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Register a new userImpact of restricted spin-ranges in the Oslo Method: The example of (d,p)$^{240}mathrm{Pu}$
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In this paper we present the first systematic analysis of the impact of the populated vs. intrinsic spin distribution on the nuclear level density and $gamma$-ray strength function retrieved through the Oslo Method. We illustrate the effect of the spin distribution on the recently performed $^{239}mathrm{Pu}$(d,p$gamma$)$^{240}mathrm{Pu}$ experiment using a 12 MeV deuteron beam performed at the Oslo Cyclotron Lab. In the analysis we couple state-of-the-art calculations for the populated spin-distributions with the Monte-Carlo nuclear decay code RAINIER to compare Oslo Method results to the known input. We find that good knowledge of the populated spin distribution is crucial and show that the populated distribution has a significant impact on the extracted nuclear level density and $gamma$-ray strength function for the $^{239}mathrm{Pu}$(d,p$gamma$)$^{240}mathrm{Pu}$ case.
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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.
Prompt fission $gamma$-rays are responsible for approximately 5% of the total energy released in fission, and therefore important to understand when modelling nuclear reactors. In this work we present prompt $gamma$-ray emission characteristics in fission, for the first time as a function of the nuclear excitation energy of the fissioning system. Emitted $gamma$-ray spectra were measured, and $gamma$-ray multiplicities and average and total $gamma$ energies per fission were determined for the $^{233}$U(d,pf) reaction for excitation energies between 4.8 and 10 MeV, and for the $^{239}$Pu(d,pf) reaction between 4.5 and 9 MeV. The spectral characteristics show no significant change as a function of excitation energy above the fission barrier, despite the fact that an extra $sim$5 MeV of energy is potentially available in the excited fragments for $gamma$-decay. The measured results are compared to model calculations made for prompt $gamma$-ray emission with the fission model code GEF. Further comparison with previously obtained results from thermal neutron induced fission is made to characterize possible differences arising from using the surrogate (d,p) reaction.
In this work, we have reviewed the Oslo method, which enables the simultaneous extraction of level density and gamma-ray transmission coefficient from a set of particle-gamma coincidence data. Possible errors and uncertainties have been investigated. Typical data sets from various mass regions as well as simulated data have been tested against the assumptions behind the data analysis.
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.
The average prompt-fission-neutron multiplicity $bar{ u}$ is of significance in the areas of nuclear theory, nuclear nonproliferation, and nuclear energy. In this work, the surrogate-reaction method has been used for the first time to indirectly determine $bar{ u}$ for $^{239}$Pu($n$,$f$) via $^{240}$Pu($alpha$,$alpha^{prime}f$) reactions. A $^{240}$Pu target was bombarded with a beam of 53.9-MeV $alpha$ particles. Scattered $alpha$ particles, fission products, and neutrons were measured with the NeutronSTARS detector array. Values of $bar{ u}$ were obtained for a continuous range of equivalent incident neutron energies between 0.25--26.25~MeV, and the results agree well with direct neutron measurements.
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