Scientific Motivation

Our research program centers on the study of nuclear reactions and structure that are of importance to understanding explosive nucleosynthesis in X-ray bursts and novae. In current models of these events, the energy generation and nucleosynthesis is initially controlled by the Hot-CNO cycles, which begin when the 13N(p,gamma)14O reaction bypasses the beta-decay of 13N. As the ambient temperature and density increase, alpha-particle and proton capture reactions on the Hot-CNO nuclei also become faster than the corresponding beta decays. In X-ray bursts where the surface gravity is high, the star may then break out of the Hot-CNO region to the rp-process, providing a way to enhance the rate of energy generation, trigger the subsequent explosion, and produce heavier elements up to roughly A=100, with important implications for the crust composition and other properties of the underlying neutron star. Based on present nuclear physics information, the initial breakout path is through the reaction sequence 15O(alpha,gamma)19Ne(p,gamma)20Na(p,gamma)21Mg…, and at higher temperatures another bridge to the rp-process opens through the sequence 14O(alpha,p)17F(p,gamma)18Ne(alpha,p)21Na…. The contribution of these reactions to the total reaction flux through this mass region depends on their cross-sections under extreme stellar conditions, and only recently have first experiments been carried out. In spite of recent progress, many of these reaction rates are still poorly understood.

In contrast, our present understanding of novae indicates that the explosion temperature may be too low for breakout to happen. For a class of novae known as O-Ne-Mg novae, the sources of energy for the thermonuclear runaway are instead the Ne-Na and Mg-Al cycles, in addition to the Hot-CNO cycles. Within these cycles, there are a number of proton capture reactions for which no direct experimental results are available. In particular, in the Mg-Al region, certain reactions are important in the context of understanding the production of 26Al in novae. Recent studies of 26Al gamma-emission in the galaxy and of 26Al / 27Al isotopic anomalies in meteorites have advanced our understanding of galactic and stellar evolution, as well as the origin of the solar system. While the observations of galactic gamma-ray emission point to massive stars as possible major contributors to the 26Al distribution, a significant contribution from novae cannot be discounted. At nova temperatures, the production of 26Al can be bypassed if the 25Al(p,gamma)26Si reaction becomes faster than the positron decay of 25Al. The strengths of 25Al + p resonances in 26Si have been estimated and recent stable beam spectroscopy studies have helped clarify the possible locations of the important resonances. The reaction rate, however, remains uncertain by potentially a factor of 100 and no direct measurements have been attempted.

Experiments

At TRIUMF-ISAC, a number of experiments with radioactive beams, addressing many of the topics mentioned above, will be performed in the next three years using either a recoil mass separator (called DRAGON) for radiative capture reaction studies, or a general-purpose scattering chamber (the TRIUMF-UK Detector Array (TUDA)) equipped with several large-scale silicon detector arrays for charged-particle reaction experiments. This equipment has been specifically designed to overcome challenges associated with the small reaction cross-sections (often in the nb range) and the need for clean separation of reaction recoils from beam contaminants. The first experiments have recently been completed: a measurement of the 21Na(p,gamma)22Mg reaction with DRAGON and a study of 21Na(p,p)21Na. The two experiments provided complementary results that elucidated the importance of the 21Na(p,gamma)22Mg reaction, a key link in the nova Ne-Na cycles.

In particular, members of our group are leading three experiments, two with DRAGON and one with TUDA, that will use 25Al and 17F beams to investigate the role of the 25Al(p,gamma)26Si reaction in the production of 26Al in novae and the role of the 17F(p,gamma)18Ne reaction in the Hot-CNO cycles. We will also be participating on an experiment to determine the 18Ne(alpha,p)21Na reaction rate by measuring the time-reversed reaction, 21Na(p,alpha)18Ne, at Argonne National Laboratory (Exp#954). The 21Na beams are produced through an in-flight technique, and will have energies higher than presently available at TRIUMF-ISAC. These higher energies are required for this measurement. The following tables summarize the approved experiments with which we will be involved over the next 3-4 years (the ones lead by members of our group are in bold):

TRIUMF-ISAC Experiments with DRAGON
Experiment	EEC#	Topic	Date
19Ne(p,gamma)20Na	E811	Breakout from the Hot-CNO cycles	2003-04
13N(p,gamma)14O	E805	Hot-CNO cycles in novae and X-ray bursts	2003-04
25Al(p,gamma)26Si	E922	Production of 26Al in nova nucleosynthesis	2004-05
17F(p,gamma)18Ne	E946	Hot-CNO cycles in novae and X-ray bursts	2006-07

TRIUMF-ISAC Experiments with TUDA
Experiment	EEC#	Topic	Date
18Ne(alpha,p)21Na	E870	Breakout from the Hot-CNO cycles	2002-03
18Ne(d,p+alpha)15O	E874	Breakout from the Hot-CNO cycles	2003-04
18Ne(3He,p)20Na	E927	Breakout from the Hot-CNO cycles	2003-04
25Al(p,p)25Al	E923	Production of 26Al in nova nucleosynthesis	2004-05

Experiments at Argonne National Laboratory
Experiment	Exp#	Topic	Date
21Na(p,alpha)18Ne	954	Breakout from the Hot-CNO cycles via 18Ne(alpha,p)21Na	2002-03

The impact of the new reaction rates on the stellar models will be investigated in collaboration with theorist colleagues. To that end, an important component of my research program will involve ensuring that the data from the experiments are compiled, evaluated, and cast into forms that are easily accessible to the astrophysicists, so that the reaction rates can be efficiently incorporated into the latest astrophysical models. Lastly, we will also be involved in developing auxiliary detectors for TRIUMF’s new gamma-ray spectrometer (TIGRESS). TIGRESS, recently funded by NSERC, will be an important tool in the study of extremely neutron-rich nuclei that might play a role in the r-process in supernovae.