Monte Carlo Simulation Study of Hot-Particle Detection in Voluminous Samples by Gamma Spectrometry

In this work, we addressed the inhomogeneity problem in gamma spectrometry caused by hot particles, which are dispersed into environment from large nuclear reactor accidents such as at Chernobyl and Fukushima. Using Monte Carlo simulation, we have determined the response of a gamma spectrometer to individual and grouped hot particles randomly distributed in a soil matrix of 1-L and 0.6-L sample containers. By exploring the fact that the peak-to-total ratio of efficiencies in gamma spectrometry is an empirical parameter, we derived and verified a power-law relationship between the peak efficiency and peak-to-total ratio. This enabled creation of a novel calibration model which was demonstrated to reduce the bias range and bias standard deviation, caused by measuring hot particles, by several times, as compared with the homogeneous calibration. The new model is independent of the number, location, and distribution of hot particles in the samples. In this work, we demonstrated successful performance of the model for a single-peak 137Cs radionuclide. An extension to multi-peak radionuclide was also derived.

Positive Effect of Noises on Sensory Systems

Stochastic resonance phenomenon in the biological sensory systems has been studied through the signal detection theories and the psychophysical experiments. In this paper, sensory systems are considered as a threshold detector including the receiver part and the classifier part. Compared with conventional models regarding the receiver part of sensory system as a linear or single non-linear system, a summing network was constructed by MacCulloch-Pitts neurons to simulate the receiver part. The simulation results show that the relevant index of the detectability of signal exhibit the stochastic resonance behaviours. The psychophysical experiments were carried out through the 2IFC (two interval two alternative forced choice) method. The experimental results qualitatively verify the conclusion in accordance with the theoretical model. These works give a proof that stochastic resonance is not only epiphenonmenon in sensory systems.

Gamma Spectrometry of Inhomogeneous Samples Using Peak-Ratio Method

In gamma spectrometry of voluminous samples, inhomogeneous distribution of radioactivity caused by the presence of hot particles can create significant Bias in the results of activity determinations. We developed a novel method to reduce this Bias using the gamma-peak ratio. We show that the peak area ratio of two gamma peaks of different energies, emitted by the same radionuclide, is a sensitive measure of emitting source location and thus the inhomogeneity. A new calibration formula was then derived for true gamma efficiency pi as a function of efficiency ratio pi/pj of two peaks. This approach was verified by Monte Carlo simulations for a sample of 1-L volume containing from 1 up to 2048 of hot particles randomly distributed in a soil matrix. A 152Eu radionuclide was selected for calculations and we used various combinations of two gamma spectral peaks selected from three gamma energies of 121.8, 344.3, and 1408.0 keV. This new method is shown to reduce the Bias range and Bias standard deviation by several times when compared with the traditional homogeneous calibration applied to measuring hot particles. The method is independent of the number, location, and distribution of hot particles in the samples, and can be applied to a mixture of radionuclides. It complements our previous calibration model based on the peak-to-total ratio.

How can survival processing improve memory encoding?

We investigated the psychological mechanism of survival processing advantage from the perspective of false memory in two experiments.Using a DRM paradigm in combination with analysis based on signal detection theory,we were able to separately examine participants’utilization of verbatim representation and gist representation.Specifically,in Experiment 1,participants rated semantically related words in a survival scenario for a survival condition but rated pleasantness of words in the same DRM lists for a non-survival control condition.The results showed that participants demonstrated more gist processing in the survival condition than in the pleasantness condition;however,the degree of item-specific processing in the two encoding conditions did not significantly differ.In Experiment 2,the control task was changed to a category rating task,in which participants were asked to make category ratings of words in the category lists.We found that the survival condition involved more item-specific processing than did the category condition,but we found no significant difference between the two encoding conditions at the level of gist processing.Overall,our study demonstrates that survival processing can simultaneously promote gist and item-specific representations.When the control tasks only promoted either item-specific representation or gist representation,memory advantages of survival processing occurred.