Nouvelle méthode de dosage du radon dans l'eau par scintillation dans l'ortho-xylène

Une nouvelle méthode de dosage du radon dans l'eau a été mise au point. Elle consiste à purger le radon par l'argon, à le dissoudre dans l'o-xylène contenant les scintillateurs organiques 2,5 diphényloxazole (PPO) et 2,2' p-phénylènebis-5-phényloxazole (POPOP) et à compter les particules alpha et bêta par scintillation induite. La méthode est rapide, relativement simple, reproductible et sans interférences. Elle est très sensible; la limite de détection est de 0,01 Bq/L pour un échantillon de 2 litres et un temps de comptage de 1 heure. La méthode a été utilisée pour mesurer les concentrations du radon dans l'eau de certains puits au Québec. Celles-ci varient de 1,6 à 627 Bq/L. Ces concentrations sont similaires à celles mesurées dans d'autres pays.A new method for the determination of radon in ground water has been developed. It consists of purging radon with argon and dissolving it in 180 mL of ortho-xylene containing the organic scintillators 2,5 diphenyloxazole (PPO) and 2,2' p-phenylenebis-5-phenyloxazole (POPOP). The scintillation yield of radon in o-xylene, toluene, benzene and hexane has been measured at - 20, 0 and + 20 °C and the highest efficiency was obtained in o-xylene at - 20 °C.The concentration of PPO in o-xylene was varied and the maximum yield occurs at a concentration of 4 g/L. The optimal purging volume of argon was Pound to be 3 liters. The counting efficiency increases from 2.7 to 3.9 counts per disintegration after 3 hours decay time due to the build-up of radon daughters and then Starts to decrease with the hall-life of radon (3.824 days). The detection limit at 95 % confidence level for a 2 liter sample and for 1 hour counting time is 0.01 Bq/L.The method is virtually interference free. Due to the argon purging only radioactive gases can interfere with the measurement. Among these, thoron (Rn-220) and action (Rn-219) will decay before counting due to their short hall-lifes (56s and 4s resp.). The concentration of Kr-85 in air is low (0.0006 Be) when compared with radon, and only 0.05 % of the Kr-85 is transfered to soil and ground water. Kr-85 is also 35 times less soluble than Rn-222 in toluene. The decay products of radon will remain in the water and it is not necessary to know the degree of equilibrium between them and Rn-222. Consequently the counting can begin soon (10 minutes) after the purging of radon and it is not imperative to wait for equilibrium.The method was used to determine the concentration of radon in ground water in Quebec. The activities Pound ranged tram 1.6 to 627 Bq/L; they are similar to these measured in other countries

Silicon Atomic Quantum Dots Enable Beyond-CMOS Electronics

We review our recent efforts in building atom-scale quantum-dot cellular automata circuits on a silicon surface. Our building block consists of silicon dangling bond on a H-Si(001) surface, which has been shown to act as a quantum dot. First the fabrication, experimental imaging, and charging character of the dangling bond are discussed. We then show how precise assemblies of such dots can be created to form artificial molecules. Such complex structures can be used as systems with custom optical properties, circuit elements for quantum-dot cellular automata, and quantum computing. Considerations on macro-to-atom connections are discussed.Comment: 28 pages, 19 figure

Inferring marine distribution of Canadian and Irish Atlantic salmon (Salmo salar L.) in the North Atlantic from tissue concentrations of bio-accumulated Caesium 137

This is a pre-copy-editing, author-produced PDF of an article accepted for publication in ICES Journal of Marine Science following peer review. The definitive publisher-authenticated version “Spares A.D., Reader J.M., Stokesbury M.J.W., McDermott T., Zikovsky L., Avery T.S., Dadswell M.J. Inferring marine distribution of Canadian and Irish Atlantic salmon (Salmo salar L.) in the North Atlantic from tissue concentrations of bio-accumulated caesium 137. (2007) ICES Journal of Marine Science, 64 (2), pp. 394–404” is available online at: http://icesjms.oxfordjournals.org/content/64/2/394peer-reviewedAtlantic salmon returning from marine migrations to eastern Canada and western Ireland during 2002 and 2003 were analysed for tissue concentrations of bio-accumulated caesium 137 (137Cs). Salmon from Canadian and Irish waters demonstrated concentrations (0.20 ± 0.14 Bq kg-1 and 0.19 ± 0.09 Bq kg-1, mean ± s.d., respectively) suggesting similar oceanic feeding distributions during migration. Canadian aquaculture escapees had a similar mean tissue concentration (0.28 ± 0.22 Bq kg-1), suggesting migration with wild salmon. However, significantly higher concentrations in 1-sea-winter (1SW) escapees (0.43 ± 0.25 Bq kg-1) may alternatively suggest feeding within local estuaries. High concentrations in some Canadian 1SW salmon indicated trans-Atlantic migration. Low concentrations of Canadian multi-sea-winter (MSW) salmon suggested a feeding distribution in the Labrador and Irminger Seas before homeward migration, because those regions have the lowest surface water 137Cs levels. Estimates of wild Canadian and Irish salmon feeding east of the Faroes (~8oW) were 14.2% and 10.0% (1SW, 24.7% and 11.5%; MSW, 2.9% and 0.0%), respectively. We propose that most anadromous North Atlantic salmon utilize the North Atlantic Gyre for marine migration and should be classified as a single trans-Atlantic straddling stock

Étude de la modification par illumination de la structure du silicium amorphe hydrogéné

Le silicium amorphe hydrogéné -- Processus physiques durant le dépôt -- Cellules photovoltaïques -- L'effet staebler-Wronski -- Résumé des théories sur l'effet SW -- Lumière pulsée -- La spectroscopie photoacoustique -- Théorie Rosencwaig-Gersho -- Spectroscopie à transformée de Fourier -- L'absorption infrarouge du A-Si:H -- Simulation du signal photoacoustique -- Méthode de mesure photoacoustique -- Résultats expérimentaux en photoacoustique -- Déconvolution du spectre PA pour matériau bot-wire -- Déconvolution du spectre PA pour matériau hot-wire -- Déconvolution du spectre PA pour matériau glow-discharge -- Évolution du spectre IR sous illumination, matériau hot-wire -- Évolution du spectre IR sous illumination, matériau glow-discharge -- Comparaison au calcul analytique -- Amélioration possibles des mesures photoacoustiques -- Mesures des énergies d'activation de conductivité du A-S:H -- Nanocalorimétrie

Trace analysis with protons [i.e. proton] and heavy ion activation

Vita.The capabilities of 12 MeV proton activation were compared with those of reactor neutron and 35 MeV photon activation. A series of biological specimens (plant and animal tissues) were chosen for this study limited to nondestructive assays. Based on y-ray spectrometry, As, Ca, Cu, Fe, Mo, Pt, Sr, Ti, Zn and Zr at levels ranging from 2 to 20,900 ppm were detected following proton activation of 1 hour. Al, Br, Ca, Cl, Cu. Mg, Mn, Rb and V. ranging from 0.4 to 20,900 ppm, were measured by neutron activation (1 min. irradiation). As, Ba, Br, Cr, Co, Fe, Hg, La, Na, Rb, Sb and Zn, .ranging from 0.2 to 2400 ppm, were determined following a 14 hour neutron irradiation. Photon activation data was obtained by computer simulation of y-ray spectra, based on published radioisotope production yields. Assuming a 4 hour irradiation, As, Ba, Br, Ca, Cl, Fe, I, Mg, Mn, Na, Pb, Rb, Sb, Sr could be detected at concentration levels ranging from 3 to 20,900 ppm. Although covering different elements, the three techniques are comparable in their scope, i.e. detection limits that can be achieved and number of elements that can be detected simultaneously. In terms of determining known essential elements, detection limits below the average concentrations of such elements in the human body were obtained for Co, Fe, Zn with neutron activation, for Cu, Fe, Zn with proton activation and for Fe, I, Zn with photon activation.