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3. Recent HRIBF Research - Study of the Elastic and Inelastic 26Al+p Reactions at HRIBF
[D. Bardayan (ORNL), spokesperson]

The detection of characteristic gamma-rays from the decay of 26Al (t1/2=7.2x105 yr) has provided clear evidence of continuing nucleosynthesis in the Milky Way Galaxy. Detailed maps of Galactic 26Al have been made by satellite gamma-ray observatories such as COMPTEL [1], RHESSI [2], and INTEGRAL [3]. The observed 26Al was concentrated along the Galactic plane, but the source is still not known. Possible sources include Wolf-Rayet stars, novae, and core-collapse supernovae. Peak temperatures in these environments vary from 0.03 to ~3 GK meaning that relevant reaction rates need to be known over a broad temperature range to estimate the contributions of these events to the Galactic 26Al abundance.

Figure 3-1: The experimental configuration used in the 26Al(p,p)26Al measurement.

Of the relevant reactions, the 26Al(p, γ)27Si reaction is thought to provide the dominant destruction mechanism over a large temperature range. Levels in 27Si above the proton threshold at 7.463 MeV provide possible resonances in the 26Al(p, γ)27Si reaction. While there have been many studies of this reaction [4-7], there are many levels in the relevant energy range (Ex=7.5-8.5 MeV) for which their spins and proton-widths are not known. Also important to know is whether the effective lifetime of 26Al is changed in the high-temperature stellar environment as a result of inelastic excitations. While the 5+ ground state has nearly a million-year half life, the 0+ metastable state only lives 6.3 s. If there are 27Si levels through which 26Al could inelastically-scatter into the metastable state, the effective half life of 26Al could be greatly reduced.

To address these uncertainties, we have measured the elastic and inelastic 26Al+p scattering reactions at HRIBF. Pure 26Al beams with intensities of 2x106 26Al/s were used to bombard 50 μg/cm2 CH2 targets (see Fig. 3-1). Scattered protons were detected in the angular range 18-41 degrees by the SIDAR Silicon Detector Array [8]. The recoil 26Al ions were detected in coincidence with the protons in an isobutane-filled ionization counter. The unscattered primary beam was prevented from entering the ionization counter by a 1-cm-diameter disk that was inserted in front of the counter entrance window during each run. The size of the disk was chosen so that for the proton angles covered by the SIDAR, the corresponding recoil 26Al ions were not blocked by the disk. The energy spectrum detected in SIDAR is shown in Fig. 3-2 in singles and coincidence demonstrating the cleanliness of the proton identification in the coincidence data. The proton yields were measured at 45 energies from Ec.m.=0.5-1.5 MeV over a period of 7 days in March 2009. The data are currently under analysis and will constitute the Ph.D. thesis of University of Tennessee graduate student Stephen Pittman.

Figure 3-2: The energy spectrum observed at 27 degrees for a 15-MeV 26Al beam bombarding a 50 μg/cm2 CH2 target in singles and in coincidence with a recoil 26Al ion.

References:

[1]R. Diehl et al., Astron. Astrophys. 298, 445 (1995).
[2]D. M. Smith et al., Astrophys. J. 589, L55 (2003).
[3]R. Diehl et al., Nature (London) 439, 45 (2006).
[4]L. Buchmann et al., Nucl. Phys. A 415, 93 (1984).
[5]R. B. Vogelaar et al., Phys. Rev. C 53, 1945 (1996).
[6]C. Ruiz et al., Phys. Rev. Lett. 96, 252501 (2006).
[7]G. Lotay et al., Phys. Rev. Lett. 102, 162502 (2009).
[8]http://www.phy.ornl.gov/hribf/equipment/sidar/




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