Test of statistical model cross section calculations forα-induced reactions onAg107at energies of astrophysical interest

Abstract

Background: Astrophysical reaction rates, which are mostly derived from theoretical cross sections, are necessary input to nuclear reaction network simulations for studying the origin of p nuclei. Past experiments have found a considerable difference between theoretical and experimental cross sections in some cases, especially for (α,γ) reactions at low energy. Therefore, it is important to experimentally test theoretical cross section predictions at low, astrophysically relevant energies.Purpose: The aim is to measure reaction cross sections of 107 Ag(α,γ) 111 In and 107 Ag(α,n) 110 In at low energies in order to extend the experimental database for astrophysical reactions involving α particles towards lower mass numbers. Reaction rate predictions are very sensitive to the optical model parameters and this introduces a large uncertainty into theoretical rates involving α particles at low energy. We have also used Hauser-Feshbach statistical model calculations to study the origin of possible discrepancies between prediction and data.Method: An activation technique has been used to measure the reaction cross sections at effective center of mass energies between 7.79 MeV and 12.50 MeV. Isomeric and ground state cross sections of the (α,n) reaction were determined separately. Results:The measured cross sections were found to be lower than theoretical predictions for the (α,γ) reaction. Varying the calculated averaged widths in the Hauser-Feshbach model, it became evident that the data for the (α,γ) and (α,n) reactions can only be simultaneously reproduced when rescaling the ratio of γ-to neutron width and using an energy-dependent imaginary part in the optical α+ 107 Ag potential. Conclusions:The new data extend the range of measured charged-particle cross sections for astrophysical applications to lower mass numbers and lower energies. The modifications in the model predictions required to reproduce the present data are fully consistent with what was found in previous investigations. Thus, our results confirm the previously suggested energy-dependent modification of the optical α+nucleus potential.

Uncertainties of α-particle optical potential assessment around and below the Coulomb barrier

“…Contrary to the use of γ-ray strength functions established by a renormalization carried out in order to achieve agreement with the (α, γ) data (e.g., Ref. [2]), we rely on the measured data of radiative strength function (RSF) and average s-wave radiation widths Γ γ [35] (Table I).…”

“…Recent high-precision measurements [1][2][3][4][5][6] of α-particle induced reaction data below the Coulomb barrier (B) provide an useful opportunity to investigate the results of a previous optical-model potential (OMP) for α-particles on nuclei within the mass number range 45≤A≤209 [7]. Actually, this potential was established by (1) analysis of α-particle elastic-scattering angular distributions above B [7,8], and (2) Hauser-Feshbach statistical model (SM) assessment of the available (α, γ), (α, n) and (α, p) reaction cross sections for incident energies below B and target nuclei either with A≤120 [9] or heavier [7,10]. Thus, starting from a semi-microscopic OMP with a doublefolding model (DFM) real part and a phenomenological energy-dependent imaginary-potential dispersive contribution [8,9], a full phenomenological analysis of the same data led to a spherical OMP to be easily involved within SM calculations for basic and applied objectives.…”

“…The (α, n) and (α, γ) reaction cross sections of several proton-rich nuclei within the region just above the A≈100 form also the object of several recent studies at low energies, with similar results for the reliability of SM predictions. So, it was possible to reproduce simultaneously the data for 107 Ag only by rescaling the ratio of γ to neutron widths [2]. A study of both elastic-scattering and αinduced reaction cross sections for 106 Cd at low energies concluded as well that these data allow to constrain the other SM ingredients except the α-particle OMP [3].…”

“…The (α, n) reaction cross-section studies performed in the meantime also for the heavier nuclei 164,166 Er [5] and 187 Re [6], with the main goal to test the low energy modification [11] of the widely-used α-particle OMP of McFadden and Satchler [16], concluded that the corresponding energy-dependence steepness assumed for a Fermi-type volume imaginary potential has the different parameter values of 2.5 and 4 MeV, respectively. Moreover, it has been considered an yet open question [2,3,5,6] whether this modification is due to a required OMP change, which affects the total-reaction cross section, or due to the ne-glection of the Coulomb excitation (CE) in the entrance channel [17]. All these new measured data and related α-particle OMP studies within so different mass ranges motivated the present work, with the aim to both check the applicability of the previous OMP [7] and to look for uncertainties and possible systematic errors of α-particle optical-potential assessment at low energies.…”

“…The results corresponding to the OMP of Ref. [7] are then compared with the recently measured cross sections [1][2][3][4][5][6] in Sec. III, followed by conclusions in Sec.…”