Patent GB-797314-A (GB)
Title: Improvements in or relating to apparatus for investigating material with neutrons
Inventor: GOODMAN CLARK
IPC: G01V5/10
Filed: 1956-02-17
Abstract: 797,314. Testing by neuclear radiation. SCHLUMBERGER WELL SURVEYING CORPORATION. Feb. 17, 1956, No. 5058/56. Class 40 (3). Apparatus for testing the material through which a bore-hole passes, comprises means for irradiating the material with neutrons during short repetitive intervals and a detection arrangement influenced by phenomenon induced in the material and operative only during a selected relatively short period in each irradiating cycle. The apparatus is arranged in two parts, that at the top of the bore-hole and that lowered down the hole inside a suitable casing 10. The two parts are connected by a screened cable 14. Power from generator 20 may be applied by means of switch 21 to lines 15 and 16 in the cable and so energize power supplies 22 and 23 in casing 10. Supply 22 feeds ion source 26 in neutron generator 25 and supply 23 is used for ion acceleration under control of pulse modulator 24. Modulator 24 is synchronized by pulses from generator 28 fed over lines 17 and 19. The generator envelope is filled with deuterium gas and the target is either of tritium of deuterium according to whether 14 mev. of 3.5 mev. neutrons are required. At the bottom end of the casing 10, shielded from the generator by plate 30 is a radiation detector 29 coupled to gated amplifier 32 at the surface through local amplifier 31. The period of time for which the gate in amplifier 32 is open is determined by an adjustable delay multivibrator 34 coupled to generator 28 and an adjustable interval determining multivibrator 33. The gate output is fed to an integrating and recording device 37. In one mode of operation the casing is lowered into the bore-hole and pulses of fast neutrons generated repetitively. Inelastic collisions occur between these neutrons and the material surrounding the bore-hole almost immediately and detector 29 in responsive to the resulting gamma radiation to produce pulses which are integrated and recorded. The gate circuit may be arranged to be open during the neutron pulse and for a few microseconds thereafter. The unit 37 may include a pulse normalizer producing standard pulses from varying input pulses and a counter. The record may indicate count in terms of the depth of the gear in the bore-hole. The detector 29 may be of the scintillation type giving a pulse output proportional in amplitude to the energy of the received gamma radiation. As the energy level of the gamma radiation is dependent on the element concerned in the collision by using amplitude selectors separating the pulses into different channels according to their amplitude and having separate integrators and parallel recordings, it is possible to assess the amounts of different elements, such as carbon and oxygen, in the material. By comparing this information with information as to the presence of hydrogen obtained by known neutron-neutron or neutron-gamma techniques, hydrocarbons may be distinguished from water. In each emission cycle there occurs neutron slowing down and diffusion over the period, which may be 1250 Á sec., before the next pulse of radiation.. To investigate the occurrences during this period the arrangement of surface apparatus shown in Fig. 4 may be used. In this a sweep generator 66 is coupled to the horizontal deflection plates of the indicator cathode-ray tube and provides a time-base of a timing and duration determined by the multivibrators 34 and 33. The output of gated amplifier 32 is fed to an integrator 60 having a relatively short time constant and connected to the vertical deflection plates of the tube. In one method of utilizing this apparatus the detector 29 is enclosed in a resonance absorbing element capable of reacting with incident neutrons of a particular energy level to produce gamma rays which activate the detector. The display produced on the tube screen is a curve d representing the time distribution of neutrons slowed to the selected energy level. The peak timing t may readily be determined and by plotting t as the housing 10 traverses the borehole information can be obtained about the materials surrounding the bore. By the use of an arrangement automatically tracking the peak time t the required plot of t against depth down the bore can be obtained directly (Fig. 6, not shown). In another method of utilization, the timing of the gamma radiation occurring when neutrons are captured may be investigated. The detector may also be made sensitive to thermal neutrons for yet another kind of investigation. A different form of neutron source, comprising a source of alpha particles separated from a beryllium target by a slotted shielding disc, may be used. Rotation of the disc causes the emission of pulses of neutrons as in the previous embodiment and the rotation is synchronized with the rest of the apparatus (Fig. 7, not shown).