Prompt Fission Neutron Uranium Logging(III): Logging Instrument

摘要: The domestically developed prompt fission neutron uranium logging(PFNUL) instrument for uranium exploration represents a significant advancement in China deep uranium mining efforts, though it comes with considerable challenges and complexity. This paper presents the development of a new prompt fission neutron uranium logging instrument(named UNL4) , integrates a domestic D-T neutron generator, two ³He proportional detectors, a lanthanum bromide (LaBr₃) gamma-ray detector, and digital multi-channel pulse amplitude analyzer. The near ³He detector is shielded with 1mm of Cadmium (Cd) and 5mm of high-density polyethylene(HDPE), enabling efficient epithermal neutron detection, while the far ³He detector measures thermal neutron. The LaBr₃ detector is employed for gamma-ray detection, primarily originating from uranium decay. High-speed ADC and FPGA technologies were used to achieve rapid acquisition and transmission of both dual neutron time spectra and gamma spectra.Moreover, this paper proposes a fast signal shaping method, which reduces the dead time effect in $^3$He detectors on neutron time spectra. Experiments conducted in standard model boreholes with varying uranium content demonstrated a strong linear relationship between the epithermal-to-thermal neutron ratio (E/T) and uranium content, with a fitting coefficient of R²>0.999, confirming the instruments accuracy. The E/T value repeatability, both in short-term (3.16% RSD) and long-term (1.2% RSD) measurements, showed excellent stability. In addition, the instrument demonstrated good performance at neutron logging speeds of 0.3∽3 m/min (E/T values) and gamma logging speeds of 1∽10 m/min. By conducting measurements in two ore sections of the PU model with lithium contents of 87.1 ppm and 45.6 ppm, the RD is less than about 10% in both logging cases, meeting the requirements for engineering applications. This marks the first successful development of a neutron logging instrument for uranium exploration based on a domestic neutron generator, signifying an important contribution to uranium resource exploration.