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Register a new userNuclear level densities and $gamma$-ray strength functions of $^{87}mathrm{Kr}$ -- First application of the Oslo Method in inverse kinematics
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V. W. Ingeberg
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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.
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The scandium isotopes 44,45Sc have been studied with the 45Sc(3He,alpha gamma)44Sc and 45Sc(3He,3He gamma)45Sc reactions, respectively. The nuclear level densities and gamma-ray strength functions have been extracted using the Oslo method. The experimental level densities are compared to calculated level densities obtained from a microscopic model based on BCS quasiparticles within the Nilsson level scheme. This model also gives information about the parity distribution and the number of broken Cooper pairs as a function of excitation energy. The experimental gamma-ray strength functions are compared to theoretical models of the E1, M1, and E2 strength, and to data from (gamma,n) and (gamma,p) experiments. The strength functions show an enhancement at low gamma energies that cannot be explained by the present, standard models.
Particle-$gamma$ coincidence experiments were performed at the Oslo Cyclotron Laboratory with the $^{181}$Ta(d,X) and $^{181}$Ta($^{3}$He,X) reactions, to measure the nuclear level densities (NLDs) and $gamma$-ray strength functions ($gamma$SFs) of $^{180, 181, 182}$Ta using the Oslo method. The Back-shifted Fermi-Gas, Constant Temperature plus Fermi Gas, and Hartree-Fock-Bogoliubov plus Combinatorial models where used for the absolute normalisations of the experimental NLDs at the neutron separation energies. The NLDs and $gamma$SFs are used to calculate the corresponding $^{181}$Ta(n,$gamma$) cross sections and these are compared to results from other techniques. The energy region of the scissors resonance strength is investigated and from the data and comparison to prior work it is concluded that the scissors strength splits into two distinct parts. This splitting may allow for the determination of triaxiality and a $gamma$ deformation of $14.9^{circ} pm 1.8^{circ}$ was determined for $^{181}$Ta.
The gamma-strength functions and level densities in the quasi-continuum of 147;149Sm isotopes have been extracted from particle-coincidences using the Oslo method. The nuclei of interest were populated via (p,d) reactions on pure 148;150Sm targets and the reaction products were recorded by the Hyperion array. An upbend in the low-energy region of the gSF has been observed. The systematic analysis of the gSF for a range of Sm isotopes highlights the interplay between scissors mode and the upbend. Shell-model calculations show reasonable agreement with the experimental gSFs and confirm the correspondence between the upbend and scissors mode.
The Oslo Method has been applied to particle-$gamma$ coincidences following the $^{239}mathrm{Pu}$(d,p) reaction to obtain the nuclear level density (NLD) and $gamma$-ray strength function ($gamma$SF) of $^{240}mathrm{Pu}$. The experiment was conducted with a 12 MeV deuteron beam at the Oslo Cyclotron Laboratory. The low spin transfer of this reaction leads to a spin-parity mismatch between populated and intrinsic levels. This is a challenge for the Oslo Method as it can have a significant impact on the extracted NLD and $gamma$SF. We have developed an iterative approach to ensure consistent results even for cases with a large spin-parity mismatch, in which we couple Greens Function Transfer calculations of the spin-parity dependent population cross-section to the nuclear decay code RAINIER. The resulting $gamma$SF shows a pronounced enhancement between 2-4 MeV that is consistent with the location of the low-energy orbital $M1$ scissors mode.
The nuclear level density and the gamma-ray strength function have been determined for 43Sc in the energy range up to 2 MeV below the neutron separation energy using the Oslo method with the 46Ti(p,alpha)43Sc reaction. A comparison to 45Sc shows that the level density of 43Sc is smaller by an approximately constant factor of two. This behaviour is well reproduced in a microscopical/combinatorial model calculation. The gamma-ray strength function is showing an increase at low gamma-ray energies, a feature which has been observed in several nuclei but which still awaits theoretical explanation.
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