| The current extensive development and utilization of offshore oil fields,mineral resources,and nuclear power generation have led to radioactive contamination of the marine ecosystem.In particular,the 2011 nuclear power plant leak in Fukushima,Japan resulted in the direct release of a substantial amount of radionuclide-contaminated wastewater into the ocean,posing a significant threat to the safety of the marine ecosystem.However,China’s current technique for measuring marine radioactivity still relies on field sampling and laboratory energy spectrum analysis,lagging behind developed nations in terms of real-time and online monitoring and early warning systems for marine radioactive environments.To address this gap and meet the needs of routine monitoring of radioactivity in China’s marine environment,as well as rapid monitoring of emergency accidents,this study proposes a research effort to develop marine radioactivity measurement sensor technology.The resulting radioactivity measurement sensor system is based on a NaI（Tl）crystal and features high energy resolution and low power consumption.This research systematically introduces both domestic and foreign marine radioactivity measurement sensor technology,elaborating on the basic principles and sensor characteristics of marineγ-ray measurement.Building upon this foundation,the study proposes an overall design scheme for the sensor system.The research work mainly consists of four main parts:hardware circuit design of the NaI（Tl）crystal sensor,firmware design of the microprogrammable control unit,software development of the functional spectrum analysis for the upper sensor,and comprehensive performance evaluation of the NaI（Tl）crystal sensor system.Firstly,the hardware circuit of the sensor is designed from three aspects:simplifying the analog signal conversion process,reducing power consumption and reducing the size of the hardware circuit.Notably,a power supply circuit is devised utilizing a power supply isolator to provide negative voltage for powering the photomultiplier.This design optimization enhances the structure of the power supply circuit and resolves the noise issues associated with the coupling capacitor in the positive voltage supply method.Moreover,a signal processing circuit for the radioactivity measurement sensor,based on MOS-tube technology,is proposed by integrating the front-end electronics system architecture with the microprogrammable control unit module.This circuit employs a front-end electronics system comprised of P-channel MOSFET transistors,capacitors,and operational amplifiers to convert the sensor’s charge pulse signal into a voltage difference signal.The voltage difference signal is subsequently processed by the microprogrammable control unit module,simplifying the complex analog signal processing,reducing circuit power consumption,and enhancing stability.Secondly,the firmware design for the microprogrammable control unit module was developed with consideration of the functional requirements of the hardware circuit in the sensor system.This design enables real-time and online monitoring of radionuclides by the sensor system.Thirdly,considering the characteristics of the marine application scenario,an upper functional spectrum analysis software is developed based on an existing energy spectrum analysis software package.This development effectively mitigates the influence of statistical fluctuations on energy spectrum analysis results,thereby improving their reliability.In addition,the marine environment can impact the detection efficiency of the system.Therefore,we implemented online calibration using the 40K natural radionuclide for ensuring measurement accuracy in long-term working conditions.Finally,the detection efficiency calibration was accomplished through a combination of Monte Carlo simulations and laboratory experiments.The sensor’s detection efficiency was scaled under controlled laboratory conditions.Three standard radioactive sources,namely22Na,137Cs and 241Am,were used for the energy scaling and calibration of the sensor.Following testing and performance analysis,the NaI（Tl）crystal sensor system achieved an energy resolution of 6.9%（661keV）,with a circuit power consumption of 0.46W. |
