Area Radiation Monitor System with Logarithmic Indication and Audio-Visual Warning

For five years a neutron detection system (utilizing BF3 detectors with moderators and vacuum tube amplifiers and count-rate circuits) has been in useful service at the Los Alamos large Van de Graaff accelerator. During this time difficulties with the need for increasing counting rate capability as well as component failures have led to a desire for improvement. In addition, the detectors were sensitive to thermal neutrons to an extent which made it difficult to get a legitimate measure of biological danger with various neutron spectra. An improved system was developed to give a more believable measure of biological danger (referred to "first collision tissue damage" described in NBS Handbook 633). The system was to be made more reliable through the minimization of vacuum tubes and relays. The problem of indicating varying levels of neutron flux without changing ranges was solved by a logarithmic count rate meter with a range from 1-5000 mrem/hour. The indicating system was to be expanded to give rapid visual information at the control room and other locations as well as audio alarms in the experimental area. By transistorizing all the electronic circuits and using only low voltages, both reliability and safety is improved. A gamma system, with a range from .01 to 10 r/hour, utilizing ionization chambers and an electrometer tube, is an integral part of the radiation system. It energizes the same alarm system as the neutron detectors. The system block diagram is shown in Fig. 1.