Reiner Kruecken

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Research Classification

Research Interests

Elementary Particles
Neutrino Physics
Nuclear Astrophysics
Nuclear Physics
Universe Structure

Relevant Degree Programs


Research Methodology

TRIUMF Accelerator Facilities

Graduate Student Supervision

Doctoral Student Supervision

Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.

A linear Paul trap for barium tagging of neutrinoless double beta decay in nEXO (2021)

nEXO is the next-generation Enriched Xenon Observatory searching for neutrinoless double beta decay (0νββ) in ¹³⁶Xe. If observed, 0νββ will validate the neutrino to be its own anti-particle and determine the absolute mass scale of the neutrinos. nEXO's sensitivity is limited by the background level. Barium tagging is the ultimate background rejection method using the coincidence detection of ¹³⁶Ba as the daughter nucleus. A linear Paul trap (LPT) is needed for the barium tagging concept in nEXO or a future gaseous experiment. The theory of an ideal LPT was studied from first principles to obtain analytical solutions of the trapped ions and to validate a simulation method. Then simulations were done to optimize the design of a realistic final LPT. Meanwhile, prototypes of key components of the LPT were built for the experimental developments. A prototype of the LPT's quadrupole mass filter (QMF) achieved mass resolving power m/Δm around 140 and exceeded its requirement. A 3D printed prototype of the novel ion cooler demonstrated successful ion cooling, trapping and ejection. Based on the progress with the prototypes, improvements were made to the design of the final setup. The final LPT will be installed between an RF funnel and a high precision mass spectrometer for barium tagging of nEXO.

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Development of a single vacuum ultra-violet photon-sensing solution for nEXO (2021)

Silicon PhotoMultiplier (SiPM) technology represents an unprecedented attempt to create an ideal solid-state photon detector, combining the low-light detection capabilities of the previous device generations with all the benefits of a solid-state sensor. For this reason, large-scale low-background cryogenic experiments, such as the next-generation Enriched Xenon Observatory experiment (nEXO), are migrating to a SiPM-based light detection system. nEXO aims to probe the boundaries of the standard model of particle physics by searching for neutrino-less double beta decay of ¹³⁶Xe. The nEXO experiment follows the same detection concept as the EXO-200 experiment, but uses 5 tonnes of liquid xenon inside a vacuum cryostat that is expected to be located at SNOLAB, the Canadian underground science laboratory. Decays in the xenon produce both light and ionization and it is important to measure both to achieve sufficient energy resolution and thus background rejection. In particular, electrons from the ionization drift in an applied electric field toward anode pads where they are measured. The light flash is simultaneously detected by an array of SiPMs. The technical goal of the proposed thesis is to study different SiPMs characteristics in order to choose the best SiPM technology for the nEXO experiment. This thesis will also introduce new mathematical models to better understand Geiger mode properties of SiPMs in order to optimize them for the next generations of double beta decay and dark matter experiments.

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Nuclear structure corrections in muonic atoms with statistical uncertainty quantification (2020)

The discovery of the proton and deuteron radius puzzles from Lamb shift measurements of muonic atoms has initiated experimental efforts to probe heavier muonic systems and casts doubt on earlier analysis based on ordinary atoms. For muonic atoms, the large muon mass results in a Bohr radius about 200 times smaller with respect to their electronic counterparts, making them sensitive to nuclear structure effects. These effects dominate the uncertainty budget of the experimental analysis and diminish the attainable accuracy of charge radii determinations from Lamb shift spectroscopy. This dissertation investigates the precision of nuclear structure corrections relevant to the Lamb and hyperfine splitting in muonic deuterium to support ongoingexperiments and shed light on the puzzles. Using state-of-the-art nuclear models, multivariate regression analysis and Bayesian techniques, we estimate the contribution of all relevant uncertainties for nuclear structure corrections in muonic deuterium and demonstrate that nuclear theory errors are well constrained and do not account for the deuteron radius puzzle. This uncertainty analysis was carried out using the “η-expansion” method that has also been applied to A ≥ 2 nuclei. This method relies on the expansion of a dimensionless parameter η, with η
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Decay spectroscopy of neutron-rich cadmium around the N = 82 shell closure (2019)

The neutron-rich cadmium isotopes around the well-known magic numbers at Z=50 and N=82 are prime candidates to study the evolving shell structure observed in exotic nuclei. Additionally, the extra binding energy observed around the nearby doubly-magic ¹³²Sn has direct implications for in astrophysical models, leading to the second r-process abundance peak at A≈130 and the corresponding waiting-point nuclei around N=82. The β-decay of the N=82 isotope ¹³⁰Cd into ¹³⁰In was investigated in 2002, but the information for states of the lighter indium isotope ¹²⁸In is still limited. Detailed beta-gamma-spectroscopy of ¹²⁸,¹³¹,¹³²Cd was accomplished using the GRIFFIN facility at TRIUMF. In ¹²⁸In, 32 new transitions and 11 new states have been observed in addition to the four previously observed excited states. The ¹²⁸Cd half-life has also been remeasured via the time distribution of the strongest gamma-rays in the decay scheme with a higher precision. For the decay of ¹³¹,¹³²Cd, results are compared with the recent EURICA data. These new results are compared with recent shell model and IMSRG calculations, highlighting the necessity to re-investigate even "well-known" decay schemes for missing transitions.

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Single particle structure of exotic strontium isotopes (2019)

The sudden onset of ground state deformation and the emergence of shape-coexisting states in the vicinity of N~60 and Z~40 has been a subject of substantial interest for many years. It has been shown that the emergence of deformed low-energy configurations can be explained in the shell model by the evolution of single particle structure and the interaction between protons and neutrons in certain valence orbitals. However, the numerous theoretical models that have been developed for this transitional region are limited by the experimental data that is available. In particular, a description of the underlying single particle configurations of low energy states is essential for a detailed description of this region. In this work, the single particle structure of states in ⁹⁵Sr and ⁹⁶Sr has been investigated through the one-neutron transfer reactions ⁹⁵ ⁹⁶Sr(d,p) in inverse kinematics at TRIUMF. In each of these experiments, a 5.5 MeV/u Sr beam was impinged on a 5.0 mg/cm² CD₂ target, and emitted particles and γ-rays were detected using the SHARC and TIGRESS detector arrays, respectively. Using an angular distribution analysis, firm spin assignments have been made for the first time of the low-lying 352 keV, 556 keV and 681 keV excited states in ⁹⁵Sr from ⁹⁴Sr(d,p), and a constraint has been made on the spin of the higher-lying 1666 keV excited state in ⁹⁵Sr. Similarly, angular distributions have been extracted for 14 states in 96Sr from ⁹⁵Sr(d,p), and new experimental constraints have been assigned to the spins and parities of 8 states in ⁹⁶Sr. Additionally, two new states in ⁹⁶Sr have been identified in this work. A measurement of the mixing strength between the 1229 keV and 1465 keV shape-coexisting states in ⁹⁶Sr was also made, which was found to be a²=0.48(17).

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Electromagnetic properties of medium-mass nuclei from coupled-cluster theory (2018)

Electromagnetic probes represent a fundamental tool to study nuclear structure and dynamics. The perturbative nature of the electromagnetic interaction allows for a clean connection between calculated nuclear structure properties and measured cross sections. Ab initio methods have long represented the gold standard for calculations of nuclear structure observables in light nuclei. Thanks to recent developments in the scientific community, ab initio calculations have finally reached the medium- and heavy-mass region of the nuclear chart. However, the challenges modern nuclear structure calculations face are multiple, ranging from the construction of nuclear forces from chiral effective field theory (χEFT) and the solution of the highly correlated quantum many-body problem, to a quantitative description of observables with solid treatment of uncertainties.The work presented in this thesis aims to contribute addressing some of these challenges, using the ab initio coupled-cluster (CC) theory formulation of the Lorentz integral transform (LIT) method. We combine the CC and LIT methods for the computation of electromagnetic inelastic reactions into the continuum. We show that the bound-state-like equation characterizing the LIT method can be reformulated based on extensions of the coupled-cluster equation-of-motion (EOM) method, and we discuss strategies for viable numerical solutions. We then focus on the calculation of the electric dipole polarizability (α_D), which quantifies the low-energy behaviour of the dipole strength and is related to critical observables such as the radii of the proton and neutron distributions. Using a variety of chiral interactions, and singles and doubles excitations, we study ⁴He, ¹⁶ ²²O and ⁴⁰ ⁴⁸Ca. Exploiting correlations between α_D and the charge radius, we predict the neutron-skin radius and the polarizability for the double-magic ⁴⁸Ca, the latter recently measured by the Osaka-Darmstadt collaboration. Finally, we study the impact of triples excitations on the dipole strength in ⁴He and ¹⁶O.

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Decay spectroscopy of N ~ Z nuclei in the vicinity of 100Sn (2017)

The nuclear shell model (SM) has been very successful in describing the properties and the structure of near-stable and stable isotopes near the magic nuclei. Today, the advent of powerful facilities capable of producing radioactive isotopes far from stability has enabled the test of the SM on very proton-rich or neutron-rich magic nuclei. 100/50Sn50 is a proton-rich doubly-magic nucleus, but is nearly unstable against proton emission. Key topics of nuclear structure in this region include the location of the proton dripline, the effect of proton-neutron interactions in N ~ Z nuclei, single-particle energies of orbitals above and below the N = Z = 50 shell gaps, and the properties of the superallowed Gamow-Teller decay of ¹⁰⁰Sn. A decay spectroscopy experiment was performed on ¹⁰⁰Sn and nuclei in its vicinity at the RIKEN Nishina Center in June 2013. The isotopes of interest were produced from fragmentation reactions of 124/54Xe on a 9/4Be target, and were separated and identified on an event-by-event basis. Decay spectroscopy was performed by implanting the radioactive isotopes in the Si detector array (WAS3ABi) and observing their subsequent decay radiations. β⁺ particles and protons were detected by WAS3ABi, and γ rays were detected by a Ge detector array (EURICA). Of the proton-rich isotopes produced in this experiment, over 20 isotopes as light as ⁸⁸Zr and as heavy as ¹⁰¹Sn were individually studied. New and improved measurements of isotope/isomer half-lives, β-decay endpoint energies, β-delayed proton emission branching ratios, and γ-ray transitions were analyzed. In general the new results were well reproduced by the SM, highlighting a relatively robust ¹⁰⁰Sn core. However, the level scheme of ¹⁰⁰Sn's β-decay daughter nucleus ¹⁰⁰In was not conclusively determined because of several missing observations which were expected from various SM predictions. Significantly higher β-decay and γ-ray statistics are required on several nuclei, including ¹⁰⁰Sn, to evaluate the limit of the current understanding of their structure.

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Master's Student Supervision

Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.

The Light only Liquid Xenon Experiment: signal production, data acquisition and commissioning (2020)

The Light only Liquid Xenon (LoLX) experiment is a small detector designed to investigate both scintillation and Cherenkov light emission in liquid xenon, validate photon transport in simulations and study Silicon Photo-Multiplier (SiPM) response. A new analytic framework for describing temporal scintillation signatures is presented, and the relationship between xenon’s refractive index and temperature is derived from literature. The energy deposition time is also calculated for relativistic alpha and beta particles, as it pertains to future phases of LoLX which will aim to measure the rise time of the scintillation signal. The characterization of the LoLX electronics shows single photon resolution and the system linearity was reported for prompt light pulses up to 200 photons. A framework is presented for describing external cross-talk (eXT), where a charge avalanche in one SiPM generates photons which trigger another device. Finally preliminary data from a gaseous nitrogen cooldown of LoLX is analyzed, which shows evidence for fluorescence in nitrogen and eXT, although no definitive conclusions are drawn.

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Thermal conductivity in ISOL target materials: development of a numerical approach and an experimental apparatus (2020)

The method of Isotope Separation On-Line (ISOL) is one of the most successful ways to produce rare nuclei. Hitting a target material with accelerated particles generates heat and reaction products which then diffuse and effuse through the target material before they are released for ionization and extraction for experiments in nuclear physics, astrophysics, materials science, pharmaceuticals, and many more. The target material temperature dominates the release process and is limiting the primary beam intensity. Heat is dissipated from the beam spot through the target material, making the effective thermal conductivity of the target material critical for the ISOL process. Specifically-engineered microstructures have the potential to achieve better release properties. To address the thermal challenges in target materials, a deeper understanding of the thermal conductivity of these engineered materials is required.In this work, a numerical method is developed for evaluating the effective thermal conductivity of representative microstructures. A combined parallel and series model agrees with numerical data of four representative microstructures when thermal radiation through pores is considered by fitting a morphological parameter and a parameter describing the portion of series connections. Separately, a steady-state experimental-numerical method is used to determine the effective thermal conductivity of porous β-SiC slip-cast material and β-SiC pressed pellet material as a function of temperature up to 1200˚C and 1050˚C respectively. In addition, a new experimental apparatus for effective thermal conductivity is presented from conceptual design to operation.This thesis works towards understanding thermal transport through target materials. The new numerical method for material analysis, effective thermal conductivity measurements on SiC, and the establishment of the CHI system at TRIUMF for thermal conductivity measurements on target materials help build towards systematic studies of engineered materials in ISOL and beyond.

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Unidentified falling objects in the Large Hadron Collider: formation, charging mechanisms and dynamics of dust particulates in a high energy proton accelerator (2020)

Micrometer-sized dust particulates present in the LHC beam pipe are known to be causing a large number of sporadic beam loss events all around the LHC, some of which are large enough to provoke protection dumps or induce magnet quenches. These so-called Unidentified Falling Objects (UFOs) remain one of the important unknowns related to LHC operation after several years of high intensity beam operation in the LHC. In this thesis, the current understanding of the UFO problem is reviewed. The dynamics of charged dust particulates interacting with the LHC proton beam is discussed based on observations, theoretical predictions and numerical simulations. Using a reviewed version of the UFO model, it is found that the time profile of proton losses from half of the observed events present a time asymmetry which can't be explained with the current understanding of UFO dynamics. Furthermore, loss profiles recorded over more than 4 years of LHC operation are analyzed. It is shown that UFOs must carry an initial negative charge to explain the length of proton losses observed experimentally. Theoretical considerations, originally developed for dust-in-plasma, are introduced to support this claim. Plausible release mechanisms of UFOs in the LHC are also discussed, and the energy required for dust particulates to leave the walls of the beam chamber is presented. Finally, the theoretical possibility of having negatively charged dust particulates orbiting the proton beam of the LHC is discussed. It is found that stable orbits exist, but that only unstable orbits could result in important proton losses.

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Decay spectroscopy of europium-160 (2019)

Nuclei far from the traditional proton and neutron shell closures with “magic numbers” often deviate from spherical shapes and are deformed. The excited- state structure of these nuclei is built on the interplay of collective motions with single-particle degrees of freedom. Neutron-rich europium (Z = 63) nuclei around N = 104 are located midshell in both proton and neutron number. These nuclei exist in a region of large deformation, and the excited- state structure of these nuclei has not been studied in-depth yet. β-decay data was taken for a number of neutron-rich europium isotopes, including ¹⁶⁰Eu, at TRIUMF-ISAC using the GRIFFIN spectrometer. For this thesis, a comprehensive β-decay study was carried out on ¹⁶⁰Eu and the daughter ¹⁶⁰Gd. 10 new excited states and 41 new transitions have been identified in the excited-state level scheme of ¹⁶⁰Gd, and for the first time, a thorough analysis on the β-decay of ¹⁶⁰Eu was completed.

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Decay spectroscopy of neutron-rich 129Cd with the GRIFFIN spectrometer (2018)

Nuclei around doubly magic ¹³²Sn are of particular interest in nuclear structure as well as nuclear astrophysics. Their properties provide important input for the r-process as waiting-point nuclei. For example, their shell structure and half-lives affect the shape of the second r-process abundance peak at A∼130. In terms of nuclear structure, the evolution of single-particle levels near shell closures is ideal for testing the current nuclear models far from stability.There have been two studies on the decay of ¹²⁹Cd, however, the level schemes of ¹²⁹In have large discrepancies. Also, many of the spins of the excited states remain unclear. Therefore, the main purpose of the present study is to resolve the disagreements in the reported level schemes and to determine the properties of the energy states.The experiment was performed at the ISAC facility of TRIUMF, Canada. A 480 MeV proton beam, which was accelerated by the main cyclotron at TRIUMF, was impinged on an uranium carbide target to produce radioactive isotopes. ¹²⁹Cd was extracted using the Ion Guide Laser Ion Source (IG-LIS). γ-rays following the decays of ¹²⁹Cd were detected with the GRIFFIN spectrometer comprising of 16 high-purity germanium (HPGe) clover type detectors, along with the β-particles detected with SCEPTAR. The high statistics and the high sensitivity of the detectors allowed us to perform detailed and precise spectroscopy.A theoretical calculation was conducted using the shell model code NuShellX@MSU, employing the realistic residual interaction model jj45pna.The results of the analysis, including 29 new transitions and 5 new excited states, will be discussed and compared to the theoretical calculations.

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