Tetsuya Oishi

Determination of Gamma-ray Efficiency Curves for Volume Samples by the Combination of Monte Carlo Simulations and Point Source Calibration

In general, radioactivity measurements for volume samples are performed by the γ-ray spectrometry method with germanium detectors. In the method, peak efficiency curves must be obtained for each kind of sample in advance. For the reason, usually, many standard volume sources have been made which have different shapes, densities, matrix compositions and so on. This paper describes about a simple method for determination of γ-ray efficiency curves by the combination of Monte Carlo simulations and a single-point calibration using a standard point source. This method enables us to determine the efficiency curves for various types of samples and detectors easily and precisely.

Simulation of the Background for Gamma Detection System in the Indoor Environments of Concrete Buildings

The indoor background has significance in determining the minimum detectable level of gamma detection systems. The gamma radiation field in a concrete building was studied by using the Monte Carlo simulations. The parameters of wall thickness, room shape and dimensions were considered in the modeling. A spherical layer model was assumed as the indoor source geometry with the intention of easy and effective calculations. The simulation model was applied to an unshielded germanium detector and the detection system with a more complex shielding configuration. As the results, the indoor radiation field in the concrete building can be predicted well by assuming the source geometry of a spherical concrete layer with a real thickness and the homogeneously distributed source of the natural major three components, the 238U series, 232Th series and 40K. The simulation model is useful for designing and optimizing gamma detection systems or shielding assemblies.

A calibration technique for gas-flow ionization chambers with short half-lived rare gases

Abstract A calibration technique for gas-flow ionization chambers was studied for implementation of reliable radioactive gas monitoring. Three radioactive gases with short half-lives of 133 Xe, 135 Xe and 41 Ar were prepared by activating stable isotopes and used for the calibration. On the basis of activity determination by the DLPC method, a gas-flow ionization chamber used as a secondary standard was precisely calibrated in terms of ionization efficiency for each radionuclide. The influence of impurities in the 133 Xe gas on calibration of gas monitoring instruments is also discussed. This technique is considered to make the easy and reliable calibration of gas monitoring instruments possible.

Design of an anti-Compton spectrometer for low-level radioactive wastes using Monte Carlo techniques

An anti-Compton spectrometer with semi-2π Compton suppression is designed to identify the photons emitted from low-level radioactive wastes from radioisotope usage and nuclear research laboratory. Since the objective sample is massive and large, the system has a full opening toward the sample position. The characteristics and features of the system concerning Compton suppression and reduction of the background component due to natural radioactive source are estimated by the Monte Carlo simulations. The anti-Compton technique is shown to be quite advantageous for the reduction of the surrounding natural background radiation, as well as the suppression of the background for the higher energy photons.

An EGS4 user code developed for design and optimization of gamma-ray detection systems

An EGS4 user code is developed to design and optimize gamma-ray detection systems for various types of radiation sources. The code is fundamentally based on the PRESTA-CG, which is the user code introducing the PRESTA algorithm and a combinatorial geometry method. The additional and existing functions are integrated in the present code, and the handling is simplified. The latest techniques related to the calculation of material cross section are also incorporated in the code for the accurate simulation. The main additional functions are classified into two parts of the definition of radiation sources and the photon tracing. The former includes the functions on the simple handling of source geometry and the use of redefined radiation source. In the latter functions, it is possible to estimate the simultaneous events among plural detectors and the trace of photon in the interested regions. The developed user code is applied to detection systems in order to demonstrate its availability. As the result, it is found that the present code allows the detailed response analysis of complicated detection systems for various radiation sources with a simple handling.

Determination of Detection Efficiency Curve for a Gas Monitor with a Built-in Germanium Detector

Calibration techniques for a gas monitor with a built-in germanium detector were studied. The peak-detection efficiencies were precisely determined with some calibration gases. In order to obtain more reliable efficiency curves from the limited data of calibration gases, an interpolation technique using a multi-γ point source was applied. And also, the feasibility of simple calibration with the point source located at an appropriate single position was discussed. As a result, the efficiencies obtained with the calibration gases show good agreement with the efficiency curves derived by the interpolation technique. The efficiencies also agreed with efficiency curves determined by the simple calibration within an error of 4%.

Development of a Plutonium Air Monitor for Emergency Environmental Monitoring.

We have developed an air monitor which is possible to measure rapidly and sensitively the concentration of plutonium for the environmental monitoring at an accident of a nuclear reprocessing plant. The monitor is designed to collect airborne plutonium by drawing the ambient air through a filter and to detect the activity by alpha spectroscopy. The following two methods are equipped with the monitor: continuous measuremeat at atmospheric pressure and batch measurement at vacuum. The description of the air monitor and the results of performance test are reported.

A Simple Method Evaluating Collection Performance of Air Filters with Imaging Plates.

Collection performance such as collection efficiency and surface collection efficiency is considerably important in order to select a suitable filter for the measurement of the concentrations of airborne radioactive particles. A simple method with imaging plates is proposed to evaluate the collection performance of air filters. By comparing the collection performance of some filters with natural airborne radioactivity as a test aerosol, it was confirmed that the method could reliably evaluate the collection performance of filters.

Uniformity of Beta-ray Plane Sources for the Calibration of Surface Contamination Monitors.

Uniformity of different types of beta-ray plane sources has been examined. Four types of plane sources, polymer, anodized aluminum, filter and ion exchange membrane, were selected. On estimation of the uniformity, the coefficient of variation with respect to surface emission rate was measured using an imaging plate or a proportional counter. As a result, polymer sources and ion exchange membrane sources showed good uniformity. The area and number of region of interest for estimation of uniformity were discussed.

Response analysis of radioactive gas monitors for short duration inflow of radioactive gas

For accurate evaluation of gaseous activity released from nuclear facilities, proper calibration of radioactive gas monitors is required. Most of gas monitors are calibrated with a closed loop method. In this method, it is implicitly assumed that radioactive gases are statically released without rapid change of radioactivity concentration. However, actual radioactive gases are sometimes released in a short time-duration and with sudden change of radioactivity concentration. To confirm the applicability of the conventional closed loop method to such a kind of gas release, the responses of three typical gas monitors were analyzed in the manner of a pulse-like injection of a radioactive gas. The gaseous behavior in the sampling chamber of the monitors was evaluated from detailed observation of response curve. The gaseous activity injected pulse-like into the monitors was compared with that calculated from their response curves with the calibration factor determined by the closed loop method. These results show that the activity concentration in the sampling chamber becomes uniform as soon as the radioactive gas flows into the monitor. That is, the calibration factors determined by the closed loop method are applicable to both static and dynamic release.