Estimating gamma fields with an ion chamber operating below saturation

Electronics Division has developed ion chambers with a sensitive volume of 650 cm3 filled with argon gas at 45 psi pressure, an all-welded stainless steel outer envelope and high purity alumina-metal insulators. With a current sensitivity of 0.25 nA r-1 h-1, the chambers can measure gamma exposure dose rates ranging from 10 mr h-1 to 250 r h-1 with an operating voltage of 500 V. Ion chambers of this type have been used earlier to detect failed fuel elements in reactors. Similar chambers have also been used in the wide range gamma monitoring applications and criticality monitoring systems for nuclear fuel storage areas. Tests have been carried out on one of the chambers to study its performance in high gamma fields up to 3000 r h-1. Theoretical calculation of the current sensitivity agrees well with the measured value. The measured values of the chamber current agree well with the theoretically expected values at various applied voltages. By applying 300 V and measuring the current, the chamber can be used to measure unknown exposure rates even under poor saturation conditions with good accuracy