Positron annihilation spectroscopy (PAS) is a sensitive probe of the shallow traps of light charged particles such as He/H embedded in solids. The nature of the shallow traps that attract positrons–i.e., whether the properties of the light charged particles or the number of particles contained in the traps affects the probability of positron capture–has so far remained unresolved. Here, the shallow traps of positron in FeCr alloy, namely (H, He)–V nano-clusters with open volume, have been investigated by first-principles calculations and a multi-grid based program package for electronic structure calculations. Various defect structures were modeled, including vacancies, interstitial helium atoms, and helium or hydrogen atoms occupying Fe vacancy sites. We calculated the charge density distribution at the (H, He)–V nano-clusters, and the results show that the charge density at the He/H–V clusters is significantly lower than around the neighboring Fe/Cr sites. The calculated lifetimes of positrons confined in the shallow traps are consistent with the effective open volume of the (H, He)–V complexes. These results suggest that a helium atom forms a more repulsive ion core than a hydrogen atom when it occupies the vacancy, resulting in a decrease in positron lifetime.

We present a four-body calculation of s-wave scattering between a positronium (Ps) and an antihydrogen atom () in the vicinity of the first Ps excitation threshold energy. We convert the Schrödinger equation to a set of coupled integro-differential equations that let us determine the multi-channel scattering wave functions. The internal region of these functions is expanded in terms of the square integrable four-body functions that can be regarded as a kind of “intermediate states” of the scattering and are obtained by the diagonalization of the full four-body Hamiltonian. The convergence of the scattering cross sections is examined with respect to the number of the intermediate states included in the coupled-channel calculation. It is demonstrated that the intermediate states are essential to describe the resonant scattering of Ps + . Around the resonance energy, the positron of Ps transfers to , and the electron is associated with the antihydrogen positive ion. We have updated our previous calculation and have obtained the converged cross sections in the collision energy interval from 3.54 eV to 5.71 eV. The energy regions where the cross section undergoes rapid variation are found to correspond to the resonance energies of the lowest four resonance states predicted by the complex coordinate rotation method.

Muon catalyzed fusion (μCF) is a cyclic reaction where a negatively charged muon itself acts like a catalyst of nuclear fusion between hydrogen isotopes. In the μCF reaction, muon transfer from deuteron to triton and muonic molecular formation are rate-limiting processes. In this work, we have investigated the role of resonance states of muonic molecule in the μCF which affects the muonic deuterium atom population. Solving simultaneous rate equations numerically by the fourth-order Runge-Kutta method, we determined the muonic molecular formation rate so that the number of fusion events reproduces a latest experimental result. It is revealed that the resonance states play a role to enhance the fusion rate by accelerating the de-excitation of the muonic atoms.

O-polar and Zn-polar ZnO films were fabricated by pulsed laser deposition. The Zn-polar sample has a lower electron concentration and higher electron mobility compared with the O-polar sample. This is due to the lower abundance of hydrogen which acts as shallow donor in the Zn-polar film. A Coincidence Doppler broadening study showed that V_Zn-2V_O is the single dominant V_Zn-related type defect in the as-grown O-polar ZnO film, while in the as-grown Zn-polar film, a mixture of V_Zn-related defects were identified. The intensity ratio of near band edge emission to defect emission for the Zn-polar ZnO sample was much larger than that of the O-polar sample. A magnetic study using a superconducting quantum interface device (SQUID) showed that both polarities were ferromagnetic at room temperature. Correlations found between the saturation magnetization, the V_Zn-2V_O defect concentration and the grain surface to volume ratio suggested that the room temperature ferromagnetism originates from V_Zn-2V_O defects residing in the vicinity of the grain boundary.

The precipitation process of tiny Cu clusters in the early stage of aging for FeCu alloys was investigated by positron annihilation lifetime spectroscopy (PALS), Doppler broadening spectroscopy (DBS) and coincidence Doppler broadening spectroscopy (CDB). The different effect of quenching and aging on the formation of Cu precipitates and Cu–V complexes in FeCu alloys was discussed. The Cu–V complexes and Cu clusters act as the shallow positron trapping sites has been confirmed. Their ability to trap positrons can be enhanced at lower temperature, which can be used to measure the behavior of fine Cu clusters at the beginning of aging. The Cu–V complexes, Cu clusters and monovacancies were formed after quenched at 1173 K. After aged at 773 K for 0.1 h, Cu–V complexes and monovacancies recovered while the Cu precipitates formed.

Inhibition of the movement of helium atoms produced by transmutation reactions under nuclear irradiation can be achieved by intentionally introduced trapping sites such as dislocations in stainless steels. In order to investigate the effect of dislocations in 316L stainless steel on helium irradiation-induced defects, well-annealed 316L stainless steel specimens were cold-rolled to either 10% or 20% reduction in thickness and then annealed at 723 K for 1 h, respectively. The non-deformed, 10% and 20% deformed specimens were irradiated with helium ions at an energy of 5 keV to a dose of 5 × 10¹⁶ ion/cm² under ambient temperature. The effects of dislocations on defects generated by helium ion irradiation and helium desorption with temperature were characterized by slow positron beam Doppler broadening spectroscopy and thermal desorption spectroscopy. The results shows that dislocations have a trend to restrict the migration and aggregation of helium atoms and vacancy-type defects, and also hinder the formation of large helium bubbles.

Point vacancy defects in hexagonal boron nitride (h-BN) were investigated by first-principles calculations based on the generalized gradient approximation method in the density-functional theory. The obtained formation energies of B vacancy (V_B) and N vacancy (V_N) with different charge states suggest that is stable for p-type h-BN in both N-rich and N-poor growth conditions, while is stable for n-type h-BN in both N-rich and N-poor growth conditions. Positron lifetime calculations were also carried out on h-BN using the atomic superposition method. The calculated positron bulk lifetime of h-BN was 180.6 ps, positron lifetime of V_B was 183.6 ps, and positron lifetime of V_N was 181.5 ps. The positron lifetime difference is small, but the tendency is clear, showing that .

Ce₇₀Ga₈Cu₂₂ bulk metallic glasses with slightly different purities of Ce element are studied by means of positron annihilation spectroscopy. It is found that the densely-packed Ce-agglomerated state in the amorphous matrix disappears upon employing the 0.38 wt.% lower purity of Ce and nanovoid is alternatively formed. The local structures susceptible to the purity of Ce atoms is discussed based on the bonding nature of solvent Ce with the impurity elements of Al and Si together with hybrid interaction between Al-3p, Si-3p, and Ce-4f orbits.

Positron annihilation lifetime spectroscopy (PALS) measurements by use of a positron source of ²²Na were performed for the polycrystalline tungsten samples that were exposed to low-energy deuterium (D) plasma. The energy of D plasma was low and then it was expected that it would affect just the near-surface region. However, we obtained the longer mean positron annihilation lifetime in the tungsten samples exposed to the low-energy D plasma than the virgin tungsten sample. Moreover, the same longer values were obtained even on the other (no exposed) side of the samples, although the thickness of the samples was 1 mm or 2 mm. The elongation of the positron lifetime probably indicates defects formation in W. Ohsawa, et al. predicted that the high concentration hydrogen creates high concentration vacancy-hydrogen clusters by the first-principles calculations. We also performed positron annihilation age-momentum correlation (AMOC) measurements to know if there are some D’s at the positron trapping sites such as vacancies and found that some of the positrons annihilate with electrons in D.

We have investigated the hydrogen trapping behavior in B2 ordered Fe-50%Al alloy with electron irradiation followed by hydrogen implantation by using positron annihilation techniques, X-ray diffraction and thermal desorption spectrometry (TDS). Positron annihilation lifetime and coincidence Doppler broadening measurement results showed that V_Fe in B2 type Fe-Al alloys are dominantly introduced by low energy (2 MeV) electron irradiation, and more complex vacancy type defects are formed by high energy (8 MeV) electron irradiation. The result of TDS measurement showed that hydrogen atoms are trapped by vacancy type defects, the resulting hydrogen desorption behavior is different for 2 MeV and 8 MeV irradiation cases.

Cerium salt conversion films on magnesium alloys with different surface roughness were investigated by SEM, EIS and positron annihilation. The results showed that the conversion films prepared from more highly smoothed AM60B magnesium alloy substrates had larger impedance arc radius and smaller corrosion current. Positron lifetime measurement showed that as the smoothness of the substrate increased, the number of vacancy defects in the film layer or interface layer increased, corresponding to better coverage rate of the conversion film. It could be inferred that the corrosion resistance of the cerium salt conversion film on the magnesium alloy increases as the surface smoothness of the substrate increases.

In this work, MOF-5, consisted of [Zn₄O]⁶⁺ clusters connected with organic linkers terephthalic acid (H₂BDC) to form a cubic network, was synthesized at different temperatures. Various measurement methods including powder X-ray diffraction patterns, scanning electron microscopy, thermos-gravimetric analysis, nitrogen adsorption-desorption isotherms at 77 K, and positron annihilation lifetime spectroscopy (PALS) were applied to investigate the properties of MOF-5. It was found that the crystallinity sizes, and porosity strongly depended on the synthesis temperature. Results showed that higher synthesis temperature was favorable to improve the porosity of MOFs because of the removal of residual ligand molecules. Remarkably, significant differences in pore radius distributions derived from nitrogen adsorption and PALS was obtained from MOF-5 synthesized at 110 °C, suggesting that there exist closed pores due to the presence of ligand molecules in the inherent pores and/or pore entrances, where nitrogen molecules could not get in.

Laser cooling of positronium (Ps) is an important technique to advance the frontier of science by, for instance, realizing the first Bose-Einstein condensation of antimatters. It was proposed that performing laser cooling on Ps trapped in a cryogenic porous material would be more efficient, but a recent study (Cooper, et al., Phys. Rev. B 97, 205302 (2018)) reported that Ps in the excited (2P) state inside nano pores of silica had a large decay rate into γ-rays, and the resonant spectrum of the transition was significantly broadened, both of which are critically problematic for the proposed cooling scheme. A possible cause of these unexplained results was proposed to be residual impurities which trapped Ps in the vicinity of the pore surface and then enhanced interactions between Ps and the material. This work reports fabrication and inspection of a new porous silica cavity without such impurities, and construction of an experimental system to induce the Lyman-α transition of Ps trapped in pores of the cavity.

In this paper, the free volumes and water barrier property of silicone rubber with different weight ratios of aluminum hydroxide (ATH) were studied by positron annihilation lifetime spectroscopy (PALS) and electrochemical impedance spectroscopy (EIS). PALS data indicate that the o-Ps intensity (I₃) decreased linearly as a function of the ATH content in silicone rubber. It was found that o-Ps are mainly formed and annihilated in the silicone rubber matrix, polydimethylsiloxane (PDMS), and are not formed in the ATH particles. The value of I₃ can thus serve as a PDMS composition characteristic in silicone rubber. When the content of ATH is more than 45.3 wt.% (I₃ ∼ 9.34%), the ATH particles percolate in the bulk of silicone rubber, resulting the failure of sample’s water barrier property.

Total-reflection high-energy positron diffraction (TRHEPD) is a novel experimental method for the determination of surface structure, which has been extensively developed at the Slow Positron Facility, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK). In this paper, a two-stage data-analysis method is proposed. The data analysis is based on an inverse problem in which the atomic positions of a surface structure are determined from the experimental diffraction data (rocking curves). The relevant forward problem is solved by the numerical solution of the partial differential equation for quantum scattering of the positron. In the present two-stage method, the first stage is a grid-based global search and the second stage is a local search for the unique candidate for the atomic arrangement. The numerical problem is solved on a supercomputer.

We have developed a Doppler broadening measurement Gamma-induced Positron Spectroscopy (GiPS) system, and performed a demonstration measurement for cm size Zr based amorphous and crystallized alloys using a laser Compton scattering (LCS) gamma-ray beam with a maximum energy of 16.9 MeV at the NewSUBARU synchrotron radiation facility, Hyogo, Japan. This method makes use of positrons generated inside the sample by implantation of highly energetic gamma-ray photons. GiPS can be a non-destructive measurement for bulk size material in air. In this study, the result of the Doppler broadening measurement is in good agreement with the density change caused by crystallization.

The possibility of on-site and nondestructive inspection of lattice defects, such as those caused by metal fatigue, through positron annihilation lifetime measurement has been investigated by simulation. In such inspection of defects, short-time measurement is essential. Therefore, it is important to extract as much as possible information on lattice defects from positron annihilation data with large statistical errors. To extract this information, a master curve representing two-dimensional change of the “mean” positron lifetime () and the starting time shift (ΔT₀) with positron trapping rate (proportional to defect density) was obtained in advance. ΔT₀ and were determined by single-component analysis of the positron lifetime spectrum, taking account of the source component. Then, the trapping rate was estimated by specifying a corresponding position on the master curve, which is based on the interpolation of ΔT₀ and , obtained by short-time measurement of about one minute. The result of the investigation elucidated that the trapping rate can be sufficiently estimated.