Nucleosynthesis of 92^{92}Nb and the relevance of the low-lying isomer at 135.5 keV

Background: Because of its half-life of about 35 million years, 92Nb is considered as a chronometer for nucleosynthesis events prior to the birth of our sun. The abundance of 92Nb in the early solar system can be derived from meteoritic data. It has to be compared to theoretical estimates for the production of 92Nb to determine the time between the last nucleosynthesis event before the formation of the early solar system. Purpose: The influence of a low-lying short-lived isomer on the nucleosynthesis of 92Nb is analyzed. The thermal coupling between the ground state and the isomer via so-called intermediate states affects the production and survival of 92Nb. Method: The properties of the lowest intermediate state in 92Nb are known from experiment. From the lifetime of the intermediate state and from its decay branchings, the transition rate from the ground state to the isomer and the effective half-life of 92Nb are calculated as a function of the temperature. Results: The coupling between the ground state and the isomer is strong. This leads to thermalization of ground state and isomer in the nucleosynthesis of 92Nb in any explosive production scenario and almost 100% survival of 92Nb in its ground state. However, the strong coupling leads to a temperature-dependent effective half-life of 92Nb which makes the 92Nb survival very sensitive to temperatures as low as about 8 keV, thus turning 92Nb at least partly into a thermometer. Conclusions: The low-lying isomer in 92Nb does not affect the production of 92Nb in explosive scenarios. In retrospect this validates all previous studies where the isomer was not taken into account. However, the dramatic reduction of the effective half-life at temperatures below 10 keV may affect the survival of 92Nb after its synthesis in supernovae which are the most likely astrophysical site for the nucleosynthesis of 92Nb.Comment: 5 pages, 3 figures; Phys. Rev. C, accepted for publicatio

alpha-cluster states in intermediate mass nuclei

Properties of intermediate mass nuclei have been investigated within the framework of the alpha-cluster model in combination with systematic double-folding potentials. Previously, this alpha-cluster model has been widely applied to light nuclei, in particular to 8Be = alpha \otimes alpha, 20Ne = 16O \otimes alpha, and 44Ti = 40Ca \otimes alpha, and to heavy nuclei, in particular to 212Po = 208Pb \otimes alpha. In the present work a wide range of nuclei is investigated with the magic neutron number N = 50 in the mass range around A \approx 80 - 100: (A+4,N=52) = (A,N=50) \otimes alpha. It is found that excitation energies, decay properties, and transition strengths can be described successfully within this model. The smooth and small variation of the underlying parameters of the alpha-nucleus potential may be used for extrapolations to predict experimentally unknown properties in the nuclei under study.Comment: 9 pages, 7 figures, TONPPJ, accepte

α\alpha-cluster states in 46,54^{46,54}Cr from double-folding potentials

$\alpha$--cluster states in $^{46}$Cr and $^{54}$Cr are investigated in the double-folding model. This study complements a recent similar work of Souza and Miyake \cite{Sou17} which was based on a specially shaped potential. Excitation energies, reduced widths, intercluster separations, and intra-band transition strengths are calculated and compared to experimental values for the ground state bands in $^{46}$Cr and $^{54}$Cr. The $\alpha$-cluster potential is also applied to elastic scattering at low and intermediate energies. Here, as a byproduct, a larger radial extent of the neutron density in $^{50}$Ti is found.Comment: 9 pages, 7 figures, Europ. Phys. J. A, accepted for publicatio

Photon-induced Reactions in Stars and in the Laboratory: A Critical Comparison

Photon-induced reactions during the astrophysical p- (or gamma-) process occur at typical temperatures of 1.8 < T9 < 3.3. Experimental data of (gamma,n), (gamma,p), or (gamma,alpha) reactions - if available in the relevant energy region - cannot be used directly to measure astrophysical (gamma,n), (gamma,p), or (gamma,alpha) reaction rates because of the thermal excitation of target nuclei at these high temperatures. Usually, statistical model calculations are used to predict photon-induced reaction rates. The relations between experimental reaction cross sections, theoretical predictions, and astrophysical reaction rates will be critically discussed.Comment: 8 pages, 2 figures, Proc. Tours Symposium Nuclear Physics V 2003, p.53

Cross sections of α\alpha-induced reactions for targets with masses A≈20−50A \approx 20-50 at low energies

A simple reduction scheme using so-called reduced energies $E_{\rm{red}}$ and reduced cross sections $\sigma_{\rm{red}}$ allows the comparison of heavy-ion induced reaction cross sections for a broad range of masses of projectile and target and over a wide energy range. A global behavior has been found for strongly bound projectiles whereas much larger reduced cross sections have been observed for weakly bound and halo projectiles. It has been shown that this simple reduction scheme works also well for $\alpha$-particle induced reactions on heavy target nuclei, but very recently significant deviations have been seen for $\alpha$+$^{33}$S and $\alpha$+$^{23}$Na. Motivated by these unexpected discrepancies, the present study analyses $\alpha$-induced reaction cross sections for targets with masses $A \approx 20-50$. The study shows that the experimental data for $\alpha$-induced reactions on nuclei with $A \approx 20-50$ deviate slightly from the global behavior of reduced cross sections. However, in general the deviations evolve smoothly towards lower masses. The only significant outliers are the recent data for $^{33}$S and $^{23}$Na which are far above the general systematics, and some very old data may indicate that $^{36}$Ar and $^{40}$Ar are below the general trend. As expected, also the doubly-magic $^{40}$Ca nucleus lies slightly below the results for its neighboring nuclei. Overall, the experimental data are nicely reproduced by a statistical model calculation utilizing the simple $\alpha$-nucleus potential by McFadden and Satchler. Simultaneously with the deviation of reduced cross sections $\sigma_{\rm{red}}$ from the general behavior, the outliers $^{23}$Na, $^{33}$S, $^{36}$Ar, and $^{40}$Ar also show significant disagreement between experiment and statistical model calculation.Comment: 41 pages, 66 figures, EPJA invited review, in pres

Uncertainty of the astrophysical 17,18^{17,18}O(α\alpha,n)20,21^{20,21}Ne reaction rates and the applicability of the statistical model for nuclei with A≲20A \lesssim 20

Background: The ($\alpha$,n) and ($\alpha$,$\gamma$) reactions on $^{17,18}$O have significant impact on the neutron balance in the astrophysical $s$-process. In this scenario stellar reaction rates are required for relatively low temperatures below $T_9 \lesssim 1$. Purpose: The uncertainties of the $^{17,18}$O($\alpha$,n)$^{20,21}$Ne reactions are investigated. Statistical model calculations are performed to study the applicability of this model for relatively light nuclei in extension to a recent review for the $20 \le A \le 50$ mass range. Method: The available experimental data for the $^{17,18}$O($\alpha$,n)$^{20,21}$Ne reactions are compared to statistical model calculations. Additionally, the reverse $^{20}$Ne(n,$\alpha$)$^{17}$O reaction is investigated, and similar studies for the $^{17}$F mirror nucleus are provided. Results: It is found that on average the available experimental data for $^{17}$O and $^{18}$O are well described within the statistical model, resulting in reliable reaction rates above $T_9 \gtrsim 1.5$ from these calculations. However, significant experimental uncertainties are identified for the $^{17}$O($\alpha$,n$_0$)$^{20}$Ne(g.s.) channel. Conclusions: The statistical model is able to predict astrophysical reaction rates for temperatures above 1 GK with uncertainties of less than a factor of two for the nuclei under study. An experimental discrepancy for the $^{17}$O($\alpha$,n)$^{20}$Ne reaction needs to be resolved.Comment: 10 pages, 9 figures, Phys. Rev. C, accepted for publicatio