Development of a secondary standard ionization chamber for environmental level ambient dose equivalent measurement

Ambient dose equivalent is the calibration quantity and type inspection quantity of environmental and workplace radiation monitoring instrument. For other unknown radiation fields other than the reference radiation field, the method of directly measuring H*(10) by secondary standard is particularly important. In order to ensure the reliability and accuracy of the value transfer of H*(10), an energy compensated high pressure ionization chamber for environmental level measurement was developed and used as a secondary standard. Firstly, the conversion coefficients from air kerma to ambient dose equivalent of X and γ-ray reference radiation field were calculated by Monte Carlo simulation. Then, the Monte Carlo model of ionization chamber was established, the thickness and energy compensation scheme of the outer wall of the chamber high voltage electrode was determined. Finally, the performance of the chamber were studied according to the calculated conversion coefficients. The results indicate that the relative deviation between calculated conversion coefficients and recommended value of ISO 4037-3 in the range of 15 keV-1.5 MeV is within ±4%. The ionization chamber meets the energy response requirements of ISO 4037-2 in the range of 65 keV-1.25 MeV. The linearity range of ambient dose equivalent rate is 0.8 μSv/h∼11.9 mSv/h, which can be used for a directly measurement of environmental level H*(10).