have been linked to a small
number of (α,p) reactions on even-even T z = -1 nuclei,
whose reaction rates are highly uncertain, but depend
critically on level densities at high excitation energies.
Predicting these level densities in this region is a current
theoretical challenge for microscopic calculations for
nuclei in this region, going beyond the traditional statistical
model (Hauser-Feshbach) approach. This latter
method is expected to become even more questionable
as the proton-drip line is approached in the subsequent
rp-process, where level densities become very low in
the burning regime. In such regions of medium to heavy
highly proton-rich nuclei, it will be important to obtain
nuclear structure information on masses, half-lives and
excited states. Although a large number of nuclei are
involved in the rp-process, sensitivity studies have shown
that only a relatively small number of nuclear reactions
(~30) have significant effects, most notably the 65 As(p,γ)
and 61 Ga(p,γ) reactions. The mass of 65 As is particularly
critical for determining the nucleosynthetic flow in a
range of X-ray burster models, since it bridges a potential
waiting point around the N=Z nucleus 64 Ge.
The specific location of the rp-process termination
point is still a matter of debate – recent nuclear structure
studies around this region suggest that photodisintegrations
in the SnSbTe-mass region are not efficient enough
to halt the extension of the nuclear path. It still remains
uncertain if material can be ejected from X-ray bursters
and contribute to the wider galactic abundances. From
the modelling point of view, probably the most critical
issue is to assess whether a mechanism, such as
a radiation-driven wind at late stages of the TNR, can
achieve this effect.
Supernovae
Stars with more than about 8 solar masses continue
their burning phases after Helium burning. Massive stars
with more than about 10 solar masses go through the
advanced burning stages hydrostatically whereas stars
in the range between 8 and 10 solar masses exhibit
degenerate conditions in the core and cannot reach
stable burning anymore. They undergo the advanced
burning phases in an incomplete manner during collapse
of the central part of the star. Ultimately, the more massive
stars face core collapse with subsequent supernova
explosion (ccSN). Silicon burning produces a stellar core,
composed of nuclei in the Ni-Fe region, which starts
contracting as more and more mass is accumulated,
while the Si burns in a surrounding shell. Increasing density
also increases the Fermi energies of the electrons
in the plasma and allows electron captures on nuclei.
The loss of electrons leads to a further decrease in the
pressure and the contraction turns into a collapse with
rapidly increasing density, allowing further electron
captures. The collapsing core decouples into an inner
and an outer core. The inner core collapses rapidly to
a proto-neutron star with the outer core following more
slowly. During the early phases of the collapse neutrinos
produced by electron captures leave the core
freely, carrying away energy and keeping the temperature
relatively low. Consequently, as the density grows,
very massive neutron-rich nuclei are produced through
electron captures.
Under supernova conditions electron captures
are dominated by Gamow-Teller (GT) transitions.
Experimentally, these transitions can be studied by
charge-exchange experiments at low momentum
transfer. With the advent of experiments based on the
(d, 2 He) reaction at KVI, it has been possible to obtain high
resolution data necessary for the constraint of theoretical
calculations of electron capture rates on iron group
nuclei. Consequently, it has been possible to validate
theoretical calculations of weak interaction rates based
in the interacting shell model in the mass range A=45-65.
Heavier nuclei become important as the collapse pro-
Perspectives of Nuclear Physics in Europe – NuPECC Long Range Plan 2010 | 135