A hybrid techinique to evaluate the line spread function

• Autor(a) principal: Almeida, Gevaldo Lisboa
• Data de Publicação: 2002
• Outros Autores: Souza, Maria Inês Silvani, Lopes, Ricardo Tadeu, Instituto de Engenharia Nuclear
• Tipo de documento: Relatório
• Idioma: eng
• Título da fonte: Repositório Institucional do IEN
• Texto Completo: http://carpedien.ien.gov.br:8080/handle/ien/2112
• topic: Line spread function; Edge response function; Tomography; Image quality

A hybrid technique to evaluate the Line Spread (LSF) has been developed. It’s based on an experimental-theoretical approach aiming the reduction of the required experimental efforts to reach an acceptable level of accuracy on the width of the Gaussian representing the LSF. Using this technique, the several spectra required to fill up the space domain with an adequate density, usually done by shifting slightly the object with respect to the source-detector system after each spectrum is taken, are replaced by few spectra and a theoretical treatment involving numerical integration and non-linear fittings. In order to accomplish this task, a function is fitted to the experimental data, allowing the accurate determination of the center of the cylindrical object in the spectrum. Once this center is determined, it becomes possible to compare channel by channel the experimental counts with the expected theoretical ones. The first ones represent an integration of the transmitted neutron beam, as automatically performed by the detector, and hence, to achieve the aimed comparison, the theoretical counts should as well arise from a similar integration. Since the function expressing the transmitted beam intensity cannot be symbolically integrated, the task in done numerically over regular intervals corresponding to each given nominal collimator aperture. The sum of quadratic differences between the experimental and calculated counts reaches a minimum at an effective collimator aperture, which in somewhat different from the nominal aperture actually used in the experiments due to the unavoidable neutron scattering process and statistical fluctuations. This effective aperture is then used to get through integration the theoretical Edge Response Function (ERF) spectrum for any displacement step. Such a feature makes possible the simulation of high density spectra which are otherwise experimentally unfeasible due to the limited mechanical tolerances of the positioning devices. A derivation of the ERF yields the aimed LSF. Several comparisons and simulations have then be made by using a computer program written in Fortran, to evaluate the applicability and advantages of the developed technique.