| Semiconductor detectors have great features such as fast time response,wide linear range,high spatial resolution and energy resolution.Thus they have widely use in the aerospace,nuclear medicine,security,environmental monitoring and other fields.The current commonly used semiconductor detectors mainly include conventional semiconductor detectors made of silicon(Si)and germanium(Ge)elements,and the new compound semiconductor detectors made of materials such as gallium arsenide(Ga As),cadmium telluride(CdTe),cadmium telluride(CdZnTe)mercury iodide(HgI2),lead iodide(PbI2)and other materials.The compound semiconductor detectors have the features of high atomic number,large band gap and high resistivity.Therefore,they have the advantages of high photon-to-electron conversion efficiency,good temperature characteristics,high detection efficiency and small volume,and can satisfy the requirements of high resolution X / ? ray radiation measurement in home temperature.At present,a large number of researches on compound semiconductor detectors have been carried out at home and abroad,and remarkable achievements have been made in crystal growth,electrode preparation,front-end electronics and back-end signal processing.Therefore,this paper starts from the crystal material of the compound semiconductor detectors,analyzes their performances,and develops a high performance detector readout circuit and a high resolution energy spectrum measurement system for their characteristics.This paper is derived from the National High Technology Research and Development Program of China(863 Program)Resource and Environment Technology Project "Research and Development of High Precision Radiation Spectrum Detection Instrument "(No.2012AA061803),Natural Science Foundation of China(NSFC)"Mathematical Construction of Nuclear Pulse Signal Chain and High-speed Real-time Digital Reconstruction Technology Research "(No.41474159).The main research contents and achievements are as follows:1.The structure of the compound semiconductor detectors and the migration characteristics of electrons and holes were analyzed to obtain the characteristics of the electrical signal.Taking the 1mm thick Cd ZnTe detector as an example,the output charge of 10 ke V ray is about 3.448 × 10-16 C,electron collection time is about 10 ns.The charge sensitivity of the designed detector readout circuit should be better than 1.45 × 1013 V / C,the circuit bandwidth should be greater than 100 MHz.2.The noise of the readout circuit was analyzed.Based on the implementation mode of the best readout circuit and the principle of the optimal filter,a differential-hybrid charge sensitive preamplifier was designed.The preamplifier adopts the dual JFETs,which form the folded cascode differential structure as the input buffer,and adopts the integrated operational amplifier to form the amplification stage circuit,its charge sensitivity is 2 × 1013 V / C,and the zero-capacitance noise measured at room temperature is 588 e,the noise slope is 4.9e / pF,and the signal to noise ratio of the ?-ray response waveform of 241 Am is 11: 1,and the signal to noise ratio of the ?-ray response waveform of 137 Cs is about 132: 1.3.A reset charge sensitive preamplifier was designed according to the implementation mode of the best readout circuit and the principle of optimal filter.The reset amplifier uses RF transistor to form cascade amplifier,uses high-speed switching transistor to form schmitt trigger circuit to achieve the reset function,its charge sensitivity is 2 × 1013 V / C,and the zero capacitance noise measured at room temperature is 475 e,the noise slope is 6.3e / pF,and the output signal amplitude range is 0 to-2.4V,the circuit reset time is 3?s;4.An analog rise time discriminator was designed based on the charge comparison principle for the "hole tailing" effect caused by the poor carrier transport characteristics of the compound semiconductor detector,which uses a threshold-adjustable rise-time discriminator circuit,compared the attenuated reference charge signal with the current signal after the deconvolution delay.It is found that the amplitude of delayed current signal is higher than that of the reference charge signal when rise time of the input signal is shorter than the discrimination time,thus generating gating signal and triggering DMCA(digital multi-channel analyzer)to receive the total integral charge signal.When rise time of the input signal is longer than discrimination time,DMCA remains inactivated and the corresponding total integral charge signal is abandoned.The circuit can be used to screen the input signal with the rise time in the range of 100 ns to 600 ns,the time precision is 50 ns.Used quasi hemispherical structure Cd ZnTe detector(size: 4mm × 4mm × 2mm),combination of the analog rise-time discriminator and energy spectrum system,measuring photoelectric peak of 137Cs(662keV),and the energy resolution was increased from 3.25% to 1.59%.5.According to the principle of charge comparison,a digital rise-time discriminator was designed.It includes fast-slow dual channel digital trapezoidal shaper.The fast channel shaping time is fixed at 120 ns,thus ensuring a very low pluse loss rate,and it also provides a standard time base signal for slow channel to extract the pluse amplitude.The shaping time of slow channel is in the range of 0.75?s to 18?s,to achieve pulse amplitude extraction,combined with the fast channel to achieve digital rise-time discriminating function.This digital rise-time discriminator can discriminate signal that rise time is in the range of 50 ns to 850 ns,the discriminating time accuracy is 25 ns.Used quasi hemispherical structure Cd ZnTe detector(size: 4mm × 4mm × 2mm),combination of the digital rise-time discriminator and energy spectrum system,measuring photoelectric peak of 137Cs(662keV),and the energy resolution was increased from 3.25% to 1.27%.6.A digital multi-channel pulse amplitude analyzer is designed for the high temperature stability,small volume and high energy resolution of the portable X / ? ray spectrum measurement system.The analog front end has gain adjustment,polarity conversion,DC bias,the data processing module uses the FPGA to design fast-slow dual channel digital trapezoidal shaper with pulse amplitude extraction,rise-time discrimination,count rate correction,baseline estimation and other functions,the multi-channel analyzer signal bandwidth can reach 100 MHz,which maximum sampling rate is 65 Mbps,sampling precision is 12 bit.The spectral resolution is 2048,the highest count rate is 232. |
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