School of Physics - Theses
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ItemEstimation of the cosmogenic activation and measurement of the quenching factor of NaI(Tl) crystal with spectrum-fitting for the SABRE experiment( 2022)Though evidence suggests that 84% of all matter consists of dark matter, its lack of substantial interaction with ordinary matter continues to obscure its exact physical qualities. With the tantalizing prospect of uncovering a rich amount of information about a seemingly fundamental aspect of our Universe, the physics community has attempted to mine this ore of knowledge for the past century. Among these attempts, the use of direct detection experiments to probe the weak interactions between dark and baryonic matter has since mostly yielded null results. An exception to these is the annual modulation signal detected by the DAMA/LIBRA experiment, whose dark matter interpretation remains inconclusive. The Sodium-iodide with Active Background Rejection Experiment (SABRE) will conduct low energy (keV) measurements of dark matter direct detection events using radiopure NaI(Tl) crystals as a model independent test of DAMA's annual modulation signal. In order to so successfully, it is vital that the radioactive background of SABRE's crystals are low enough so that they are more sensitive to WIMP-like events than DAMA/LIBRA and that the crystals' response is properly understood by measuring their scintillating properties beforehand. In this thesis, the radioactive background of SABRE's crystal, due to cosmogenic activation while stored on the surface and during transport to its laboratory site, is estimated. This estimation takes into account the amount of cosmic ray flux and geomagnetic shielding for two possible freight travel scenarios, either by air or sea. The subsequent decay of each considered isotope at their underground site is also considered in order to determine how significantly they would contribute to the background over the lifetime of the experiment. In light of these calculations, recommendations for the storage time and method of travel of SABRE's crystals can be motivated with knowledge on whether the cosmogenic background produced will be sufficiently low for SABRE's purposes. Additionally, the quenching factors of SABRE's NaI(Tl) crystal must also be known to low uncertainty in order to determine the energies of the nuclear scattering interactions. A novel spectrum-fitting methodology was developed and tested to extract the quenching factor from sodium nuclear recoil measurements in NaI(Tl). The method employs Monte Carlo simulated recoil energy spectra to fit measured data in order to account for experiment-specific systematics. This was employed to measure the sodium quenching factors of a commercial NaI(Tl) crystal for recoil energies between 36 and 401 keV. The SABRE experiment will use this method for the measurement of their own crystal's quenching factors.
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ItemSpin-down signatures of young neutron stars( 2022)The spin down of neutron stars has been invoked to explain a wide variety of electromagnetic and gravitational-wave signals. This thesis explores two different signals associated with the spin down of neutron stars, one electromagnetic signal and one gravitational-wave signal. Binary neutron star coalescences, confirmed as the progenitor of at least some short Gamma-ray bursts (sGRBs) in 2017, are predicted to form either a black hole or a highly magnetized neutron star. Up to 20% of sGRBs observed by the Neil Gehrels Swift telescope display prolonged X-ray emission, sometimes called a ``canonical'' afterglow, consisting of three phases: an initial power-law luminosity decay; a 'plateau', lasting between 10 s and 105 s, during which the X-ray luminosity is approximately constant; and a final power-law decay. Previous authors have noted that the evolution of the canonical light curve is broadly consistent with the expected spin-down luminosity of a neutron star. Key ideas from analytic, one-zone models of plerions (also called pulsar wind nebulae) can be used to model the evolution of a synchtrotron nebula fuelled by the the spin-down luminosity of a neutron star formed in an sGRB. An analytic expression for time-dependent, spatially-averaged electron energy distribution in the nebula is found and used to calculate the light curve and the point-in-time spectra. The light curves predicted by the plerionic model are consistent with the shape and luminosity of the X-ray light curves and reproduce the observed correlation between plateau duration and luminosity (i.e. brighter plateaux end sooner). Furthermore, Bayesian parameter estimation comparing the point-in-time spectra to time-averaged spectra of six Swift sGRBs with canonical X-ray afterglows and of known redshift allows estimation of the parameters of the neutron-star central engine, including its poloidal field strength Bp and its rotation period P0 at birth, and injection parameters within the shock, including the energy range of the relativistic electrons and their power-law index. All six sGRBs favour a neutron star with Bp ~ 1011 T and P0 ~ s, consistent with the prediction the neutron star should be highly magnetized and rapidly spinning. We also apply the point-in-time spectra to four time-averaged spectra taken at four separate epochs in the X-ray afterglow of GRB130603B and infer the evolution of the magnetic field in the synchrotron bubble B. We find the evolution of B is slower than the expected evolution of the far-field limit of the stellar magnetic field. Rotating, non-axisymmetric neutron stars spin down via the emission of continuous gravitational waves which may be detectable by current terrestrial interferometers such as the advanced Laser Interferometric Gravitational-wave Observatory (LIGO) and advanced Virgo. Young core-collapse supernova remnants are likely hosts of young neutron stars and are common targets for wide-band directed searches for continuous gravitational waves targeting non-pulsating neutron stars. In this work, we present the results for two searches for continuous waves from neutron stars in young supernova remnants using a hidden Markov model (HMM). The HMM tracking scheme models the frequency evolution as a random walk with secular spin down and remains sensitive in the presence of stochastic spin wandering similar to that observed in pulsar timing observations. A search targeting twelve neutron stars in young supernova remnants in the second observing run (O2) of advanced LIGO using an HMM tracking scheme identifies 1012 potential candidates, 18 of which survive a series of standard vetoes. Further assessment of the 18 survivors based on their dependence on sky position and Doppler modulation confirms they are all consistent with terrestrial noise. A second search, conducted with the the LIGO-Virgo-KAGRA (LVK) collaboration, targets fifteen neutron stars in young supernova remnants in the first half of the third observing run (O3a) of advanced LIGO and advanced Virgo using three search pipelines, including an HMM tracking scheme, and reports no candidates consistent with an astrophysical origin after a rigorous veto and follow-up process. The HMM tracking scheme sets the first 95% confidence limits on gravitational-wave strain, h095%, for these targets with a random-walk signal model, reaching a sensitivity of h095% = 2.64 x 10-25 at 172 Hz for G353.6-0.7. The constraints on h095% are converted to upper limits on neutron-star ellipticity below 10^-5 above 150 Hz and constrain the maximum amplitude of internal r-mode oscillations below 10^-3 above 150 Hz.