Konverterat

An Excel sheet to analyse data from beta-gamma coincidence xenon spectra was already developed several years ago to provide a benchmark for the coding of the automatic analysis routines at the International Data Centre of the CTBTO. At that time a few small covariance terms were taken out, which, however, had some impact on the calculations of variances and thereby detection criteria and detection limits. The fact that 133mXe decays into 133mXe during processing and data acquisition, was also ignored, as in the standard IDC scheme, with no transport times, the "internal decay" does not matter very much. With the Swedish sampling in South Korea in the days after the nuclear test in the Democratic People's Republic of Korea on 9 October 2006 and the fairly long delay between sampling in South Korea and analysis in Stockholm the problem of mass 133 internal decay had to be looked at. As further the Korean samples were of great interest it was reasonable to check that none of the other simplifications were questionable. The results of applying the new algorithms on ten spectra (five originally used for benchmarking and the five Korean samples) are that the simplifications could be well defended for routine surveillance in the IMS network. Disregarding the covariance in the Gas Background subtraction could yield up to some 10% higher detection criteria and disavowing internal mass 133 decay changed the 133Xe results less than 1% for the non-Korean, IMS type samples. For one of the Korean samples, not observing internal decay gave a three times too high 133Xe concentration. The IDC (and the Swedish) analysis scheme included decision points such that when a background nuclide was found below the detection criterion its contribution to the background of other nuclides was fully disregarded. This reduces in principle the variances and improves the apparent detection limits. This improvement was, however, shown to be quite insignificant, and it does not motivate the complexity it introduces in the code. For 133Xe there are several regions of interest contributing to the final result. In a fair estimation of the variance of the weighted average covariance terms should be considered. It can reduce the detection criteria by up to 50%. The report focuses on the effects of mathematical simplifications, but it must be pointed out that uncertainties in calibration data, electronic stability and background variations over time in most cases are essentially more prominent. In a bonus chapter a Mathematica? program is presented for Bateman calculations of the mass 131, 133 and 135 fission product decay chains. It is a handy tool to analyse leakage scenarios and to study the ratio of different xenon isotopes and meta-stable states as a function of time and type of nuclear fuel. The Excel sheet including the ten samples as well as the Mathematica? Bateman program are attached on a CD