| Due to the substantial increase in demand for uranium resources in China's nuclear power industry,hydrothermal uranium deposits have once again become the key types of exploration.In order to accurately measure the contents of uranium and thorium in the borehole formation of hydrothermal uranium deposits,it is urgent to carry out gamma spectrometry logging in uranium exploration.In the absence of a wide-range,high-precision gamma spectrometry logging tool for small-diameter uranium exploration at home and abroad,this paper has carried out research work on key technologies in high-precision small-diameter gamma spectrometry logging in uranium exploration.Based on gamma spectrometry logging system constructed in this paper,uranium and thorium contents are measured in the range of(1.0×10-6,1.0×10-2)and(1.0×10-6,1.5×10-2)respectrvely when the relative errors of uranium and thorium content measurement are less than 5%,featuring wide-range and high-precision measurement.Based on the model wells and MCNP simulation data,the gamma energy deposition spectrum in the BGO crystal at the center of the wellbore is obtained,which provides basic data for gamma spectrometry data acquisition and processing.Based on the digital nuclear pulse signal processing technology,"ADC+FPGA"is adopted.Based on the architecture,the gamma spectrometry logging data acquisition system is developed.Based on the fast and slow dual-channel pulse processing technology,the correction of the count rate such as dead time and pulse accumulation is effectively solved.The scheme of constructing the detector of the gamma spectrometry logging system using"long+short"double BGO crystals is proposed innovatively.The rationality and feasibility of the dual crystal scheme are verified by the model on-hole measurement.The problem that the high and low content measurement can not be considered at the same time in uranium deposit is solved.A real-time automatic stabilization method without radiation source is proposed by establishing a functional relationship between the high voltage of the photomultiplier tube and the relative drift of the peak value of the natural source.The method above solves the problems of effective counting reduction and radiation safety hazard introduced by using the stable spectrum source.By analyzing the gamma spectrometry data of the model well,a sensitivity coefficients calculation method by using multi-scale model wells based on the least square method is innovatively proposed.The sensitivity coefficients obtain the optimal value with this method,which effectively improves the calculation accuracy of the contents of uranium and thorium in the borehole formation.By comparing the decomposed results of the measured data on the model wells,the inverse matrx decomposing method based on the standard three-window scheme is select.The error source of gamma spectrometrylogging is analyzed.Based on the error synthesis calculation results,the best logging velocity in uranium anomaly section and the section in which the uranium content is above 100×10-6 in field work is determined.In summary,this paper achieves the goal of wide range and high precision measurement under the condition of small diameter borehole by breaking through key technologies such as efficient acquisition,count rate correction,real-time automatic stabilization without radiation source and gamma spectrometry analysis of energy spectrum data in gamma spectrometry logging in uranium exploration.By that achievement mentioned above,powerful technical supports for gamma spectrometry logging in the field of uranium exploration are provided. |
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