Study of Cold Neutron Beam Intensity Stability with Low-Efficiency N₂ Monitor at HFIR Reactor in ORNL

This study is being conducted to observe the behavior of the cold neutron beam over days-long experiments. The High Flux Isotope Reactor (HFIR) at Oak Ridge National Lab (ORNL) is used to observe the behavior of neutrons in varying states. In a four-day long experiment performed in January 2024 at the HFIR reactor, the transformation of neutrons to sterile mirror neutrons was searched with the cold-neutron beam of the General Purpose – Small Angle Neutron Scattering (GP-SANS) instrument. The intensity of the beam was monitored by the ORDELA 4511N Low Efficiency N2 monitor. The monitor shows a small decline over time due to the burning of fuel in the reactor. However, the stability is better shown through the root mean square error values. From observing the intensity over different runs from the 4-day experiment, we determined the accuracy of the count rates received by the monitors and applied the correction factor for deadtime. The average RMSE value for each run was approximately 1.52 counts/second for the linear fit showing a steady stability for each run of the experiment. This refines the knowledge we currently have about the GP-SANS monitor as well as contributes to our efforts to find the deadtime correction factor of the monitor. The stability results provide data for the experiment for a more accurate value of the monitor.