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Proton-Induced Reactions

General Description

Many of the reactions that power novae and X-ray bursts are proton-induced reactions on proton-rich unstable nuclei. These include Proton Capture Reactions (p,gamma) and reactions such as (p,alpha) and its inverse, (alpha,p). While proton- and alpha-capture reactions serve to synthesize ever-heavier nuclei in these astrophysical environments, (p,alpha) reactions act to cycle the heavier nuclei back down to lower masses. This often acts to close reaction "cycles" such as those in the Neon-Sodium, Magnesium-Aluminum, Silicon-Phosphor, and other nuclei that occur in energetic novae, as shown in the figure to the right. (alpha,p) reactions, on the other hand, are prevalent in the high temperatures occurring at the onset of X-ray bursts. Here, sequences of (alpha,p) reactions followed by (p,gamma) proton captures, the "alpha-p process", serve as a bridge into the successive proton captures that occur in the rapid proton capture process (rp-process). Direct measurement of these (p,alpha) and (alpha,p) reactions on proton-rich unstable nuclei is at the forefront of nuclear astrophysics.

Detailed Description

Measurements of capture reactions have very stringent requirements on detector systems -- due to the similarity of incident and product heavy nuclei and the presence of a gamma ray in the exit channel. More modest systems can, however, be used to directly measure (p,alpha) and (alpha,p) reactions. These same systems can also be used for indirect studies -- specifically, for measurements of elastic (p,p) and inelastic (p,p') scattering reactions, which enable the determination of properties of resonances that may dominate a reaction rate. The figure to the right shows a typical setup, one we used to simultaneously measure the 18F(p,p) and 18F(p,alpha) reactions with a radioactive 18F beam at ORNL. 18F(p,p) enabled us to determine the energy and total width of strong resonances in the (p,alpha) reaction, as shown in this figure, then we determined the strength of these resonances by the direct (p,alpha) measurement. Light products of these reactions were measured in arrays of Silicon strip detectors, while the heavy products were measured in a gas-filled ionization counter. Other configurations used three layers of strip detectors -- the first two enabling the measurement and identification of the light ions, the third for the measurement of the heavy fragment, and a hole in the middle to enable unreacted beam particles pass through without damaging the detector. This was, for example, utilized in our measurement of 17F(p,alpha)14O, the inverse of the astrophysically important 14O(alpha,p)17F reaction that occurs helps trigger thermonuclear burning in novae and X-ray bursts.

Technical Description

We utilize a number of different detector systems in our work. The first was the SIlicon Detector ARray SIDAR, consisting of Micron YY1 - configuration Silicon strip detectors arranged in either a flat geometry (8 detectors forming a circle) or "lampshade" configuration with 6 angled detectors covering more solid angle at slightly worse angular resolution. To these, we have added many additional detectors (see this presentation for details). Our future work in this area will involve using the SuperORRUBA detector system with SIDAR endcaps, shown at right, as well as the JENSA gas jet target system.


• "Analysis of 25Al energy levels observed in the 28Si(p,gamma)25Al reaction", S. T. Pittman, D. W. Bardayan, J. C. Blackmon, R. L. Kozub, and M. S. Smith, Phys. Rev. C 80, 038801 (2009)

• "Direct studies of low-energy resonances in 31P(p,α) 28Si and   35Cl(p,α)32S", B. H. Moazen, C. Matei, D. W. Bardayan, J. C. Blackmon, K. Y. Chae, K. A. Chipps, R. Hatarik, K. L. Jones, R. L. Kozub, M. Matos, C. D. Nesaraja, P. D. O’Malley, S. D. Pain, T. Pelham, S. T. Pittman, and M. S. Smith, Eur. Phys. J. A 47 (2011) 66

• "Inelastic 17F(p,p)17F scattering at Ec.m.=3 MeV and the 14O(alpha,p)17F reaction rate", D. W. Bardayan, J. C. Blackmon, K. Y. Chae, M. E. Howard, C. Matei, W. Martin, M. Matos, B. H. Moazen, C. D. Nesaraja, W. A. Peters, S. T. Pittman, M. S. Smith, and I. Spassova, Phys. Rev. C 81, 065802 (2010)

• "Comment on 'Low-energy 18F(p,alpha)15O cross section measurements relevant to nova gamma-ray emission' ", J. F. Shriner, Jr., D. W. Bardayan, J. C. Blackmon, K. Y. Chae, R. L. Kozub, and M. S. Smith, Phys. Rev. C 81, 039801 (2010)

• "Measurement of the 330 KeV resonance in 18F(p,alpha)15O", B.H. Moazen, J.C Blackmon, D.W. Bardayan, K.Y. Chae, K. Chipps, K.L. Grzywacz, R.L. Kozub, C. Matei, C.D. Nesaraja, S.D. Pain, J.F. Shriner Jr., M.S. Smith, Acta Physica Polonica B 40, 699 (2009)

• "Measurement of the 183 keV resonance in 17O(p,alpha)14N using a novel technique", B. H. Moazen, D. W. Bardayan, J. C. Blackmon, K. Y. Chae, K. Chipps, C. P. Domizioli, R. Fitzgerald, U. Greife, W. R. Hix, K. L. Jones, R. L. Kozub, E. J. Lingerfelt, R. J. Livesay, C. D. Nesaraja, S. D. Pain, L. F. Roberts, J. F. Shriner Jr., M. S. Smith, and J. S. Thomas, Phys. Rev. C 75, 065801 (2007)

• "A new technique for measuring astrophysically important (α,p) reactions", K. Y. Chae, D. W. Bardayan, C. D. Nesaraja, M. S. Smith, S. H. Ahn, A. Ayres, A. Bey, K. L. Jones, S. T. Pittman, M. E. Howard, P. D. O’Malley, R. L. Kozub, M. Matos, B. H. Moazen, W. A. Peters, Proceedings of Science, PoS (NICXI) 217 (2010)

• "An analysis of 25Al energy levels observed in the 28Si(p,α)25Al reaction", S. T. Pittman, D. W. Bardayan, J. C. Blackmon, R. L. Kozub, M. S. Smith, Proceedings of Science, PoS(NIC X)159 (2009)

• "First experimental constraints on the interference of 3/2+ resonances in the 18F(p,alpha)15O reaction", K.Y. Chae et al., Phys. Rev. C 74 (2006) 012801R

• "Elastic scattering of the proton drip-line nucleus 17F", J.C. Blackmon et al., Phys. Rev. C72 (2005) 034606

• "Strength of the 18F(p,alpha)15O Resonance at Ecm = 330 keV", D. W. Bardayan et al., Phys. Rev. Lett. 89 (2002) 262501

• "Nuclear Astrophysics Measurements with Radioactive Beams", M.S. Smith, K.E. Rehm, Ann. Rev. Nucl. Part. Sci. 51 (2001) 91.

• "The astrophysically important 3+ state in 18Ne and the 17F(p,gamma)18Ne stellar rate", D. W. Bardayan et al., Phys. Rev. C 62 (2000) 055804.

• "Observation of the Astrophysically Important 3+ State in 18Ne via Elastic Scatering of a Radioactive 17F Beam from 1H", D. W. Bardayan et al., Phys. Rev. Lett. 83 (1999) 45


• Presidential Early Career Award for Scientists and Engineers, Dan Bardayan, 2005 - 2010

• Presidential Early Career Award for Scientists and Engineers, Jeff Blackmon, 2004 - 2009

• Presidential Early Career Award for Scientists and Engineers, Michael Smith, 1996 - 2001

For More Information

The following links will let you learn more about this topic:

Nuclear Astrophysics Measurements with Radioactive Beams - M.S. Smith, K.E. Rehm, Ann. Rev. Nucl. Part. Sci. 51 (2001) 91


Dan Bardayan, bardayandw at ornl.gov
Steve Pain, painsd at ornl.gov
Michael Smith, smithms at ornl.gov