Search Talks

The DEAP-3600 single-phase liquid argon detector at SNOLAB will increase the sensitivity to spin-independent WIMP-nucleon scatters by two orders of magnitude, allowing for the possibility of dark matter particle detection. The spherical detector will contain 3600 kg of liquid argon in an 85 cm radius acrylic vessel surrounded by 255 photomultiplier tubes (PMTs). After a collision between a WIMP and an Ar-40 nucleus, the scintillation light from the recoiling nucleus will be collected by PMTs. The separation of background events from WIMP events is critical. Detector materials contain levels of uranium and thorium, and these decay chains contain alpha, beta, and gamma decays. Alpha particles near the surface of the acrylic vessel are perhaps the most difficult background. A fraction of the alpha energy, or the recoiling nucleus from the alpha decay, could misreconstruct in the fiducial volume and result in a false candidate dark matter event. The maximum concentrations in the DEAP-3600 acrylic are 0.3 ppt, 1.3 ppt, and 1.1 x 10^-8 ppt for U-238, Th-232, and Pb-210, respectively. The concentrations of U-238, Th-232, and Pb-210 in the bulk acrylic will be measured by vaporizing acrylic, collecting the residue, and counting the contamination in a high-purity germanium well detector.

The goal of Total Body Irradiation (TBI) is to deliver a uniform dose of radiation to the entire body, to destroy cancerous cells. Since the human body is not uniform in either density or thickness, it is difficult to deliver a uniform dose. A novel, Aperture Modulated, Total Body Irradiation (AMTBI) technique was introduced by researchers at the Tom Baker Cancer Centre to address this problem. The AMTBI technique reduces the dose deviation along the midline in the longitudinal direction to less than 5%, as compared to 15% with conventional TBI. This improvement in dose homogeneity is achieved by dynamically changing the apertures of the Multi-Leaf Collimator (MLC) according to the radiative area

This research focuses on finding analytical solutions to the mechanical bidomain model of cardiac tissue. In particular, a perturbation expansion is used to analyze the equations, with the perturbation parameter being inversely proportional to the spring constant coupling the intracellular and extracellular spaces. The results indicate that the intracellular and extracellular pressures are not equal, and that the two spaces move relative to each other. This calculation is complicated enough to illustrate the implications of the mechanical bidomain model, but is nevertheless simple enough to solve analytically. The zeroth-order of the perturbation expansion reveals that the intracellular and extracellular displacements are equal, thus making it unnecessary to account for either space on an individual basis. Yet, in the first-order of the expansion we see a shift and the intracellular and extracellular displacements are unequal. One application of the calculation is to the mechanical behavior of active cardiac tissue surrounding an ischemic region. Also, a hypothesis for the physical meaning of the pressure inequality is if this inequality is held for an extended period of time it may cause fluid to flow across the cell membrane and in the tissue.

3D moment invariants (3DMIs) are mathematical spatial descriptors designed to be invariant to scaling, translation and rotation. We propose to characterize the spatial distribution of positron emission tomography (PET) images using 3DMIs. We have used 3DMIs to characterize the spatial distribution of PET brain images recorded from subjects with Parkinson's Disease (PD) and healthy controls. 3DMIs were found to accurately describe the 3D texture of PET images despite changes in the size and orientation of the participating subjects in the PET scanner. In addition, we were able to find differences in the 3DMIs of PD patients distinct from those of healthy volunteers. These changes suggest that disease-related variations in the spatial distribution measured using PET can be quantitatively described with the proposed method. Therefore, this method shows great promise to extract additional information from PET data with a wealth of potential applications to disease diagnosis, staging, treatment assessment and more. The quantification of the observed disease-related changes for PD subjects is currently under way.

Melanie will discuss how she has and collaborators have applied physics techniques to advance the understanding of the optics of the eye, and to develop novel diagnostic and therapeutic approaches for eye diseases. Her work includes the application of inverse methods used to characterise optical fibres, waveguide theory applied to cone photoreceptors, sinusoidal analysis of circadian rhythms in the eye, adaptive optics, confocal and polarisation imaging used to improve images of the rear of the eye, characterisation of deposits by atomic force microscopy and drug excitation by two photons as a therapy for eye disease. Using an Abel integral inversion technique applied in optical fibres, Campbell measured for the first time, the gradient refractive index variation in the crystalline lens of the eye. She and her collaborators demonstrated that this distribution can be modified by visual experience. Campbell and her collaborators have also shown that the optical quality of the lens varies with age and that the progressive loss of near vision is lens based. These findings inspired a new design for an IOL lens which replaces the living lens during cataract surgery. In another example, adaptive optics, originally developed for astronomy, offers a powerful tool for localizing light within the eye. In turn, this has resulted in the correction of the optical imperfections of the eye, giving images of structures at the rear of the eye with improved resolution and contrast. In addition, adaptive optics can precisely localize light stimuli for therapeutic purposes within the eye. The precise localization of light energy in other at the retina is limited by the optics of the eye. Adaptive optics may enable precise light based therapies in the crystalline lens and retina of the eye.

The magnetic properties of ceramic samples of Gd1.98R0.02CuO4 R= Ca Sr Th were studied and compared with Gd2CuO4. The results showed weak ferromagnetic ordering in all samples. We observed two magnetic ordering temperatures in the heat capacity measurement a sharp peak at TN(Gd) 6.5 K that can be attributed to the Neel temperature of Gd3+ ions and the second transition temperature at about 20 K that suggested to the magnetic interactions of Gd-Cu. The third anomaly was seen at TN(Cu)=280 K in susceptibility measurements. Investigations indicated that 0.02% mole substitution for Gd was not much effective on the transition temperature of compounds although we bserved significant change in the magnitude of heat Capacity susceptibility and magnetization of samples as well as their conductivities.

Epitaxial MnSi grown on Si (111) offers new opportunities in the development of spin-dependent transport in helical magnets. Helical magnets are a class of noncollinear structures that have shown promise as a material for spin-dependent electron transport studies.The helical magnets are of particular interest in spintronics because in these magnets the electron spins spiral about a particular crystallographic direction, this property can allow for control over electron spin. Many interesting magnetic properties can be studied with the combination of thin-film heterostructures and helical magnets. Through use of x-ray diffraction, SQUID magnetometry and transmission electron microscopy, we have observed the structural and magnetic properties of crystalline MnSi thin-films to determine the effects of strain on the magnetic properties. As a result, we have found that epitaxially induced tensile strain results in an increase in the unit-cell volume, and that the atypical strain relaxation behaviour is correlated with a magnetic response.The talk will give a brief outline of the theory/techniques used, and the results gathered.

Pb2CrO5 have received considerable interests due to their potentials applications in UV radiation measuring devices, visible and UV light photodetectors. In this research we are examining the structural, electronic, magnetic, and thermal properties of polycrystalline Pb2-xLaxCrO5. Samples have been prepared using a solid state solution technique. The temperature dependent magnetic measurements reveal a transition in the Pb2CrO5 and La doped samples near 300 K. To understand the possible origin of such transition, we measured thermal properties using Differential Scanning Calorimetry (DSC) technique. These results reveal an endothermic transition close to 285 K in the parent sample and in La doped sample. We have also measured the temperature dependent resistance in 300K-900K range.

Heavy-hole spin states have been proposed as a robust qubit candidate. Nevertheless, the coupling of the hole spins to nuclei in the surrounding medium likely limits hole-spin coherence and has, until very recently, been overlooked. We describe the spin decoherence of a heavy-hole in a semiconductor quantum dot, subject to spin echo pulses. We do so both analytically and numerically for an experimentally realistic number (10^4) of nuclear spins. Including the (previously neglected) nuclear Zeeman term in the Hamiltonian, we observe novel effects uniquely characterizing the decoherence mechanisms under study. In particular, we find a nontrivial dependence of the decay on the applied magnetic field, as well as novel predictions for motional narrowing and envelope modulation, which could significantly extend the hole-spin memory time in near-future experiments.

Quantum Key Distribution is a form of public-key cryptography where the security comes from the unique properties of quantum mechanical systems: entanglement and the no-cloning theorem, rather than computational complexity. With increased adoption of fibre optic networks, it may be possible to implement QKD in parallel with classical data traffic. Many research projects have demonstrated QKD over fibre optic networks at the same wavelengths as existing network traffic. These projects require sophisticated noise cancellation due to wave mixing between quantum and classical signals, as well as having to use complex non-silicon based photodiodes. Our research uses lower wavelengths for QKD over active telecom fibres to avoid these problems. Entangled lower-wavelength photons are combined with telecom wavelength laser signals carrying a large amount of traffic, and passed through single mode telecom fibres. We show that data bandwidth usage has a negligible effect on the quantum bit error rate (QBER) and visibility for distances up to 6km. We find key rates of 61 bits per second with QBER rates of 10% at 6km. This research demonstrates the simplicity and applicability of QKD to existing fibre optic infrastructure in corporate, government, and academic campuses.