Search

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.

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.

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.%.

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

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 ).

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.

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.

Different thermal treatments on bulk undoped and S doped InP were carried out. The ambient atmosphere, temperature, heating, and cooling conditions were varied to study the influence of each individual parameter on sample characteristics. It is apparent that annealing in a P atmosphere is preferable for electrical and crystallographic homogeneity. The physical and structural properties of InP annealed under vacuum suggest that indiffusion of In takes place. The In at the surface is available because of the rapid P evaporation during the sample heating. Depending on the cooling conditions, the diffused In atoms can be gettered by dislocations and cause micro defects. A hypothesis of Inp antisite formation (and corresponding suppression of P vacancies) is also proposed to justify the Hall, photoluminescence, and positron lifetime data.MST/3337

Summary In order to select between various pharmacological activities related to the 1-benzazepine nucleus, a set of 1-benzazepine derivatives bearing different N-1 or C-2 substituents were synthesized and tested in vitro as ATP-dependent K + -channel antagonists on isolated guinea-pig trachea. Cumulative concentration curves to cromakalim were constructed in the absence or presence of the various compounds tested in comparison with glibenclamide. Products bearing N-1 substituents, especially (2-imidazolinyl)methyl showed a non-competitive antagonism towards ATP-dependent K + channels.

We report positron lifetime results on electron-irradiated (2.0 MeV at 20 K) Zn-, Fe- and S-doped indium phosphide. In all samples, the average positron lifetime at 300 K increases after irradiation and annealing at 300 K. In electron-irradiated InP(S) (n = 3.9 × 10 18 cm −3 ) , a defect identified as the indium vacancy recovers between 150 and 200 K. After annealing at 300 K, another defect lifetime is detected, interpreted as the upper limit for the positron lifetime at the phosphorus vacancy. The phosphorus vacancies remain intact up to annealing temperatures of 300 K.

The effect of irradiation is a key point in the knowledge of the behavior of some compounds such as uranium-based ones. Regarding this topic, the authors developed an original in-situ Raman spectroscopy device coupled to a cyclotron ion beam. This original instrument allows observing the kinetics of uranium dioxide weathering by α radiolysis of water. The authors then observed that an altered layer made of Studtite and Schoepite phases grows linearly during the irradiation and extends for several hours after irradiation. Kinetics of production of some molecular species during radiolysis were also reported.

Positron lifetime measurements give evidence on vacancies in n-type CdTe crystals. The crystals are either nominally undoped or In doped and grown by the travelling-heater method (THM) or the Bridgman technique. In THM CdTe(In) crystals grown using Te as the solvent, the concentration of vacancies correlates with the concentration of In and electrons. This correlation is in aggreement with the model of self-compensation where the In donors are compensated by In-vacancy complexes. In addition to (VCd−In)− complexes, it is found that positrons are trapped by negative ions.

Positron-beam experiments have been carried out to characterize defects in metal-organic vapor-phase-epitaxially grown GaAs layers on (100)GaAs and (100)Si substrates. The Te-doped GaAs/GaAs layer (n=2×10 15 cm -3 ) appears to be free of vacancies which are present in similarly doped bulk n-type GaAs crystals. The heteroepitaxial GaAs/Si layer with the low (

After electron irradiation two positron lifetimes 250+or-1 and 268+or-3 ps, are observed in phosphorus-doped Si ((P)=3*1016 cm-3) depending on the temperature and the electron fluence. The lifetimes are assigned to the negative and neutral charge states of the phosphorus-vacancy pair (P-V). The longer lifetime at the neutral charge state indicates a larger open volume of the vacancy and implies an outward relaxation of the phosphorus-vacancy pair in the charge-state transition (P-V)- to (P-V)0. A comparison with the theoretical lifetimes yields a breathing-mode relaxation of the nearest-neighbour atoms Delta r/r0 approximately 5.1% (r0 is the Si-Si bond length), or Delta r approximately 0.12 AA. The positron trapping coefficients 2*1016 s-1 at 20 K and 2*1015 s-1 at room temperature at the negatively charged state (P-V)- are estimated. A weakly bound Rydberg-like precursor state is invoked to explain the temperature dependence of positron trapping at the negatively charge centres (P-V)- and V- in Si. The binding energy at the precursor state is 15-30 meV.

In weakly n-type CdTe(In) crystals grown by the travelling heater method, positrons annihilate in vacancy-type defects with a lifetime of 320 ± 4 ps. The concentration of these native defects varies with the concentration of indium and electron in agreement with the model of self-compensation where the indium donors are compensated by indium-vacancy complexes. These defects are assumed to be (VCdIn)- complexes. The positron trapping in these complexes disappears at low temperature. This phenomenon is attributed to competing trapping of positrons by negative ions which are either residual impurities or intrinsic defects.

Positron lifetime experiments show the formation of voids in deformed n-type GaAs. In addition, the concentration of monovacancies is increased from the native population by a factor of three to six, depending on the deformation and annealing conditions of the material. The strong temperature dependency of the positron lifetime spectrum also indicates that the concentration of negative ions in deformed GaAs is increased compared to the as-grown material. The authors attribute this to the formation of Ga antisite defects during deformation.

In the hypothesis of a nuclear waste geological disposal, zirconolite is a candidate host material for minor tri- and tetra-valent actinides arising from enhanced nuclear spent fuel reprocessing and partitioning. Its chemical durability has been studied here under charged particle-induced radiolysis (He2+ and proton external beams) to identify possible effects on dissolution rates and mechanisms in pure water. Two experimental geometries have been used to evaluate the influence of the following parameters: solid irradiation and total deposited energy. Results on the evolution of the elemental releases due to the enhanced dissolution of the zirconolite surface during charged particle-induced irradiation of water are presented. Under radiolysis, elemental releases are first kinetically controlled. When the titanium and the zirconium releases reach (or exceed) their corresponding hydroxide solubility limits, the zirconolite dissolution becomes thermodynamically controlled.

Expérience GANIL; The effect of external irradiation on $\beta$-thorium phosphate-diphosphate (tetravalent actini des bearing storage matrix) and the influence on its dissolution in aqueous media were studied. Highly energetic heavy ions were used to get the amorphization of the crystalline structure of the ceramic. The ex-situ dissolution expenments showed an increase of the dissolution versus amorphous fraction in several pH and temperature conditions. The in-situ dissolution experiments highlighted the primordial importance of radiolytic produced free radical species. From these results, the ceramic presents a good resistance to aqueous alteration even in amorphous state.

The effects of hydrogen peroxide, H2O2, on UO2 Corrosion is investigated in aerated deionized water in two types of situations. The H2O2 species is either added to water or produced by radiolysis at UO2/H2O interfaces. The concentrations vary in the range 10(-5)-10(-1) mol l(-1). The radiolysis is induced by irradiating the UO2/H2O interfaces with a He2+-beam emerging from the UO2 discs into the solutions. Both the evolution of the aqueous solutions and the UO2 surfaces are characterised. In both types of experiments, the alteration Of UO2 results in the formation of the same secondary phase, an hydrated uranium peroxide called studtite (UO2(O)(2) center dot 4H(2)O). However, the uranium release at the interface differs strikingly. It is much higher when H2O2 is produced by irradiation than when it is simply added. Furthermore, it varies in opposite direction as a function of the H2O2 concentration. This gives evidence that the chemistry at the UO2 interface under irradiation differs significantly from the chemistry induced by simply adding H2O2 to the solution. Rutherford backscattering spectrometry is used to determine the growth rate of the corrosion layer. For H2O, addition, the layer thickness increases with increasing leaching time, although as time increases, the U release tends towards zero. It is possible to establish the first empirical equation relating the corrosion rates to the added H2O2 concentrations. For H2O2 radiolytic production, the growth is continuous as irradiation time increases but the growth rate seems to decrease as the layer grows and to reach a limit

Positron annihilation experiments have been performed to investigate the compensating defects in silicon δ-doping superlattices in (001) GaAs. The results reveal vacancies and ion-type defects, which are located between the delta planes in undoped GaAs. The vacancy defect is identified as the Ga vacancy and the negative ion is attributed to the Ga antisite. The concentrations of these defects increase strongly, when the areal concentrations of free carriers are reduced at the delta planes.

We used a slow positron beam to investigate the depth dependence of the positron–electron pair momentum distribution in ZnSe layers grown on a GaAs substrate. We report evidence that positrons annihilate in lattice in undoped ZnSe and at vacancies in heavily n-type ZnSe. It is also demonstrated that positrons in semi-insulating ZnSe are drifted to GaAs by fields of 1–3 kV/cm.

Leaching experiments were performed on UO2 pellets doped with alpha-emitters (238/239Pu) and on spent fuel, in the presence of an external gamma irradiation source (A60Co = 260 Ci, D ˙ γ = 650 Gy h−1). The effects of α, β, γ radiation, the fuel chemistry and the nature of the cover gas (aerated or Ar + 4%H2) on water radiolysis and on oxidizing dissolution of the UO2 matrix are quantified and discussed. For the doped UO2 pellets, the nature of the cover gas clearly has a major role in the effect of gamma radiolysis. The uranium dissolution rate in an aerated medium is 83 mg m−2 d−1 compared with only 6 mg m−2 d−1 in Ar + 4%H2. The rate drop is accompanied by a reduction of about four orders of magnitude in the hydrogen peroxide concentrations in the homogeneous solution. The uranium dissolution rates also underestimate the matrix alteration rate because of major precipitation phenomena at the UO2 pellet surface. The presence of studtite in particular was demonstrated in aerated media; this is consistent with the measured H2O2 concentrations (1.2 × 10−4 mol L−1). For spent fuel, the presence of fission products (Cs and Sr), matrix alteration tracers, allowed us to determine the alteration rates under external gamma irradiation. The fission product release rates were higher by a factor of 5–10 than those of the actinides (80–90% of the actinides precipitated on the surface of the fragments) and also depended to a large extent on the nature of the cover gas. No significant effect of the fuel chemistry compared with UO2 was observed on uranium dissolution and H2O2 production in the presence of the 60Co source in aerated conditions. Conversely, in Ar + 4%H2 the fuel self-irradiation field cannot be disregarded since the H2O2 concentrations drop by only three orders of magnitude compared with UO2.

ACE, NIMB; The aim of this paper is to study the effect of water radiolysis under 12 MeV proton irradiation on the corrosion behaviour of pure iron. Oxygen and hydrogen playing a crucial role during the corrosion process have been specifically investigated. Heavy desaerated water (enriched at 99.9% in D) was also used to determine the origin of hydrogen at the iron surface. Proton irradiations were performed at the CERI cyclotron in Orléans. Both sides of the Fe foil (respectively in contact with air and with water) were analysed with ion beam techniques: Alpha Rutherford Backscattering Spectrometry was used to profile oxygen, Elastic Recoil Detection analysis has allowed to profile hydrogen. The use of D$_2$O gives evidence that the hydrogen concentration present on the water face could originate from wet air. In addition, in case of the aerated deionised H$_2$O media, it is shown that the irradiation process induces a strong corrosion. Scanning electron microscopy experiments confirm the formation of oxide precipitates.

Positron lifetime spectroscopy is used to detect vacancy-related defects in as-received and 12-MeV proton-irradiated $6H\ensuremath{-}\mathrm{SiC}$ crystals. We can infer from the temperature dependence of the lifetime spectra decomposition that neutral and negatively charged vacancy defects exist in crystals before and after proton irradiation at low fluence $(l~4\ifmmode\times\else\texttimes\fi{}{10}^{15}{\mathrm{H}}^{+}{\mathrm{cm}}^{\ensuremath{-}2}).$ Neutral vacancies are detected after irradiation at high fluence $(g~4\ifmmode\times\else\texttimes\fi{}{10}^{16}{\mathrm{H}}^{+}{\mathrm{cm}}^{\ensuremath{-}2}).$ Negatively charged ${V}_{\mathrm{Si}}$ silicon monovacancies with $202\ifmmode\pm\else\textpm\fi{}8\mathrm{ps}$ lifetime are detected at low temperature in as-received n-type $6H\ensuremath{-}\mathrm{SiC}$ and after irradiation at low fluence. Neutral ${V}_{\mathrm{S}\mathrm{i}\ensuremath{-}\mathrm{C}}$ divacancies associated with the $(225\ifmmode\pm\else\textpm\fi{}5)\ensuremath{-}\mathrm{ps}$ lifetime are produced by irradiation and dominate the positron trapping at room temperature. In addition, different types of ionic acceptors are detected. One of them acts as a strong trapping center even at room temperature.

Positron annihilation spectroscopy has been applied to identify Si and C vacancies as irradiation-induced defects in 6H-SiC. Si vacancies are shown to have ionization levels at E C -0.6 eV and E C -1.1 eV below the conduction-band edge E C by detecting changes of positron trapping under monochromatic illumination. These levels are attributed to (2-/1-) and (1-/0) ionizations of the isolated Si vacancy. In as-grown n-type 6H-SiC, a native defect complex involving V S i is shown to have an ionization level slightly closer to conduction band at roughly E C -0.3 eV. These results are used further to present microscopic interpretations to effects seen in optical-absorption spectra and to electrical levels observed previously by deep-level transient spectroscopy.

The behaviour of UO2/water interface under irradiation has been investigated as a function of alpha flux using an alpha beam provided by a cyclotron. The effects of alpha-radiolysis on UO2 alteration in aerated deionized water were studied by characterizing both the chemistry of irradiated aqueous solutions and the UO2 surface. Uranium (as uranyl ion UO22+) and hydrogen peroxide concentration (H2O2) increased whereas pH decreased in the irradiated solutions when alpha-beam flux increased. The formation of hydrated uranium peroxide (metastudtite UO4·2H2O) on UO2 leached surface was identified by X-ray diffractometry. The production of metastudtite can be considered as a direct effect of water radiolysis due to the production of radiolytic species H2O2, since its formation is known to occur out of irradiation via a precipitation reaction between hydrogen peroxide and uranyl ion. From both these observations and literature about hydrated uranium peroxide occurrence, the possibility of metastudtite formation on nuclear spent fuel in storage conditions is discussed.

For a potential performance assessment of direct disposal of spent fuel in a nuclear waste repository, the chemical reactions between the fuel and possible intruding water must be understood and the resulting radionuclide release must be quantified.Leaching experiments were performed with five spent fuel samples from French power reactors (four UO2 fuel samples with burnup ratings of 22, 37, 47 and 60 GWd·tHM−1 and a MOX fuel sample irradiated to 47 GWd·tHM−1) to determine the release kinetics of the matrix containing most (over 95%) of the radionuclides. The experiments were carried out with granitic groundwater on previously leached sections of clad fuel rods in static mode, in an aerated medium at room temperature (25°C) in a hot cell.After 1000 or 2000 days of leaching, the Sr/U congruence ratios for all the UO2 fuel samples ranged from 1 to 2; allowing for the experimental uncertainty, strontium can thus be considered as a satisfactory matrix alteration tracer. No significant burnup effect was observed on the alteration of the UO2 fuel matrix. The daily strontium release factor was approximately 1 à 10−7 d−1 for UO2 fuel, and five to six times higher for MOX fuel. Several alteration mechanisms (radiolysis, solubility, precipitation/clogging) are examined to account for the experimental findings.

This work uses an external alpha beam to irradiate an uranium oxide/water interface and investigates the release of uranium in aerated deionized water under alpha irradiation. A high energy alpha beam delivered by a cyclotron (CERI-CNRS) goes through the oxide and emerges in the water with a 20 MeV energy. First results are reported here showing that the uranium mass loss rate increases by three orders of magnitude in aerated deionized water under high flux (≥3.3×1010 α.cm−2s−1).

We describe the two dimensional angular correlation of positron annihilation radiation (2D-ACAR) method and show that it can be advantageously used to study the electronic structure of defects, in addition to standard positron lifetime and Doppler broadening measurements. Using annihilation fractions determined by lifetime measurements, we separate 2D-ACAR distributions for negatively charged and neutral arsenic vacancies in n-type GaAs. In electron-irradiated semi-insulating GaAs, we present 2D-ACAR results for the negatively charged gallium vacancy and for positron Rydberg states induced by gallium antisites. The 2D-ACAR for delocalized positrons, needed in the separation process, has been obtained from measurements on as-grown semi-insulating GaAs. Our results outline the capabilities of the 2D-ACAR technique when it is applied to the study of defects: the electronic structure in the vacancies are found to be well described by recent molecular dynamics calculations. The method also provides information about atomic relaxations around the vacancy sites, they are in agreement with the calculated relaxations. We conclude that the 2D-ACAR method is a promising tool to investigate defects in semiconductors.

Positron-lifetime measurements show that acceptors are produced in semi-insulating GaAs by 1.5-MeV electron irradiation at 20 K. Two types of acceptors can be separated. The first ones are negative vacancy-type defects which anneal out over a very broad range of temperature between 77 and 500 K. The second ones are negative ion-type defects which are stable still at 450 K. The data show that these two types of defects are independent and do not form close pairs. We attribute both to gallium-related defects. We identify the ion-type acceptors as isolated gallium antisites. The vacancy-type acceptors are identified as gallium vacancies which are isolated or involved in negatively charged complexes. The introduction rate of the gallium antisite is estimated to be 1.8\ifmmode\pm\else\textpm\fi{}0.3 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in the fluence range ${10}^{17}$--${10}^{18}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ for 1.5-MeV electron irradiation at 20 K.

A beam of low-energy positrons together with ion-beam techniques have been used to profile defect and hydrogen distributions and their annealing behavior after the room-temperature implantation of 3\ifmmode\times\else\texttimes\fi{}${10}^{15}$ to 1\ifmmode\times\else\texttimes\fi{}${10}^{17}$ 60-keV ${\mathrm{H}}^{+}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ in semi-insulating GaAs. A monovacancy overlayer is observed close to the surface at 0\char21{}330 nm after 1\ifmmode\times\else\texttimes\fi{}${10}^{17}$ ${\mathrm{H}}^{+}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ implantation. The vacancies in the stopping range of the implanted H at 330\char21{}720 nm are seen only after the annealing at 200 \ifmmode^\circ\else\textdegree\fi{}C, probably due to a release of hydrogen from the vacancies. The total vacancy depth of 720 nm correlates well with the distribution of displaced atoms and with the energy deposited in the H implantation. Above 350 \ifmmode^\circ\else\textdegree\fi{}C, the monovacancies in the region of 0\char21{}350 nm start to disappear, whereas at the depth of 350\char21{}750 nm stable vacancy-hydrogen agglomerates are formed at 400 \ifmmode^\circ\else\textdegree\fi{}C. The redistribution of the implanted hydrogen at 0\char21{}350 nm was observed to be connected with the migration of complexes containing implantation-induced defects.

Positron annihilation measurements are performed on low temperature electron irradiated pure bismuth. Different stages during recovery are found. These stages are compared with literature results on the annealing of pure bismuth. The discrepancies in the appearing of the different annealing stages are discussed, as is positron trapping in defects in bismuth. Des mesures d'annihilation des positrons ont ete faites sur des echantillons du bismuth pur irradie aux electrons. Plusieurs stages de recuit ont ete mesures. Ces stages sont compares avec les resultats de la litterature sur le recuit du bismuth pur. Les contradictions apparentes concernant (a position des differents stages sont discutees, ainsi que le piegage des positrons dans des defauts dans le bismuth.

Gold and gold doped with small amounts of tin, antimony, germanium and iron have been quenched in liquid helium. Isochronal annealing studies of the quenched-in resistivity show that there are important interactions between vacancies and the dopant.

The lifetime of the delocalized positrons in α-HgI 2 crystals is found to be 320±4 ps. Some crystals grown from the vapor phase in an excess of iodine have native mercury vacancies. The lifetime of positrons localized in mercury vacancies is 360±5 ps. Some crystals grown in unstabilized solutions with respect to added iodine show shorter lifetime components of 180–200 ps attributed to positron annihilation inside mercury-rich and/or impurity-rich clusters. Other crystals show both shorter and longer lifetime components than the lifetime in the bulk, indicating the coexistence of mercury vacancies with clusters rich in mercury interstitials and/or impurities. The energy resolution of the γ-ray detectors is correlated to the positron-lifetime spectra of the crystals.