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

powers many of the objects we observe in the cosmos.
These same nuclear reactions which generate the energy
also transmute the atoms to create new elements. The
early universe which emerged some minutes after the
Big Bang comprised only hydrogen and helium. This
would have remained a sterile and uninteresting place,
and one certainly not conducive to the development of
life, were it not for the gradual transformation of these
light elements into heavier ones through nuclear reactions
in stars.
The rates at which the reactions occur depend particularly
on the temperature in the particular site, as the
reaction cross sections depend on the energy in the collision.
At the (relatively) low temperatures during hydrogen
and helium burning the rates can be very low, with nuclei
existing for millions of years before being changed in a
reaction. For this reason only stable nuclei are important,
since any unstable nuclei produced in the reactions have
time to decay. For massive stars, as the temperature
in the stellar core grows photodissociaton reactions
become more and more important. Finally, after Silicon
burning processes mediated by the strong and electromagnetic
interactions reach equilibrium. However, weak
interaction processes never reach such an equilibrium
and should be explicitly considered. Initially, electron
captures and β decays are the most relevant processes
but as the density increases neutrino interactions also
became important. The neutrinos produced in the stellar
core play a very important role in the explosion of
massive stars, the nucleosynthesis of heavy elements
and if detected on Earth may provide us with valuable
information about the dynamics of the stellar core.
In explosive sites like nova, X-ray burst and supernova,
the interval between individual nuclei undergoing a reaction
drops dramatically and here the nuclear reactions
can be dominated by unstable nuclei. This has posed a
challenge to us until recently, firstly because we don’t
know much about the structure of these “exotic” nuclei
and secondly because we did not have the ability to
mimic collisions here on Earth. The recent development
of radioactive beam facilities has removed this restriction
and will result in rapid advances in the field with early
progress already present.
Neutron capture processes followed by β-decay are
also important and are responsible for the production
of most of the elements heavier than Iron. In particular
the r-process remains a major challenge both for experimentalists
(to measure the rates) and for theorists and
modellers (to understand in which sites in the universe
this process occurs). Moreover, it is quite possible that
we have not yet uncovered all the reaction processes
which occur in the universe – as exemplified by the recent
discovery of the υp-process.
Figure 2. An artist’s impression of a Nova, Type Ia Supernova,
or X-ray Burster in a binary star system. Material streaming from
an evolving star feeds down onto its companion white dwarf
(Nova, SNIa) or neutron star (X-ray burster) to become compressed
and ignited in a runaway thermonuclear explosion.
If we want to understand how this energy generation
determines the evolution of a particular astrophysical
site then we need to understand the details of the various
nuclear reactions that can occur – how probable
are they, how much energy do they release, what new
nuclei do they produce To do this we recreate the
collisions here on Earth, using accelerators to produce
fast moving nuclei of one type which we can then collide
with a target containing atoms of the other type. Various
detector systems can then be placed around the collision
point to determine the reaction probability, the energy
release and the various nuclei produced. By varying the
energy of the accelerator we can select the energy of the
collision appropriate to the conditions (temperature) in
the astrophysical site that we are interested in.
The facilities that we need for these studies can vary
enormously. Sometimes small accelerators housed
in University labs will suffice, while for other studies
extremely large, multi-stage facilities are required whose
technological complexity requires the resource of major
international laboratories. The detection systems can also
vary tremendously in complexity and scale, depending
on the particular reaction being studied. Other challenges
arise if we are looking at very low probability
reactions when sometimes the very infrequent collisions
of interest may be masked by signals in the detectors
from natural background radioactivity or cosmic rays.
In extreme cases this may require the measurements to
be carried out in deep underground laboratories where
these backgrounds can be screened out. Furthermore,
not all of the reactions of interest involve two colliding
nuclei of the type that can be accelerated in conventional
accelerators – some involve particles like neutrons and
neutrinos, or high energy photons, which may require
much more complex facilities. In a later section we will
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