Patent GB-784335-A (GB)
Title: Neutron logging apparatus
IPC: G01V5/10
Filed: 1955-03-08
Abstract: 784,335. Measuring radiation. SCHLUMBERGER WELL SURVEYING CORPORATION. March 8, 1955 [March 8, 1954], No. 6841/55. Class 40 (3). [Also in Group XL (a)] Apparatus for investigating earth formations traversed by a bore-hole comprises a pressureresistant housing which may be lowered into the bore-hole and which contains a source of monoenergetic neutrons and a detector and means responsive to some phenomenon external to the device to control the flux of neutrons emitted by the source. As shown (Figs. 1A, 1B, 1C) the device comprises a neutron generator 14 (described below in detail with reference to Fig. 2) situated with various power supply and control arrangements above a partition 16 of lead, if a neutron-gamma log to be made, and of a boron compound such as boron carbide in paraffin if a neutron-neutron log is desired. Also in the housing above partition 16 is a detector 125 which is used for controlling automatically the neutron output or if situated below the partition may be responsive to thermal neutrons. Neutron generator and ionization gauge.- Fig. 2 shows one embodiment of a neutron. generator with an ionization gauge in the upper section, both mounted in a glass housing 18. The neutron generator consists of a glass cylinder 23 surrounded at its upper and lower ends by electrodes 26, 27 to which a radio frequency is applied. A cylindrical magnet 24 surrounds the outer housing producing an axial field within the envelope. Radio-active material 29 is provided to initiate ionization. Situated at the lower end of cylinder 23 is a frusto-conical glass disc 30 into which projects a probe electrode 33 also frusto-conical and having an enlarged base portion 35 to which connection is made by means of lead 36 fixed in the wall 18. A conical focusing electrode 39 is also provided to which connection is made by a lead passing through extension 35. Below the focusing electrode is target electrode 41 aligned with apertures in members 31, 33 and 39 and having a cap 43 plated with zirconium hydrided with tritium. Deuterium may also be used if a deuterium-deuterium reaction is desired. The upper part of housing 18 contains an ionization gauge and filaments 69, 70. Filament 69 of zirconium deuteride is arranged to produce or absorb deuterium when heated while filament 70 of zirconium scavenges oxygen and nitrogen but not deuterium. Each is surrounded by a heat shield 71, 72. The ionization gauge consists of an anode constituted by magnet 57 and electrode 61 and cathodes 62, 63 parallel to the magnet pole faces by layers of glass insulation. A baffle-plate 68 intercepts particles moving towards the magnet from the ion source. In operation when switch 101 at the surface is closed a radio frequency generator 113 in the housing is energized and its output delivered to electrodes 24, 25 of ion source 21. High-voltage power supply 121 delivers its output voltage to the accelerating gap defined by the periphery of aperture 34 and lug 44 of target 41 and an intermediate potential to focus electrode 39. A positive potential is applied by source 130 to anode 57, 61 of the ionization gauge relative to cathodes 62, 63. Initial ionization of the gas by the radio-active material 29 is followed by a continuous ionic discharge in the R.F. field. Since probe electrode 33, 38 is at the potential of the housing 13 (Fig. 1) while magnet 57 is positive, deuterium ions from source 21 drift towards the probe. Some drift through the opening in glass member 31 and enter the accelerating gap defined by the probe and target 41, 43 reach a high velocity and react with the tritium in target 44 to produce neutron of energies of 14 million electron volts. By varying the voltage applied to the focusing electrode the ion beam may be distributed over the target to prevent localized overheating. The secondary electrons produced are prevented by baffleplate 68 from striking magnet 57. Alternative construction of neutron generator (Fig. 4).-The anode of the ion generator comprises a spiral anode 191a and plate cathodes 191c and 191j. Cathode 191j is supported by a spiral 191k on a disc-shaped metallic electrode 191f spaced from the lower end of helix 191a. A magnet assembly 192 surrounds the part of the envelope within which the ion source is situated. Below plate 191f is a suppressor electrode assembly 196 comprising a disc with a central opening 196b through which extends connector 192. Support rods 196c end in a suppressor electrode or disc 196d having a central aperture and downwardly flared edges. The target 195c is supported on a cylindrical glass insulator 195a having its lower end fixed to cylinder 194 which is fixed to the glass body of the generator. In operation deuterium gas in the upper envelope is ionized in source 191. Some of the ions pass through the opening in cathode 191i and enter an area of accelerating potential between probe plate 191f and electrode 196c. These ions are accelerated to high velocities and pass through the opening in suppressor 196 and strike the target layer. A voltage drop is developed by the accelerating gap current in resistor 199 which maintains suppressor 196d negative relative to the target, thereby repelling secondary electrons towards the target. Pressure control system.-Positive ions are created in the pole gap of magnet 57 by spiralling electrons emitted from cathodes 62, 63 when positive ions strike these cathodes. Further electrons are emitted which in turn produce further positive ions and a continuous discharge occurs. The resulting current is a function of gas pressure and feeds pressure control circuit 144 which supplies the power applied to the deuterium-emitting filament. An alternative construction of ionization gauge is shown in Fig.3 consisting of disc-shaped cathodes 181, 182 surrounded by a cylindrical magnet 180 constituting the anode and provided as in the previous construction with a baffle-plate 68. Automatic control of neutron flux.-Detector 125 is mounted in the vicinity of the target electrode of the neutron generator. The detector is provided with a layer of hydrogenous material on the side facing the generator. The output of the detector is connected to a conventional pulse amplifier and integrator unit 129 the output of which is applied to the input circuit of voltage control unit 119. The control unit includes a magnetic amplifier in a servo circuit and compares the output potential of unit 129 with a reference potential and adjusts the potential supplying high voltage power supply unit 121 to automatically maintain generated neutron flux constant. The pulses from unit 129 are fed to an integrator voltmeter 9 at the surface. Instead of controlling the neutron flux by adjusting the accelerating gap voltage the target layer may be unevenly loaded with tritium so that defocusing of the beam increases the neutron flux or a deflection plate may be provided to move the beam along a path of increasing tritium content. Detector 125 may be mounted below partition 16 so as to respond to the neutrons slowed by the earth formations. Measuring output of #-ray detector.-To energize the #-rays detector 15 an A.C. source 160 at the surface is coupled to a power supply unit 166 by transformer 164 in series with a winding of pulse transfer 165. The detector output is amplified in pulse amplifier 171 and fed through pulse transformer 165 to a further pulse transformer 174 at the surface the primary winding of which is connected through a filter to eliminate the supply frequency. The secondary of the pulse transformer operates an integrator and recorder 177. Controlling neutron flux in accordance with thermal neutrons.-Monitor 125 may be placed in the vicinity of detector and made responsive to thermal neutrons, thus it may adjust the neutron output of the neutron generator so as to maintain the flux of reflected thermal neutron contacts. Thus the log of gamma rays produced by counter 15 represents a condition of constant thermal neutron flux.