Construção, determinação das características ótimas de funcionamento e aplicação de um detector de eletrons de conversão Mossbauer

Abstract

The use of scattering technique instead of the convencional transmission geometry provides two new resources to Mossbauer Spectroscopy. Firstly, the sample can be studied in its natural form, that is, on does not need either powdred it or reduced it do a thin foil. Secondly, the scattering geometry is the ideal technique for surface analysis. However, the verly low signal to noise ratio in the detection process has limited the use of the scattering technique. In this work, a gás-flow proportional detector for Mossbauer conversion electrons has been designed and constructed in such a way that it was possible to deternine the best values of the various parameters which give the optimum signal to noise ratio and energy resolution. The constructed detector consists of a external cylindrical chamber at ground potential, having 30 mm diameter and a height varying between 3 and 10 mm. The high positive potential was apllied at 3 copper anode of 75 um in dameter, which were equally spaced and parallel to the vase of the cylinder. In order to find out the option operatioon conditions of the detector, the following parameters were investigated as far as their influences in the Mossbauer percentage effect and the counting time are concern: Source-absorber distance, Collimation of the incident gamma radiation , Gas-flow rate, Chamber volume, High positive potential detected energy discrimination. The detector was operated using a premixed helium 6% methane gás. All testing were carried out with 310 stainless steel and natural iron absorbers and a 10 mCi 57Co-Pd source. The best average volues for the investigated parameters which optimized the wording condtions of the detector are: Source absorber distances..........30 mm, Collimator diameter.................10 15 mmChamber height........................3,5 6,0 mm, Gas-flow rate..........................4 ml/min. detected energy discrimation cutting dow the initial region of the puse-heigt spectra. A more refined choice for the values of the above parameters will depend on the characteristics of the absorber, because a trade-off must be made between optimizing the Mossbauer percentage effect, so as to minimize the count rate for a given experiment. Mossbauer spectra of meteorite and low carbon alloy steel samples were taken in order to test the efficiency and versatillity of the constructed detector.