Design of radiation shielding for the PBP-CMU electron Linac laboratory

Abstract

At the PBP-CMU Electron Linac Laboratory, we aim to generate three types of coherent radiation: coherent terahertz transition radiation (THz-TR), mid infrared free electron laser (MIR-FEL) and super-radiant terahertz free-electron laser (THz-FEL). The accelerator system can produce the electron beam with maximum energy of up to 25 MeV and a maximum charge of about 80 pC per bunch. During the accelerator operation, when electrons hit beam dump or vacuum chambers, it is possible to generate secondary particles such as bremsstrahlung photons and neutrons, which are the ionizing radiation that harmful for cells of a living body. The radiation shielding is therefore essentially important to prevent the escaping radiations from the accelerator hall, especially at the roof, which has only a very thin gypsum ceiling. In the radiation safety work, the radiation area classification is required to limit access to specific areas in the facility. In our laboratory, there are three types of radiation areas consisting of controlled area, supervised area, and public or uncontrolled area. The ambient dose equivalent is an important quantity, which used for the area classification and compared with the measured value using an radiation area monitor. In this work, the Monte-Carlo simulation program GEANT4 is employed to calculate the radiation fluence to ambient dose equivalent conversion coefficient (F) to construct the annual ambient dose equivalent map. The result of F for photons with energy ranging from 0.01 to 30 MeV was benchmarked with the ICRP report and simulation using the FLUKA program. The method to calculate the ambient dose equivalent rate from pulsed radiation was presented together with the suggestion for simulation set up. Furthermore, the designs of radiation shielding were obtained by using GEANT4 simulation. The study results including the annual ambient dose equivalent map and design of local shielding for the three beam dumps inside the accelerator hall are reported. The main shielding materials consisted of lead, iron, concrete, and polyethylene to absorb both photons (X-ray, gamma) and neutrons. With the local shielding for the three beam dumps, 2-cm iron ceiling and concrete wall are added to limit the radiation dose rate outside the accelerator hall to be below 1 uSv/year. As a result from this study, we purpose the shielding design to control the radiation dose rate to the safety limit as required by International Atomic Energy Agency (IAEA) and Thailand’s government regulation. The construction of the proposed shielding is presently ongoing at our laboratory