Positron implantation profiles in various materials for22 Na isotope source are discussed. The spatial distribution of positrons in materials with densities from 1 to 21 g/cm 3 are simulated using GEANT4 code. The results indicate that depth distributions of thermalized positrons in any homogeneous medium can be described by the sum of two exponential functions. Contribution of the short-range exponent is about 11% in the materials. The both absorption coefficients can be well represented as a function of material density and atomic number. Proposed empirical model agrees well with available experimental data.
Chemical carcinogens are the main cause of cancer. Many thousands of new chemicals are synthesized annually in the world, unknown part of them is carcinogens. There are several methods, which are currently used for testing carcinogenicity. The fastest are physicochemical methods. We suggest the method of fast carcinogen screening using the positron annihilation spectroscopy (PAS). It is based on the fact that the presence of carcinogens, being strong electrophiles, significantly decreases positronium (Ps) formation probability. This quantity can be measured by means of conventional PAS techniques: positron annihilation lifetime spectroscopy (PALS), angular correlation of annihilation radiation (ACAR) or Doppler broadening of annihilation radiation (DBAR).It is also possible to evaluate anti-carcinogenic properties of chemicals using the anti-inhibition effect of Ps formation.
Biocompatibility and biodegradability of porous silicon (por-Si) nanoparticles (NPs), as well as the fact that they can selectively accumulate in tumor tissues, allow using them as containers for delivery of diagnostic markers or drugs for therapy of cancer tumors. Advantages of por-Si NPs as carriers of drugs are also favorable due to the high surface area and large pore volume. To apply por-Si NPs as nanocontainers it is necessary to have the comprehensive information about their porosity. In our work we use the positron annihilation lifetime (PAL) spectroscopy for porosity investigation. Samples of por-Si were prepared by electrochemical etching of heavily boron doped crystalline Si wafers in a hydrofluoric acid solution. The prepared por-Si films were dried and mechanically milled to obtain powder of NPs, which was pressed into tablets for PAL investigation. Ortho-positronium components of the measured positron lifetime spectra allowed us to evaluate the pore size distribution in por-Si NPs as continuous bimodal one with two peaks near 1 nm and 3 nm.
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