Method and apparatus for separating gamma and neutron signals from a radiation detector and for gain-stabilizing the detector

A method for stabilizing the gain of a spectroscopy system for gamma rays and neutrons in a downhole tool comprising:

• detecting radiation events in a radiation field comprising neutrons and gamma rays;
• in a processor, converting the detected radiation events into a pulse amplitude spectrum;
• in the processor, decomposing the detector pulse amplitude spectrum into data sets from detected gamma rays and detected neutrons using gamma ray standard spectra and neutron standard spectra and a spectral fitting procedure which results in a best fit between a weighted sum of the data sets and the detector pulse amplitude spectrum;
• wherein the fitting procedure comprises determining, in the processor, fitting parameters for each of the gamma ray standard spectra and neutron standard spectra, at least one of the fitting parameters is a spectral gain;
• and
• in the processor, using the spectra gain fitting parameter to adjust a gain of a radiation detector spectroscopy system, determining a property of a subterranean formation from which the gamma rays and neutrons are detected.

A system for detecting neutrons and gamma rays using single detector in a downhole tool and distinguishing the neutron contribution from the gamma ray contribution in a pulse height spectrum comprising:

• a neutron and gamma ray detector wherein the neutrons are detected through a neutron reaction inducing heavy charged particles;
• a detector pulse amplitude analyzer in signal communication with the neutron and gamma ray detector;
• a processor in signal communication with the detector pulse amplitude analyzer configured decompose an output thereof into spectral data sets from gamma rays and neutrons using a fitting procedure establishing a best fit between a set of standard spectra and the output of the detector pulse amplitude analyzer;
• the processor configured to calculate at least one fitting coefficient for the fitting procedure wherein the at least one of the fitting coefficients applied to the decomposed output is different between the neutron and the gamma ray standard spectra;
• the processor further configured to determine elemental gamma ray data sets and neutron data sets from the decomposed output and the determined fitting coefficients, and the processor further configured to determine a property of a subsurface formation from which the neutrons and gamma rays are detected.