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This paper focus on the corrosion behaviour of austenitic 316L stainless steel in PWR primary water under the influence of radiolysis, including experimental devices and main obtained results. After a short overview of the literature in the field, the approach is described with the use of a high energy proton beam to control the production of radiolytic species at the interface between stainless steel (316L type) and PWR water solution in a high temperature and high pressure electrochemical cell working up to 300°C and 100 bar. The corrosion potential of stainless steel was enhanced by the radiolysis causing by high energy proton beam. A high similarity of electrochemical behaviour was also observed under electron beam. The electrochemical oxidative response of the 316L/PWR solution interface under radiolysis is related to the surface characterization analysis (SEM, XPS, Raman spectroscopy, NRA...) on the oxide layers of 316L which are formed under or without irradiation. The radiolysis effect on the oxide film includes micron scale cavities which were observed in a highly irradiated oxide film. The observation of a-Fe2O3 hematite on the outer oxide film where cavities were formed is in accordance with the electrochemical oxidative response. Radiolysis influences not only the electrochemical behaviour but also the oxide formation of stainless steel in high temperature water environments, like primary water of Pressurised Water Reactors (PWR).

International audience; The ground penetrating radar WISDOM on board the Rover of the ExoMars 2022 mission (ESA/Roscosmos) will be a pioneer in the exploration of the Martian subsurface from the surface (until now, Martian sounding radars have been operated from orbit). WISDOM will image the first meters below the surface of Oxia Planum — the ExoMars 2022 landing site — with the objectives of revealing its geological history and identifying safe and promising scientific targets for subsurface sampling by the Rover drill. In this paper, we present the qualification, characterization and calibration tests that have been conducted on WISDOM flight model in order to assess its performance, build the data processing pipeline and prepare scientific return of this experiment. In most favorable but geologically plausible cases (low loss and homogeneous subsurface, smooth interface), WISDOM can detect a buried interface down to a depth of 8 m with a vertical resolution of 3 cm (for a subsurface dielectric constant of 4). Its penetration depth is typically 2 m in less favorable environments. For safety reason, WISDOM antennas are accommodated 38 cm above the ground; the amplitude of the surface echo will be used to estimate the top layer dielectric constant with an accuracy of 13% which translates into an accuracy of 6% on the distance/depth assessment. WISDOM data processing chain includes corrections aiming at removing parasitic signals of various origins (electronic coupling, antenna crosstalk, multiple surface echoes, etc.) and at correcting the data to a reference temperature and antenna elevation; it has been designed to automatically produce calibrated radargrams in less than 20 min as required for the mission operations. Additional more sophisticated processing will be manually run in parallel. The impact of the Rover structure on measurements has been investigated and can be partially removed.

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We report experiments in which positronium (Ps) atoms were created in thin, porous silica films containing isolated voids with diameters ranging from 5 to 75 nm. Ps lifetimes in the pore structures were measured directly via time-delayed laser excitation of ${1}^{3}{S}_{1}\ensuremath{\rightarrow}{2}^{3}{P}_{J}$ transitions. In a film containing 5-nm pores Ps was predominantly emitted into vacuum, with a small component of confined Ps with a lifetime of 75 ns also observed. In films with larger pores Ps atoms were not emitted into vacuum except from the film surface, and confined Ps lifetimes of $\ensuremath{\approx}90$ ns were measured with no dependence on the pore size. However, for such large pores, extended Tao-Eldrup (ETE)-type models predict Ps lifetimes close to the 142-ns vacuum value. Moreover, ${1}^{3}{S}_{1}\ensuremath{\rightarrow}{2}^{3}{P}_{J}$ excitation of Ps atoms inside the pores was found to result in annihilation and exhibited an extremely broad $(\ensuremath{\approx}10$ THz) linewidth. We attribute these observations to a process in which nonthermal Ps atoms in the isolated voids become temporarily trapped in a series of surface states that dissociate following excitation. The occurrence of this mechanism is not necessarily apparent from ground-state Ps decay rates without some prior knowledge of the sample structure, and it precludes the application of ETE-type models as they do not take into account surface interactions other than pickoff annihilation.

The electrochemical behaviour of 316L stainless steel has been investigated in representative primary PWR environment under proton irradiation. The electrochemical potential of 316L stainless steel and the environmental parameters (temperature, hydrogen pressure, etc.) have been measured continuously under irradiation. Highly reproductive electrochemical results have been observed with fast increases of the potentials under irradiation. This oxidative response increases with increasing flux and decreases with increasing hydrogen pressure. Depending on the temperature, the oxidative response can either be reduced or enhanced. Moreover, the synergetic effect of thermal ageing and cumulated fluence on 316L stainless steel can influence the electrochemical potential with or without irradiation.

We have studied the variation in the work function of the surface of sputtered cleaned 316L stainless steel with only a very thin residual oxide surface layer as a function of grain orientation using X-ray photoelectron emission microscopy (XPEEM) and Electron Backscattering Diffraction. The grains are mainly oriented [1 1 1] and [1 0 1]. Four distinct work function values spanning a 150 meV energy window are measured. Grains oriented [1 1 1] have a higher work function than those oriented [1 0 1]. From core level XPEEM we deduce that all grain surfaces are Cr enriched and Ni depleted whereas the Cr/Fe ratio is similar for all grains. The [1 1 1] oriented grains show evidence for a Cr2O3 surface oxide and a higher concentration of defective oxygen sites.

This Letter relates the clear evidence of Yb

In situ Raman scattering studies allow following real‐time evolutions of volume or surface structures under extreme conditions. In nuclear materials sciences, ion irradiation‐induced atomic organization modification and water radiolysis are of a major interest. In order to better understand these phenomena, we have developed an in situ versatile portable Raman spectroscopy system coupled with a cyclotron accelerator, allowing monitoring of a solid/liquid interface under irradiation and thus giving access to effects of radiolysis. The different parts of the system and their improvements are described in details. The system efficiency is highlighted by a comparative study of the time dependence of UO2 surface modification induced, on one hand by contact with water under irradiation by 5 MeV He2+ particles, and on the other hand by pure chemical alteration, through contact with a hydrogen peroxide solution.

Few data are available in the literature on the role of the water radiolysis on the corrosion of stainless steel core components in PWR operating conditions (300 °C, 155 bar). The present approach uses a high energy proton beam to control the production of radiolytic species at the interface between a stainless steel sample and water in a high temperature and high pressure (HP–HT) electrochemical cell working in the range 25 °C/1 bar–300 °C/90 bar. The cell is designed to record the free corrosion potential of the AISI 316L/water interface mounted in line with a cyclotron delivering the proton beam. The evolution of the potential is compared before, during and after the proton irradiation. The first results are obtained with an aqueous solution containing boron, lithium and dissolved hydrogen, as in PWR primary coolant circuit. The stainless steel/water interfaces are irradiated between 25 °C and 300 °C with protons emerging at 22 MeV at the interface. The flux is varied by five orders of magnitude, from 6.6 × 10 11 to 6.6 × 10 15 H + m −2 s −1 . The evolution of the free corrosion potential is highly dependent on the temperature and/or pressure. For a given temperature and pressure, it evolves with the flux and the ageing of the AISI 316L/water interfaces. An important role of the temperature of irradiation on the electrochemical response was observed. These results give a better understanding of the role of radiolysis on stainless steel corrosion in high temperature conditions.

Positron annihilation gamma energy distribution, lifetime spectroscopy and time-of-flight method were used to study surfactant-templated mesoporous silica films deposited on glass. The lifetime depth profiling was correlated to Doppler broadening and 3γ annihilation fraction measurements to determine the annihilation characteristics inside the films. A set of consistent fingerprints for positronium annihilation, o-Ps reemission into vacuum, and pore size was directly determined. The lifetime measurements were performed in reflection mode with a specially designed lifetime spectrometer mounted on a slow positron beam system. The intensity of the 142 ns vacuum lifetime component was recorded as a function of the energy of the positron beam. In a film with high porosity a reemission efficiency of as high as 40 % was found at low positron energy. Positron lifetime in samples capped by a thin silica layer was used to determine the pore size. The energy of the reemitted o-Ps fraction was measured by a time-of-flight detector, mounted on the same system, allowing determination of both o-Ps re-emission efficiency and energy in the same sample. We demonstrate the potential of the simultaneous use of different positron annihilation techniques in the study of thin porous films.

Model experiments are reported where proton beams delivered by the cyclotron located at CERI (CNRS-Orleans) are used for irradiating AISI 316L/water and Au/water high purity interfaces with 6 MeV protons. The free exchange potentials at the interfaces are recorded as a function of time at room temperature in situ before, under, and after proton irradiation. The evolutions are compared to those calculated for the Nernst potentials associated with the radiolytic RedOx couples. It is shown how the comparison gives evidence that five radiolytic species – O2 , H2O2, HO2−, HO2 and O2 − – exchange electrons at the Au interfaces in a range of dose rates that vary over three orders of magnitudes, i.e. 0.0048 and O2 − radical disproportionations play a key role and control the potential at the interfaces under 6 MeV proton flux. This role is generally mostly overlooked for gamma irradiation.

In this work both 22Na based positron lifetime spectroscopy (PALS) and slow positron beam based Doppler annihilation-ray broadening spectrometry (SPBDB) have been used to characterize respectively the bulk and the first micron under the surface of sintered UO2 disks that have been polished and annealed at high temperature (1700 °C/24 h/ArH2). Results show the presence of negative ions that are tentatively identified to negatively charged oxygen atoms located in interstitial sites. The positron annihilation characteristics in the UO2 lattice have been determined and are equal to SL(UO2) = 0.371(5), WL(UO2) = 0.078(7), τL(UO2) = 169 ± 1 ps. Such disks have been irradiated at room temperature with electrons and α particles at different fluences. After irradiation SPBDB and PALS measurements show the formation of U-related vacancy defects after a 2.5 MeV electrons irradiation whereas no defects are detected for an irradiation at 1 MeV. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Evolution of vacancy defects as a function of temperature was studied by positron annihilation spectroscopy in 1 MeV helium implanted sintered UO2 disks. A slow positron beam coupled with a Doppler broadening spectrometer was used to measure the low and high momentum annihilation fractions, S and W respectively, as a function of positron energy E in UO2 disks implanted at the fluence of 1 × 10163He cm−2 before and after annealing. The S(E) and W(E) behaviours change with the annealing temperature in the range from 160 to 1300 °C indicating that the vacancy defects distribution evolves. The positron trapping rate at vacancy defects decreases with the increasing annealing temperature with two stages around 250 °C and 800 °C. We will discuss the phenomena that could explain these annealing stages in comparison with other positron annihilation spectroscopy results and with the 3He depth profile evolution as the function of temperature measured by Nuclear Reaction Analysis.

A slow positron beam coupled with Doppler broadening (DB) spectrometer was used to measure the low- and high-momentum annihilation fractions, S and W, respectively, as a function of positron energy in UO2 disks implanted with different 1 MeV 3He fluences and annealed in ArH2 or in vacuum. The S(E) and W(E) behaviors indicate that for fluences in the range from 2 × 1014 to 2 × 1016 3He cm−2, the vacancy defects distribution evolves with the annealing temperature in the range from 264 to 700 °C under ArH2. This evolution is found to be dependent on the 3He fluence implanted in the sintered UO2 disks. For the lowest fluence of 2 × 1014 3He cm−2, the S(W) plot with positron energy as the running parameter suggests that only the concentration of vacancy defects decreases when annealing temperature increases. For the highest implantation fluences (from 5 × 1015 to 2 × 1016 3He cm−2) the S(W) plot suggests that the nature of the vacancy defects changes in the annealing temperature range from 260 to 400 °C. Measurements performed in implanted UO2 disks annealed in vacuum have revealed a partial recovery of the vacancy defects possibly due to their recombination with mobile oxygen interstitials. The role of the hydrogen infusion into the disk is also discussed.

Vacancy defects have been investigated in sintered polished and annealed uranium oxide disks. Slow positron beam coupled with Doppler broadening spectrometer was used to probe the track region of 1 MeV 3 He ions implanted in uranium dioxide (UO 2 ) disks. The low and high momentum annihilation fractions, S and W , respectively, were measured in the first micrometer near surface region of the disks as a function of positron energy. The S and W values indicate that the 1 MeV He ions induce vacancy defects in the track region of their range. The vacancy defect depth distribution is heterogeneous. The positron trapping at these vacancy defects increases with the depth and with the implantation fluence indicating an increase of the vacancy defect concentration. The nature of the induced vacancy defects does not change with the fluence.

We have implemented a Monte Carlo simulation method to study electron emission from solid surfaces under electron irradiation at energies of up to 100 keV. Calculations are performed for the yields and energy distributions of the emitted electrons in backscattering and transmission geometries for Al and Au metals. For transmission studies the film thicknesses range from a few nanometres to micrometres. The method is demonstrated to predict correctly the electron backscattering and transmission properties, and gives a qualitatively reasonable description of the low-energy (E < 50 eV) secondary electron emission. For the exit surface of thin films we predict a universal behaviour according to which the maximum low-energy secondary electron yield is obtained when ~50–70% of the incident electrons are transmitted through the film. This corresponds to film thicknesses of ~0.75 times the mean depth of penetration of the electron with the same kinetic energy in a bulk target.

The option of direct disposal of spent nuclear fuel in a deep geological formation raises the need to investigate the long-term behavior of the UO2 matrix in aqueous media subjected to α-β-γ radiation. The β-γ emitters account for most of the activity of spent fuel at the moment it is removed from the reactor, but diminish within a millennial time frame by over three orders of magnitude to less than the long-term activity. The latter persists over much longer time periods and must therefore be taken into account over a geological disposal time scale. Leaching experiments with solution renewal were carried out on UO2 pellets doped with alpha emitters (238Pu and 239Pu) to quantify the impact of alpha irradiation on UO2 matrix alteration. Three batches of doped UO2 pellets with different alpha flux levels (3.30×104, 3.30×105, and 3.2×106 α cm-2 s-1) were studied. The results obtained in aerated and deaerated media immediately after sample annealing or interim storage in air provide a better understanding of the UO2 matrix alteration mechanisms under alpha irradiation. Interim storage in air of UO2 pellets doped with alpha emitters results in variations of the UO2 surface reactivity, which depends on the alpha particle flux at the interface and on the interim storage duration. The variation in the surface reactivity and the greater uranium release following interim storage cannot be attributed to the effect of alpha radiolysis in aerated media since the uranium release tends toward the same value after several leaching cycles for the doped UO2 pellet batches and spent fuel. Oxygen diffusion enhanced by alpha irradiation of the extreme surface layer and/or radiolysis of the air could account for the oxidation of the surface UO2 to UO2+x. However, leaching experiments performed in deaerated media after annealing the samples and preleaching the surface suggest that alpha radiolysis does indeed affect the dissolution, which varies with the flux at the UO2/water interface.

The behaviour of helium implanted in sintered uranium dioxide disks has been investigated as a function of annealing temperature. UO2 disks have been implanted with 1 MeV 3He ions at a fluence of 1 × 1016 3He cm−2 using a Van der Graaff accelerator. These implantation conditions lead to a local helium concentration of 0.2 at.% at a depth of 1.9 μm in UO2. The 3He(d,α)1H Nuclear Reaction Analysis method was used to determine the helium depth profile after the various annealing stages. The experimental results measured after 1100 °C anneal were analysed using a simple model which satisfactorily reproduces the observed helium depth profile changes. The intragranular helium diffusion coefficient is estimated and the result is assessed against other data published in the open literature.

Profiling of 3He implanted in sintered uranium dioxide pellets was performed using 3He(d, α)1H Nuclear Reaction Analysis technique. To accurate profile measurements, the cross-section of the 3He(d, p)4He nuclear reaction was carefully measured with a relative standard deviation of 5%. The total cross-section represented by a fit function is compared with data from literature. With the described coincidence arrangement, we obtained a reduction by a factor ≈4 × 104 of the signal of deuterons elastically scattered in uranium and a total suppression of parasite signals in the region of helium profiling. This technique enables us to measure helium profile in UO2 a couple of microns below the surface with a resolution of 0.1 μm and a detection limit of 0.01 at.%.

Strains of enterobacteria that cause urinary tract infections are able to grow in urine with high tonicity. In such conditions bacterias adapt to osmotic forces by incorporation of osmoprotectant compounds including glycine betaine. Accumulation of toxic analogues in bacteria, using inducible betaine transporters, has been previously proposed for development of antibiotics. In this study we report antibacterial effect of two analogues against 82 strains of Escherichia coli isolated form urinary tract infections. Minimal inhibitory concentrations have been measured with and without osmotic stress. The betaine analogues have antibacterial effect against E. coli strains, but only in presence of an osmotic stress.

The behavior of helium implanted in sintered uranium dioxide disks has been investigated as a function of annealingtemperature. UO 2 disks have been implanted with 1 MeV 3 He at a nominal fluence of 5·10 16 3 Hecm 2 using a Van deGraaff accelerator. The 3 He(d,a) 1 H nuclear reaction analysis method was used to determine the helium depth profile inthe UO 2 disks. Partial flaking was observed after annealing at 500 C for local He concentration of 1 at.%. Afterannealing at 600 C flaking has affected the whole surface. The formation of helium bubbles is discussed. 2003 Elsevier B.V. All rights reserved. PACS: 66.30.h; 61.82.m; 81.05.Je 1. IntroductionThe release of fission gases, mostly Xe, from UO 2 and spent fuels has been extensively studied since themiddle of the 1960s [1–5]. The solubility of these gases isextremely low in UO 2 and, as a consequence, the gasatoms tend to precipitate into bubbles. In irradiatedfuels, a high density ( 10 17 cm 3 ) of small intragranularbubbles of about 2 nm uniformly distributed in thematrix are observed [6,7]. The size of the bubbles in-creases and the concentration decreases slightly withincreasing temperature and burnup. At higher burnupand/or temperatures a second bubble population is cre-ated with a larger mean diameter (10–20 nm) [7]. Theprecipitation of bubbles has also been demonstrated foruranium dioxide samples implanted with Kr and Xe evenat temperatures as low as 300–350 C [8,9].Fewer experiments have been performed on the be-havior of helium gas produced by alpha decay of acti-nides [10,11]. Yet the amount of helium produced afterirradiation are large in particular in the case of MOXfuels: the amount of helium produced in MOX (burnup47.5 GWd/tU) after 10000 years is evaluated at 6700cm

We have used slow positron beam-based positron lifetime spectroscopy to study positron diffusion in thick epitaxial n-type 6H-SiC layers. The layers are considerably thicker than the maximum positron penetration depth, and can therefore, be treated as homogeneous semi-infinite bulk material in an analysis including the time-dependent diffusion of a single group of probe particles. Temperature-dependent measurements show a reduction in positron diffusion at low temperatures, which has been interpreted by an increase in trapping to negatively charged defect states.

Integrin alphaIIb is a cell adhesion molecule expressed in association with beta3 by cells of the megakaryocytic lineage, from committed progenitors to platelets. While it is clear that lymphohemopoietic cells differentiating along other lineages do not express this molecule, it has been questioned whether mammalian hemopoietic stem cells (HSC) and various progenitor cells express it. In this study, we detected alphaIIb expression in midgestation embryo in sites of HSC generation, such as the yolk sac blood islands and the hemopoietic clusters lining the walls of the major arteries, and in sites of HSC migration, such as the fetal liver. Since c-Kit, which plays an essential role in the early stages of hemopoiesis, is expressed by HSC, we studied the expression of the alphaIIb antigen in the c-Kit-positive population from fetal liver and adult bone marrow differentiating in vitro and in vivo into erythromyeloid and lymphocyte lineages. Erythroid and myeloid progenitor activities were found in vitro in the c-Kit(+)alphaIIb(+) cell populations from both origins. On the other hand, a T cell developmental potential has never been considered for c-Kit(+)alphaIIb(+) progenitors, except in the avian model. Using organ cultures of embryonic thymus followed by grafting into athymic nude recipients, we demonstrate herein that populations from murine fetal liver and adult bone marrow contain T lymphocyte progenitors. Migration and maturation of T cells occurred, as shown by the development of both CD4(+)CD8- and CD4-CD8(+) peripheral T cells. Multilineage differentiation, including the B lymphoid lineage, of c-Kit(+)alphaIIb(+) progenitor cells was also shown in vivo in an assay using lethally irradiated congenic recipients. Taken together, these data demonstrate that murine c-Kit(+)alphaIIb(+) progenitor cells have several lineage potentialities since erythroid, myeloid, and lymphoid lineages can be generated.

The release of uranium in aerated deionized water at a uranium oxide/water interface under He2+ irradiation is investigated as a function of the fluence by using an external ion beam. A high-energy He2+ beam delivered by a cyclotron (CERI–CNRS) goes through the thin oxide and emerges in the water with 20 MeV energy. First results are reported here showing that the release of uranium increases by three orders of magnitude in aerated deionized water under high flux (⩾ 3.3×10 10 α cm −2 s −1 ).

Le comportement d'une interface UO 2 /eau sous irradiation a ete etudie en utilisant un faisceau d'ions 4 He 2+ produit par un cyclotron. Les effets de la radiolyse alpha sur l'alteration de UO 2 dans de l'eau aeree deionisee ont ete examines en caracterisant parallelement la chimie des solutions irradiees et la surface de UO 2 . Un effet du faisceau d'ions 4 He 2+ sur la chimie de la solution a ete observe: la quantite d'uranium relâchee augmente tandis que le pH diminue sous irradiation. De plus, de fortes concentrations de peroxyde d'hydrogene produites par radiolyse ont ete mesurees dans les solutions irradiees. La caracterisation de la surface du solide a permis de mettre en evidence la formation d'un produit d'alteration, identifie comme etant un peroxyde d'uranium hydrate (UO 4 .2H 2 O), lie directement a la presence en solution de fortes concentrations d'uranyl et de peroxyde d'hydrogene au voisinage de l'interface UO 2 /eau.

The RR5 monoclonal antibody (mAb) was obtained after immunization of mice with hemopoietic cells from chicken embryos. The cDNA encoding the protein recognized by RR5 was cloned using COS-7 cells transfected with an embryonic bone marrow (BM) cDNA library. The epitope recognized by the RR5 mAb was located on the non-polymorphic MHC class II beta-chain molecule. In the embryonic BM, RR5 labeled 50% of the c-kit expressing cells. Previous experiments have shown that the T-cell progenitors are present in the MHC class II(+)/c-kit(+) BM population along with myeloid progenitors and that T-cell and myeloid progenitors also express the integrin alphaIIbbeta3. In this study, using intrathymic cell transfer experiments in chicks, we have tested the T-cell differentiation potential of MHC class II/alphaIIbbeta3 double positive cells. It proved to be similar to that of the c-kit/MHC class II positive cells. However, injection of triple positive cells resulted in a selection of cells with an increased T-cell potential. Most of the MHC class II positive cells which do not express c-kit are prone to apoptosis, indicating that these progenitors might need a survival signal via c-kit. Interestingly, the MHC class II positive progenitors lose this expression after intrathymic transfer. Taken together our data suggest that the presence of the MHC class II beta-chain molecule on the surface of BM progenitor cells could be implicated in differentiation toward myeloid and lymphoid lineages.

We have used a low-energy positron beam to investigate the defects in N-doped and undoped ZnSe layers grown on GaAs:Si or ZnSe substrates. In N-doped ZnSe we observe positron trapping at vacancies, identified as VSeNSe complexes, and at negative ions. Based on positron data, we give estimates for the concentrations of vacancies and negative ions. The results suggest that in addition to forming isolated NSe acceptors or VSeNSe pairs, incorporated nitrogen also forms donor-type defects leading to compensation of holes. In undoped ZnSe layers, the results show presence of Zn vacancies in concentrations 10–10 cm. The Zn vacancy concentration correlates with the dislocation density at the ZnSe/GaAs interface.

We show that the detailed atomic structure of vacancy–impurity complexes in Si can be experimentally determined by combining positron lifetime and electron momentum distribution measurements. The vacancies complexed with a single impurity, V–P and V–As, are identified in electron irradiated Si. The formation of native vacancy defects is observed in highly As-doped Si at the doping level of 10 20 cm −3 . The defects are identified as monovacancies surrounded by three As atoms. The formation of V–As 3 complex is consistent with the theoretical descriptions of As diffusion and electrical deactivation in highly As doped Si.

We use a low-energy positron beam to study the influence of doping and stoichiometry on the native defects in GaAs grown by molecular-beam epitaxy at 250 °C. Ga vacancies are identified in all samples by measuring the momentum distribution of annihilating core electrons. The charge of VGa is negative in Si-doped samples but neutral in undoped and Be-doped material. We propose that the Ga vacancies are complexed with As antisites in undoped and Be-doped samples and with Si impurities in n-type material. The concentration of Ga vacancies depends on the doping and stoichiometry of growth conditions. It follows generally the trends in the VGa formation energy as a function of the Fermi level position and stoichiometry. The strong loss of free carriers in the As-rich Si-doped samples is attributed to the formation of Ga vacancy complexes, negative ion defects and inactive clusters of Si atoms.

Resume Chlamydia trachomatis (CT) est le microorganisme sexuellement transmissible le plus frequemment responsable d’infection genitale. Si la frequence de l’infection a CT est connue chez la femme enceinte et chez la femme a risque de maladies sexuellement transmissibles, peu de donnees existent chez l’adolescente non exposee en milieu africain. Nous avons etudie la prevalence des anticorps anti-CT dans une population de 295 adolescents (154 filles, 141 garcons) scolarises âges de 13 a 19 ans, parallelement a celle d’un groupe temoin constitue de 190 enfants âges de 6 a 12 ans. Les anticorps anti-CT apparaissent vers l’âge de 15 ans. La prevalence est de 6,1% dans cette population et elle predomine davantage chez la fille (10,4%) que chez le garcon (1,4%). Les donnees suggerent que l’adolescent apparemment asymptomatique est concerne par l’infection a CT et que les sujets doivent etre depistes et traites. La frequence chez l’adolescente merite toute l’attention car le jeune organisme est davantage predispose a des infections recidivantes et a leurs complications.

We show that the detailed atomic structure of vacancy-impurity complexes in Si can be experimentally determined by combining positron lifetime and electron momentum distribution measurements. The vacancies complexed with a single impurity, V -P and V -As, are identified in electron irradiated Si. The formation of native vacancy defects is observed in highly As-doped Si at the doping level of 1020 cm23. The defects are identified as monovacancies surrounded by three As atoms. The formation of a V -As3 complex is consistent with the theoretical descriptions of As diffusion and electrical deactivation in highly As-doped Si. [S0031-9007(99)08546-4]

The isochronal recovery of irradiation induced defects was investigated in γ-TiAl intermetallic compounds (with 50 and 54 at% Al) by positron lifetime measurements after 2.5 MeV electron irradiation at 21 K. In the as-irradiated condition, the analysis of the results and the comparison with published data led to a value τd=230±5 ps for the lifetime of vacancy-trapped positrons. The lifetime variations observed during isochronal anneals at increasing temperatures are consistent with vacancy migration around 450 K. The observation of a progressive decrease in the lifetime of trapped positron, during the migration and elimination of vacancies, suggests that they do not form unrelaxed three-dimensional clusters, and that another type of positron traps is simultaneously present.

Une etude du vieillissement du polypyrrole dope par du para-hydroxybenzene sufonate a ete effectuee par annihilation de positons. Le polymere conducteur est prepare par oxydation chimique et vieilli a 140°C a l'air durant 48 jours. Les resultats experimentaux sont interpretes par le piegeage du positon au voisinage des dopants actifs (charges negativement) dont la concentration diminue au cours du vieillissement conduisant a la perte de conductivite du polypyrrole.

The optical ionization of the silicon divacancy in 2-MeV electron-irradiated Si was studied by using positron-lifetime and positron-electron momentum distribution measurements under illumination with monochromatic light. Upon irradiation at room temperature, negative and neutral divacancies are detected in both float zone and Czochralski Si by positron-annihilation measurements in darkness. The positron-annihilation characteristics of the divacancy are determined as t d5300(5) ps51.35(2)3t b , Sd 51.055(3)3Sb , and Wd50.75(2)3Wb . Illumination at 15 K with monochromatic 0.70‐1.30 eV light has a strong effect on the positron trapping rate to the divacancies. The results can be understood in terms of optical electron and hole emission from the electron levels V 2l0 and V 22l2 of the divacancy. The changes in the positron trapping rate are due to the different sensitivities of the positron to the charge states V2 , V2 , and V 22 . The spectral shape of the positron trapping rate under illumination reveals an electron level at Ev 10.75 eV, which is attributed to the ionization level V22/2 of the divacancy. @S0163-1829~98!10819-6# Point defects are created in the silicon lattice during the crystal growth as well as in the processing of the material, e.g., by ion implantation. To introduce and study simple point defects in Si in a controlled way, one generally uses electron irradiation. The most important defects created upon irradiation and room-temperature annealing are the divacancy, vacancy-oxygen complexes, and vacancy dopantimpurity pairs. The abundances of these defects depend on the oxygen and dopant concentrations in the material. Their structures and the electron levels in the forbidden gap have been investigated by electron paramagnetic resonance ~EPR!, infrared ~IR! absorption, and deep-level transient spectros

We present a method based on positron annihilation to investigate defects at interfaces formed between a thin 10 - 50 A overlayer and a substrate. The method applied to 38 interfaces gives evidence that (i) high concentrations of divacancies exist in Si domains at the interfaces formed with natural oxides obtained after etching and that (ii) positrons see the same annihilation state at various interfaces where the oxides are either commercial, thermally grown, native or obtained after rapid thermal annealing treatments.

Ion-implantation is a widely used doping technique in II–VI semiconductors. Nevertheless, ion-implantation damage has to be better understood to properly control this process. In order to investigate the implantation-induced defects in such compounds, room-temperature implantations of 320 keV Al ions have been performed on crystalline samples of [111] Hg1−xCdxTe (x≈20%) for doses ranging from 1013 to 1015 cm−2. We report the first measurements of x-ray diffuse scattering close to different Bragg reflections on such as-implanted samples. The evolution of the diffuse intensity as a function of the dose has been observed. The defect-induced diffuse intensity arises mainly from interstitial dislocation loops. Nevertheless, vacancy loops are observed above 3×1014 Al/cm2. The mean radius of the dislocation loops increases in size by three to four times when the dose rises from 1013 to 1015 cm−2. Finally, the saturation of point defects has been observed independently of their clustering at about 5×1013 Al/cm2, that is in the same range as the saturation dose of the sheet electron concentration.

Doppler-broadening of the 511 keV positron annihilation line was used for defect identification in CdTe materials. In electrically compensated lightly n-type CdTe(In) and lightly p-type or semi-insulating CdTe(Cl) crystals positron lifetime measurements show vacancy defects with characteristic positron lifetimes of 323 ps and 370 ps, respectively. The shapes of the high-momentum parts of the measured electron-momentum distributions indicate that both defects contain a cadmium vacancy . The defects are assigned to vacancy-donor complexes and , respectively. A vacancy in MBE-grown CdTe(I) layers observed with a low-energy positron beam is also identified as a cadmium vacancy which is most likely complexed with I-donors.

Systematic correlation of infrared absorption and positron annihilation experiments allows us to identify two metastable defects in electron-irradiated GaAs. The first one is present already in as-grown material, and can be identified as the native EL2 defect by its photoquenching and subsequent thermal-annealing properties. The second one is introduced by irradiation at 300 K, and it disappears in thermal annealing at 520 K. Its metastable state is optically active, anneals at 70 K, and exhibits optical recovery at 25 K. Positron experiments indicate that both of these defects have a vacancy in their metastable state in good agreement with the vacancy-interstitial model of the As antisite defect. Therefore, the native defect is here attributed to an As antisite and the irradiation-induced one to an As antisite complex. The metastable state of the ${\mathrm{As}}_{\mathrm{Ga}}$ complex can be generated with 1.075-eV photons and recovered with 1.35-eV photons with optical cross sections of 2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}20}$ and 5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}19}$ ${\mathrm{cm}}^{2}$, respectively. The metastable state of the ${\mathrm{As}}_{\mathrm{Ga}}$ complex absorbs 0.7-1.2-eV photons, indicating that it has ionization levels in the band gap.

Defect profiles were determined in proton-implanted low-doped ([P]=1\ifmmode\times\else\texttimes\fi{}${10}^{14}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$) n-type silicon layers by performing positron-electron pair momentum-distribution measurements with a slow-positron beam, conventional positron lifetime, and ${\mathrm{e}}^{+}$-${\mathrm{e}}^{\mathrm{\ensuremath{-}}}$ pair momentum-distribution measurements with a $^{22}\mathrm{source}$ and spreading resistance measurements. The dominant positron trap induced by 1.15 and 3.0 MeV proton implantations is the silicon divacancy ${\mathrm{V}}_{2}$. Compared to the values in bulk, the characteristic positron lifetime and the characteristic low- and high-momentum parameters of the ${\mathrm{e}}^{+}$-${\mathrm{e}}^{\mathrm{\ensuremath{-}}}$ pair momentum distribution at the divacancy are ${\mathrm{\ensuremath{\tau}}}_{\mathrm{d}}$=300 ps=1.35${\mathrm{\ensuremath{\tau}}}_{\mathrm{b}}$, ${\mathrm{S}}_{\mathrm{d}}$=1.${052\mathrm{S}}_{\mathrm{b}}$, and ${\mathrm{W}}_{\mathrm{d}}$=0.${78\mathrm{W}}_{\mathrm{b}}$, respectively. The divacancy is observed in the negative charge state ${\mathrm{V}}_{2}^{\mathrm{\ensuremath{-}}}$. The divacancy profile is determined in n-type Si implanted with 1.15-MeV (20 \ensuremath{\mu}m) protons to a dose 1\ifmmode\times\else\texttimes\fi{}${10}^{14}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ and the maximum concentration [${\mathrm{V}}_{2}^{\mathrm{\ensuremath{-}}}$]=4--8\ifmmode\times\else\texttimes\fi{}${10}^{15}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ is observed at depths 16--18 \ensuremath{\mu}m. The resistivity increases with increasing divacancy concentration. After annealing at 400 \ifmmode^\circ\else\textdegree\fi{}C the spreading resistance measurements reveal a region of shallow hydrogen-related donors at depths 15--21 \ensuremath{\mu}m. The positron annihilation results support the idea that the introduction of shallow donors is due to the formation of hydrogen-vacancy complexes during the annealing.

We will show that positron lifetime spectroscopy provides information on the atomic structure of vacancy and ion-type defects in semiconductors. We will further demonstrate that positrons can be used to study electrical and optical properties of defects as well as their thermal stability in the heat treatments of the material. Especially, we will review information that positron experiments have provided on the point defects formed in 1–2 MeV electron irradiation of GaAs.

Positron annihilation is used to determine the vacancy ionization levels and the atomic structure of acceptors, vacancy-type or ion-type, in GaAs and InP materials. Positron annihilation yields information on the size of the vacancy open volume via the annihilation lifetime in the vacancy τV, on its charge state via the positron trapping coefficient μV(T) and the concentration CV via the positron trapping rate KV = μVCV. It is shown that V(As) and V(Ga) vacancies play a role as compensating centers in semi-insulating (SI) and n-type GaAs. V(As) vacancies absorb near-band edge light and interact with holes or electrons depending on their charge state V(As)+, V(As)0 or V(As)−. The conversion EL2 to EL2* generates metastable vacancies which are part of the EL2* structure.

Positron annihilation experiments have been performed to study the type and concentration of compensating defects in highly Si-doped GaAs grown by molecular-beam epitaxy (MBE). The results show the presence of both Ga vacancies and negative ion defects, each of which act as acceptors in $n$-type GaAs. The concentrations of both types of defects increase strongly for Si concentrations exceeding 5 \ifmmode\times\else\texttimes\fi{} ${10}^{18}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. At [Si] \ensuremath{\ge}5 \ifmmode\times\else\texttimes\fi{} ${10}^{19}$ ${\mathrm{cm}}^{\ensuremath{-}3}$, the concentrations of Ga vacancies and negative ions are comparable, and their sum represents a substantial fraction of the total concentration of Si itself. The results provide direct evidence that Ga vacancies play an important role in the electrical deactivation of highly Si-doped MBE-grown GaAs.