Design and Calibration of a Chair-Type Whole Body Counter

A whole body counter is used for in vivo measurement of body radioactivity to access the internal exposure of radiation workers, as well as, the internal exposure of the public. The chair-type WBC is more suitable than the other geometry for rapid monitoring in radiation accidents. In this study, a transportable in vivo monitoring system with the chair-type geometry has been designed, and evaluation of the counting efficiency has been performed to calibrate the system effectively. The counter incorporates Nal (Tl) detectors (5cm dia.×5cm), housed in a collimator, and the chair is shielded using 2cm thick lead layers. For calculating the seat angle of the chair and collimator angle, a Bottle Mannkin Absorber (BOMAB) phantom model has been used. The counting efficiencies have been obtained by using Monte Carlo simulation and from the conventional calibration experiment. For the calibration experiment, a torso phantom containing a point source is used. The minimum detectable activity (MDA) for 137Cs for 10min counting interval is 448±47 Bq. The results indicate that the calibration of the whole body counter by the MCNPX code can be substituted for the actual calibration. It is shown that, the Monte Carlo simulation is an inexpensive alternative method to obtain the counting efficiency of the chair type WBC for design and accurate calibration purposes.