New measurement of the Sm144(α,γ)Gd148 reaction rate for the

Abstract

Background: Most of the heavier p isotopes are believed to be produced in the γ process whose reaction path crucially depends on the proton and α-particle penetrability at sub-Coulomb energies. Both nuclei of the samarium p-process chronometer, 146 Sm and 144 Sm, are produced in the γ process, and their initial abundance ratio is very sensitive to the (γ , n) and (γ , α) branching ratio on 148 Gd. The 148 Gd(γ , α) 144 Sm reaction rate was measured roughly 20 years ago by means of the activation technique and its surprising results triggered adjustments to the global low-energy α+nucleus optical-model potentials (OMPs). Purpose: We want to take advantage of modern α-particle spectroscopy techniques in order to constrain the controversial previous results on the 148 Gd(γ , α) 144 Sm reaction rate. Method: The 148 Gd(γ , α) 144 Sm reaction rate has been determined by measuring the cross section of the reverse reaction 144 Sm(α, γ) 148 Gd, applying the activation technique to the α decay of 148 Gd. Targets have been irradiated at the cyclotron of the Physikalisch-Technische Bundesanstalt in Braunschweig, Germany. The α-particle spectroscopy has been carried out with a state-of-the-art low-background ionization chamber of the Technische Universität Dresden, Germany. Results: Cross sections for the 144 Sm(α, γ) 148 Gd reaction have been measured between 10.66 and 12.66 MeV with much higher precision than in the previous measurement. The results agree with earlier results within their uncertainties. The statistical-model analysis has been carried out using the TALYS code on the basis of the latest parametrizations of α-OMPs. The best reproductions of the experimental results within the statistical model have been used to calculate the reaction rates. Conclusion: The values presented here suggest a steeper increase in the astrophysical S factor towards lower center-of-mass energies. Different parametrizations of the α-OMP were able to describe the experimental values sufficiently. Further measurements at energies below 11.0 MeV are suggested.

Cross section measurement of the 144Sm(alpha,n)147Gd reaction for studying the alpha-nucleus optical potential at astrophysical energies

“…The widely used simple 4-parameter AOMP by McFadden and Satchler [28] fits the new data at higher energies; however, at lower energies close above the threshold this AOMP clearly overestimates the experimental data. Such an overestimation towards lower energies has been found also for the 144 Sm(α,γ) 148 Gd reaction [8,9]. An explanation for the overestimation of low-energy cross sections was provided in [25] which is related to the tail of the imaginary potential at large radii above 10 fm (far beyond the colliding nuclei).…”

“…This was the first isotope on which (α, γ) cross section measurement has been carried out in relation to the γ-process and the problem with the AOMP has been identified [8]. A recent experiment has confirmed those results [9]. Elastic α-scattering experiment on 144 Sm has been carried out very recently at α-energies of 16, 18 and 20 MeV [10].…”

“…Any change in the level density affects whether the (α,n) cross section is dominated by low-lying excited states or by high-lying excited states, but does not affect the total (α,n) cross section (as measured in the activation experiment). In the case of the (α,γ) cross section, the contribution of high-lying states is increasing with energy and exceeds 50% around 9 MeV; i.e., all available experimental data points of [8,9] are governed by transitions to higher-lying excited states in 148 Gd. Nevertheless, a significant relevance of the level density appears only above the (α,n) threshold, see also Eq.…”

“…It is the aim of the present study to provide further constraints for the calculation of α-induced cross sections on 144 Sm. These constraints will be based on the new 144 Sm(α,n) 147 Gd data from this work which will be complemented by further information from experiments on 144 Sm(α,γ) 148 Gd capture [8,9] and 144 Sm(α,α) 144 Sm elastic scattering [10,22].…”

“…(α,X)LD shows the contribution of the higher-lying states above E * 0 for the two dominating (α,X) channels which are the (α,γ) and the (α,n) channel. The experimental data points represent total cross sections from (α,α) elastic scattering (black open diamonds, [10]), (α,n) cross sections from this work (blue full squares), and (α,γ) cross sections (red full circles [8] and red open triangles [9]). The arrows indicate the standard Gamow window for typical temperatures of T9 = 2 − 3.…”