A Gamma-Ray Perimeter Alarm System

A prototype perimeter alarm system using a beam of gamma rays from 137Cs is extremely sensitive to interruptions of the beam. Monte Carlo calculations indicate that a 1-Ci source is adequate to protect an interval of 93 m. A gamma-ray source can easily be made bidirectional, which would allow about 200 m of perimeter to be guarded. A system using a gamma ray having an energy in the range of 500-1000 keV would result in a minimum number of false alarms per year.