M. Döbeli

Behaviour of implanted xenon in yttria-stabilised zirconia as inert matrix of a nuclear fuel

A zirconia-based fuel is studied for use of plutonium in light water reactors. Among the relevant properties for a nuclear fuel, efficient retention of fission products is required since the fuel matrix constitutes the first barrier against fission product release. To study the retention of xenon, its stopping power and its diffusion properties within (Er,Y,Pu,Zr)O2 potential inert matrix fuel (IMF) are investigated. Stopping and range of ions in matter (SRIM) calculations were carried out to estimate the average penetration depth of Xe ions as a function of their incident energy and of the material composition. To study its diffusion properties, Xe was implanted into yttria-stabilised zirconia (YSZ) to a depth of around 100 nm from the surface. After successive heat treatments to a maximum temperature of 1773 K, quantitative Xe depth profiles were determined by Rutherford backscattering. No profile modification by diffusion was observed. The behaviour of Xe is investigated at the subnanoscopic level and compared with the results obtained with zirconia samples implanted with Cs or I, as well as with Xe in UO2.

Behavior of cesium implanted in zirconia based inert matrix fuel

Retention of cesium in yttria stabilized zirconia (YSZ) was studied experimentally and discussed on the basis of its solubility estimated on a thermodynamic basis. The retention was investigated experimentally by determination of the temperature dependence of diffusion and the cesium release. Cesium was introduced using 1 MeV ions or by chemical reaction in yttria stabilized zirconia. The distribution of the implanted cesium was evaluated by using TRIM computer calculation and measured by Rutherford backscattering spectroscopy (RBS) using 5 MeV 4He ions. The cesium profile was quantified after successive annealing up to 1373 K. Cesium release was rather weak up to 1173 K. The retention of Cs chemically introduced by Cs2ZrO3 was also experimentally investigated for different calcination temperatures. The chemically introduced cesium followed similar release behavior as the implanted one. Thermodynamic modelling of the system cesium, zirconium and oxygen was performed in a comprehensive way and as a function of temperature.

Defect production by MeV cluster impacts

Abstract Monocrystalline silicon has been irradiated by MeV carbon and germanium clusters (Cn, n = 1, 2, 3, 4, 6, 8 and Gen, n = 1, 2, 3) with fluences up to 3 × 1016 atoms per cm2. The energy has been varied between 0.2 and 4 MeV per atom. The produced defect concentration profiles have been measured by channeling Rutherford backscattering (RBS). While the damage at the end of range of the particles (where the fragments of a cluster have straggled far apart) is independent of the cluster size, the defect concentration in the first few hundred nm below the sample surface depends significantly on the size of the molecule. Up to a size of approximately 6 the swift carbon clusters (for which electronic stopping is clearly dominant) produce fewer defects per incident constituent than single carbon atoms of the same velocity. For larger carbon clusters the defect production per atom is increased. The appreciably slower polyatomic Ge particles, however, show a strong enhancement of the defect production close to the sample surface. This enhancement increases strongly with cluster size, but is reduced for higher energies. From the shape of the defect profile a radius of interaction between the individual fragment tracks of one cluster can be estimated.

Surface tracks by MeV C60 impacts on mica and PMMA

Abstract Muscovite mica has been bombarded by C60 ions with energies between 2 and 15 MeV. The produced surface tracks have been analyzed by atomic force microscopy (AFM). The particle impacts formed hillocks with a height which increases with energy from 2 to 6 nm. For impacts of I, Ge3, C8, C10, Cu9, and Cu10 MeV ions, hillocks smaller than 0.6 nm have been found. Irradiation with GeV monoatomic particles at similar energy loss performed by other research groups has only changed the frictional properties of the mica surface but did not produce any hillocks. The results can be understood qualitatively by taking into account the large difference in the velocity of the projectiles and the secondary particles along the track. PMMA irradiated with MeV Cn, Gen, and I ions exhibited holes with diameters between 18 and 45 nm after development. The measured track area as a function of energy loss shows an approximately linear behavior, pointing towards a diffusive spreading of energy.

Accelerator SIMS at PSI/ETH Zurich

The accelerator SIMS system at PSI/ETH Zurich has been completed and has been run successfully in first tests. A new ion source has been added to the existing AMS facility. The main concept of the ion source is the suppression of the contamination of the sample and of the secondary ion beam. All parts close to the sample have therefore been gold plated. With a commercial mass filtered Cs gun as used for secondary ion mass spectrometry (SIMS) a pure Cs ion beam is obtained for sputtering. In first experiments some routinely used AMS techniques have been shown to be very effective for accelerator SIMS as well. In many cases a fast beam-pulsing system allows normalization of trace element yields to the substrate current. In order to avoid contamination of the accelerator SIMS chamber, a pilot beam from the adjacent AMS sputtering source can be used for tuning of the accelerator mass spectrometer to the trace element of interest. Since the AMS ion source yields a more intense current, tuning becomes easier and, in addition, a good reference spectrum for the gas ionization counter is obtained. First results are presented.

Characterization of ultra thin oxynitrides: A general approach

The determination of nitrogen depth profiles in thin oxynitride layers (1.5‐3 nm) becomes more and more important in microelectronics. The goal of this paper is to investigate a methodology for the characterization of thin oxynitride layers with the aim to establish in a quantitative manner the layer thickness, N-content and detailed N-depth profile. For this study ultra thin oxynitride films of 2.5 nm on Si were grown by oxygen O2 annealing of Si followed by a NO annealing. The global film characteristics were measured using spectroscopic ellipsometry (SE) (thickness), atomic force microscopy (AFM) for roughness and X-ray photoelectron spectroscopy (XPS) for total O- and N-content. Depth profiles of oxygen, silicon and nitrogen were obtained using (low energy) secondary ion mass spectroscopy (SIMS) and time of flight (TOF)-SIMS, high resolution-Rutherford backscattering (H-RBS) (magnetic sector and TOF) and high resolution-elastic recoil detection (H-ERD). A comparison of the results obtained with the diAerent techniques is presented and discussed. ” 2000 Elsevier Science B.V. All rights reserved.

MEV-CLUSTER IMPACTS AND RELATED PHENOMENA

Point defect creation in 8.7 MeV C5-cluster irradiated LiF crystals have been studied in a dose range from 2 × 1010 up to 1 × 1013 C5+/cm2 and compared to the defect creation in similar conditions with single energetic carbon ions. An enhancement of the primary defect creation (F-centres) and aggregation (F2-centres) has been measured in a near surface region as deep as about 0.3 μm corresponding to an energy per carbon atom of the incident cluster decreasing from 1.74 MeV at the impact down to 1.3 MeV. In this region, the carbon atoms of a cluster are confined in a track with a radius (∼ 100 nm) comparable to that of a single carbon ion track. This leads to a very high locally deposited energy density which is directly responsible of the enhanced defect production measured.

Retention of iodine in yttria stabilized zirconia

The diffusion behavior of iodine in yttria stabilized zirconia (YSZ) was investigated as a function of the temperature. Iodine implantation was performed in this cubic solid solution by irradiating material samples with 1 MeV iodine ions. The distribution of iodine was evaluated by using TRIM computer calculation and measured by Rutherford backscattering spectrometry (RBS) using 5 MeV 4He particle. The iodine profile was quantified after successive temperature steps. At each step the maximum temperature was held for 2 h. It was found, that the iodine profile remained unchanged up to 1373 K. At 1573 K iodine diffused significantly. The iodine profile was measurable up to 1773 K. Even at this high temperature a significant iodine retention is observed. The diffusion coefficient of iodine (3.5 ± 1.4 × 10−15 cm2 s−1) is of the same order as the temperature extrapolated data of iodine in monoclinic/tetragonal zirconia.

Defect creation in tracks produced by high energy carbon clusters in LiF

Abstract LiF single crystals have been bombarded at room temperature with 3 and C 5 clusters with energies of 1.74 MeV/carbon atom and fluences ranging from 10 12 to 10 14 carbon atoms/cm 2 . Point defects (color centers: F, F 2 etc.) resulting from electronic excitation processes were measured by optical absorption spectroscopy. The defect concentrations were compared to those produced by irradiations with single 12 C ions with the respective energy and dose. Also using a differential optical absorption technique, it was possible to determine the defect concentrations close to the sample surface, when the tracks associated to each carbon atom of the cluster overlap. In this zone where the “cluster effect” is maximum due to the very high density of electronic excitations, enhanced defect production is observed. Defect concentrations as large as those obtained previously by Kr and Xe irradiations at GANIL have been measured. In addition to the high production rates of defects observed, aggregation laws (i.e. F → F 2 ) characteristic of cluster irradiations are also deduced.

Sputtering and defect production by focused gold cluster ion beam irradiation of silicon

Abstract A focused ion beam (FIB) system with a liquid metal ion source (LMIS) has been used to irradiate monocrystalline silicon with Au clusters. Energies of 6 keV per gold atom ( = 1,…,5) and 10 keV per gold atom ( n = 1, 2, 3) were applied and fluences ranged from 10 10 to 10 17 cm −2 . Laser induced modulated reflectivity at normal incidence has been used for characterization of the optical properties of the sample surface and the defect density after irradiation. The removal of substrate material induced by the cluster beam was measured by atomic force microscopy (AFM). The number of produced surface defects detected by the optical measurement did not significantly depend on the cluster size n . A small nonlinear increase of the total sputtering yield per incident gold atom was observed as a function of cluster size.