Optimisation of the PGAA instrument at FRM II for low background and 2D measurements

Content Provider	Semantic Scholar
Author	Canella, Lea
Copyright Year	2011
Abstract	At the beginning of 2008, the new Prompt Gamma-ray Activation Analysis (PGAA) facility started operation at the Forschungs-Neutronenquelle Heinz Maier-Leibnitz (FRM II). The main characteristic of this facility is the relatively intense cold neutron beam. This property is due to the special construction of the neutron beam guide; the last 7m are elliptically tapered, which means that the neutrons are focused on the sample. This arrangement allows for a max. neutron flux of 6.07 · 1010 cm−2s−1, which is currently the highest cold neutron flux worldwide. Due to this high flux, the main problem encountered was the beam background, i.e. the radiation background created from irradiation of construction materials. The first part of this work was dedicated to the optimisation of the instrument. The goal achieved was a reduction of the background by a factor of 15. Once the instrument was optimised, measurements were dedicated to special elements like Cd, Sm, Eu, and Gd, that have very good characteristics for this method and to archaeological samples (old greek coins). Another improvement of the instrument was the development of a 2D imaging system. A new setup was installed in order to obtain spacial information about the distribution of elements inside samples. This imaging method was first applied to a small piece of the Allende meteorite with a different setup developed in the frame of the European Project ANCIENT CHARM. This setup was thought for 3D imaging, so the conditions were not optimal for 2D mapping. With this insight a second setup was built later specially dedicated for this application. In particular, the neutron field was reduced to a small spot of about 2x2mm2 and a two stage motor was built in order to allow the movement of the sample in two dimensions. Moreover, the possibility to evacuate the sample chamber was added. With this second setup the measurements on the Allende meteorite were repeated for a comparison. The 2D-setup was also applied in the study of the new alloy U-Mo for nuclear research. Through a sputtering process, thin foils were produced from the target material. The goal of these measurements was to determine the distribution of U and Mo in these foils, and the local element ratio, in order to check the homogeneity. Nearby the foils, measurements on thin slices of the target material were also carried out in order to check eventual changes in the elemental distribution due to the sputtering process.
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Alternate Webpage(s)	http://mediatum.ub.tum.de/doc/1004594/1004594.pdf
Language	English
Access Restriction	Open
Content Type	Text
Resource Type	Article