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4.4 Nuclear Astrophysics
from severe theoretical problems. Although all β-decay
and EC rates of relevance in the s process are known
under terrestrial conditions, the contribution of thermally
populated excited states, as well as atomic effects in
the strongly ionized stellar plasma can drastically modify
the laboratory values. With the exception of isomeric
states, the half-lives of excited states cannot easily be
measured, and the complexity of the nuclear structure
makes the prediction of the required β - decay matrix
elements a real challenge for nuclear theory. Charge
exchange reactions like (p,n) can be used to determine
β-decay rates under stellar conditions indirectly, which
is most valuable for modern s-process networks.
Underground facilities
At present, only one deep underground facility dedicated
to nuclear astrophysics exists: LUNA (Laboratory
Underground for Nuclear Astrophysics), located at the
Laboratori Nazionali del Gran Sasso (LNGS) in Italy.
As outlined earlier, this facility has carried out crucial
measurements on key pp chain reactions and the first
measurements of CNO-cycle reactions. There is now
a recognised need for a higher energy underground
accelerator so that the key reactions involved in more
advanced stages of stellar burning can be measured,
including the neutron sources for the r-process. This
requires an accelerator with multi-MV capability.
Recently, the LUNA Collaboration presented to LNGS
a Letter of Intent (LoI) for a long-term scientific programme
based on a new 3 MV accelerator. The proposed
programme received very positive response but the decision
of LNGS is still pending. A similar LoI has recently
been submitted by a group of European researchers to
the Scientific Committee of the Canfranc Laboratory
with a scientific programme partially complementary to
the LUNA project. The Canfranc Scientific Committee
expressed a first positive opinion encouraging the proponents
to submit a full proposal. Other underground
accelerator projects being considered in Europe are at
Boulby in the UK and Praid in Romania and there is also
a low background facility being considered at Dresden
in Germany. In the USA plans are well advanced for
the DIANA project (Dakota Ion Accelerator for Nuclear
Astrophysics), a very high intensity, several MV accelerator
at the new deep underground DUSEL facility.
The experience of the last 20 years demonstrates
that underground nuclear astrophysics is one of the
key approaches in the solution of the most important
questions still open in the field. The list of experiments
in the LoIs quoted above corresponds to a two to three
decade project at one MV machine. In some cases the
experiments are exceptionally challenging and they will
need to be carried out in the framework of a collaboration
of suitable strength. The effort to put into operation a
machine of several MV in a European deep underground
laboratory should be considered with the highest priority.
This could be achieved in the next three to five years
with the opportunity to measure one or two key reactions
within the next decade. Considering the high scientific
interest in measuring several more nuclear reactions,
the case could be made to complete the programme
with a second facility designed for a complementary
set of measurements.
Neutron measurements
Measurements of neutron capture reactions are essential
for a better understanding of the slow neutron capture
process (s process). Stellar network simulations require
as input neutron capture cross sections for many nuclei
along the valley of stability. Cross sections of the lightest
nuclei, especially the CNO group nuclei, are important
because they act as neutron poison and consume neutrons,
which are then not available for the s process.
Cross sections for nuclei in the mass range 56≤A≤90
are needed for the weak component and nuclei in the
range 90≤A≤210 are crucial for the main component of
the s process. There has been a lot of progress in the
last decades, which has led to an extensive database
that can be used in s-process simulations. However,
some neutron capture cross sections, in particular those
for unstable nuclei are still not known with sufficient
accuracy. This includes for example important branching
point nuclei such as 63 Ni, 79 Se, 95 Zr, etc. but also
neutron capture reactions which lead to the production
of certain radioisotopes which can be observed by γ-ray
astronomy (e.g. 60 Fe).
The first milestone of these challenging experiments
involves the production and preparation of radioactive
targets, which requires the development of hot target
handling laboratories, such as the CACAO (Chimie des
Actinides et Cibles radioActives à Orsay) which is currently
under construction in Orsay. New facilities like
FAIR, SPIRAL2, or ALTO could contribute to the production
of the radioactive material, either by neutron induced
reactions on stable materials or by direct production of
neutron rich-nuclei. Finally, next generation neutron timeof-flight
facilities such as n_TOF2 (CERN, Switzerland),
SARAF/LiLiT (Weizmann Institute, Israel), LENOS (INFN,
Italy), or FRANZ (University Frankfurt, Germany) with
unsurpassed neutron fluxes are necessary to perform
such measurements.
Indirect methods might also play an important role
in the determination of neutron reaction cross sections.
The Coulomb dissociation (CD) method would use the
virtual photon field in the vicinity of a heavy target
nucleus to break a given nucleus into a residual plus
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