C. Baraldi

An empirical approach to K-shell ionisation cross section by electrons

The working out of an empirical expression for the cross section for production of atomic K-shell vacancies by electrons is described. The basis is a critical analysis of the existing experimental data. The ranges for which the function is evaluated are 6<or=Z<or=79 and 1<or=U<or=20 (U being the ratio between the incident electron energy and the binding energy of electrons in the K shell). The possible meanings of the various factors present in the expression are examined by comparison with the asymptotic Bethe expression. The results obtained by the empirical function can also be applied to both Z and U intervals even wider than those first considered.

Molecular differential cross sections for x-ray coherent scattering in fat and polymethyl methacrylate

Molecular differential cross sections for the coherent scattering of x-rays in polymethyl methacrylate (PMMA) and fat were determined from measured diffraction patterns in the interval chi = 0 to 6.4 nm-1 (chi = [sin theta/2)]/lambda; lambda being the incident wavelength having the units of nm and theta the scattering angle). All measurements were performed at a controlled temperature of 23 degrees C. The final results for PMMA show overall agreement when compared with the data existing in the literature. However, some discrepancies with the results reported by Kosanetzky et al in 1987 are found at the first three maxima. The data for filtered fat material are reported here for the first time. Finally, data sets of molecular form factors for fat and PMMA were compiled from the smoothed corrected experimental results by assuming these materials to be mono-molecular.

Updating of form factor tabulations for coherent scattering of photons in tissues.

An updating of photon transport modelling in tissues is carried out by including the effect of molecular interference in the coherent (Rayleigh) scattering. To this end, the present tabulations--which permit us to obtain the linear differential scattering coefficient of compounds from a simple weighted sum of the elemental components--are integrated by adding files for a limited set of molecular interference functions. This set originates from a four-component model which is found to be capable of reproducing human tissues in situations involving bony and soft tissues. The proposed procedure overcomes, in the computation, the hindrance that the dependence on molecular interference effects leads every tissue to have its own diffraction pattern, which is not easily obtained by means of measurements or calculations.

Compton scattering profile for in vivo XRF techniques

The contribution from single Compton scattered photons to the background in in vivo x-ray fluorescence analysis is evaluated by taking into account the energy broadening of the scattered photons which reflects the momentum distribution of the target electrons. A general-purpose Monte Carlo evaluation of multiple scattering components, as well as accurate experimental verifications with 59.54 keV photons impinging on various targets of interest for real-life irradiation, confirm that the single Compton scattering profiles of the elements composing the biological matrix dominate the trend and amplitude of the background in the region of interest with near-backscatter configurations. Step features are likewise explained in terms of single Compton phenomenology. Other probable sources of background, such as photoelectron Bremsstrahlung and pile-up distribution, are studied both theoretically and experimentally in order to compare their amplitude and features with those of single Compton scattered photon profiles.

On the angular dependence of L x-ray production cross sections following photoionization at an energy of 59.54 keV

The anisotropy of L x-ray fluorescence induced by 59.54 keV unpolarized photons is investigated by means of an experimental procedure which allows the relative L x-ray production cross section to be evaluated without taking account of the angular set-up and the instrumental efficiency. Thick targets of Yb, Hf, Ta, W and Pb are considered, and the angular trend of the relative experimental ratios, IL?/IL?, is calculated by simple evaluations of the peak area alone. Within the experimental uncertainties, which are found to be of the order of 1.6% in the worst cases, the results do not show any significant angular dependence of the L? emission lines if the isotropy of the L? lines is assumed. This fact contrasts with the results of some groups, which report evidence of strong angular dependence at this level of photon energy.

Photon backscattering tissue characterization by energy dispersive spectroscopy evaluations

Techniques for in vivo tissue characterization based on scattered photons have usually been confined to evaluating coherent and Compton peaks. However, information can also be obtained from the energy analysis of the Compton scattered distribution. This paper looks at the extension of a technique validated by the authors for characterizing tissues composed of low-atomic-number elements. To this end, an EDXRS (energy dispersive x-ray spectrometry) computer simulation procedure was performed and applied to test the validity of a figure of merit able to characterize binary compounds. This figure of merit is based on the photon fluence values in a restricted energy interval of the measured distribution of incoherently scattered photons. After careful experimental tests with 59.54 keV incident photons at scattering angles down to 60degrees, the simulation procedure was applied to quasi-monochromatic and polychromatic high-radiance sources. The results show that the characterization by the figure of merit, which operates satisfactorily with monochromatic sources, is unsatisfactory in the latter cases, which seem to favour a different parameter for compound characterization.

Feasibility of in vivo tissue characterisation by Compton scattering profile measurements

Abstract The authors investigate a technique for the characterisation of biological tissues, based on energy analysis of Compton scattered photons. The incident beam consists of 59.54 keV photons and a backscatter geometry is used. Compared to the technique based on the ratio of photons coherently scattered due to the Rayleigh effect and those incoherently scattered due to the Compton effect, the present technique provides similar results for mineral content evaluation, with an error lower than 1%. However it allows the determination of fat content in a soft tissue matrix - even in the cases in which the Rayleigh peaks is not measurable - although the error can rise up to 3%.

Photon cross-section data from X-ray diffraction pattern measurements: Correction procedure evaluations

Abstract The feasibility of evaluating absolute differential cross-sections of coherently scattered X-rays by exploiting diffraction pattern measurements has been tested in the case of plastic and biological samples. The investigation takes into account three powder diffractometers with different irradiation facilities. The main relevant correction factors have been evaluated by means of Monte Carlo and computational algorithm approaches. A careful evaluation of the effect of non-monochromatic incident photon beams allows this procedure to be valid without resorting to sophisticated irradiation facilities. Moreover, in comparing the experimental data with the theory, it is shown that some assumptions usually made when calculating theoretical scattering cross-sections are not valid when a scattering angle ranging from forward to backscattering values is to be considered. These results, together with other usual instrumental corrections, permit us to evaluate the differential scattering cross-sections within acceptable uncertainties.

The effect of delta rays on the ionometric dosimetry of proton beams

The interface effects arising in the measurement of absorbed dose by ionization chambers, owing to the inhomogeneity between the walls and the gas, have been evaluated by an analytical model. The geometrical situation considered here is appropriate for representing the behaviour of a plane-parallel ionization chamber exposed to a radiotherapeutic beam of protons. Two gases, dry air and tissue equivalent gas (methane based), as well as six materials commonly used in ionization chamber walls, i.e. graphite, A-150 tissue equivalent plastic, C-522 air equivalent plastic, nylon type 6, polymethyl methacrylate and polystyrene, have been examined. The analysis of the results shows that, within the limits of the detector dimensions and proton energies commonly used in the dosimetry of radiotherapeutic beams, these effects, if not taken into account in the measurement interpretation, can entail deviations of up to about 2% with respect to the correct absorbed dose in gas.

Molecular coherent scattering data for tissue in photon transport Monte Carlo codes

The suitability of introducing molecular interference effects into coherent scattering phenomenology is considered here in the case of X and gamma photon transport in biological tissues. In order to make its implementation practicable in most of the existing open user Monte Carlo codes, a self-consistent basic set of tabulations for the linear differential scattering coefficient was proposed. The tabulations were critically reviewed by testing their validity over the entire range of the momentum transfer variable from 0 to 10(10) nm(-1). Implementation in three different Monte Carlo codes was performed, and they proved to be reliable.