A range-based method to calibrate a magnetic spectrometer measuring the energy spectrum of the backward electron beam of a plasma focus.

The electron beam emitted from the back of plasma focus devices is being studied as a radiation source for intraoperative radiation therapy applications. A plasma focus device is being developed for this purpose, and there is a need for characterizing its electron beam, particularly, insofar as the energy spectrum is concerned. The instrument used is a magnetic spectrometer. To calibrate this spectrometer, a procedure relying on the energy-range relation in Mylar® has been devised and applied. By measuring the transmission through increasing thicknesses of the material, electron energies could be assessed and compared to the spectrometer readings. Thus, the original calibration of the instrument has been extended to higher energies and also to better accuracy. Methods and results are presented.

Structural characterization of the human cerebral myelin sheath by small angle x-ray scattering.

Myelin is a multi-lamellar membrane surrounding neuronal axons and increasing their conduction velocity. When investigated by small-angle x-ray scattering (SAXS), the lamellar quasi-periodical arrangement of the myelin sheath gives rise to distinct peaks, which allow the determination of its molecular organization and the dimensions of its substructures. In this study we report on the myelin sheath structural determination carried out on a set of human brain tissue samples coming from surgical biopsies of two patients: a man around 60 and a woman nearly 90 years old. The samples were extracted either from white or grey cerebral matter and did not undergo any manipulation or chemical-physical treatment, which could possibly have altered their structure, except dipping them into a formalin solution for their conservation. Analysis of the scattered intensity from white matter of intact human cerebral tissue allowed the evaluation not only of the myelin sheath periodicity but also of its electronic charge density profile. In particular, the thicknesses of the cytoplasm and extracellular regions were established, as well as those of the hydrophilic polar heads and hydrophobic tails of the lipid bilayer. SAXS patterns were measured at several locations on each sample in order to establish the statistical variations of the structural parameters within a single sample and among different samples. This work demonstrates that a detailed structural analysis of the myelin sheath can also be carried out in randomly oriented samples of intact human white matter, which is of importance for studying the aetiology and evolution of the central nervous system pathologies inducing myelin degeneration.

Radiation protection of PFMA-1, a plasma focus for medical applications.

A plasma focus is being developed for breeding short-lived radionuclides. The different radiation protection issues and concerns posed by the machine once in operation are analysed and discussed. Activation is shown to be totally negligible and likewise neutron emission is found to pose no concern at all. The only source of radiation risk is found to rest in the radionuclides produced, 18F and 15 O, generating a peak exposure of 1.114 Sv y(-1) at the distance of closest approach of 2.5 m. Shielding to protect against this hazard is calculated to be 5 cm Pb or 54 cm concrete for the operation area and 5.5 cm Pb for the transportation flask.

Preliminary results on the production of short-lived radioisotopes with a Plasma Focus device.

An experimental campaign was conducted to assess the feasibility of short-lived radioisotope (SLR) production within the pulsed discharges of a Plasma Focus (PF) device. This so-called "endogenous production" technique rests on the exploitation of nuclear reactions for the creation of SLR directly within the plasma, rather than on irradiating an external target. Until now only one research group has published data relevant to PF endogenous production of SLR, and the data seem to confirm that the PF has the capability to breed SLR. The campaign demonstrated production of (15)O, (17)F and (13)N from the (14)N(d,n)(15)O, (12)C(d,n)(13)N and (16)O(d,n)(17)F reactions. A 7kJ, 17kV Mather-type PF was operated with natural nitrogen, oxygen, CO(2) and deuterium in the vacuum chamber. Results to date confirm that, with a PF of this type, up to 1microCi of SLRs per discharge can be obtained.

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.

Principal component analysis of large layer density in Compton scattering measurements

A multivariate approach based on Principal Component Analysis (PCA) was used to analyze the energy distribution of n Monte Carlo simulated Compton scattered photon spectra describing the electron density of large layers. Three to five layers with different density distribution were used to test the algorithm; each layer was obtained by collecting 25 Compton spectra coming from sensitive volumes (SVs) where the complementary conditions of high and low density were realized (respectively full and void SV). The density variation inside a single layer is described by a two principal components (PCs) linear model that depicts the electron density of each SV: the layer density distribution appears to be correctly described even in the presence of very low signal-to-noise Compton spectra. Density profiles for layers at different depths were comparatively analyzed in order to show that, at least within one mean-free-path distance, it is possible to describe the layer density distribution by the PCA without any correction for the beam attenuation.

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.

Fat and water photon scattering data for in vivo lean and fatty tissue composition studies.

Photon scattering cross-section data for freshly excised and filtered liquid pig fat was measured in the interval chi = 0.02 to 0.64 A-1 (chi = E*[sin(theta/2)]/12.4; E being the photon incident energy (keV) and theta the scattering angle). The experimental results demonstrate that the marked intermolecular effects of coherent scattering in the forward direction can be exploited as a tool for characterizing lean and fatty tissue.

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 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.

In vivo monitoring of bone-Pb and retrospective exposure: an assessment in occupationally exposed subjects.

In this work we present the preliminary results of an investigation in to the bone burden of lead in occupationally exposed subjects. The assessment of bone lead levels normalised to the mineral content (mg/kg bone mineral) were carried out by means of an in vivo X ray fluorescence techniques (Pb-XRF). The measurements were performed on the second phalanx of the right index finger and the minimum detectable limits of the XRF techniques was estimated around 25-30 mg/kg. Although the blood lead level monitoring over long time seems to indicate working activities in environments with low sources of lead contamination, high Pb-XRF levels, and not well correlated with the exposure time, were found. These anomalous values could be explained in term of the dependence of zinc protoporphyrin assessment against the measurement time.

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.

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.