Improved STEREO simulation with a new gamma ray spectrum of excited gadolinium isotopes using FIFRELIN

Abstract

The Stereo experiment measures the electron antineutrino spectrum emitted in a research reactor using the inverse beta decay reaction on H nuclei in a gadolinium loaded liquid scintillator. The detection is based on a signal coincidence of a prompt positron and a delayed neutron capture event. The simulated response of the neutron capture on gadolinium is crucial for comparison with data, in particular in case of the detection efficiency. Among all stable isotopes, 155 Gd and 157 Gd have the highest cross sections for thermal neutron capture. The excited nuclei after the neutron capture emit gamma rays with a total energy of about 8 MeV. The complex level schemes of 156 Gd and 158 Gd are a challenge for the modeling and prediction of the deexcitation spectrum, especially for compact detectors where gamma rays can escape the active volume. With a new description of the Gd(n, γ) cascades obtained using the Fifrelin code, the agreement between simulation and measurements with a neutron calibration source was significantly improved in the Stereo experiment. A database of ten millions of deexcitation cascades for each isotope were generated and are now available for the user. PACS. 23.20.Lv γ transitions and level energies -95.55.Vj Neutrino detectors -28.20.-v Neutron physics

Improved sterile neutrino constraints from the STEREO experiment with 179 days of reactor-on data

“…Figure 18 displays a neutron capture spectrum for an AmBe source, obtained after coincidence search and accidental subtraction. A good agreement between data samples and MC simulations have been reached due to the implementation of the FIFRELIN code [57], improving the description of the de-excitation cascades of the relevant Gd isotopes [58]. The distribution of coincidence times of neutron captures are represented for both data and MC simulations in Fig.…”

“…This approach allows to take into account nuclear structure uncertainties, and provides an accurate description of the energy and multiplicity distributions of the de-excitation products. An high-statistic sample of deexcitation products generated from FIFRELIN is used in the STEREO simulation on an event-by-event basis for each cascade following a neutron capture on the 155 Gd or 157 Gd isotopes [58,59].…”

Sterile Neutrino Constraints from the STEREO Experiment with 66 Days of Reactor-On Data

“…This approach allows to take into account nuclear structure uncertainties, and provides an accurate description of the energy and multiplicity distributions of the de-excitation products. An high-statistic sample of de-excitation products generated from FIFRELIN is used in the Stereo simulation on an event-by-event basis for each cascade following a neutron capture on the 155 Gd or 157 Gd isotopes [46,47].…”

Latest results of the STEREO sterile neutrino search at the ILL Grenoble

“…2 shows the improvement with respect to the previously used GLG4sim package for the neutron capture spectrum. Further details can be found in [3].…”