2. Recent HRIBF Research - Measurement of the ²⁶Al(d,p)²⁷Al Reaction for the Study of the Astrophysical ²⁶ Al(p, γ)²⁷Si Rate
[S. D. Pain (Univ. of the West of Scotland), spokesperson]

Astronomical gamma-ray mapping, by charting the distribution of specific isotopes, yields information which can constrain the rate of explosive nucleosynthesis events (such as novae and supernovae) within the galaxy. One of the landmarks in observational astronomy has been the detailed galactic mapping of the decay of ²⁶Al, achieved by the observation of the 1809-keV gamma rays emitted following its beta decay using the COMPTEL instrument aboard the Compton Gamma Ray Observatory (see Fig.2-1), and more recently the European Space Agency's INTEGRAL satellite. With a half-life of ~105 years, the distribution of ²⁶Al provides an insight into the galactic nucleosynthesis over a timescale of about the last million years. The mechanisms contributing to the formation and destruction of ²⁶Al are consequently of direct interest to the interpretation of these gamma-ray maps [2].

Figure 2-1: COMPTEL galactic map of the 1809-keV gamma-ray line from the decay of ²⁶Al [1].

In most astrophysical environments, ²⁶Al is destroyed via the ²⁶Al(p,γ)²⁷Si reaction. Understanding states near the proton threshold in ²⁷Si is crucial for constraining this reaction rate, and thus for elucidating the amount of ²⁶Al which survives to enrich the interstellar medium. The difficulty in measuring directly the strengths of these resonances, due to their small cross sections, means that indirect approaches are required. Furthermore, due to the difficulties inherent in measuring proton transfer reactions, an alternative is to measure mirror states in ²⁷Al to obtain information about the ²⁷Si structure.

The ²⁶Al(d,p)²⁷Al reaction has been measured in inverse kinematics at the HRIBF, in order to study states in ²⁷Al which are mirror to those in ²⁷Si. A batch-mode beam of ²⁶Al, of typically 5 million particles per second, impinged on a ~150 mg/cm² CD₂ target for ~5 days. Proton ejectiles were detected in the SIDAR and ORRUBA silicon detector arrays, subtending angles from ~90-165 degrees in the laboratory frame. These data represent the first measurement performed with a complete barrel of ORRUBA. Elastic scattering was monitored in ORRUBA detectors mounted at angles just forward of θ_lab = 90°. A forward array for recoil tagging, comprised of annular segmented silicon detectors of Micron QQQ2 design, covering angles from ~1.5 to 10 degrees, was used to detect ²⁷Al ions coincident with (d,p) protons.

Figure 2-2 shows an online spectrum from a single strip of SIDAR, for a subset of the data taken during the experiment. Transitions at these backward angles are strong candidates for ℓ = 0 transfer, pending a full analysis of angular distributions. These data are under analysis by, and will contribute to the PhD thesis of Stephanie Brien from the University of the West of Scotland.

Figure 2-2: Proton energy spectrum from the ²⁶Al(d,p)²⁷Al reaction, for a single SIDAR strip, for a subset of the data.

References:

[1] S. Pluschke et al., "The COMPTEL 1.809 MeV Survey" in Proceedings of the 4th INTEGRAL Workshop, A. Gimenez, V. Reglero, and C. Winkler, eds. (ESA Publications, Noordwijk, 2001).
[2] J. Jose, A. Coc and M. Hernanz, Astrophys. J. 520, 347 (1999).