[en] The STARS/LiBerACE collaboration has been exploring the surrogate technique with success in the actinide region. This method uses a direct reaction to measure the decay probability of the same compound nucleus produced via a neutron-induced channel. This paper serves as an overview of these activities. Using the STARS array at 88-inch Cyclotron at Lawrence Berkeley National Laboratory we have explored the following surrogate reactions: ²³⁴U(α, α(prime)f), ²³⁵U(³He, αf), ²³⁶U(α, α(prime)f), ²³⁸U (α,α(prime)f), ²³⁸U(³He,αf), ²³⁸U(³He, tf) surrogates for ²³³U(n,f), ²³³U(n,f), ²³⁵U(n,f), ²³⁷U(n,f), ²³⁶U(n,f), and ²³⁷Np(n,f), respectively

[en] We measured the ratio of the fission probabilities of ²³⁴U* relative to ²³⁶U* formed via an (α,α(prime)) direct reactions using the STARS array at the 88-inch cyclotron at the Lawrence Berkeley National Laboratory. This ratio has a shape similar to the ratio of neutron capture probabilities from ²³³U(n; f) and ²³⁵U(n; f), indicating the alpha reactions likely formed a compound nucleus. This result indicates that the ratios of fission exit channel probabilities for two actinide nuclei populated via (α, α(prime)) can be used to determine an unknown fission cross section relative to a known one. The validity of the External Surrogate Ratio Method (ESRM) is tested and the results support the conclusions of Burke et al. [1]

[en] Excited states in ¹⁵²Sm have been investigated with the ¹⁵²Sm(n,n^'γ) reaction. The lowest four negative-parity band structures have been characterized in detail with respect to their absolute decay properties. Specifically, a new K^π=0^- band has been assigned with its 1^- band head at 1681 keV. This newly observed band has a remarkable similarity in its E1 transition rates for decay to the first excited K^π=0⁺ band at 684 keV to the lowest K^π=0^- band and its decay to the ground-state band. Based on these decay properties, as well as energy considerations, this new band is assigned as a K^π=0^- octupole excitation based on the K^π=0₂⁺ state. An emerging pattern of repeating excitations built on the 0₂⁺ level similar to those built on the ground state may indicate that ¹⁵²Sm is a complex example of shape coexistence rather than a critical point nucleus.

[en] Extensive studies of the low-spin excited states in ₄₂⁹⁶Mo₅₄ with the (n,n^'γ) reaction have clarified the level scheme below 3.7 MeV excitation energy and determined detailed information about ⁹⁶Mo, including lifetimes from the Doppler-shift attenuation method, branching ratios, and multipole mixing ratios. Also, B(E2) and B(M1) values were determined for many transitions, multiphonon states were identified, and several low-spin states were characterized in terms of collective, mixed-symmetry states

[en] Properties of low-spin states in ¹¹⁴Cd have been studied with the (n,n^'γ) reaction. Gamma-ray angular distributions and excitation functions have been used to characterize the decays of the excited levels. Level lifetimes have been obtained with the Doppler-shift attenuation method. Sixteen new levels and many new transitions have been suggested below 3.5 MeV in excitation energy. Levels belonging to the phonon multiplets have been proposed based on their decay patterns and collectivity, and the existing intruder structure has been extended. A two-phonon 1_ms⁺ state has been suggested. Excitation of the hexadecapole moment has been considered. Data have been compared with the theoretical calculations of the interacting boson model

[en] This paper highlights some of the current basic nuclear physics research at Lawrence Livermore National Laboratory (LLNL). The work at LLNL concentrates on investigating nuclei at the extremes. The Experimental Nuclear Physics Group performs research to improve our understanding of nuclei, nuclear reactions, nuclear decay processes and nuclear astrophysics; an expertise utilized for important laboratory national security programs and for world-class peer-reviewed basic research.

[en] The ^171,173Yb(d,pγ) reactions have been measured to determine the efficacy of this reaction as a surrogate for neutron capture on radioactive nuclei. Preliminary results for the surrogate cross section ratios, with gating conditions that best mimic the spin distribution of neutron capture, reproduce the Wisshak et al., (n,γ) cross section ratios within 15%.

[en] The lifetime of the 2₁⁺ state in ¹⁶C has been measured with the recoil distance method using the ⁹Be(⁹Be,2p) fusion-evaporation reaction at a beam energy of 40 MeV. The mean lifetime was measured to be 11.7(20) ps corresponding to a B(E2; 2₁⁺→0⁺) value of 4.15(73)e² fm⁴ [1.73(30) W.u.], consistent with other even-even closed shell nuclei. Our result does not support an interpretation for ''decoupled'' valence neutrons

[en] To benchmark the validity of using the (d,pγ) reaction as a surrogate for (n,γ), the ^171,173Yb(d,pγ) reactions were measured and compared with the neutron capture cross sections measured by Wisshak et al. The (d,pγ) ratios were measured using an 18.5 MeV deuteron beam from the 88-Inch Cyclotron at LBNL. Preliminary results comparing the surrogate ratios with the known (n,γ) cross sections are discussed

[en] We have indirectly determined the ²³⁷Np(n,f) cross section over an equivalent neutron energy range from 10 to 20 MeV using the surrogate reaction ²³⁸U(³He,tf). A self-supporting ∼761 μg/cm² metallic ²³⁸U foil was bombarded with a 42 MeV ³He²⁺ beam from the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory (LBNL). Outgoing charged particles and fission fragments were identified using the Silicon Telescope Array for Reaction Studies (STARS) consisted of two 140 μm and one 1000 μm Micron S2 type silicon detectors. These results were compared with the ²³⁷Np(n,f) cross section data from the direct measurements, the Evaluated Nuclear Data File (ENDF/B-VII.0), and the Japanese Evaluated Nuclear Data Library (JENDL 3.3) and found to closely follow those datasets