Comparison of measured and computed values for the ionization in various gases irradiated with 15 MeV neutrons

Ionization measurements are reported for 15 MeV neutrons obtained with a parallel plate ionization chamber employing different wall materials. The chamber consisted of a graphite back electrode and interchangeable front electrode. It was irradiated with neutrons incident normally upon the front plate. Since recoils from the chamber wall are produced in a forward direction, the influence of the graphite back plate was very small. Hence by changing the front electrode and filling gas, measurements were made for chambers of different atomic composition. Measured values of the ionization in different gases relative to that in acetylene show agreement to better than 5% with the calculations of Dennis, providing confidence both in relative values of cross sections and the W values used as input data in the calculations. Relative values of kerma derived from the experimental measurements were also in good agreement with the earlier calculations of Bach and Caswell. Values of neutron flux density derived from the ionization measurements were within about 10% of the value measured independently with a calibrated 238U fission chamber. This satisfactory agreement between ionization and flux density measurements increases confidence in the methods employed for neutron dosimetry