Patent Number: 052934101
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows a neutron generator 10 which may be used in a logging tool such as described e.g. in U.S. Pat. Nos. 4,794,792, 4,721,853 or 4,600,838, which are herein incorporated by reference. The major components of the neutron generator 10 are a hollow cylindrical tube 11 made of an insulating material such as alumina ceramic and having its respective longitudinal extremities fixed to a ceramic ring 12 and a conductive ring 13, an ion source 45, a gas supply means 25, an extracting electrode 50, and a massive copper target electrode 15. A transverse header 14 and the target electrode 15 close the rings 12 and 13, respectively, to provide a gas-tight cylindrical envelope. Ring 12 comprises parallel transversely disposed flanges 6, 7, 8, and 9, providing electrically conductive paths and sturdy support for the generator components as described subsequently in more complete detail. Flanges 6-9 are substantially equally spaced along ring 12, between header 14 and the corresponding extremity of tube 11. The gas supply means 25 is disposed transversely to the longitudinal axis I--I of the generator 10, between first flange 6 and second flange 7, closest to header 14. The gas supply means 25 comprises a helically wound filament 26 of tungsten, which may be heated to a predetermined temperature by an electric current from a gas supply power means 105 to which both ends 26a and 26b of filament 26 are connected. A film 44 of zirconium or the like, for absorbing and emitting deuterium and tritium, is coated on the intermediate turns of the filament 26 in order to provide a supply of these gases and to control gas pressure during generator operation. Due to physical isolation, a substantially uniform temperature can be maintained along the coated intermediate turns of the filament helix 26. As the gases released from the film 44 are withdrawn from the atmosphere within the envelope for ion generation, more gases are emitted to restore the envelope gas pressure to a level commensurate with the temperature of the intermediate portion of the filament helix 26. The gases emitted by the film 44 diffuse through holes provided in flanges 7-9, i.e. a hole 31 in second flange 7, a hole 33 in third flange 8 and holes 34, 35 in fourth flange 9. The gases emitted finally enter an ion source 45 interposed between the gas supply means 25 and the extremity of tube 11 facing ring 12. An annular shaped electrical insulator 90 is interposed between tube 11 and ring 12. More details on the structure of the neutron generator can be found e.g. in U.S. Pat. Nos. 3,756,682; or 3,775,216; or 3,546,512, which are herein incorporated by reference. The ion source 45 comprises a cylindrical hollow anode 57 aligned with the longitudinal axis I--I of the generator 10 and made out of either a mesh or a coil. Typically, a positive ionizing potential (either direct or pulsed current) comprised in the range of 100-300 volts relative to the cathode, is applied to the anode 57. In one exemplary embodiment of the invention, the anode 57 is about 0.75 inch (1.9 cm) long and has a diameter of approximately 0.45 inch (1.14 cm). The anode 57 is secured rigidly to flange 9, e.g. by conductive pads 60. The ion source 45 also includes a cathode 80 disposed close to the outside wall of the anode 57, in a substantially median position with respect to the anode. The cathode 80 comprises an electron emitter 81 consisting of a block of material susceptible, when heated, to emit electrons. Emitter 81 is fixed (e.g. by brazing) to the U-shaped end 82 of an arm 84 being itself secured to flange 8. The arm 84 provides also an electrical connection between the emitter 81 and a hot cathode heater current means 100 able to generate e.g. a few watts for heating the emitter. Heater current 100 is known per se (see U.S. Pat. Nos. 3,756,682, 3,775,216 or 3,546,512) and thus does not need to be further described. According to an alternate embodiment shown on FIG. 2A, the cathode 80 could also comprise two arms (similar to arm 84), each provided at one of its ends with a block of dispenser material, both arms being disposed outside the hollow anode 57. This embodiment (cathode disposed outside the anode) prevents the material evaporated from the cathode from coating the surface of suppressor 75 causing enhanced field emission. In a further alternate embodiment shown on FIG. 2B, the cathode 80 may also comprise a single arm provided at one end with an emitter, the arm being disposed inside the hollow anode 57, substantially in the center thereof. According to this embodiment, the cathode emitting surfaces are so arranged that electron emission is perpendicular to the axis of the ion source. This embodiment reduces the amount of cathode material being deposited on the suppressor surface. Now described in more detail is the structure of the cathode 80. The thermionic cathode 80 comprises an emitter block including a material forming a substratum and a material susceptible to emit electrons. Thermionic cathodes here mean heated cathodes, as opposed to cold cathodes which emit electrons when not heated. The thermionic cathodes can be broken down into: (i) those with inherent electron emission capability if they can be heated high enough in temperature without melting (e.g. pure tungsten or tantalum or lanthanum hexa boride), and (ii) those to which use a low work function material is applied, either to the surface of a heated substratum (such as thoria coated tungsten, oxide coated) [called "oxide cathode"], or impregnated by bulk into a porous substrate [called "dispenser" cathode]. General information on thermionic cathodes can be found in the book "Materials and Techniques for Electron Tubes" by W. Kohl, Reinhold Publishing, 1960, pages 519-566, which is herein incorporated by reference. In other words, "oxide" cathode involve what could be called a "surface" reaction, whereas in a "dispenser" cathode there occurs what could be called a "volume" reaction. General information on "dispenser" or "volume" type cathodes can be found e.g. in the article "Surface Studies of Barium and Barium Oxide on Tungsten and its Application to Understanding the Mechanism of Operation of an Impregnated Tungsten Cathode" by R. Forman, in Journal of Applied Physics, vol. 47, No 12, December 1976, pages 5272-5279; or in the article "A Cavity Reservoir Dispenser Cathode for CRT's and Low-cost Traveling-wave Tube Applications" by L. R. Falce, in IEEE transactions on electron devices, vol 36, No 1, January 1989. Cathodes of the "oxide" or "surface" type are described in the article "Compact Pulsed Generator of Fast Neutrons" by P. O. Hawkins and R. W. Sutton, The Review of Scientific Instruments, March 1960, Vol. 31, Number 3, Pages 241-248; in "Focused Beam Source of Hydrogen and Helium Ions" by G. W. Scott, Jr., in Physical Review, May 15, 1939, vol 55, pages 954-959; in U.S. Pat. No. 3,490,944 or U.S. Pat. No. 3,276,974; or in the article "Operation of Coated Tungsten Based Dispenser Cathodes in Nonideal Vacuum" by C. R. K. Marrian and A. Shih, in IEEE Transactions on Electron Devices, vol. 36, No 1, January 1989. All of the above mentioned documents are incorporated herein by reference. The thermionic cathode 80 of the ion source of the present invention is preferably of the "dispenser" or "volume" type. A dispenser cathode used in a hydrogen environment maximizes electron emissions per heater power unit compared to other thermionic type cathodes (such as LaB.sub.6 or W), while operating at a moderate temperature. The emitter block 81 comprises a substrate made of porous tungsten, impregnated with a material susceptible to emit electrons, such as compounds made with combinations of e.g. barium oxide and strontium oxide. Each cathode has different susceptibility to their operating environment (gas pressure and gas species). Dispenser cathodes are known to be the most demanding in terms of the vacuum requirements and care that is needed to avoid contamination. One, among others, of the (novel and non-obvious) features of the invention includes using, in a neutron generator, a dispenser cathode which works as long as several hundred hours in a hydrogen gas environment of pressure on the order of several mTorr, providing an average electron emission current of from 50 to 80 mA yet requiring only a few watts of heater power. The cathode 80 according to the invention is provided with hot cathode heater current 100 which is distinct from the ion source voltage supply 102. Such implementation permits a better control of both heater current means 100 and voltage supply 102. The extracting electrode 50 is disposed at the end of the ion source 45 facing target electrode 15, at the level of the junction between tube 11 and ring 12. The extracting electrode 50 is supported in fixed relation to the ring 12 by a fifth flange 32. The extracting electrode 50 comprises a massive annular body 46, e.g. made of nickel or an alloyed metal such as KOVAR (trademark), and which is in alignment with the longitudinal axis I--I of the tube 11. A central aperture 47 in the body 46 diverges outwardly in a direction away from the ion source 45 to produce at the end of body 46 facing target electrode 15 a torus-shaped contour 51. The smooth shape contour 51 reduces a tendency to voltage breakdown that is caused by high electrical field gradients. Moreover, the extracting electrode 50 provides one of the electrodes for an accelerating gap 72 that impels ionized deuterium and tritium particles from the source 45 toward a deuterium- and tritium-filled target 73. The target 73 comprises a thin film of titanium or scandium deposited on the surface of the transverse side, facing ion source 45, of the target electrode 15. The potential that accelerates the ions to the target 73 is established, to a large extent, between the extracting electrode 50 and a suppressor electrode 75 hereafter described. The suppressor electrode 75 is a concave member that is oriented toward the target electrode 15 and has a centrally disposed aperture 78 which enables the accelerated ions to from the gap 72 to the target 73. The aperture 78 is disposed between the target 73 and the extracting electrode 50. The suppressor electrode 75 is connected to a high voltage supply means 103 which is also connected, through a resistor "R" to the ground. In order to prevent electrons from being extracted from the target 73 upon ion bombardment (these extracted electrons being called "secondary electrons"), the suppressor electrode 75 is at a negative voltage with respect to the voltage of the target electrode 15. The velocity of the ions leaving the ion source 45 is, on an average, relatively lower than ion velocity in a known Penning source. Consequently, these slow moving ions tend to generate a tail in the neutron pulse, at the moment the voltage pulse is turned off. The presence of an end tail is detrimental to the pulse shape which, as already stated, is of importance. The present invention remedies this situation by adding to the extractor a cut-off electrode, in the form of a mesh screen 95, which is fixed, e.g. by welding, to the aperture 47 of the extracting electrode 50, facing the ion source 45. The mesh screen 95 (cut-off electrode) is e.g. made of high transparency molybdenum. The cut-off electrode 95 is submitted to voltage pulses synchronized with and complementary to the voltage pulses applied to the anode 57. The pulses applied to cut-off electrode 95 are positive and e.g. of 100 to 300 volts. In an alternate embodiment, the cut-off electrode 95, instead of being submitted to voltage pulses, is maintained at a positive voltage, of e.g. a few volts. This low positive voltage prevents the slow ions produced at the end of the pulse in the ion beam from leaving the ion source, and thus allows one to truncate the terminal part of the ion beam, which in turn provides a sharp cut-off at the end of the neutron pulse (i.e. a short fall time). The cut-off electrode 95 is preferably made of a metallic grid in the form of a truncated sphere, and its concavity turned towards the target 73. Part of the mesh screen 95 might protrude inside cylindrical hollow cathode 57. FIG. 3 shows two examples of neutron pulses obtained respectively with cut-off electrode (solid line) and without cut-off voltage (dotted line), everything else being equal. The benefit to the neutron pulse shape (especially the fall time) derived from the cut-off electrode is easily appreciated from FIG. 3. In an alternate embodiment, (wherein the extractor 50 is not provided with the cut-off screen 95), the end tail of the ion beam is truncated by applying a positive voltage pulse to the extracting electrode 50. In order to generate a controlled output of neutrons, continuously or in recurrent bursts, an ion source voltage supply means 102 provides power for the bombarding ion beam. For pulse operation, an ion source pulser 101 is provided at the output of ion source voltage supply 102 to regulate the operation of voltage supply to the ion source. The ion source pulser 101 has a direct output connected to the anode 57 (via flange 9) and a complementary output connected to extracting electrode 50. The high voltage supply 103, the ion source voltage supply 102, and the ion source pulser 101 may be of any suitable type such as e.g. described in U.S. Pat. Nos. 3,756,682 or 3,546,512, already referred to. A gas supply means regulator 104 (connected to the high voltage supply means 103) regulates, through a gas supply power means 105, the intensity of the ion beam by controlling the gas pressure in the envelope. The current flowing through resistor r provides a measure of ion beam current which enables the gas supply regulator 104 to adjust the generator gas pressure accordingly. The voltage developed by the high voltage supply 103, moreover, is applied directly to the suppressor electrode 75 and through a resistor R to the target electrode 15. The voltages thus developed provide the accelerating and suppressor voltages, respectively. During operation, current is passed through the filament 26 of the gas supply 25 in an amount regulated by the gas supply regulator means 104 to achieve a deuterium-tritium pressure within the generator envelope that is adequate to obtain a desired ion beam current and ad hoc conditions for the generator to operate. The high voltage established between the extracting electrode 50 and the suppressor electrode 75 produces a steep voltage gradient that accelerates deuterium and tritium ions from the electrode aperture 47 in extracting electrode 50 toward the target 73. The energy imparted to the ions is sufficient to initiate neutron generating reactions between the bombarding ions and the target nuclei and to replenish the target 73 with fresh target material. Initial bombardment of a fresh target 73 by, for example, a half-and-half mixture of deuterium and tritium ions, produces relatively few neutrons. As increasing quantities of impinging ions penetrate and are held in the lattice of the target, however, the probability for nuclear reactions increases. Thus, after a short period of ion bombardment, a continuous or pulsed output ranging from 10.sup.7 to 10.sup.9 neutrons per second is reached. As previously described, the regulator 104 regulates the power supplied to the filament 26 and thereby manipulates the tube gas pressure and the ion beam intensity to produce the desired neutron output. If the neutron output should increase as a result of an increase in the current, a corresponding increase in current through the resistor causes the regulator 104 to decrease the filament power supply and thereby reduce the gas pressure within the generator. The lower gas pressure in effect decreases the number of ions available for acceleration, and thus restores the neutron output to a stable, predetermined value. Similarly, a decrease in the current through the resistance causes the regulator 104 to increase the generator gas pressure. If desired, the neutron output can be monitored directly, and either the ion source voltage supply or the high voltage power supply can be controlled automatically or manually to achieve stable generator operation. In the event the generator is supplied only with deuterium gas, neutrons are produced as a result of deuterium-deuterium interactions, rather than the deuterium-tritium reactions considered in the foregoing illustrative description. The present invention provides the following advantages, as compared to the prior art neutron generators. Since no magnet is necessary, the neutron generator is lighter and of smaller dimensions than the prior art generators. This is a substantial improvement for logging applications due to the limited space available in the logging tools. The use of a dispenser cathode virtually cancels, or at least substantially reduces, the delay between the time the generator is turned on and the production of neutrons, and thus provides a sharp rise of neutron burst. This also results in an improved burst timing control. Also, the thermionic cathode operates without troublesome plasma mode transitions responsible for disturbing jumps in the neutron output, and for difficulties in using the beam control feedback loop with the reservoir heater. The erosion of the extractor and consequent coating of insulator surfaces, by sputtered metal due to ion bombardment, is substantially reduced because of the relatively low anode voltage. The reduced anode voltage allows one to use simplified pulsing circuitry. The voltage applied to the cut-off screen-electrode 95 allows the tail of the ion beam to be cut-off, made mainly of slow ions, and thus allows the generation of a neutron pulse showing a sharp end edge. Finally, the lifetime of the cathode is in the range of several hundred hours in a hydrogen gas environment of pressure on the order of several mTorr providing an average electron emission current of from 50 to 80 mA, yet requiring only a few watts of heater power. Above all, the invention is beneficial in term of pulse shape. In particular, the neutron pulse shows the following characteristics, as can be seen from FIG. 3: the time required for the instantaneous neutron output to reach its maximum, called plateau, measured from the instant when the voltage is applied to said cathode, is less than 1.5 microsecond; PA1 the fall time, i.e. the period of time between the instant when the voltage applied to said cathode is turned off and the instant when the instantaneous neutron output falls to 10% of the plateau, is less than 0.5 microsecond; PA1 the neutron output reaches a plateau which remains constant within a 10% range thereof, over a pulse time width comprised between 5 and 500 microseconds; PA1 the time lag between the instant when the voltage is applied to said cathode and the instant when the instantaneous neutron output reaches 10% of the plateau, is less than 0.5 microsecond; another benefit is that the time lag is independent of operational parameters of the ion source, such as gas pressure; and PA1 the rise time for the neutron output to reach 90% of the plateau, measured from the time when the neutron output is 10% of said plateau, is less than 1 microsecond.