The γ-ray strength function and level density in the quasi-continuum of 151,153 Sm have been measured using BGO shielded Ge clover detectors of the STARLiTeR system. The Compton shields allow for an extraction of the γ strength down to unprecedentedly low γ energies of ≈ 500 keV. For the first time an enhanced lowenergy γ-ray strength has been observed in the rare-earth region. In addition, for the first time both the upbend and the well known scissors resonance have been observed simultaneously for the same nucleus. HauserFeshbach calculations show that this strength enhancement at low γ energies could have an impact of 2-3 orders of magnitude on the (n,γ) reaction rates for the r-process nucleosynthesis.

No abstract

The effect of the production mechanism on the decay of a compound nucleus is investigated. The nucleus 90 Zr was produced by three different reactions, namely 90 Zr(p, p ) 90 Zr, 91 Zr(p, d) 90 Zr, and 92 Zr(p, t) 90 Zr, which served as surrogate reactions for 89 Zr(n, γ). The spin-parity (J π ) distributions of the states populated by these reactions were studied to investigate the surrogate reaction approach, which aims at indirectly determining cross sections for compound-nuclear reactions involving unstable targets such as 89 Zr. Discrete γ-rays, associated with transitions in 90 Zr and 89 Zr, were measured in coincidence with light ions for scattering angles of 25-60 • and 90 Zr excitation energies extending above the neutron separation energy. The measured transition systematics were used to gain insights into the J π distributions of 90 Zr. The 90 Zr(p, p ) reaction was found to produce fewer γ-rays associated with transitions involving high spin states (J = 6-8 ) than the other two reactions, suggesting that inelastic scattering preferentially populates states in 90 Zr that have lower spins than those populated in the transfer reactions investigated. The γ-ray production was also observed to vary by factors of 2-3 with the angle at which the outgoing particle was detected. These findings are relevant to the application of the surrogate reaction approach.

No abstract

The240 Am(n,f) cross section has been measured for the first time above 4 MeV, using the surrogate ratio method over the neutron energy range of 200 keV to 14 MeV. The reactions 243 Am(p,tf) and 238 U(p,tf), which proceed through the fissioning excited nuclei 241 Am* and 236 U*, were used as surrogates for the desired 240 Am(n,f) and 235 U(n,f) reactions. The experiment was fielded using the STARLiTeR detector system with a recently commisioned VME-based data acquisition system. The 38.4 MeV proton beam used in these measurements was provided by the K150 cyclotron at the Texas A&M Cyclotron Institute. The measured 240 Am(n,f) cross section disagrees with many of the most recent evaluations, and a reevaluation is recommended.