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

Abstract

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.

Popular Abstract

Scientists at the European Spallation Source (ESS) in Sweden are designing a new research facility. The ESS will provide the highest number of neutrons produced per second for various experiments in the World. The neutrons produced at the ESS will enter a moderator, where they are slowed down before they reach beamports to beamlines. The beamlines deliver neutrons to experiments and modify their properties. The focus of my master’s project was on delivering neutrons from the moderator into the High-Intensity Baryon Extraction and Measurement (HIBEAM) experiment by creating a computer geometric model of the proposed ANNI beamline. An integral part of the project was also to conduct simulations using the computer geometric model to estimate how the ANNI beamline was efficient to block high-energy (fast) neutrons before they reach HIBEAM. In addition, the effectiveness of the proposed shielding against fast neutrons for the ANNI beamline was tested in the simulations to keep the level of radiation safe for the ESS personnel. All of these studies of the ANNI beamline are important in order to achieve the experimental goals of HIBEAM.

Specifically, HIBEAM will study rare oscillations of the neutrons into sterile neutrons which do not interact with electromagnetic, weak, or strong force. These forces are fundamental forces that we use to describe particles and their interactions around us. The sterile neutrons may come from a hidden world of particles. Therefore, they are assumed to be one of the dark matter candidates. The dark matter corresponds to a significant fraction of particles present in the Universe whose origin is unknown. These particles should interact with the ordinary world of particles via gravity to explain the observations, for example, of galaxies and clusters of galaxies that feel the additional gravitational force from an unknown source to keep their form. Although no experimental evidence exists, the sterile neutrons can be part of the mirror world of particles. These would be duplicates of the ordinary world neutrons that we know. However, the interaction between the ordinary and mirror world neutrons would be possible only via gravity. If the hypothesis about the existence of the mirror world of particles is correct, it would have implications on, for example, observed ultra-high-energy cosmic rays hitting the Earth atmosphere.

In addition, HIBEAM may also observe oscillations of neutrons into antineutrons. This is unlikely, but such a process can help to explain another important question of modern cosmology: why the ratio of matter over antimatter became asymmetric in the early times of the Universe such that most of the antimatter disappeared from the Universe. The NNBAR experiment, which will be constructed based on the experience gained in the HIBEAM experiment, should observe the oscillations of neutrons into antineutrons due to a significant increase of the experimental sensitivity in comparison to the similar experiment already conducted at the Institut Laue-Langevin (ILL) in France.

• author: Rataj, Blahoslav
• supervisor: Valentina Santoro; David Silvermyr
• organization: Particle and nuclear physics; Department of Physics
• course: FYSM30 20211
• year: 2021
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• language: English
• id: 9062042
• date added to LUP: 2021-07-26 16:04:17
• date last changed: 2021-07-26 16:04:17