O. Goncalves

X-RAY DIFFRACTION TOMOGRAPHY USING INTERFERENCE EFFECTS

Abstract When the electromagnetic wave excites more than one electron, the coherent scatter from different electrons gives rise to interference effects. X-rays scattered from a crystalline solid can constructively interfere, producing a diffracted beam at well-defined Bragg angles. The aim of this work is to describe a new imaging method based on the detection of diffracted X-rays. Diffraction patterns of polycrystalline solids (lead, silver and copper) were measured. A selective discrimination of a given element in a scanned specimen can be realized by fixing the Bragg angle which produces an interference peak and then, to carry out the computed tomography in the standard mode. The images obtained show the feasibility of this selective tomography

Elastic scattering of 662 keV γ-rays on atoms

Differential cross sections of Rayleigh scattering were measured for Cu, Zn, Cd, Sn, W, Pt, Pb and U at scattering angles between 10,5° and 60° using the 662 keV γ-rays of a137Cs source. The experimental results are compared with other measurements and with calculations of modified form factor theories and second order perturbation theory. Agreement is obtained with the latter one.

Some experiments on γ-scattering between 122 and 145 keV at small angles

Differential cross section for scattering of 145.4 keV gamma rays by B, C, Al, Cu and Cd have been measured from 5–25°. For angles <10° it was not possible to separate Rayleigh and Compton scattering; therefore, the sum of the cross sections is given. Rayleigh cross sections have been measured for Pb at 122.1 and 136.5 keV at angles between 20° and 70°. The experimental results are compared with the form factor theory for Rayleigh scattering and the incoherent scattering factor theory for Compton scattering.

A method to measure the pressure profile inside evacuated tubes with high voltage electrodes

Abstract We developed a method to determine the pressure distribution in the electrode region of electrostatic accelerators. Direct measurement of this distribution is not possible: the electrodes are maintained inside a high pressure vessel and the presence of vacuum gauges would disturb the beam optics. The method is based on measuring the energy distribution of the atomic ion beams and identifying features associated to collisions with residual gas molecules. The procedure is also applicable to any other evacuated tube in the presence of a static longitudinal electric field. The feasibility of the method was verified in a 1.7 MV commercial particle accelerator using its facilities (accelerator tubes, particle beam, stripper, etc.). The measurements also revealed characteristics of the field structure inside the accelerator tube and the occurrence of leaks.

Comparison between measurements on Rayleigh scattering and form-factor theories for 1 Å−1 ≤ x ≤ 50 Å−1

Abstract From our measured differential cross section of Rayleigh scattering, the form factor for momentum transfers in the range 1 A −1 ≤ x ≤ 50 A −1 was deduced. Data for U, Pb, Pt, W, Sn, Cd, Ag, Mo and Cu at energies from 60 to 662 keV and scattering angles ranging from 5° to 140° were used. In addition, for the region 1 A −1 ≤ x ≤ 50 A −1 of momenta, experimental data from other authors at higher energies were analyzed. The experimental values obtained were compared with various form-factor theories and limits of validity are established. For the relativistic modified form factor G a good agreement always exists for θ ≤ 65°, independently of energy and atomic number.