The light emitted by the stars is a direct result of the nuclear reactions that naturally occur when
massive clouds of gas collapse and heat, whether it be the constant shine of our
Sun, or the newly observed flares of gamma-ray bursters. The abundances of the elements
here on Earth are also the result of nuclear reactions that have occurred in previous generations of stars.
The understanding of nucleosynthesis processes and of the energy generation in astrophysical objects is the
subject of Nuclear Astrophysics.
The Edinburgh Nuclear Physics Group has a very strong record in nuclear astrophysical research. A selection of currently active research areas is summarised below:
Accretion from a hydrogen rich envelope of one star onto the electron degenerate
surface of a compact evolved companion can generate the conditions required
for a thermonuclear runaway. In a binary system such a case might be
when material from the outer layers of a red giant
is transferred on to the surface of a white dwarf. Such an event is what we observe as a classical nova, and
understanding the sequence of nuclear reactions that occur is the key to understanding the observed
light curves and elemental abundances in the ejecta. Similarly, X-ray bursters can be interpreted as
accretion on to a neutron star. In this case the deeper gravitational potential wells lead to
hotter temperatures and higher pressures, and consequently the nuclear reaction
pathways follow faster, more extreme routes.
The rate at which various reactions occur is determined
not only by the environment, but also by the
properties of the nuclei involved, typically their masses, halflives, modes of decay and properties
of any excited states.
Many of the nuclei involved are generated in the explosions themselves, and can be unstable nuclei with quite short halflives. Hence, the Edinburgh group has taken the opportunities afforded by the recent developments of radioactive ion beam technology to investigate reactions involving unstable nuclei. Experiments have recently been performed at TRIUMF in Canada, the CRC at Louvain-la-Neuve in Belgium, and the HRIBF at Oak Ridge in the US.
In addition, the Edinburgh group has exploited its expertise in using silicon detectors to complement the new accelerator technology with world leading charged particle detectors, such as the LEDA array.
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The vast majority of any star's life is spent in an almost perfect steady state of thermal equilibrium, for example stars in the main sequence such as our own Sun, or those that are in their red giant or AGB phases. Their luminosity is still powered by nuclear reactions such as those in the p-p chain or the CNO cycle occurring deep in the stellar interior, and later stages of evolution can depend on the nucleosynthesis that occurs in this quiescent period. However the lower temperatures and pressures of these stellar environments mean that the reaction rates are very small for the nuclear reactions and it is this that leads to the longevity of the stars. This can be understood in terms of the Coulomb barrier: at the temperatures involved, maybe a few millions of degrees, the corresponding kinetic energies of the particles are only a few keV, much smaller than the Coulomb barrier. This makes the direct measurement of these reaction rates in the laboratory extremely difficult.
However, novel experiments are beginning to be able to reach the sensitivity necessary to probe these reactions at the relevant energies. Even with intense beams, event rates around 1 per day may be encountered, and hence removal of all possible sources of background must be achieved. To remove the flux of events induced by cosmic rays passing through or near the detector apparatus, these experiments may be performed underground.
The laboratory for underground nuclear astrophysics (LUNA) essentailly consists of a low energy accelerator that supplies isotopes such as 3He nuclei for measurements of the reaction rates of p-p chain interactions.
Of particular importance for these measurements is correctly taking in to account the screening effects of plasmas. In the stellar environment it is well known that the presence of free electrons in a plasma acts as an enhancement factor for the reaction rate, especially at lower temperatures. A similar effect is present in the laboratory where the interacting species are typically in the form of ions and atoms.To calculate the effect of the screening, one needs to know the exact energy loss of the ions passing through target, but recent high precision measurments of this have shown marked deviations from the theoretical predictions. A tentative possibility is that this is due to atomic shell effects not previously included. Further research continues.
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Although the origin of the abundances of the elements is generally well understood, significant uncertainties still exist. A case in point is that of the so-called p-process nuclei, that is, nuclei that are blocked from production via the r- and s-processes by stable nuclides.
Nucleosynthesis of the p-process nuclei and thus the nature of the p-process remains puzzling. Proton-rich nuclides can be produced by the photodisintegration of heavy seed nuclei. This is thought to take place in oxygen-shell burning and explosive nucleosynthesis in massive stars which end in type II supernovae. However, it is found that p-nuclides are underproduced in certain mass regions. Because of this short-coming of the type-II supernova production, other scenarios have been suggested. Proton-capture reactions in type Ia supernovae have been proposed to build up the light p-isotopes but this remains difficult unless one has a large enhancement of s-process nuclei in the material accreted on the white dwarf. In extremely proton-rich, high-temperature environments an rp-process can ensue, i.e. rapid proton capture faster than beta decays on light seed nuclei. This moves the synthesis path to the proton-rich side far away from stability. Since an endpoint of the rp-process was recently found around mass 100, the light p-nuclides could be produced without altering the abundances of the heavier ones. The rp-process is thought to power so-called X-ray bursts but it remains unclear how much material is actually ejected. A combination of all the processes described previously can also be envisaged.
The theoretical description remains largely untested and depends upon complex network calculations within a given astrophysical environment that involves hundreds of reactions. In the case of p-process calculations especially, many of the reactions involve unstable proton-rich nuclei. Consequently, only a handful of reaction rates in the region of interest have been measured and so p-process studies are forced to rely on reaction models to estimate the reaction rates.
In an effort to reduce the nuclear uncertainties in the above models, we have made cross section measurements of a range of different (p,g) and (a,g) reactions using activation techniques with HPGe gamma-ray detection. Measurements use the 11 MV FN Tandem at the Nuclear Structure Lab of the University of Notre Dame and are performed within the relevant Gamow windows.
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