The shift in RT/sub NDT/ caused by radiation: a study of the marginal distribution

There have now been numerous statistical studies relating the shift in the brittle-ductile transition temperature to the fluence and various chemical elements. In this study, we have been examining the distribution of observed shifts about one of the statistical equations. (The computer codes developed for this work could be applied to any such function.) First, the residuals (differences between observation and prediction) are shown in histogram form, revealing that they are close to the normal (Gaussian) distribution. Next, the functional dependence of the residuals upon features other than those included in the equation were studied using stepwise linear regression techniques. No important dependencies which are not currently considered were revealed. Next, the square of the residuals, each an estimate of variance, was studied in a similar manner. This quantity was found to vary systematically. The variance was shown to relate to the standard error of the shift measurement, and to the maximum capsule temperature recorded in the surveillance reports. Further study showed that the standard error of the shift measurement related in a linear fashion to the amount of radiation shift. This indicates that weighted regression procedures should ideally be used prior to the generation of marginal distributions for probabilistic studies. (author)