| The detection of gamma rays is a standard technique in a wide variety of applications in science, nuclear medicine, industry and other fields. Not only the energy of the gamma radiation but also its intensity playing an important role in detections. And the gamma radiation detection instrument is the most specific, efficient and convenient detection instrument. For an efficient detection of penetrating gamma rays scintillation detectors are usually used. The combination of a high density, high Z scintillation crystal which converts gamma photon energy into light photons coupled to an efficient light detector, such as a photomultiplier tube (PMT), is the classical and most straightforward way of gamma ray detection. The classical PMT is a large bulky instrument that requires high voltage and a complex divide circuit and shows considerable sensitivity to temperature drift and the change of electromagnetic field. For some applications this is a serious problem. In the last decades, a number of alternative detection techniques have been developed, especially, the new type detector base on new type scintillator and the silicon photodiode, but the research of coupling NaI(Tl) crystal, the traditional scintillator material, and silicon photodiode was very few. For these reasons, we committed a photodiode scintillation detectors base on silicon PIN photodiode and NaI(Tl) crystal,In this research, we have designed a gamma radiation detection instrument which using a new type scintillator detector. The instrument is designed to cover a wide rang of energies. It's main components include a detector, a preamplifier, a shaper, linear amplifiers, a discriminator, the main processor unit and the power supply unit.The detector was made of one 10×10mm2 sensitive area silicon PIN photodiode (HAMAMATSU S3590-08) and aΦ30×25mm3 NaI(Tl) scintillation crystal. The NaI(Tl) crystal has the highest light yield, and it emission spectrum located from 320nm to 540nm(the wavelength of the emission maximum is 415nm). S3590-08 is sensitive in this spectrum. A cone of optics (A light guide) was made to couple between the two object to improve photodiode-to-scintillator coupling efficiency.Photodiode provides electronic pulse when scintillator emits emission the light excited by gamma radiation. But the signal is very weak, a special attention must be paid in order to minimize electronic noise, which is higher than statistical noise at low energies. Especially the first level of electronic circuit, a charge sensitive preamplifier (CSP) was developed specifically to be the interface with the photodiode to collect the charges from detector and output a voltage signal. To design a low noise CSP, we used a low noise CMOS operational amplifier AD8606, an IC that is excellently suited for our application, a very low feedback capacitor and a very large resistance.But signal output from CSP is mixed with a lot of noise. To reduce the noise and to discriminate energy of gamma rays the preamplifier signal is subsequently shaped by a CR-RC filter. In this way the bandwidth of the signal is limited and the signal is shaped into a Gaussian curve. Then, At the same time, signal is linearly amplified by two level high speed, low noise operational amplifier. The next circuit is a voltage follower to be the buffer. Then the analog signal is input to a comparator which is the interface of analog circuit and digital circuit, and signals become to digital pulses. This circuit is a integral discriminator. At last, digital pulses are counted by a high-performance, low-power AVR microcontroller ATtmega-8. ATmega-8 controls the count time, LCD displays, key board input, alarm buzzer. And all data processing function is accomplished by it.The instrument's power is supplied by two 7# batteries. And the power supply units convert the power to required voltage. Further more, a suitable bias voltage is indispensable if a silicon PIN photodiodes can be good performance. In our research, we used an DC-DC converter, TPS61040, to boost the +5V to +24V, applied the electronic filter to circuit to minimize ripples of power.During tests, the instrument can operate steady at a wide range of radiation intensity and a wide range of energy in a high electromagnetic field circumstance for a long time. And it is a small, convenient and efficient instrument in gamma rays detection activities. We can conclude that the NaI(Tl) scintillation detector base on silicon PIN photodiode is suitable for gamma radiation detection in circumstance that the classical detect can not.
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