Graduate Seminar Series - Fridays

Investigating Nuclear Shell Evolution in Neutron-Rich Calcium Isotopes

Speaker:  Robin Coleman
Date: Friday February 10th, 2023

Abstract

Nuclei away from the line of stability have been found to demonstrate behavior that is inconsistent with the traditional magic numbers of the spherical shell model. This has led to the concept of the evolution of nuclear shell structure in exotic nuclei, and the neutron-rich calcium isotopes are a key testing ground of these theories; there have been conflicting results from various experiments as to the true nature of a sub-shell closure for neutron-rich nuclei around $^{52}$Ca. An experiment was performed at the ISAC facility of TRIUMF; $^{52}$K, $^{53}$K, and $^{54}$K were delivered to the GRIFFIN gamma-ray spectrometer paired with the SCEPTAR and the ZDS ancillary detectors for beta-tagging, as well as DESCANT for neutron-tagging. Using this powerful combination of detectors, we combine the results to construct level schemes for the isotopes populated in the subsequent beta-decay. Preliminary results from the analysis of the gamma, beta, and neutron spectra will be presented and discussed in the context of shell model calculations in neutron-rich nuclei.
 

How to consistently use modern nuclear forces in an ab initio technique

Speaker:  Ryan Curry
Date: Friday February 10th, 2023

Abstract

One of the challenges faced while studying the nuclear many-body problem is the nature of the nucleon-nucleon interaction. The full details are described by the theory of Quantum Chromodynamics (QCD), but for realistic calculations approximate models must be used. Historically these have been phenomenological potentials fit to experimental data. However, in recent decades, models for the nucleon-nucleon interaction were produced from a power counting expansion in Chiral Effective Field theory (EFT). As a result, these modern nuclear interactions have an advantage over the phenomenological potentials, since they have a connection to the symmetries of the underlying theory of QCD.

To investigate the nuclear many-body problem, we employ an ab initio approach. Quantum Monte Carlo (QMC) consists of a family of powerful stochastic methods for solving the many-body Schrodinger equation. QMC methods provide very accurate results, at the cost of being computationally expensive. In addition to their accuracy, QMC methods have the benefit that we can build the appropriate physics, such as pairing, directly into them.

Combining these two tools, QMC methods and the Chiral-EFT derived nucleon-nucleon interaction, leads to something of a contradiction. The Chiral-EFT potential is designed to be used perturbatively at the many-body level, but this is almost never the case, due to the fact that combining a non-perturbative technique (QMC) with a perturbative potential is not a trivial problem. This work attempts to remedy this inconsistency. To show this, we explore a variety of low-density neutron matter systems that have a direct application to neutron-rich systems such as the inner crust of neutron stars.

For any graduate students interested in presenting in the future please use the google sheets link below to claim a slot. If you are presenting at an upcoming conference and would like to practice a shorter talk, we can book two speakers on a single date.

Development work for The Detector Array for Energy Measurement of Neutrons (DAEMON) 

Speaker: Zarin Ahmed
Date: Friday February 3rd 2023

Abstract

As one moves away from stable isotopes and deeper into the neutron-rich region, the likelihood of \(\beta-delayed\) neutron (\(\beta n\)) emission decay increases. The ability to understand the neutron emission probabilities and the neutron energy spectrum can reveal highly sensitive detail of the nuclear structure that a conventional \(\beta-decay\) study using only \(\gamma-ray\) detection cannot. We propose to build the Detector Array for Energy Measurements of Neutrons (DAEMON) that will employ the timeof-flight technique to enable high-resolution energy measurements of the neutrons emitted following \(\beta n\) emission. The initial trials, performed at the University of Guelph, testing the rudimentary geometries of EJ200 plastic scintillators and various electronic parameters of silicon photomultiplier (SiPM) arrays for the foundation of DAEMON will be presented. Upon successful comparison of tests with gamma sources with simulations data, the DAEMON prototype will be tested with the monoenergetic neutron beam at the University of Kentucky Accelerator Laboratory. Used in conjunction with the GRIFFIN Decay Station at TRIUMF in Vancouver, BC, DAEMON will establish a frontier for \(\beta n\) studies currently non-existent at the facility and therefore initiating a road to strong international collaborations. From shaping the abundance curve of the astrophysical rapid neutron capture process, as well as controlling the neutron induced fission in nuclear reactions, the building of a neutron detector will address a broad arena of physics.

Hoyle's Steady State Cosmology

Speaker: Paul Deguire
Date: Friday February 3rd 2023

Abstract

Since the first quarter of the 20th century and the development of the theory of general relativity by Einstein, many attempts have been made to describe the universe. The cosmological model we use today, the big-bang theory, was first stated by George Lemaître and Arthur Eddington around 1930. Basically, Lemaître and Eddington’s model describes a universe in constant expansion. However, this model raised more questions than answers. For instance, this implied there was a beginning to the universe, and nothing before. Therefore, Fred Hoyle proposed the steady-state cosmology in 1948. In order to keep the density constant, and taking into account the expansion universe, matter needs to be created from nothing. During the ’50s, Lemaître-Eddington and Hoyle’s cosmologies divised the cosmology community in two. But during the ’60s, the big-bang model triumphed over its main competitor thanks to the refinement of observational astronomy.

The Hydrodynamic Stability of Black Hole Mimickers

Speaker: Joshua Cadogan
Date: Friday January 27th 2023
Time: 4:30pm 
Location: MacNaughton Room 415

Abstract

Let’s get heavy. Since the initial proposal of Einstein’s theory of general relativity (GR), there has been significant debate over the physicality of a black hole, leading physicists to propose less “mathematically convenient” black hole models. This debate continued until experimental endeavours such as the LIGO-Virgo-KAGRA and Event Horizon Telescope collaborations provided significant evidence towards the existence of a true black hole as predicted by GR. However, in the pursuit of unified theories, alternative black hole structures which mimic the classical black hole persist. One such model, the Gravastar, had been previously shown through thermodynamic arguments to have a stable structure heavy enough and small enough to resemble the features of a black hole.
In this talk we will investigate this model from a GR perspective, starting from a summary of GR and stellar structure fundamentals, followed by a perturbative analysis of this equilibrium state. By investigating how sensitive the state is to perturbations, arguments can be drawn as to the physicality of the Gravastar’s existence, as it is known that stars form in extreme astrophysical systems.

Pairing in nuclear and cold-atomic systems

Speaker: George Palkanoglou
Date: Friday, January 20th 2023
Time: 4:30pm 
Location: MacNaughton Room 415

Abstract

Let's talk about pairing. In the context of many-body quantum physics, pairing refers to the tendency of particles to form pairs. It is a phenomenon encountered at low temperatures and it lays at the heart of superfluids and superconductors. In nuclear systems, pairing effects have a long history, with the first investigations dating back to the 1950s. Today, we know that pairing is responsible for effects seen in the lab, such as the strong binding of certain nuclei, as well as in the sky, like the irregular periods of pulsars: nuclear matter can be superfluid. In this talk, I will start with a brief historical overview of the theories that have been developed to describe pairing, focusing on the ones that are still in use today. We will then discuss the recent investigations that have demonstrated the possible existence of a new type of pairing in heavy nuclei. I will present preliminary results from new investigations on the effect of nuclear deformation on these novel superfluids and discuss their signatures in nuclear experiments. Indirectly, such nuclear effects can be probed in cold-atomic experiments by emulation: tuning the inter-atomic interactions to resemble the nuclear ones, via the so-called Feshbach resonances, allows for indirect studies of nuclear systems. On that note, I will discuss preliminary results on similar phenomenological investigations of novel pairing mechanisms in cold-atomic gases, and how they can provide insight for the physics of nuclei and neutron stars.