The NPL air kerma primary standard free air chamber for medium energy X-rays: summary of factors

The National Physical Laboratory (NPL) air kerma primary standard for x-rays generated at kilovoltages from 40 kV up to 300 kV is a parallel plate, guarded field free-air ionisation chamber that covers the range of x-ray qualities from approximately 1.0 mm Al HVL to 20.0 mm Al HVL for therapy level qualities and from 0.085 mm Cu to 6.2 mm Cu for protection level qualities. The usable air kerma rate range is from about 0.2 mGy min-1 to 90 mGy min-1. This standard has been in use at NPL as the national primary standard since the spring of 1998. This report summarises the factors applicable to the chamber response in order to realise air kerma and calibrate a secondary standard. The associated uncertainties are also presented.