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Design of an experiment to search for neutron oscillations at the European Spallation Source

Persson, Mats Bror Linus LU (2024) FYSM63 20241
Department of Physics
Particle and nuclear physics
Abstract
This work is aimed at presenting a design for the proposed HIBEAM beamline which seeks to perform neutron oscillation measurements at the European Spallation Source (ESS). The design is informed by beamline simulations performed with PHITS 3.33 and detector simulations performed with both PHITS and Geant4 11.2. The shielding, consisting of 45 cm of steel and 85 cm of heavy concrete at its thickest point, is designed to satisfy the ESS dose rate requirement of 1.5 μSv/h. This work has also demonstrated the applicability of multivariate kernel density estimation for neutron beamline simulations, showing better reproduction of the neutron energy spectrum in the beamline than the source term methodology previously employed at ESS. This work... (More)
This work is aimed at presenting a design for the proposed HIBEAM beamline which seeks to perform neutron oscillation measurements at the European Spallation Source (ESS). The design is informed by beamline simulations performed with PHITS 3.33 and detector simulations performed with both PHITS and Geant4 11.2. The shielding, consisting of 45 cm of steel and 85 cm of heavy concrete at its thickest point, is designed to satisfy the ESS dose rate requirement of 1.5 μSv/h. This work has also demonstrated the applicability of multivariate kernel density estimation for neutron beamline simulations, showing better reproduction of the neutron energy spectrum in the beamline than the source term methodology previously employed at ESS. This work further provides estimates of the spallation background for the HIBEAM detector systems. It illustrates how the addition of LiF cladding to the vacuum pipe reduces the electron background in the time projection chamber by two orders of magnitude, making the background levels manageable for the experiment to proceed. (Less)
Popular Abstract (Swedish)
Vanlig materia består huvudsakligen av protoner, neutroner och elektroner. Genom att kollidera partiklar med varandra vid höga energier kan man även skapa antiprotoner, antineutroner och positroner. Dessa så kallade antipartiklar har samma egenskaper som sina vanliga motsvarigheter fast med motsatt elektrisk laddning. Eftersom universum behandlar partiklar och antipartiklar på ett liknande sätt kan man fråga sig varför det finns mer materia än antimateria i universum. I skrivande stund har vetenskapen inget entydigt svar på denna fråga. Av någon anledning har universum utvecklat en preferens för materia framför antimateria, ett fynd som inte kan förklaras av den nuvarande formuleringen av standardmodellen för partikelfysik.

... (More)
Vanlig materia består huvudsakligen av protoner, neutroner och elektroner. Genom att kollidera partiklar med varandra vid höga energier kan man även skapa antiprotoner, antineutroner och positroner. Dessa så kallade antipartiklar har samma egenskaper som sina vanliga motsvarigheter fast med motsatt elektrisk laddning. Eftersom universum behandlar partiklar och antipartiklar på ett liknande sätt kan man fråga sig varför det finns mer materia än antimateria i universum. I skrivande stund har vetenskapen inget entydigt svar på denna fråga. Av någon anledning har universum utvecklat en preferens för materia framför antimateria, ett fynd som inte kan förklaras av den nuvarande formuleringen av standardmodellen för partikelfysik.

HIBEAM-experimentet kan vara en viktig pusselbit i strävan att lösa detta mysterium. Experimentet kommer att söka efter spontana oscillationer mellan neutroner och antineutroner. Det kommer också att leta efter oscillationer till sterila neutroner, en ännu oupptäckt partikel som, om den finns, interagerar mycket svagt med vanlig materia. Oscillationsprocessen lär vara särskilt trolig för fria neutroner som inte är bundna i atomkärnor. För att maximera sannolikheten för oscillationer behöver vi därför studera ett stort antal fritt flygande neutroner. Lyckligtvis kommer European Spallation Source (ESS), belägen i Lund, att vara en av de mest kraftfulla neutronkällorna i världen. Genom att studera neutronerna när de rör sig i ett långt vakuumrör och placera neutron- och/eller antineutrondetektorer i änden av röret går det att avgöra om en oscillation har skett.

I detta examensarbete har en geometrisk modell av ett nytt ESS-strålrör utvecklats. Modellen har använts för att simulera stråldosen som genereras av neutronerna och fotonerna i strålröret. Detta är nödvändigt för att avgöra hur mycket strålskydd som behövs för att följa relevanta hälso- och säkerhetsföreskrifter. Simuleringarna genomförs med hjälp av Monte Carlo-simuleringsprogram, där slumptal kombineras med fysiska modeller och experimentella reaktionssannolikheter för att bestämma partiklarnas öde när de fortplantar sig i modellen.

Det är av yttersta vikt att de sällsynta neutronoscillationerna kan urskiljas av detektorn med hög effektivitet. Vi måste därför ha en god förståelse för bakgrundsstrålningen som når detektorn. I detta arbete har en detaljerad modell av detektorn utvecklats och bakgrunden som härstammar från neutronkällan har uppskattats. Dessutom visas hur tillägget av ett starkt neutronabsorberande material, LiF, inuti vakuumröret kan avsevärt minska bakgrunden i detektorn. (Less)
Popular Abstract
Ordinary matter mainly consists of protons, neutrons and electrons. By colliding particles with each other at high energies, you can also create antiprotons, antineutrons and positrons. These so-called antiparticles have the same properties as their normal counterparts but with the opposite electrical charge. Since the universe treats particles and antiparticles in a similar way, one might ask why there is more matter than antimatter in the Universe. At the time of writing, science has no clear answer to this question. For some reason, the Universe has developed a preference for matter over antimatter, a finding that cannot be explained by the current formulation of the Standard Model of particle physics.

The HIBEAM experiment may be an... (More)
Ordinary matter mainly consists of protons, neutrons and electrons. By colliding particles with each other at high energies, you can also create antiprotons, antineutrons and positrons. These so-called antiparticles have the same properties as their normal counterparts but with the opposite electrical charge. Since the universe treats particles and antiparticles in a similar way, one might ask why there is more matter than antimatter in the Universe. At the time of writing, science has no clear answer to this question. For some reason, the Universe has developed a preference for matter over antimatter, a finding that cannot be explained by the current formulation of the Standard Model of particle physics.

The HIBEAM experiment may be an important puzzle piece in the quest to solve this mystery. The experiment will search for spontaneous oscillations between neutrons and antineutrons. It will also search for oscillations into sterile neutrons, a yet undiscovered particle which, if it exists, interacts very weakly with ordinary matter. The neutron oscillation process is thought to be particularly likely for free neutrons that are not bound in nuclei. To maximise the probability of transitions, we therefore need to study a large number of free-flying neutrons. Fortunately, the European Spallation Source (ESS), situated in Lund, Sweden, will be one of the most powerful neutron sources in the world. By studying the neutrons as they propagate in a long vacuum pipe and placing neutron and/or antineutron detectors at the end of the pipe, it is possible to determine whether oscillation has taken place.

Within this thesis, a geometrical model of a new ESS beamline has been developed. The model has been used to simulate the radiation dose generated by the neutrons and photons in the beamline. This is necessary to determine how much shielding is needed to abide by relevant health and safety regulations. The simulations are performed using Monte Carlo simulation programs, where random numbers are combined with physical models and experimentally evaluated reaction probabilities to determine the fate of the particles as they traverse the model.

It is of utmost importance that the rare neutron oscillations can be distinguished by the detector with high efficiency. We must therefore have a good understanding of the background radiation that reaches the detector. In this thesis, a detailed model of the detector has been developed and the background rates in the detector stemming from the ESS neutron source have been estimated. In addition, it is shown how the addition of a strongly neutron-absorbing material, LiF, inside the vacuum pipe can significantly reduce the background in the detector system. (Less)
Please use this url to cite or link to this publication:
author
Persson, Mats Bror Linus LU
supervisor
organization
course
FYSM63 20241
year
type
H2 - Master's Degree (Two Years)
subject
keywords
ESS, European Spallation Source, PHITS, Geant4, CombLayer, beamline, neutron oscillation, sterile neutron, simulation, radiation transport, HIBEAM, kernel density estimation, annihilation, spallation, background, baryon number violation, Sakharov conditions, beam stop, dose rate
language
English
id
9165108
date added to LUP
2024-06-19 08:48:43
date last changed
2024-06-19 08:53:00
@misc{9165108,
  abstract     = {{This work is aimed at presenting a design for the proposed HIBEAM beamline which seeks to perform neutron oscillation measurements at the European Spallation Source (ESS). The design is informed by beamline simulations performed with PHITS 3.33 and detector simulations performed with both PHITS and Geant4 11.2. The shielding, consisting of 45 cm of steel and 85 cm of heavy concrete at its thickest point, is designed to satisfy the ESS dose rate requirement of 1.5 μSv/h. This work has also demonstrated the applicability of multivariate kernel density estimation for neutron beamline simulations, showing better reproduction of the neutron energy spectrum in the beamline than the source term methodology previously employed at ESS. This work further provides estimates of the spallation background for the HIBEAM detector systems. It illustrates how the addition of LiF cladding to the vacuum pipe reduces the electron background in the time projection chamber by two orders of magnitude, making the background levels manageable for the experiment to proceed.}},
  author       = {{Persson, Mats Bror Linus}},
  language     = {{eng}},
  note         = {{Student Paper}},
  title        = {{Design of an experiment to search for neutron oscillations at the European Spallation Source}},
  year         = {{2024}},
}