Relationship Between Low-energy Scattering Spectrum And Lithology Of Airborne Gamma-ray Spectrometry

The low-energy spectrum(energy<400keV) of Gamma-ray spectrometry hasrelationship with the lithology of stratum medium, as some authors reported. Thestudy on the low-energy spectrum of airborne gamma-ray spectrometry may help usto abtain more information from the spectrum, and will improve airborne gammameasurement technique.By contrasted the three low-energy scattering spectrum, whose initial particle’senergy was emitted from Cs-137、 Ra-226artificial source and natural sourcerespectively, author proved that the energy of initial particle’s had little influence onthe low-energy scattering spectrum. So, it is feasible to substitute natural source forartificial source. In this paper, author verified the relationship between the effectiveatomic number、density of the medium and the low-energy scattering spectrum, bydetected the spectrum after the photons, emitted from Cs-137, passed throughdifferent mediums, such as iron、marble and concrete, with a NaI(Tl) detector.At the same time, the author analysed the integral area of low-energy scatteringspectrum from50keV to400keV by Monte Carlo method(MCNP5), and found thatthe energy ranges were50-150keV and150-230keV, which respectively related withthe effective atomic number and the density of the formation medium. Then authorestablished a mathematical equation between the integral area of low-energyscattering spectrum and the effective atomic number、density of the medium, as wellas the peak position with the effective atomic number.The test on the Emei Mountain showed that the error of the effective atomicnumber and density which calculated according to the equation abtained from MonteCarlo simulation was less than14%. It is feasible for us to identify the lithology offormation medium by analysing the low-energy gamma scattering spectum. Since the low-energy scattering gamma ray attenuated greatly due tointeraction with the air, it is necessary to take some correction of the atmosphereinfluence.The author made some correction of the intensity of gamma ray by threemethods such as theoretical calculation、Monte Carlo method and experiment. Theresult show that the intensity of gamma ray of the low-energy scattering spectrum inthe analysed window varies inversely as the exponential of the detecting height. Usingthe data of AGS-863airborne gamma-ray spectrometry, author abtained the correctioncoefficient of intensity of low-energy scattering gamma ray in three different energywindows.Finaly, author used the equation on some aera of Inner Mongolia AutonomousRegion. The result showed that in the region where there was a great difference in thelithology, author could distinguish the lithology by analysing the low-energyscattering spectrum of airborne gamma-ray spectrometry; but in the region wherethere was little difference in the lithology, the effective atomic number abtained fromthe low-energy scattering spectrum distributed discretely, it was difficult to divide thestratigraphic boundary according to the effective atomic number. The densitiesabtained from the caulation of low-energy spectrum has little difference in the region.