Aluminium 26
The 26gAl(ρ,γ)27Si Reaction at DRAGON
The astrophysically important 26gAl(p,γ)27Si radiative proton capture reaction was recently investigated using the ISAC-DRAGON facility at TRIUMF. In this experiment, an intense radioactive 26gAl beam produced at the ISAC radioactive beam facility was used in conjunction with a windowless H2 gas target at the DRAGON facility to investigate narrow resonances which are believed to dominate the rate of this reaction in explosive stellar environments such as novae and supernovae explosions. The 188 keV resonance in 27Si was investigated over a 3 week running period, during which approximately 250 runs were taken. From the data collected, the thick target yield of the reaction will be determined, which will then be used to calculate an experimental value for the resonance strength, a value that can be used in astrophysical models attempting to describe the reactions occurring in explosive stellar nucleosynthesis. The purpose of this project was to work on determining two quantities critical to the calculation of the thick target yield and resonance strength: the normalized beam particles on target over the run, and the BGO gamma array detection efficiency. Two methods of beam normalization were used and refined in the analysis of the experimental runs, and validated one another, showing agreement within 8%. BGO efficiency was evaluated using GEANT simulations for a number of different angular distributions and thresholds, to provide averaged efficiency values. Further work on incorporating angular distributions of emitted gamma radiation into the GEANT simulation is ongoing, and will improve the accuracy of efficiency calculations.
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Summer report on the 26Al(ρ,γ)Si reaction at DRAGON
The 26Al(ρ,γ)27Si reaction has begun at DRAGON, the Detector of Recoils And Gammas Of Nuclear reactions. The experiment is done in inverse kinematics, meaning an aluminum ion produced and accelerated is incident on a hydrogen gas target. The resulting mix of beam ions and 27Si then moves through the DRAGON, which separates the two, allowing silicon recoils to hit the end detectors.
In order to suppress the leaky beam incident of the end detectors, a coincidence measurement is made. For the 26Al(ρ,γ) experiment, however, futher methods of suppression were examined. An ion chamber was used to aid in element separation, and suppression were examined. In order to understand these options, a number of simulations were completed. Also of importance to the experiment was an understanding of the composition of the incoming beam. Certain contaminents of mass 26 were expected to be present in the beam, notably magnesium, sodium and a metastable state of aluminum which would not react in the same way as the ground state. New hardware and analysis was required to look for these contaminents. Also, some programs were written in FORTRAN to aid with the data analysis.
The reaction is of astrophysical interest as 26Al decays via a characteristic gamma-ray which can be readily observed. The 26Al(ρ,γ)27Si reaction is the only direct 26Al destruction process besides β-decay. The reaction rate is currently known to within a factor of four, far from the 20% uncertainties required for consistent astrophysical modelling. Of increasing interest is the extent of 26Al in explosive stellar environments, such as novae and supernovae, environments which the DRAGON facility was made to examine.
In this report, a brief overview of DRAGON will be given, then the results of the various simulations and contamination measurements, and, finally, some preliminary results from the experiment.
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Production of 26Al in Oxygen-Neon-Magnesium Novae
In the beta-decay of the ground state of 26Al (denoted 26gAl, t1/2 = 7.2 x 105 y), a characteristic 1.809 MeV gamma-ray is emitted. This signature of the presence of 26gAl has been widely observed throughout the Galaxy. Indeed, the observation of this gamma-ray proves the ongoing nucleosynthesis of 26gAl in astrophysical environments, given its short half-life on cosmological timescales. Reproduction of the Galactic 26gAl steady-state abundance implied by the observations (~ 3 M␣) provides a powerful constraint on nucleosynthesis model calculations. These calculations may also be used to determine the relative contributions to the 26gAl abundance by different types of astrophysical phenomena.
The amount of 26gAl produced in nova explosions on oxygen-neon-magnesium white dwarfs is thought to be relatively minor (~ 0.1 - 0.4 M␣). Nuclear uncertainties in the 25Al(p,γ)26Si and 26gAl(p,γ)27Si reactions may change this by a factor of ~2, however. A direct study of the 25Al(p,γ)26Si reaction has been proposed and accepted at the TRIUMF- ISAC radioactive beams facility in Vancouver, Canada, and is awaiting the production of a sufficiently-intense 25Al beam. To both guide this direct study, and to improve the accuracy of the current 25Al(p,γ)26Si calculations (based on indirect measurements), we have made a new measurement of the 26Si mass. We find the mass excess of 26Si to be∆(26Si) = -7139.5 ± 1.0 keV; this new mass leads to a reduction in the 25Al(p,γ)26Si rate by as much as ~30% at nova temperatures. We have also made new measurements of the energy and strength of a key resonance for the 26gAl(p,γ)27Si reaction: we find ERCM = 184 ± 1 keV and ωγ = 35 ± 4stat ± 5sys μeV. These results lead to a decrease in the 26gAl(p,γ)27Si rate by as much as ~15% at nova temperatures.
Our measurements of the 26Si mass and the resonance in 26gAl(p,γ)27Si both imply an increase in the 26gAl yield from novae, but still confirm the secondary nature of their contribution to the Galactic abundance of 26gAl.
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Measurement of the Ec.m.=184 keV Resonance Strength in the 26gAl(ρ,ϒ)27Si Reaction
The strength of the Ec.m.=184 keV resonance in the 26gAl(ρ,ϒ)27Si reaction has been measured in inverse kinematics using the DRAGON recoil separator at TRIUMF's ISAC facility. We measure a value of ωϒ=35 ±7 µeV and a resonance energy of Ec.m.=184±1 keV, consistent with p-wave proton capture into the 7652(3) keV state in 27Si, and discuss the implications of these values for 26gAl nucleosynthesisin typical oxygen-neon white-dwarf novae.
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Experimental developments for the study of explosive nucleosynthesis in stars
For several years now, the -SNS collaboration has been working to place a small neutrino detector at the Spallation Neutron Source at Oak Ridge National Lab. If successful, the experiment may produce the needed neutrino-nucleus cross sections on solid targets such as iron and aluminum. These reaction probabilites are of great interest for a number of reasons, including: neutrino astronomy, explosive nucleosynthesis, and nuclear structure.
However, success for this project requires a very efficient cosmic ray detector to exclude backgrounds. The system would need to be 99% efficient while remaining affordable in a difficult financial climate for basic science. The first half of this thesis addresses a prototype cosmic ray veto based on extruded scintillator with embedded wave-length-shifting fibers. This approach has been successfully used before, and may provide the performance needed for this project. However, our results suggest some additional research and development would be required to meet the requirements for the -SNS experiment.
The second half of this thesis relates to experimental work to study the resonance strength of the 23Mg(p,)24Al reaction. For this purpose a radioactive ion beam experiment has been conducted at TRIUMF using the DRAGON experiment. This reaction is thought to play an important role during explosive nucleosynthesis such as novae and X-ray bursts. If so, then accurate knowledge of this break-out reaction would help explain the isotopic abundances around that mass range in the universe.
Our results suggest the rate of this reaction at astrophysically relevant energies is lower than predicted and might further exclude explosive binary systems as the production site for such elements as 26Al.
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