Wanno Lee

A radiation monitoring system with capability of gamma imaging and estimation of exposure dose rate

A new radiation monitoring system, which has the capability of both gamma imaging and estimation of real-time exposure dose rate, was developed by an integration of a visual image and a dose image. It is composed of a portable gamma imaging system for visualizing gamma-source distribution and a dose conversion unit (DCU) for real-time dose conversion from gamma spectrum. Using Monte Carlo simulation, gamma-ray spectra for energies from 10 keV to 5 MeV with energy intervals of 10 keV were calculated for a cylindrical NaI(Tl) detector used in our developed system. These calculated spectra were compared with the measured spectra for several monoenergetic gamma rays using a multichannel analyzer. Based on the simulation data and the dose conversion coefficients obtained from ICRU 47, the dose conversion factors were also calculated by matrix calculation. The feasibility of the DCU using the dose conversion factors was evaluated by measurement of exposure dose rates as a function of distance between the gamma ray and detector for several standard sources. The exposure dose rates measured by the DCU are within an 11.5% deviation with analytical values based on gamma factor. They are also similar to the dose rates measured by a high-pressure ionization chamber with maximum difference of six. We finally visualized two-dimensional distributions of gamma source and measured the exposure dose rate simultaneously.

A study on the sensitivity of self-powered neutron detectors (SPNDs)

Self-Powered Neutron Detectors (SPND) are widely used in reactors to monitor neutron flux. While they have several advantages such as small size, and relatively simple electronics required in conjunction with those usage, they have some intrinsic problems of the low level of output current, a slow response time, and the rapid change of sensitivity which makes it difficult to use for a long term. Monte Carlo simulation was used to calculate the escape probability as a function of the birth position of emitted beta particle for geometry of rhodium-based SPNDs. A simple numerical method calculated the initial generation rate of beta particles and the change of generation rate due to rhodium burn-up. Using the simulation result, the burn-up profile of rhodium number density and the neutron sensitivity were calculated as a function of burn-up time in reactors. In addition, for improvement of some properties of rhodium-based SPNDs which are currently used, a new material and modified geometry were proposed in this work. Ag/sup 109/ was chosen as a replacing material for rhodium. Also, this work compared the initial sensitivity of a solid type with its of a tube type.

Step filter detector module for monitoring of diagnostic X-ray

Using Monte Carlo simulation and semi-empirical model, we designed new detector module for monitoring of diagnostic X-ray. The new detector module consists of the filters of various thickness material, X-ray converters, photo-sensors, and data treatment system. In this paper we performed computer simulation and proved that it is possible to use new detector module for monitoring diagnostic X-ray spectrum using simple method. The semi-empirical model is used to design filter geometry as a reference criteria. The Monte Carlo simulation was performed to decide filter, X-ray converter, and compensation procedure of lost data. The new detector module is available for reappearing X-ray spectrum with designed parameters and algorithm. The proposed simple method, which calculate the diagnostic X-ray spectrum by Monte Carlo and semi-empirical model, is evaluated by the comparison of the X-ray spectra by other data and the spectra by simulation results.