Release of Al from SiC targets used for radioactive ion beam production

On-line targets of silicon carbide have been used to produce radioactive ion beams (RIB) at TRIUMF’s ISAC facility. Surface ionized beams of Li, Na and Al from SiC targets have been delivered to a variety of experiments. While yields of Li and Na radionuclide beams have been high enough to meet experimental requirements, the yields of radio-aluminums have been very low in comparison to in-target production estimates. The properties of SiC relevant to radioactive Al ion beam production are reviewed and on-line measurements of radiogenic Li, Na and Al from SiC are presented. Diffusion, effusion and formation of stable phases are found to strongly affect the release of radio-aluminum from SiC, greatly reducing overall Al RIB production efficiency. In comparison, the production of radio-sodium beams from SiC is found to be relatively efficient.     

Kinetics of precipitation of U4O9 from hyperstoichiometric UO2+x

For safe and reliable operation of fission reactors in space, the phase diagrams and reaction kinetics of systems used as nuclear fuels, such as U-O, U-N, U-C, are required. Diffraction allows identification of phases and their weight fractions as a function of temperature in situ, with a time resolution of the order of minutes. In this paper, we will provide results from a neutron diffraction experiment studying the U-O system. Using the neutron diffractometer HIPPO, the decomposition of UO_2+x into UO₂ and U₄O₉ as a function of temperature was investigated in situ. From the diffraction data, the participating phases could be identified as UO_2+x, UO₂ and U₄O_8.94 and no stoichiometric U₄O₉ was found. Results of the experiment were used to improve existing thermodynamic models. The presented techniques (i.e., neutron diffraction and thermodynamic modeling) are also applicable to the other systems mentioned above.     

Near threshold vibrational excitation of molecules by positron impact: A projection operator approach

We report vibrational excitation (ν_i=0→ν_f=1) cross-sections for positron scattering by H₂ and model calculations for the (ν_i=0→ν_f=1) excitation of the C-C symmetric stretch mode of C₂H₂. The Feshbach projection operator formalism was employed to vibrationally resolve the fixed-nuclei phase shifts obtained with the Schwinger multichannel method. The near threshold behavior of H₂ and C₂H₂ significantly differ in the sense that no low lying singularity (either virtual or bound state) was found for the former, while a e⁺-acetylene virtual state was found at the equilibrium geometry (this virtual state becomes a bound state upon stretching the molecule). For C₂H₂, we also performed model calculations comparing excitation cross-sections arising from virtual (-iκ₀) and bound (+iκ₀) states symmetrically located around the origin of the complex momentum plane (i.e. having the same κ₀). The virtual state is seen to significantly couple to vibrations, and similar cross-sections were obtained for shallow bound and virtual states.     

Polarization effects on the cross-section of ion-atom collisions

Coupled-channel cross-sections for electron capture, ionization and electron loss due to polarization effects are calculated. The maximum impact parameter for electron escape is analyzed within the classical framework. The probabilities of ionization and capture are analyzed simultaneously by a semi-empirical method. Differing from the n-body classical trajectory Monte Carlo method, the condition for electron escape is determined by Coulomb forces related to the two nuclei. This method can be used to calculate coupled-channel cross-sections rather than single-channel ones in other methods. Therefore the calculated results can be compared with experimental data directly. In the low energy range, neglecting the ionization effect, the single-capture cross-sections of hydrogen atoms induced by various partially-stripped ions were calculated. In the high energy range, neglecting the capture effect on ionization, the pure-ionization cross-sections of neon atoms induced by Ne^q+ (q = 4, 6, 8) and Ar^q+ (q = 4, 6, 8, 10) at an incident energy E = 1.05 MeV/u were calculated. Good agreement was found between our calculation and experimental data in the literature. This method had been partially applied for intermediate energy successfully.     

Nuclear resonant reflectivity with standing waves for the investigation of a thin Fe layer buried inside a superconducting Si/[Mo/Si]45/Fe/Nb multilayer

A special multilayer sample Si/[Mo/Si]₄₅/⁵⁷Fe/Nb has been prepared for the depth selective investigations of the hyperfine fields in thin iron layer at low temperatures above and below the superconducting transition in the top Nb layer (T_c ∼ 8 K) by means of the nuclear resonant reflectivity with standing waves. The periodic multilayer [Mo/Si]₄₅ below the iron layer in our sample was used as “a standing wave generator”. A weak magnetic hyperfine splitting in the ⁵⁷Fe layer was detected just at low temperature. A slight variation of the nuclear resonant reflectivity time spectra measured above and below T_c was observed. At first it was supposed that this change of the spectrum shape is caused by the spatial modulation of ferromagnetic domains in the ⁵⁷Fe layer caused by a proximity effect. A closer analysis, however, reveals that the spectrum variations are due to just the changes of the relative weights of the magnetic and paramagnetic phases in ⁵⁷Fe layer.     

Evaluated activation cross sections of longer-lived radionuclides produced by deuteron induced reactions on natural iron up to 10 MeV

Charged particle induced reactions can be used to monitor in real time the wear of mechanical structures such as internal combustion engines and turbines. Charged particle induced reactions with suitable properties also can be used for monitoring the actual parameters of bombarding particle beams. One of these reactions is the ^natFe(d,x)⁵⁷Co process. The available experimental data in the literature are well established around and above the maximum of the excitation function, however only a little, scattered data are available in the threshold energy region of the reaction. In this work we measured the excitation functions of relevant processes resulting in ^55,56,57,58Co, ⁵⁹Fe and ^52,54Mn induced by deuteron beams on natural iron in the energy range from threshold up to 10 MeV. The new experimental data collected are used to determine with high confidence recommended values for the missing low energy part of the investigated excitation functions.     

Determination of bulk etch rate for CR-39 nuclear track detectors using an X-ray fluorescence method

The thickness of a CR-39 detector is determined using an energy dispersive X-ray fluorescence (EDXRF) method of analysis. The method is based on exciting a suitable target and measuring the intensity of its fluorescence X-ray lines passing through the CR-39 sample in a fixed geometry. By properly selecting the target material, the method succeeds in assessing the thickness change of CR-39 detectors etched for different time intervals. The bulk etch rate (V_b) may thus be obtained, which is an important parameter for any solid state nuclear track detector. Application of the EDXRF method yielded a value of V_b = (2.01 ± 0.04) μm h⁻¹ for etching in a 6 N NaOH solution at 75 °C. This value agrees with the bulk etch rate of (1.90 ± 0.03) μm h⁻¹, obtained by the conventional mass-change method.     

Study of ion emission from a germanium crystal surface under impact of fast Pb ions in channeling conditions

A thin germanium crystal has been irradiated at GANIL by Pb beams of 29 MeV/A (charge state Q_in = 56 and 72) and of 5.6 MeV/A (Q_in = 28). The induced ion emission from the sample entrance surface was studied, impact per impact, as a function of Q_in, velocity v_in and energy loss ΔE in the crystal. The Pb ions transmitted through the crystal were analyzed in charge (Q_out) and energy using the SPEG spectrometer. The emitted ionized species were detected and analyzed in mass by a time-Of-flight multianode detector (LAG). Channeling was used to select peculiar ΔE values in Ge and hence peculiar Pb ion trajectories close to the emitting entrance surface. The experiment was performed in standard vacuum. No Ge emission was found. The dominating emitted species are H⁺ and hydrocarbon ions originating from the contamination layer on top of the crystal. The mean value 〈M〉 of the number of detected species per incoming Pb ion (multiplicity) varies as (Q_in/v_in)^p, with p values in agreement with previous results. We have clearly observed an influence of the energy deposition ΔE in Ge on the emission from the top contamination layer. When selecting increasing values of ΔE, we observed a rather slow increase of 〈M〉. On the contrary, the probabilities of high multiplicity values, which are essentially connected to fragmentation after emission, strongly increase with ΔE.     

Ion mass dependence for low energy channeling in single-wall nanotubes

An Monte Carlo (MC) simulation program has been used to study ion mass dependence for the low energy channeling of natural- and pseudo-Ar ions in single-wall nanotubes. The MC simulations show that the channeling critical angle Ψ_C obeys the (E)^−1/2 and the (M₁)^−1/2 rules, where E is the incident energy and M₁ is the ion mass. The reason for this may be that the motion of the channeled (or de-channeled) ions should be correlated with both the incident energy E and the incident momentum (2M₁E)^1/2, in order to obey the conservation of energy and momentum.     

Safe radioisotope thermoelectric generators and heat sources for space applications

Several isotopes are examined as alternatives to ²³⁸Pu that is traditionally used in radioisotope thermoelectric generators (RTGs) and heating units (RHUs). The radioisotopes discussed include ²⁴¹Am, ²⁰⁸Po, ²¹⁰Po, and ⁹⁰Sr. The aim of this study is to facilitate the design of an RTG with a minimal radiation dose rate and mass including any required shielding. Applications of interest are primarily space and planetary exploration. In order to evaluate the properties of the alternative radioisotopes a Monte Carlo model was developed to examine the radiation protection aspect of the study. The thermodynamics of the power generation process is examined and possible materials for the housing and encapsulation of the radioisotopes are proposed. In this study we also present a historical review of radioisotope thermoelectric generators (RTGs) and the thermoelectric conversion mechanism in order to provide a direct comparison with the performance of our proposed alternative isotope systems.     

Negative differential conductivity of positrons in gases

This paper reports on a new series of calculations of positron transport properties based on current experimental cross section data. It is found that negative differential conductivity (NDC) occurs in the bulk drift velocity W but not the flux drift velocity w. The origin of the phenomenon lies in the “reactive” nature of positron collisions associated with positronium Ps formation, and is quite different in origin to the better known NDC effect in w arising from certain combinations of inelastic-elastic cross sections. Moreover, while the Ps formation process is qualitatively similar (at least from a kinetic theory perspective) to electron attachment, it is characterized by a cross section several orders of magnitude larger and hence the “reactive” NDC effect is correspondingly more pronounced. In this paper we test both established conditions for NDC, and develop new criteria, using simple mathematics and physical arguments where possible.     

Cross-sections and transport properties of F ions in Ar, Kr and Xe

The aim of the current work is to determine with relatively high accuracy the elastic momentum-transfer cross-sections of F⁻ ions in collisions with noble gases Ar, Kr and Xe. We have applied a simple form of momentum-transfer theory (MTT) based on elastic collisions as the first step in order to develop negative fluorine ion/Ar, Kr, Xe elastic momentum-transfer cross-sections based on the available data for reduced mobilities at 300 K as a function of E/N. In our procedure MTT is used only as the initial step to make adjustments and thus save a lot of computation time. However, our final results have been all obtained by exact Monte Carlo (MC) technique that has been well tested and documented. The unfolded cross-sections were validated or further improved by assuring a good agreement between our MC calculated transport data and the available experimental results for reduced mobility and longitudinal diffusion in the range from 1 Td to 500 Td. We have also calculated the net rates of elastic scattering and detachment.     

Evaluation of alloy 690 process pot at the contact with borosilicate melt pool during vitrification of high-level nuclear waste

Understanding the material behaviour under service conditions is essential to enhance the life span of alloy 690 process pot used in vitrification of high-level nuclear waste. During vitrification process, interaction of alloy 690 with borosilicate melt takes place for substantial time period. Present experimental studies show that such interactions may result in Cr carbide precipitation along grain boundaries, Cr depletion in austenitic matrix and intergranular attack close to alloy 690/borosilicate melt pool interfaces. Widths of Cr depleted zone within alloy 690 is found to follow kinetics of the type x = 10.9 × 10⁻⁶ + 1 × 10⁻⁸t^1/2 m. Based on the experimental results it is recommended that compositional modification of alloy 690 process pot adjacent to borosilicate melt pool need to be considered seriously for any efforts towards reduction and/or prevention of process pot failures.     

Equilibrium and nonequilibrium molecular dynamics simulations of heat conduction in uranium oxide and mixed uranium-plutonium oxide

The thermal conductivity of nuclear fuels such as UO_2+x and (U,Pu)O_2−x has been calculated by the molecular dynamics (MD) simulation in terms of oxygen stoichiometric parameter x, temperature and Pu content. In the present study, the MD calculations were carried out in both equilibrium (EMD) and nonequilibrium (NEMD) systems. In the EMD simulation, the thermal conductivity was defined as the time-integral of the correlation function of heat fluxes according to the Green-Kubo relationship. Meanwhile, in the homogeneous NEMD, it was given by the ratio of the time-averaged heat flux to the perturbed external force subjected to each particle in the simulated cell. NEMD, as compared with EMD, gave somewhat precise results efficiently. Furthermore, both MD calculations showed that the thermal conductivity of these oxide fuels decreased with increase of temperature and defects, i.e. excess oxygen or vacancy, and was rather insensitive to Pu content for the stoichiometric fuel.     

Polarization effects in non-relativistic ep scattering

The cross section which addresses the spin-flip transitions of a proton (antiproton) interacting with a polarized non-relativistic electron or positron is calculated analytically. In the case of attraction, this cross section is greatly enhanced for sufficiently small relative velocities as compared to the result obtained in the Born approximation. However, it is still very small, so that the beam polarization time turns out to be enormously large for the parameters of e^± beams available now. This practically rules out a use of such beams to polarize stored antiprotons or protons.     

Analysis of the potential for sub-eV Ps beams

Understanding the interactions of Ps at low energies has challenged researchers due to the inability to create Ps near thermal energies in a directed fashion. As a result, previous attempts to understand interactions such as the scattering cross-section (σ) between Ps and other atoms have relied on indirect methods. Examples are the measurements of the thermalization rate of Ps in a gas environment or lifetime measurements in various systems. In analyzing the pore size of certain nanotube materials we have observed 3-10% of incident positrons form long lived Ps within the sample. Due to the alignment of these tubes we expect Ps emitted from them to be highly directional. By focusing a time-tagged positron beam on these samples we will use the resulting directional Ps emitted at near thermal energies for various scattering experiments.     

Angular anisotropy of the RCE X-rays under planar channeling as manifestation of geometric properties of the in-crystal electric field

Planar channeled ion under resonant coherent excitation conditions experiences an action of the oscillating electric field arising in the ion rest frame. We show how the alignment of the angular momentum of coherently excited ions and, hence, the angular anisotropy of their characteristic X-ray radiation are connected with the geometrical and symmetry properties of this field. The consideration is based on two examples of (k,l)=(2,-1) and (1, 3) resonances with 423 MeV/u Fe²⁴⁺ ions in planar channel of Si crystal, corresponding to different symmetries of the resonant field. In both cases the resonant electric field is elliptically polarized. A choice of an appropriate coordinate frame allows us to show the connection between the geometrical properties of the resonant field and the X-ray angular distributions especially clear. To illustrate, we calculate angular distributions of X-rays for individual ionic trajectories using density matrix formalism and then consider formation of the angular distributions not resolved by ion trajectory. Comparison with recent experimental data is done.     

The (He, tf) as a surrogate reaction to determine (n, f) cross sections in the 10-20 MeV energy range

The surrogate reaction ²³⁸U(³He, tf) is used to determine the ²³⁷Np(n, f) cross section indirectly over an equivalent neutron energy range from 10 to 20 MeV. A self-supporting ∼761 μg/cm² metallic ²³⁸U foil was bombarded with a 42 MeV ³He²⁺ beam from the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory (LBNL). Outgoing charged particles and fission fragments were identified using the Silicon Telescope Array for Reaction Studies (STARS) consisted of two 140 μm and one 1000 μm Micron S2 type silicon detectors. The ²³⁷Np(n, f) cross sections, determined indirectly, were compared with the ²³⁷Np(n, f) cross section data from direct measurements, the Evaluated Nuclear Data File (ENDF/B-VII.0), and the Japanese Evaluated Nuclear Data Library (JENDL 3.3) and found to closely follow those datasets. Use of the (³He, tf) reaction as a surrogate to extract (n, f) cross sections in the 10-20 MeV equivalent neutron energy range is found to be suitable.     

The influence of f-type function in positron-He/positron-H2 scattering with the Schwinger multichannel method

In this paper we present results for positron-Helium and positron-H₂ scattering with the inclusion of the f-type Cartesian Gaussian functions in our computer codes of the Schwinger multichannel method (SMC). The effects of this modification can be noticed in the integral cross-section for both studied targets, with our new curves being closer to the most recent experimental measurements. The inclusion of the f-type function in the scattering wave function expansion also helped us to obtain a better set of results with the SMC method for the annihilation parameter. Data for differential cross-section (DCS) for helium is presented as well as our improvement in the DCS data in the forward scattering angles for the hydrogen molecule.