Measurement of the H2-D mass difference with a nuclear microprobe

A 1.24 MeV deuteron (D) beam mixed with a H₂ molecular beam was separated with a microslit system of a nuclear microprobe consisting of a 100 μm diameter object and a 1 mm diameter aperture diaphragm. D was distinguished from H₂ by Rutherford backscattering (RBS) on a thin Au film. By slightly changing the magnetic field strength of the beam steerer installed in front of the object diaphragm, the maximum and the minimum RBS D/H₂ ratios were found to be 50.3 and 1.5, respectively. M/ΔM = 3.9 × 10³ was obtained as the mass resolution of the nuclear microprobe. The transmission of this system was 2 × 10⁻³.     

Ion traps — recent applications and developments

Paul and Penning traps are now widely applied in chemistry and physics laboratories. They are used as storage devices, as tools for precision spectroscopy and metrology, and as mass spectrometers. Direct mass measurements of short-lived Rb, Sr, Cs, Ba, Fr and Ra isotopes were performed at the on-line mass separator ISOLDE at CERN, Geneva, by means of a tandem Penning trap system. The ions from ISOLDE are captured and cooled in a first trap and trasnferred to a second trap. Here the mass of the trapped ions is determined by measuring their cyclotron frequency. Resolving powers exceeding m/Δm (FWHM) = 10⁶ could be achieved. Mass values of about 60 isotopes have been determined with accuracies of typically δm/m = 10⁻⁷. For the first time in the history of mass spectrometry the isomeric and ground states of a nucleus have been resolved.     

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.     

Measurements of the rate of the uranium-water vapour reaction

The rate of the uranium-water vapour reaction has been measured between 30 and 80°C. The measured reaction rate obeys the rate equation: k = 3.0 × 10⁹r^1/2 exp(−15.5 kcal/RT) mg U/cm ² H = 4.0 × 10⁸r exp(−15.5 kcal/RT) mg weight gain/cm² h, where r is the fractional relative humidity.

This rate equation agrees remarkably well with the literature equation which was derived from much more limited experimental evidence and so the present equation is preferred.     

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.     

Contribution of uranium diffusion on creep behavior of uranium dicarbide

Compressive creep tests of uranium dicarbide (UC₂) have been conducted. The general equation best describing the creep rate over the temperature range 1200–1400°C and over the stress range 2000–15000 psi is represented by the sum of two exponential terms ge =A(σ/E)^0.9 exp(−39.6 ± 1.0/RT) + B(σ/E)^4.5 exp(−120.6 ± 1.7/RT), where pre-exponential factors are A(σ/E)^0.9 = 12.3/h at low stress region (3000 psi) and B(σ/E)^4.5 = 3.17 × 10¹³/h at high stress region (9000 psi), and the activation energy is given in kcal/mol. Each term of this experimental equation indicates that important processes occurring during the steady state creep are grain-boundary diffusion of the Coble model at low stress region and the Weertman dislocation climb model at high stress region. Both mechanisms are related to migration of uranium vacancies.     

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.     

Depth profiling study on the migration of tritium in titanium induced by deuterium-ion bombardment

A thin titanium layer with uniformly absorbed tritium (T/Ti ˜1.0) was bombarded by 390 keV D₃⁺ ions (130 keV per deuteron). Bombardment was performed at low (111 K) and room temperatures up to fluences of 5.9 × 10¹⁸ D/cm² and 3.0 × 10¹⁸ D/cm², respectively. Depth profiles of tritium up to a depth of 0.8 mg/cm² (˜1.8 μm) were measured and the change of the profile with fluence was investigated by means of the T(d, )n nuclear reaction. At both of the temperatures, a dip was formed on the depth profile of tritium at the depth around the projected range, indicating that the deuteron bombardment induced the migration of tritium against the concentration gradient. At the low temperature, the dip showed a gradual growth with fluence and saturation of the growth at the higher fluences, which could not be described by the existing model for isotope mixing. The spectrum of protons from the D(d, p)T reaction obtained in the same measurement suggested that the release of deuterium suddenly started at the final stage of the present bombardment. The dip formed at room temperature was larger than that at the low temperature. The migration of tritium induced by the bombardment is discussed on the basis of the experimental results obtained.     

Research of Si and GaAs diode structures by Ion Beam Induced Charge (IBIC) collection

A Si pn junction diode and a GaAs Schottky diode were prepared for studying the basic mechanism of charge collection followed by high energy charged particle incidence in order to improve the resistance against single event upset. A 2 μm wide and 20 μm long rectangular Al electrode attached to a circular Al electrode with a 50 μm diameter was made on a 2.5 μm thick epilayer (minority carrier density 2 × 10¹⁵ /cm³). Both a Schottky electrode of Al (5 μm × 110 μm) and two ohmic electrodes of AuGe/Ni (110 μm × 110 μm) were made on a 2 μm thick epilayer (7.3 × 10¹⁵ /cm³) grown on a semi insulator GaAs substrate (1 × 10⁷ Ω cm). The internal device structure was examined by the IBIC (Ion Beam Induced Charge) method using a 2 MeV He⁺ ion microbeam. IBIC images clearly show an Al electrode, the SiO₂, and an epilayer. These results were then used to improve the qualities of the test diodes.     

Investigation of in-reactor creep and irradiation growth of zirconium-2.5 wt% niobium channel tubes

We summarize the diametral creep results obtained in the MR reactor of the Kurchatov Institute of Atomic Energy on zirconium-2.5 wt% niobium pressure tubes of the type used in RBMK-1000 power reactors. The experiments that lasted up to 30 000 h cover a temperature range of 270 to 350°C, neutron fluxes between 0.6 and 4.0 ×10¹³ n/cm² · s (E > 1 MeV) and stresses of up to 16 kgf/mm². Diametral strains of up to 4.8% have been measured. In-reactor creep results have been analyzed in terms of thermal and irradiation creep components assuming them to be additive. The thermal creep rate is given by a relationship of the type ε_th = A₁ exp [(A₂ + A _3σt) T] and the irradiation component by ε_rad = A_4σtø(T − A₅), where T = temperature, σ_t = hoop stress, ø = neutron flux and a₁ to A₅ are constants. Irradiation growth experiments carried out at 280° C on specimens machined from pressure tubes showed a non-linear dependence of growth strain on neutron fluence up to neutron fluences of 5 × 10²⁰ n/cm². The significance of these results to the elongation of RBMK reactor pressure tubes is discussed.     

Extraction efficiency of N(T1/2 = 9.96 min) atoms from a graphite target: comparison between off- and on-line obtained results

An off-line release study ¹³N(T1/2 = 9.96 min) produced by proton induced reaction on a graphite target (POCO-graphite EDM3, density = 1.84 g/cm³, grain size /t 3 μm) has been performed. The activation energy for the diffusion process is determined to be 6.15(16)×10⁵ J/mol. With this activation energy, extraction efficiencies for ¹³N are obtained at different temperatures and are compared to on-line measured extraction efficiencies.     

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.     

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.     

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.     

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.     

Fast neutron irradiation of A-203 steel — recovery studies

A-203 steel has been irradiated by fast neutrons to fluences of 1 × 10²² and 1.25 × 10²² n/m². The isochronal annealing studies have been carried out from 333 to 913 K. The results show a recovery peak at ˜ 385 K, which has been explained to arise due to migration of carbon and carbon-vacancy complexes. After this stage the resistivity instead of decreasing starts increasing. This has been explained on the basis of increase of short range ordering on annealing. The influence of nickel and carbon on the recovery behaviour has also been discussed.     

Carbon clustering in Si1−xCx formed by ion implantation

Silicon-carbon alloys were formed by multiple energy implantation of C⁺ ions in silicon and in Silicon on Sapphire (SOS). The ion fluence ranged between 5 × 10¹⁶ − 3 × 10¹⁷ ions/cm² and the energy between 10–30 keV in order to obtain constant carbon concentration into a depth of 100 nm. The carbon atomic fraction (x) was in the range 0.22–0.59 as tested by Rutherford backscattering spectrometry (RBS). Thermal annealing of the implanted films induced a transition from amorphous to a polycrystalline structure at temperatures above 850°C as detected by Infrared spectrometry (IR) in the wavenumber range 600–900 cm⁻¹. The optical energy gap and the intensity of the infrared signal after annealing at 1000°C depended on the film composition: they both increased linearly with carbon concentration reaching a maximum at the stoichiometric composition (x = 0.5). At higher carbon concentration the IR intensity saturated and the optical energy gap decreased from the maximum value of 2.2 to 1.8 eV. The behaviour at the high carbon content has been related to the formation of graphitic clusters as detected by Raman spectroscopy.     

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.     

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.     

Ion-optical correction elements for on-line mass separators

To adjust on-line mass separators, variable magnetic or electrostatic multipoles are most desirable. Such elements were built and subsequently included in the ISOLDE-3 on-line mass separator at CERN. In first tests a mass resolving power m/†m > 10 000 (FWHM) was achieved, which allowed the isobaric separation of ³⁷Ca from ³⁷K using a tungsten-surface ionization source.