Radiation Hazard Detectors

Basically you want to put a "hold" on the signal to output a warning and maintain the output. Once the system is reset it will go about its scanning. Everything depends on what the background radiation may be / compared to what threshold you want to meet. The system we use is "Drive Through". Once a trailer goes through the portal and trips the alarm it senses and records how high the object is and how far from the front and rear of the trailer. It can also detect the intensity of the radiation as to whether protective equipment is required. The system is designed so that drivers who have had radioactive medical testing within the last 48 hours will set an alarm off. These drivers are not allowed to pass through the portal to avoid false alarms.
Many companies can sell you amplifiers that can fit the range and sensitivity your procedure calls for. It all depends on how sensitive and what range you wish to record. It also should be considered as to where and how the detection is taking place. Contact any good company and they can make recommendations for their chips and boards. Once the amplification is chosen, the treshold can then be adjusted for when the alarm should trip. Not sure if this helps much, but again it depends on a lot of factors.

OOps just saw you were looking for RF detector. Is this for transmission towers, radar or what? Whole new ballgame! I'd still contact a company that makes the chips and boards to assist with setup. Many are already on the market, unless this is what you are looking to manufacture.