Since the incident air kerma to be measured is generally only slightly higher than background levels, the standards and guidance documents in Table 1 generally recommend, for sensitivity, using a largevolume, free-air (thin-walled) ionization chamber (IC) with a high-quality electrometer with 0.
On the left is the Radcal IC used to measure incident air kerma from x-ray backscatter systems that were installed at NIST for extended study.
Both FACs are proven national-standard instruments deemed suitable for the realization of air kerma for these sources.
The quantities exposure and air kerma can be related through use of the mean energy expended in a gas per ion pair formed, divided by the elementary charge, W/e, where W is the mean energy expended in air per ion pair formed when the initial kinetic energy of a charged particle is completely dissipated in the air, and e is the elementary charge.
The results reveal the degree to which the participating calibration facility can demonstrate proficiency in transferring air kerma calibrations under the conditions of the said facility at the time of the measurements.
The objective of this proposal was to compare the air kerma calibration coefficients of a reference class chamber determined in reference beams of x rays and gamma rays.
primary standards for air kerma (formerly exposure) for x rays generated at potentials in the range from 10 kVp to 300 kVp using a series of free-air ionization chambers.
Primary standards for the air kerma from photon-emitting radionuclides have been developed by the NBS/NIST as well as by other national metrology institutes.
The National Institute of Standards and Technology (NIST), formerly the National Bureau of Standards (NBS), maintains the primary standards for exposure and air kerma for x rays and gamma rays.
For graphite as the wall material and air as the cavity gas, one can then write for the air kerma [K.
The data shown are converted from dose in mrad to air kerma
rate in nGy/h, without correction factors (4) applied.
A direct comparison was made between the air kerma
primary standards used for the measurements of low-energy x rays at the National Institute of Standards and Technology (NIST) and the National Physical Laboratory (NPL).