Key Techniques Of High Sensitive Gamma Camera Over A Long Distance

This paper introduces the needs of gamma camera in the nuclear accidents and the basic principles of gamma camera. According to the nuclear accidents, the author has done a lot of research work on scintillating arrays, collecting data by multi-anode photomultiplier tubes and the data processing algorithm, pulse screening, and counting statistical analysis. What is more, we design a gamma camera which is appropriate for the special needs of the nuclear accidents, and analyze the camera application prospect.Many Literature demonstrated that most of nuclear accidents were caused by Sudden events, such as earthquake, fire and mechanical failure. Due to the urgency of emergency action, the gamma camera should have following features:high sensitivity, short detecting time, small volume, and light weight. In order to improve the sensitivity and reduce the detecting time, using the method of combining the alarm criterion with the gamma camera, the author design the gamma camera based on the multi-anode photoelectric multiplier tube. The camera can detect radioactive pollution sources which were close to and even lower than the environmental background radiation.The methods of improving the sensitivity are:shielding measures to reduce the impact of environmental gamma-ray; increasing the aperture; signal screening technology to improve the signal to noise ratio; analyze the read-out counts based on the probability of the statistical distribution of the environmental gamma-ray background counting statistics. Due to the urgency of the nuclear emergency action, the gamma camera must be small size, light weight, so the lead shield can not be too thick, otherwise the system would be unportable. For the rays wich have high energy, like gamma ray, the main way to increase the aperture is encoded-hole imaging. the penumbra encoding imaging method can increase the system’s field of view and shorten the time to find sources of nuclear pollution. Fast decay time, high light yield of scintillation array can provide a single pulse amplitude, studied the luminescence properties of scintillators like Ce:YAP crystal and terbium-cerium-scintillating glass. Contrast to photoelectron pulse generated by gamma-ray, the amplitude of the electronic thermal noise is slightly lower. Part of the electronic thermal noise can be eliminated by the amplitude discriminator.The FWHM of electronic thermal noise only1-2ns,but Ce:YAP crystal light-emitting FWHM is10～20ns. Waveform shaping and width recognition method can further reduce the impact of electronic thermal noise. Radiation monitoring equipment give alarm signal according to the radiation intensity; monitoring the current count and the background average count, and gives a warning signal when the difference between the two over a certain range. The formula used to determine the deviation from the degree of probability is called the alarm criterion expression. According to alarm criterion,Counter array composed of multi-anode photoelectric multiplier tube can detect the position and image of radio-active contaminated areas.the detection limit close to environmental background radiation dose rate.This paper adopt a series methods to improving the sensitivity.As a result,this system can find out1microcurie137Ce, can find out100millicurie137Ce.