Speaker
Description
Beta-delayed neutron emission probabilities (P₁ₙ) of neutron-rich fission products are key inputs to r-process nucleosynthesis calculations, determining the detour probabilities in β-decay chains back to stability and the quantity of neutrons recaptured during freeze-out in the A ≈ 130 peak region. Despite their importance, experimental data remain sparse and, in several cases, inconsistent between independent measurements. For ¹³⁵Sb, a tension of approximately 2.3σ exists between the only modern precision measurement (Wang et al., PRC 101, 025806 (2020): P₁ₙ = 14.6 ± 1.3%) and the evaluated literature value (~22 ± 3%), motivating an independent determination with a complementary technique.
We report the first test of a P₁ₙ measurement at the Fragment Separator Ion Catcher (FRS-IC) at GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt, performed in June–July 2025. The method, originally demonstrated for α-decay and isomeric transition by Miskun et al. (EPJA 55, 148 (2019)), requires no neutron detection. Instead, ¹³⁵Sb ions produced via ²³⁸U in-flight Coulomb fission are stopped in a Cryogenic Stopping Cell (CSC), stored at the extraction nozzle for a controlled time of 2 seconds, and the resulting decay daughters (¹³⁵Te from β-decay, ¹³⁴Te from β-delayed neutron emission) are identified and counted by their masses using a Multiple-Reflection Time-of-Flight Mass Spectrometer (MR-TOF-MS) with resolving power R ≈ 590,000 FWHM. P₁ₙ is extracted from the ratio of daughter yields, with orthogonal systematic uncertainties to existing methods.
Key results include: (i) successful commissioning of the nozzle storage technique for β-delayed neutron emitters; (ii) experimental confirmation that the ²⁵²Cf spontaneous fission source background is independent of nozzle storage time, validating the background subtraction strategy. The technique is particularly well-suited for future multi-neutron emission probability (P₂ₙ) measurements at the Super-FRS during FAIR Early Science.