Background correction method for portable thyroid dose monitor using gamma-ray spectrometer developed at JAEA in high dose rate environment

Abstract A portable thyroid dose monitoring system has been developed at the Japan Atomic Energy Agency (JAEA) to assess the thyroid equivalent dose for workers and members of the public in a high dose rate environment. The background (B.G.) photon correction is required for an accurate measurement in a high dose rate environment at an early stage after a nuclear accident. The responses of the system to the B.G. photons were calculated with the simplified cylindrical phantoms made of PMMA or the voxel phantoms using the PHITS code. Then they were compared each other among the respective age groups. It was found that the cylindrical phantom with an appropriate size and properly arrangement can mimic the human body in the high dose rate caused by 131I sources. As such we successfully developed the B.G. photon correction method using the cylindrical phantoms; one of those is used as an effective way to calibrate the monitoring system by inserting a133Ba point source into it. As an additional phantom is not required, this method has a great advantage in the practical operation of the B.G. photon correction.