Nuclear-reactor-control ionization chambers

The paper describes the design and the electrical characteristics of a neutron-sensitive d.c. ionization chamber suitable for a wide range of applications in nuclear-reactor-control instrumentation systems. Thermal-neutron sensitivities of 10¿15¿10¿14amp/n/cm2/sec can be obtained using boron trifluoride filling or boron-coated electrodes with hydrogen filling. The corresponding ¿-radiation sensitivities are of the order of 10¿12¿10¿11amp/r/h. Virtually complete collection of the ionization current is possible, up to neutron flux levels of 1011 n/cm2/sec, with a polarizing voltage of a few hundred volts. The residual current due to neutron-induced activities in the chamber falls to less than 10¿6 times the full-power neutron current some 20 min after shut-down. The siting of ionization chambers has an important bearing on their range of utilization in control-instrumentation systems, and this is discussed briefly in relation to the electrical characteristics of the chambers described.