Development Of High Resolution Airborne Gamma Spectrometer Based On Array Detector

Airborne gamma energy spectrum survey technology,as an important method of geophysical exploration,is applied widely in mineral resources exploration,regional environmental radiation assessment,emergency response to sudden nuclear event and other fields.Currently,domestic and foreign airborne gamma energy spectrometer are functioning by applying large-area NaI?Tl? scintillation counter with its direct energy spectrum readout technique which is efficient,simple and economical friendly.But NaI?Tl? crystal has flaws like low energy resolution,high minimum detection limit and restricted efficiency of to-ground detection that cannot be applied on measurement which requires high accuracy and sensitivity.This research focuses on accurate radioactivity measuring techniques based on high resolution array detector and the design method of it,implementation method of List-mode multi-parameter nuclear pulse signal processing technology and eventually fulfill the development of high resolution airborne gamma spectrometer based on array detector.This detector can significantly improve both efficiency and accuracy of system detection thus replace traditional energy spectrum measuring method by its particle event unit collection,transmission,storage and procession of array detector output signal.Main research content and achivements are as follow:1.The ground-air interface natural gamma ray energy is between 30keV-3MeV,of which the fluence rate of gamma ray energy that below 1MeV is 85% out of total fluence rate.Natural gamma energy spectrum reaches“spectro-balance”condition when thickness of surface media is more than 50cm,therefore the fluence rate of gamma ray decreases with the height increases.Established mathematical expression about parameters like to-ground minimal detection activity?MDA?,energy resolution of detector,volume of detector and flight altitude for the airborne gamma spectrometry system and developed design of new airborne gamma spectrometer probe.This probe applied 8×8 high energy resolution CeBr₃ and LaBr₃ scintillation counter as array detector,and the energy resolution raise to 4%@662keV from 8%and detection efficiency raised 263.39%?1.33MeV?compare to Nal?Tl? scintillation counter in same volume.Mechanical structure of probe is using carbon fiber shell filled with Teflon material to reduce the gamma ray absorption of probe thus increase the sensitivity of detecting system without changing mechanical strength and thermo-stability of it.2.Proposed List-mode multi-parameter nuclear pulse signal processing technology and developed 64-channel multi-parameter gamma energy spectrum collector according to collecting and processing difficulty of the time/amplitude synchronization of nuclear pulse signals outputted by array detector and mark parameters such as time,location?detector ID?,amplitude and intensity of those signals,packaged every single event?particle?that gamma photon acted in detector as a data package and output to host computer.Host computer can combine energy spectrum and process data into synthetic energy spectrum,anti-coincidence energy spectrum,energy spectrum generated by anti-Compton and enhance photoelectric peak energy spectrum,background energy.Realized time-extraction of nuclear pulse signal by apply constant-ratio electric circuit.In the experiment using CeBr₃ detector,the maximum probability of signal time difference from zero to set time occurs in 280.8ns,and the root mean square?RMS? value of time jitter is 1.6ns.Realized measurement of location data by ID marked every incident particle and adds ID information into List package.Amplitude extraction of nuclear pulse is realized by apply 80MSPS sampling rate and 16-digit ADC to digitalize nuclear pulse signal and then realize amplitude extraction through symmetric zero-area trapezoidal shaping digital algorithm.Applied 59.56keV gamma ray produced by Am-241 source as internal standard to realize automatic stabilization of energy spectrum,ensure the consistency and stability of gamma ray energy extraction?nuclear pulse amplitude?thus under experiment condition the analog bandwidth can reach 100MHz,500kcps pulse throughput rate,360MB/s maximum data transmission rate,and differential nonlinearity?1.0%.By applying this collector for energy spectrum response test of ?50mm×50mm Ce Br₃ scintillation counter,the energy resolution is approximately 4.2%@662keV and spectrum drift is less than±1 channel?@2048 channel?in the range of 0??50?at 662keV.3.By analyze the action law of compound scintillation counter and gamma ray,proposed anti-Compton and enhance photoelectric peak technique.Re-superpose nuclear pulse amplitude?energy of scattering photon?generated by single/multiple scattering photons from Compton scattering process in compound detector to the recoil electron deposition spectrum of the detector as contribution to the photoelectric peak counts of incident gamma photon.This technique can reduce low-energy scattering background of airborne gamma spectrometer detector,increase the counts of photoelectric peak and peak/background ratio of photoelectric peak efficiently,and therefore increase the sensitivity of airborne gamma spectrometer detector to-ground detection.4.Developed software of compound airborne gamma spectrometer measurement based on particle-model multi-parameter nuclear pulse signal processing technology on which Visual C++Language Programming is applied to realize hardware equipment control and interface function,WinC#.NET Language is applied to realize interface programming,Sqlite is applied to realize data analysis layer.This software is capable for particle data collection,energy/time spectrum display,coincidence/anti-coincidence measurement,anti-Compton and enhance photoelectric peak,and spectrum data processing etc.5.Set up a 4-channel airborne gamma spectrometer measurement system with an1L NaI crystal bar,a 2L NaI crystal bar and two 1.5 inch CeBr₃ crystal bars.And done 220km² field flight experiment of airborne gamma spectrometer measurement by F-120unmanned aerial vehicle at a prospective area of uranium deposit in a province of China.The average altitude of the flight is 80m,velocity is 10m/s and sampling time interval is 3s.By set uranium?1.66-1.86MeV?,thorium?2.41-2.81MeV?,potassium?1.37-1.57MeV? and full spectrum counts 4 energy windows respectively in synthetic spectrum of 2-bar NaI scintillation counter and 2-bar CeBr₃scintillation counter,get 2662 energy window data including coordinate,GPS altitude and radar altitude parameters.In the contour map of NaI and CeBr₃ scintillation counter full channel energy window,shows clear high value concentration center and gamma high field,moreover,result of CeBr₃ has a more elaborate distribution of gamma high field than NaI.