A study on characteristics of background gamma spectrum from LaCl3 detector in pulsed neutron logging. (May 2023)

• Record Type: Journal Article
• Authors: Qiu, Fei; Zhang, Feng; Zhang, Xiaoyang; Chen, Qian; Fan, Jilin; Lu, Baoping
• Is Part Of: Applied radiation and isotopes. Volume 195(2023)
• Publication Date: 2023-05
• Subjects: LaCl3 detector -- Background -- Capture spectrum -- Si/Ca -- MCNP simulation -- Oil saturation; Radiology -- Periodicals; Radiation -- Industrial applications -- Periodicals; Nuclear chemistry -- Periodicals; Internet resource; Periodical; 660.298
• DOI: 10.1016/j.apradiso.2023.110738
• Languages: English
• ISSNs: 0969-8043

Abstract

Abstract: The scintillator detectors such as LaCl3 (Ce) play an important role in some fields of scientific research, environment, safeguards, medicine, security and industry due to its superior energy resolution and exceptional luminescence properties, etc. However, Cl element in a LaCl3 crystal produces uncertainty of determining oil saturation in pulsed neutron logging because of the background spectrum caused by secondary gamma ray from the reaction of Cl nuclei with the neutron. In this paper, we employed Monte Carlo method to simulate secondary gamma ray generated LaCl3 crystal induced by thermal neutron with different borehole and formation conditions and establish a reference spectrum of Cl element. The relations between elemental window or peak areas counts and borehole and formation conditions were also investigated. The background was obtained by combining the reaction rate derived from thermal neutron capture cross section for Cl element and neutron flux with the reference spectrum. The results indicate that the contribution of secondary gamma ray to measuring spectrum decreases with formation porosity, limestone content, borehole diameter, and water salinity increasing. Nevertheless, the relative peak areas of Cl at different energies remain constant, indicating that the logging conditions have less of an effect on the background spectrum shape. As evidenced by the measured spectra in the sandstone and limestone calibration wells processed, the peaks of Si and Ca elements are enhanced while the peaks of Cl element are weakened. After subtracting detector background, the computations of oil saturation based on calibration wells are 38% more accurate than the original method. Highlights: The characteristic response of secondary gamma background induced by LaCl3 crystal in different borehole and formation conditions are cleared based on MCNP simulation. The background was obtained by combining the reaction rate derived from thermal neutron capture cross section for Cl element and neutron flux with the reference spectrum. The Cl peak area at 1.16 MeV was utilized for background deduction and the accuracy of formation oil saturation determination was improved after precessed.