Background Studies of the High-Intensity Baryon Extraction and Measurement (HIBEAM) Experiment at the European Spallation Source

Sammanfattning: The High-Intensity Baryon Extraction and Measurement experiment (HIBEAM) is proposed as one of the experimental stations at the European Spallation Source (ESS). This experiment will mainly focus on the neutron into sterile neutron oscillations which violate the baryon number (B) by one unit. The sterile neutrons are hypothetical dark matter candidates. The neutrons suitable for the experiment have energy lower than approximately 1 eV and are transported from the moderator of the ESS into HIBEAM by the ANNI (a pulsed cold neutron beam facility) beamline. The goal of this thesis was to estimate the neutron background and radiation dose rate for the ANNI beamline using simulations performed with the Particle and Heavy Ion Transport code System (PHITS). This study is supported by the need to reach a high sensitivity of the neutron into sterile neutron oscillation experiments. The sensitivity of such experiments strongly depends on the magnitude of the background. The detailed model of the ANNI beamline was implemented and integrated into the ESS target model. The neutron spectrum at 22 m from the center of the ESS target monolith was calculated with a two-step calculation process in order to transport the neutrons through such a long guiding system that extends from 2 m relative to the ESS target monolith. It was found that the flux of high-energy (fast) neutrons that are the source of background significantly reduced between the ANNI opening and the entrance to the HIBEAM experimental area at 22 m relative to the target monolith. Nevertheless, the design of the neutron guide system of ANNI could be improved because a significant fraction of the fast neutrons were not stopped inside the bunker wall. Consequently, the fast neutrons contributed to observed large dose levels outside the bunker wall edge located at a distance of 15 m from the target monolith. Different thicknesses of the shielding material outside the bunker wall edge were tested, but even 70 cm thick block of high-density concrete was not capable of stopping fast neutron below the acceptable value of the effective dose of 1.5 μSv/h. This project can be used as a ground for the optimization of the geometric model of the ANNI beamline. The results of this project can also support the future ANNI beamline proposal.