M.-F. Barthe

Alpha-radiolysis effects on UO2 alteration in water

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.

Investigation of laser sputtering of iron at low fluence using resonance ionization mass spectrometry

Emission of neutral and ionized Fe atoms induced by N2 laser irradiation (λ=337 nm) of iron targets was investigated for laser fluences ranging from the sputtering threshold (a few tens mJ/cm2) up to 320 mJ/cm2. The unique sensitivity of resonance ionization mass spectrometry permitted to identify the laser‐sputtered neutral particles, to study their velocity distribution and excitation state, and to measure the laser‐sputtering yield at a very low emission level (100 Fe atoms removed per laser shot). In this soft ablation mode, it appears that the sputtered atoms carry direct information on the primary mechanisms involved in the laser sputtering of an iron surface. The present study corroborates a thermal process in which the laser beam acts as a pulsed heat source. Energy characteristics of the emitted neutral atoms reflect rather well the thermal state of the surface, the atom temperature increasing from the ambient at threshold to melting and boiling temperatures for growing laser energies.

Positronium reemission yield from mesostructured silica films

The reemission yield of ortho-positronium (o-Ps) into vacuum outside mesoporous silica films on glass is measured in reflection mode with a specially designed lifetime (LT) spectrometer. Values as high as 40% are found. The intensity of the 142 ns vacuum LT is recorded as a function of reemission depth. The LT depth profiling is correlated to the 2gamma and 3gamma energy ones to determine the annihilation characteristics inside the films. Positron lifetime in capped films is used to determine the pore size. For the first time, a set of consistent fingerprints for Ps annihilation, o-Ps reemission into vacuum, and pore size, is directly determined in CTACl-TEOS films.

Helium behavior in UO2 polycrystalline disks

AbstractThe 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

Optimal conditions forNV−center formation in type-1b diamond studied using photoluminescence and positron annihilation spectroscopies

We studied the parameters to optimize the production of negatively-charged nitrogen-vacancy color centers (NV-) in type~1b single crystal diamond using proton irradiation followed by thermal annealing under vacuum. Several samples were treated under different irradiation and annealing conditions and characterized by slow positron beam Doppler-broadening and photoluminescence (PL) spectroscopies. At high proton fluences another complex vacancy defect appears limiting the formation of NV-. Concentrations as high as 2.3 x 10^18 cm^-3 of NV- have been estimated from PL measurements. Furthermore, we inferred the trapping coefficient of positrons by NV-. This study brings insight into the production of a high concentration of NV- in diamond, which is of utmost importance in ultra-sensitive magnetometry and quantum hybrid systems applications.

FT-ICR with laser ablation and AMS combined with X-ray detection, applied to the measurement of long-lived radionuclides from fission or activation: preliminary results

Abstract The measurement of long-lived radionuclides from fission or activation is difficult when isobars are present. Preliminary results are reported here on two methods that can solve the problem in some instances. The first one, FT-ICR with laser ablation, has a very high mass resolution; the second one, AMS combined with X-ray detection, discriminates isobars using characteristic projectile X-rays (“inverse PIXE”).

In-Situ Raman Observation of the First Step of Uranium Dioxide Weathering Exposed to Water Radiolysis

ABSTRACT 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.

Increase of the uranium release at an UO2/H2O interface under He2+ ion beam irradiation

Abstract 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 ).

Improved tungsten moderator structures for slow positron beams

Tungsten filaments from commercial incandescent light bulbs have been tested as slow positron moderators. The filaments were annealed by electric current in the neutral gas atmosphere of the bulbs at or above the normal operating temperature of the lamp. The total efficiency of the positron source/moderator assembly was found to be as high as 4 � 10 � 4 . This value is almost doubled at high extraction voltage (3000 V). Neither the annealing temperature (i.e. the heating current) nor the filament thickness (related to the nominal power of the lamp) affected significantly the moderation efficiency, most probably due to the relatively thick filaments used and the difficulties in controlling the source geometry. It is suggested that the moderation efficiency can be further improved by using structures with thinner tungsten patterns, optimised configuration and extraction electric field. # 2002 Elsevier Science B.V. All rights reserved.

Pulsed laser sputtering of the (100)GaAlAs surface

We have studied the pulsed laser sputtering of (100)Ga1−xAlxAs (x=0.545) surface with 337 nm photons, starting from the threshold for particle emission (a few tens of mJ/cm2) up to about 300 mJ/cm2. Atoms and molecules sputtered from the irradiated surface are detected, their relative number measured, and their time of flight determined using laser resonant ionization mass spectrometry. After laser irradiation the surface is examined by scanning electron microscopy and electron microprobe analysis. From the shot number and the fluence dependencies of the sputtering yield, it is shown that two sputtering regimes exist. For low fluence (<150 mJ/cm2), the sputtering results mainly from the absorption and excitation of defect sites. At higher fluences, the process is quite similar to thermal evaporation. One observes preferential emission of As in the form of As2 molecules and the correlated Ga and Al enrichment of the surface with formation of GaAl microsized structures. However, As preferential laser sputte...