Near-threshold positron impact ionization of hydrogen

The hyperspherical hidden crossing method (HHCM) is used to investigate positron impact ionization of hydrogen near threshold. An important feature of this method is that it can provide valuable insight into scattering processes. In the calculation of positron-hydrogen ionization, the adiabatic Hamiltonian is expanded about the Wannier saddle point; anharmonic corrections are treated perturbatively. The S-wave results are consistent with the Wannier threshold law and with the extended threshold law that was previously derived using the HHCM. We have extended the previous HHCM calculation to higher angular momenta L and have calculated the absolute ionization cross-section for L = 0, 1 and 2. The HHCM calculation confirms that the S-wave ionization cross-section is small and provides the reason why it is small. The HHCM ionization cross-section (summed over the lowest partial waves) is compared with a convergent close-coupling calculation, a 33-state close-coupling calculation and experimental data.     

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.     

Absolute molecular flux and angular distribution measurements to characterize DNA/RNA vapor jets

Vapor jets of DNA and RNA bases (adenine, cytosine, thymine, and uracil) from an oven with a capillary exit have been studied in the intermediate regime between molecular and viscous flow corresponding to Knudsen numbers in the range 0.1 < K_n < 10. The temperature control method ensured stationary flow. Assuming the Knudsen hypothesis, the pressure of sublimated molecules in the oven was determined as a function of temperature and the transmission probability of the capillary (Clausing factor). Thus it was possible to relate the oven temperature and pressure to the total flux through the capillary, determined by measuring the total mass of DNA/RNA base molecules condensed on a cold surface intersecting the jet. The angular distribution of molecules in the jet has been also studied experimentally using an optical interference method. The measured profiles are in good agreement with Troïtskii’s [Sov. Phys. JETP 7 (1962) 353] analytical law for (cos θ)^3/2 angular dependence in the intermediate regime with error functions associated with the mean free path between intermolecular collisions.     

A study on bactericidal properties of Ag coated carbon nanotubes

A silver layer is coated on carbon nanotubes (CNTs) by ion beam assisted deposition (IBAD). Standard agar dilution method is used to evaluate the bactericidal rate against Gram positive S. aureus and negative E. coli. The structure and the chemical states are investigated by scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). The results show that Ag coated CNTs possess very high bactericidal rate. In comparison with the Ag coated pyrolytic carbon sample, the Ag coated CNTs show stronger bactericidal property.     

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.     

Thermal ionization of Cs Rydberg states

Rates P_nl of photoionization from Rydberg ns-, np-, nd-states of a valence electron in Cs, induced by black-body radiation, were calculated on the basis of the modified Fues model potential method. The numerical data were approximated with a three-term expression which reproduces in a simple analytical form the dependence of P_nl on the ambient temperature T and on the principal quantum number n. The comparison between approximate and exactly calculated values of the thermal ionization rate demonstrates the applicability of the proposed approximation for highly excited states with n from 20 to 100 in a wide temperature range of T from 100 to 10,000 K. We present coefficients of this approximation for the s-, p- and d-series of Rydberg states.     

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.     

Enhanced multiple scattering in crystals with effective LR length in the micron range

Multiple Coulomb scattering in amorphous media can be characterised by radiation length L_R. We find in computer simulations that particles can scatter in bent crystal lattices with effective L_R length down to a few microns or up to 700 times shorter than in the corresponding amorphous media. The effect exists only within the angular range of the bent crystal arc. Outside this range L_R is back to normal value. This makes the crystal a material with unique enhanced multiple scattering property easily switched on/off. We derive a theoretical estimate independent of energy for the effective L_R that is in agreement with our simulations for C, Si, Ge and W crystal lattices. We show that in the ongoing collimation experiment at the Tevatron a crystal-based “super-scattering material” could outperform a channeling crystal in collimation efficiency reducing the local background rate by a factor of 40. The introduced “enhanced multiple scattering” materials could serve for new approaches to beam collimation in accelerators, especially at the high-energy frontier machines like the LHC and ILC.     

The concentration and distribution of essential elements in brown rice associated with the polishing rate: Use of ICP-AES and Micro-PIXE

The concentration and distribution of essential elements in brown rice grains (Oryza sativa L. var. japonica) associated with the polishing rate was determined. Rice samples were collected in Japan and polished to 5%, 10%, 15% and 20% loss of the total weight of brown rice. Concentrations of eight essential elements (P, K, Ca, Mg, Cu, Fe, Mn and Zn) were measured by inductively coupled plasma atomic emission spectrometry (ICP-AES), and distributions of the elements in a single grain were visualized as elemental distribution maps of a cross section by micro particle induced X-ray emission (Micro-PIXE) analysis. Results of ICP-AES analysis indicated that in rice which polished from 0% to 10% loss of weight, there were three patterns in the P/B ratio, which is the mean concentration of an element in polished rice divided by that of the element in the brown rice: no change (Cu and Zn), a gradual decrease (P, Mg, Mn and Fe), and a decrease after a constant phase (Ca and K). There was no remarkable change of the P/B ratio in rice grains which polished from 10% to 20% loss of weight. Micro-PIXE analysis images showed that P, K, Ca, Mg, Fe and Mn were present in large amounts in the surface layer (approx. <200 μm thickness) of brown rice. Two sub-layers were mainly recognized in the grain surface layer in the elemental distribution maps of a cross section. The first sub-layer was approximately 130-170 μm thick. The second sub-layer was approximately 20-50 μm thick, and the primary part of the grain (endosperm cells and starch granules) was under it. The images showed Cu and Zn were uniformity distributed in brown rice, and their concentrations of polished rice were not affected by the polishing rate. Although ICP-AES measurements could not provide the detail structure of the surface layer of the rice grains, the trend of concentration of the elements generally agreed with the elemental distribution maps obtained Micro-PIXE analysis.     

Thermal conductivities of hypostoichiometric (U, Pu, Am)O2−x oxide

The thermal conductivities of (U_0.68Pu_0.30Am_0.02)O_2.00−x solid solutions (x = 0.00-0.08) were studied at temperatures from 900 to 1773 K. The thermal conductivities were obtained from the thermal diffusivities measured by the laser flash method. The thermal conductivities obtained experimentally up to about 1400 K could be expressed by a classical phonon transport model, λ = (A + BT)⁻¹, A(x) = 3.31 × x + 9.92 × 10⁻³ (mK/W) and B(x) = (−6.68 × x + 2.46) × 10⁻⁴ (m/W). The experimental A values showed a good agreement with theoretical predictions, but the experimental B values showed not so good agreement with the theoretical ones in the low O/M ratio region. From the comparison of A and B values obtained in this study with the ones of (U,Pu)O_2−x obtained by Duriez et al. [C. Duriez, J.P. Alessandri, T. Gervais, Y. Philipponneau, J. Nucl. Mater. 277 (2000) 143], the addition of Am into (U, Pu)O_2−x gave no significant effect on the O/M dependency of A and B values.     

Online time-differential perturbed angular correlation study with an O beam - Residence sites of oxygen atoms in highly oriented pyrolytic graphite

The online time-differential perturbed angular correlation (TDPAC) method was applied to a study of the physical states of a probe ¹⁹F, the β⁻ decay product of ¹⁹O (t_1/2 = 26.9 s), implanted in highly oriented pyrolytic graphite. The observed magnitude of the electric field gradient at the probe nucleus, ∣V_zz∣ = 2.91(17) × 10²² V m⁻², suggests that the incident ¹⁹O atoms are stabilized at an interlayer position with point group C_3v. Exhibiting observed TDPAC spectra having a clear sample-to-detector configuration dependence, we demonstrate the applicability of the present online method with a short-lived radioactive ¹⁹O beam.     

Guiding and blocking of highly charged ions through a single glass capillary

Highly charged ions produced in an electron beam ion trap, I^q+, q = 10-50, were transmitted through a tapered glass capillary having diameter of at the end. We found that for a particular beam current, there exists an optimum tilting angle of the capillary in which a steady output of ions is observed, while for smaller angles, the ion counts first rise, then gradually decay on a time scale of minutes. In the case of steady transmission, the charge state distribution is found to be slightly towards the lower side.     

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.     

Measurement of thermal neutron cross-sections and resonance integrals for Hf(n,γ)Hf and Hf(n,γ)Hf reactions at the Pohang neutron facility

The thermal-neutron cross-sections and the resonance integrals for the ¹⁷⁹Hf(n,γ)^180mHf and the ¹⁸⁰Hf(n,γ)¹⁸¹Hf reactions have been measured by the activation method. The high purity Hf and Au metallic foils within and without a Cd shield case were irradiated in a neutron field of the Pohang neutron facility. The gamma-ray spectra from the activated foils were measured with a calibrated p-type high-purity Ge detector.In the experimental procedure, the thermal neutron cross-sections, σ₀, and resonance integrals, I₀, for the ¹⁷⁹Hf(n,γ)^180mHf and the ¹⁸⁰Hf(n,γ)¹⁸¹Hf reactions have been determined relative to the reference values of the ¹⁹⁷Au(n,γ)¹⁹⁸Au reaction, with σ₀ = 98.65 ± 0.09 barn and I₀ = 1550 ± 28 barn. In order to improve the accuracy of the experimental results, the interfering reactions and necessary correction factors were taken into account in the determinations. The obtained thermal neutron cross-sections and resonance integrals were σ₀ = 0.424 ± 0.018 barn and I₀ = 6.35 ± 0.45 barn for the ¹⁷⁹Hf(n,γ)^180mHf reaction, and σ₀ = 12.87 ± 0.52 barn and I₀ = 32.91 ± 2.38 barn for the ¹⁸⁰Hf(n,γ)¹⁸¹Hf reaction. The present results are in good agreement with recent measurements.     

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.     

Magnetic texture determination by conversion electron Mössbauer spectroscopy with circularly polarized beam

A new construction of a CEMS detector is presented. The detector is dedicated to resonant polarimetric studies of samples in an external magnetic field. The construction permits investigations at different angles of radiation with respect to the sample surface. A possibility of magnetic texture measurements using circularly polarized radiation is demonstrated. An average square of the cosine 〈(γm)²〉 and an average cosine 〈γm〉 were determined and compared with results of independent measurements of the magnetic field inhomogeneity by a Hall probe. CEMS measurements performed on an isotopically enriched sample show a substantial influence of thickness effects on the measured spectra. An appropriate correction for a saturation effect was proposed and tested.     

The design and calibration of a phantom for depth profiling measurements of entrained radioactivity in silica-based media

The development of a phantom which replicates the effect of concrete on radioactivity entrained within it is described. The phantom was designed as a basis on which methods can be developed to measure the depth of radioactive contamination in the concrete of defunct nuclear facilities. In particular, this apparatus has been used to validate a differential attenuation method for the profiling of radioactive contamination at depth. Entrained radioactive contamination is a significant issue in defunct nuclear facilities where in situ, non-destructive assay of radioactive waste arisings is a routine requirement.The phantom comprises a polymethylmethacrylate structure filled with high-purity silica-sand which, for the purposes of the application, is an effective analogue of fully-hydrated concrete paste. A void was created within the silica sand which incorporates a sliding mechanism for the insertion of a radioactive source to a required depth. The sealed source represents the entrained radioactivity in the phantom but is also specifiable, removable and poses no long-term contamination risk beyond the expected life of the apparatus. The remainder of the void either side of the source is filled with silica-sand to complete the homogeneity of the phantom. The void was situated near the front of the phantom constructed at a 5.14° angle with respect to the front scanning surface; thus the apparent depth within the silica-sand can be varied by changing the position of the source along the phantom's void.The steps taken to develop the concrete phantom are described. The design has been validated with a set of radiation transport simulations affording comparison with an exemplar of the concrete found used in nuclear facilities. Some initial results from measurements taken with the phantom and a caesium-137 γ-radiation source in combination with a sodium iodide radiation detector are provided. These measurements are used to validate the differential attenuation method and compared with data from previous measurement attempts with concrete slices.     

Theoretical investigations of electron emission after water vapour ionization by light ion impact

An ab initio quantum-mechanical treatment is applied for treating the ionization process of water vapour by light ions. In this theoretical model, the initial state of the system is composed of a projectile and a water target described by a plane wave and an accurate one-centre molecular wave function, respectively, whereas the final state is constituted by a slow ejected electron and a scattered projectile represented by a Coulomb wave and a plane wave, respectively. The obtained results are compared to available experimental data in terms of doubly differential cross sections (DDCS), singly differential cross sections (SDCS) and total cross sections (TCS). A good agreement is generally found especially for the SDCS and the TCS.     

Measurement of electrical conductivity of Pb-Bi alloys in the melting-solidification region

The electrical conductivity, σ(T), of Pb-Bi alloys of eutectic and near eutectic compositions was investigated in the melting-solidification temperature region. The revealed discrepancies between the heating and cooling σ(T) curves as well as a hysteresis observed in course of heating-cooling cycles suggest a metastable microheterogeneous structure of the Pb-Bi melts.     

Computer simulation of high-energy-beam irradiation of single-crystalline silicon

The structural relaxation caused by the high-energy-beam irradiation of single-crystalline silicon was simulated by the molecular dynamics method. As the initial condition, high thermal energy was supplied to the individual silicon atoms within a cylindrical region of nanometer-order radius located in the center of the specimen. The Tersoff potential was assumed as the interaction mechanism between silicon atoms. The supplied thermal energy was first spent to change the crystal structure into an amorphous one within a short period of about 0.3 ps; then it diffused in the specimen by an ordinary thermal dissipation process. The amorphized track radius R_a was determined as a function of the energy density of the thermalized region. It was found that the relationship between R_a and the effective stopping power gS_e follows the relation , which is similar to the formula derived on the basis of the thermal spike model [G. Szenes, J. Nucl. Mater. 336 (2005) 81]. It was also found that the mechanism of structural transition changes at the thermalized region of 1 nm radius.