First direct measurement of the unexpectedly large neutron capture cross-section of 88Zr at CERN's n_ToF facility EAR2.*

In 2019, the isotope zirconium-88 (88Zr) was discovered by Shusterman et al at Lawrence Livermore National Labs to have the second largest neutron cross-section, second only to 135Xe. Additionally, a preprint by the DICER collaboration at Los Alamos National Laboratories (LANL) measured the 88Zr total cross-section from 0.0253 eV to 500 eV, confirming the existence of a massive thermal cross-section but producing a factor 200 discrepancy in the 2019 reported resonance integral. We report on our group's attempt to make the first ever direct measurement of the neutron capture cross-section of 88Zr at CERN's Neutron Time-of-Flight (n_ToF) facility. Due to 88Zr's short half-life of 83 days and the required mass for such a measurement equivalent to about 1e16 88Zr atoms, the sample production and the experimental setup are customized. The 30mCi sample, produced and separated by LANL and deposited at the Paul-Scherrer Institute (PSI), is sealed between two kapton foils. At n_ToF, a thin lead ring sufficiently reduces the intensity of the 393 keV gamma rays from the 88Zr decay seen by an array of nine gamma ray detectors allowing the detectors to be placed just 4.5cm from the sample center, improving the detection efficiency for the (n,g) reaction. During the measurement, the continuous buildup of 88Y necessitates and at the same time enables permanent gain monitoring to correct for potential gain drifts induced by the high activity of the sample. This ensures accurate detection and analysis to confirm the shape of the large resonance measured at DICER and potentially the integral value.