The effects of I-127 on a high-granularity LiI(Eu) Bonner Sphere Spectrometer response matrix

The measurement of neutron energy spectra using a Bonner Sphere Spectrometer (BSS) requires spectrum unfolding. One necessary component of spectrum unfolding is an accurate neutron response matrix, which describes the energy shift experienced by neutrons when passing through different moderation configurations prior to detection. Leveraging the recently-released ENDF/B-VIII.0 cross-section library, this work details the development of a new LiI(Eu) scintillator response matrix that features improved energy binning granularity as well as increased isotopic and statistical accuracy. Using MCNP6.2, detector responses were modeled at 105 discrete log-equi-spaced incident neutron energies from 1.000 × 10 −9 to 2.512 × 10 1 MeV with a maximum relative error of 1.0%. Comparison with corresponding data from previous work reflects an average variation within 0.025%, reinforcing confidence in the expanded measurements. The impact of 127 I within the scintillator crystal and resulting structure within these novel response functions is also explored, along with previously unexplained variations between prior published works. • Novel, more accurate response function for LiI(Eu) BSSs featuring more precise detector modeling, recently released ENDF-VIII cross-section data, and decreased relative error. • Increased energy bin granularity reveals 127 I cross-section impacts; test case demonstrates resulting improved unfolded solutions. • Data provided in tabular form as a community resource for experimental data interpretation of Ludlum 42-5 BSS measurements.