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The purpose of this LDRD was to establish a program at LLNL in radioactive ion beam (RIB) experiments that would use these experiments to address a wide range physics issues in both stellar nucleosynthesis and stockpile stewardship radiochemistry. The LDRD was funded for a total of two years (fiscal years 2000 and 2001) and transferred to the Physical Data Research Program in fiscal year 2002. Reactions on unstable nuclei and isomeric states play a central role in the formation of elements in both stars and nuclear devices. However, the abilities of reaction models to predict cross sections on radioactive nuclei are uncertain at best. This can be attributed to the lack of experimental data to guide reaction-modeling efforts. Only the 10% of all bound nuclei that can be formed with stable targets and beams have been accessed and studied. The proposed Rare Isotope Accelerator (RIA) and existing RIB facilities offer an unprecedented opportunity to address many of the outstanding questions in nuclear structure, reactions and astrophysics by enabling the observation of nuclear reactions with radioactive targets and/or beams. The primary goal of this LDRD is to develop three experimental capabilities for use with RIB experiments: (1) Level density and [gamma]-ray strength function measurements using statistical [gamma]-rays. (2) Charged particle-induced cross sections measurements on radioactive nuclei. (3) Neutron-induced cross section measurements on a radioactive target. RIA and RIB based experiments are the new frontier for nuclear physics. The joint DOE/NSF nuclear science advisory committee has named development of a RIA facility in the United States as the highest new construction priority. In addition to addressing the questions presented above, this LDRD has helped to establish a position for LLNL at the forefront of the international nuclear science community.
In this paper we discuss efforts underway at LLNL to develop the technology for the measurement of proton and alpha-particle reactions with unstable nuclei which are necessary for understanding the nucleosynthesis and energy generation in hot hydrogen-burning environments. 16 refs., 5 figs.
The availability of radioactive ion beams represents a major advance in the capbaility to attack important problems of basic and applied nuclear physics. For the first time we are able to study nuclear reactions on nuclides outside the valley of stability. These nuclides represent about 80% of particle-stable isotopes. Questions of nuclear reaction mechanisms, nuclear properties of bound and continuum states, and basic symmetries for these unstable nuclides can therefore be answered by direct mesurement rather than by speculation. The applications of radioactive ion beams are widespread and extensive. In astrophysical studies, nuclear reactions of radioactive ion beams are essential to understanding a wide range of problems in energy generation, nucleosynthesis, neutrino effects, and the implications of gamma-ray astrophysical measurements. In material studies, the implantation of radioactive ions has already seen important use. Well defined beams of these ions will greatly increase the specifity and sensititivity of this technology. Radioactive ion beams may also have significant applications in medical diagnostics and therapy. Finally, standard sources of beta and gamma radiation can be prepared with much greater precision than formerly available.
Several modifications and additions have been made to improve the radioactive beam facility at Livermore with the main aim of measuring the cross section for 7Be(p, .gamma.)8B (which is important in determining the solar neutrino flux) and other reactions of astrophysical interest. The quadrupole sextuplet spectrometer has been upgraded by inserting an electrostatic deflection element near the midpoint and by installing a movable beam stop near the 7Be production target. These changes have allowed an improvement in the purity, and a large increase in the intensity, of the 7Be beam. Six large NaI(Tl) detectors and the gas cell from the OSU system along with its active and passive shielding have been incorporated into the Lawrence Livermore facility. True events are to be identified by a multiple coincidence. The first requirement is the detection of a .gamma.-ray from the proton capture 7Be(p, .gamma.)8B. After the candidate capture gamma is observed the 8B decay signature is required. This signature is a positron (from 8B .-->. 8Be* + e + .nu.) along with the two .cap alpha.'s from 8Be .-->. .cap alpha. + .cap alpha. observed in a CaF2 detector into which the 8B have implanted. Also a detector telescope inside the gas cell monitors the incoming 7Be beam. The current status of the 7Be(p, .gamma.)8B measurement is discussed.