Deducing primary γ -ray intensities and the dipole strength function in Mo94 via radiative proton capture

Abstract

Background: The nucleosynthesis of heavy nuclei is affected by the reaction rates of radiative capture reactions. Many of the astrophysical relevant rates cannot be obtained from experiments but are obtained from theoretical models. The $\ensuremath{\gamma}$-decay widths that are derived from radiative strength functions are one of the key nuclear physics input parameters in those calculations. The explicit study of $\ensuremath{\gamma}$-ray strength functions has been thoroughly addressed in the last decade and various methods have been established to extract the dipole strength in atomic nuclei.Purpose: The investigation of primary $\ensuremath{\gamma}$-ray transitions from the $^{93}\mathrm{Nb}(p,\ensuremath{\gamma})^{94}\mathrm{Mo}$ reaction allows deducing the $\ensuremath{\gamma}$-ray strength function in $^{94}\mathrm{Mo}$. The results are compared to results obtained using other techniques and the impact of level densities, excitation mechanism, and excitation energy will be thoroughly investigated.Method: The proton beam was delivered by the 10 MV FN-Tandem accelerator located at the Institute for Nuclear Physics at the University of Cologne, Germany. By means of in-beam $\ensuremath{\gamma}$-ray spectroscopy the intensity of primary $\ensuremath{\gamma}$ rays is determined. Additionally, the first generation $\ensuremath{\gamma}$-ray intensity is deduced from the secondary $\ensuremath{\gamma}$ rays using their branching and the coincidence detection efficiency. Absolute $\ensuremath{\gamma}$-ray strength function values as well as relative values using the ratio method will be extracted.Results: Numerous states in $^{94}\mathrm{Mo}$ at excitation energies above 3 MeV have been identified for the first time and their decay behavior has been studied. We disentangled the effects of $M1$ and $E1$ radiation for the $\ensuremath{\gamma}$-ray emission channel in $^{94}\mathrm{Mo}$ and extracted strength function curves for both radiation types. Our results are in good agreement with experimental results using the Oslo method and photoinduced experiments as well as with recent theoretical quasiparticle random-phase approximation (QRPA) calculations. An enhancement of the $\ensuremath{\gamma}$-ray strength below the neutron separation energy was found that is most likely of $E1$ character. In addition, a significant increase of the dipole strength at low $\ensuremath{\gamma}$-ray energies was found which is most likely due to $M1$ strength.Conclusion: Radiative proton capture reactions are a well-suited tool to study the $\ensuremath{\gamma}$-ray strength function in atomic nuclei. From the intensity of primary $\ensuremath{\gamma}$-ray transitions as well as from secondary $\ensuremath{\gamma}$-rays, information about the radiative dipole strength can be extracted. The $\ensuremath{\gamma}$-ray emission seems to be independent of the excitation energy in the studied mass and energy range. Detailed knowledge about the intrinsic properties of the nuclear states is very important and their uncertainties affect the uncertainty of the extracted $\ensuremath{\gamma}$-ray strength functions heavily.