β-delayed neutrons from ¹³⁵In at IDS with NEXT

We performed β-delayed neutron spectroscopy of ¹³⁵In at the ISOLDE Decay Station (IDS), measuring neutrons emitted from neutron-unbound states in ¹³⁵Sn. The experiment was done using 40 position-sensitive plastic-scintillator detectors (4×8 segmented, multi-anode PMTs with Anger-logic) of the Neutron dEtector with multi-neutron (Xn) Tracking (NEXT) array, together with three double-layer β-trigger detectors. This setup allows for efficient β tagging with reduced γ-induced background. Using β–n and β–n–γ coincidences, we isolated the β1n and β2n branches in ¹³⁵In→¹³⁵Sn(→¹³⁴Sn), determined P_1n and P_2n, and obtained neutron energy spectra with various neutron multiplicities. The results reveal β feeding to neutron-unbound states in ¹³⁵Sn above S_n and S_2n, which was then used to benchmark the Hauser–Feshbach model and the nuclear shell model beyond ¹³²Sn, providing valuable inputs for r-process calculations around A≈130.