Tritium is a radioactive hydrogen isotope that is typically produced via neutron interaction with heavy water (D2O), producing tritiated water (DTO). As a result of this, tritium accounts for roughly a third of all occupational exposures at a CANDU type nuclear power plant. This identifies a need to study the biological effects associated with tritium (and low energy electrons in general). However, there are complications regarding the dosimetry of tritium, as well as difficulties in handling and using tritium for the purposes of biophysics experiments. To avoid these difficulties, an experiment has been proposed using photons to mimic the beta decay energy spectrum of tritium. This would allow simulation of the radiation properties of tritium, so that a surrogate photon source can be used for biophysics experiments. Through experimental and computational means, this work has explored the use of characteristic x-rays of various materials to modify the output spectrum of an x-ray source, such that it mimics the tritium beta decay spectrum. Additionally, the resultant primary electron spectrum generated in water from an x-ray source was simulated. The results from this research have indicated that the use of characteristic x-rays is not a viable method for simulating a tritium source. Also, the primary electron spectrum generated in water shows some promise for simulating tritium exposure, however further work must be done to investigate the slowing down electron spectrum. / UOIT

An advanced simulation code, LEEPS (Low Energy Electron Positron Simulation), has been adapted to simulation of electrons and positrons in liquid water for energies down to 50 eV. Different scattering parameters and results from simulations are compared with existing data in the literature.   Several programs including a subroutine package for simulation of secondary electrons created in binary like collisions have been developed in purpose of charting different characteristics of the energy deposition.   A toy model for DNA damage is presented as an example of how LEEPS possibly can be used for future investigation of cellular damage due to radiation.

Résumé : Lors de l’utilisation de la radiation pour le diagnostic et le traitement du cancer, l’ADN est une cible importante due à son rôle dans la division cellulaire. La radiation y dépose de l’énergie par production abondante (10[indice supérieur 5] e[indice supérieur −]/MeV) d’électrons de basse énergie (EBE) (<50 eV) menant à la production de radicaux et à la dissociation de molécules. Une meilleure compréhension de ces phénomènes physico-chimiques mènera au développement de nouvelles stratégies en radioprotection et en radiothérapie. Il est primordial d’identifier et de quantifier ces dommages initiaux. Suite à des résultats obtenus par des expériences récentes (Li et al., 2010) sur des couches minces d’ADN irradiées par des EBE dans le vide, nous suggérons que certains produits désorbent en quantité significative. Nous proposons une méthode pour mesurer cette perte de matière en utilisant une balance à quartz pour mesurer in situ les changements de masse totale. Ce mémoire présentera la conception et la construction de l’appareil ainsi que les résultats d’irradiation de la thymine et de la thymidine. À 25 ◦ C, le taux de perte de masse spontanée des échantillons joue un rôle important pour les petites molécules comme la thymine (126 uma). L’irradiation augmente d’abord ce taux qui diminue d’un facteur 5 à 15 après une exposition prolongée, signe de modifications notables de l’échantillon. Pour des molécules plus imposantes comme la thymidine (242 uma), il n’y a pas de désorption spontanée et le taux de désorption induite par des électrons de 50 eV est de 0,4 ± 0,1 uma/e[indice supérieur -]. Cette méthode, nécessaire à la calibration d’autres expériences réalisées par HPLC et spectrométrie de masse, permet de compléter la quantificationdes fragments, qui peuvent aussi être l’origine de lésions subséquentes. / Abstract : DNA is the principle target of radiotherapy (RT) due to its crucial role in cellular growth and function. Ionizing radiation (IR) delivers its energy into the cell and its nucleus via sequential ionization events that produce many low-energy electrons (LEE)(10[superscript 5]e[superscript −] per MeV) which drive subsequent molecular dissociations and the formation of radicals and other reactive species. Since a better understanding of these mechanisms is needed to develop new strategies for radioprotection and RT, it is essential to identify and to quantify the initial damage induced by IR. Recent chromatographic (HPLC) analysis of short oligonucleotide irradiated with LEE in vacuo (Li et al., 2010) revealed that only ∼30 % of the loss of intact molecules could be explained by the formation of identifiable radiation products. We hypothesize that electron stimulated desorption (ESD) may account for some of the unexplained loss of the missing molecules. Here we propose a new experimental method to quantify this loss using a quartz crystal microbalance to measure in situ the total mass change due to ESD. This thesis describes the design and the construction of the novel apparatus and presents results for LEE irradiated thymine (thy) and thymidine (dT). We find that at 25 ◦ C, the thermal-induced mass loss is important for small molecules such as thy (126 amu). Upon irradiation at 50 eV, the rate of mass loss initially increases, but then decreased by factors between 5 and 15 indicating structural changes occurring at the sample surface. For larger molecules such as dT (242 amu), there is no thermal evaporation at 25 ◦ C and the LEE induced rate of desorption at 50 eV is 0.4 ± 0.1 amu/e[superscript -]. This work is needed to calibrate HPLC and mass spectrometry experiments allowing us to quantify the fragment species produced by LEE that are expected to induce further and biologically significant damage.

No description available.

The focus of the presented research is to examine the fundamental physics and chemistry of low-energy electron-stimulated reactions on adsorbate covered single crystal surfaces. Specifically, condensed SiCl₄ on the Si(111) surface and condensed H₂O on the TiO₂ (110) surface have been studied. By varying adsorbate film thicknesses, the coupling strength of the target molecule to the substrate and surrounding media dictates the progression of the electron induced reactions. To investigate the electron interactions with SiCl₄ on the Si(111) surface, desorbing cations and neutrals were detected via time of flight mass spectrometry (ToF-MS) where neutral chlorine atoms were ionized using a resonance enhanced multi-photon ionization (REMPI) technique. Structure in the cation and neutral yields were assigned to molecular excitations. At an incident electron energy of 10 eV, a resonance structure in the neutral yields was attributed to a negative ion resonance and observed in thick and thin films of SiCl₄. With monoenergetic electrons, specific surface reactions can be controlled which have implications for film growth, surface patterning and masking, and etching. For the H₂O/TiO₂ (110) system, the water interactions with the TiO₂ surface are revealed through the strong electron induced reaction dependencies on the water coverage. Understanding the nonthermal reaction landscape of H₂O on the TiO₂ (110) surface is crucial for developing the system as a catalytic source of hydrogen. The electron-stimulated oxidation of the TiO₂ (110) surface and electron induced sputtering of H ₂O was investigated. Irradiation of water films ([coverage]< 3 ML) oxidized the TiO₂ (110) surface similarly as surface oxidation via O₂ deposition. Each H₂O molecule in the first monolayer seems to be a target for the incoming electron initiating the oxidation. However, water coverages greater than a monolayer limited the oxidation process. The electron-stimulated desorption and sputtering yields of water from the TiO₂ (110) surface were measured as a function of water coverage. Surprisingly, the amount of water sputtered from the surface is nonlinearly dependent on water coverage.

Abstract : The major objective of our study is to investigate DNA damage induced by soft X-rays (1.5 keV) and low-energy electrons (˂ 30 eV) using a novel irradiation system created by Prof. Sanche’s group. Thin films of double-stranded DNA are deposited on either glass and tantalum substrates and irradiated under standard temperature and pressure surrounded by a N[subscript 2] environment. Base release (cytosine, thymine, adenine and guanine) and base modifications (8-oxo-7,8-dihydro -2’-deoxyguanosine, 5-hydroxymethyl-2’-deoxyuridine, 5-formyl-2’-deoxyuridine, 5,6-dihydrothymidine and 5,6-dihydro-2’-deoxy uridine) are analyzed and quantified by LC-MS/MS. Our results reveal larger damage yields in the sample deposited on tantalum than those on glass. This can be explained by an enhancement of damage due to low-energy electrons, which are emitted from the metal substrate. From a comparison of the yield of products, base release is the major type of damage especially for purine bases, which are 3-fold greater than base modifications. A proposed pathway leading to base release involves the formation of a transient negative ion (TNI) followed by dissociative electron attachment (DEA) at the N-g lycosidic bond. On the other hand, base modification products consist of two major types of chemical modifications, which include thymine methyl oxidation products that likely arises from DEA from the methyl group of thymine, and 5,6-dihydropyrimidine that can involve the initial addition of electrons, H atoms, or hydride ions to the 5,6-pyrimidine double bond. / Résumé: L'objectif majeur de ce projet étude est d'étudier les lésions d'ADN induites par les rayons X mous (1,5 keV) et des électrons de faible énergie (˂ 30 eV) à partir d'un nouveau système d'irradiation créé par le groupe du Pr. Sanche. De minces couches d'ADN double brin sont déposées soit sur du verre ou sur les substrats de tantale. Celles-ci sont irradiées sous une température et pression environnante, mais dans une atmosphère de N[indice inférieur 2]. Les bases relâchées (cytosine, la thymine, l'adénine et la guanine) et les produits de modification de base (8-oxo-7,8-dihydro-2'-désoxyguanosine, 5-hydroxyméthyl-2'-désoxyuridine, 5-formyl-2'-désoxyuridine, 5,6-dihydrothymine et 5,6-dihydrouridine) sont analysés et quantifiés par LC-MS/MS. Nos résultats révèlent un plus grand rendement de dommages dans les échantillons déposés sur le tantale que celles sur le verre. Cette différence peut être expliquée par l’interaction des électrons de faible énergie qui sont photo émis des substrats métalliques. D'après les résultats obtenus, la libération de bases est un produit majeur en comparaison avec la modification de bases. Ceci provient, en particulier, surtout des purines qui libèrent la base a un niveau trois fois plus grand que la modification de la base. Une voie proposée, conduisant à la libération de base, implique la formation d'ions négatifs transitoires (TNI), suivie par l'attachement d'électrons dissociatifs (DEA) à la liaison N-glycosidique. En outre, les produits de modification de base sont composés en deux grands types de modifications chimiques. L’un des produits est l’oxydation du groupe méthyle de la thymine, qui probablement consiste de en d'hydrure (-H[indice supérieur -]) par l'intermédiaire de DEA. Alors que l’autre modification chimique est la formation de 5,6-dihydropyrimidine qui implique l'addition d'hydrure à la double liaison du 5,6-pyrimidine.

This dissertation has investigated the adsorption, thermal behavior, and radiation (both photon and electron) processing of prebiotically-relevant ices and surfaces. A custom ultra-high vacuum (UHV) chamber has been built that is coupled with a Fourier Transform-Infrared (FT IR) spectrometer and a Temperature Programmed Desorption (TPD) system that utilizes Quadrupole Mass Spectrometry (QMS) to study selected organic:surface systems. Formamide (HCONH₂) has been studied in two related but distinct studies relevant to primitive Earth and interstellar chemistry. First, in collaboration with a theory group, formamide’s interaction with kaolinite (Al6Si6O36H30), a clay mineral relevant to early Earth chemistry, has been studied experimentally and theoretically. Experimental infrared results are compared with calculated infrared frequencies obtained by our collaborators. TPD analysis is compared with the calculated values of adsorption energy, and the optimal kaolinite termination site for adsorption is reported. Second, the first thermal and radiation damage study of pure formamide and HCONH₂:H₂O mixed ices on an interstellar icy grain analog (SiO₂) is reported. A discussion of the pure formamide ice phases identified with FT-IR upon warm-up, as well as the TPD binding energies of HCONH₂ on SiO₂, is presented. The observed Lyman-alpha photochemical products and proposed formation mechanisms from pure formamide ice is reported and discussed. In addition, results of Lyman alpha processing of mixed HCONH₂:H₂O ices are provided. Low-energy electron irradiation of pure HCONH₂ and HCONH₂:H₂O mixed ices has also been reported for the first time. A third investigation has studied acetylene (C₂D₂) and acetonitrile (CH₃CN) interactions and radiation stability in mixed low-temperature ices to simulate possible prebiotic reactions that may occur on Saturn’s moon, Titan. This investigation contributes to understanding the possible consumption, trapping, and degradation of these species on the surface of Titan.

Lorsque la matière condensée est soumise à des rayonnements de haute énergie, des électrons secondaires de basse énergie (0-20 eV) sont produits en grande quantité. Ces électrons participent à part entière aux dommages induits dans la matière, incluant les processus d’érosion et de modifications chimiques. Les fragments produits au sein du milieu réagissent et de nouvelles espèces sont formées. Plusieurs domaines d’application sont concernés par ces processus, et plus particulièrement le design de dispositifs par lithographie ou par dépôts assistés par faisceaux focalisés et l’astrochimie. Les enjeux concernent l’identification des mécanismes induits par les électrons lents, le contrôle des fragments réactifs et espèces stables formés, ainsi que la détermination de grandeurs quantitatives permettant d’apprécier l’efficacité des processus impliqués. L’approche développée dans ce travail de thèse consiste à irradier des surfaces et interfaces directement avec des faisceaux d’électrons de basse énergie afin d’étudier les processus induits. Les réponses de films moléculaires supportés modèles (d’épaisseur variable) sont étudiées en fonction de l’énergie incidente des électrons et des doses délivrées. Dans les cas favorables, des méthodologies ont pu être proposées pour accéder à l’estimation de sections efficaces effectives. Pour ce faire, trois techniques expérimentales sont combinées. Les films déposés et les résidus formés sont analysés par spectroscopie de perte d'énergie d’électrons à haute résolution (HREELS) et désorption programmée en température (TPD). Les fragments neutres (et non pas ioniques comme le plus souvent) désorbant sous irradiation sont analysés en masse afin de mener une étude de désorption stimulée par impact d’électrons (ESD).Dans le contexte de la fonctionnalisation de surface, le greffage de centres carbonés hybridés sp2 sur un substrat de diamant poly-cristallin hydrogéné a été réalisé par irradiation électronique d’une couche mince de benzylamine. A 11 eV, le mécanisme dominant implique la dissociation en neutres du précurseur. La section efficace effective de greffage a pu être déterminée par HREELS suite à une unique irradiation, en tirant avantage du profil du faisceau d’irradiation. Dans le contexte de l’astrochimie, la réponse à l’irradiation par électrons lents de glaces d’ammoniac amorphes et cristallisées a été étudiée. La désorption de molécules d’ammoniac a été observée. Elle peut résulter de l’érosion directe du film et de mécanismes de désorption induite par excitation électronique (DIET). Différents processus de fragmentation/recombinaison ont été mis en évidence via la désorption des espèces neutres NHx (x = 1,2), H2 et N2. Une chimie particulièrement riche est induite par irradiation électronique à 13 eV. L’analyse temporelle des rendements ESD a permis la détermination de la section efficace de la désorption de NH3, et l’observation de la formation retardée de N2 et H2. L’analyse TPD des résidus a démontré la synthèse de diazène (N2H2) et d’hydrazine (N2H4) dans le film. Ces résultats peuvent aider à l’élucidation des écarts observés dans les abondances de NH3 et N2 dans les régions denses de l'espace. Enfin, les premiers travaux réalisés pour fonctionnaliser un substrat de façon résolue à l’échelle micrométrique sous irradiation d’électrons lents sont également présentés. La faisabilité de la procédure utilisant un microscope électronique à basse énergie (LEEM) a été démontré sur une monocouche de terphenylthiol (TPT). Des motifs de 5 μm de travaux de sortie différents ont été imprimés en travaillant à des énergies de 10-50 eV. Ensuite la réponse de films modèles de résines lithographiques (PMMA, polyméthacrylate de méthyle) à des irradiations électroniques a été étudiée, afin d’identifier les énergies favorables en vue d’une modification de surface résolue spatialement. / High-energy irradiation of condensed matter leads to the production of copious amounts of low-energy (0-20 eV) secondary electrons. These electrons are known to trigger various dissociative processes leading to observed damages including erosion and chemical modifications. The resulting reactive species within the condensed media can also lead to the synthesis of new molecules. This has implications in several applications most especially in the design of lithographic methods, focused beam-assisted deposition, as well as in astrochemistry. In all these applications, it is important to identify the processes induced by low-energy electrons, study the reactive fragments and stable molecules produced to determine possibilities of controlling them, and generate quantitative data to gauge the efficiencies of these processes. The approach developed for this PhD work consists of directly irradiating surfaces and interfaces using low-energy electrons and studying the processes that arise. The responses of different model molecular films (of varying thickness) were studied as a function of incident electron energy and dose. In favorable cases, methodologies proposed herein can be used to estimate effective cross sections of observed processes. Three complementary surface-sensitive techniques were utilized for this purpose. To characterize the deposited films and formed residues, the High Resolution Electron-Energy Loss Spectroscopy (HREELS) and Temperature Programmed Desorption (TPD) were used. Neutral fragments (as opposed to their often-detected ionic counterparts) desorbing under electron irradiation were monitored using a mass spectrometer in a technique called Electron Stimulated Desorption (ESD).Within the context of surface functionalization, the grafting of sp2-hybridized carbon centers on a polycrystalline hydrogenated diamond substrate was realized through electron irradiation of a thin layer of benzylamine precursor deposited on its surface. At 11 eV, the dominant mechanism is proposed to be neutral dissociation of the precursor molecules. The effective cross section of the grafting process was estimated in only a single measurement from the HREELS map of the sample surface, taking advantage of the electron beam profile. Within the context of astrochemistry, on the other hand, the responses of crystalline and amorphous NH3 ices were studied under electron impact. The desorption of intact NH3 was observed which resulted in the direct erosion of the film proceeding through a mechanism consistent with desorption induced by electronic transitions (DIET). Different fragmentation and recombination processes were also observed as evidenced by detected neutral species like NHx (x=1,2), N2, and H2. Aside from desorption, a wealth of chemical processes was also observed at 13 eV. Temporal ESD at this energy allowed for the estimation of the effective cross section of NH3 desorption and observing the delayed desorption of N2 and H2. TPD analysis of the residues also provided evidence of N2H2 and N2H4 synthesis in the film. These results can help explain the observed discrepancies in abundances of NH3 and N2 in dense regions in space. Lastly, this PhD work will present prospects for these electron-induced processes to be constrained spatially in microscopic dimensions for lithographic applications. The feasibility of the procedure utilizing Low-Energy Electron Microscope (LEEM) was demonstrated on a terphenylthiol self-assembled monolayer (TPT SAM) specimen. Spots of 5 μm in diameter with different work functions were imprinted on the surface using energies from 10-50 eV. Electron-induced reactions in thin-film resists (PMMA, poly(methyl methacrylate)) were also studied at low-energy identifying opportunities for energy- and spatially-resolved surface modification.