Capture Gamma Spectrum Measurement With Neutron Moderation
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Solution Overview
Problem
Existing methods for measuring elemental capture gamma ray spectra in oil-gas exploration suffer from low accuracy due to discrepancies between laboratory measurements and international nuclear database standards, and the use of neutron sources and detectors that differ from actual subsurface equipment, especially for elements with small thermal neutron capture cross sections.
Innovation Solution
A method and device for measuring formation elemental capture gamma ray spectra involving neutron moderation, adjustment of sample position, and simulation calculation to obtain standard spectra, using Am-Be neutron sources and BGO detectors, with moderation layers and shields to optimize thermal neutron flux, and verification against IAEA nuclear data.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If prior art methods are used to measure standard capture gamma ray spectra in the laboratory, then measurement can be performed, but the accuracy is low due to great differences from IAEA nuclear database standards
Solution Approach 1:
The patent changes the measurement parameters by using an Am-Be neutron source instead of other neutron sources, and by optimizing the thermal neutron flux density to a specific range (10^4-10^6 n/(cm²·s)). It also changes the detector type to BGO and optimizes its parameters (diameter 3-5 cm, length 5-8 cm). These parameter changes enable laboratory measurements to achieve accuracy consistent with IAEA nuclear database standards.
Solution Approach 2:
The patent introduces a moderation layer as an intermediary between the neutron source and the test sample. This moderation layer converts fast neutrons from the Am-Be source into thermal neutrons, creating the appropriate neutron energy spectrum for accurate capture gamma ray measurement. The moderation layer is crucial for achieving measurement conditions that match international standards.
2Ease of manufacture
If neutron sources and detectors different from actual subsurface equipment are used in laboratory measurements, then measurement can be performed, but the results differ significantly from field conditions
Solution Approach 1:
The patent creates a scaled-down copy of the actual logging environment by using an Am-Be neutron source that replicates the neutron spectrum characteristics of downhole tools, and a BGO detector that mimics the detector properties in field equipment. The measurement geometry and thermal neutron flux density are also copied to match actual logging conditions, ensuring laboratory results accurately represent field measurements.
3Device complexity
If no moderation optimization is performed, then measurement setup is simple, but thermal neutron flux density is insufficient for accurate measurement of elements with small capture cross sections
Solution Approach 1:
The patent performs preliminary moderation of neutrons using a specifically designed moderation layer before the neutrons reach the test sample. This preliminary action converts fast neutrons into thermal neutrons in advance, ensuring that elements with small capture cross sections can be accurately measured. The moderation layer is positioned and dimensioned optimally to achieve the required thermal neutron flux density without complicating the overall setup.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The method and device provide accurate and consistent standard capture gamma ray spectra for various elements, enabling precise elemental capture spectrum logging and improving spectrum unfolding calculations.
Implementation Method 1
performing moderation on neutrons emitted from a neutron source according to a neutron capture cross section of a test sample
Implementation Method 2
a standard capture gamma ray spectrum of the test sample is measured and obtained at a position where a thermal neutron flux reacting with atomic nucleuses of the test sample reaches a first predetermined value
Implementation Method 3
using Am-Be neutron sources and BGO detectors
Data Source
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AI summary
The present invention provides a method and a device for measuring a formation elemental capture gamma ray spectrum, pertaining to the field of oil-gas exploration and well logging technologies. The method comprises: performing moderation on neutrons emitted from a neutron source according to a neutron capture cross section of a test sample; and adjusting a relative location of the test sample with respect to the neutron source, so that a standard capture gamma ray spectrum of the test sample is measured and obtained at a location where a thermal neutron flux reacting with atomic nucleuses of the test sample reaches a first predetermined value. The present invention performs moderation on a neutron source according to neutron capture cross sections of different test samples to measure and obtain elemental capture gamma ray spectra of the test samples, thereby obtaining various formation elemental capture gamma ray spectra with higher accuracy.