Nuclear Astrophysics and Nuclear Structure

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It was my pleasure to be at Hacienda Cocoyoc for StuFiesta honoring the 60th. Birthday of Stuart Pittel. I met Stuart at Brookhaven National Laboratory when I ...
Nuclear Astrophysics and Nuclear Structure Ani Aprahamian

Citation: 726, 173 (2004); doi: 10.1063/1.1805934 View online: http://dx.doi.org/10.1063/1.1805934 View Table of Contents: http://aip.scitation.org/toc/apc/726/1 Published by the American Institute of Physics

Nuclear Astrophysics and Nuclear Structure Ani Aprahamian Institute for Structure and Nuclear Astrophysics University of Notre Dame, Notre Dame, IN 46556 USA Abstract. We explore the impact of nuclear structure on nucleosythesis processes via the nuclear mass.

It was my pleasure to be at Hacienda Cocoyoc for StuFiesta honoring the 60th Birthday of Stuart Pittel. I met Stuart at Brookhaven National Laboratory when I was just starting my graduate studies in various aspects of Nuclear Structure. Today, my research interests have drifted to nuclear astrophysics looking at the impact of nuclear structure on stellar processes. One of the most important goals of nuclear astrophysics is the attempt to understand nucleosynthesis processes that take place in the cosmos by the simulation of various astrophysical scenarios. These scenarios are strongly dependent on nuclear structure which sets the time scale for the stellar processes from giga-years of stellar evolution to milli-seconds of stellar explosions. In each case, they leave signatures in stellar luminosities, elemental and/or isotopic abundances, and neutrino fluxes from distant supernovae. One of the most basic nuclear structure effects is the nuclear mass. There have been a number of excellent reviews recently on Nuclear Masses [1] reporting on the status of present day mass models. We also heard from Jorge Hirsch at StuFiesta that one cans set bounds on the chaotic motion of nuclei and possibly allow the prediction of nuclear masses with higher accuracy. Masses or massdifferences in the form of reaction Q-values come in exponentially into reaction rates. Mass differences determine the energy generation in nucleosynthesis processes as well as the decay channels which are open for compound states and determine reaction branchings that affect the reaction path. The latter part naturally guides the abundance patterns that result from a specific nucleosynthesis scenario. An example of this is the waiting point phenomenon. Whether we speak about a thermonuclear runaway ignited in the high temperature and density conditions of an accretion disk that may be a part of the rp-process (rapid sequential proton capture reactions) or the high neutron densities and temperatures that characterize the r-process (rapid sequential neutron capture reactions) thought to occur on the shockfronts of core collapse supernovae. In the rp-process, the p-capture reactions are thought to drive the nucleosynthesis path towards the proton drip-line passing through the N=Z nuclei beyond 56Ni. The Qvalues for p-capture decrease significantly and sometimes become negative as N=Z nuclei are approached. This causes an enrichment in the N=Z nuclei as an equilibrium is established between (p,γ)-(γ,p) reactions. The N=Z nucleus becomes a “waiting

CP726, Nuclear Physics, Large and Small: International Conference on Microscopic Studies of Collective Phenomena, edited by R. Bijker, R. F. Casten, and A. Frank © 2004 American Institute of Physics 0-7354-0207-8/04/$22.00

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point” and the reaction flow is delayed by the beta-decay half-life of the N=Z nucleus unless 2p-capture reactions become feasible enabling the nucleosynthesis to jump over the waiting-point. One of the waiting points for the rp-process occurs at N=Z=34, 68Se. The mass of 68Se is particularly important in determining whether further p-capture is likely to occur within the rp-process. The Q value for proton capture on 68Se leading to 69 Br that can be predicted from various mass models differs from a positive +70keV [2] to a maximum of -730keV [3] with the Audi extrapolations lying somewhere in between. There have been several attempts to search for 69Br by fragmentation studies at GANIL and MSU. No 69Br was observed but an upper limit of 150ns was set for the half-life indicating that 69Br may be p-unbound. The only way that the rp-process can proceed is via 2p-capture or β-decay of 68Se. The mass of 68Se was measured [4] via the β−decay endpoint. 68Se was produced by the 12C(58Ni,2n) 68Se reaction and subsequently implanted onto a moving tape system using the Fragment Mass Analyzer at the ATLAS facility of Argonne National Laboratory. A mass excess value of (-54189+/-240) keV was determined from the b-endpoint measurement of QEC=4710(200) keV. Evaluations of proton separation energies based on the measured mass were used in a one zone type I x-ray burst model. It is concluded, that 2p-capture reactions are not very likely to play a significant role at N=Z=34 isotope and that 68Se remains a waiting point for rp-process nucleosynthesis. While we have shown that the likelihood of jumping over the waiting point via 2p-capture reactions is low in this case, there is still the possibility that a long lived isomeric state in 68Se can itself capture protons decreasing the reaction Q value by the excitation energy of the isomer and still provide a path for the rp-process. If the isomeric state is at approximately 1.5 MeV in excitation, the Q value changes and p-capture on 68Se becomes feasible. To date, there is no information on the existence of any isomeric states in the spectrum of 68 Se. I conclude by emphasizing that in addition to nucleus via nuclear structure, we can also see transformed into time scales of stellar events, scenarios, as well as the energy production nucleosynthesis processes.

describing global properties of the the signatures of nuclear structure abundance distributions of various from both steady and explosive

Acknowledgments This work was supported by the National Science Foundation under contract PHY0140324 and the Joint Institute of Nuclear Astrophysics under contract PHY02-16783. REFERENCES 1.Lunney,D. Pearson,J.M., Thibault,C, Reviews of Modern Physics 75, (2003) 1021 2. Aprahamian,A. Gadala-Maria,A. and Cuka,N., Rev. Mex. Fis. 42 (1996) 1 3. Brown,B.A. et al., Phys. Rev. C 65 (2002) 045802 4. Woehr,A. et al., Nucl. Phys. A (2004) in press.

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