Isotopic Effect in Ion-Induced Re-Emission of Hydrogen Implanted into Graphite

The re-emission of H and D atoms, implanted into graphite, induced by 1.5 MeV He⁺bombardment has been studied by means of the elastic recoil detection (ERD) technique by changing the initial peak concentration of H and D, the temperature and the He⁺ ion flux. The experimental re-emission profiles were analyzed solving analytically the mass balance equations. The whole re-emission profiles are shown to be excellently reproduced in terms of the ion- induced detrapping (σd), trapping (or retrapping) (Σ_T ) and local molecular recombination (k) between an activated (free) hydrogen atom and a trapped one and that the effective detrapping cross sections Σ_dK/Σ_T for H and D are determined as the best fitting parameter to be 2.9xl0-¹⁸ and 7.7x 10-¹⁹ cm², respectively. It is also shown from the experimental data on little dependence of the re-emission profile on the He⁺ ion flux that the local molecular recombination plays a major role in the ion-induced re-emission. Furthermore, the difference between the effective detrapping cross sections for H and D is reasonably explained in terms of isotopic difference in the values of K for H and D.     

Isothermal Re-Emission of Hydrogen Implanted into Isotropic Graphite

Isothermal re-emission of hydrogen from graphite implanted with 5 keV H₂ ⁺ ion beam up to saturation (3×10¹⁸/cm²) at room temperature has been studied by means of the elastic recoil detection (ERD) technique at temperatures of 450, 500, 550 and 600°C. It is found that the concentration of retained hydrogen decreases rapidly in the beginning and then decreases very gradually as the annealing time increases.

The re-emission profiles have been analyzed taking into account local molecular recombination between activated hydrogen atoms and that between an activated hydrogen atom and a trapped one together with retrapping of the activated hydrogen atom. It is shown that the re-emission of hydrogen by isothermal annealing occurs mainly due to the former type of local molecular recombination and that the activation energy of the thermally activated detrapping rate constant is 0.50±0.04eV. Moreover, it is shown that an analytical expression for the re-emission profile reproduces reasonably well the observed thermal desorption spectra.     

Low-energy helium implantation of aluminum

A series of 20 keV He⁺ implantations was conducted on well-annealed MARZ grade aluminum at fluxes of 6 × 10¹⁴ and 8 × 10¹³He⁺/cm² sec. Three distinct, temperature dependent He release mechanisms were found by He re-emission measurements during implantation, and by subsequent SEM and TEM investigations. At 0.08 of the melting temperature (T_m) gas re-emission rose smoothly after a critical dose of 3 × 10¹⁷He⁺/cm², with extensive blistering. The intermediate temperature range (~0.3 T_m) was characterized by repeated flake exfoliation and bursts of He after a dose of 3 × 10¹⁷He⁺/cm². Rapid He evolution, with hole formation, was found above 0.7 T_m. No significant differences in either gas re-emission or surface deformation were found between the two fluxes employed.     

Deuteron–nucleus total reaction cross sections up to 1 GeV

Total reaction cross sections of deuteron, σ^R_d, are calculated by a microscopic three-body reaction model. The reaction model has no free adjustable parameter and applicable to reactions at various deuteron incident energies E_d and with both stable and unstable nuclei. The predicted σ^R_d are consistent with those evaluated by a phenomenological optical potential for E_d ? 200 MeV in which the potential has been parametrized. A simple formula of σ^R_d up to E_d = 1 GeV, as a function of E_d, the target mass number A and its atomic number Z, is given.     

Helium and hydrogen re-emission during implantation of molybdenum,vanadium and stainless steel

He re-emission has been measured during 300 keV He⁺ implantation from 400°C to 1200°C in V and Mo between ?170°C and 700°C in 316 SS. H re-emission has been measured during 150 keV H⁺ implantation of Mo, V, and 316 SS between ?90°C and 115 °C. In general the He re-emission is low until a critical dose is reached, at which time the re-emission abruptly rises to higher values. Both the critical dose and re-emission are strongly temperature dependent. The H re-emission generally rises smoothly to an equilibrium value which is both temperature and material dependent. Only at the lowest temperature does the H re-emission in Mo bear a qualitative resemblance to He re-emission.     

Room-Temperature Reactor Packed with Hydrophobic Catalysts for the Oxidation of Hydrogen Isotopes Released in a Nuclear Facility

A room-temperature reactor packed with hydrophobic catalysts for the oxidation of hydrogen isotopes released in a nuclear facility will contribute to nuclear safety. The inorganic-based hydrophobic Pt catalyst named H1P has been developed especially for efficient oxidation over a wide concentration range of hydrogen isotopes at room temperature, even in the presence of saturated water vapor. The overall reaction rate constant for hydrogen oxidation with the H1P catalyst in a flow-through system using a tritium tracer was determined as a function of space velocity, hydrogen concentration in carriers, temperature of the catalyst, and water vapor concentration in carriers. The overall reaction rate constant for the H1P catalyst in the range near room temperature was considerably larger than that for the traditionally applied Pt/Al₂O₃ catalyst. Moreover, the decrease in reaction rate for H1P in the presence of saturated water vapor was slight compared with the reaction rate in the absence of water vapor due to the excellent hydrophobic performance of H1P. Oxidation reaction on the catalyst surface is the rate-controlling step in the range near room temperature and the rate-controlling step is shifted to diffusion in a catalyst substratum above 313K due to its fine porosity. The overall reaction rate constant in the range near room temperature was dependent on the space velocity and hydrogen concentration in carriers. The overall reaction rate constants in the range of 1;000=T greater than 3.2 correlated to k _overall[s^?1] = 5.59 × 10⁷ × SV[h^?1] × exp (?67.7 [kJ/mol]/R_gT), where the space velocity range was from 600 to 7,200 h^?1.     

Heavy hydrogen and neutron isotopes

The article discusses the latest experimental data on the states of four-nucleon and five-nucleon nuclei. They confirm the existence of three unbound excited states of He⁴: He⁴ (20.1 MeV, 0⁺, T = 0), He⁴ (22 MeV, 2^–, T = 0), He⁴ (24–25 MeV, 1^–, T = 1). The H⁴ and Li⁴ nuclei formed in the above processes have no bound states, and their lifetimes are of the order of 10^–22 sec. The H⁵ nucleus also has no bound state and decays to H³ + 2n with an energy of Q 1 MeV and a lifetime of the order of 10^–22 sec. The report that radioactivity has been observed in H⁵ is erroneous. An analysis of the binding energies of nuclei of known masses indicates that the tetraneutron also has no bound state. The H⁴, H⁵, and n⁴ nuclei, as well as the heavier hydrogen and neutron isotopes, lie beyond the limit of stability for decay with the emission of nucleons.Translated from Atomnaya Énergiya, Vol. 17, No. 1, pp. 3–9, July, 1964     

The elastic He+d cross section of relevance for knock-on effects in radio frequency heated (He)D plasmas

The cross section for d+³He elastic scattering has been determined for the angular range 20-180° (CM) for beam energies E_d = 0.05 to 11 MeV through combined use of experimental data, Coulomb scattering and extrapolations. The results are used to study, for instance, how the cross section is affected by nuclear interaction contributions. Implications of these results on the calculation of knock on effects in (³He)D plasmas subjected to RF heating and their manifestations in the spectrum of the d + d fusion neutron emission are discussed.     

Experimental determination of <Superscript>80,82,83</Superscript>Kr(α, <Emphasis Type="Italic">xn</Emphasis>)<Superscript>82</Superscript>Sr cross sections for reaction threshold to 60 MeV α-particles

The activation method has been used to determine the experimental cross sections of the nuclear reactions ⁸⁰Kr(α, 2n)⁸²Sr, ⁸²Kr(α, 4n)⁸²Sr, and ⁸³Kr(α, 5n)⁸²Sr for α-particle energies from the reaction threshold to E_α = 60 MeV on targets which are highly enriched with krypton isotopes. The structural and geometric parameters are established for a “thick” cascade target with krypton isotopes, which gives maximum ⁸²Sr yield for E_α ≤ 60 MeV. The ⁸²Sr yield in the indicated target for initial energy E_α = 60 MeV is estimated to be 3.4·10⁶ Bq/(μA·h), which gives a basis for commercial applications of this isotope in medium-energy accelerators.     

1988 National Symposium on Atomic Energy,Tokyo, Japan

Sediment-water distribution coefficients (K_ds) were obtained in estuarine areas of four A-class rivers in Japan. The total concentrations of stable elements and naturally occurring radionuclide (i.e., Na, Mg, K, Ca, V, Mn, Fe, Co, Ni, Cu, Rb, Sr, Y, Mo, Cd, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Pb, and U) in the estuarine water at each sampling point and in the corresponding sediment sample were measured. K^ds of most of the elements varied within one order of magnitude regarding their differences between minimum and maximum values of each element at all the stations. However, a wide variation of K^ds of Mn, Fe, Co, Cu, Rb, and La was observed. In addition, geometric means (GMs) of observed K^ds were compared with the recommended values in the IAEA Technical Report Series 422 (TRS-422). The results showed that GMs of K^ds for most of the elements agreed well with the recommended values, but GMs of K^ds for Mn, Fe, and Cd were more than 10 times lower than the recommended values. The obtained K^d values could be important to investigate the behavior, transport, and fate of artificial radionuclide and to assess the radiological doses in estuarine areas.     

Measurements of Double-Differential Neutron Emission Cross Sections of 6Li, 7Li and 9Be for 11.5 MeV and 18.0 MeV Neutrons

Double-differential neutron emission cross sections (DDXs) of ⁶Li, ⁷Li and ⁹Be were measured for 18.0 MeV and 11.5 MeV incident neutrons produced by the T(d, n) and ¹⁵N(d, n) reactions respectively, using the Tohoku University Dynamitron time-of-flight (TOF) spectrometer. The data were obtained at 13 laboratory angles, and angular-differential cross sections (ADXs) of elastic and inelastic scattering neutrons were derived from the DDXs. For 11.5 MeV neutrons, we obtained the neutron emission spectra over the secondary neutron energies by newly employing the double TOF method as well as the conventional one. In the measurements at 18.0 MeV, we achieved better energy resolution than in our previous studies by using a neutron detector that has a larger solid angle and a thinner tritium target. The experimental results of DDXs and ADXs were compared with our previous results and the evaluated data given in JENDL-3.2, JENDL Fusion File and ENDF/B-VI. It is found that the JENDL data reproduce the experimental ones very well.     

Measurement and Analysis of Fission Rate Distributions of 237Np and 238U in a Polyethylene System with a 65 MeV Neutron Source

Fission rates of ²³⁷Np and ²³⁸U in a polyethylene (CH₂) system were measured with a 65MeV quasi monoenergetic neutron source. Relative fission rate distributions dependent on polyethylene thickness up to about 70 cm were obtained for both nuclides with the experimental error within 7%. The present experiment was analyzed by the NMTC/JAERI code that has been employed for designing accelerator-driven transmutation systems. The fission rates of both ²³⁷Np and ²³⁸U calculated by the NMTC/JAERI did not agree with the experimental ones. The C/E values for both were about 2.0 at 71.8cm of polyethylene thickness when both experimental and calculated values were normalized to 1.0 at 0.0 cm of polyethylene thickness. A sensitivity analysis of the NMTC/JAERI was performed by changing cross sections and angular distributions of hydrogen and carbon and by employing three options of the intra-nuclear cascade/evaporation calculation of the NMTC/JAERI. The disagreement of the NMTC/JAERI calculation with the experimental values was partially improved by increasing the nonelastic-scattering cross section of carbon and by broadening the elastic-scattering angular distribution of carbon.     

Sputtering of materials by H+, D+, T+ and He+ Ions

The sputtering yields of various materials by H⁺, D⁺, T⁺, He⁺ ions were calculated using the theoretical dependence of sputtering yields on energy and ion mass and taking into account experimental sputtering yield values, whenever available, as measured for H⁺, D⁺, T⁺, He⁺ ions.     

Effect of Absorbed Hydrogen on the Stress Corrosion Cracking (SCC) Susceptibility of Unirradiated Zircaloy Cladding

Effect of absorbed hydrogen on the stress corrosion cracking (SCC) susceptibility of unirradiated Zircaloy cladding was examined. The data obtained from literatures showed that the normalized ratios of SCC threshold stress (σ_th ) to 0.2% yield stress (σ_0.2.) claddings, from which the influence of σ_0.2 had been eliminated, increased with increasing hydrogen contents below 50ppm in unirradiated Zircaloy-2 and -4. For Zircaloy-4, the break point was observed in the relationship between normalized ratios of σ_th to σ_0.2 and hydrogen content in sample at hydrogen content of approximately 50ppm. Thermodynamic calculations were carried out for the reaction between iodine gas and zirconium containing hydrogen. The results suggested that the reactions hardly occurred at increased hydrogen content and zirconium reacted with iodine gas only below 100 ppm of hydrogen. Since these tendencies correspond to those of the normalized ratios of σ _th to σ_0.2 on the hydrogen content, it is considered that hydrogen affects the reactions between iodine gas and zirconium and reduces the SCC susceptibility of Zircaloy cladding.     

An experimental investigation of a passive cooling unit for nuclear plant containment

A set of condensation experiments in the presence of noncondensables (e.g. air, helium) was conducted to evaluate the heat removal capacity of a passive cooling unit in a post-accident containment. Condensation heat transfer coefficients on a vertically mounted smooth tube have been obtained for total pressure ranging from 2.48×10⁵ Pa(abs) to 4.55×10⁵ Pa(abs) and air mass fraction ranging from 0.30 to 0.65. An empirical correlation for heat transfer coefficient (h), has been developed in terms of a parameter group made up of steam mole fraction (Xs), total pressure (P_t), temperature difference between bulk gas and wall surface (dT). This correlation covers all data points within 20%. All data points are also in good agreement with the prediction of the diffusion layer model (DLM) with suction and are approximately 2.2 times the Uchida heat transfer correlation. Experiments with an axial shroud around the test tube to model the restriction on radial flow experienced within a tube bundle demonstrated a reduction of the heat transfer coefficient by a factor of about 0.6. The effect of helium (simulating hydrogen) on the heat transfer coefficient was investigated for helium mole fraction in noncondensable gases (X_He/X_nc) at 15, 30 and 60%. It was found that the condensation heat transfer coefficients are generally lower when introducing helium into noncondensable gas. The difference is within 20% of air-only cases when X_He/X_nc is less than 30% and total pressure is less than 4.55×10⁵ Pa(abs). A gas stratification phenomenon was clearly observed for helium mole fraction in excess of 60%.     

Dechanneling by lattice defects

Backscattering measurements were used to study the dechanneling of ¹H⁺ and ⁴He⁺ ions in irradiated crystals of Al and Cu. The effect of the annealing of self-interstitials and vacancies on dechanneling was correlated with the formation of Al-Ag mixed dumbbells and vacancy-Sn atom complexes in dilute alloys of Al. The depth profile of damage produced in Al-0.1 at.% Ag by 0.5 MeV He⁺ irradiation was measured by ¹H⁺ dechanneling. The rate of dechanneling of ¹H⁺ ions in ⁴H⁺-irradiated Al-0.1 at.% Ag Cu-0.06 at.% Au and Cu-0.25 at.% Be crystals exhibited ion energy dependences between E^?0.5 and E^?0.9. Dechanneling from small defect clusters in Cu-0.06 at.% Au increased linearly with increasing sample temperature. The dechanneling cross-sections for Al-Ag mixed dumbbells in Al, and Cu-Be mixed dumbbells in Cu were σ_d～- 8 × 10^?18 cm². The results were compared with dechanneling theories.     

Stopping power measurements of heavy ions (3 ? Z1 ? 14) in Mylar foil by time-of-flight spectrometry

Heavy ions elastic recoil detection analysis coupled with time of flight spectrometer (HIERDA_ToF-E) have been used to measure energy loss of charged particles in thin absorber. The stopping power of heavy ions has been determined in Mylar for ²⁸Si, ²⁷Al, ²⁴Mg, ¹⁹F, ¹⁶O, ¹²C and ⁷Li ions over a continuous range of energies 0.14-0.80 MeV/nucleon. The ions were recoils from the bombardment of different samples (Si, MgO, Al₂O₃, LiF and C) with a 27.5 MeV Kr⁺ beam. The energy loss of the recoil atoms is measured with and without additional foils placed in front of a Surface Barrier Detector (SBD). The energy of individual ions is determined from its ToF data; the exit energy after the stopping foil is measured using the SBD detector. We have compared our stopping values to those predicted by SRIM-2008 computer code, ICRU-73 stopping data tables, MSTAR calculations and to the published data from literature. The results show good agreement with limited existing data but indicate a large deviation among the predicted theoretical values at the low energy side of the stopping maximum peak.     

Distribution Coefficient of Cesium and Cation Exchange Capacity of Minerals and Rocks

The sorption behavior of cesium on a variety of minerals and rocks has been investigated by batch equilibration using ¹³⁷Cs. The distribution coefficient (K_d) of cesium increased with decreasing concentration of cesium and approached a constant value in dilute regions. The distribution coefficients determined at a trace level (K_dt,tr) differed more than four orders of magnitude strongly depending on the kinds of samples, for example, about 30 on quartz and 10⁵ on tuffs. The values of K_d,tr are related to the cation exchange capacity (CEC; meq/g) determined from the saturated sorption of cesium for individual samples as : log K_{d, tr} =log CEC + constant. Most K_{d, tr} values were found to approximately follow this linear relationship.     

Hydrogen Concentration Dependence of Tritium Tracer Diffusion Coefficient in Alpha Phase of Niobium

In order to examine influences of coexistent hydrogen isotopes on diffusion behavior of tritium in niobium, tracer diffusion coefficients D_t of tritium in alpha phase of hydrogenated and deuterized niobium (α-NbH_xT_y and α-NbD_xT_y (x<0,8,y<<x)) have been measured at 473 K, 493 K and 553 K. The data on D_t show typical hydrogen concentration dependence: D_t of tritium for both α-NbH_xT_y, and α-NbD_xT_y, decreases with hydrogen concentration under all experimental conditions. The obtained concentration dependence of D_t of tritium differs from that of D_t of protium in α-NbH_x or of deuterium in α-NbH_xT_y. On the other hand, no appreciable differences in the concentration dependence of D_t of tritium between α-NbH_xT_y, and α-NbD_xT_y, are observed: there are no definite isotope effects due to the coexisting hydrogen isotopes. This result suggests that D_t of tritium for a tritiated niobium (α-NbT_x) is not very different from that for α-NbH_xT_y and α-NbD_xT_y. The chemical diffusion coefficient D* of tritium is also evaluated on the basis of the obtained D_t of tritium and of a literature value of a thermodynamic factor F for Nb-H and Nb-D systems.