Ionisation measurements and the derivation of Wn in acetylene and carbon dioxide irradiated with neutrons.

The use of ionisation chambers for neutron dosimetry requires a knowledge of Wn, the mean energy loss per ion pair. Experimental measurements of the ionisation in acetylene and carbon dioxide have been made for monoenergetic neutron beams of mean energies 0.236, 0.554, 1.12 and 1.72 MeV. The neutron fluences were measured with a De Pangher long counter calibrated at the National Physical Laboratory, and spectra determined by the time-of-flight technique. The measurements are compared with calculations based on the work of Dennis. Ionisation values were computed using a program which calculates kerma and uses different empirical models to describe the energy dependence of the W value of heavy ions. For the agreement to better than +/- 8% was obtained between the measured ionisation values and the calculated values based on these models. Larger discrepancies of up to 25% were observed between the measured and calculated ionisation in carbon dioxide. This is thought to reflect the larger uncertainties in the calculation of kerma in this gas due to resonances in the oxygen cross-section. Representative mean values for Wn/Wbeta of about 1-14 for acetylene and 1.58 for carbon dioxide were derived from the experimental measurements and calculated kerma for the neutron energy range 0.2--2 MeV.