Well logging tool with an accelerator neutron source

A well logging tool for investigating an earth formation surrounding a borehole comprising:

• a substantially monoenergetic D-T accelerator source of 14-MeV neutrons;
• neutron source monitoring means responsive primarily to unmoderated neutrons incident thereon directly from said neutron source for monitoring the output thereof, the sensitive volume of said monitoring means being located externally of but closely adacent to the neutron source;
• a first He-3 proportional counter spaced from but close to said neutron source, without subtantial intervening high density shielding, and being substantially insensitive to neutrons below approximately 0.5 eV in energy;
• first shielding means, having both neutron moderating and neutron absorbing properties, for shielding said first He-3 proportional counter so as to increase the low energy neutron detection threshold of said first He-3 proportional counter to at least approximately 10 eV;
• a second He-3 proportional counter located farther from said neutron source than said first He-3 proportional counter, said second He-3 proportional counter being eccentered towards one side of the well logging tool and being substantially insensitive to neutrons below approximately 0.5 eV in energy;
• second shielding means, having both neutron moderating and neutron absorbing properties, for shielding said second He-3 proportional counter from neutrons incident thereon from a side thereof away from said one side of the well logging tool;
• and
• a third He-3 proportional counter located farther from said neutron source than said second He-3 proportional counter and being eccentered towards said one side of the well logging tool and shielded on the side thereof away from said one side of the well logging tool by a neutron absorbing material.

Apparatus for investigating a characteristic of an earth formation traversed by a borehole, comprising:

• a source of high energy neutrons for irradiating said earth formation so as to give rise to a population of neutrons within the borehole including (1) a first flux of relatively low energy neutrons carrying information about said formation as a result of scattering reactions with formation constituents, (2) a second flux of neutrons carrying information predominately about the borehole contents as a result of scattering reactions with said borehole contents, and (3) a third flux of relatively high energy neutrons carrying information predominately about neutrons emanating directly from said neutron source;
• first and second neutron detectors spaced at near and far distances, respectively, from said neutron source for detecting said neutron population and for generating first and second signals, respectively, representative thereof;
• neutron shielding means for altering the energy characteristics of the neutron flux reaching said first detector so that said first signal is predominately indicative of said third flux and relatively less indicative of said first and second fluxes;
• and
• means for combining said first and second signals to provide an indication of said characteristic of said formation.

Well logging apparatus for investigating a characteristic of an earth formation traversed by a borehole, comprising:

• a well tool adapted to be moved through a borehole;
• a neutron accelerator carried by said well tool for irradiating an earth formation with high energy neutrons so as to give rise to a population of neutrons within the borehole including (1) a first flux of relatively low energy neutrons carrying information about said formation as a result of scattering reactions with formation constituents, (2) a second flux of relatively intermediate energy neutrons carrying information predominately about the borehole contents as a result of scattering reactions with said borehole contents, and (3) a third flux of relatively high energy neutrons carrying information predominately about neutrons emanating substantially directly from said neutron source;
• first and second He-3 gas-filled neutron detectors carried by said well tool at near and far distances, respectively, from said accelerator for detecting said neutron population and for generating first and second signals, respectively, representative thereof;
• neutron shielding means located within said well tool in surrounding relation to the sensitive volume of said first detector for (1) moderating high energy neutrons to levels more readily detected by said first detector and intermediate level neutrons to low energy levels and (2) absorbing low energy neutrons, so as to modify the energy characteristics of the neutron flux reaching said first detector so that said first signal is predominately indicative of said third flux and relatively less indicative of said first and second fluxes;
• and
• means for combining said first and second signals to provide an indication of said characteristic of the formation.

A neutron source monitor for monitoring the output of a high-energy neutron source, comprising:

• an organic scintillator for detecting high-energy neutrons incident thereon substantially directly from the neutron source and for producing outputs related to the incident energies thereof, said scintillator having a characteristic detector dimension that is large relative to the average range of recoil protons produced in said scintillator by substantially source-strength neutrons and small relative to the average range of those Compton-scattering electrons produced in said scintillator which would appear to the scintillator to have energies within the energy range of said recoil protons;
• a photomultiplier optically coupled to said scintillator and responsive to the outputs thereof for producing signals representative of the energies of the recoil protons produced by detected neutrons;
• first pulse height discriminator means coupled to said photomultiplier for passing photomultiplier output signals having magnitudes corresponding to a range of the pulse height spectrum located on the flat region of the scintillator spectrum and sufficiently high thereon comparatively to eliminate scintillator outputs produced by scattered-back neutrons and by gamma rays;
• and
• means for counting the signals passed by said first pulse height discriminator means as an indication of neutron source intensity.

In a well logging tool including a high-energy neutron source and means for monitoring the intensity of the source, the improvement in said source monitoring means comprising:

• an organic scintillator for detecting high-energy neutrons incident thereon substantially directly from the neutron source and for producing outputs related to the incident energies thereof, said scintillator having a characteristic detector dimension of a size that is large relative to the average range of recoil protons produced in said scintillator by substantially source-strength neutrons and small relative to the average range of those Compton-scattering electrons produced in said scintillator which would appear to the scintillator to have energies within the energy range of said recoil protons;
• said scintillator being located with respect to the neutron source so as substantially to reduce the incidence thereon of scattered-back neutrons relative to the incidence thereon of neutrons emanating directly from the source;
• a photomultiplier optically coupled to said scintillator and responsive to the outputs thereof for producing signals representative of the energies of the recoil protons produced by detected neutrons;
• first pulse height discriminator means coupled to said photomultiplier for passing photomultiplier output signals having magnitudes corresponding to a range of the scintillator pulse height spectrum located on the flat region of the spectrum and sufficiently high therein comparatively to eliminate scintillator outputs produced by scattered-back neutrons and by gamma rays;
• and
• means for counting the signals passed by said first pulse height discriminator means as an indication of neutron source intensity.