To open this Application Note, click here

Time is of the essence when dealing with radioactive material releases or incidents. The use of portable portal monitors, like CANBERRA’s MiniSentry, allows users to monitor a large amount of people in a very short amount of time. This process allows the users to make fast and accurate decisions based on the amount of gamma radioactivity that is present on the individual being monitored.

Prior to 9/11, portal monitors were used by state and local authorities to monitor individuals in and around nuclear power plants in case of an accident. These portal monitors were typically setup at a local school gymnasium or similar large location to monitor the general public for radioactivity. All of these protocols were written into the NRC-approved Emergency Response Plan. Post 9/11, we now have the possibility of intentional misuse and hazardous release of radioactive materials in an act of terrorism. Either way, the original and new threats are still present and require monitoring. In 1995, FEMA released REP-21, “Contamination Monitoring Standard for a Portal Monitor Used For Radiological Emergency Response”. This document was specifically created by FEMA in response to decontamination decision criteria for individuals when using portal monitors for radiological emergency response to commercial nuclear power plant accidents. Up until the early 1990s, people were checked for contamination by hand-held instruments that took too long to use and were potentially unreliable.

Why screen for gamma radiation? As seen from the figure below, Gamma is the most penetrating form of ionizing radiation that is released from “radioactive contamination deposited on people’s clothing”. Alpha Contamination can travel approximately two inches in air and can be stopped by a piece of paper. Beta Contamination can travel several hundred feet in the air, and can be stopped by most protective clothing. With Gamma Contamination being the most “penetrating” of the three, it is of the utmost importance to initially screen for the worst. The object of mass screening by using a portable portal monitor is not to find all contamination but only that which presents a hazard. Fortunately for the detection systems, there are only a select few radioisotopes that decay with no gamma-ray emission accompanying the transition. And as a result, the detection of gamma-rays is the best first emission to detect. Upon request, CANBERRA can provide some “source term” calculations showing nuclear power plant release fractions, and the resulting gamma-ray emissions that accompany the contamination.