Determination of the structure of subsurface layers by means of coaxial time-of-flight scattering and recoiling spectrometry (TOF-SARS)

It is demonstrated that both surface and subsurface structural information can be obtained from Si{100}-(2 × 1) and Si{100}-(1 × 1)-H by coupling coaxial time-of-flight scattering and recoiling spectrometry (TOF-SARS) with three-dimensional trajectory simulations. Experimentally, backscattering intensity versus incident angle scans at a scattering angle of 180° have been measured for 2 keV He⁺ incident on both the (2 × 1) and (1 × 1)-H surfaces. Computationally, an efficient three-dimensional version of the Monte Carlo computer code RECAD has been developed and applied to simulation of the TOF-SARS results. An R (reliability) factor has been introduced for quantitative evaluation of the agreement between experimental and simulated scans. For the case of 2 keV He⁺ scattering from Si{100}, scattering features can be observed and delineated from as many as 14 atomic layers ( 18 Å) below the surface. The intradimer spacing D is determined as 2.2 Å from the minimum in the R-factor versus D plot.     

Impact-parameter dependence of the electronic energy loss and angle-dependent energy loss for ions traversing thin foils

The impact-parameter dependence of the electronic energy loss has been investigated using the quantal harmonic oscillator model and the shell-wise model of atomic stopping for ions with velocity v > v₀ and v < v₀, respectively. The electronic energy losses obtained by the impact-parameter dependent formalism have been used in a Monte Carlo simulation code to give the angular dependence of the energy loss of protons, helium ions and heavy ions transmitted through thin foils. The model calculations are compared with available experimental data.     

Delayed neutron study using ENDF/B-VI basic nuclear data

The basic nuclear data of the latest releases of ENDF/B-VI were used in preliminary calculations with the CINDER'90 nuclide inventory code to simulate the activity of fission delayed-neutron precursors. Total delayed-neutron production was obtained at times during and following pulse (0.1-ms) and equilibrium (4-hr) fission histories for each of the sixty fission systems having fission-product yields in ENDF/B-VI. The equilibrium studies — at unit fission rate for constant fission periods sufficiently long that all precursors reached saturation inventories — yielded the value for each system. Delayed-neutron production rates at 54 decay times t, extending to 500 s following a fission pulse, were fit using the STEPIT code to the pulse function R(t) = ∑a_iλ_ie^−λ_it. Results following equilibrium irradiations were fit to the equilibrium function R(∞, t) = ∑a_iλ_ie^−λ_it. It was observed that functions from fits to pulse results did not well represent equilibrium results at long cooling times. Similarly, functions fit to the equilibrium results did not well represent pulse results at short cooling times.

A comprehensive series of CINDER'90 calculations was then made for irradiation times T of 0.1 ms, 1 s, 10 s, 100 s, 1000 s, and 4 hours; results were obtained at 60 decay times t extending to 800 s following irradiation. Comprehensive calculations were made using both the 1989 Pn data of England and Brady and the new Pn data of Pfeiffer, Kratz and Möller described elsewhere in this issue. The body of results for each system was included in fits to obtain the neutron production rate R(T, t) = ∑a_ie^−λ_it(1 − e^−λ_iT) for each system. Fits were made for the traditional sum of six exponentials, with all variables free to vary; additional fits were made for a sum of eight exponentials with decay constants set to values suggested by Piksaikin. The resulting pulse functions R(t), defined by the a_i and λ_i thus obtained, accurately represent calculated delayed-neutron production when integrated with any irradiation history.

The pulse functions thus produced and other published pulse functions fit to past measurements and calculations are compared numerically at several times after fission. Reactivity effects of all functions from measurements and calculations for each of the sixty systems are indicated by the asymptotic periods following positive 10¢– 50¢ reactivity steps simulated in point-reactor kinetics calculations using the AIREK-10 code.     

Measurement of the mechanical properties of thermally-shocked zirconia-based simulated inert matrix fuel

The Vickers micro-hardness (H_V) was measured by an indentation technique of simulated ZrO₂-based Inert Matrix Fuel (IMF) material with a composition of Er_0.07Y_0.10Ce_0.15Zr_0.68O_1.915 in two different densities on sintered specimens and specimens thermally shocked with the quenching temperature differences (ΔTs) between 473 and 1673 K and compared with those of simulated MOX, namely, U_0.92Ce_0.08O₂. The H_V values obtained for two IMF materials were found higher, ranging from 6.37 GPa to about 7.84 GPa, depending on ΔT and the sintered density, than those obtained for the simulated MOX which are quasi-constant in the same range of ΔT with a mean value of 6.37 GPa. The fracture toughness (K_IC) was calculated from the measured H_V and the crack length, and it was found to exhibit a slight increase with increasing ΔT, ranging between 1.4 and 2.0 MPa m^1/2, while that of simulated MOX specimen is ranging between 0.8 and 1.1 MPa m^1/2. The thermally shocked specimens were observed with an optical microscope and analyzed in terms of microstructural changes and cracking patterns.     

Influence of the target thickness on the backward and forward electron emission characteristics induced by protons incident on thin carbon foils

The forward (γ_F) and backward (γ_B) electron emission yields have been measured for protons incident on thin carbon foils for incident energies between 2 and 7 MeV as a function of the target thickness. Comparisons with theoretical results obtained by Monte Carlo simulations are presented. In particular, the Meckbach factor R_γ = γ_F/γ_B is discussed.     

Release of nitrogen from SiOxNy films during RBS measurement

We have found that nitrogen atoms are released very rapidly from ultrathin SiO_xN_y films (2.6 nm) during RBS measurement with 500 keV He⁺ ions. The release behavior strongly depends on the preparation technique of the SiO_xN_y films. There is no release from the film prepared by thermal nitridation of SiO₂, while 80% of the nitrogen atoms are released from the film prepared by plasma nitridation at a fluence of 1×10¹⁶ cm⁻². The release cross-section for plasma SiO_xN_y films is of the order of 10⁻¹⁶ cm². This large cross-section cannot be explained by a simple recoil mechanism. The nitrogen release is also observed under irradiation with 5–10 keV electrons though the cross-section is of the order of 10⁻¹⁹ cm². These findings suggest that the observed nitrogen release is an electronic excitation induced process.     

Channeled ion beam synthesis: a new technique for forming high-quality rare-earth silicides

High dose ¹⁶⁶Er or ¹⁶⁰Gd implantations are used to form rare-earth (RE) silicides in Si. After implanting 0.8−2.0 × 10¹⁷ at./cm² with 90 keV into Si(111) substrates kept at 450 to 530°C, we found that using conventional non-channeled implantation (tilted over 7°), it is impossible to form a continuous RESi_1.7 layer. On the contrary, using channeled implantation, a continuous epitaxial ErSi_1.7 layer with very good crystalline quality can be synthesized; a lowest χ_min value of 1.5% for a surface ErSi_1.7 layer is obtained. This different behaviour is explained using a model based on the difference in implantation depth, defect density and sputtering yield between random and channeled implantation, and the results are compared with Monte Carlo simulations. Such a high-quality RESi_1.7/Si system offers a rare opportunity to study the structure, orientation and strain comprehensively using Rutherford backscattering and channeling spectrometry, X-ray diffraction and TEM. We found that the azimuthal orientation of the hexagonal RESi_1.7 layer to the cubic Si substrate is RESi_1.7[0001]/t|Si[111] and RESi_1.7{11 0}/t|Si{110}. It is further observed that the ErSi_1.7 epilayer is compre strained and quasi-pseudomorphic. In the case of GdSi_1.7, the most difficult rare-earth silicide to form, and enhanced stabilization of the hexagonal over the orthorhombic phase is observed.     

Simulation of momentum flux ratios in the cascade in the sputtering of Mo---Mo

Momentum flux ratios ⁹²Mo/¹⁰⁰Mo towards the surface and the bulk through marker planes have been studied by a static Monte Carlo simulation program for an isotopic mixture ⁹²Mo_0.5¹⁰⁰Mo_0.5 bombarded with 5 keV Xe ions. The calculations show that the momentum flux ratios in the cascade deviate far from one at different surface depths. The calculations also show that the zero-fluence isotopic ratio in the near-normal direction and its difference between small and large polar emission angles are positive.     

Structure and kinetic studies of U(VI)-benzamidoxime complex in non-aqueous solutions by H- and C-NMR

The structural and kinetic studies of U(VI) complex with benzamidoxime(Hba) as ligand in CD₃COCD₃ have been studied by means of ¹H and ¹³C NMR. The Hba molecule was found to coordinate to UO₂²⁺ in the form of anionic benzamidoximate (ba), and the number of ba coordinated to UO₂²⁺ was determined to be 3 by analyzing the chemical shift of ¹³C NMR signal for Hba in the presence of UO₂²⁺. The exchange rate constants(k_ex) of ba in [UO₂(ba)₃] were determined by the NMR line-broadening method. The kinetic parameters were obtained as follows: k_ex(25°C) = 3.1 × 10³s⁻¹, ΔH^≠ = 35.8 ± 3.5 KJ mol⁻¹, and ΔS^≠ = −65 ± 13.7 J K⁻¹ mol⁻¹. The UV-visible absorption spectra of solutions containing UO₂²⁺ and Hba were also measured. The molar extinction coefficient of the complex was found to be extremely large compared with those of UO₂(L)₅²⁺ (L = unidentate oxygen donor ligands) complexes. This is due to the strong electron withdrawing of UO₂²⁺ from Hba and suggests that an interaction between UO₂²⁺ and Hba is very strong. Such a high affinity of monomeric amidoxime to UO₂²⁺ reasonably explains the high adsorptibility of amidoxime resin to U(VI) species, and is considered to result in the high recovery of U(VI) species from sea water using amidoxime resin.     

Applying elastic backscattering spectrometry when the nuclear excitation function has a fine structure

For some nuclei the (non-Rutherford) elastic scattering cross-section has a fine structure with a typical width of the resonances of 1–10 keV. It is shown that bulk sample backscattering spectra can be calculated correctly in these cases only if these are carefully taken into account. The proton elastic backscattering (EBS) spectrum from bulk aluminium was measured just above the interval 1.35–1.75 MeV where a strong resonance structure in the ²⁷Al(p,p₀)²⁷Al cross-section has a relatively high density. Different sets of the Al cross-section data were employed in the simulation: the high resolution proton resonance measurement results retrieved by R-matrix calculations and experimental data obtained later especially for use in IBA. These data sets were consistent, but the latter sets did not resolve the fine structure of the cross-section. The simulations reproduce the spectra well only for the high resolution data: that is, EBS spectra can be simulated provided that the scattering cross-section include all the detail of the fine structure.     

Thermodynamic studies on Cs4U5O17(s) and Cs2U2O7(s) by emf and calorimetric measurements

The oxygen potentials over the phase field: Cs₄U₅O₁₇(s)+Cs₂U₂O₇(s)+Cs₂U₄O₁₂(s) was determined by measuring the emf values between 1048 and 1206 K using a solid oxide electrolyte galvanic cell. The oxygen potential existing over the phase field for a given temperature can be represented by: Δμ(O₂) (kJ/mol) (±0.5)=−272.0+0.207T (K). The differential thermal analysis showed that Cs₄U₅O₁₇(s) is stable in air up to 1273 K. The molar Gibbs energy formation of Cs₄U₅O₁₇(s) was calculated from the above oxygen potentials and can be given by, Δ_fG⁰ (kJ/mol)±6=−7729+1.681T (K). The enthalpy measurements on Cs₄U₅O₁₇(s) and Cs₂U₂O₇(s) were carried out from 368.3 to 905 K and 430 to 852 K respectively, using a high temperature Calvet calorimeter. The enthalpy increments, (H⁰_T−H⁰₂₉₈), in J/mol for Cs₄U₅O₁₇(s) and Cs₂U₂O₇(s) can be represented by, H⁰_T−H⁰_298.15 (Cs₄U₅O₁₇) kJ/mol±0.9=−188.221+0.518T (K)+0.433×10⁻³T² (K)−2.052×10⁻⁵T³ (K) (368 to 905 K) and H⁰_T−H⁰_298.15 (Cs₂U₂O₇) kJ/mol±0.5=−164.210+0.390T (K)+0.104×10⁻⁴T² (K)+0.140×10⁵(1/T (K)) (411 to 860 K). The thermal properties of Cs₄U₅O₁₇(s) and Cs₂U₂O₇(s) were derived from the experimental values. The enthalpy of formation of (Cs₄U₅O₁₇, s) at 298.15 K was calculated by the second law method and is: Δ_fH⁰_298.15=−7645.0±4.2 kJ/mol.     

Threshold oxygen levels in sodium necessary for the formation of NaCrO2 in sodium-steel systems

A knowledge of the threshold oxygen level in liquid sodium necessary for the formation of NaCrO₂ in sodium-steel systems is useful in the operation of fast breeder reactors. There is considerable discrepancy in the data reported in the literature. In order to resolve this, the problem was approached from two sides. Direct measurement of oxygen potential in the Na(l)-Cr(s)-NaCrO₂(s) phase field using the galvanic cell In, In₂O₃/YDT/Na, Cr, NaCrO₂ yielded: _o2 = −800847 + 147.85 T J/mol O₂ (657–825 K). Knudsen cell-mass spectrometric measurements were carried out in the phase field NaCrO₂(s)-Cr₂O₃(s)-Cr(s) to obtain the Gibbs energy of formation of NaCrO₂ as: ΔG^o_f,T(NaCrO₂) = −870773 + 193.171 T J/mol (825–1025 K). The threshold oxygen levels deduced from G^o_f,T (NaCrO₂) data were an order of magnitude lower than the directly measured values. The difference between the two sets of data as well as differing experimental observations from operating liquid sodium systems are explained on the basis of the influence of dissolved carbon.     

Three-dimensional depth profiles of boron ions implanted in SiO2

Silicon dioxide films were implanted at room temperature with boron ions at 7° and 35°. Implantation energies ranged from 20 to 250 keV and the dose was 5×10¹⁴ ions/cm². The depth profiles of ion-implanted boron in SiO₂ were measured using secondary ion mass spectrometry and least-squares fitted to a Pearson distribution. The results demonstrated that the measured depth profiles are well approximated by Pearson distributions, while the experimentally determined range parameters correspond fairly well to the theoretical predictions yielded by the SRIM (stopping and range of ions in matter) Monte Carlo simulation code. The overall differences between the measured and calculated values are 4%, 7%, 15%, 22% and 10%, for projected range, longitudinal range straggling, skewness, kurtosis and transversal range straggling, respectively. Rapid thermal annealing (1050°C for 30 s) of the as-implanted specimens revealed that radiation-enhanced diffusion tends to increase projected range, longitudinal range straggling and transversal range straggling, but decrease (in absolute values) skewness and kurtosis. Notably, the increase in transversal range straggling is smaller than that of longitudinal range straggling.     

133Ba活性炭滤盒自吸收修正方法研究

为了¹³³Ba滤盒体源活度的准确定值以及提高对碘监测仪探测效率刻度的准确性,本研究采用粒径为20~40目,堆积密度为0.47 g/cm³的优质椰壳活性炭,对体源不同活性炭层厚度的自吸收修正进行研究。使用MCNP蒙卡模拟程序,计算面源在不同活性炭层厚度t下的探测效率ε_t,以及无填充介质时的探测效率ε₀,建立随活性炭层厚度变化的自吸收修正曲线F（t）,并通过实验验证模拟计算结果的有效性。通过模拟计算与实验,得到了面源在不同活性炭层厚度下的活性炭自吸收修正因子F（t）,结果表明,模拟计算与实验结果相差在1%左右符合。对于¹³³Ba的356 keV γ能点,得出滤盒中活性炭的自吸收修正因子F（t）,给出了活性炭滤盒体源自吸收修正的技术方法,并采用自吸收修正因子F（t）对自制的¹³³Ba活性炭滤盒体源的自吸收修正进行了理论计算,与实验测量结果比较,两者在2%以内符合,可利用实验得到的自吸收修正曲线F（t）进行活性炭滤盒体源的自吸收修正。     

Mass spectrometric investigation of the vaporization of li2tio3(s)

The vaporization of Li₂TiO₃(s) has been investigated by the mass spectrometric Knudsen effusion method. Partial pressures of Li(g), LiO(g), Li₂O(g), Li₃O(g) and O₂(g) over Li₂TiO₃(s) have been obtained in the temperature range 1180–1628 K. When the vaporization of Li₂TiO₃(s) proceeds, the content of Li₂O in the Li₂TiO₃(s) sample decreases. The phase of the sample is a disordered Li₂TiO₃ solid solution above 1486 K. The enthalpies of formation and the atomization energies for LiO(g) and Li₃O(g) have been evaluated from the partial pressures to be ΔH^o_f0(LiO, g) = 65.4 ± 17.4 kJ/mol, ΔH^o_f0(Li₃O, g) = − 207.5 ± 56.6 kJ/mol, D^o₀(LiO) = 340.5 ± 17.4 kJ/mol and D^o₀(Li₃O) = 931.6 ± 56.6 kJ/mol, respectively.     

A comparative EPR study of ion implantation induced damage in Si, Si1−xGex (x ≠ 0) and SiC

Electron Paramagnetic Resonance (EPR) measurements have been made to investigate the build up of damage in silicon in relaxed crystalline Si_1−xGe_x (x = 0.04, 0.13, 0.24, 0.36) and in 6H-SiC as a result of increasing the ion dose from low levels (10¹² cm⁻²) up to values (10¹⁵ cm⁻²) sufficient to produce an amorphous layer. Si, Si_1−xGe_x (x ≠ 0) and SiC were implanted at room temperature with 1.5 MeV Si, 2 MeV Si and 0.2 MeV Ge ions respectively. A comparison is made between the ways in which the type and population of paramagnetic defects depend on ion dose for each material.     

The kinetics and mechanism of the uranium-water vapour reaction — an evaluation of some published work

The published results of Grimes and Morris on the rate of the uranium-water vapour reaction which were obtained using interferometry have been recalculated using the best values derived from the literature for the complex refractive indices of uranium and uranium dioxide (3.1–3.91 for uranium and 2.2-0.51 for uranium dioxide). The kinetics have been described by Haycock's model and the linear rate constant is given by K₁ = 1.3 × 10⁴P^1/2_H2O exp( − 9.0 kcal/RT )mg U/cm² h, where P_H2O is the water vapour pressure in torr or K₁ = 3.48 × 10⁸r^1/2 exp( −14.1 kcal/RT)mg U/cm² h, where r is the fractional relative humidity, R is the gas constant and T is the absolute temperature.

A mechanism is described which accounts for the observed dependence of the rate of uranium-water vapour reaction on the square root of the water vapour pressure.     

The elastic properties of lithium oxide and their variation with temperature

The energies of the acoustic phonon modes in a single crystal of ⁷Li₂O have been measured in the temperature range 293–1603 K using the technique of inelastic neutron scattering. The slopes of the phonon energy dispersion curves as they approach the Brillouin zone centre give values for the cubic elastic stiffness constants, C_ij. C₁₁ is found to undergo a sharp decrease above ˜ 1350 K similar to that observed in structurally related compounds, such as CaF₂ and SrCl₂, as they undergo a transition to a fast-ion phase. The Reuss and Voigt averaging methods have been used to calculate the temperature dependence of the adiabatic Young's modulus, shear modulus, bulk modulus and Poisson's ratio of polycrystalline Li₂O. Estimates of the corresponding isothermal values are obtained using an expression for the linear thermal expansion coefficient of Li₂O obtained in this work, together with thermodynamic data available from specific heat measurements. These results represent the first experimental data describing the elastic properties of Li₂O at elevated temperatures and are important in predicting the behaviour of this material in its potential role as a tritium breeding blanket material for future fusion reactors.     

Sputtering yield of Ti1−xBx films by 2 keV deuterium bombardment

We deposited titanium borides (Ti_1−xB_x; 0.40 < x < 0.77) by the co-sputter coating method and measured their sputtering yield by 2 keV deuterium ion bombardment as a function of their chemical composition at room temperature. The total sputtering yield is found to increase with increase of the boron content in Ti_1−xB_x. The total sputtering yield of stoichiometric TiB₂ is estimated to be 2.8 × 10⁻², about the same as those reported previously. Concerning the partial sputtering yield, that of the titanium does not depend on the chemical composition, but that of the boron increases with increase of the boron content. These experimental results could be explained by assuming that the partial sputtering yield is proportional to the spatial concentration of each atom in the Ti_1−xB_x matrix.     

Analysis of wall conditioning by using plasma impact desorption in the tandem mirror GAMMA 10

A method for evaluating wall condition by using plasma impact desorption (PID) technique has been developed and successfully applied to the tandem mirror GAMMA 10 as a monitor for wall conditioning. A magnetically shielded quadrupole mass spectrometer was installed in the vacuum chamber of the GAMMA 10 central cell. The behaviour of the partial pressure of various gas molecules desorbed by ICRF-heated plasma discharges were analyzed. The predominant increase of the partial pressure due to PID (ΔP_PID) was hydrogen (M = 2) and a small amount of impurity as CO (M = 28), CH₃ (M = 15), H₂O (M = 18) and CO₂ (M = 44) was observed in the wall-conditioning discharges. The reduction of hydrogen pressure as well as ΔP_PID of the above impurities was observed with the progress of wall conditioning. This behavior has a good correlation with the increase of partial pressures due to electron-impact desorption measured at the same period. The relation between ΔP_PID and the charge-exchange flux was investigated.