GB724441A - Improvements in or relating to neutron well logging apparatus - Google Patents

Abstract

724,441. Producing neutrons; positive ion tubes SCHLUMBERGER WELL SURVEYING CORPORATION. March 10, 1953 [March 11, 1952], No. 6631/53. Classes 39 (1) and 39 (4). [Also in Group XL(b)] A device for lowering into a bore hole for determining the nature of the strata through which the hole passes comprises a pressure resistant housing in which are means for producing a beam of charged particles, and a target responsive to the charged particles for emitting a flow of neutrons. Neutron production. As shown the logging apparatus comprises a pressure resistant housing 10 in which is the neutron source consisting of a further evacuated system within the main housing. The bombarding particles for the target are positively charged deuterium ions, the deuterium being contained in a container 24. The rate of leakage of the deuterium from the chamber is determined by means of a conventional palladium plug 29 (Fig. 2), provided with a heater winding 28 the current through the winding being adjustable by means of a variable tap on an auto-transformer 32. The deuterium passing the palladium plug is ionized by a discharge between an annular electrode 38 provided with a hole at both ends and a fixed electrode 41, the arcing potential being controllable by a potentiometer 45, supplied by a rectified D.C. voltage from the secondary of transformer 48. The ionization of the gas is confined to the central axis of the chamber by means of a magnetic field produced by a solenoid 51 and adjustable by a potentiometer 53. The ions are withdrawn from the ion chamber into a conducting probe 54 maintained negative with respect to a chamber 27 housing electrode 38. Potentiometer 58 permits adjustment of this potential. The positive ions pass through an aperture in probe 54 into an accelerating column 19. The column consists of a number of funnel shaped electrodes 75 having flanges 76 which are separated from each other by insulators 74 of outgassed glass resting on the flanges. The top of the accelerator column is closed by the target chamber inside which is supported a cup-shaped member 78a carrying a target plate 79. The target plate is a tungsten disc 2 cms. in diameter and .025 inches thick with a coating of zirconium .002 inches thick containing 2 to 3 ccs. of tritium. A focusing electrode 81 is positioned between the two bottom electrodes and is maintained at potential between that of either. Each part of the accelerating column is made negative with respect to the preceding column by means of a potential divider. Gettering arrangements. To remove gas at periods when the neutrons are not being produced a corrugated tube 68 of out-gassed zirconium, titanium or uranium heated by a filament 67 is provided. Also the parts of the accelerating column and the electrodes 54 and 81 are made of the same out-gassed getter materials which absorb gases at the temperature encountered in bore holes. Detecting arrangements. Situated in the pressure resistant housing above the neutron producing tube is a Geiger Muller tube 112 which may be made responsive to gamma rays or to neutrons of various energy levels. If neutrons are to be detected plate 15 separating the detection chamber for the neutron producing tube is of hydrogenous material; if gamma rays, of lead. Circuit and metering arrangements. The neutron source may be modulated in intensity. Three methods of doing this are described in the Specification. In one an alternating voltage source such as a transformer secondary is included in series with the H.T. supply to the target electrode (Fig. 4 not shown). In Fig. 4A (not shown) the potential is applied to the target electrode by means of a second discharge from an electrode vibrating near the housing 20. Thus the potential on the target varies as the electrode nears or parts from the target electrode. In Fig. 4B (not shown) deflector plates are provided being two electrodes 75 the electrodes being fed with a square pulse produced by a pulse generator. In Fig. 4C (not shown) a motor driven switch is provided operating solenoid 102 (Fig. 1B) to allow a member 96 to enter the ion beam and at the same time to complete a circuit with an electrode 101 thus enabling a measure of the ion current to be made. The output of the detectors may be fed to a number of amplifiers which are gated in turn to indicate the intensity of the received radiation after the peak value of neutron emission from the source, thus the decay rate for material in the formation may be measured. A pulse generator may operate to gate the amplifiers the operation of this generator being arranged to occur at a fixed point after each deflection or reduction of the ion beam. In the arrangement of Fig. 4C (V.S.) the same motor rotates contacts to produce the amplifier gating pulses.