The growth of Pb film on Si(111) is an unusual metal-semiconductor system. For a certain temperature range, Pb films have been found to grow in steep-edge and flat-top islands with uniform height on Si(111). This specific film morphology has been correlated to Quantum Size Effect (QSE) that the object size or film thickness affects the electronic structure of the films and results in certain thicknesses more stable than others. The X-ray diffraction technique has the advantages of long penetration length and high statistics, therefore it has been used to investigate the influence of QSE on the growth of Pb on the Si(111) 7x7 surface. It is demonstrated that the structure of Pb islands and the associated wetting layer are consistent with effects of quantum confinement. Specular reflectivity of 3 monolayer (ML) Pb films grown on the substrate at 227K has conclusively shown that the Pb islands do not reside on top of a Pb wetting layer, but directly on top of the Si substrate. The nucleating Pb nanocrystals transform the highly disordered Pb wetting layer beneath the islands into well-ordered fcc Pb. The surface then consists of fcc Pb islands directly on top of the Si surface with the disordered wetting layer between the islands. Moreover, it is found that QSE leads to novel behavior for the coarsening evolution of the Pb islands. The diffuse X-ray scattering experiments have been carried out as functions of temperature, deposition rate and coverage. A structural evolution of Pb islands was observed after deposition at very low coverages (0.2 -- 1.0 ML above the wetting layer coverage). Contrary to the classical scaling theory of nucleation and Ostwald ripening, a much lower island density is achieved with coarsening after deposition at high rather than low flux rates. The time constants of coarsening are found to be orders of magnitudes shorter than what is expected from the Gibbs-Thompson analysis. The rapid evaporation of unstable 3-layer islands shown in complementary STM suggests the role of QSE in the more efficient decay mechanism operating at low temperatures. These results have important applications for the controlled growth of nanostructures.

The objective of this research is the statistical study of space weather e ects on im- age detectors in Low Earth Orbit. The Hubble Space Telescope is used as a resource for acquiring proton a ected images for statistical analysis. For the purpose of the present work, the space weather environment will consist of cosmic as well as solar proton particles. The proton occurrences evident in images from the Hubble Charge Coupled Device (CCD) have been used to calculate the probability of proton events, which is related to the local space weather particle ux. The proton particles transfer energy to the CCD silicon, which ultimately results in measured signal that is not originating from photon illumination. The signal due to the proton interactions is rst separated from the noise contribution and subsequently used in the determi- nation of a pulse height probability distribution. Separation of the noise from the proton events also leads to the measurement of proton streak lengths and orientations along with the associated probability distributions. The directionality of the space weather environment in Low Earth Orbit is examined using the distribution of proton streak angles. Statistics found from the Hubble are also used as a starting point for simulations that create synthetic proton signal images. The distributions resulting from the Hubble CCD analysis give the probability of the: number of proton events, which is related to the ux of the space weather protons; energy of proton events, which allows estimates of damaging proton interactions; length of proton streaks on the CCD, which shows the relative probability of a long traversing proton event; angle of proton event, which indicates the directionality of the space weather environment.

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Genetic improvement has been an integral part of loblolly pine plantation forestry in southern United States for about 60 years with focus on improving timber yield, wood quality, and pest and disease resistance. Advances in techniques of genetic selection, breeding, and propagation of planting material have made it possible to achieve genetic gains that are likely to result in significant changes in loblolly pine stand dynamics. Height-age relationships, height and diameter relationships and distributions, and intraspecific competition were investigated in second generation open-pollinated, controlled-pollinated, and clonal loblolly pine with an objective of characterizing the nature and magnitude of changes in these characteristics due to genetic improvement and clonal forestry. Genetic improvement and/or clonal forestry had no practical effect on parameters of the height-age and height-diameter relationships beyond the effect on the asymptote parameter of the Chapman-Richards and Korf equations that were used to model these relationships. Genetic improvement resulted in an increase in the mean of height distribution without a corresponding increase in the mean of the diameter distribution, but had no effect the variance and skewness of the distributions. Thus, growth and yield models in which basal area is a function of height at a specific age (site index) are likely to over predict genetic gains in basal area growth and volume yield. Increase in stand density resulted in an increase in variance of the diameter distribution of non-clonal stands but had no effect on the variance of the diameter distribution of clonal stands. Thus, diameter distribution of clonal stands may differ from that of non-clonal stands after crown closure despite the distributions not being different before and during early stages of crown closure. This study also evaluated methods that may be used to predict height growth of new genetic varieties and those that may be used to asses intraspecific competition in forest stands. Mixed-model approach of calibrating a height-age relationship to a new loblolly pine clone gave biased estimates for clones that were at the extremes of the distribution of the groups. The use of maximum likelihood with simulated annealing (MLSA) to evaluate competitive interactions among trees in loblolly pine stands gave non-unique estimates of optimum competitor selection radius. A simpler technique that uses Pearson correlations was proposed and was found to work better than MLSA. / Ph. D.

En dosimétrie, la distribution énergétique des photons émis par une source constitue un paramètre incontournable. Dans la gamme des basses et moyennes énergies (i.e. E<150 keV ici), le LNHB possède 5 tubes à rayons X ainsi que des grains de curiethérapie à l’iode 125, présentant des hauts débits de fluence. La détermination du spectre émis par calcul (déterministe ou Monte-Carlo) est limitée, dans la gamme d’énergie considérée, par les incertitudes élevées sur les bases de données ainsi que par les approximations du modèle. La mesure directe avec un détecteur au germanium ultra-pur a donc été retenue, bien que nécessitant de lourds moyens. De plus, le spectre mesuré est le produit de convolution du spectre émis recherché par la réponse du système. Une fois la réponse du détecteur modélisée, il est possible de « déconvoluer» la mesure, i.e. de remonter au spectre réellement émis en corrigeant (par stripping, model-fitting, inférence bayésienne…) les déformations spectrales induites par le processus de détection. Pour la curiethérapie, le modèle de grain-source a ainsi pu être ajusté. Pour les tubes à rayons X, les résultats obtenus avec différents codes Monte-Carlo et 4 logiciels déterministes ont été comparés à un spectre dit de référence obtenu par mesure et déconvolué. Ainsi l’impact sur certaines grandeurs dosimétriques de la méthode utilisée a pu être quantifié. / In the field of dosimetry, the knowledge of the whole photon fluence spectrum is an essential parameter. In the low-to-medium energy range (i.e. E<150 keV), the LNHB possess 5 X-ray tubes and iodine-125 brachytherapy seeds, both emitting high fluence rates. The performance of calculation (either Monte Carlo codes or deterministic software) is flawed by increasing uncertainties on fundamental parameters at low energies, and modelling issues. Therefore, direct measurement using a high purity germanium is preferred, even though it requires a time-consuming set-up and mathematical methods to infer impinging spectrum from measured ones (such as stripping, model-fitting or Bayesian inference…). Concerning brachytherapy, the knowledge of the seed’s parameters has been improved. Moreover, various calculated X-ray tube fluence spectra have been compared to measured ones, after unfolding. The results of all these methods have then be assessed, as well as their impact on dosimetric parameters.

The structure of a forest stand is characterized by: (a) species composition, (b) age, (c) size (diameter and height), and (d) spatial (horizontal and vertical) arrangement of the trees. Depending on the species, site, and disturbance history, the stand structure varies with time, thus providing a snapshot of a particular development stage. Research on growth and stand structure has shown that the spatial distribution of trees is one of the key determinants of stand productivity. Forest inventories and ecological surveys carried out in British Columbia (BC) have shown that the structure of naturally established, unmanaged stands varies from simple (single-species, single-storied, and even-aged) to complex (multi-species, multi-storied, and uneven-aged). Only a few studies have quantitatively characterized this range of structural complexity, with nearly all studies focusing on old-growth stands. BC forest policy requires that harvested areas be regenerated with a mixture of tree species whenever a mixture is suited to the site. This policy is based upon the assumption that under appropriate conditions, increases in stand productivity, reliability, and/or biodiversity can be attained in mixed-species stands. This assumption has not yet been tested for forest ecosystems. One mechanism by which different tree species can reduce crown competition for light is through vertical separation (the development of multiple canopy strata). Canopy stratification is not easily recognized in mixed-species stands, particularly when species have similar shade tolerance and height growth patterns, and no quantitative methods have been developed to detect stratification. The diameter frequency distribution of two-storied stands have been characterized by inverted J-shaped as well as modal curves. Although it would be more appropriate to characterize stand structure by height frequency distributions, these distributions have not been developed. We suggest that (i) a stand is stratified if there are distinct, quantitatifiable modes in the size distribution; either diameter, height, or crown height, and (ii) height or crown height distributions will be the most sensitive measures. To characterize the structure of western hemlock (Tsuga heterophylla (Raf.) Sarg.) (Hw) and western redcedar (Thuja plicata Donn ex D. Don in Lamb.) (Cw) second-growth stands, and to investigate its influence on tree growth, we (1) described and compared size (diameter, height, and crown height) frequency distributions in single- and mixed-species stands, (2) determined whether mixed-species stands develop a stratified canopy, and (3) examined whether interactions between hemlock and redcedar affect tree growth.