Configuration interaction calculations of positron binding to molecular oxides and hydrides and its effect on spectroscopic constants

Ab initio multireference single- and double-excitation configuration interaction (MRD-CI) calculations have been carried out for magnesium oxide (MgO) and lithium oxide (LiO) and their positronic complexes. These results are compared with previous theoretical data obtained earlier for beryllium oxide (BeO) and the series of alkali hydrides with and without an additional positron. Potential curves have been constructed for each of the systems, MgO, e⁺MgO, LiO and e⁺LiO. Positron affinities (PAs) of 0.472 eV and 0.304 eV, respectively, have been computed for the ground states of MgO and LiO. Because of the relatively low ionization potential of the Li atom, it is found that the dissociation limit in the latter case is Li⁺ + PsO (Oe⁺e⁻), whereas it is Mg + e⁺O in the former case, in close analogy to what has been found for e⁺BeO. Significant changes in bond lengths, vibrational frequencies and dissociation energies are indicated as a result of binding a positron to each of these oxides as well as to the alkali hydrides studied earlier. The general trend observed is toward increased bond length and decreased frequency as a result of attaching a positron to these systems in their various low-lying electronic states.     

Molecular model for annihilation rates in positron complexes

The molecular approach for positron interaction with atoms is developed further. Potential energy curves for positron motion are obtained. Two procedures accounting for the nonadiabatic effective positron mass are introduced for calculating annihilation rate constants. The first one takes the bound-state energy eigenvalue as an input parameter. The second is a self-contained and self-consistent procedure. The methods are tested with quite different states of the small complexes HPs, e⁺He (electronic triplet) and e⁺Be (electronic singlet and triplet). For states yielding the positronium cluster, the annihilation rates are quite stable, irrespective of the accuracy in binding energies. For the e⁺Be states, annihilation rates are larger and more consistent with qualitative predictions than previously reported ones.     

The interaction of antihydrogen with simple atoms and molecules

In this paper, we describe calculations that we have carried out of cross sections for rearrangement processes in very low-energy helium + antihydrogen scattering that result in or or . A significantly more accurate method from that used previously [E.A.G. Armour, S. Jonsell, Y. Liu, A.C. Todd, Nucl. Instr. and Meth. B 247 (2006) 127] is used to calculate the entrance channel wave function. Results are presented for the first two processes. Mention is made of the use of the method in calculations of low-energy e⁺H₂ scattering.     

Rearrangement in antihydrogen-atom scattering

I discuss rearrangement in low-energy collisions between atoms (ions) and antihydrogen. For many atoms there exists a critical internuclear distance below which the leptons are no longer bound to the heavy particles. At this critical distance the Born-Oppenheimer approximation breaks down. I briefly discuss the case of general atoms. Numerical examples are given for the simple proton-antihydrogen system, which has the advantage that the Born-Oppenheimer approximation can be solved exactly. I study the convergence of the optical potential approach for this system.     

Electron-positronium scattering in Debye plasma environment

Electron-positronium scattering has been investigated in the Debye plasma environment employing the close-coupling approximation. Three models, viz. 3-state CCA, 6-state CCA and 9-state CCA, have been employed. The 2s²¹S^e autodetaching resonant state of the positronium negative ion has been successfully predicted for various plasma environments. The position of the resonance for different Debye lengths are in close agreement with those of Kar and Ho [S. Kar, Y.K. Ho, Phys. Rev. A 71 (2005) 052503].     

Dispersion coefficients for interactions between positronium and light atoms with pure Coulomb and screened Coulomb potentials

The Van der Waals two-body dispersion coefficients for interactions between the positronium (Ps) atom and light atoms have been investigated using highly correlated exponential basis functions in the framework of both pure Coulomb and screened Coulomb potentials. The C₆, C₈ and C₁₀ coefficients for Ps-Ps and Ps-H interactions, and the C₆ coefficients for Ps-He interactions for both the screened and unscreened cases, are reported.     

Towards trapped antihydrogen

Substantial progress has been made in the last few years in the nascent field of antihydrogen physics. The next big step forward is expected to be the trapping of the formed antihydrogen atoms using a magnetic multipole trap. ALPHA is a new international project that started to take data in 2006 at CERN’s Antiproton Decelerator facility. The primary goal of ALPHA is stable trapping of cold antihydrogen atoms to facilitate measurements of its properties. We discuss the status of the ALPHA project and the prospects for antihydrogen trapping.     

Anti-hydrogen - Meta-stable helium elastic scattering at thermal energies

This article analyses the results for ground state anti-hydrogen scattering off meta-stable helium (He(2^1/3S)) targets at thermal energies using atomic orbital expansion technique. The variation of phase shifts indicates that the effective leptonic potential for each system has neither any barrier nor any hump. The zero energy cross sections for both the targets are much greater than the ground state target: and . The maxima of the p-wave cross sections occur in the vicinity of the minima of s-wave result.     

Collisional dynamics in antiprotonic helium

The ASACUSA collaboration at CERN has measured some collisional dynamics and chemical-physics properties of antiprotonic helium atoms and ions by laser spectroscopy. These include measurements of the primary populations, collisional lifetimes against antiproton annihilation in the helium nucleus, and Auger decay rates of some antiprotonic states. The experimental results are compared with theoretical calculations.     

Design study of a rotating gantry for the HIMAC new treatment facility

A project to construct a new treatment facility as an extension of the existing HIMAC facility has been initiated for the further development of carbon-ion therapy. In the new facility, one of the treatment rooms will be equipped with an iso-centric gantry system employing a 3D pencil beam scanning irradiation method. In order to realize a compact design, the final 90° bending magnet is divided into two bending magnets of 60° and 30°. The scanning magnets are set between these two bending magnets, although the aperture of the 30° magnet increases. Finally, we achieved 350 tons of total weight for 400 MeV/u carbon beams with a 150 mm square field.     

Evaluation of specimen preparation techniques for micro-PIXE localisation of elements in hyperaccumulating plants

Hybanthus floribundus subsp. floribundus, a rare Australian Ni-hyperaccumulating shrub and Pityrogramma calomelanos var. austroamericana, an Australian naturalized As-hyperaccumulating fern are promising species for use in phytoremediation of contaminated sites. Micro-proton-induced X-ray emission (μ-PIXE) spectroscopy was used to map the elemental distribution of the accumulated metal(loid)s, Ca and K in leaf or pinnule tissues of the two plant species. Samples were prepared by two contrasting specimen preparation techniques: freeze-substitution in tetrahydrofuran (THF) and freeze-drying. The specimens were analysed to compare the suitability of each technique in preserving (i) the spatial elemental distribution and (ii) the tissue structure of the specimens. Further, the μ-PIXE results were compared with concentration of elements in the bulk tissue obtained by ICP-AES analysis.In H. floribundus subsp. floribundus, μ-PIXE analysis revealed Ni, Ca and K concentrations in freeze-dried leaf tissues were at par with bulk tissue concentrations. Elemental distribution maps illustrated that Ni was preferentially localised in the adaxial epidermal tissues (1% DW) and least concentration was found in spongy mesophyll tissues (0.53% DW). Conversely, elemental distribution maps of THF freeze-substituted tissues indicated significantly lower Ni, Ca and K concentrations than freeze-dried specimens and bulk tissue concentrations. Moreover, Ni concentrations were uniform across the whole specimen and no localisation was observed.In P. calomelanos var. austroamericana freeze-dried pinnule tissues, μ-PIXE revealed statistically similar As, Ca and K concentrations as compared to bulk tissue concentrations. Elemental distribution maps showed that As localisation was relatively uniform across the whole specimen. Once again, THF freeze-substituted tissues revealed a significant loss of As compared to freeze-dried specimens and the concentrations obtained by bulk tissue analysis.The results demonstrate that freeze-drying is a suitable sample preparation technique to study elemental distribution of ions in H. floribundus and P. calomelanos plant tissues using μ-PIXE spectroscopy. Furthermore, cellular structure was preserved in samples prepared using this technique.     

Dependence of resonant positron-molecule annihilation on molecular temperature

Measurements of positron-on-molecule annihilation as a function of positron energy have established that positrons can bind to hydrocarbon molecules via vibrational Feshbach resonances. The magnitudes of these resonances, which grow rapidly with increasing molecular size, are still not well understood. In this paper, the role of molecular temperature is investigated using a specially designed temperature-regulated annihilation cell. Only relatively small changes in annihilation rate are observed when the molecular temperature is varied by a factor of two, from 300 K down to approximately half that value for the alkane molecules pentane and heptane. These results place important constraints on theories of the annihilation rates and positron-molecule binding energies.     

Effect of swift heavy ion irradiation on hydrothermally synthesized hydroxyapatite ceramics

The effect of swift heavy ion irradiation on hydroxyapatite (HAp) ceramic - a bone mineral was investigated. The irradiation experiment was conducted using oxygen ions at energy of 100 MeV with three different fluences of 10¹², 10¹³, 10¹⁴ ions/cm². The irradiated samples were characterized by glancing angle X-ray diffraction (GXRD), atomic force microscopy (AFM), dynamic light scattering (DLS), photoluminescence spectroscopy (PL), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDAX). GXRD confirmed incomplete amorphisation of HAp with increase in fluence. There was considerable reduction in particle size on irradiation leading to nanosized HAp (upto 53 nm). PL studies showed emission in the visible wavelength region. The irradiated samples exhibited better bioactivity than the pristine HAp.     

Effect of oxygen on gamma irradiation of polypropylenes studied by positron annihilation spectroscopy

The aim of this paper is to study the effect of oxygen on the microstructure of PP irradiated by gamma ray. The released gases from the sample in irradiation process were measured by gas chromatography. The results showed that the hydrogen release from the irradiated PP is independent on the irradiation atmosphere. It is only dependent on the irradiation dose that could be interpreted by a simple model. The microstructure of PP samples irradiated under different atmospheres is studied by positron annihilation spectroscopy. It is found that there are two sub-peaks in free volume distribution of PP irradiated under helium, while only one peak in the distribution of PP irradiated under air. It is the direct evidence to the argument that the new voids are formed in the irradiation process under inert gas and the oxygen may induce the voids to merge together. The influence of oxygen on the crystalline of PP irradiated by gamma is also discussed.     

Implementation of the beamline controls at the Florence accelerator laboratory

The new Tandetron accelerator in Florence, with many different beamlines, has required a new organization of all the control signals of the used equipment (slow control).We present our solution, which allows us the control of all the employed instruments simultaneously from a number of different workplaces. All of our equipment has been designed to be Ethernet based and this is the key to accomplish two very important requirements: simultaneous remote control from many computers and electrical isolation to achieve a lower noise level. The control of the instruments requires only one Ethernet network and no particular interfaces or drivers on the computers.     

Dating of aragonite fossil shell by ESR for paramagnetic species assignment of Mae Moh basin

Electron spin resonance (ESR) spectroscopy technique has been employed to date the aragonite freshwater fossil shells in bottom sediments collected from Mae Moh basin of northern Thailand in the southwest margin of the coal pit. The samples were given an accumulated dose (AD) of 1100.46 ± 52.72 Gy by the additive irradiation method. The dose response of the ESR signals were found to be suitable for an age determination using a signal at g = 2.0016 corresponding to . The ESR signal growth curve on additional γ irradiation has been best fitted by a linear saturation function. Based on this model, the accumulated dose value for dating is obtained. The ESR age of the aragonite fossil shells was found to be about 13.02 ± 1.03 Ma. The results show that the ESR age within the Middle Miocene, Mae Moh Group of the geological time scale, which agreed with the stratigraphic level of the fauna.     

The total cross section and resonance parameters for the 0.178 eV resonance of Cd

It has been suggested that the capture and scattering cross sections of natural cadmium are not well described by the resonance parameters that are given in the evaluated data files. In particular, doubts on the parameters of the first resonance of ¹¹³Cd at 0.178 eV have been raised. This resonance is of high importance in the interpretation in many integral experiments, such as neutron activation analysis, in which cadmium foils are used to shield from thermal neutrons. A new set of experiments has been designed and performed at the neutron time-of-flight facility GELINA, to determine the total cross section and to extract a set of resonance parameters. The covariance information of the experimental data is propagated and the correlation between the resonance parameters is derived. The obtained parameters are then compared to the data available in the literature. Finally a set of criticality experiments from the international handbook of evaluated critical safety benchmark experiments is used to quantify the influence of the change in the resonance parameters.     

A SU-8 dish for cell irradiation

The objective of the CELLION project is radiation research at low doses. The main cell responses to low dose irradiation are bystander effects, genomic instability and adaptive responses. In order to study these effects it is convenient to make the cells addressable in space and time through locking the cell position. A new alternative dish has been developed for irradiation procedures at the Lund Nuclear Probe. The versatile dish can be used both to cultivate and to hold the cells during the irradiation procedure.The irradiation dish is made of an epoxy-based photopolymer named SU-8 chosen by its flexibility, non-toxicity and biological compatibility to cell attachment. It has been fabricated using a UV lithographic technique. The irradiation dish forms a 2 × 2 mm² grid which contains 400 squares. Each square has 80 μm side and is separated from neighbouring ones by 20 μm wide walls. The location of each square is marked by a row letter and column number patterned outside the grid.The Cell Irradiation Facility at the Lund Nuclear Probe utilizes protons to irradiate living cells. A post-cell detection set up is used to control the applied dose, detecting the number of protons after passing through the targeted cell. The transmission requirement is fulfilled by our new irradiation dish. So far, the dish has been used to perform non-targeted irradiation of Hepatoma cells. The cells attach and grow easily on the SU-8 surface. In addition, the irradiation procedure can be performed routinely and faster since the cells are incubated and irradiated in the same surface.     

Quantitative elemental localisation in leaves and stems of nickel hyperaccumulating shrub Hybanthusfloribundus subsp. floribundus using micro-PIXE spectroscopy

Hybanthusfloribundus (Lindl.) F.Muell. subsp. floribundus is a native Australian nickel (Ni) hyperaccumulating shrub and a promising species for rehabilitation and phytoremediation of Ni tailings. Spatial localisation and quantification of Ni in leaf and stem tissues of H.floribundus subsp. floribundus was studied using micro-proton-induced X-ray emission (micro-PIXE) spectroscopy. Young plants, grown in a potting mix under controlled glasshouse conditions were exposed to Ni concentrations of 0 and 26 mM kg⁻¹ for 20 weeks. Leaf and stem samples were hand-sectioned and freeze-dried prior to micro-PIXE analysis. Elemental distribution maps of leaves revealed Ni concentration of 7800 mg kg⁻¹ dry weight (DW) in whole leaf sections, which was identical to the bulk tissue analysis. Elemental maps showed that Ni was preferentially localised in the adaxial epidermis (10,000 mg kg⁻¹ DW) and reached a maximum of up to 10,000 mg kg⁻¹ DW in the leaf margin. Freeze-dried stem sections from the same plants contained lower Ni than leaf tissues (1800 mg kg⁻¹ versus 7800 mg kg⁻¹ DW, respectively), however did not resolve a clear pattern of compartmentalisation across different anatomical regions. Our results suggest localisation in epidermal cells is an important physiological mechanism involved in Ni accumulation and tolerance in leaves of H.floribundus subsp. floribundus.