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Scheduled Experiments l Special Events l Contacts Overview The main focus of our research program is a basic investigation of the important areas of nuclear physics discussed below. In parallel, application-oriented research is pursued in nuclear transmutations, biophysics and nuclear medicine. Precision (n,2n) cross section measurements on Actinides at TUNL An experimental program has been developed at TUNL to study (n,2n) excitation functions on Actinide nuclei using monoenergetic and pulsed neutron beams. Measurements have been performed on a 238U target, with incident neutron energies ranging from 5 to 20 MeV and using different types of HPGe detectors. Our goal is to improve the partial cross section data for the NNSA Stockpile Stewardship Program with a special emphasis on the cross sections of the 239Pu(n,2n)238Pu reaction. Accurate measurements of the 239Pu(n,2n)238Pu cross sections are needed to better understand radiochemical data from past experiments at the Nevada Test Site. Fig. 1. NNSA experimental area. Neutron Capture Cross Sections Measurements using DANCE and GEANIE Array GEANIE (GErmanium Array for Neutron Induced Excitations) is the first large-scale, escape-suppressed, high-resolution gamma-ray spectrometer to be used at a white neutron source. It is now installed at the WNR high-energy neutron facility at LANSCE. Fig. 2. GEANIE array. The DANCE offers an exceptional capability to do elemental and isotopic identification of materials that might be recovered following an act of terrorism. Samples on the order of a milligram could be rapidly assayed to determine isotopic composition by identifying characteristic neutron capture parameters associated with specific isotopes. This isotopic fingerprint of a sample could provide information to establish attribution of the source a the material, as well as identifying potentially dangerous compositions. The analysis could be done with minimal pre-processing, and since only small quantities are required, they could be obtained from terrorist debris or clandestine operations. Fig. 3. DANCE array. Nuclear Structure Study at HIGS In this project, our focus is the study of the different modes of excitation at energies below the neutron threshold. A wide variety of nuclear structure phenomena have been investigated with the Nuclear Resonance Fluorescence (NRF) technique during the last four years at HIGS. These include two-phonon excitations of even-even, nearly closed shell nuclei with large magnetic dipole (M1) strengths, the so-called "scissors mode". Large electric dipole (E1) transitions to the ground states have been observed in spherical nuclei near Z = 50 and N = 82, which are assumed to arise from the coupling of quadrupole and octupole vibrational modes of the nucleus. Recent experiments at HIGS concerned the so-called Pygmy Dipole Resonances (PDR), observed as a clustering of states close to the neutron threshold at excitation energies from 5.5 MeV to 8.0 MeV. Although carrying only a small fraction of the full dipole strength, these states are of particular interest because they reflect the motion of the neutron skin against the isotopic symmetric core. Fig. 4. Excitation of different modes in the Giant Dipole Resonance. More then 100 new parities have been measured at HIGS in neutron closed-shell nuclei, including 138Ba, 88Sr, 92Zr, 96Mo, 172,174,176Yb, 164,162Dy, 208Pb, 40Ar, 11B, and 124Sn. These experiments show HIGS to be an excellent dipole "parity meter". More information about the NRF technique can be found here. Study of the nucleosynthetic p-process at HIGS The stable neutron-deficient and proton-rich isototopes of elements with Z > 34 are referred to as p-nuclei and the main stellar process synthesizing them is called the p- (or gamma-) process. The HIGS facility, with its quasi-monoenergetic gamma-ray beam, is an ideal tool for the study of photodisintegration reactions that are relevant for the nucleosynthesis of proton-rich nuclei in the gamma-process. The origin of the 35 nuclei lying in the proton-rich side of the valley of the b-stability between 74Se and 196Hg can be explained by the burning of the pre-existing s-and r-nuclei. During a stellar explosion, temperatures of about T = 2-3 x10^9 K are reached, and the resulting thermal photon bath contains enough high-energy photons to initiate a sequence of (g,n), (g,p), and (g,a) reactions, which lead to the production of the p-nuclei. The process develops largely in the O-Ne rich layers of Type II supernovae. The production and destruction of p-nuclei through the (g,n) reaction occur in the low-energy tail of the GDR (near the neutron threshold) and should be measured with great accuracy. A particularly interesting case is the 181Ta(g,n)180mTa reaction which is considerably underproduced in theoretical predictions of p-processes. Fig. 5. Region of the periodic table relevant to 180Ta investigation. The s-, r-, and p-process are shown.