Energy absorption buildup factors of human organs and tissues at energies and penetration depths relevant for radiotherapy and diagnostics.

Energy absorption geometric progression (GP) fitting parameters and the corresponding buildup factors have been computed for human organs and tissues, such as adipose tissue, blood (whole), cortical bone, brain (grey/white matter), breast tissue, eye lens, lung tissue, skeletal muscle, ovary, testis, soft tissue, and soft tissue (4-component), for the photon energy range 0.015-15 MeV and for penetration depths up to 40 mfp (mean free path). The chemical composition of human organs and tissues is seen to influence the energy absorption buildup factors. It is also found that the buildup factor of human organs and tissues changes significantly with the change of incident photon energy and effective atomic number, Z(eff). These changes are due to the dominance of different photon interaction processes in different energy regions and different chemical compositions of human organs and tissues. With the proper knowledge of buildup factors of human organs and tissues, energy absorption in the human body can be carefully controlled. The present results will help in estimating safe dose levels for radiotherapy patients and also useful in diagnostics and dosimetry. The tissue-equivalent materials for skeletal muscle, adipose tissue, cortical bone, and lung tissue are also discussed. It is observed that water and MS20 are good tissue equivalent materials for skeletal muscle in the extended energy range.

The effective atomic numbers of some biomolecules calculated by two methods: a comparative study.

The effective atomic numbers Z(eff) of some fatty acids and amino acids have been calculated by two numerical methods, a direct method and an interpolation method, in the energy range of 1 keV-20 MeV. The notion of Z(eff) is given a new meaning by using a modern database of photon interaction cross sections (WinXCom). The results of the two methods are compared and discussed. It is shown that for all biomolecules the direct method gives larger values of Z(eff) than the interpolation method, in particular at low energies (1-100 keV) At medium energies (0.1-5 MeV), Z(eff) for both methods is about constant and equal to the mean atomic number of the material. Wherever possible, the calculated values of Z(eff) are compared with experimental data.

Energy dependence of effective atomic numbers for photon energy absorption and photon interaction: studies of some biological molecules in the energy range 1 keV-20 MeV.

Effective atomic numbers for photon energy absorption, Z(PEA,eff), and for photon interaction, Z(PI,eff), have been calculated by a direct method in the photon-energy region from 1 keV to 20 MeV for biological molecules, such as fatty acids (lauric, myristic, palmitic, stearic, oleic, linoleic, linolenic, arachidonic, and arachidic acids), nucleotide bases (adenine, guanine, cytosine, uracil, and thymine), and carbohydrates (glucose, sucrose, raffinose, and starch). The Z(PEA,eff) and Z(PI,eff) values have been found to change with energy and composition of the biological molecules. The energy dependence of the mass attenuation coefficient, Z(PEA,eff), and the mass energy-absorption coefficient, Z(PI,eff), is shown graphically and in tabular form. Significant differences of 17%-38% between Z(PI,eff) and Z(PEA,eff) occur in the energy region 5-100 keV. The reasons for these differences, and for using Z(PEA,eff) rather than Z(PI,eff) in calculations of the absorbed dose, are discussed.

Studies on effective atomic number, electron density and kerma for some fatty acids and carbohydrates.

The effective atomic number, Z(eff), the effective electron density, N(el), and kerma have been calculated for some fatty acids and carbohydrates for photon interaction in the extended energy range from 1 keV to 100 GeV using an accurate database of photon-interaction cross sections and the WinXCom program. The significant variation of Z(eff) and N(el) is due to the variations in the dominance of different interaction processes in different energy regions. The maximum values of Z(eff) and N(el) are found in the low-energy range, where photoelectric absorption is the main interaction process. The minimum values of Z(eff) and N(el) are found at intermediate energies, typically 0.05 MeV < E < 5 MeV, where Compton scattering is dominant. In this case, Z(eff) is equal to the mean atomic number of the bio-molecule. Wherever possible, the calculations are compared with experimental results. A comparison is also made with the single values of the Z(eff) and N(el) provided by the program XMuDat. It is also observed that carbohydrates have a larger kerma than fatty acids in the low-energy region, where photoelectric absorption dominates. In contrast, fatty acids have a larger kerma than carbohydrates in the MeV range, where Compton scattering is the main interaction process.

High-pressure study of binary thorium compounds from first principles theory and comparisons with experiment.

The high-pressure structural behaviour of a series of binary thorium compounds ThX (X = C, N, P, As, Sb, Bi, S, Se, Te) is studied using the all-electron full potential linear muffin-tin orbital (FP-LMTO) method within the generalized gradient approximation (GGA) for the exchange and correlation potential. The calculated equlibrium lattice parameters and bulk moduli, as well as the equations of state agree well with experimental results. New experiments are reported for ThBi and ThN. Calculations are performed for the ThX compounds in the NaCl- and CsCl-type crystal structures, and structural phase transitions from NaCl to CsCl are found in ThP, ThAs, ThSb and ThSe at pressures of 26.1, 22.1, 8.1 and 23.2 GPa, respectively, in excellent agreement with experimental results. ThC, ThN and ThS are found to be stable in the NaCl structure, and ThBi and ThTe in the CsCl structure, for pressures below 50 GPa. The electronic structures of the ThX compounds are studied using the quasiparticle self-consistent GW method (G: Green function, W: dynamically screened interaction).

The effective atomic number revisited in the light of modern photon-interaction cross-section databases.

The effective atomic number, Z(eff), has been calculated for fatty acids and cysteine. It is shown that Z(eff) is a useful parameter for low-Z materials at any energy above 1 keV. Absorption edges of medium-Z elements may complicate the energy dependence of Z(eff) below 10 keV. The notion of Z(eff) is perhaps most useful at energies where Compton scattering is dominating, and where Z(eff) is equal to the mean atomic number, Z, over a wide energy range around 1 MeV.

Temperature- and pressure-dependent lattice behaviour of RbFe(MoO4)2.

Trigonal RbFe(MoO(4))(2) is a quasi-two-dimensional antiferromagnet on a triangular lattice below T(N) = 3.8 K, The crystal exhibits also a structural phase transition at T(c) = 190 K related to symmetry change from P3m1 to P3. We present the temperature- and pressure-dependent characteristics of this material in the context of ambiguous opinions on the symmetry and crystal properties below T(c). A single-crystal x-ray diffraction shows that the temperature-dependent evolution of the unit cell in the range 100-300 K is strongly anisotropic with markedly discontinuous changes at T(c). The transition is connected with a spontaneous strain developing in effect of the volume decrease. The structure releases the strain by rotation of corner-sharing rigid MoO(4) and FeO(6) polyhedra in the (a,b) basal plane. The temperature dependence of the IR vibrational wavenumbers exhibits weak changes near T(c), which are consistent with the symmetry transformation from P3m1 to P3. High-pressure x-ray powder diffraction indicates that the material is extremely soft but with some stiffening at high pressure. The zero-pressure bulk modulus is B(0) = 7.9(6) GPa and the pressure derivative is B(0)' = 10(1). The compression curve can be described by a single equation of state, corresponding to the trigonal cell, up to 5 GPa. An amorphization appearing above 5 GPa and increasing gradually on further pressure increase suggests the thermodynamic instability of the high-pressure structure.

Amorphouslike diffraction pattern in solid metallic titanium.

Amorphouslike diffraction patterns of solid elemental titanium have been detected under high pressure and high temperature using in situ energy-dispersive x-ray diffraction and a multianvil press. The onset pressure and the temperature of formation of amorphous titanium is found to be close to the alpha-beta-omega triple point in the P-T phase diagram. Amorphous Ti has been found to be thermally stable up to 1250 degrees C for at least 3 min at some pressures. By analyzing the conditions for producing amorphous elemental Zr and Ti, we observed a multi-phase-point amorphization phenomenon for preparing single-element bulk amorphous metals. The results reported may open a new way to preparing single-element bulk amorphous metals with a high thermal stability.

The high-pressure phase of zincite.

Synchrotron-radiation X-ray diffraction studies of ZnO have been performed with special emphasis on the rock-salt-type high-pressure phase. The wurtzite (B4) to rock salt (B1) phase transformation is accompanied by a volume collapse of 18%, which is a very large effect. The lattice constant of the B1 phase at ambient pressure is a(0) = 4.280 (4) A, and the bulk modulus is B(0) = 170 (10) GPa. Comparisons are made with a recently published theoretical equation of state obtained from ab initio perturbed-ion calculations.

Microfocus x-ray spectra measurement.

The applied equipment, the experimental procedures, and the results obtained in the experimental measurement of x-ray spectra from a microfocus x-ray unit are described. The measured spectra of x rays passing through different absorbers are compared to calculated spectra, based on calculation of the linear absorption coefficient, µ, and the spectrum of the incident radiation. The nice fit between the measured and the calculated spectra testifies to the applicability of the presented method for determination of the linear attenuation coefficient. Such calculations form the basis for a more accurate experimental determination of the contrast reducing build-up factor in industrial radiography.