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.     

The external beam microprobe facility in Florence: Set-up and performance

An external beam microprobe facility, based on a quadrupole doublet supplied by Oxford Microbeam Ltd, has been installed on a new beamline at the 3 MV single-ended Van de Graaff accelerator in Florence. The goal was to obtain a beam with a spot size on target of 10–20 μm and a current in the order of at least 1 nA, in order to allow PIXE, PIGE and RBS elemental analysis in air or in a helium atmosphere. The beam was extracted from the vacuum lines through a 0.1 μm thick Si₃N₄ window to minimise lateral straggling. The design goals have been successfully achieved; the measurements of the beam spot characteristics in vacuum as well as in air and in helium atmosphere, are here reported.     

L-shell ionization of Cd, Sb, Te, Ba, La, Eu, Tb and Yb by O ions in the energy range from 0.19 to 0.75 MeV u

L_1,2, Lβ_1,3,4, Lβ_2,15, Lγ₁₍₅₎ and Lγ_2,3(4,4′) X-ray production cross-sections have been measured on selected medium-Z elements using ¹⁶O ions with energies from 0.19 to 0.75 MeV u⁻¹. Calculated L-subshell ionization cross-sections were compared with the predictions of the direct ionization theories: the ECPSSR theory, the semiclassical approximation (SCA) in the united atom limit (SCA-UA) and SCA coupled-channel calculations (SCA-CC) involving a few lowest lying states. For the ECPSSR theory, large discrepancies were found for all three subshells. Better overall agreement between the experiment and theory was found for the SCA-UA theory. SCA-CC calculations improved the agreement between experiment and theory for the L₂-subshell in the very adiabatic region. By measuring the L X-ray production cross-sections on W targets of different thickness, possible contribution of electron capture (EC) to the X-ray production cross-section was estimated to be less than 20%. The degree of multiple ionization of outer shells caused by heavy ion bombardment was estimated from the X-ray line energy shifts and by comparing the X-ray intensity ratios for heavy ions and protons.     

Cobalt cross sections for 14 MeV neutrons

14 MeV cross sections for the reactions ⁵⁹Co(n,2n) ^58m+gCo, ⁵⁹Co(n,p) ⁵⁹Fe and ⁵⁹Co(n,) ⁵⁶Mn were measured relative to the ⁵⁶Fe(n,p) ⁵⁶Mn reaction employing the activation technique. Accuracies of about 1% were achieved for the (n,) reaction and 2% for the others. The isomeric cross section ratio was measured for the ⁵⁹Co(n,2n) reactions.

Nuclear reactions – ⁵⁹Co(n,2n) ^58m+gCo, ⁵⁹Co(n,p) ⁵⁹Fe, ⁵⁹Co(n,) ⁵⁶Mn, E_n = 14.3, 14.7 MeV: measured activation cross sections relative to ⁵⁶Fe(n,p) ⁵⁶Mn.

⁵⁹Co(n,2n) ^58m,gCo, E_n = 14.3 MeV; measured isomeric cross section ratio. Natural targets. Ge, NaI and 4πβ detectors.     

Use of the nuclear microprobe at the University of Arizona for the study of heavy metal deposition in rabbit renal tissue

Industrial wastes consigned to disposal sites frequently contain substantial amounts of heavy metals. We have successfully applied proton induced X-ray emission analysis (PIXE) in the conduct of heavy metal (Hg, Cd, Cr, As) toxicity studies using precision cut rabbit renal cortical slices. The large beam diameter (4000 μm) of the proton macroprobe at The University of Arizona Ion Beam Analysis facility allowed an overall concentration of the metal(s) of interest in the samples to be determined, but lacked the ability to resolve point concentrations in the tissue. The ability to locate these areas has now been made available to us with the addition of a rastering microprobe (μ-PIXE) to the facility. Studies now being conducted in our laboratory using this micro-technique include analysis of renal tissue taken from rabbits injected intraperitoneally with HgCl₂, K₂Cr₂O₇, and NaAsO₂. The small beam size (3 μm) and the ability to raster this beam over areas of up to 125 μm × 125 μm has allowed regional mapping of endogenous and non-endogenous metal concentrations and revealed trends in heavy metal deposition in in vivo treated renal tissue, significantly increasing the amount of information obtained from these animal studies using PIXE alone. The combination of small beam size, high resolution, and multi-element detection makes μ-PIXE a powerful tool for investigating the impact of non-endogenous metals on the kidney.     

Lower and upper bounds on M-shell X-ray production cross sections by heavy ions

In inner-shell ionization by heavy ions, a significant shift of X-ray lines to the higher energy side and broadening of the peaks indicate that simultaneous multiple ionization of the M and higher shells can dramatically change the values of the atomic parameters. ₁₄Si^3,4+ and ₁₆S^3,4+ ions in the energy range of 5-10 MeV were used to bombard gold (200 μg/cm²) and bismuth (80 μg/cm²) targets. Eight main M X-ray lines have been detected with a Si(Li) detector. Without a possibility for a realistic way to modify the atomic parameters and an accurate extraction of M-subshell ionization cross sections, theoretical cross sections for M-shell ionization are converted to X-ray production cross sections with two extreme choices that presume (i) no multiple ionization and (ii) the certainty that all shells outer to the M-shell are completely ionized in the full multiple ionization. These choices impose the lower and upper limits on theoretical predictions. The X-ray production data should be bracketed by the bounds calculated with any theory. We test this proposition by comparison of the measured cross sections for production of the main X-ray lines and their sum for the total M-shell X-ray production with the predictions of the First Born and ECUSAR [G. Lapicki, Nucl. Instrum. Meth. B 189 (2002) 8] theories in those two extreme limits. With the extreme assumption of no multiple ionization, the First Born approximation shows overall satisfactory agreement with the data while the ECUSAR theory drastically underpredicts our measurements. With the opposite extreme assumption of the full multiple ionization, the ECUSAR exhibits better agreement with the data than the First Born approximation. While neither agreement suggests sure preference for either of these theories, such extreme conversions - as they would have for of any ionization theories - set the lower and upper bounds on their predictions.     

Nuclear microprobe analysis of U-doped (Bi,Pb)2Sr2Ca2Cu3Oy/Ag superconducting tapes

Superconducting (Bi,Pb)₂Sr₂Ca₂Cu₃O_y/Ag tapes are doped with uranium compounds to introduce flux pinning defects from neutron-induced fission. The composition and distribution of elements in cross sections of the tapes were probed with a scanned 3 MeV proton microbeam using proton induced X-ray emission (PIXE). Distributions of the constituent elements were found to be heterogeneous on a scale of 10 μm. By combining the PIXE analysis with Rutherford backscattering spectrometry, the stoichiometry of the superconductor within the tape was measured, including oxygen from elastic scattering, revealing a departure from the desired 2223 composition. In one of the tapes, PIXE elemental maps of the Ag distribution showed diffusion of Ag into the superconductor from the enclosing jacket. Crystals of the same material, not fabricated into tapes, did not contain the contaminants and had a more ideal stoichiometry. Correlation maps between the constituent elements, deduced from the elemental maps, indicate the presence of secondary or unreacted phases.     

Hertzian indentation of advanced lmfbr fuels with simulated burn-up

Hertzian indentation fracture of advanced fast breeder reactor fuel materials [mixed carbonitrides, (U_0.8, Pu_0.2)C_0.8N_0.2, and nitrides (U_0.8Pu_0.2)N was evaluated to yield the fracture surface energy, γ, and the fracture toughness, K_Ic. Both technological grade fuels and fuels with added fission products to chemically simulate burn-up values of 3 and 10 at.% were used. As in previous self-diffusion studies on the same materials, identical behavior (identical critical loads, P_c for crack formation) was observed for 3 and 10% b.u. Simulated M(C, N), whereas the 10% b.u. Simulated MN showed a cracking behavior identical with that of the undoped MN. In contrast, the 3 at.% b.u. Simulated MN showed lower P_c values. This is compatible with differences in fission product solubilities in these materials. The effect of fission products on γ was < 20% whereas γ increased from (U, Pu)(C, N) to (U, Pu)N by up to 80 to 90%, depending on content in fission products.     

Plasma nitridation of thin SiO2 films: AES, ELS and IR study

Auger electron spectroscopy, low-energy electron loss spectroscopy and infrared spectroscopy are used to investigate the nitridation of thin (10–22 nm) thermal SiO₂ in RF soft NH₃ plasma. It is found that plasma action at a substrate temperature of 573 K can completely nitridate the thermal oxide to an oxynitride layer. The layers obtained are macroscopic mixtures of two phases SiO₂ and Si₃N₄, rather than amorphous polymers of Si, O and N.     

Phase transition temperature in the Zr-rich corner of Zr–Nb–Sn–Fe alloys

The influence of small composition changes on the phase transformation temperature of Zr–1Nb–1Sn–0.2(0.7)Fe alloys was studied in the present work, by electrical resistivity measurements and metallographic techniques. For the alloy with 0.2 at.% Fe we have determined T_↔+β=741°C and T_+β↔β=973°C, and for the 0.7 at.% Fe the transformation temperatures were T_↔+β=712°C and T_+β↔β=961°C. We have verified that the addition of Sn stabilized the β phase.     

Thermochemical study of vaporization of LiTiO by a mass spectrometric Knudsen effusion method

The vaporization of Li₄TiO₄ has been studied by a mass spectrometric Knudsen effusion method in the temperature range 1082–1582 K. Identified vapors are Li(g), LiO(g), Li₂O(g) and Li₃O(g). When the vaporization proceeds, the content of Li₂O in the Li₄TiO₄ sample decreases and the condensed phase of the sample changes to β-Li₄TiO₄ plus l-Li₂TiO₃ below 1323 K, to β-Li₄TiO₄ plus h-Li₂TiO₃ in the range 1323–1473 K and to h-Li₂TiO₃ plus liquid above 1473 K. On the basis of the partial pressure data, the enthalpies of formation for β-Li₄TiO₄ from elements and from constituent oxides have been determined to be ΔH_f,298^°(β-Li₄TiO₄,s) = −2247.8 ± 14.3 kJ mol⁻¹ and Δ_fox,298^°(β-Li₄TiO₄, s) = −107.3 ± 14.3 kJ mol⁻¹, respectively.     

Thermodynamic stability of Na2ZrO3 using the solid electrolyte galvanic cell technique

The sodium potential in the test electrode (a) Pt,O₂,Na₂ZrO₃,ZrO₂ was measured by using the emf technique employing Na-β^″-alumina as the solid electrolyte in conjunction with (b) Pt,O₂,Al₂O₃,NaAl₁₁O₁₇, (c) Pt,O₂,Na₂MoO₄,Na₂Mo₂O₇ and (d) Pt,Na₂CO₃,CO₂,O₂ as the reference electrodes over the ranges 880–1045, 700–800 and 850–940 K, respectively. The emf results between electrodes (b) and (c) were utilized for internal consistency checks. From the results on cells formed between (a) and (b) and those on (a) and (c), the standard Gibbs energy of formation, Δ_fG^o (kJ/mol) of Na₂ZrO₃ was determined to be −1699.4+0.3652T (K) valid over the temperature range 700–1045 K. The break in the emf data at 1045 K was corroborated by independent TG/DTA measurements carried out on Na₂ZrO₃ which exhibited an endotherm at 1055 K indicative of a phase transition in Na₂ZrO₃.     

PESA as a complementary tool to PIXE at CTU Prague

Proton Elastic Scattering Analysis (PESA) is a simple convenient method for hydrogen analysis in thin samples. A Proton Induced X-ray Emission (PIXE) target chamber was equipped with a PIPS detector for detection of forward-scattered protons. One of the objectives was to perform PIXE and PESA analyses of air particulate targets simultaneously. Tests and calibrations were fulfilled mainly with Mylar foils 1.5–6.5 μm thick in the proton energy region between 1.35 and 2.3 MeV. The energy dependence of scattering cross section is different from the Rutherford formula. Comparison of PIXE/PESA analysis with the Guelph Scanning Proton Microprobe Laboratory at the University of Guelph, Canada on seven aerosol samples was carried out. The intercomparison results validated our PESA/PIXE quality assurance protocol. In addition, repeated measurements of Gelman Teflo^TM filters indicated a gradual increase of hydrogen content by 1 ng/cm² per 1 μC/cm² proton dose.     

Investigation of phase equilibria related to fusion reactor materials: II. The ternary system Hf-Al-N

Phase equilibria in the ternary system Hf-Al-N are investigated. At 1273 K, the ternary phase Hf₃AlN (isotypic with Zr₃AlN, monoclinic, a = 0.59011 nm, 6 = 0.88646 nm, c = 0.33185 nm, β = 106.33°) coexists with HfN_1−x, A1N, Hf₂Al₃, HfAl, Hf₄Al₃, Hf₃Al₃ and -Hf(s.s.). At 1673 K, a second ternary phase Hf₅Al₃N (isotypic with Zr₅Al₃N_1−x, hexagonal, a = 0.8062 nm, c = 0.5603 nm) occurs. A1N in contact with hafnium or hafnium-rich Hf-Al alloys decomposes at this temperature into HfN_1−x and HfAl₂. No ternary solubilities are observed further to the extended solid solutions of -Hf.     

Electron Capture in M-Shell X-Ray Production from Heavy Elements by 25- and 35-MeV Fluorine Ions

M-shell x-ray production cross sections have been measured for thin solid targets (~2 ?g/cm2) of Au, Pb, Bi, and U for 25 and 35 MeV incident 199Fq+ ions (q = 4, 5, 6, 8, 9). The target M-shell x-ray production cross sections by projectiles with one or two K-shell vacancies are found to be enhanced over those by projectiles without K-shell vacancies. The cross sections averaged over target thickness were measured as a function of target thicknesses. Electron capture (EC) to the L-, M-, ...shells and EC to the K-shell of the projectile, have been extracted from the data. These are compared with the predictions of first Born theories, i.e., OBK of Nikolaev for EC. The data are also compared with the theory by Brandt and Lapicki that goes beyond the first Born approximation, i.e., the ECPSSR approach which accounts for the Energy loss, Coulomb deflection and Relativistic effects in the Perturbed Stationary State theory.     

Production of cosmogenic nuclides in thick targets by alpha bombardment. Part I — short-lived radioisotopes

Production of short-lived cosmogenic nuclides in planetary surfaces and remote spacecraft detectors was simulated by bombarding “thick” C, Mg, Al, Si, SiO₂, Fe, Ni, and Ge targets with 60, 90 and 120 MeV alpha particles. Gold foils were used to monitor alpha particle fluence; product nuclides were measured by gamma ray spectroscopy. The results were used to calculate production yields for each alpha energy, as well as cross sections averaged over the energy ranges 60–90 and 90–120 MeV.     

Measurement of the cross sections for the reactions Cr(n,2n)Cr, Zn(n,2n)Zn, Y(n,2n)Y and Zr(n,2n)Zr from 13.5 to 14.8 MeV

Cross section measurements for the reactions ⁵²Cr(n,2n)⁵¹Cr, ⁶⁶Zn(n,2n)⁶⁵Zn, ⁸⁹Y(n,2n)⁸⁸Y and ⁹⁶Zr(n,2n)⁹⁵Zr were carried out in the neutron energy range 13.47–14.79 MeV applying the activation technique. Neutrons were produced via the T(d,n)⁴He reaction, making use of the variation of neutron energy with the emission angle. The neutron fluences incident on the samples were determined relative to the well-evaluated cross section for the reaction ⁹³Nb(n,2n)^92mNb.

The induced γ-ray activities of the irradiated Zn, Zr and Y₂O₃ samples and their monitor foils were measured by means of a calibrated Ge(Li) γ-ray detector at the KFI, Debrecen. At the IRK, Vienna, relative γ-ray measurements using a high-purity Ge detector were combined with integral γ-ray counting by means of a NaI(TI) well-type detector on the Cr, Zn and Zr foils of highest activity and on some Nb monitor foils; integral γ-ray counting only was applied in the case of the Y₂O₃ samples. All necessary corrections were taken into account.

The results are compared to the corresponding results of cross section measurements published in the literature. The uncertainties obtained in this work are considerably smaller in most cases than the uncertainties given by other authors.     

Analysis of atmospheric aerosols by PIXE: the importance of real time and complementary measurements

Particle-Induced X-ray Emission (PIXE) has been used for more than 30 yr in many urban and background air pollution studies. The technique has certainly contributed to the understanding of source-receptor relationship for aerosol particles as well as to aerosol physics and chemistry. In the last few years, where aerosol issues were strongly linked to global climate change through the relationship between aerosol and atmospheric radiation points to new challenges in atmospheric sciences, where PIXE could play an important role. Also the recognition for the inter-relationship between aerosol and liquid and gas phases in the atmosphere makes important to integrate PIXE aerosol analysis with other complementary measurements. The use of Nephelometers and Aethalometers to measure scattering and absorption of radiation by aerosol particles can be done in parallel with particle filter collection for PIXE analysis. Parallel measurements of trace gases using traditional monitors as well as with new techniques such as Differential Optical Absorption Spectroscopy (DOAS) that can provide concentration of O₃, SO₂, NO₃, NO₂, HCHO, HNO₃, Benzene, Toluene, and Xylene, is also important for both urban and remote aerosol studies. They provide information that allows a much richer interpretation of PIXE data. Recently developed instruments that provide real time aerosol data such as the Tapered Element Oscillating Microbalance (TEOM) PM₁₀ monitor and automatic real time organic and elemental carbon analyzers provide extremely useful data to complement PIXE aerosol analysis. The concentrations of trace elements measured by PIXE comprise only 10–30% of the aerosol mass, leaving the organic aerosol characterization and measurement with an important role. The aerosol source apportionment provided by PIXE analysis can be extended with other aerosol measurements such as scattering and absorption, estimating for example, the radiative impact of each discriminated aerosol source. The aerosol bulk PIXE measurements can be complemented with soluble concentrations provided by Ion Chromatography (IC) and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). Recent developments in remote sensing techniques and products also enhance significantly regional aerosol studies. Three-dimensional air mass trajectories should be integrated in aerosol studies for urban and remote areas. The applications of these techniques to study urban aerosols from São Paulo and Santiago de Chile have broadened extensively the scientific scope of these studies.     

Optical properties of γ-irradiated synthetic sapphire and yttria-stabilized zirconia spectroscopic windows

Synthetic sapphire and yttria-stabilized zirconia single-crystals were irradiated by increasing doses of γ-radiation to study the changes of their optical properties. The optical transparency of -Al₂O₃ was nearly constant up to the γ-radiation dose of 150 kGy for the spectral range of 400–1000 nm, while yellowish-brown coloration of (Zr_0.89Y_0.11)O_1.94 appeared for irradiation above 1 kGy. However, after a short-term heating in the temperature of 210^oC stable discoloration of zirconia can be achieved.     

Dynamic response of target electrons upon low and medium energy ion impact

We found that differential scattering cross sections for medium and low energy He⁺ and Ne⁺ impact on high Z-atoms were significantly enhanced compared with those calculated from the inter-atomic potential based on the Hartree–Fock–Slater atomic model coupled with the bare nuclear charge of a projectile. The enhanced scattering cross sections determined experimentally are reproduced well by a simple model that the center of gravity of target electrons is shifted toward the projectile during a large-angle collision. The shift from the target nucleus is expressed as a function of inter-nuclear distance in terms of a dipole moment (Z₁ and Z₂: atomic numbers of projectile and target, : polarizability, e: electron charge). The effective polarizability β (≡Z₁/Z₂) is expressed as a function of ion velocity v [10⁷ cm/s], in the form β = 0.079 exp[−0.46v].