Statistical gamma-ray decay studies at iThemba LABS

Abstract. A program to study the γ -ray decay from the region of high-level density has been established at iThemba LABS, where a high-resolution gamma-ray detector array is used in conjunction with silicon particle-telescopes. Results from two recent projects are presented: 1) The 74Ge(α, α γ ) reaction was used to investigate the Pygmy Dipole Resonance. The results were compared to (γ,γ ) data and indicate that the dipole states split into mixed isospin and relatively pure isovector excitations. 2) Data from the 95Mo(d,p) reaction were used to develop a novel method for the determination of spins for low-lying discrete levels utilizing statistical γ -ray decay in the vicinity of the neutron separation energy. These results provide insight into the competition of (γ ,n) and (γ,γ ) reactions and highlights the need to correct for angular momentum barrier effect

Nuclear level densities and γ -ray strength functions of 180,181Ta and neutron capture cross sections

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Resonances in odd-odd 182Ta

Abstract: Enhanced γ -decay on the tail of the giant electric dipole resonance, such as the scissors or pygmy resonances, can have significant impact on (n,γ ) reaction rates. These rates are important input for modeling processes that take place in astrophysical environments and nuclear reactors. Recent results from the University of Oslo indicate the existence of a significant enhancement in the photon strength function for nuclei in the actinide region due to the scissors resonance. Further, the M1 strength distribution of the scissors resonances in rare earth nuclei has been studied extensively over the years. To investigate the evolution and persistence of the scissor resonance in other mass regions, an experiment was performed utilizing the NaI(Tl) γ -ray detector array (CACTUS) and silicon particle telescopes (SiRi) at the University of Oslo Cyclotron laboratory. Particle-γ coincidences from the 181Ta(d,p)182Ta and 181Ta(d,d’)181Ta reactions were used to measure the nuclear level density and photon strength function of the well-deformed 181Ta and 182Ta systems, to investigate the existence of resonances below the neutron separation energy

Re-estimation of 180Ta nucleosynthesis in light of newly constrained reaction rates

Recent measurements of the nuclear level densities and γ-ray strength functions below the neutron thresholds in 180,181,182Ta are used as input in the nuclear reaction code TALYS. These experimental average quantities are utilized in the calculations of the 179,180,181Ta radiative neutron capture cross sections. From the latter, astrophysical Maxwellian-averaged (n,γ) cross sections (MACS) and reaction rates are extracted, which in turn are used in large astrophysical network calculations to probe the production mechanism of 180Ta. These calculations are performed for two scenarios, the s-process production of 180,181Ta in Asymptotic Giant Branch (AGB) stars and the p-process nucleosynthesis of Tam180 in Type-II supernovae. Based on the results from this work, the s-process in stellar evolution is considered negligible in the production of Tam180 whereas 181Ta is partially produced by AGB stars. The new measurements strongly constrain the production and destruction rates of Tam180 at p-process temperatures and confirm significant production of nature's rarest stable isotope Tam180 by the p-process. Keywords: Nuclear level density, γ-ray strength function, (n,γ) cross sections, Maxwellian-averaged cross sections, Nucleosynthesi

Nuclear level densities and γ-ray strength functions of 180,181Ta and neutron capture cross sections

The γ -ray strength functions and nuclear level densities in the quasi-continuum of 180,181Ta are extracted from particle-γ coincidence events with the Oslo Method, below the Sn . The data were used as input in the TALYS reaction code for calculations of the astrophysical Maxwellian-averaged (n, γ ) cross-sections to investigate nucleosynthesis of nature’s rarest stable isotope 180Ta

Resonances in odd-odd 182Ta

Enhanced γ -decay on the tail of the giant electric dipole resonance, such as the scissors or pygmy resonances, can have significant impact on (n,γ ) reaction rates. These rates are important input for modeling processes that take place in astrophysical environments and nuclear reactors. Recent results from the University of Oslo indicate the existence of a significant enhancement in the photon strength function for nuclei in the actinide region due to the scissors resonance. Further, the M1 strength distribution of the scissors resonances in rare earth nuclei has been studied extensively over the years. To investigate the evolution and persistence of the scissor resonance in other mass regions, an experiment was performed utilizing the NaI(Tl) γ -ray detector array (CACTUS) and silicon particle telescopes (SiRi) at the University of Oslo Cyclotron laboratory. Particle-γ coincidences from the 181Ta(d,p)182Ta and 181Ta(d,d’)181Ta reactions were used to measure the nuclear level density and photon strength function of the well-deformed 181Ta and 182Ta systems, to investigate the existence of resonances below the neutron separation energy

First application of the Oslo method in inverse kinematics

International audienceThe $\gamma$-ray strength function ($\gamma$SF) and nuclear level density (NLD) have been extracted for the first time from inverse kinematic reactions with the Oslo method. This novel technique allows measurements of these properties across a wide range of previously inaccessible nuclei. Proton–$\gamma$ coincidence events from the $\mathrm {d}(^{86}\mathrm {Kr}, \mathrm {p}\gamma )^{87}\mathrm {Kr}$ reaction were measured at iThemba LABS and the $\gamma$SF and NLD in $^{87}\mathrm {Kr}$ was obtained. The low-energy region of the $\gamma$SF is compared to shell-model calculations, which suggest this region to be dominated by M1 strength. The $\gamma$SF and NLD are used as input parameters to Hauser–Feshbach calculations to constrain $(\mathrm {n},\gamma )$ cross sections of nuclei using the TALYS reaction code. These results are compared to $^{86}\mathrm {Kr}(n,\gamma )$ data from direct measurements

Nuclear level densities and γ\gamma-ray strength functions of 87Kr^{87}\mathrm{Kr} -- First application of the Oslo Method in inverse kinematics

The $\gamma$-ray strength function ($\gamma$SF) and nuclear level density (NLD) have been extracted for the first time from inverse kinematic reactions with the Oslo Method. This novel technique allows measurements of these properties across a wide range of previously inaccessible nuclei. Proton-$\gamma$ coincidence events from the $\mathrm{d}(^{86}\mathrm{Kr}, \mathrm{p}\gamma)^{87}\mathrm{Kr}$ reaction were measured at iThemba LABS and the $\gamma$SF and NLD in $^{87}\mathrm{Kr}$ obtained. The low-energy region of the $\gamma$SF is compared to Shell Model calculations which suggest this region to be dominated by M1 strength. The $\gamma$SF and NLD are used as input parameters to Hauser-Feshbach calculations to constrain $(\mathrm{n},\gamma)$ cross sections of nuclei using the TALYS reaction code. These results are compared to $^{86}\mathrm{Kr}(n,\gamma)$ data from direct measurements