| Introducing gamma spectroscopy to radiation detection systems can help to obtain information about medium properties such as elemental concentration,gas and fluid saturation,neutron porosity,and bulk density,etc.Therefore,gamma spectroscopy is still a research hotspot in nuclear science.Because traditional radioactive sources still have radiation hazards during storage,transportation and operation,controllable sources like pulsed neutron generator are increasingly being used in well logging.During the measurement of the pulsed neutron instrument with 14 MeV neutrons as the radiation source,neutrons interact with various common nuclides in the medium and produce different induced gamma rays.It provides ideas for calculating the elemental concentration,but also makes the measurement and analysis of energy spectrum signals challenging.The theory and implementation of pulsed neutron tools in gamma spectra measurement and element concentration calculation are introduced.Based on the Monte Carlo method,the theoretical model and the numerical simulation model of the tool are built,and the gamma spectral response simulation scheme and tool verification method are studied.By solving the Gaussian energy broadening parameters and inelastic-tocapture ratio of the measured spectra,the verification process of the tool response is improved;a complete elemental standards is established by combining measured and simulated data.The simulation scheme suitable for the neutron-photon transport problem and the verified tool modeling can ensure the accuracy of the simulated element standards and reduce the systematic error of element concentration acquisition.The spectral response of the element logging tool can be regarded as the linear superposition of the standards of each element,and the element yield expresses the contribution of different elements to the measured energy spectra,which can reflect the concentration of the corresponding element.To improve the accuracy of the element yield results,the statistical characteristic of the EMT tool measurement is studies by quantitatively assessing the error of gamma spectra using the spectra samples measured in certain test pits.Accordingly,based on Bayesian theory,an spectral deconvolution algorithm suitable for the measurements of the elemental logging tool is proposed to obtain the elemental yield.Both field tests and theoretical analysis have proved the feasibility and prospects of the gamma spectra simulation and interpretation method of the pulsed neutron tool as the results shows improvement in the accuracy of elemental yield calculation and the correctness of the calculated formation elemental concentration. |
