Direct Determination of Neutron Capture Relevant to the <i>s</i> -process of ¹²⁷ I and ¹³³ Cs at the CSNS Back-n Facility

Abstract The cross sections of 127 I( n , γ ) 128 I and 133 Cs( n , γ ) 134 Cs play a critical role in the s -process of nuclear astrophysics, the iodine–xenon chronometer on early meteorite evolution, and understanding the performance of CsI crystal detectors for Dark Matter Detection Experiment and Coherent Elastic Neutrino-Nucleus Scattering (CE ν NS) experiments. The neutron capture cross sections of 127 I and 133 Cs were measured from 1 eV to 1 MeV using the time-of-flight method at China Spallation Neutron Source Back-n facility. The neutron capture yield was derived via the pulse-height weighting technique based on the total energy detection principle of C 6 D 6 detectors. In the resolved resonance region (RRR), some new resonance peaks of 133 Cs( n , γ ) were observed for the first time. Resonance parameters for the neutron capture cross section were extracted using the SAMMY code from 1 eV to 4 keV, while averaged capture cross sections in the unresolved resonance region (URR) from 4 keV to 1 MeV were analyzed with TALYS. Results were compared with evaluated libraries and previous work in both RRR and URR. Astrophysical Maxwellian-averaged cross sections (MACS) were calculated for kT = 5–100 keV. At kT = 30 keV, the MACS of both 127 I( n , γ ) 128 I and 133 Cs( n , γ ) 134 Cs were slightly higher than those of the KADoNiS v0.3 recommended value. And the reaction rates were derived over the astrophysically relevant temperature range of s -process nucleosynthesis models. The uncertainties of present reaction rates are significantly reduced.