A Self-Checking Radiation Monitor

A self-checking system of circuits is expected to provide a new high in operational reliability for the set of radiation monitors described. Developed under sponsorship of the Bureau of Ships, these monitors can readily be adapted for application in any nuclear installation where radiation monitoring must be performed. Three basic channels are provided to measure accurately the gamma radiation in the range from 1 to 1000 mr/hr, the thermal neutron flux from 20 to 20,000 thermal neutrons/cm
²
sec, and the air-particle concentration from 10
^-9
to 10
^-6
, μC/cm
³
or from 10
^-8
to 10
^-5
μC/cm
³
. Checking of each channel is performed by periodically (and automatically) placing a small radioactive source next to the detecting element in each channel. This results in a signal near the maximum range of each channel. During such checks, the alarm-indicating circuit is disconnected and the channel output applied to a comparator circuit; there will result a "circuit fault" indication if any part of the signal-handling or the checking circuits has failed. The philosophy and the limitations of the checking scheme are discussed. The calibration of the air-particle channel is in terms of a specific isotope. Calibrations for other isotopes can be prepared. Shielding is applied to the detecting element in the air-particle channel in order to minimize counts due to the ambient gamma background. The gamma detector has shielding to equalize its response to radiation at energies between 80 kev and 2 mev.