Unexpected properties of the33S(α,p)36Clreaction cross section at low energies

Abstract

New experimental data for the 33 S(α,p) 36 Cl reaction show a very unusual energy dependence. Contrary to common findings for many other α-induced reactions, statistical model calculations underestimate the measured cross sections at very low energies. The relatively huge cross sections at these low energies require a significant amount of single-particle strength in the measured energy range which exceeds by far 100 % as soon as the additional strength from the competing 33 S(α,n) 36 Ar reaction is taken into account. In addition, the new data deviate from a general trend for the energy dependence of α-induced reaction cross sections.

Cross sections of α-induced reactions for targets with masses A ≈ 20–50 at low energies

“…This trend becomes nicely visible when so-called reduced cross sections σ red are plotted versus the reduced energy E red as suggested by [19]. However, very recently huge discrepancies have been found for the light nuclei 23 Na and 33 S where much larger values for σ red have been observed; a detailed dis-cussion of α-induced reactions of 33 S was provided recently in [20].…”

“…Finally, the total reaction cross section σ reac is determined from the sum of (α,p) and (α,n) cross sections. Because the ratio between the (α,n) and (α,p) cross sections is well constrained by theory [20], the uncertainties for the (α,n) cross section and the total reaction cross section σ reac remain acceptable.…”

“…At the highest energies the results for 33 S lie [102] for the 33 S(α,p) 36 Cl reaction. In addition, the 33 S(α,n) 36 Ar cross section has been estimated using a theoretical branching ratio from the StM [20], and the total reaction cross section is calculated from the sum of the (α,p) and (α,n) contributions. Further discussion see text and [20].…”

“…36; here the cross section is presented as a function of E α (i.e., in forward kinematics). The [102]; the (α,n) cross section was calculated from the experimental (α,p) cross section and theoretical ratios between the (α,n) and (α,p) cross section [20]. Further discussion see text.…”

“…But at somewhat higher energies around 6 MeV the (α,p) cross section becomes dominant because of the larger number of states in the exit channel towards the odd-odd nucleus 36 Cl. The 33 S(α,n) 36 Ar cross section was estimated in [20] from the experimental 33 S(α,p) 36 Cl cross section [102] and the theoretical ratio between the (α,n) and (α,p) cross section (for details see [20]). Finally, the total reaction cross section σ reac is determined from the sum of (α,p) and (α,n) cross sections.…”