Patent Number: 043107656
Section: description

DESCRIPTION OF SPECIFIC EMBODIMENTS Turning now to FIG. 1, the invention will be described with respect to a preferred application in a radioactive well logging system and particularly one in which the neutron source is operated in a pulsed mode. The well logging system comprises a logging tool 3 which is suspended from a cable 4 within a well 5 traversing a subterranean formation of an interest indicated by reference numeral 6. The well bore may be lined or unlined with casing but will normally be filled with a fluid such as drilling mud, oil or water. Signals from the logging tool are transmitted uphole via suitable conductors in the cable 4 to an uphole analysis and control circuit 8. Circuit 8 operates on the downhole measurements and applies one or more output functions to a recorder 9. In addition, Circuit 8 transmits certain control functions to the logging tool via conductors in cable 4. As the logging tool is moved through the hole, a depth recording means, such as measuring sheave 10, produces a depth signal which is applied to recorder 9, thus correlating the downhole measurements with the depths at which they are taken. The logging tool 3 comprises a pulsed neutron source 12, a downhole power supply 14 for the source, and a radiation detector 15, which responds to primary or secondary radiation in the formation in response to the output of the pulsed neutron source. For example, the detector 15 may be a gamma ray detector, a thermal neutron detector or an epithermal neutron detector. While only one detector is shown, it will be recognized that such logging tools may comprise a plurality of detectors responsive to similar or dissimilar radiation. The pulsed neutron source is an accelerator-type neutron tube comprising a replenisher section 16, an ionization section 18, and a target section 19. Replenisher section 16 may comprise replenisher element 16a which releases deuterium gas in response to an applied DC or AC voltage from power supply 14. Target section 19 comprises a tritium target 19a. The target section will also typically include an extraction-focusing electrode assembly and a negative high voltage supply (not shown) which functions to direct ions from the ionization section 18 to the target 19a while suppressing the counter current flow of secondary electrons produced by ion impact on the target. The ionization section 18 includes anode means 18a and cathode means 18b and 18c. The neutron source 12 may be operated in a continuous or in a pulsed mode. In either mode of operation, deuterium gas released upon the application of power to the replenisher element 16 enters the ionization section 18 where the gas molecules are ionized by a positive (with respect to cathodes 18b and 18c) ionization voltage applied across anode 18 and cathodes 18b and 18c. The deuterium ions formed in the ionization section are then accelerated toward the target 19a by a negative voltage applied to the target section. For example, a positive voltage or voltage pulse with an amplitude from a few hundred volts to a few kilovolts may be applied to anode member 18a and a -100 kilovolt voltage applied to target section 19. The Penning ionization section of the accelerator-type neutron tube may be of the "cold cathode" or of the "hot cathode" type. In the cold cathode source, the primary electrons are produced by field emission when a positive voltage pulse is applied to the anode. In the hot cathode type of source, electrons are initially produced by thermionic emission from an electrically heated filament. The cold cathode source suffers the disadvantage, which is of particular significance when the neutron source is operating in a pulsed mode, of having a time lag before the electron flux reaches a sufficient value for optimum ionization of the accelerator gas. Thus, upon applying a positive voltage pulse to the anode there normally is a period from about 3-10 microseconds in which the electron flux builds up to an equilibrium value. The hot cathode source, because electrons are instantly available from thermionic emission, does not suffer this disadvantage. However, the hot cathode source requires an additional high voltage power supply which is particularly significant in the case of downhole logging tools where the power requirements must be met by transmission from the surface. The concentration of ionized accelerator gas is dependent upon the accelerator gas pressure, i.e., the concentration of gas molecules in the replenisher section and the efficiency of the ionization section. Ionization efficiency is directly related to the flux and energy of free electrons in the ionization section. Therefore, a relatively inefficient ionization process would require a relatively high accelerator gas pressure, i.e., a higher concentration of gas molecules. Conversely, the accelerator gas pressure can be significantly reduced by increasing the electron flux in the ionization section. A very significant increase in neutron production is realized by having an ion source that operates efficiently at low gas pressure in the range of a few microns of Hg pressure. This relationship holds true for continuous ion sources as well as for the pulsed ion sources. In one aspect of the present invention, the efficiency of the ionization section of the neutron source is significantly increased by formulating the active surface of at least one of the cathodes with a material having a secondary electron emission factor of 2 or more. Preferably, both cathodes are provided with active surfaces formulated of a material having a secondary electron emission factor of at least 2. Thus, one or more active cathode faces may be formulated of alkali metal halides such as potassium, sodium, and rubidium chlorides, potassium and sodium iodines, sodium bromine, and lithium and sodium fluorides. Alkaline earth metal halides such as barium and calcium fluorides may also be used. However, the alkali metal and alkaline earth metal halides described above suffer from the practical disadvantage of deteriorating rapidly due to ion bombardment. Other materials which are more useful in formulating the cathode surfaces include metallic oxides such as aluminum oxide, beryllium oxide, barium oxide, and magnesium oxide. Preferably, the cathode surface is formulated of an oxide layer selected from the group consisting of beryllium oxide, aluminum oxide, and magnesium oxide since these materials are relatively stable in a low pressure environment of hydrogen or its heavy isotopes and, if sufficiently thick, are stable to ion impact. Of the oxides mentioned, beryllium oxide is most resistant to reduction in a hydrogen environment, and thus it is especially preferred to employ beryllium oxide in the present invention. Beryllium oxide provides a secondary electron emission factor which is in excess of 3 which is preferred in carrying out the present invention. The preferred metallic oxide electrode surfaces are formulated by heating the metal layer in the presence of air or oxygen to form a relatively thick oxide layer. For example, beryllium may be heated in the presence of oxygen at a temperature of a few hundred .degree. C. to form a uniform beryllium oxide layer of at least two atom layers in thickness. The metal layer with oxidized surfaces is mechanically secured to the cathode. The use of the high secondary electron emission materials such as the metallic oxides may be contrasted with the use in the prior art of materials such as copper, iron, nickel, aluminum, magnesium and tantalum which have secondary electron emission factors of about 1.3. The prior art practice with respect to those metallic materials such as described above which are subject to oxidation has been to remove any oxide layers from the cathode surfaces. In a further aspect of the invention which may be employed in combination with cathode materials of a relatively high secondary electron emission factor as described previously or which may be employed in conjunction with more conventional cathode materials, one of the cathode members is formed with an active surface having a protuberant portion which extends axially into the chamber of the ionization section. Preferably, the protuberant portion of the cathode surface is in a closer proximity to the ionization section anode than the remainder of the active cathode surface. This configuration of the cathode member functions to increase the electrical field at the peripheral edge of the protuberant portion and through the central interior of the ionization section, thus enhancing field emission of electrons at the peripheral edge, and therefore increasing the probability that the electron emitted from the cathode will travel to the opposing cathode rather than being collected by the anode. Turning now to FIG. 2 of the drawing, there is illustrated a sectional view of the ionization section 18 of the neutron accelerator tube. The ionization section comprises primary and secondary cathode members 22 and 24 which define the upper and lower ends of the chamber 25 in which ionization of the accelerator gas actually takes place. Extending peripherally about the interior of the chamber 25 and located intermediate the cathode members 22 and 24 is an anode member 27. The anode member 27 is mounted on ceramic insulating collars 29 and 30 and thus is insulated from metallic sleeves 32 and 33 which, together with the cathode members, defines the remainder of the ionization chamber. Sleeves 32 and 33 and anode 27 are formed of a nonmagnetic or relatively low permeability metal such as an AISI 300 series austenitic stainless steel. An annular magnet 35 extends around the exterior of the ionization chamber and extends beyond the upper and lower ends of the anode member as shown. Extending from the anode member and between the ceramic collars 29 and 30, and through magnet 35, and an annular ring 37 described in greater detail hereinafter is an electric lead 38 to a high voltage power supply for the anode. The cathode member 22 is provided with channels 36 through which accelerator gas from the replenisher section 16 flows into the ionization chamber. The cathode member 24 is provided with an aperture 40 along the same axis as the protuberant portion 42 of cathode member 22 which provides an opening from the ionization chamber into the target section of the neutron tube. The target (not shown) is enclosed within a chamber formed in part by a glass collar 44 which is held in place by a Kovar or other metallic sleeve 45 which is welded to the cathode member 24. The cathode member 24 is provided with an insert 46 which is formulated of beryllium with an oxide layer or another suitable material having a high secondary electron emission factor as described previously. The active surface of the cathode member 22 exposed to the interior of chamber 25 may be formulated of a material similarly having a high secondary electron emission factor in one embodiment of the invention, or it may be formulated of another suitable material. Cathode member 24, cathode member 22, and annular ring 37 are all formed of materials which are highly permeable to magnetic flux. For example, these elements may be formed of soft iron or certain stainless steels such as AISI series 410 stainless steel. The upper portion of cathode member 24 and the ring 37 together with the lower portion of cathode member 24 and cathode member 22 thus establish a high permeability flux path which extends initially outwardly from the ends of magnet 35 and then turns inwardly to the active surfaces of the cathode members. The high permeability paths established by cathodes 22 and 24 and ring 37 direct most of the magnetic flux between the north and south poles of magnet 35 into the interior of the chamber 25. In addition, it will be recognized from an examination of FIG. 2 that the strongest electric field established upon the application of a positive voltage to anode member 27 will extend from the peripheral edge 47 of the protuberant portion 42 of cathode member 22 and to a lesser extent at the interior peripheral edge 48 of insert 46. The intense electric field at the peripheral edges 47 and 48 increases the number of field emission electrons and in a region of chamber 25 that increases the probability that electrons emitted from cathode members 22 and 24 will impact the opposing cathode surface rather than being collected by anode member 27. Together with the spiraling action imparted by the increased magnetic field within the chamber and the cathode surface with an electron emission ratio greater than 2 the probability that the electrons will impact accelerator gas molecules to produce the desired ions is greatly increased for lower gas pressure, and the time between applying a positive voltage pulse to the anode and the electron flux building up to an equilibrium value is reduced.