Determining of the optimized dimensions of the Marinelli beaker containing source with inhomogeneous emission rate by using genetic algorithm coupled with MCNP and determining distribution type by neural networks.

In order to determine the activity of C137s in soil resulting from nuclear accidents or fallouts, the best choice is to use HPGe detectors due to their best energy resolutions. In this regard, in order to enhance the detection efficiency, the Marinelli beakers have been used to increase the radiation interaction with the sensitive volume of the detector. In previous works, to optimize the dimension of Marinelli beakers, the assumption was that the emission rate of the source is homogeneous in beaker volume. In the present study, to investigate the effect of the inhomogeneous emission rate of the source on the optimum dimensions of the beaker, in a simple case, the beaker was divided into two sections (upper and lower) containing different C137s emission rates. A code based on MCNPX2.6 coupled with genetic algorithm was developed and used to investigate the effects of different emission rates of C137s at the upper and lower section of the beaker on optimum dimensions of the Marinelli with different volumes (300-1500 cm3). The results showed that the inhomogeneity in the emission rate greatly affects the optimal dimensions of the beaker. By using the C137s spectra simulated for a conventional beaker at different emission probabilities in the upper and lower section in training a neural network, the emission probabilities of a beaker containing C137s was estimated.

Optimization of the marinelli beaker dimensions using genetic algorithm.

A computational code, based on the genetic algorithm and MCNPX version 2.6 code was developed and used to investigate the effects of some important parameters of HPGe detector (such as Al cap thickness, dead-layer thickness and Ge hole size) on optimum dimensions of marinelli beaker. In addition, the effects of detector material on optimal beaker dimensions were also investigated. Finally, the optimized beaker dimensions at various beaker volumes (300, 500, 700, 1000 and 1500 cm3) were determined for some conventional Ge detectors with different crystal sizes (16 sizes). These sets of data then were used to drive mathematical formulas (obtained by best fitting to data sets). The results showed that, there is no meaningful correlation between the optimum dimensions of the beaker and each of the dead-layer thickness, Al cap thickness and the Ge-crystal hole size. On the other hand, the optimum beaker radius increases with decreasing the density of the detector material while the beaker height decreases.