A primary standard for the absorbed dose rate to water in a ⁶⁰Co radiation field has been newly established at the National Metrology Institute of Japan. This primary standard combines the calorimetric measurements using a graphite calorimeter with the ionometric measurements using a thick-walled graphite cavity ionisation chamber. The calorimeter is operated in the constant temperature mode using AC Wheatstone bridges. The absorbed dose rate to water was determined to be 12 mGy s⁻¹ at a point of 1 m from the radiation source and at a water depth of 5 g cm⁻². The uncertainty on the calibration coefficient in terms of the absorbed dose to water of an ionisation chamber using this standard was estimated to be 0.39 % (k=1).

Measurements Concerning the collection efficiency were made for a parallel-plate ionization chamber which has a variable space between the polarizing electrode and collector. To do so. the chamber was exposed to -CO y rays at several different exposure rates and the value of m-which is equal towhere OL is the recombination coefficient, e is the charge per ion, and K, and K . are the mobilities of positive and negative ions respectively-was deduced from the collection efficiencies which were determined at various applied voltages. It was found that m depends upon the lifetime of ions in the chamber (i.e. it increases from 17.1 to 18.5 MVA-1/2m-'/' with a decrease in the ratio of the voltage to the square of the chamber space from 600 to 20 kVm-*).

Collection efficiencies for ion loss due to initial recombination and back-diffusion were measured for several humidities using a parallel-plate cavity ionization chamber irradiated with 60Co gamma-rays. It was shown, from measurements in a range of inverse electric field strengths from 0.05 to 14 mm V(-1), that initial recombination took place both in clusters and columns of ions produced along the path of the secondary electrons ejected by the y-rays. The ion loss due to recombination in clusters was found to increase with humidity, but that in columns did not. Effects of ion clustering reactions on recombination may be reduced after longer periods of ion drift, when recombination in columns takes place. Ion loss due to back-diffusion was also found to have no dependence on humidity.

A method for calculating mobilities in gas mixtures is described. The method is based upon an assumption that the momentum transfer cross sections of ions in a gas mixture and those in pure component gases are the same if mean relative velocities of ions and gas atoms/molecules are equal. Deviations from Blanc's law of the calculated reciprocal mobilities agree well with experimental values for K+ ions in mixtures of He+Ne, Ne+Ar and H2+N2 and with those for Li+ ions in mixtures of H2+N2.

The signal charge from a free air ionisation chamber for the measurement of air kerma and exposure consists of not only the charge of ion pairs produced by secondary electrons (i.e. photoelectrons, Compton electrons and Auger electrons), but also the charge of the secondary electrons and single and multiple charged ions formed by the release of the secondary electrons. In the present work, correction factors for air kerma and exposure for the charge of the secondary electrons and ions were calculated for photons with energies in the range from 1 to 400 keV. The effects of an increase in the W value of air for low-energy electrons were also taken into consideration. It was found that the correction factors for air kerma and exposure have a maximum value near a photon energy of 30 keV; in the lower energy region, the correction factor for exposure monotonically decreases with a decrease in photon energy except for a small dip due to K-edge absorption by argon atoms in air. The values of the correction factors were found to be 0.9951 and 0.9892, respectively, for a spectrum with a mean energy of 7.5 keV, the reference X-ray spectrum with the lowest mean energy in ISO 4037-1. The air kerma correction is smaller than that for exposure, because for air kerma the signal due to the charge of secondary electrons and ions is partly compensated by the decrease in the number of ion pairs produced by the secondary electrons due to the increase of the W value of air for lower energy electrons.