The coefficient of detector efficiency is an essential while we use lung counting system to assess the internal exposure dose. This paper used an anthropomorphic phantom which was made accordance with Chinese Reference Man to calibrate our system. We cooperated with China Institute for Radiation Protection to manufacture the phantom which was made of tissue equivalent materials. In the lung there are 246 club-shaped uranium sources. Each source includes (20.12±0.32) Bq 238U and (0.53±0.01) Bq 235U. There are 4 parts in this paper. We start with a brief introduction to our lung counting system. Then the method of how to manufacture the lung and the result of the parameters such as density, linear absorption coefficient of the lung are provided in detail. The efficiency curve under different Chest Wall Thickness (CWT) is obtained. In our application, about 22 workers have been measured finally. We find in our research that the Minimum Detectable Activity (MDA) of natural uranium changes from 2.5mg to 5.9mg as the CWT varies from 16.2mm to 40.5mm for our lung counting system in 4000s detecting time by analyzing 63.3keV photo-electricity peak of gamma ray. Based on the least limited value of annual inhalation is about 220mg while the lung slowly absorbed uranium aerosol with particles about 1 μm, we may conclude that our lung counting system can be well used to detect the activity of natural uranium in a short time after a worker inhaled uranium aerosol.

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