Positron annihilation anomaly in the high-temperature superconductor YBa2Cu3O7−x

We have measured the ac susceptibility of a wire with a Nb core (1.27 mm diam.) and a Cu cladding (0.37 mm thickness) atT50 K andB0.1 mG. Due to its proximity to Nb, the Cu becomes fully superconducting. From the data we find a breakdown fieldH _b =1.2 (mG) and a coherence length =2.2T ^–1/2 (m) for the Cu, as well as a field penetration depth -34T ^1/2 (m) at the Cu/Nb interface.     

Electrical properties of some crystalline salts of adenine

In the present paper we report the first experimental results on ac and dc conductivity and permittivity of adenine hemisulphate hydrate and adenine sulphate measured at atmospheric and high hydrostatic pressures. For both materials ac conductivity is of ^s type, where:s 1·1· Room temperature dc conductivity of adenine hemisulphate hydrate equals approximately 5×10^{–15 –1} cm^–1 with an activation energy of 0·86 eV; dc conductivity of adenine sulphate is less than 10^–16 cm^–1. On the basis of these measurements and those carried out at high pressure, it is concluded that conductivity of adenine hemisulphate hydrate is of electronic type.The authors wish to thank Dr. J. Zachová for the preparation of adenine salts single crystals.     

On the expansion of hole in main chain polymers determined by positron annihilation

The temperature dependence of positron annihilation characteristics, ₃ andI ₃, has been studied on sample of poly(butadiene), poly(isobutylene) and poly(chloroprene). The temperature range was between 15 and 470 K. The rate of expansion of holes or free-volume in all samples was deduced belowT _g as well aboveT _g as appr. 3·10^–3 K^–1 and 2·10^–2 K^–1, respectively. These values are very close to the rate of the mean squared displacement of scatterer<r ²>observed in neutron scattering experiments. A possibility to use an inverse value of free-volume,V _f ^–1 for study of viscoelastic state of polymers is demonstrated.     

The structure of83Sr excited states

The -decay of⁸³Y isomers (7.06 min and 2.85 min) have been studied by means of Ge (Li) detectors. The proposed level scheme of the⁸³Sr nucleus is based on the coincidence measurement and the analysis of energy sums. The intensity balance requirement leads to _T (35·5 keV)=3.2 which is consistent with M1 multipolarity of this transition. A probable structure of some of the excited states in the⁸³Sr nucleus is discussed in the framework of Alaga's model and Kuriyama's model.     

Rearrangement of hexadecane molecules confined in the nanopores of a controlled pore glass using positron annihilation and differential scanning calorimetry

The solidification/melting of cetane confined in nine silica glasses with specific controlled pores (CPG) with different pore sizes (r _p ranging from 3.7–72 nm) has been studied. Samples with variable coefficient of filling of cetane were prepared and analyzed. Combining the indication of the evolving solidification heats (via differential scanning calorimetry, DSC) with the lifetime spectra of positron annihilation, the mechanism of filling of cetane molecules in the pores of the hosting silica glass matrices has been searched. Three independent solidification effects—three independent DSC peaks—were observed in each measuring cycle, namely one crystallization exotherm Rb of the bulk-cetane (if present) and two crystallization sub-peaks Rc1 and Rc2 of the confined cetane in the case of continuous cooling. The temperatures of both Rc1 and Rc2 effects decrease on decreasing the r _p of the CPG matrix. The mutual relation between Rc1 and Rc2 has been analyzed. The kinetics of all three steps of the solidification in the continuous-cooling as well as isothermal regimes have been specified.     

Thermal properties of poly-ε-caprolactam and copolyamides based on ε-caprolactam

The copolyamides consisting of ε-caprolactam and 6.1–24.5 wt.% of nylon salt prepared from adipic acid and 1-(2-aminoethyl) piperazine were synthesized. Physical and thermal characteristics of polyamide 6 and the copolyamides were compared. Nylon salt does not influence the polyreaction equilibrium so it is possible to prepare the copolyamides with high molecular weight and with the content of low-molecular compounds comparable with that of pure PA 6. Melting temperatures of the copolyamides are lower in comparison with PA 6 and decrease proportionally to the amount of the nylon salt. The thermal stability of the copolyamides is good and equal to that of PA 6. The melting enthalpies indicate that the process of crystallization of the copolyamides is influenced by the time of crystallization and the amount of comonomer present. Longer time of the crystallization assures higher degree of crystallization. The kinetics and the level of crystallization are positively influenced by the mobility of copolyamide segments mainly up to 10 wt.% of comonomer.     

Tritium in organic matter around Krško Nuclear Power Plant

The aim of the research was to obtain first results of tritium in the organic matter of environmental samples in the vicinity of Kr?ko NPP. The emphasis was on the layout of suitable sampling network of crops and fruits in nearby agricultural area. Method for determination of tritium in organic matter in the form of Tissue Free Water Tritium (TFWT) and Organically Bound Tritium (OBT) has been implemented. Capabilities of the methods were tested on real environmental samples and its findings were compared to modeled activities of tritium from atmospheric releases and literature based results of TFWT and OBT.     

The decay of the 23min131Sb

The direct and the coincidence spectra of gamma-rays from the¹³¹Sb decay have been measured by means of different Ge(Li) detectors. The Sb sources have been prepared from mixture of²³⁵U thermal neutron fission products. The proposed level scheme of the¹³¹Te nuclei is based on the coincidence measurements, the analysis of the energy sums and the reaction data. The 49 of the total number of 64 gamma-rays observed have been placed in the present decay scheme. Properties of the some levels in¹³¹Te are compared with those observed in other oddmass Te isotopes. A probable structure of some of the excited states in¹³¹Te is discussed.