J. Chaumont

ARAMIS: An ambidextrous 2 MV accelerator for IBA and MeV implantation

Abstract We will present the 2 MV accelerator that we built at our laboratory. ARAMIS is a tandem accelerator with a positive-ion source in the high-voltage terminal so that it can be operated both in the tandem and in the single-ended mode. Tuning from one mode to the other is quite easy so that the implantation or irradiation of samples can be followed periodically with RBS. We show some results.

Progress report on Aramis, the 2 MV tandem at Orsay

Abstract Aramis is a home built multipurpose 2 MV electrostatic tandem accelerator. A large variety of ions are available for high energy implantation. Characterization possibilities are also quite large in the Van de Graaff mode owing to the Penning positive ion source in the terminal. A second beam line is now available that sends the beam into the target chamber of the 200 kV medium current implanter. We will provide a progress report on the machine and present some results regarding in situ studies of multilayer mixing and implanted silicide layers

Competition between disorder creation and annealing in fluoroapatite nuclear waste forms

Abstract He-ion irradiation simulating α-particle emission, induces crystal defect recovery in fluoroapatite Ca10(PO4)6F2 pre-damaged by Pb-ions. In fluoroapatite loaded with α-emitter actinides, a similar effect is expected. Therefore, the amorphization level in fluoroapatite has been modeled versus disposal time, α-decay fluence and temperature. When loaded with 239 Pu and 244 Cm , the structural disorder reaches a plateau, which level depends only slightly on α-energies. This level corresponds to an amorphous fraction of approximately 20% of disorder and is independent of the dose rate. The time delay in reaching the equilibrium regime in the disorder production is inversely proportional to the dose rate whereas the dose or the actinide content is not a critical parameter for the magnitude of the equilibrium level.

Simulation of the α-annealing effect in apatitic structures by He-ion irradiation: Influence of the silicate/phosphate ratio and of the OH−/F− substitution

In fluoroapatite, the α-annealing is a crystal recovery process due to the electronic energy loss of α-particles emitted by radionuclides. This effect, which can be accompanied by a thermal recovery, is accounting for the crystalline state of natural calcium phosphate apatites containing long lifetime actinide isotopes. The aim of the present study is to determine the influence of the silicate–phosphate grouping substitution, on the α-annealing efficiency. In addition, measurements have been performed on hydroxyapatite Ca10(PO4)6(OH)2 and compared with those obtained on fluoroapatite Ca10(PO4)6F2 to evaluate the influence of the OH−/F− substitution on the annealing efficiency. Using a transmission electron microscope (TEM) on line with an ion implanter, the main result obtained in this study is that α-annealing is strongly dependent on the chemical composition of the mineral. Our results show that this effect is decreasing when the SiO4/PO4 ratio increases and when F− is substituted for OH− anions.

EVIDENCE OF IONIZATION ANNEALING UPON HELIUM-ION IRRADIATION OF PRE-DAMAGED FLUORAPATITE

Abstract In order to study the behaviour of fluorapatite towards irradiation due to incorporated alpha-emitters, single crystals of this mineral have been bombarded with 220 keV Pb ions, simulating alpha recoil nuclei, and subsequently irradiated with He ions. The defect concentration measured by RBS associated with channeling steadily decreases upon He-ion irradiation. By changing the energy of the incident He ions in the range 0.3–3.2 MeV, we have shown that this defect annealing phenomenon can be ascribed to the electronic energy loss, while the use of B ions which have slightly higher electronic energy loss in the target induces the opposite effect, i.e., increases the defect concentration.

MeV gold irradiation induced damage in α-quartz: Competition between nuclear and electronic stopping

Abstract Damage creation in crystalline α-quartz by irradiation is studied using gold ions of energies between 0.5 and 10 MeV. For all ions, the total stopping power (d E /d x ) tot has a value of about 4.5 keV/nm, whereas the contribution of the electronic stopping power ranges from 0.93 keV/nm at 0.5 MeV to 3.6 keV/nm at 10 MeV. This variation allows us to test which role the nuclear and the electronic collisions plays for the damage processes. The kinetic of the ion induced damage was determined by channeling RBS and the volume increase by profilometry. Single ion impacts create damage when electronic stopping dominates, while several impacts are necessary to achieve damage in the nuclear stopping regime. A detailed analysis allows us to deduce the damage cross-sections of the two processes. The electronic stopping power of damage creation appears above an electronic d E /d x threshold of 1.4±0.3 keV/nm.

Dose rate and temperature effects in radiation disorder creation in SrTiO3

Abstract An important aspect of assessing the physical and chemical stability of SrTiO 3 phase under radiation is to evaluate the disorder caused by α-decays in the actinide loaded nuclear waste form during the repository time. Indeed, radiation can affect considerably the chemical durability of the solid waste forms, especially for α-emitter immobilisation. It is already known that amorphisation in SrTiO 3 can be described at room temperature, by the cascade-overlap model with an additional term representing the initial disorder creation. In the present work, it is shown by using Pb-ion beam to simulate the α-recoil nuclei for creating the disorder that Pb-dose rate and sample temperature during irradiation are two important factors that affect SrTiO 3 susceptibility to proceed toward amorphisation.

Determination of the defect creation mechanism in the mono-silicated fluoroapatite. Disorder modeling under repository conditions

Abstract Amorphization resulting from α-decay events can strongly reduce the chemical durability of nuclear waste matrices. The creation rate of defects produced by α recoils in mono-silicated fluoroapatite was determined using a transmission electron microscope (TEM) on line with an ion implanter and compared to previous results obtained in fully phosphated fluoroapatite. In both materials, the defect creation is controlled at room temperature, by the amorphization process directly in the cascade. Furthermore, it has been shown previously that in mono-silicated fluoroapatite, the disorder recovery proceeds mainly via α-annealing. Taking into account our already published data and new results on the defect creation in the mono-silicated fluoroapatite, we have modeled the amorphization level evolution versus time under repository conditions. The main conclusion is the following: thanks to α-annealing, mono-silicated fluoroapatite loaded with 244 Cm, will maintain a disorder at a level low enough to prevent the total amorphization of the host lattice during the long-term disposal.

Defect creation by MeV clusters in LiNbO3

Abstract Single crystals of LiNbO 3 (Y-cut orientation) were irradiated at room temperature with 3 MeV C 6 + and 2.4 MeV C 8 + clusters provided by CSNSM “ARAMIS” accelerator. The electronic stopping power was about 6.5 keV nm −1 in both cases. The fluences extended from 10 10 to 4 × 10 12 C 6 + or C 8 + cm −2 . Rutherford Backscattering Spectrometry in Channeling geometry (RBS/C) revealed the presence of extended defects, which cannot be due solely to nuclear stopping and which we ascribe to the high density of electronic energy associated with the correlated electronic stopping of the cluster components. The RBS/C analysis, providing a lattice disorder profile, confirms this interpretation: the damaged thickness closely corresponds to the cluster components correlation length (between 50 and 100 nm). The fluence evolutions of the disorder ratio, measured at the sample surface for C 6 and C 8 projectiles, correspond to damage cross sections close to 1.8 × 10 −13 cm 2 in the two cases.

Defect creation in LiNbO3 irradiated by medium masses ions in the electronic stopping power regime

Abstract Single crystals of LiNbO3 (with y-cut orientation) were irradiated at the ARAMIS tandem using two different ions (35Cl and 79Br) accelerated in the few MeV energy range. All the irradiations were performed at room temperature, with fluences extending from 1012 to 1.2 × 1013 ion cm−2. The damage resulting from these irradiations was characterized by Rutherford Backscattering Spectrometry in Channeling geometry (RBS-C) in conjunction with Transmission Electron Microscopy (TEM) observations. Two different mechanisms concerning the disorder kinetics were discussed: (i) single overlap of a predamaged zone; (ii) nucleation and growth of amorphous phases. Depending on both the electronic stopping power (dE/dx)e and the velocity of the incident ions, the damage cross-section A e varies between 1.6 × 10−13 and 5.8 × 10−13 cm2.