We test seven different local density approximation (LDA) or generalized gradient approximation (GGA) forms of the enhancement factor and correlation potential for positron-lifetime calculations by using a useful database of experimental values based on the all-electrons approach: full-potential linearized augmented plane wave (FLAPW). To make a numerical assessment of these calculation methods, we use the mean-deviation and the reduced chi-squared as model selection criterions. We find that the tworecent LDA forms of the enhancement factor make distinct improvements upon the calculations for positron-lifetime compared with the older LDA form proposed by Arponen and Pajanne. However, all the LDA forms are still disfavored by the experimental data compared with the GGA forms. In addition, the two recent GGA forms do not yield any improvement when compared to experimental data over the original from given by Barbiellini et al., which is found to give the best agreement.

We examined the effect of constraining the positron source lifetime parameter during a least-square fitting (LSF) of positron annihilation lifetime (PAL) data on the resultant lifetimes, as determined from the fit result. The effect was assessed using calculated PAL spectra and also experimentallyobtained spectra for single-crystal silicon with different measurement statistics, in order to discuss the uncertainty of the resultant lifetime for a sample having a relatively short lifetime of around 220 ps. The obtained results suggest that constraining the positron source lifetime during the LSF analysis results in a relatively minor uncertainty of the sample lifetime in comparison with that due to the measurement statistics.

Positronium bubble oscillation before the stabilization of the bubble was successfully observed by the change of the triplet Ps (ortho-Ps) pick-off annihilation rate in a room temperature ionic liquid, N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide (TMPA-TFSI). It was reported that the Ps bubble stabilization in TMPA-TFSI took more than 500 ps. For materials having Ps formation, singlet Ps (para-Ps) gives the fastest annihilation mean lifetime of about 125 ps. Although it is intrinsic annihilation, the Ps bubble oscillation affects the para-Ps annihilation rate. The oscillation on the ortho-Ps pick-off annihilation was expected to appear at positron ages older than about 400 ps after the para-Ps annihilation fraction becomes small enough. An oscillation with 5.85 GHz frequency of the ortho-Ps pick-off annihilation rate was successfully observed at the positron ages older than 500 ps in 25 °C TMPA-TFSI.

We performed positron age-momentumcorrelation measurements to investigate the positron slowing down process in Ar gas at 7.5 MPa. Increase in the S parameter was observed up to 1 ns, and then stayed on the same level. By comparing with the calculated energy loss of positrons via elastic scattering, it was concluded that the increase in the S parameter corresponded to the positron slowing down from 11.6 eV to 2.5 eV. The positron slowing down process just below the first electronic excitation energy of Ar gas can be observed using the S parameter.

Lithium and sodium atoms can interact with positronium to from a positronic alkali atom in which the ion core of the alkali atom loosely binds to positronium. The systems are suitable to investigate mechanisms of positron binding to atoms because the systems can be described with a three-body model and atomic interactions are included in a model potential between the ion core and the valence electron in the atoms. In order to clarify the details of the loosely bound states, we estimated relativistic effects by reconstructing the model potential. A major contribution to the relativistic effects on the binding energy are ascribed to the electron in the alkali atomic orbital whose contribution to the total wavefunction is small. It was found that the relativistic effects appear largely in the binding energy and geometry of LiPs⁺ and NaPs⁺ compared with those of Li and Na atoms.

The formation of Cu precipitates in Fe–0.3%Cu binary model alloy after hydrogen ion irradiation at 400 °C was investigated by positron annihilation spectroscopy using a slow positron beam. In order to study the effect of elevated temperature on the Cu precipitates, annealing treatment at 400 °C in Fe–0.3%Cu alloy was also investigated. The S-parameters of specimens increased with increasing irradiation dose, especially in the damage peak region. H⁺ implantation produced a large number of vacancy-type defects in Fe–0.3%Cu alloy. Compared to the unirradiated samples, the irradiated specimens show an overall major increase in the W parameter values. The experimental results indicate that Cu precipitates formed easily under a low irradiation dose at elevated temperature. No obvious Cu precipitates formed when unirradiated Fe–0.3%Cu alloy was annealed for 2 h at 400 °C.

We have developed a positron moderator annealing system with a handmade electron gun which allows the pre-assembled moderator temperature to be monitored directly during heating. The moderator, composed of strips of tungsten arranged in a regular lattice, was heated by bombarding electrons emitted from a hot tungsten filament. Moderator annealing was successfully achieved up to a temperature of 2600 °C in the central region of the moderator with a thermionic power of about 1200 W. Positron annihilation lifetimemeasurements of as-received and annealed tungsten foils were performed using a pulsed slow positron beam. As a result, it was confirmed that residual defects contained in the as-received tungsten foil were eliminated by conducting the annealing process.

We report β⁺–γ coincidence positron annihilation lifetime spectroscopy of in situ observation of ion damage in polycarbonate under irradiation by MeV-energy H⁺ ions. Ion damage was investigated from changes in the relative intensity of the long-lived ortho-positronium pick-off annihilation lifetime component measured under irradiation and non-irradiation conditions. It was found that at fluences of less than 10¹⁵ ions·cm⁻² the relative intensity of this component during irradiation was significantly reduced compared to that after irradiation. This reduction disappears at fluences higher than 10¹⁵ ions·cm⁻². Results suggest that at fluences up to 10¹⁵ ions·cm⁻², transient damage structures are formed under irradiation.

The rheological behavior of ethylene vinyl acetate copolymers (EVA) with different vinyl acetate (VA) contents during the extrusion process was investigated by means of viscosity and positron lifetime measurements. The viscosity of EVAs in the liquid state under shearing stress initially decreased with processing time before passing through a minimum at the transition point. The variation of processing time at this transition point with extrusion temperature showed that higher extrusion temperature leads to lower viscosity and faster degradation. The relationship between the inverse of the processing time for the transition point and the corresponding process temperature suggested that the VA content is a key factor for the degradation process of EVA. The positron results indicated that the free-volume size exhibits a corresponding variation to the rheological behavior.

Positron gamma ray spectroscopy coupled with an energy-tunable positron beam was utilized to the study of the microstructure in polyethylene (PE) modified by the radio-frequency (RF) plasma. The energy dependence of the line-shape S parameter, a measure of the Doppler broadening of the positron radiation, confirmed that the change in the hydrophilic state of the PE surface after plasma modification was due to the altered chemical structure in the near-surface region. Moreover, only a region at a depth shallower than 700 nm was influenced by the plasma modification. The recovery in the surface hydrophobicity after plasma modification was observed through contact angle measurements. The variation of the contact angle of water can be fitted by a two-factor decay model, suggesting that the surface recovery is caused by the diffusion of two different groups.

The rheological mechanism of long-term self-assembly triggered by H₂O molecules was studied for unmilled and mechanochemically-milled saponite nanoparticles by means of thermogravimetry and differential thermal analysis (TG-DTA), dilatometry (DLT), and positronium (Ps) lifetime spectroscopy. For unmilled saponite, the adsorption of H₂O molecules due to hydration caused volume expansion arising from an increase in the basal spacing as well as weight gain with a time scale of ~10 h. Ps lifetime spectroscopy revealed two kinds of voids with sizes of ~0.3 and ~0.9 nm for unmilled saponite before hydration. The intensity of the larger void component in the annihilation spectra decreased from ~9 to ~5% with increasing time up to ~100 h and correspondingly the intensity of the smaller void component increased from ~5 to ~9% due to long-term rheological self-assembly. Both the weight gain and volume expansion were largely suppressed for milled saponite, indicating that the adsorption of H₂O molecules is reduced. Furthermore, the larger void disappeared and a single void component, corresponding to a void size slightly larger than the original smaller void, was formed for milled saponite. The intensity of this void, created as a result of destruction, decreased with increasing time up to ~100 h.

Porous silica films were synthesized via a sol–gel method using tetraethyl orthosilicate with mixing hexadecyltrimethylammonium bromide (CTAB) as a structural template. Doppler broadening of positron annihilation radiation spectroscopy based on a slow positron beam and ellipsometry were applied to the study of the prepared silica films. The obtained results suggested that a nanoscopic structure change for the porous silica films takes place around 15 wt % of the CTAB loading.

Flow-type ellipsometric porosimetry (EP) and low-energy positron annihilation lifetime spectroscopy (PALS) were applied to the pore characterization for two types of nanoporous methyl silsesquioxane thin films fabricated on silicon wafers, in order to examine the consistency between the porosities characterized by both techniques. The sizes of the mesopores in the films were evaluated from the respective pore size distributions, obtained using flow-type EP from n-hexane adsorption isotherms at 26 °C based on the Barrett–Joyner–Halenda (BJH) model, while the longest-lived ortho-positronium (o-Ps) lifetimes for the films were measured using low-energy PALS at an incident positron energy of 1.5 keV. The relationship between the pore size and the o-Ps lifetime is discussed in comparison with previously reported measurements for various porous substances.

Undoped and Sb doped tin oxide thin films were fabricated by the dip-coating technique through a propylene oxide assisted sol–gel method. Atomic force microscope measurements reveal that the grain size increased after being calcined at higher temperature; while increasing the dopant content leads to a reduction in grain size and a corresponding increase in the concentration of grain boundaries. Positron annihilation spectroscopy analysis shows the defects are reduced with increasing Sb content to 5%; however, further increasing the doping level to 10% introduces more defects to the films. At all doping levels, the defects in Sb doped tin oxide films decreases significantly upon elevating the calcination temperature.

The photodegradation progress of epoxy cured with polyamide and the effect of the curing agent content under UV-A irradiation have been investigated using positron annihilation spectroscopy with an energy tunable positron beam. After 88 h of irradiation, a post-cure process and the generation of carbonyl groups reduce the value of the S parameter, compared with the virgin samples. As the irradiation time increases from 208 to 399 h, the S parameter decreases, which may be due to the growth of carbonyl groups and the generation of free radicals. After 543 h of irradiation, a dead layer with very low S value appears near the sample surface probably induced by a dramatic decrease in the Ps formation probability. The positron results also reveal that epoxy cured with an appropriate amount of polyamide has a smaller dead layer suggesting that the amount of curing agent is a key factor affecting the photodegradation of epoxy resin.

The effect of small molecules in poly(dimethyl siloxane) (PDMS) on the hydrophobic recovery has been studied. Soxhlet extraction was employed to remove the small molecules. The original and extracted samples were probed by positron annihilation and scanning electron microscopy (SEM). The results confirmed that the surface of unmodified PDMS is covered by the small molecules. PDMS with varying octamethylsiloxane (D4) contentwas modified by argon plasma. The variation of contact angle with the ageing time for different samples was studied by contact angle measurement. As a result it was shown that all the samples can recover to the original hydrophobic surface state after sufficient ageing time. Samples with higher D4 content exhibit a faster hydrophobic recovery. For the sample extracted first and then plasma modified, the hydrophobic recovery rate is very low, and such samples did not return to the untreated hydrophobic state.

The reversible process of self-assembly involving the rheological motion of 2-dimensional nanosheets with the aid of water molecules was studied for saponite inorganic layered nanoparticles by dilatometry and positronium lifetime spectroscopy. The two nanosheet insertion type local molecular structure, dominant in the dehydrated state, was not fully reproducible upon dehydration for the self-assembled sample. The two nanosheet insertion type local structure could thus disappear when saponite goes through self-assembly several times. It was furthermore found that a local structure with a void size slightly larger than that in the two nanosheet insertion type structure exists before self-assembly. This structure, presumably due to curved nanosheets, is metastable and gradually disappears during self-assembly.

Variable-energy positron annihilation spectroscopy was applied to the investigation of nanopore formation in hydrocarbon-siliconoxide hybrid films fabricated through plasma enhanced chemical vapor deposition. Positron lifetime and Fourier transform infrared absorption measurements revealed that the pore size increases from 0.4 to 0.8 nm in radius with increasing annealing temperature, illustrating that the formed pores are in the micropore range. Based on a one-dimensional diffusion model, the ortho-positronium diffusion length was estimated from the variation of the positron 3γ decay probability for the films annealed with various temperatures as a function of positron incident energy. The result suggested that the pore interconnectivity during the pore formation by annealing depends on the gradual decomposition of the carbonaceous component.

A new positron production and measurement apparatus has been developed at a synchrotron radiation facility. Highly energetic positrons were created via pair creation in a Pb target by implantation of 16.7 MeV photons generated via inverse Compton scattering of a Nd laser beam from a 1 GeV electron beam circulating in the storage ring at the New SUBARU synchrotron radiation facility. These positrons, with an energy of around 8 MeV, are separated using a magnetic field and directly implanted into a thick sample to detect defects. By using laser Compton scattered (LCS) photon generated positrons, we performed positron annihilation Doppler broadening measurement for fatigue stress applied iron with a thickness of 2 mm. Vacancy type defects in nondestructive fatigue stress applied iron specimens were successfully detected by this LCS-positron apparatus.

We have measured the emission stability of a coniferous carbon nano-structure (CCNS) field emission electron source. Stable emission over the 1274 h measurement was observed at an emission current density of 20 mA·cm⁻². The CCNS emitter can generate an emission current of more than 10 mA making it a practical choice for many applications requiring high electron current. For example, we are currently developing a CCNS based electron gun for electron accelerators and a portable X-ray source. Recent progress on both these applications is presented.

Recent progress on the development of an energy-tunable positronium beam is reviewed. We describe experimental results for the efficient emission of positronium negative ions from alkali-metal coated tungsten surfaces. The ions generated using this technique have been accelerated by an electric field and photodetached in order to produce positronium beams with a hitherto unrealized energy range up to 1.9 keV.

A new slow positron beamline with two measurement ports has been installed at AIST. Positrons are generated using the 70 MeV AIST LINAC and are guided to the measurement ports using a solenoid magnetic field. Both beam ports are arranged vertically with the positron beam incident on the sample from above and samples loaded horizontally. Port No. 1 is designed for positron annihilation lifetime spectroscopy (PALS) and Doppler broadening of annihilation radiation (DBAR) measurements with a ∼10 mm diameter positron beam. The beamline on port No. 2 contains a transmission type remoderator for a brightness enhanced positron microbeam, similar to the existing positron probe microanalyzer (PPMA) at AIST.

A spin-polarized positron beam generated using a ²²Na source, solid neon moderator and magnetic lens was developed. The beam diameter, beam flux and spin polarization were 0.5 mm, 1 × 10⁵ e⁺ s⁻¹ and 27%, respectively. An electromagnet system, which can generate ±1 T magnetic field, was also developed and equipped with the beam apparatus. Using this apparatus, Doppler broadening of annihilation radiation spectra under a high magnetic field were measured for bulk polycrystalline Fe, Co, and Ni samples and a thin film Co₂MnSi sample. These spectra showed a clear asymmetry upon field reversal.

The concept of combinatorial defect/pore analysis using an intense positron microprobe is proposed. This combinatorial method is suitable to analyze defects/pores for a large number of samples systematically. A test of this method was performed by applying it to the analysis of ion beam irradiated Fe film samples.