Positronic Formaldehyde—the Configuration Interaction Study

Positron binding to the formaldehyde molecule has been determined within the Hartree–Fock and configuration interaction approaches. Rough estimation of the positron affinity of formaldehyde, obtained with the CI method, is 684 micro-Hartree. Problems of building the appropriate positronic basis set in molecular calculations are discussed.     

Defect Characterization of 6H-SiC Studied by Slow Positron Beam

The defect formation and annealing behavior in as-grown and electron-irradiated 6H-SiC wafers were investi-gated by variable-energy slow positron beam. For the n-type as-grown samples, it was found that annealing decreased the defect concentration due to recombination with interstitial, and when it was annealed at 1400 ±C for 30 min in vacuum, a 20 nm thick Si layer was found on the top of SiC substrate, which is a direct proof of the Si atom diffusing to the surface when annealed at the high temperature stages. During the high temperature annealing stage, we found an obvious surface effect occurred that induced the higher S parameter close to the surface. This may be caused by the diffusion of the Si atoms to the surface during annealing. After 10 MeV electron irradiation of the n-type 6H-SiC, the positron effective diffusion length decreased from 86.2 nm to 39.1 nm. This shows that there are some defects created in n-type 6H-SiC. But in the p-type 6H-SiC irradiated by 10 MeV electrons, the change is very small. This may be because of the opposite charge of the vacancy defects. The same annealing behavior as that of as-grown 6H-SiC samples was also observed for the 1.8 MeV electron-irradiated 6H-SiC samples except that after being annealed at 300 ±C, its defect concentration increased. This may be explained as the generation of carbon vacancies, due to either the recombination between divacancies and silicon interstitial, or the charge of the charge states.     

An EPR study of Ni+ centers in SrF2:Ni and BaF2:Ni

An EPR study of RT X-irradiated SrF2:Ni and BaF2:Ni has been performed. Different kinds of Ni⁺ centers showing small deviations with respect to a basic structure have been observed. This basic structure consists of a Ni⁺ ion displaced along a <100> direction from the cation substitutional position toward the center of a face of the cube of fluorines. These results are similar to those previously reported on CaF2:Ni.

The spin Hamiltonian parameters of the different centers obtained from the analysis of the corresponding rotational diagrams are also reported.     

An improved,regioselective synthesis of the radiolabelling precursor for the translocator protein targeting positron emission tomography imaging radiotracer [F]GE-180

[¹⁸F]GE-180 has been demonstrated to be a promising new positron emission tomography radiotracer for targeting translocator protein. PET imaging of TSPO will enable measurement of neuroinflammation and microglia activity in vivo. The synthetic route used in the initial discovery of GE-180, whilst enabling the rapid evaluation of the structure–activity relationships (SAR) in this molecular class, was not high yielding and not suitable for scale-up. Here we present an optimised route towards GE-180 and the radiolabelling precursor of [¹⁸F]GE-180 with significantly improved yields due to a strategy which improves the regioselectivity of the key indole formation step of the synthesis.     

Preparation of Benz[b]Indeno[1,2-e]Pyran-11(6H)-Ones and Benz[b]Indeno[1,2-e]Thiopyran-11(6H)-One from Dilithiated 2-Indanone and Lithiated Methyl Salicylates or Lithiated Methyl Thiosalicylate

Dilithiated 2-indanone was prepared with excess lithium diisopropylamide, and the resulting intermediate was condensed with several lithiated methyl salicylates or lithiated methyl thiosalicylate, which was followed by acid cyclization to benz[b]indeno[1,2-e]pyran-11(6H)-ones 3--9 or benz[b]indeno[1,2-e]thiopyran-11(6H)-one 10, which are rare fused-ring indeno-chromones and a new indeno-thiochromone, respectively.     

Electrical properties of MOS structures on nitrogen-doped Czochralski-grown silicon: A positron annihilation study

Measurements of interface trap density, effective generation lifetime (GL) and effective surface generation velocity have been performed using different methods on selected MOS structures prepared on nitrogen-doped Czochralski-grown (NCz) silicon. The application of the positron annihilation technique using a pulsed low energy positron system (PLEPS) focused on the detection of nitrogen-related defects in NCz silicon in the near surface region. In the case of p-type Cz silicon, all the results could be used for the testing of homogeneity. In n-type Cz silicon, positron annihilation was found insensitive to nitrogen doping.     

基于捕获的新型正电子束及应用

基于捕获的新型正电子束技术是通过潘宁阱中捕获、约束和积累正电子而发展的新一代正电子束技术。本文介绍正电子的捕获、冷却,压缩技术,基于捕获的正电子束形成技术,以及该技术将来发展展望,最后讨论该技术在原子物理学和材料科学等多个领域的应用。     

On the measurement of structural relaxation in polymers using positron annihilation lifetime spectroscopy

Positron annihilation lifetime spectroscopy has been identified as an effective means of characterizing the free volume content of amorphous polymers. The lifetime and intensity of the ortho-positronium (o-Ps) pick-off annihilation has been found to correlate with the average size and density of free volume sites, respectively. Recently, PALS has been used to evaluate and monitor the physical aging and structural relaxation of polymers in terms of both initial state and evolution in state with time. However, during extended PALS measurements in insulating materials, an electric field can build up due to positron-electron annihilation and can effectively reduce the probability of positronium formation. In this paper, an observed decrease in intensity associated with the o-Ps annihilation component in the glassy polymers polycarbonate and polystyrene is found to be unrelated to structural relaxation of the materials over the time periods examined as reported earlier by others, and, instead, to be more likely a result of electric charge build-up. © 1993 John Wiley & Sons, Inc.     

PAL spectroscopy of rare-earth doped Ga–Ge–Te/Se glasses

Positron annihilation lifetime (PAL) spectroscopy was applied for the first time to study free-volume void evolution in chalcogenide glasses of Ga–Ge–Te/Se cut-section exemplified by glassy Ga₁₀Ge₁₅Te₇₅ and Ga₁₀Ge₁₅Te₇₂Se₃ doped with 500 ppm of Tb³⁺ or Pr³⁺. The collected PAL spectra reconstructed within two-state trapping model reveal decaying tendency in positron trapping efficiency in these glasses under rare-earth doping. This effect results in unchanged or slightly increased defect-related lifetimes τ₂ at the cost of more strong decrease in I₂ intensities, as well as reduced positron trapping rate in defects and fraction of trapped positrons. Observed changes are ascribed to rare-earth activated elimination of intrinsic free volumes associated mainly with negatively-charged states of chalcogen atoms especially those neighboring with Ga-based polyhedrons.