Improvements in the determination of radionuclide distribution in the analysis of irradiated samples using double-differentiation method

The spatial response of an 8x4 block detector made up of 5.6-mm-wide, 12.9-mm-high, 30-mm-thick individual detector crystals to a collimated line source of 511 keV annihilation photons was examined. The response of each crystal showed a spread around the average positioning values and distributions from adjacent crystals overlapped as the collimated source scanned the individual detectors. This leads to possible errors in the event assignment. The implementation of double differentiation or the second derivative method was proposed for the removal of scattered photons so as to reduce the overlap and, hence, avoid mispositioning. This method is a mathematical solution implemented when analysing the results. A curve in a spatial spectrum could be considered to be a function f(x), wherex is the position. When double differentiation of f(x) is carried out, then the normalized curve d²f(x) appears with some reduction in the wings. It was shown that a reduction of the scattering contribution in the tails without overestimating the contribution of scattered events could be achieved by implementing a double-differentiation process.