Pumping experiment of water on B and LaB6 films with an electron beam evaporator

The pumping characteristic of water vapor on boron and lanthanum hexaboride films formed with an electron beam evaporator have been investigated in high vacuum between 10⁻⁴ and 10⁻³ Pa. The measured initial maximum pumping speeds of water for the fresh B or LaB₆ films with a deposition amount from 2.3 × 10²¹ to 6.7× 10²¹ molecules/m² separately formed on a substrate are 3.2–4.9 m³/sm², and the saturation values of adsorbed water on these films are 2.1 ×10²⁰−1.3 × 10²¹ H₂O molecules/m².     

The thermal conductivity of UO2 vapor

The thermal conductivity, λ of a saturated vapor over UO_1.96 is calculated in the temperature range 3000–6000 K. The calculation shows that the contribution to λ from the transport of reaction enthalpy dominates all other contributions. All possible reactions of the gaseous species UO₃, UO₂, UO, U, O, and O₂ are included in the calculation. We fit the total thermal conductivity to the empirical equation λ = exp(a+ b/T+cT+dT² + eT³), with λ in cal/(cm s K), T in kelvins, a = 268.90, B = − 3.1919 × 10⁵, C = −8.9673 × 10⁻², d = 1.2861 × 10⁻⁵, and E = −6.7917 × 10⁻¹⁰.     

Impurity diffusion of Fe in stoichiometric UC

Measurements have been made between 1800 and 2260°C of the diffusion coefficient D of ⁵⁹Fe tracer in coarse polycrystalline near-stoichiometric UC. The results give very large values for D, around 5 × 10⁻⁸ cm²/s, comparable with the C diffusion rate and 10⁵ to 10⁶ times greater than the self-diffusion of the U in the same temperature range. The enhancement of the U-self-diffusion rate by the presence of Fe, previously observed by Matzke, is much too small to be compatible, on the basis of a vacancy mechanism, with the very large Fe diffusion rates we report here. Accordingly it is proposed that Fe may dissolve, at least partially, in interstitial solution and diffuse by a Miller-type interstitial-vacancy pair mechanism. The likely similar behaviour of other solutes is also briefly discussed.     

Effect of tin on the irradiation growth of polycrystalline zirconium

Measurements of irradiation growth of polycrystalline Zr-1.5% Sn and Zr-0.1% Sn alloys at 353 K and 553 K have been made following fast neutron irradiation with fluences up to 3.1 × 10²⁵ n/m². At 353 K, growth of Zr-1.5% Sn virtually saturated at a strain of 4.5 × 10⁻⁴ after a fluence of ˜10²⁴ n/m². At this temperature, Zr-0.1% Sn continued to grów until ˜ 2 × 10²⁵ n/m², when the strain levelled off at ˜ 1.2×10⁻³. At 553 K, Zr-1.5% Sn initially grew about twice as fast as the 0.1% Sn alloy, but both eventually reached the same steady state rate of ˜ 2.4 × 10⁻²⁹ m²/n. Comparison of the data for the 1.5% Sn material with those for Zircaloy-2 from earlier work reveals that at 353 K, growth is suppressed by the presence of Sn atoms, which may serve as vacancy traps. However, at 553 K, minor additions and impurities in Zircaloy-2 (such as Fe, Ni, Cr and O) play an important role and cannot be neglected. The growth behaviour of Zr-0.1% Sn is similar to that of pure polycrystalline zirconium, especially at 353 K, indicating that the addition of Sn at this concentration does not strongly influence the growth of zirconium.     

The diffusivities of fission-created or thermally-doped tritium in UO2

The diffusion behavior of tritium in UO₂ was studied. Two methods were adopted for the introduction of tntium into UO₂: one via ternary fission of ²³⁵U and the other via thermal doping. In the former, the diffusion constants decreased with increase in sample weight. The diffusion constants obtained from the pellet with the same specification (9 mm in diameter, 5 mm high) were D″_bulk = 3.03 × 10⁻³(^+0.369_−0.003) exp[−163±43(kJ/mol)/RT](cm²/s) for fission-created tritium and D′_bulk = 0.15(^{+ 0.94}_−0.13) exp[−76±13 (kJ/mol)/RT](cm²/s) for thermally-doped tritium. The difference of the diffusion constants between two systems was discussed in terms of the effects associated with the recoil processes of energetic tritium.     

The release of fission-recoiled xenon from Zr-2.5 wt% Nb alloy

The release of fission-recoiled ¹³³Xe from Zr-2.5 wt% Nb alloy was measured in the temperature range 640–1080 K. In the range 640–880 K, where purely phase exists, a linear relationship between log D versus 1/T is observed and can be represented by the equation: D(640–880 K) = 6.24 × 10⁻⁹exp(−142.7 kJmol/RT)m²/s. The release has been attributed to the non-volume diffusion process.

In the temperature range 930–1080 K where both and β phases coexist, the linearity in the plots of log D versus 1/T is violated.

The present values of the release parameters have been compared with the corresponding values for the release of fission-recoiled ¹³³Xe from Zircaloy-2. Alloying elements seem to have very small effect on the release kinetics. The results have been presented and discussed.     

Measurements of natural concentrations of I in uranium ores by accelerator mass spectrometry

An accelerator mass spectrometry system is described and utilized for measurements of ¹²⁹I concentrations in natural and environmental samples. We report here on measurements of ¹²⁹I isotopic abundances in iodine reagents and in iodine of mineral origin and of ¹²⁹I concentrations in uranium ores of different origins. The ¹²⁹I isotopic abundances for two measured contemporary iodine reagents and for iodine from a deep underground brine are 1.3 × 10⁻¹³ and about 4 × 10⁻¹⁴, respectively. ¹²⁹I/U ratios in the range 10⁻¹³–10⁻¹² are measured and compared to a simple model of ¹²⁹I production by spontaneous and induced fission of uranium. No clear correlation with the uranium concentrations or residence times is observed.     

Defect evolution in MeV ion-implanted silicon

Lightly doped silicon samples of both n- and p-type have been implanted with low doses of H, B and Si ions using energies between 1 and 6 MeV. The resulting electrically active point defects were characterized by deep level transient spectroscopy (DLTS) and several of these defects involve oxygen and/or carbon, two major impurities in as-grown crystalline silicon. Both interstitial- and vacancy-type defects are observed; in particular, interstitial carbon is found to migrate at room temperature with a diffusion constant of 1 × 10⁻¹⁵ cm² s⁻¹ and is effectively trapped by interstitial oxygen atoms. The concentration of implantation-induced defects increases linearly with dose but the defect production decreases at high enough dose rates. This dose rate effect depends on the ion mass and is qualitatively predicted by computer simulations of the defect reaction kinetics.     

Mechanism of dynamic strain aging and characterization of its effect on the low-cycle fatigue behavior in type 316L stainless steel

Low-cycle fatigue tests were carried out in air in a wide temperature range from 20 to 650 °C with strain rates of 3.2 × 10⁻⁵–1 × 10⁻² s⁻¹ for type 316L stainless steel to investigate dynamic strain aging (DSA) effect on the fatigue resistance. The regime of DSA was evaluated using the anomalies associated with DSA and was in the temperature range of 250–550 °C at a strain rate of 1 × 10⁻⁴ s⁻¹, in 250–600 °C at 1 × 10⁻³ s⁻¹, and in 250–650 °C at 1 × 10⁻² s⁻¹. The activation energies for each type of serration were about 0.57–0.74 times those for lattice diffusion indicating that a mechanism other than lattice diffusion is involved. It seems to be reasonable to infer that DSA is caused by the pipe diffusion of solute atoms through the dislocation core. Dynamic strain aging reduced the crack initiation and propagation life by way of multiple crack initiation, which comes from the DSA-induced inhomogeneity of deformation, and rapid crack propagation due to the DSA-induced hardening, respectively.     

Angular effects in the sputtering of Cr from stainless steel by 10 keV H3

Energy and angular distributions of Cr⁺ sputtered from stainless steel by 1.6 × 10⁻¹⁵ J (10 keV) H⁺₃ are reported as a function of angle of incidence. For more normal incidence, the peak in the energy distribution occurs in the vicinity of 3.2 × 10⁻¹⁹ J (2 eV), the average energy is approximately 1.12 × 10⁻¹⁸ J (7 eV), and the angular distribution is close to cosine. Toward glancing incidence, the peak energy increases to ˜6.4 × 10⁻¹⁹ J (4 eV), the average energy increases to ˜1.28 × 10⁻¹⁸ J (8.0 eV), and the angular distribution shows a distinct maximum in the forward direction. These results are discussed in terms of the increasing role of surface recoils in the sputtering mechanism at glancing incidence.     

Effects of ion implantation on poly(dimethylsilylene-co-methylphenylsilylene)

The effects of ion implantation on the electrical and structural properties of poly(dimethylsilylene-co-methylphenylsilylene), (DMMPS) thin films have been investigated. Ionic species of krypton, arsenic, fluorine, chlorine, and sulfur were implanted at energies ranging from 35 to 200 keV and with doses of up to 1 × 10¹⁶ ion cm². The conductivity of the polymer increased upon implantation reaching a maximum value of 9.6 × 10⁻⁶ (Ω cm)⁻¹ for the case of arsenic ion at a dose of 1 × 10¹⁶ ion cm² and energy of 100 keV. The results showed that ion implantation induced conduction in DMMPS was primarily due to structural modifications of the material brought about by the, energetic ions. Infrared analysis and Auger electron spectroscopy showed evidence for the formation of a silicon carbide-like structure upon implantation.     

Ion beam synthesis and recrystallization of amorphous SiGe/SiC structures

Si_1−xGe_x amorphous layers implanted with different doses of carbon (between 5 × 10¹⁵ and 2 × 10¹⁷ cm⁻² and annealed at 700°C and 900°C have been analyzed by Raman and Infrared spectroscopies, electron microscopy and Auger electron spectroscopy. The obtained data show the synthesis of amorphous SiC by implanting at the highest doses. In these cases, recrystallization only occurs at the highest annealing temperature (900°C). The structure of the synthesized SiC strongly depends on the implantation dose, in addition to the anneal temperature. For the highest dose (2 × 10¹⁷ cm⁻²), crystalline β-SiC is formed. Finally, a strong migration of Ge towards the Si substrate is observed from the region where SiC precipitation occurs.     

Neutron beam design for both medical neutron capture therapy and industrial neutron radiography for TRIGA reactor

Neutron beam designs were studied for TRIGA reactor with a view to generating thermal, epithermal and fast neutron beams for both medical neutron capture therapy (NCT) and industrial neutron radiography (NR). The beams are delivered from thermal and thermalizing columns, and also horizontal beam hole. Several prospective neutron filters (high-density graphite (G), bismuth (Bi), single-crystal silicon (Si), aluminum (Al), aluminum oxide (Al₂O₃), aluminum fluoride (AlF₃) and lead fluoride (PbF₂)) were examined for obtaining sufficiently intense neutron beam for various applications. Monte Carlo calculations indicated that with a suitable neutron filter arrangement, thermal and epithermal neutron beams attaining 2×10⁹ and 7×10⁸ n cm⁻²S⁻¹, respectively, could be obtainable from thermal and thermalizing columns with the reactor operating at 100 kW. These neutron beams could be adopted for boron neutron capture therapy. Compared with these columns, horizontal beam port would deliver neutron fluxes of 10⁻² 10⁻³ lower intensity, but produced thermal and neutron beams would be adequate for different application of nondestructive inspection by neutron radiography.     

A study of the blue photoluminescence emission from thermally-grown, Si-implanted SiO2 films after short-time annealing

Thermal SiO₂ films have been implanted with Si⁺ ions using double-energy implants (200 + 100 keV) at a substrate temperature of about −20°C to total doses in the range 1.6 × 10¹⁶−1.6 × 10¹⁷ cm⁻² followed by short-time thermal processing, in order to form a Si nanostructure capable of yielding blue photoluminescence (PL). The intensity and the peak position of the PL band have been investigated as a function of ion dose, manner of heat treatment, anneal time and anneal temperature. For the formation of blue PL emitting centres, optimum processing conditions in terms of excess Si concentration and overall thermal budget are mandatory. The nature of the observed blue emission is discussed.     

Measurements of the erosion of stainless steel, carbon, and SiC by hydrogen bombardment in the energy range of 0.5 – 7.5 keV

Total erosion yields by sputtering and blistering for 1 to 15 keV H₂⁺ bombardment at normal incidence have been measured by weight loss of 304 stainless steel, pyrolytic graphite, carbon fibres, glassy carbon and SiC. The erosion yields are in the range of 3 × 10⁻³ to 2.6 × 10⁻² atoms per incident hydrogen atom. Observation in the scanning electron microscope shows that blisters occur in stainless steel and SiC at doses of 5 × 10¹⁸ particles/cm², but disappear at doses of 5 × 10 particles/cm² . The surface roughening observed depends largely on grain orientation. On carbon no blistering could be found. After bombardment the carbon surfaces are generally more smooth than before.     

Surface morphology of the Ar ion-irradiated Ag/Si(1 1 1) system

In the present study, a 500 Å thin Ag film was deposited by thermal evaporation on 5% HF etched Si(1 1 1) substrate at a chamber pressure of 8×10⁻⁶ mbar. The films were irradiated with 100 keV Ar⁺ ions at room temperature (RT) and at elevated temperatures to a fluence of 1×10¹⁶ cm⁻² at a flux of 5.55×10¹² ions/cm²/s. Surface morphology of the Ar ion-irradiated Ag/Si(1 1 1) system was investigated using scanning electron microscopy (SEM). A percolation network pattern was observed when the film was irradiated at 200°C and 400°C. The fractal dimension of the percolated pattern was higher in the sample irradiated at 400°C compared to the one irradiated at 200°C. The percolation network is still observed in the film thermally annealed at 600°C with and without prior ion irradiation. The fractal dimension of the percolated pattern in the sample annealed at 600°C was lower than in the sample post-annealed (irradiated and then annealed) at 600°C. All these observations are explained in terms of self-diffusion of Ag atoms on the Si(1 1 1) substrate, inter-diffusion of Ag and Si and phase formations in Ag and Si due to Ar ion irradiation.     

The Stuttgart positron beam, its performance and recent experiments

A monoenergetic MeV positron (e⁺) beam, with a flux at present of 6 × 10⁴ e⁺/s in the energy range of 0.5 to 6.5 MeV, has been installed at the Stuttgart Pelletron accelerator. The stabilization and the absolute calibration of the energy E is monitored by a Ge detector with real-time feedback; a relative energy stability of ΔE/E 10⁻⁴ is obtained. So far, e⁺e⁻ scattering and annihilation-in-flight experiments for investigating the low-energy e⁺e⁻ interaction as well as β⁺ γ positron lifetime measurements in condensed matter have been performed. The advantages of the β⁺ γ method compared to the conventional γγ coincidence technique have been demonstrated. Recently, triple-coincidence positron “age-momentum correlation” measurements have been carried out on fused quartz. A brief account is given on the development of a “positron clock” aiming at a substantial improvement of the time resolution of the β⁺ γ positron lifetime measurements.     

Room temperature migration of ion beam injected point defects in crystalline silicon

We have investigated the room temperature diffusion and trapping phenomena of ion beam generated point defects in crystalline Si by monitoring their interaction with dopants, native contaminants such as C and O, and other defects. Spreading resistance measurements show that a small fraction ( 10⁻⁷–10⁻⁶) of the defects generated at the surface by a 40 keV Si implant is injected into the bulk. These defects undergo trap-limited diffusion and produce dopant deactivation and/or partial annihilation of preexisting deep (several micron) defect markers, produced by MeV He implants. It is found that in highly pure, epitaxial Si layers, these effects extend to several microns from the surface, demonstrating a long range migration of point defects at room temperature. A detailed analysis of the experimental evidences allows us to identify the Si self-interstitials injected into the bulk as the major responsible of both dopant deactivation and partial annealing of vacancy-type defects (divacancies, phosphorus-vacancy and oxygen-vacancy) generated by the implants. Finally, a lower limit of 6 × 10⁻¹¹ cm²/s is obtained for the room temperature diffusivity of Si self-interstitials.     

Irradiation growth during fission fragment and proton bombardment of uranium

Irradiation growth results are reported for annealed -uranium at 373 K under 3.5 MeV proton bombardment. Two such experiments were performed at damage rates of 6.9 × 10⁻⁸ and 9.3 × 10⁻⁸ dpa/s to doses of 0.0072 and 0.0077 dpa, respectively. In each case the growth rate remained constant throughout the experiment. The respective damage normalised growth rates were 5.6 × 10⁻³ and 7.1 × 10⁻³ dpa⁻¹. Comparison between proton growth rates and published in-reactor growth rates is made by converting the more usual fuel damage parameters, such as burn-up, to dpa. Damage calculations, using the NRT damage model, are presented which indicate that, in uranium, each fission event produces 100 000 displacements. The reported growth rate of annealed, polycrystalline -uranium at 353 K, during thermal neutron irradiation, represents a damage normalised growth rate of 9.6 × 10⁻³ dpa⁻¹, which is not substantially different from the present proton results. This similarity of proton and fission growth rates appears to be contrary to the earlier finding of Thompson (1960), who deduced that proton bombardment produced two orders of magnitude less growth than fission fragments. Thompson concluded that thermal spikes played a dominant role in irradiation growth. Thompson's results and analysis are reassessed in the light of recent range data and damage models and found to be consistent with the present results in both magnitude and direction. The results are also inconsistent with Buckley's original model to the extent that thermal spikes were thought to play an important role. From a consideration of primary recoil spectra it is shown that the concept of the anisotropic aggregation of point defects to form vacancy and interstitial clusters, which is at the centre of that model, remains viable. Furthermore, similar though slightly less growth would be expected during proton bombardment. This was indeed found to be the case, the growth rate with protons being about half that with fission fragments.     

Investigation of pulsed irradiation effects on interstitial loop growth in metals

A computer program for the solution of non steady-state diffusion equations describing the evolution of point defects and interstitial dislocation loops during pulsed and continuous irradiation is developed. The equations take into account mutual recombination of point defects, defect migration to dislocation loops and line dislocations, and the existence of equilibrium thermal vacancies. It is shown that interstitial loops grow from 2 to 9 run in diameter due to the surplus flux of interstitials in the non steady-state regime (dynamic preference) at 573 K. At 873 K the dislocation loops begin to shrink owing to line tension forces. Comparison of interstitial loop and vacancy behaviour for pulsed and continuous irradiation at 573 and 873 K is performed. It is shown that at pulse duration 2 × 10⁻⁶ s and repetition rate 100 pulses/s, pulsing does not affect the interstitial loop behaviour.