Boron shielding design for neutron and gamma detectors of a pulsed neutron tool

Shielding materials are critical for downhole pulsed neutron tool design because they directly influence the accuracy of formation measurements. A well-designed shield configuration ensures that the response of the tool is maximally representative of the formation without being affected by the tool and borehole environment. This study investigated the effects of boron-containing materials on neutron and gamma detectors based on a newly designed logging-while-drilling tool that is currently undergoing manufacturing. As the boron content increased, the ability to absorb thermal neutrons increased significantly. Through simulation, it was proven that boron carbide (B4C)\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$(\hbox {B}_4\hbox{C})$$\end{document}can be used as an effective boron shielding material for thermal neutrons, and is therefore employed in this work. To shield against thermal neutrons migrating from the mud pipes, the optimal shielding thicknesses for the near- and far-neutron detectors were determined to be 5 and 4 mm. At a porosity of 25 p.u., near-neutron sensitivity exhibited a 5.6% increase. Furthermore, to shield the capture gamma generated by thermal neutrons once they enter the tool from the mud pipe and formation, internal and external shields for the gamma detector were evaluated. The results show that the internal shield requires a boron content of 75%, whereas the external shield has a thickness of 14.2 mm thickness and a boron content of 25% to minimize the tool effect.