Determination of absorbed dose and kerma in a neutron field from measurements with a tissue-equivalent ionisation chamber

Equations are derived for the determination of absorbed dose and kerma free-in-air from measurements with a tissue-equivalent ionisation chamber in a neutron field. Three methods of calibration of the chamber are considered; an exposure calibration in air in a photon beam, an absorbed-dose calibration in a phantom in a photon beam, and an absorbed-dose calibration using a calorimeter in a neutron beam. An uncertainty analysis of the results obtained using the different methods of calibration is made and it is concluded that the calorimetric method gives the greatest overall accuracy, the major uncertainty in this technique being due to uncertainties in the kerma data used to convert absorbed dose in the wall material to absorbed dose in tissue. A detailed discussion is given of the correction factors that must be applied to the chamber reading to account for its finite size during calibration and measurements in air as well as in a phantom.