Estudo de monocromadores assimetricos de raios-X
AUTOR(ES)
Irineu Mazzaro
DATA DE PUBLICAÇÃO
1979
RESUMO
Asymmetric X-ray monochromators, firsts introduced by I. Fankuchen ( 1937 ) with the purpose of concentrating an X-ray beam have been the subject of numerous investigations. Evans, Hirsch and Kellar as early as 1948, based ther well known and hitherto unchallenged study on the hypothetic existence of a non-ref1ecting surface layer with an absorption coefficient different from that of the bulk crystal which they used to fit a "theoretical" curve to their experimental data. In this Thesis it is shown that the physical behavior of asymmetric monochromators can be explained without recourse to any kind of artificial hypothesis, by means of the theory of multiple scattering of X-rays ( Caticha-Ellis, 1969 ). The differential equations governing the interchange of power among the incident and the diffracted waves were solved in the two-beam asymmetric case and the solutions expressed in terms of the asymmetry parameters: R = sen( qB - f ) / Sen( qB + f ) or B = 1 - R / 1 + R, where qB is the Bragg angle and f0 the angle between the reflecting plane and the reflecting surface measured on the plane of incidence. f0 is the dihedral angle between the o plane surface and the reflecting planes. The solutions were found to depend not just on one but on two parameters, one of assimmetry and k = tgf0 / tgqB which was called a "cutting parameter". The experimental method used here is based in that of Evans et al. A crystal cut with f0 >qB, turns around the normal to the reflecting planes thus generating different asymmetry f angles. The experimental advantage is th at only one crystal has to be cut. The theoretical complications involved, most of which are discussed here in detail, are the price for that simplifications. A new effect, the rotation of the asymmetrically diffracted beam was predicted and confirmed experimentally. The theory also demonstrates that the intensity in the concentrated beam can not be move than twice the intensity of the symmetrically reflected beam, a fact already known to previous authors. The measured diffracted power agrees well with the maximum discrepancies occur when approaching of maximum asymmetry b = +-1, where the experimental values are largely affected by surface irregularities as well instrumental factors, which how ever have not yet been studied. w - profiles taken on different regions of the crystal showed pronounced differences according to the suface treatment; even after extremely careful polishing with corrudum powder of 0.05 mm grain size. Etching worsened the situation by splitting the w - peak in several smaller anes. Only a hybrid mechanical - chemical treatment, developped for this purpose was able to produce surfaces whose w - profiles possess a reasonable peak shape and are practically invariant for the differente regions of the crystal. In conclusion the theory used irl this thesis explains well the power and intensity diffracted asymmetrically by a crystal cut with an angle f0 with respect to the reflecting planes except under extreme conditions of asymmetri where the instrumental and surface effects are dominant
ASSUNTO(S)
radiação cristalografia de raio x
