First experimental results of the 33S(α,p)36Cl cross section for production in the Early Solar System

Short-lived radionuclides (SLRs), with t1/2 < ∼5 Ma, were extant in the Early Solar System (ESS). Their relatively short lifetimes and different chemical behavior make them useful for dating processes in the ESS. The existence of the SLR 36Cl (t1/2 = 3.01 × 105 a) has been determined from correlation between isotopic enrichment of its daughter isotope, 36S, and Cl abundance in meteorites. The relatively high inferred initial Solar System 36Cl/Cl ratio strongly suggests that 36Cl was produced in the ESS by bombardment of solar energetic particles on gas and dust in the protoplanetary disc. A major uncertainty in the irradiation models is the absence of experimental data. Experimental data on key 36Cl production reactions will help constrain the calculated yields. Of the reactions considered in the ESS models, the 33S(α,p)36Cl reaction is one of the most important. Here we report first results of a measurement of the averaged reaction cross section in the energy range 1.84-2.04 MeV/A. Our result, 192 ± 33 mb (1σ), is significantly higher than results of previous calculations, 102 and 34 mb.