Photon energy absorption and exposure buildup factors for deep penetration in human tissues

Using photons in therapeutic and diagnostic medicine requires accurate computation of their attenuation coefficients in human tissues.The buildup factor,a multiplicative coefficient quantifying the ratio of scattered to primary photons,measures the degree of violation of the Beer-Lambert law.In this study,the gamma-ray isotropic point source buildup factors,specifically,the energy absorption buildup factor (EABF) and exposure buildup factor,are estimated.The computational methods used include the geometric progression fitting method and simulation using the Geant4 (version 10.4) Monte Carlo simulation toolkit.The buildup factors of 30 human tissues were evaluated in an energy range of 0.015-15 MeV for penetration depths up to 100 mean free paths (mfp).At all penetration depths,it was observed that the EABF seems to be independent of the mfp at a photon energy of 1.5 MeV and also independent of the equivalent atomic number (Zeq)in the photon energy range of 1.5-15 MeV.However,thebuildup factors were inversely proportional to Zeq for energies below 1.5 MeV.Moreover,the Geant4 simulations of the EABF of water were in agreement with the available standard data.(The deviations were less than5%.) The buildup factors evaluated in the present study could be useful for controlling human exposure to radiation.     

Photon energy absorption parameters for some polymers

Some photon energy absorption parameters viz. mass energy absorption coefficient (μ/ρ)_en, photon energy absorption effective atomic number (Z_PEA), electron density (N_e) and KERMA relative to air has been computed in the energy range from 1 keV to 20 MeV for some polymers such as nylon, poly-acrylo-nitrile, poly-methyl-acrylate, poly-vinyl-chloride, poly-styrene, synthetic rubber and poly-tetra-fluro-ethylene. The dependence of different parameters on incident photon energy and chemical composition of the selected polymers has been studied .     

Modeling of gamma-ray energy absorption buildup factors using neural network

This paper presents a new approach based on multilayered perceptrons (MLPs) to compute energy absorption buildup factors. The MLP has been trained by a Levenberg–Marquardt learning algorithm. The model is fast and does not require tremendous computational efforts. The results obtained by using the proposed model are in good agreement with the ANSI/ANS-6.4.3 standard data set.     

Be careful when using X-ray exposure analysis for polymers

The techniques of X-ray diffraction analysis (XDA) and X-ray photoelectron spectroscopy (XPS) for investigation of the structure and surface constitution of materials respectively are considered to be free from radiation-induced artifacts. But earlier a very strong effect of the X-ray flux on the surface constitution of the polytetrafluoroethylene (PTFE) was observed. Later we studied a crystalline structure of the PTFE by the methods of XDA and differential scanning calorimetry (DSC) simultaneously. Quite unexpectedly the values of crystallinity degree measured by the two methods for as-received samples of the same polymer differed more than two times (26% for DSC and 68% for XDA). It was established that during an analysis by XDA the samples were irradiated by the X-ray doses 0.1-1 kGy depending on exposure. The irradiation of PTFE with such small doses of both X-ray and ⁶⁰Co γ-radiation results in a significant increase of crystallinity that is identified by the DSC method but not by the XDA one. Only after irradiation with doses higher than 10 kGy the measured crystallinity degrees for the two methods become equal. We believe the reason of the observed difference is in the capability of the DSC method to register only thermodynamically stabilized crystallites while the XDA procedure fixes the total response from crystal and paracrystal phases.     

A comprehensive study on energy absorption and exposure buildup factors for some essential amino acids, fatty acids and carbohydrates in the energy range 0.015-15 MeV up to 40 mean free path

The gamma ray energy absorption (EABF) and exposure buildup factors (EBF) have been calculated for some essential amino acids, fatty acids and carbohydrates in the energy region 0.015-15 MeV up to a penetration depth of 40 mfp (mean free path). The five parameter geometric progression (G-P) fitting approximation has been used to calculate both EABF and EBF. Variations of EABF and EBF with incident photon energy, penetration depth and weight fraction of elements have been studied. While the significant variations in EABF and EBF for amino acids and fatty acids have been observed at the intermediate energy region where Compton scattering is the main photon interaction process, the values of EABF and EBF appear to be almost the same for all carbohydrates in the continuous energy region. It has been observed that the fatty acids have the largest EABF and EBF at 0.08 and 0.1 MeV, respectively, whereas the maximum values of EABF and EBF have been observed for aminoacids and carbohydrates at 0.1 MeV. At the fixed energy of 1.5 MeV, the variation of EABF with penetration depth appears to be independent of the variations in chemical composition of the amino acids, fatty acids and carbohydrates. Significant variations were also observed between EABF and EBF which may be due to the variations in chemical composition of the given materials.     

Study of relation between the gamma flux buildup factors and source geometry by M-C simulation

This paper is to achieve a gamma-ray source with the lowest rate of buildup factor, which is of great importance in medical, industrial and agricultural sciences.The flux buildup factor of gamma rays is calculated by the MCNP code for point, linear, surface and volume sources with shield layers of lead, iron and aluminum. The results show that for the high Z shielding material, the flux buildup factor of coaxial cylindrical sources is the lowest(1.6–2.3)of all sources, while for low Z shielding materials, the coaxial disk surface sources have smaller buildup factor(1.45–1.6).     

Temperature dependence of dislocation bias factors in metals

Recently, it has been shown that edge dislocations are not stable sinks for vacancies. Trapping and detrapping of vacancies occur as a thermally-activated process. In this paper, the temperature dependence of vacancy absorption coefficient of edge dislocations under irradiation was calculated by using rate equations in Fe and Ni. The temperature dependence was almost the same in both Fe and Ni and did not depend on the damage rate between 10⁻¹⁰ dpa/s and 10⁻⁶ dpa/s. At low temperatures such as room temperature, the coefficient was low and with increasing irradiation temperatures, it had a peak (500 K) and decreased.     

Permeability of fluorine and some fluorine-containing gases through nonporous fluorine-stable polymers

The experimental coefficients of permeability of fluorine, hydrogen fluoride, and uranium and tungsten hexafluorides through nonporous films consisting of certain fluorine resistant polymer materials are presented. It is shown that the membrane method of removing hydrogen fluoride from fluorine and uranium and tungsten hexafluorides is attractive. __________ Translated from Atomnaya énergiya, Vol. 102, No. 6, pp. 367–370, June, 2007.     

肝单光子发射计算机断层若干物理因素的讨论

我们于1985年对我院1984年开展肝单光子发射计算机断层以来的52例肝病人进行了分析总结。结果表明,肝单光子发射断层在诊断肝内占位性病变方面比普通肝静态γ照相有许多优点。需要指出的是,肝单光予发射断层的若干优点及诊断的准确性是与物理条件的正确选择和应用密切相关的。本文根据至今积累的近100例断层经验,从技术方面介绍了影响单