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Register a new userExperimentally constrained ($p,gamma$)$^{89}$Y and ($n,gamma$)$^{89}$Y reaction rates relevant to the $p$-process nucleosynthesis
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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.
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Unknown neutron-capture reaction rates remain a significant source of uncertainty in state-of-the-art $r$-process nucleosynthesis reaction network calculations. As the $r$-process involves highly neutron-rich nuclei for which direct ($n,gamma$) cross-section measurements are virtually impossible, indirect methods are called for to constrain ($n,gamma$) cross sections used as input for the $r$-process nuclear network. Here we discuss the newly developed beta-Oslo method, which is capable of providing experimental input for calculating ($n,gamma$) rates of neutron-rich nuclei. The beta-Oslo method represents a first step towards constraining neutron-capture rates of importance to the $r$-process.
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.
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.
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 photodisintegration cross sections for the 94Mo({gamma},n) and 90Zr({gamma},n) reactions have been experimentally investigated with quasi-monochromatic photon beams at the High Intensity {gamma}-ray Source (HI{gamma}S) facility of the Triangle Universities Nuclear Laboratory (TUNL). The energy dependence of the photoneutron reaction cross sections was measured with high precision from the respective neutron emission thresholds up to 13.5 MeV. These measurements contribute to a broader investigation of nuclear reactions relevant to the understanding of the p-process nucleosynthesis. The results are compared with the predictions of Hauser-Feshbach statistical model calculations using two different models for the dipole {gamma}-ray strength function. The resulting 94Mo({gamma},n) and 90Zr({gamma},n) photoneutron stellar reaction rates as a function of temperature in the typical range of interest for the p-process nucleosynthesis show how sensitive the photoneutron stellar reaction rate can be to the experimental data in the vicinity of the neutron threshold.
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