脉冲中子源密度测井俘获伽马射线强度的数值模拟

由于传统密度测井仪器中同位素中子源具有潜在的放射性危害,可控的脉冲中子源得到越来越广泛的应用。脉冲中子源发射的快中子,经过地层的减速作用和扩散作用被原子核俘获(吸收),并释放出俘获伽马射线。运用蒙特卡洛方法模拟快中子离开源后与地层之间的作用过程,得到了俘获伽马射线的强度分布；将俘获伽马射线作为伽马射线源进行密度测井研究,模拟经过地层吸收后剩余伽马射线强度的分布与地层密度的关系,以及测量条件对剩余伽马射线强度的影响。研究结果表明,剩余伽马射线强度随着地层密度的增加而减小。建立了剩余伽马射线强度与地层密度、孔隙度以及探测源距的拟合关系,可用于指导脉冲中子源密度测井仪器的设计。

To reduce the radiological hazards of isotopic neutron source used in conventional density logging instrument,controlled pulsed neutron source is widely used instead of conventional chemical source. A fast neutron emitted from the pulsed neutron source is captured by atomic nucleus after decelerating and diffusing of stratum,and releases captured gamma ray. We use Monte Carlo method to simulate the process of fast neutron departed from source and getting through stratum to get the intensity distribution of captured gamma ray,regard the captured gamma ray as the gamma source of density logging,simulate the relations between stratum density and the intensity distribution of remaining gamma ray after stratum absorption and study on the influence measurement conditions to the remaining gamma ray intensity. The results show that as the stratum density increases,the remaining gamma ray intensity decreases. The fitting relationship between the remaining gamma ray intensity and stratum density is built,which can guide to design pulsed neutron source density logging instrument.