Prompt fission neutron uranium logging (II): dead-time effect of the neutron time spectrum

The acquisition of neutron time spectrum data plays a pivotal role in the precise quantification of uranium via prompt fission neutron uranium logging (PFNUL). However, the impact of the detector dead-time effect remains paramount in the accurate acquisition of the neutron time spectrum. Therefore, it is imperative for neutron logging instruments to establish a dead-time correction method that is not only uncomplicated but also practical and caters to various logging sites. This study has formulated an innovative equation for determining dead time and introduced a dead-time correction method for the neutron time spectrum, called the "dual flux method." Using this approach, a logging instrument captures two neutron time spectra under disparate neutron fluxes. By carefully selecting specific "windows" on the neutron time spectrum, the dead time can be accurately ascertained. To substantiate its efficacy and discern the influencing factors, experiments were conducted utilizing a deuterium-tritium (D-T) neutron source, a Helium-3 (3\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$<^>{3}$$\end{document}He) detector, and polyethylene shielding to collate and analyze the neutron time spectrum under varying neutron fluxes (at high voltages). The findings underscore that the "height" and "spacing" of the two windows are the most pivotal influencing factors. Notably, the "height" (fd\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${f}_{\text{d}}$$\end{document}) should surpass 2, and the "spacing" twd\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${t}_{\text{wd}}$$\end{document} should exceed 200 mu\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upmu$$\end{document}s. The dead time of the 3\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$<^>{3}$$\end{document}He detector determined in the experiment was 7.35 mu\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upmu$$\end{document}s. After the dead-time correction, the deviation of the decay coefficients from the theoretical values for the neutron time spectrum under varying neutron fluxes decreased from 12.4% to within 5%. Similarly, for the PFNUL instrument, the deviation in the decay coefficients decreased from 22.94 to 0.49% after correcting for the dead-time effect. These results demonstrate the exceptional efficacy of the proposed method in ensuring precise uranium quantification. The dual flux method was experimentally validated as a universal approach applicable to pulsed neutron logging instruments and holds immense significance for uranium exploration.