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Register a new userThe gamma-ray strength function of $^{89}$Y and $^{90}$Y
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In this work, we present new data on the $^{89}$Y($gamma$,n) cross section studied with a quasi-monochromatic photon beam produced at the NewSUBARU synchrotron radiation facility in Japan contributing torwards resolving a long standing discrepancy between existing measurements of this cross section. Results for $gamma$-ray strength function below threshold obtained by applying the Oslo method to $^{89}$Y($p,pgamma$)$^{89}$Y coincidences combined with the $^{89}$Y($gamma$,n) data this providing experimental data for the $gamma$-ray strength function of $^{89}$Y for $gamma$ energies in the range of $approx 1.6$ Mev to $approx$ 20 MeV. A low-energy enhancement is seen for $gamma$-rays below $approx 2.5$ MeV. Shell-model calculations indicate that this feature is caused by strong, low-energy $M1$ transitions at high excitation energies. The nuclear level density and $gamma$-ray strength function have been extracted from $^{89}$Y($d,p gamma$)$^{90}$Y coincidences using the Oslo method. Using the ($gamma,n$) and ($d,pgamma$) data as experimental constraints, we have calculated the $^{89}$Y($n,gamma$)$^{90}$Y cross section with the TALYS reaction code. Our results have been compared with directly measured (n,$gamma$) cross sections and evaluations. The $N=50$ isotope $^{89}$Y is an important bottleneck in the s-process and the magnitude of the $^{89}$Y(n,$gamma)$ cross section is key to understanding how s-process stars produce heavy isotopes.
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Partial cross sections of the $^{89}$Y(p,$gamma$)$^{90}$Zr reaction have been measured to investigate the $gamma$-ray strength function in the neutron-magic nucleus $^{90}$Zr. For five proton energies between $E_p=3.65$ MeV and $E_p=4.70$ MeV, partial cross sections for the population of seven discrete states in $^{90}$Zr have been determined by means of in-beam $gamma$-ray spectroscopy. Since these $gamma$-ray transitions are dominantly of $E1$ character, the present measurement allows an access to the low-lying dipole strength in $^{90}$Zr. A $gamma$-ray strength function based on the experimental data could be extracted, which is used to describe the total and partial cross sections of this reaction by Hauser-Feshbach calculations successfully. Significant differences with respect to previously measured strength functions from photoabsorption data point towards deviations from the Brink-Axel hypothesis relating the photo-excitation and de-excitation strength functions.
The nuclear level density and the $gamma$-ray strength function have been extracted for $^{89}$Y, using the Oslo Method on $^{89}$Y($p,p gamma$)$^{89}$Y coincidence data. The $gamma$-ray strength function displays a low-energy enhancement consistent with previous observations in this mass region ($^{93-98}$Mo). Shell-model calculations give support that the observed enhancement is due to strong, low-energy $M1$ transitions at high excitation energies. The data were further used as input for calculations of the $^{88}$Sr($p,gamma$)$^{89}$Y and $^{88}$Y($n,gamma$)$^{89}$Y cross sections with the TALYS reaction code. Comparison with cross-section data, where available, as well as with values from the BRUSLIB library, shows a satisfying agreement.
Elastic scattering cross sections of the $^{89}$Y($alpha$,$alpha$)$^{89}$Y reaction have been measured at energies E$_{c.m.}$ = 15.51 and 18.63 MeV. The high precision data for the semi-magic $N = 50$ nucleus $^{89}$Y are used to derive a local potential and to evaluate the predictions of global and regional $alpha$-nucleus potentials. The variation of the elastic alpha scattering cross sections along the $N = 50$ isotonic chain is investigated by a study of the ratios of angular distributions for $^{89}$Y($alpha$,$alpha$)$^{89}$Y and $^{92}$Mo($alpha$,$alpha$)$^{92}$Mo at E$_{c.m.} approx$ 15.51 and 18.63 MeV. This ratio is a very sensitive probe at energies close to the Coulomb barrier, where scattering data alone is usually not enough to characterize the different potentials. Furthermore, $alpha$-cluster states in $^{93}$Nb = $^{89}$Y $otimes$ $alpha$ are investigated.
Photoneutron cross sections were measured for $^{137}$Ba and $^{138}$Ba at energies below two-neutron threshold using quasi-monochromatic $gamma$-ray beams produced in laser Compton-scattering at the NewSUBARU synchrotron radiation facility. The photoneutron data are used to constrain the $gamma$-ray strength function on the basis of the Hartree-Fock-Bogolyubov plus quasi-particle random phase approximation using the Gogny D1M interaction. Supplementing the experimentally constrained $gamma$-ray strength function with the zero-limit E1 and M1 contributions which are unique to the deexcitation mode, we discuss radiative neutron capture cross sections relevant to the s-process nucleosynthesis of barium isotopes in the vicinity of the neutron magic number 82.
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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