Realization of the Bose–Einstein condensation (BEC) of positronium is a long-standing challenge of positron physics. Since the positron is the antimatter of the electron, the positronium is the antimatter of itself, and its gravity interaction is a sum of matter and antimatter components. In this sense, it can be used to study antimatter gravity. It can also be used as a source of a γ-ray laser. We have proposed a new method to realize a positronium BEC: a combination of thermalization in a cold silica target and laser cooling using 1S-2P transitions. We have started some basic studies based on our new idea. Here we report a preliminary result of our positronium thermalization measurement in cryogenic environment and development status of a new laser system for positronium cooling.

In this work, we calculated the electronic properties and formation energies of various defects in BiOBr using first principles calculations based on density functional theory. The calculated formation energies of Bi, O, and Br vacancies are 9.85, 3.66, and 1.9 eV, respectively, which suggests that the Br vacancy has the highest formation probability. We also calculated the positron trapping states of BiOBr in the perfect bulk state and vacancy trapping state. The positron bulk lifetime in BiOBr crystal is 221 ps, and the positron wave function is distributed in the layer gap. Positrons are insensitive to O vacancies, with lifetime the same as the bulk lifetime, and the positron wave function is delocalized and distributed in the layer gap region. However, the positron lifetimes in Bi and Br vacancies are 234 and 265 ps, respectively, and the positron wave function is localized at vacancy sites.

The trapping and annihilation characteristics of positrons in nanosized solute atom clusters (Mg, Ag, Au) embedded in Al crystals were studied by theoretical calculations based on the atomic superposition model (ATSUP). The results confirm the effective confinement of the positron wavefunction inside clusters with a diameter less than 1 nm, which contain only a few atoms. With increasing nanocluster size, the trapping of positrons is gradually enhanced leading to an increased positron annihilation lifetime. Our results suggest that positron lifetime spectroscopy should be a sensitive probe for Ag, Au, and Mg nanoclusters in Al matrix.

We report a theoretical study of antihydrogen-hydrogen molecular resonance states consisting of a positron, an antiproton, an electron and a proton. The four particles strongly correlate and show different character from the hydrogen molecule. Because of the non-separability of the positron and electron motions from the antiproton and proton motions, the adiabatic approximation breaks down at the short antiproton-proton distance. Based on a non-adiabatic method, we directly solve the four-body problem and obtain the resonance energies and widths. In order to examine the roles of positron–antiproton and electron–proton correlations as well as positron–electron and antiproton–proton correlations, we introduce two different types of coordinate systems. One is suited for describing an antihydrogen–hydrogen configuration, and the other is a positronium–protonium configuration. The antihydrogen–hydrogen configuration contributes to the existence of the molecular resonance states, and the positronium–protonium configuration makes the resonance states unstable. Mixing of the two configurations results in an antihydrogen-hydrogen molecular resonance state, and the resonance state has an energy of −0.077 9(3) a.u. from the antihydrogen–hydrogen dissociation threshold with its lifetime 16 (2) fs.

The effects of ion flux on radiation defect production are studied for single crystal silicon bombarded by 6.7 MeV carbon ions. The resultant damage was characterized by X-ray diffraction analysis and positron annihilation Doppler broadening spectroscopy. The results showed that lattice shrinkage occurs after irradiation although the amount of shrinkage decreases with increasing flux at a fixed fluence. This implies that defect concentration is decreased at higher flux. The major defect is identified as a divacancy. To evaluate this flux effect, we consider the flux dependence of defect recombination by defect reaction rate theory. The calculation suggests that the experimental results can be explained by considering the flux effect on the defect recombination process except thermal annealing. This suggests that the reaction rate constant varies by ion flux i.e., the rate of displacements per atom.

A desktop positron beam apparatus combined with a β⁺-γ coincidence positron annihilation lifetime spectrometer was used for investigating thermal equilibrium vacancies in a TiAl intermetallic compound which is expected to find uses as a lightweight, heat-resistant structural material. The vacancy formation energy of the TiAl was derived from high-temperature in-situ measurements of positron lifetimes, and its value is in good agreement with a previously-reported value measured using a sophisticated internal positron source method. The measurement method used in this study makes it possible to investigate the vacancy formation energies in any material at high temperatures easily, even in unweldable ceramics, semiconductors, and brittle intermetallics for which conventional internal positron source methods cannot be applied.

Positron lifetime and coincident Doppler broadening (CDB) spectroscopy were conducted for Ce₇₀Al₁₀Cu₂₀ bulk metallic glass (BMG) to study the local atomic structure. A single component positron lifetime of ∼246 ps corresponding to annihilation in the free volume intrinsic to the local structure of BMG glassy matrix was obtained. CDB spectroscopy revealed that the free volume is dominantly surrounded by Ce atoms. Positron lifetime for Ce₇₀Al₁₀Cu₂₀ BMG is much longer than that of Ce₆₈Al₁₀Cu₂₀Nb₂ BMG, which is caused by the diffusion of Nb atoms into the vacancy-sized free volumes in the Ce–Al–Cu matrix. In addition, the positron lifetime for Ce–Al–Cu BMG is much different from those of Ce–Ga–Cu BMGs where two positron lifetime components τ₁ ∼ 129 ps and τ₂ ∼ 261 ps attributable to the densely-packed glassy state and free volume, respectively, are found. The present results imply that Ga plays an important role in triggering off the formation of the densely-packed glassy state.

Ion implantation was used to introduce N-ions into a ZnO film, which was deposited on sapphire by pulsed-laser deposition. The implantation fluence of N-ions was about 5 × 10¹⁶ cm⁻². The annealing behavior of ferromagnetism and structures of the N-implanted ZnO sample were determined by a vibrating sample magnetometer and X-ray diffraction, respectively. Positron annihilation spectroscopy and Raman spectroscopy were also employed to investigate the defect conditions in the sample. We observed that room temperature ferromagnetism can be introduced by V_Zn-related defect-complexes instead of only by substitutional N-ions. The results were supported by ab initio calculations based on density functional theory. Also, the possibility of oxygen vacancies as the origin of the ferromagnetism was clearly ruled out.

Polycrystalline nickel was irradiated using 50 keV He ions at room temperature. The irradiated fluences were 5 × 10¹³, 5 × 10¹⁴, and 5 × 10¹⁵ He⁺ cm⁻², respectively. Positron annihilation Doppler broadening spectroscopy (DBS) was used to characterize the irradiation-induced defect evolution. The DBS results show that a large amount of vacancy defects were introduced in the specimens after helium irradiation. In addition, the DBS data could be also interpreted as the formation of helium-vacancy (He_nV_m) clusters due to combination between vacancies and helium atoms.

ZnS : Cu thin films were prepared at 440 °C by sulfuring Zn : Cu thin films which were grown by magnetron sputtering with zinc targets covered with different areas of copper foil to vary the Cu content. As the area of copper increases, the morphology of the ZnS : Cu thin films becomes more uniform and dense. Owning to lower mobility of copper atoms that inhibit the mass aggregation of zinc, the aggregation in Zn : Cu films gradually disappears. After sulfuring the thin films with higher copper content, the concentration of both surface-holes and defects decreases. Annealing the prefabricated Zn : Cu thin films can improve the quality of ZnS : Cu thin films. Notably, the increasing Cu content contributes more to the quality of ZnS : Cu thin films than annealing. Among the samples with increasing cooper content, the defects in the thin films tends to be of only one type.

Pd/chitosan/polyacrylate sodium composite fibers (Pd@CS/PAA) with average diameter of (152 ± 30) nm were prepared by electrospinning. The composite fibers were subsequently cross-linked at 160 °C to improve their solvent stability. TEM results showed that entrapment of palladium species into the composite fibers could increase its dispersion. The composite fibers were found to be very active and stable to catalyze the Mizoroki–Heck reaction of iodobenzene with n-butyl acrylate. Therefore, a highly active and stable heterogeneous palladium catalyst can be prepared by entrapping the palladium nanoparticles in CS/PAA composite fibers.

A series of cellulose acetate membranes with different porosities were prepared. The porosity of the membrane was adjusted by adding different molecular weights and contents of polyethylene glycol (PEG), which acts as pore-forming agent in the preparation process. Positron annihilation lifetime measurement and γ-ray energy spectroscopy were employed to evaluate the porosities of the membranes. The lifetime results indicate the size of the free-volume holes in all the membranes are nearly the same. The 3γ annihilation results suggest that, with the increase of porogen content: if the molecular weight of the porogen is low (PEG200, PEG400), pore combination is dominant in the preparation process; while if the molecular weight of the porogen is higher (PEG1000, PEG2000), new pores are formed and/or pores are enlarged.

Pore interconnectivities of mesoporous silica prepared with precursor sols aged for various times were investigated by N₂ adsorption–desorption measurement and cyclic voltammetry. With increasing aging time of the precursor sol to 4 days, an abnormal decrease in both total open volumes and specific surface areas was found. However, with further increasing the aging time to 15 days, the total open volumes and specific surface areas dramatically increased, indicative of the formation of highly interconnected pores and a remarkable increase in the silica porosity. This was further confirmed by cyclic voltammetry measurements using aqueous potassium iodide solutions on the corresponding silica films, which showed a significant rise in redox current and total charge across the films prepared after a longer time aging of precursor sols. This rise can be attributed to the diffusion of I⁻ across the silica films with enhanced pore interconnectivity, despite of the decline in total porosity for some silica samples.

Poly(vinyl alcohol) coated urea films were prepared by simple tape casting. The crystallinity of the films decreased with increasing urea weight content, and hydrogen bonds were formed between PVA and urea. The size of nanoscaled free volume holes in the amorphous region were measured by positron annihilation lifetime spectroscopy, and it was found that the mean size of the free volume increased with increasing urea content. About 80% of the urea was released from the composite film after 25 h in liquid water. The increased free volume holes supplied diffusion channels for water and urea molecules, ensuring efficient urea release from the PVA coating.

To examine the effect of elapsed time on the nanoporosity, subnanoporous silica thin films, fabricated by plasma-enhanced chemical vapor deposition (PECVD), were investigated by means of low-energy positron annihilation lifetime spectroscopy and vapor-adsorption ellipsometric porosimetry. The structural change of the subnanoscaled pores was elucidated by comparison of the as deposited and 6-month-old films. It is expected that the change in the subnanoscaled pores of the present films, after exposure to air for half a year, is due to the adsorption of water molecules from air, followed by the filling up the nanoscaled pores as well as partial polycondensations between silanol groups at the silica grain boundaries of the films.

A low-energy positron diffraction (LEPD) experiment station was developed at the Slow Positron Facility, High Energy Accelerator Research Organization (KEK). An electron-accelerator-based slow-positron beam with an energy of 5 keV was magnetically transported and focused on a transmission-type Ni remoderator to achieve a 50 eV–500 eV brightness-enhanced beam in a non-magnetic space. A LEPD detector with center holed retarding meshes, Chevron-type microchannel plates, and a delay-line detector (DLD) was installed. A newly developed pulse stretching system was used to stretch the initial positron-pulse width of 1.2 µs to 200 µs–20 ms to avoid multiple hit events within the position analysis time of the DLD.

In this work, an energy-variable positron age-momentum correlation (AMOC) measurement system with a pulsed, ²²Na-radioisotope-based beam at AIST was developed for investigating thin materials. Preliminary results for fused silica and polyethylene were obtained using the developed system. The obtained AMOC data agreed well with the previous data for similar bulk samples.

The Muon Spin Rotation/Relaxation/Resonance (MuSR) technique uses a muon beam to probe the structure and dynamics of matter at a microscopic level. An experimental muon source (EMuS) will be built at the China Spallation Neutron Source (CSNS). A MuSR spectrometer was proposed to be constructed in a sub-branch of EMuS. In this contribution, the size of the scintillator and light guides of the polarized MuSR spectrometer were optimized by Geant4. The simulation shows that a scintillator with a length of 175 mm can cover a large solid angle (43%) and get a good detection efficiency and collection efficiency. Double counts can be minimized with a scintillator thickness of 5 mm.

We have developed an apparatus, which can output a high-quality, energy-tunable positronium (Ps) beam, for fundamental studies on Ps interactions with surfaces. The beam was produced through a two-step process, in which Ps negative ions were efficiently generated by the impact of slow positrons onto a Na-coated W thin-film and were then photodetached by an infrared laser beam. An experimental system for surface scattering experiments was also constructed and tested. The Ps beam was incident on a LiF (100) crystal surface in a grazing-angle geometry and the scattered components were projected onto a position sensitive detector. We show the current status of the Ps beam apparatus and some of the test results of the surface scattering experiment.