Low-temperature irradiation effects in lithium orthosilicates

The emission spectra of lithium orthosilicates (Li₄SiO₄₎ ceramics have been measured in the range of 1.8–5.8 eV under irradiation by 6–30 eV photons or 1–30 keV electrons at 6–300 K. The tunnel recombination phosphorescence, as well as luminescence, stimulated by 1.5–2.5 eV photons has been detected in the sample preliminarily irradiated at 6 or 80 K. The main peaks of thermally stimulated luminescence (TSL) in the irradiated ceramics have been observed at 72, 118 and 265 K. The creation spectra of the 118 K TSL peak, as well as the excitation spectrum of photostimulated luminescence (PSL) span the region of the intrinsic absorption of a lithium orthosilicate (9–30 eV). The intensity of PSL and the TSL peaks in Li₄SiO₄ ceramics prepared in hydrogen/argon atmosphere is several times lower than that in the mainly investigated Li₄SiO₄ ceramics prepared in the atmosphere of dry argon. The optical characteristics of Li₄SiO₄ are compared with the ones known for Li₂O and SiO₂. Low-temperature luminescent methods are promising for the investigation of electron–hole processes and radiation defects serving as the traps for tritium released in D–T fusion reactor blanket systems.     

Effects of secondary electrons emitted from surroundings on defect formation in silica glass under γ-ray irradiation

We have investigated the effects of secondary electrons and photons emitted from surrounding materials on defect formation in silica glass under γ-ray irradiation. SiO₂ (silica) glass plates and those sandwiched in a pair of various material disks (carbon, stainless steel or lead) were irradiated by γ-ray, and the optical absorption spectra (UV–vis spectra) of the silica glass plates before and after the irradiation were examined. UV–vis spectra of the glass plates after the irradiation showed three absorption bands peaked around 2 eV, 4 eV and 5.8 eV being assigned to color centers relate metal impurities (Al and Ge) and oxygen-deficient centers like E′ center, respectively. All three bands were found to grow with γ-ray irradiation dose and saturated at higher doses, and absorbance of the bands at the saturation for the sandwiched glass plates was higher than that for the bare glass plate. Moreover, the saturated absorbance was higher for the glass plate sandwiched with heavier materials. Employing Monte Carlo N-Particle (MCNP) code for the simulation of the photon–electron transport process, enhanced energy deposition and numbers of secondary electrons and photons emitted from sandwiching material disks to a silica glass plate were calculated. The higher deposition energy correlates well to the higher saturated absorbance, indicating that the secondary electrons and photons emitted from the disks clearly enhanced the defect formation in the sandwiched silica glass plates. This suggests the existence of the dose effect above a critical does, i.e. the irradiation with higher dose will result in higher saturated absorbance.     

Core concept of a passive-safety fast reactor “METAL-KAMADO” and reactivity coefficients

New concept of a passive-safety simple fast reactor “METAL-KAMADO” with metallic fuels is presented, which has same concept as a passive-safety thermal reactor “KAMADO”. A fuel element of the “METAL-KAMADO” consists of metallic fuel (U–10%Zr) and cooling holes of He gas flow. These fuel elements are located in a reactor water pool of atmospheric pressure (0.1 MPa) and low temperature (<60 °C). In case of LOF, decay heats of fuel elements are removed by natural heat transfer from surfaces of the fuel elements to the reactor water pool.

Preliminary neutronic calculations of the “METAL-KAMADO” show possibility of high burn-up of more than 120 GWd/t with 10% enriched U–Zr fuel. Reactivity coefficients of the core are also discussed.     

Fermi shuttle acceleration in atomic collisions: the case of ion induced electron emission

The “Fermi shuttle” acceleration of electrons in ion–atom collisions, i.e. multiple collision sequences of electrons bouncing off the projectile and target nuclei, can lead to the emission of very energetic electrons (i.e. with velocities higher than the binary encounter electrons). We performed measurements of the evolution of the Fermi shuttle electron yield with the induced perturbation and the target atomic number. The yield increases with the perturbation parameter (ratio of projectile charge and projectile velocity q/v_P), which was varied from q/v_P ≈ 0.2 (weak perturbation) to q/v_P ≈ 2 (strong perturbation). We also introduce a more realistic scaling parameter, which accounts for the re-bouncing of the electrons on target and projectile, and show that the yields increase as a function of this parameter. For a given projectile, the Fermi shuttle electron yield increases with the target atomic number. Furthermore, we show that the velocity distribution of the high-energy electrons is exponentially decreasing N(v)  exp(−nv) and exhibits the same evolution of the slope n with projectile velocity as in the case of Fermi accelerated deuterons.     

Raman and optical absorption studies of proton bombarded CsI

CsI single crystals, which have the simple cubic structure, have been bombarded with 1 MeV protons at a temperature close to 300 K. Optical absorption and Raman studies have identified most of the defects created. These include F and F₂ centres, and V centres having absorption bands at 2.7 eV and 3.4 eV, which grow together during irradiation. The Raman studies identify these latter centres as having the structure. Isochronal and isothermal annealing experiments show a mutual decay of the F-type centres and these V centres in a second order reaction with an activation energy of 1.28 eV. The results are discussed in relation to the excitonic mechanism of defect production and the formation of interstitial iodine aggregates of various types in alkali iodides.     

Electronic excitation effects in CeO2 under irradiations with high-energy ions of typical fission products

In order to understand the formation mechanism of a crystallographic re-structuring in the periphery region of high-burnup nuclear fuel pellets, named as “rim structure”, information on the accumulation process of radiation damage and fission products (FPs), as well as high-density electronic excitation effects by FPs, are needed. In order to separate each of these processes and understand the high-density electronic excitation effects, 70–210 MeV FP ion (Xe^10–14+, I⁷⁺ and Zr⁹⁺) irradiation studies on CeO₂, as a simulation of fluorite ceramics of UO₂, have been done at a tandem accelerator of JAEA-Tokai and the microstructure changes were determined by transmission electron microscope (TEM). Measurements of the diameter of ion tracks, which are caused by high-density electronic excitation, have clarified that the effective area of electronic excitation by high-energy fission products is around 5–7 nm  and the square of the track diameter tends to follow linear function of the electronic stopping power (S_e). Prominent changes are hardly observed in the microstructure up to 400 °C. After overlapping of ion tracks, the elliptical deformation of diffraction spots is observed, but the diffraction spots are maintained at higher fluence. These results indicate that the structure of CeO₂ is still crystalline and not amorphous. Under ion tracks overlapping heavily (>1 × 10¹⁵ ions/cm²), surface roughness, with characteristic size of the roughness around 1 μm, is observed and similar surface roughness has also been observed in light-water reactor (LWR) fuels.     

Neutron-irradiation effect in ceramics evaluated from macroscopic property changes in as-irradiated and annealed specimens

Macroscopic length (linear swelling) and thermal diffusivity changes were measured for heavily neutron-irradiated -Al₂O₃, AlN, β-Si₃N₄ and β-SiC that were irradiated under the same capsule to compare the difference between these materials. And in addition, several capsules were irradiated under different temperatures (646–1039 K) and to different neutron doses (0.4–8.0 × 10²⁶ n/m²) in the Japanese experimental fast reactor JOYO. The swelling and the thermal diffusivity of as-irradiated specimens showed some dependence on the neutron-irradiation dose and the irradiation temperature, and that indicates stability under neutron-irradiation environments. Alpha-Al₂O₃ and AlN showed relatively large swelling and degradation of thermal diffusivity than β-Si₃N₄ and β-SiC. This difference is related to the crystal structure of each material. The dependence of swelling on irradiation dose, that is, -Al₂O₃ showed linear inclination but β-Si₃N₄ and β-SiC showed saturation, supports the model of defect structures. In addition, annealing behaviors of swelling and thermal diffusivity were compared to analyze the behavior of defects at higher temperature.     

Low energy nitrogen ion doping into GaAs using combined ion-beam and molecular-beam epitaxy method

Low energy nitrogen (N) ions were irradiated during the epitaxial growth of GaAs using combined ion beam and molecular beam epitaxy (CIBMBE) method as a function of N⁺ ion acceleration energy (E_a) and N⁺ ion beam current density (I_N). E_a was varied from 70 to 170 eV I_N from 900 pA/cm² to 75 nA/cm². GaAs growth rate was fixed to 1 μm/h. In 2 K photoluminescence (PL) spectra of the samples with I_N = 3 nA/cm² and E_a = 70–100 eV, two sharp emissions at 1.508 eV (X₁) and 1.495 eV (X₂), which have been attributed to the emissions of excitons bound to isolated N atoms, and another one at 1.443 eV (X₅) were observed. These results show that nitrogen (N) atom in GaAs becomes optically active as an isoelectronic impurity at least in as-grown condition. For N⁺ ion-irradiated samples with rather high I_N, e.g., with I_N = 75 nA/cm² and E_a = 100 eV, a broad emission together with multiple sharp ones were observed after furnace annealing at 750°C which were ascribed to emissions of excitons bound to nitrogen-nitrogen (N---N) pairs.     

Temperature dependence of luminescence from a silica glass under in-reactor irradiation

We have investigated in-reactor luminescence (IRL) from a silica glass at temperatures ranging from 100 K to 250 K. The IRL consists mainly of a broad emission band peaked at 2.7 eV assigned to oxygen deficient centers produced in the silica glass under the in-reactor irradiation. The 2.7 eV emission intensity linearly increased with the irradiation time and its increasing rate was larger for higher irradiation temperatures. However, this temperature dependence is inconsistent with that for the defect production rate and the cause is not clear at present. The initial intensity of the 2.7 eV IRL band increases with temperature, showing an activation energy of ca 21 meV. This value is much lower than those observed in the temperature dependence of the 2.7 eV photoluminescence (PL) and the cathodeluminescence (CL) induced by 8 keV electron irradiation. These results suggest that in IRL, some electrons excited to higher energy levels than the luminescence level are likely transferred to the luminescence state without thermal activation, resulting in a lower activation energy in their temperature dependence.     

Ion-velocity dependence of high-density electronic excitation effects in oxide superconductors

Thin films of EuBa₂Cu₃O_y oxide superconductor have been irradiated with high energy heavy ions (80 MeV I, 125 MeV Br, 1.1 GeV Mo and 3.5 GeV Xe) having same electronic stopping power, S_e, in order to investigate the ion-velocity dependence of the electronic excitation effects under the constant electronic energy deposition. Although S_e is constant, a strong reduction in the irradiation effect on lattice parameter with increasing ion-velocity is observed in the low ion-velocity region around E  1 MeV/nucleon, while the ion-velocity dependence is hardly observed in the high ion-velocity region of E > 10 MeV/nucleon. If the observed velocity-dependence is assumed to be due to the change in the fraction of S_e contributing to defect creation, the fraction in the low velocity region (E  0.6 MeV/nucleon) is estimated to be about two times larger than that in the high velocity region (E > 10 MeV/nucleon).     

Radiation-Induced Charge Transport and Charge Buildup in SiO2 Films at Low Temperatures

Studies of the temporal, temperature, and electricfield dependences of radiation-induced charge transport have been performed for radiation-hardened SiO2 films. At room temperature for high applied fields, nearly all electrons and holes generated in the oxide by a pulse of ionizing radiation (5-keV electrons) drift to the interfaces, whereas at low temperatures only electrons contribute to observed transport for relatively low fields. Below ~130°K at high fields, field-induced emission of trapped holes occurs, giving rise to collection within seconds of a significant fraction of the total number of holes generated. The present hole transport data are accounted for quite well in terms of a multiple-trapping model with a spread in trap levels ranging from ~0.3 to ~0.5 eV from the valence band. Comparison with the stochastic hopping transport model is made and that model is found to be less satisfactory in explaining these data. Charge buildup was examined in a Co60 environment and it is demonstrated that oxides exhibiting radiation tolerance at room temperature display severe radiation-induced changes at 77°K. It is also demonstrated that low-temperature charge buildup problems can be alleviated either by employing an ion-implanted oxide or by applying a relatively high field to the oxide during irradiation.     

Energy window effect in chemisorption of C36 on diamond (0 0 1)-(2×1) surface

In this paper, the collision of a C₃₆, with D_6h symmetry, on diamond (0 0 1)-(2×1) surface was investigated using molecular dynamics (MD) simulation based on the semi-empirical Brenner potential. The incident kinetic energy of the C₃₆ ranges from 20 to 150 eV per cluster. The collision dynamics was investigated as a function of impact energy E_in. The C₃₆ cluster was first impacted towards the center of two dimers with a fixed orientation. It was found that when E_in was lower than 30 eV, C₃₆ bounces off the surface without breaking up. Increasing E_in to 30–45 eV, bonds were formed between C₃₆ and surface dimer atoms, and the adsorbed C₃₆ retained its original free-cluster structure. Around 50–60 eV, the C₃₆ rebounded from the surface with cage defects. Above 70 eV, fragmentation both in the cluster and on the surface was observed. Our simulation supported the experimental findings that during low-energy cluster beam deposition small fullerenes could keep their original structure after adsorption (i.e. the memory effect), if E_in is within a certain range. Furthermore, we found that the energy threshold for chemisorption is sensitive to the orientation of the incident C₃₆ and its impact position on the asymmetric surface.     

Loop formation by ion irradiation in yttria stabilized zirconia

Yttria stabilized zirconia (YSZ) is a candidate material focused as optical and insulating materials in nuclear reactors. Therefore, it is useful to investigate defect formation during irradiation, in order to assess YSZ resistance to radiation damage. In the present study, in situ transmission electron microscopy (TEM) observations were performed on YSZ during 30 keV Ne⁺ ion irradiation in the temperature range of 723–1123 K (using 100 K intervals). Results revealed that damage evolution morphology depends on irradiation temperature. For irradiations below 1023 K, defect clusters and bubbles were formed simultaneously. On the other hand, at 1123 K, only bubbles were formed in the initial stage of irradiation. Loops formed later following the bubble formation. It was also observed that, in the early stage of irradiation above 923 K, larger bubbles were formed along the loop planes compared with other areas.

TEM observations indicated that dislocation loops formed on three kinds of crystallographic planes: namely, {1 0 0}, {1 1 1} and {1 1 2} planes.     

Deduction of the He–Fe interaction potential in eV-range from experimental data by computer simulation in grazing ion-surface scattering: Row-model

In glancing-angle scattering of keV-ions from a crystal surface, the ion reflection takes place in the eV-part of the interaction potentials. The elastic interactions are determined by the energy transverse to atomic rows, which can be of the order of 10 eV. A row-model using averaged potentials according to the Lindhard cylindrical potential has been developed using step-by-step integration of Newton's equations of motion. Previously [D. Danailov, K. Gärtner, A. Caro, Nucl. Instr. and Meth. B 153 (1999) 191; presented on COSIRES, Okayama, 1998] we reported that zig–zag trajectories within surface channels and the corresponding multimode azimuthal angular distributions of reflected ions are very sensitive to the interaction potential used in the simulation. Here we simulate the scattering of 15 keV He-atoms from Fe(1 0 0) surfaces at different angles of incidence comparable with previously published experimental results [D. Danailov, T. Igel, R. Pfandzelter, H. Winter, Nucl. Instr. and Meth. B 164–165 (2000) 583]. Our results show that for interaction energies below about 4 eV the well-known “universal” potential works well. However, for energies between 4 and 13 eV the “individual” He–Fe potential (D. Danailov, K. Gärtner, A. Caro, Nucl. Instr. and Meth. B 153 (1999) 191; presented on COSIRES, Okayama, 1998) gives a better agreement with the experimental data. For interaction energies above 13 eV both potentials are similar. We have constructed a mixed He–Fe potential, which describes the experimental observations well. The row-model enables us to deduce the He–Fe interaction potential in the eV-range. In addition, a shift in the experimental angular spectra compared with the calculated spectra indicates that the atomistic rows undergo an elastic horizontal bend due to the scattering and an order of magnitude smaller vertical displacement.     

Mean excitation energies extracted from stopping power measurements of protons in polymers by using the modified Bethe–Bloch formula

Recent stopping power measurements in thin polymeric films have been performed for protons of 0.4–3.5 MeV energies using the indirect transmission technique [H. Ammi, S. Mammeri, M. Chekirine, B. Bouzid, M. Allab, Nucl. Instr. and Meth. B 198 (2002) 5]. Experimental stopping data have been analyzed with the modified Bethe–Bloch formula and the mean excitation energies I have been then extracted from the data. Resulting values for each thin film are 76 ± 1 eV in Mylar, 70.8 ± 1 eV in Makrofol, 82.2 ± 1.2 eV in LR-115 and 55.4 ± 1 eV in Polypropylene. The I-extracted values are compared to those I_B calculated by using the Bragg’s rule.     

Molecular dynamics modelling of radiation damage in normal, partly inverse and inverse spinels

The radiation response of perfect crystals of MgAl₂O₄, partially inverted MgGa₂O₄ and fully inverse MgIn₂O₄ were investigated using molecular dynamics. Dynamical cascades were initiated in these spinels over a range of trajectories with energies of 400 eV and 2 keV for the primary knock-on event. Collision cascades were set up on each of the cation and anion sublattices and were monitored up to 10 ps. Simulations in the normal MgAl₂O₄ spinel for the 2 keV energy regime resulted in similar defect structures as obtained at the post-threshold 400 eV energies, with little clustering occurring. The predominant defect configurations were split interstitials and cation antisites. For the inverse spinels, a much wider variety of lattice imperfections was observed. More defects were also produced due to the formation of interstitial–vacancy cation chains and oxygen crowdions.     

Surface excitation parameter for semiconducting III–V compounds

In the rapid development of mesoscopic science, the study of surface excitations in solids and overlayer systems plays a crucial role. The surface excitation parameter which describes the total probability of surface plasmon excitations by an electron traveling in vacuum before impinging on or after escaping from a semiconducting III–V compound has been calculated for 200–2000 eV electrons crossing the compound surface. These calculations were performed using the dielectric response theory with sum-rule-constrained extended Drude dielectric functions established by the fits of these functions to optical data. Surface excitation parameters calculated for InSb, InAs, GaP, GaSb or GaAs III–V compounds were found to follow to a simple formula, i.e. P_s = aE^−b, where P_s is the surface excitation parameter and E is the electron energy. These surface excitation parameters were then applied to determine the elastic reflection coefficient for electrons elastically backscattered from III–V compounds using the Monte Carlo simulations. Good agreement was found for the electron elastic reflection coefficient between calculated results and experimental data.     

Effects of 160 MeV Ni ion irradiation on polypyrrole conducting polymer electrode materials for all polymer redox supercapacitor

Electronically conducting polymers are suitable electrode materials for high performance supercapacitors, for their high specific capacitance and high dc conductivity in the charged state. Supercapacitors and batteries are energy storage and conversion systems which satisfies the requirements of high specific power and energy in a complementary way. Ion beam {energy > 1 MeV} irradiation on the polymer is a novel technique to enhance or alter the properties like conductivity, density, chain length and solubility.

Conducting polymer polypyrrole thin films doped with LiClO₄ are synthesized electrochemically on ITO coated glass substrate and are irradiated with 160 MeV Ni¹²⁺ ions at different fluence 5 × 10¹⁰, 5 × 10¹¹ and 3 × 10¹² ions cm⁻². Dc conductivity measurement of the irradiated films showed 50–60% increase in conductivity which is may be due to increase of carrier concentration in the polymer film as observed in UV–Vis spectroscopy and other effects like cross-linking of polymer chain, bond breaking and creation of defects sites. X-ray diffractogram study shows that the degree of crystallinity of polypyrrole increases in SHI irradiation and is proportionate to ion fluence. The capacitance of the irradiated films is lowered but the capacitance of the supercapacitors with irradiated films showed enhanced stability compared to the devices with unirradiated films while characterized for cycle life up to 10,000 cycles.     

Core performance of new concept passive-safety reactor “kamado” - safety, burn-up and uranium resource problem -

New concept of a passive-safety reactor “KAMADO” has a negligible possibility of core melting and flexibility of total reactor power. The reactor core of KAMADO consists of fuel elements of graphite blocks, which have UO₂ fuel rods and cooling water holes. These fuel elements are located in a reactor water pool of atmospheric pressure (1 atm) and low temperature (< 60°C). In case of LOCA, decay heat from fuel rods is removed by conduction heat transfer to the reactor water pool. Since the cooling water does not contact a fuel rod directly, core design has much flexibility without considering dry-out limitation and Minimum Critical Power Ratio (MCPR). Additionally an effective use of spent fuel is expected.     

Nucleation and growth of defect clusters in CeO2 irradiated with electrons

Nucleation and growth process of defect clusters in cerium dioxide (CeO₂) with fluorite-type crystal structure has been investigated in situ under electron irradiation by using high voltage transmission electron microscopy. Planar defect clusters were formed with electron irradiation ranging from 200 to 1000 keV at temperatures below 450 K. The defect clusters were determined to be faulted-interstitial type dislocation loops lying on {1 1 1} planes. The growth rate of dislocation loops was found to increase with decreasing electron energy. An analysis of the fluence dependence of the growth process of dislocation loops suggests an increase in the vacancy mobility with decreasing electron energy. The rate of the electronic excitation is discussed in terms of the radiation-induced diffusion of oxygen-ion vacancies.