Novae and X-ray bursts are violent explosions occurring in close binary star systems where one star is gravitationally pulling material off the surface of its companion. As the material accumulates, the temperatures and densities increase to the point where thermonuclear reactions are ignited. These reactions, primarily involving proton captures due to the hydrogen-rich nature of the accreted material, synthesize heavier elements and generate sufficient energy to blow the accumulated layer off into space. The high temperatures and densities of the explosive environment ensure that many of the proton-rich unstable nuclei formed by proton capture on stable nuclei do not have a chance to decay before they have a subsequent reaction. For this reason, sequences of proton capture reactions on proton-rich unstable nuclei are the power source of these explosions. The determination of these rates is one of the frontiers of nuclear astrophysics.
Since the capture reactions of interest involve proton-rich unstable nuclei with lifetimes too short to fabricate into a target, direct laboratory measurements of these reactions require use of a radioactive beam incident on a hydrogen target. Such "inverse kinematics" reactions can be efficiently measured by placing a series of electromagnets and electrostatic devices immediately downstream of the target to collect all of the products of the capture reaction. This device also captures the intense flux of unreacted nuclei in the beam -- which is 10 billion to 1 trillion times more intense than the recoils of interest. This utilization of a "recoil separator", pioneered at Caltech [M.S. Smith et al., Nucl. Inst. Meth. A306 (1991) 233], is more efficient than detection of the gamma rays emitted in all directions from the capture reaction, because the recoils are all focused in the forward direction in the laboratory frame. The challenge is to separate out the weak flux of recoil nuclei from the intense flux of unreacted beam nuclei -- but these differ in mass by only 1 amu and have nearly the same momentum and charge. At ORNL, we use a device optimized for such measurements, the Daresbury Recoil Separator (DRS) (see the image at right). The DRS has a windowless, differentially-pumped extended Hydrogen gas target system that has enabled measurements of 7Be(p,gamma)8B [to learn about neutrinos from our Sun] and 17F(p,gamma)18Ne [to learn about the synthesis of 18F in nova explosions]. References for these measurements are given below. The windowless extended gas target system is now being converted into a gas jet target system with a significantly higher density and very compact target region (a few mm in diameter). This new system is the JENSA gas jet target system.
Using the DRS, we directly measured the 17F(p,gamma)18Ne resonant reaction with a mixed beam of 17F [35% to 70% of the total] and 17O at ORNL's Holifield Radioactive Ion Beam Facility. We determined that the astrophysically important 3+ resonance at approximately 600 keV above the proton threshold in 18Ne has a partial width of = 56 +/- 24 (stat) +/- 30 (sys) meV, in reasonable agreement with the theoretically predicted width. The beam energy on resonance was 10.8 MeV, the beam current was between 1 - 10 million particles per second, the H gas target thickness was 16.6 keV wide [roughly the width of this broad resonance], corresponding to a pressure of about 4 torr in the center of the target. We measured roughly 50 counts in 6 weeks of running (see figure at right). We also measured off-resonance and determined a 2 sigma upper limit on the direct capture of S(E) as 65 keV b at an energy of 800 keV (beam energy of 14.3 MeV). Our limiting systematic uncertainty was the determination of the beam current, which was made by regularly inserting a plate into the beam for a set period, extracting it, and counting the decays of the implanted 17F beam particles. More details are provided in the references [K.A. Chipps et al.] given below.
"The 17F(p,gamma)18Ne resonant cross section", K. A. Chipps, D. W. Bardayan, C. D. Nesaraja, M. S. Smith, J. C. Blackmon, K. Y. Chae, B. H. Moazen, S. T. Pittman, U. Greife, R. Hatarik, W. A. Peters, R. L. Kozub, J. F. Shriner, Jr., C. Matei, and S. D. Pain, Phys. Rev. C 80, 065810 (2009)
"First Direct Measurement of the 17F(p,gamma)18Ne Cross Section", K. Chipps, D. W. Bardayan, J. C. Blackmon, K. Chae, U. Greife, R. Hatarik, R. L. Kozub, C. Matei, B. Moazen, C. D. Nesaraja, S. D. Pain, W. Peters, S. Pittman, J. F. Shriner , M. S. Smith, Phys. Rev. Lett. 102 (2009) 152502
"Direct measurement of (p,gamma) cross sections at astrophysical energies using radioactive beams and the Daresbury Recoil Separator", D.W. Bardayan, K.A. Chipps, R.P. Fitzgerald, J.C. Blackmon, K.Y. Chae, A.E. Champagne, U. Greife, R. Hatarik, R.L. Kozub, C. Matei, B.H. Moazen, C.D. Nesaraja, S.D. Pain, W.A. Peters, S.T. Pittman, J.F. Shriner, Jr., M.S. Smith, Eur. Phys. J. A 42, 457 (2009)
"Direct measurements of (p,gamma) cross sections at astrophysical energies using radioactive beams and the Daresbury Recoil Separator", D.W. Bardayan, K.A. Chipps, R.P. Fitzgerald, J.C. Blackmon, K.Y. Chae, A.E. Champagne, U. Greife, R. Hatarik, R.L. Kozub, C. Matei, B.H. Moazen, C.D. Nesaraja, S.D. Pain, W.A. Peters, S.T. Pittman, J.F. Shriner, Jr., M.S. Smith, Proceedings of The Eleventh International Symposium on Nuclei in the Cosmos, PoS (NIC-XI) 202 (2011)
"Design of a New Recoil Separator for Measurements of Radiative Capture Reactions in Astrophysics", G.P.A. Berg, J.C. Blackmon, M. Couder, U. Greife, F. Montes, K.E. Rehm, H. Schatz, M.S. Smith, M. Wiescher, A. Zeller, Proc. Int. Symp. Origin Matter Evolution Galaxies 2010, AIP Conf. Proc. 1269 (2010) 445
"First Direct Measurement of the 17F(p,gamma)18Ne Cross Section K.A. Chipps, D.W. Bardayan, J.C. Blackmon, K.Y. Chae, U. Greife, R. Hatarik, R.L. Kozub, C. Matei, B.H. Moazen, C.D. Nesaraja, S.D. Pain, W.A. Peters, S.T. Pittman, J.F. Shriner, M.S. Smith, Proc. Thirteenth Int. Symp. on Capture Gamma-Ray Spectroscopy, AIP Conf. Proc. 1090, 471 (2009)
"First Direct Measurement of the 17F(p,gamma)18Ne Cross Section K. Chipps, D.W. Bardayan, C.D. Nesaraja, S.D. Pain, M.S. Smith, J.C. Blackmon, K.Y. Chae, B.H. Moazen, S.T. Pittman, U. Greife, R. Hatarik, W.A. Peters, R.L. Kozub, J.F. Shriner, Jr., C. Matei, Proc. Tenth Int. Symp. on Nuclei in the Cosmos, PoS (NIC-X) 059 (2008)
"Studies of (p,gamma) reactions with the Daresbury Recoil Separator at ORNL's HRIBF", R. Fitzgerald et al., Nucl. Phys. A 748 (2005) 351.
"Nuclear Astrophysics Measurements with Radioactive Beams", M.S. Smith, K.E. Rehm, Ann. Rev. Nucl. Part. Sci. 51 (2001) 91.
"A recoil separator for use in radioactive ion beam experiments", M.S. Smith, C. Rolfs, C.A. Barnes, Nucl. Inst. Meth. A306 (1991) 233.
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: Separator for Capture Reactions SECAR
Nuclear Astrophysics with Radioactive Beams at FRIB
Dan Bardayan, bardayandw at ornl.gov
Steve Pain, painsd at ornl.gov
Michael Smith, smithms at ornl.gov