The positron density distributions in C60 and K6C60 have been evaluated using the positron lifetime and Doppler-broadening spectroscopy for the annihilation radiation.In C60, positrons are distributed in the interstitial sites between the C60 molecules,which has been demonstrated by measurements of the temperature dependence of the Doppler-broadening of the annihilation radiation. On the other hand, the positron density distribution must be greatly changed in K6C60, because positrons are repelled by Coulomb interactions by the positively charged K atoms. It has been observed that there is an extremely short lifetime and a small Doppler-broadening component for the positron annihilation in K6C60. This component is considered to reflect the positron annihilation inside a C60 molecule.
Considering that Ps formation in condensed media has to take place in the teminal positron spur which, comprising many positive ions, provides a strong oxidizing environment for Ps, it is suggested that Ps formation via the epithemal process has to be mixed up with that of the spur process. Eventually in condensed media the observable aspect is those involving slowed down electrons and positron. When the size of the terminal positron spur is large, on the other hand, Ps via the epithermal process can survive. It appears to be possible to overcome the apparent inconsistency between the two models in this way. With this assumption admitted, we propose to investigate the intra-spur positronic processes in more detail. The spur process is an evolution of the energy level and the number of excess electrons, and Ps can be formed from various electron states in the course of the evolution: free excess electrons molecular anions, and excited molecular states. Ideas of Ps formation via resonant energy absorption and that involving vacancies are also presented. The importance of the non-exponential decay of o-Ps in the study of the positronic spur processes is also demonstrated with some recent results of scavenger effects.
The kinetics of the thermal dehydration of yttrium formate dihydrate was studied by means of isothermal gravimetry under various water vapour pressures from 5×10−4 to 8 torr. On the whole, the dehydration was described as the three dimensional phase boundary reaction, R3. An unusual dependence of the rate of dehydration on the atmospheric water vapour pressure was observed: with increasing water vapour pressure, the rate increased at first, passed through a maximum, and then decreased gradually to a constant value. These phenomena were similar to the Smith-Topley effect. The mechanism of the phenomena can be described on the basis of the crystallinity of the dehydrated product phase.
The separation of technetium(VII) from uranium(VI) has beeen studied through experiments on the coprecipitation of Tc(VII) with ammonium diuranate precipitate, and in the extraction of Tc(VII) from an aqueous hydrofluoric acid solution using a primary amine (Primene JMT) as an extractant dissolved inn-heptane. The extraction of Tc(VII) reached a quantitative level after several repetitions of the extraction procedure. Also, the stripping of Tc(VII) into 3M aqueous solution of ammonium carbonate was enhanced to a level over 99% by repeating the stripping cycles.
Authors:Y. Uchiyama, K. Ito, H. Li, Y. Ujihara, and Y. Jean
The variations of size, intensity, and size distribution of free volumes generated in the network of molecular chains of gelatin at the sol-gel transition were studied by means of the positron annihilation lifetime technique. Although variation in average free-volume radius was not recognized, a variation of free-volume content was observed at the sol-gel transition point of gelatin with an addition of saccharose.
PAL were measured for several low molecular organic compounds, normal and cyclic-hydrocarbons and their perfluorinated ones, from room temperature down to about 40 K, and information about the vacancies in them has been extracted from
3 andI3. Normally the size of vacancies were larger in the solids comprising larger molecules, showing thato-Ps can represent the vacancy size. In a special case of perfluorocyclohexane whose solid had to be prepared by sublimation the vacancy size was larger than expected. In all the solid molecules studied twoo-Ps states could be observed and their relative importance could easily change showing a hysteresis-like behavior. At further lower temperatures of around 40 K theo-Ps state with longer lifetime and larger intensity became overwhelming for all molecules. Also
3 andI3 were not very sensitive to most of the phase transitions, including the melting points.
Authors:Y. Sawada, K. Omika, Y. Ito, F. Muta, and M. Momota
The formation process of a ceramic (indium oxide) thin film (thickness: approximately 20 nm to several microns) was investigated
by thermal analyses. Thermal changes of an organic precursor, indium(III) 2-ethylhexanoate, dip-coated on a glass substrate
was successfully detected by DSC in air. Exothermic phenomena were observed at marked lower temperatures for the thin films
than for the bulk material; thinner films had slightly lower peak temperatures. The reaction mechanism is discussed with reference
to mass spectra of the evolved gases.
Authors:Y. Sakai, C. Yonezawa, H. Matsue, H. Sawahata, and Y. Ito
The spectral lines of the 478 keV prompt -ray from7*Li produced in the10B(n, )7*Li reaction were measured for a silicon wafer coated with a thin boron layer using neutron beams. The shapes of the Doppler broadened lines were found to depend greatly on the angle between the measurement axis and the boron layer surface. The angular dependence was interpreted according to whether the energetic7*Li ions are ejected into air or into the substance in the forward or backward recoil of7*Li to the -ray detector.
Authors:K. Ito, Y. Ujihira, T. Yamashita, and K. Horie
Discontinuous and continuous volume phase transitions of organic polymer hydrogels, such as polyacrylamide (PAAm) and poly(N-isopropylamide) (PNIPA) gels, uponpH and temperature were studied by the positron annihilation lifetime measurement, which allows the estimation of size, intensity and size distribution of the free volume. Microscopic changes of physical and chemical interactions between gel network and solvent molecules and among conjugated solvent molecules at volume phase transitions of polyacrylamide gels were discussed.
Authors:Hamdy Mohamed, Y. Ito, A. El-Sayed, and E. Abdel-Hady
Positron annihilation lifetime spectra were measured forpolyvinylalcohol (PVA) doped with CuCl2 (0.5 to 5.0 wt%) at temperature range from room temperature to 160°C. For a fresh pure PVA (without annealing)
3 belowTg was larger in the heating runs than in the cooling runs, but aboveTg,
3 was the same for both runs. The larger
3 values in the heating run were considered to be due to the existence of strain brought forth in the process of preparation. For annealed pure PVA
3 was the same for the heating and the cooling runs andTg was shifted to lower temperature, 88 and 80°C for fresh and annealed samples, respectively. This shift was attributed to loss of H-bonded water in the annealed pure PVA by heating above 100°C. BothI30
3 were decreased by the added CuCl2, being interpreted as the result of inhibition and quenching, respectively. The results show that both the inhibition coefficient and the quenching rate constantk were smaller than the corresponding values in liquids. The small diffusion constant ofo-Ps estimated fromk implies thato-Ps is not very mobile in the polymer.