PHITS simulations for CEvNS experiments at ESS

Abstract: Recently the interest for coherent elastic neutrino-nucleus scattering experiments has increased since it is believed that through these experiments, a wide range of new physics, e.g., sterile neutrinos, could be discovered. This experiment is ideally performed at spallation sources and in this project we look into an experiment, which is planned at the European Spallation Source. However, the measurement is sensitive to background and therefore we specifically seek to describe the neutron background in the area of the room, which the experiment is planned to take place in. This is done through a tool called PHITS (Particle and Heavy Ion Transport code System), which is used to simulate the neutrons generated from the tungsten target. In this work, we expand an already existing PHITS geometry to include the location of the experimental room and also study the properties of neutrons that leak from the target. The energy spectra and the momenta of the leaking neutrons were extracted together with the relative amount of neutrons that would directly irradiate the room from the target. Furthermore, it was found that additional work to improve the used variance reduction technique is needed in order to sufficiently populate the experimental area in the simulation.