An intense pulsed positron beam

A positron pulsing system for an intense positron beam generated by an electron linac is reported. The pulsing system generates an intense pulsed positron beam of variable energy and variable pulse period. The pulsed positron beam is used as a non-destructive probe for various material research. In this paper, we also discuss applications of the pulsed positron beam: positron lifetime spectroscopy, age-momentum correlation spectroscopy, positronium time-of-flight measurement, and positron annihilation-induced Auger electron spectroscopy with a time-of-flight technique.     

Stretching of slow positron pulses generated with an electron linac

A facility for generating a high intensity slow positron beam using an electron linear accelerator has been constructed. A conversion efficiency of 6×10^–7 slow positrons per incident electron has been obtained for 75 MeV electrons. Storage and stretching of pulsed slow positrons have been successfully carried out with a Penning trap.     

Slow positron study on dielectric-coated mirror for free-electron-laser experiments

A positron lifetime study has been done on dielectric multilayer cavity mirrors for free-electron-laser experiments by the use of a variable-energy pulsed positron beam. A long-lived ortho-positronium component has been observed at low positron energy region, corresponding to the depth of the top amorphous SiO₂ layer. The intensity of the positronium component correlates with the degradation and restoration of the mirrors. We discuss the relation between the positronium intensity and degradation mechanism of the mirrors. The present studies revealed that the slow positron lifetime technique is highly sensitive to the properties of the mirrors and is useful for the evaluation of the mirrors.     

Asymmetric synthesis of isoquinuclidines by Diels–Alder reaction of 1,2-dihydropyridine utilizing a chiral Lewis acid catalyst

The chiral isoquinuclidine derivative, 2-azabicyclo[2.2.2]octane ring system, endo-(7R)-3 was obtained in good yield with excellent diastereoselectivity (up to 92% de) by Diels–Alder reaction of 1-(phenoxycarbonyl)-1,2-dihydropyridine 1 with N-acryloyl-(4S)-4-benzyloxazolidin-2-one (4S)-2 using titanium-(2R,3R)-TADDOLate 4 as a chiral Lewis acid catalyst in toluene at 0 °C. On the other hand, endo-(7S)-3 was obtained in good yield with excellent diastereoselectivity (up to 97% de) by Diels–Alder reaction of 1 with (4R)-2 using Cu(OTf)₂/(4S,4′S)-bis(oxazoline) catalyst 8 as a chiral Lewis acid catalyst in dichloromethane at 0 °C. In these reactions, the choice of solvent and the combination of titanium-(2R,3R)-TADDOLate 4 {or Cu(II)/(4S,4′S)-bis(oxazoline) 8} and dienophile (4S)-2 {or (4R)-2} are very important. The stereochemistry of endo-(7R)-3 has been established to be (1R,4S,7R) and the reaction mechanism is proposed.     

Purification of potassium dichromate and its assay by precise coulometric titration

Potassium dichromate was purified and crystallized by a zone-melting technique, and the purity of the crystal obtained was determined by precise coulometric titration. The dichromate obtained by zone melting was not a single crystal but lumpy material (aggregate of small crystals). The purity of the dichromate obtained by six zone passages in the upward or downward direction was 99.990 (s=0.015%). The water adsorbed on the surfaces of the lumpy crystals which were stored in a desiccator for 3.5 h was about 0.002%. The zone-melting technique is satisfactory for the purification of potassium dichromate and the purified crystals are suitable for use as a primary standard in titrimetric analysis.