The ionization produced by a collimated beam of fast neutrons, filtered for gamma-rays, resulting from neutron collisions in various hydrogenous and non-hydrogenous gases at pressures ranging from 3 mm to 3 atmospheres or resulting from collisions in various hydrogenous and non-hydrogenous wall materials has been measured in ionization chambers of various types. In large wire-defined gas-walled chambers the ionizing particles are the recoiling gas nuclei. The long range protons of hydrogenous gases expend part of their energy in the walls of the container at ordinary pressures; hence their ionization-pressure curves are quasiparabolic, becoming linear at higher pressures in accordance with theoretical predictions. The ionization-pressure curves for non-hydrogenous gases are linear except at low pressures. The limiting pressures at which linearity sets in lead to maximum values of the range and energy of the recoiling nuclei and indicate that in the beam of the 37-inch Berkeley cyclotron 5-Mev neutrons predominate. The slopes of the linear portions of the $i-p$ curves permit the calculation of the rate, $E_j$, at which energy is transferred from the neutron beam to nuclear constituents of the gas. The component of the ionization resulting from gamma-rays, produced in the target and in the walls of the collimator and chamber, was found less than 1 percent in hydrogenous gases and only 2-6 percent in other gases. From the rate of energy transfer and the neutron energy flux an approximate mean value of the $n-p$ cross section for neutron energy distribution of the beam was calculated; also the lower limits of similar cross sections for other nuclei have been estimated, these values containing both the scattering cross sections and those due to neutron absorption followed by disintegrations. In thimble chambers with 1-cm and in cylindrical chambers with 2-mm wall separations, the ionization results in large part from the recoiling wall nuclei. Ranges and energies of the heavier recoiling wall nuclei are indicated by the limiting pressures revealed in $i-p$ curves. The gamma-ray percentage is greater than in large volumed chambers. Neutron responses from most non-hydrogenous walls are practically independent of wall material. The excess response from hydrogenous walls is proportional to proton content. From the $E_j$ values measured in large chambers the energies absorbed per g of various biological substances and of hydrogenous and non-hydrogenous wall materials have been calculated, and (1) predict the respective responses measured in thimble chambers and (2) indicate that the energy absorption per g of tissue must be similar to that for a wall material like amber. Finally, analysis of the relative x-ray and neutron energy absorptions in such materials yields a factor $k_n>2\mathrm{}$, which must be applied to reduce neutron exposures measured in certain hydrogenous-walled chambers to tissue doses.

• Received 12 August 1945

DOI:https://doi.org/10.1103/PhysRev.69.1