Measurement of the ${}^{187}\mathrm{Re}(\alpha ,n){}^{190}\mathrm{Ir}$ reaction cross section at sub-Coulomb energies using the Cologne Clover Counting Setup

Background: Uncertainties in adopted models of $\text{particle}+\text{nucleu}\mathrm{s}$ optical-model potentials directly influence the accuracy in the theoretical predictions of reaction rates as they are needed for reaction-network calculations in, for instance, $\gamma$-process nucleosynthesis. The improvement of the $\alpha +\text{nucleu}\mathrm{s}$ optical-model potential is hampered by the lack of experimental data at astrophysically relevant energies especially for heavier nuclei.

Purpose: Measuring the ${}^{187}\mathrm{Re}(\alpha ,n){}^{190}\mathrm{Ir}$ reaction cross section at sub-Coulomb energies extends the scarce experimental data available in this mass region and helps understanding the energy dependence of the imaginary part of the $\alpha +\text{nucleus}$ optical-model potential at low energies.

Method: Applying the activation method, after the irradiation of natural rhenium targets with $\alpha$-particle energies of 12.4 to 14.1 MeV, the reaction yield and thus the reaction cross section were determined via $\gamma$-ray spectroscopy by using the Cologne Clover Counting Setup and the method of $\gamma \gamma$ coincidences.

Results: Cross-section values at five energies close to the astrophysically relevant energy region were measured. Statistical model calculations revealed discrepancies between the experimental values and predictions based on widely used $\alpha$+nucleus optical-model potentials. However, an excellent reproduction of the measured cross-section values could be achieved from calculations based on the so-called Sauerwein–Rauscher $\alpha +\text{nucleus}$ optical-model potential.

Conclusion: The results obtained indicate that the energy dependence of the imaginary part of the $\alpha +\text{nucleus}$ optical-model potential can be described by an exponential decrease. Successful reproductions of measured cross sections at low energies for $\alpha$-induced reactions in the mass range $141\le A\le 187$ confirm the global character of the Sauerwein–Rauscher potential.

Figure 1

Sensitivity of the laboratory cross section for the ${}^{187}\mathrm{Re}(\alpha ,n){}^{190}\mathrm{Ir}$ reaction to the variation of the $\alpha$ (red line), $\gamma$ (black dotted line), and neutron widths (blue dashed line) as a function of center-of-mass energy [22]. The cross section is insensitive to the proton width. See text for details.

Figure 2

A computer-aided design drawing of the Cologne Clover Counting Setup. Two high-purity clover-type germanium detectors are mounted on a movable rail system. The target is placed between them. In the closest geometry, the detectors cover a solid angle of almost $4\pi$.

Figure 3

Total full-energy peak efficiency of the Cologne Clover Counting Setup as a function of $\gamma$-ray energy. Additionally, a simulation of the full-energy peak efficiency obtained with geant4 with and without addback is shown. In favor of a better readability, the fitted efficiency curve is not shown.

Figure 4

Comparison of Compton background of the different Clover operation modes. See text for details.

Figure 5

Example of the measured $\gamma$-ray spectra for an alpha-particle energy of 13.7 MeV. For comparison, the spectrum obtained by the application of an addback as well as the measured background spectrum are shown in addition.

Figure 6

Simplified level scheme of ${}^{190}\mathrm{Os}$. Only the strongest $\gamma$-ray transitions are shown in the level scheme. All data are adopted from Ref. [23]. Note that there are two 407 keV $\gamma$-ray transitions contributing to the specific peak in the $\gamma$-ray spectra. Besides the 557.9 keV $\gamma$-ray transition, all visible transitions are in coincidence with the $2^{+}\to 0^{+}$ ground-state transition with a $\gamma$-ray energy of 186.6 keV. The parameters of the $\gamma$-ray transitions are given in Table 2.

Figure 7

Illustration of the $\gamma \gamma$-coincidence method. The upper panel shows the sum of the singles spectra of all clover leaves in an energy range between 360 and 615 keV. The measuring time was 202 h. The spectrum in the lower panel shows only those events which were in coincidence with an event with an energy of 186.68 keV. The peak-to-background ratio is increased by a factor of two to ten depending on the specific $\gamma$-ray transition. See text for details.

Figure 8

Total cross section as a function of the $\alpha$-particle energy in comparison to statistical model calculations using the talys code [34]. The standard $\alpha$ OMP of talys is the one of Watanabe [35] which overestimates the measured values by a factor of four. Also the widely used $\alpha$ OMP of McFadden and Satchler [37] leads to a prediction of cross-section values which are too large. However, as pointed out in Ref. [38], this may be due to the neglect of the Coulomb excitation in the determination of the $\alpha$ optical potential. In contrast to this, the optical-model potentials of Ref. [36] lead to predictions of cross-section values which are too low (OMP1). The potential of Ref. [20] with a varied value for the “steepness” of the Fermi-type function can reproduce the experimental values properly. See text for details.