Neutron stars are amongst the most exotic objects known in the universe; more than a solar mass of material is squeezed into an object the size of a city, leading to a density comparable to that of an atomic nucleus. They have a surface magnetic field which is typically around a trillion times stronger than the magnetic field here on Earth, and we have observed them to spin up to around 700 times per second. The existence of neutron stars was first proposed by Baade and Zwicky in 1934 but later graduated from theory to fact in 1967 as the first pulses were detected by Jocelyn Bell-Burnell, a then graduate student at the University of Cambridge. There are now well over 2000 neutron stars whose radio emission beams point at, and have been detected on Earth. We call these objects pulsars. Because of their remarkable properties, pulsars are very useful to physicists, who can employ them as precision timing tools due to the unwavering nature of their emission and of their rotation. Having an array of ultra-accurate clocks scattered throughout our galaxy is very useful for performing astrophysical experiments. In particular, precise pulsar timing measurements and the models that explain them, will permit the direct detection of gravitational radiation; a stochastic background initially, and potentially the individual signals from supermassive black hole binaries. Our models of pulsar behaviour are so precise that we are now able to notice even slight departures from them; we are starting to see that unmodelled variability in pulsars occurs over a broad range of timescales, both in emission and in rotation. Any unmodelled variability is, of course, detrimental to the pulsar's utility as a precision timing tool, and presents a problem when looking for the faint effects of a passing gravitational wave. We are hoping that pulsar timing arrays will detect gravitational radiation in the coming decade, but this depends, in part, on our ability to understand and mitigate the effects of the unmodelled intrinsic instabilities that we are observing. One important clue as to the nature of the variability in pulsar emission and rotation, is the emerging relationship between the two; we sometimes observe correlation on timescales of months and years. We have been observing pulsars for almost fifty years and our expanding datasets now document decades of pulsar behaviour. This gives us the ability to investigate pulsar variability on a range of timescales and to gain an insight into the physical processes that govern these enigmatic objects. In this thesis I describe new techniques to detect and analyse the emission and rotational variability of radio pulsars. We have employed these techniques on a 24 year pulsar dataset to unearth a striking new example of a dramatic and simultaneous shift in a pulsar's emission and rotation. We hypothesise that this event was caused by an asteroid interaction, although other explanations are also possible. Our variability techniques have also been used to analyse data from 168 young, energetic pulsars. In this thesis we present results from the nine most interesting. Of these, we have found some level of correlated variability in seven, one of which displays it very strongly. We have also assessed the emission stability of the NANOGrav millisecond pulsars and have found differing degrees of variability, due to both instrumental and astrophysical causes. Finally, we propose a method of probing the relationship between emission and rotation on short-timescales and, using a simulation, we have shown the conditions under which this is possible. Throughout the work, we address the variability in pulsar emission, rotation and links between the two, with the aim of improving pulsar timing, attaining a consolidated understanding of the diverse variable phenomena observed and elucidating the evolutionary path taken by pulsars.

Doctor of Philosophy / Department of Mechanical and Nuclear Engineering / Douglas S. McGregor / Interest in high-efficiency replacements for thin-film-coated thermal neutron detectors led to the development of single-sided microstructured semiconductor neutron detectors (MSNDs). MSNDs are designed with micro-sized trench structures that are etched into a vertically-oriented pvn-junction diode, and backfilled with a neutron converting material, such as 6LiF. Neutrons absorbed by the converting material produce a pair of charged-particle reaction products that can be measured by the diode substrate. MSNDs have higher neutron-absorption and reaction-product counting efficiencies than their thin-film-coated counterparts, resulting in up to a 10x increase in intrinsic thermal neutron detection efficiency. The detection efficiency for a single-sided MSND is reduced by neutron streaming paths between the conversion-material filled regions that consequently allow neutrons to pass undetected through the detector. Previously, the highest reported intrinsic thermal neutron detection efficiency for a single MSND was approximately 30%. Methods for double-stacking and aligning MSNDs to reduce neutron streaming produced devices with an intrinsic thermal neutron detection efficiency of 42%. Presented here is a new type of MSND that features a complementary second set of trenches that are etched into the back-side of the detector substrate. These dual-sided microstructured semiconductor neutron detectors (DS-MSNDs) have the ability to absorb and detect neutrons that stream through the front-side, effectively doubling the detection efficiency of a single-sided device. DS-MSND sensors are theoretically capable of achieving greater than 80% intrinsic thermal neutron detection efficiency for a 1-mm thick device. Prototype DS-MSNDs with diffused pvp-junction operated at 0-V applied bias have achieved 53.54±0.61%, exceeding that of the single-sided MSNDs and double-stacked MSNDs to represent a new record for detection efficiency for such solid-state devices.

The principal goal of this thesis is to highlight a possible "neutron skin" effect in the nuclear reaction mechanisms between heavy ions at energies around 6 MeV/nucleon. To this end, the reactions 58Ni+122Sn at 354 and 375.5 MeV and 64Ni+116Sn at 382.5 MeV were carried out at the CYCLONE accelerator in Louvain-la-Neuve. The projectile Ni and the target Sn were selected because of their magicity in proton. This characteristic implies the existence of a proton core more bound and of a neutron peripheral distribution in the isotopes 64Ni and 122Sn, rich in neutrons. The first goal was, to observe the possible influence of these neutrons on the fusion probability between the projectile and the target. In addition, the two types of reactions were selected in order to produce the same compound nucleus (180Pt) with the same excitation energy or with the same angular momentum. Thus, the effect of a neutron skin could be observed from the point of view of the production as from the point of view of the de-excitation of the compound nucleus. This de-excitation can proceed by evaporation of light particles or by fission. So, on the one hand, the fusion, fusion-evaporation and fusion-fission cross sections, and, on the other hand, the multiplicities of the light particles (neutrons, protons or alpha) emitted by the compound nucleus or by the fission fragments should have been compared. The experimental realization of this project required the use of the neutron detection system DEMON made up of 90 liquid scintillators laid out on a 4 m in diameter sphere which contain an aluminium chamber under vacuum. The chamber contained two multi-wire proportionnal counters XY intended for the detection of the fission fragments, two systems of Silicon junctions coupled with microchannels intended for the detection of evaporation residues and finally six triple Silicon telescope intended for the detection of the light charged particles (protons and alpha). At the end of the analysis, it was shownthat the cross sections of fusion are identical, within the error bars (about 10 %), for the two types of studied reactions.Also, it was observed that the possible compound nucleus de-excitations are independent of its production, as well on the competition between fission and evaporation as on the light particle emission. No effect of neutron skin was thus highlighted in the nuclear reaction mechanisms leading to fusion of the target and the projectile at energy around 6 MeV/nucleon. An effect seems nevertheless to appear in the nucleons transfer reactions in which the compound nucleus does not amalgamate. ------------------------------------------------ L'objectif principal de cette thèse est de mettre en évidence un éventuel effet de "peau de neutrons" dans les mécanismes des réactions nucléaires entre ions lourds à des énergies de l'ordre de 6 MeV/nucléon. Dans ce but, les réactions 58Ni+122Sn à 354 et à 375.5 MeV et 64Ni+116Sn à 382.5 MeV ont été réalisées auprès de l'accélérateur CYCLONE de Louvain-la-Neuve. Le projectile Ni et la cible Sn ont été choisis en raison de leur magicité en proton. Cette caractéristique implique l'existence d'un coeur de protons plus liés et d'une distribution plus périphérique des neutrons dans les isotopes 64Ni et 122Sn, riche en neutrons. Il s'agissait donc, dans un premier temps, d'observer l'influence possible de ces neutrons excédentaires sur la probabilité de fusion du projectile et de la cible. D'autre part, les deux types de réactions ont été choisis afin de produire le même noyau composé 180Pt de même énergie d'excitation ou de même moment angulaire. Ainsi, l'effet de peau de neutrons pouvait être observé tant du point de vue de la production que du point de vue de la désexcitation du noyau composé. Cette désexcitation peut se faire par évaporation de particules légères ou par fission. Plus précisement, il s'agissait de comparer, d'une part, les sections efficaces de fusion, de fusion-évaporation et de fusion-fission, et, d'autre part, les multiplicités des particules légères (neutrons, protons ou alpha) émises par le noyau composé ou les fragments de fission. La réalisation expérimentale de ce projet a nécessité l'utilisation du système de détection de neutrons DEMON constitué de 90 scintillateurs liquides disposés sur une sphère de 4 m de diamètre au centre de laquelle une chambre en aluminium était mise sous vide. Celle-ci contenait deux chambres à gaz et à multi-fils XY destinés à la détection des fragments de fission, deux systèmes de jonctions au Silicium couplées à des galettes à micro-canaux destinés à la détection de résidus d'évaporation et enfin six triple-téléscopes au Silicium destinés à la détection des particules chargées légères (protons et a). Au terme de l'analyse des données récoltées, il s'avère que les sections efficaces de fusion du projectile et de la cible sont identiques, dans une barre d'erreur de l'ordre de 10 %, pour les deux types de réactions étudiées. De même, il a été observé que les désexcitations possibles du noyau composé sont indépendantes de sa production, tant sur la compétition entre fission et évaporation que sur l'émission de particules légères. Aucun effet de peau de neutrons n'a donc été mis en évidence dans les mécanismes des réactions nucléaires impliquant la fusion du projectile et de la cible. Un effet semble néanmoins apparaître dans les réactions d'échanges de nucléons dans lesquelles les noyaux ne fusionnent pas.

No description available.

No description available.

Thesis (Ph. D.)--University of Missouri-Columbia, 1996. / Typescript. Vita. Includes bibliographical references (leaves 262-263). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 1996. / Typescript. Vita. Includes bibliographical references (leaves 262-263). Also available on the Internet.

Made available in DSpace on 2014-10-09T12:36:47Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:57:29Z (GMT). No. of bitstreams: 1 01485.pdf: 2055071 bytes, checksum: d7a431e820f8793828ac54edaacbd1d2 (MD5) / Tese (Doutoramento) / IPEN/T / Univ. North Carolina State

Made available in DSpace on 2014-10-09T12:29:17Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:00:50Z (GMT). No. of bitstreams: 1 01343.pdf: 8187182 bytes, checksum: 1a795b521139f6efcc47c46e35075982 (MD5) / Dissertacao (Mestrado) / IEA/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Made available in DSpace on 2014-10-09T12:31:43Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:00:31Z (GMT). No. of bitstreams: 1 02573.pdf: 3007952 bytes, checksum: c5ad0aa94fbfe8573f958b6952ebfe9e (MD5) / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP