Patent GB-862434-A (GB)
Title: Measurement of gamma ray energy due to inelastic neutron scattering
Inventor: CALDWELL RICHARD LOUIS
IPC: G01V5/10
Filed: 1959-02-20
Abstract: 862,434. Neutron well logging. SOCONY MOBIL OIL CO. Inc. Feb. 20, 1959, No. 6005/59. Class 40(3) It has been found that certain chemical elements occurring in earth formations, e.g. oxygen, carbon and sulphur, emit gamma radiation of a characteristic energy level when bombarded with neutrons of energy at least equal to said characteristic level, the characteristic energy levels being fairly distinctive for such elements, e.g. 6 and 7 M.e.v. for oxygen, 4.5 M.e.v. for carbon and 2.3 M.e.v. for sulphur, Fig. 3 (not shown). According to the present invention the concentration of such an element in the earth surrounding a bore hole is recorded as a function of position along the hole by traversing the hole with an exploring unit 10 containing a source of light energy neutrons and means for detecting gamma radiation emitted by the surrounding earth in response to irradiation by the neutrons, means being also provided for recording the intensity of the component of the detected gamma radiation having energies within a restricted energy band which includes the characteristic energy level of the element being tested for. As shown the neutron source 14 comprises a cylindrical target electrode 15 having an inner surface layer of zirconium upon which tritium gas (H<SP>3</SP>) has been adsorbed and an axial electrode 16 maintained at a positive potential greater than 5,000 v, the space between the electrodes being filled with deuterium gas (H<SP>2</SP>) at a-pressure of 10<SP>-3</SP> to 10<SP>-5</SP> mm of mercury. The resultant bombardment of the tritium by the deuterium results in 14 M.e.v. monoenergetic neutrons without the direct production of gamma radiation. The voltage for the axial electrode 16 may be produced by a Van de Graaf generator, Fig. 4 (not shown). The gamma radiation produced by irradiating the surrounding earth with the neutrons is detected by a scintillation crystal 25 (thallium actuated sodium iodide) and photomultiplier tube 26 whose electrical pulse output is applied to a differential pulse light analyser 27 which is adjusted to produce an output proportional to the intensity of the gamma radiation within the appropriate energy band. The output from the analyser 27 is transmitted uphole by a cable 11 and applied to a recorder 32 whose recording medium is moved in correspondence with the exploring unit 10 by means of a mechanical linkage 33, 34. A multi-channel recorder may be employed for the simultaneous detection of a plurality of elements having different characteristic energy levels. The crystal 25 is shielded from the neutron source 14 by a copper slug 24. In a modification, Fig. 5 (not shown), the neutron source comprises a radium-beryllium or polonium-berylium mixture. Such a source also produces direct gamma radiation and the gamma ray detector is shielded from such radiation by a lead shield. In this embodiment the differential pulse light analyser is located at the surface and the output from the gamma ray detector is transmitted thereto by modulation of a carrier oscillation.