Neutron generator having multiple extractors with independently selectable potentials

A radiation generator includes at least three extractor electrodes, with an ion source upstream of the at least three extractor electrodes to emit ions in a downstream direction toward the at least three extractor electrodes. There is a target downstream of the at least three extractor electrodes. The at least three extractor electrodes have independently selectable potentials so as to allow direction of an ion beam, formed from the ions, by the independently selectable potentials, toward different longitudinal and lateral regions of the target.

FIELD OF THE DISCLOSURE

This disclosure relates to the field of radiation generators, and, more particularly, to radiation generators with ion beams that can be deflected in desired longitudinal and lateral directions.

BACKGROUND

Well logging instruments that utilize radiation generators, such as sealed-tube neutron generators, have proven incredibly useful in oil formation evaluation. Such a neutron generator may include an ion source or ionizer and a target. An electric field, which is applied within the neutron tube, accelerates ions generated by the ion source as an ion beam toward an appropriate target at a speed sufficient such that, when the ions are stopped by the target, fusion neutrons are generated and irradiate the formation into which the neutron generator is placed. The neutrons interact with elements in the formation, and those interactions can be detected and analyzed in order to determine characteristics of interest about the formation.

Traditional neutron generators tightly focus the ion beam to help reduce a number of undesirable effects that can shorten their lifetime. However, a tightly focused ion beam may cause the formation of an area or hole in the target depleted in the reactants used for producing fusion neutrons. This can lower the neutron output of the neutron generator, or even lead to failure of the neutron generator.

Another type of radiation generator is an x-ray generator, and includes an electron source and a target. An electric field applied in the x-ray tube accelerates electrons emitted by the electron source as an electron beam toward the target at a speed sufficient such that, when the electrons are stopped by the target, Bremsstrahlung x-rays are released.

Traditional x-ray generators tightly focus their electron beams, which can cause a hole to be burned in the target. This can lower the x-ray output of the x-ray generator, or even lead to failure.

Thus, radiation generators capable of mitigating these undesirable effects of tightly focused beams are desired.

SUMMARY

In accordance with the present disclosure, a radiation generator may include at least three extractor electrodes, and an ion source upstream of the at least three extractor electrodes to emit ions in a downstream direction toward the at least three extractor electrodes. In addition, a target may be downstream of the at least three extractor electrodes. The at least three extractor electrodes may have independently selectable potentials so as to allow direction of an ion beam, formed from the ions by the three independently selectable potentials, toward different longitudinal and lateral regions of the target.

Another aspect is directed to a well logging tool for determining at least one property of a subsurface formation. The well logging tool may include a radiation generator. The radiation generator may include at least three extractor electrodes, and an ion source upstream of the at least three extractor electrodes to emit ions in a downstream direction toward the at least three extractor electrodes. In addition, a target may be downstream of the at least three extractor electrodes. The at least three extractor electrodes may have independently selectable potentials so as to allow direction of an ion beam, formed from the ions by the independently selectable potentials, toward different longitudinal and lateral regions of the target. The well logging tool may also include at least one radiation detector to detect incoming radiation resulting from interactions between the outgoing radiation and the subsurface formation, and processing circuitry coupled to the at least one radiation detector to determine the at least one property of the subsurface formation based upon the detected incoming radiation.

A method aspect is directed to a method of operating a radiation generator that may include emitting ions from an ion source in a downstream direction toward at least three extractor electrodes. The method may also include selecting potentials of the at least three extractor electrodes so as to direct of an ion beam, formed from the ions by the selected potentials, toward different longitudinal and lateral regions of a target downstream of the at least three extractor electrodes.

DETAILED DESCRIPTION

One or more embodiments of the present disclosure will be described below. These described embodiments are only examples of the presently disclosed techniques. Additionally, in an effort to provide a concise description, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions may be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill in the art having the benefit of this disclosure. In the drawings, like numbers separated by century denote similar components in other configurations, although this does not apply toFIG. 9.

For clarity in descriptions, when the term “downstream” is used, a direction toward the target of a radiation generator tube is meant, and when the term “upstream” is used, a direction away from the target of a radiation generator tube is meant. “Interior” is used to denote a component carried within the sealed envelope or housing of a radiation generator tube, while “exterior” is used to denote a component carried outside of the sealed envelope of a radiation generator tube. An “active” cathode is used to describe a cathode which is designed to emit electrons, as opposed to a “passive” cathode, which is an electrode of negative potential relative to the ion source.

In addition, when any voltage or potential is referred to, it is to be understood that the voltage or potential is with respect to a reference voltage, which may or may not be ground. The reference voltage may be the voltage of the active cathode as described below, for example. Thus, when a “positive” voltage or potential is referred to, that means positive with respect to a reference voltage, and when a “negative” voltage of potential is referred to, that means negative with respect to a reference voltage.

Referring initially toFIG. 1, a radiation generator100is now described. The radiation generator100includes an ion source101. The ion source101includes a portion of a hermetically sealed envelope or housing, with one or more insulator(s)102forming a part of the hermetically sealed envelope or housing. The insulator102may be an insulator constructed from ceramic material, such as Al2O3. At least one ionizable gas, such as hydrogen, deuterium, or tritium, is contained within the hermetically sealed envelope at a pressure of 1 mTorr to 20 mTorr, for example. A gas reservoir (not shown), such as a getter, stores and supplies this gas and can be used to adjust this gas pressure. It should be understood that the gas reservoir may be located anywhere in the ion source101since the hermetically sealed envelope helps to prevent gas loss, and in some cases, may be positioned outside of the ion source101. The ion source101includes an ion generator103which generates positive deuterium and/or tritium ions, and emits them in a downstream direction.

Downstream of the ion generator103are four extractor electrodes or extractor electrode segments110a-110d, although it should be understood that there may be more or less than four such segments (i.e., there may be two extractor electrode segments, three extractor electrode segments, four extractor electrode segments, five extractor electrode segments, six extractor electrode segments, etc.). Respective support structures111a-111dextend axially through feedthroughs or vias (not shown) in the insulator102to carry (i.e., physically support and electrically couple to their respective voltage sources or feeds) the extractor electrode segments110a-100d. Other configurations may be used as well. For example, as shown inFIG. 2, the support structures211a-211dmay extend radially through feedthroughs or vias in the insulator202to carry the extractor electrode segments210a-210d.

The extractor electrode segments210a-210dmay be axially collocated, or may be axially spaced apart as shown by the extractor electrode segments310a-310dinFIG. 3. Likewise, the support structures311a-311dmay be axially spaced apart as shown inFIG. 3, or may be axially collocated as shown by the support structures111a-111dinFIG. 1. Axial collocation of the extractor electrode segments110a-110dofFIG. 1may be more mechanically complex to accomplish than the axially spaced extractor electrode segments310a-310dofFIG. 3, but helps to reduce the overall length of the radiation generator100, which may be desirable. An example of one configuration of the extractor electrode segments810a-810dbeing axially collocated is shown inFIG. 8.

Referring again toFIG. 1, the potentials of the extractor electrode segments110a-100dhelp to define and shape the electric field within the ion source101, and to accelerate the ions out of the ion source101as an ion beam directed downstream toward and through suppressor electrode112, and onto a target114. The ions are further accelerated as they travel downstream by the voltage differences between the extractor electrode segments110a-110dand the suppressor electrode112as well as the target114. When the ions of the ion beam strike isotopes embedded in the target114, fusion reactions such as deuterium-deuterium (d-D), deuterium-tritium (d-T), and tritium-tritium (t-T) reactions, may occur, depending upon what types of ions are contained in the ion beam, and depending upon what types of ions are embedded within the target114. A byproduct of these fusion reactions is the creation of neutrons, with a d-D fusion reaction creating a 2.45 MeV neutron, a d-T fusion reaction creating a 14.1 MeV neutron, and a t-T fusion creating a pair of neutrons of an undefined energy (but less than 11.3 MeV combined between the pair).

The potential of each extractor electrode segment110a-100dis separately selectable. For example, the extractor electrode segment110amay be a first potential, while the extractor electrode segment110bis at a second potential different than the first potential. By having unequal potentials between certain (or between each) extractor electrode segments110a-100d, a voltage difference between certain (or between each) extractor electrode segment is created, and thus electric fields with a component orthogonal to the direction of the charged particle beam can be formed. This enables deflection or “steering” of the charged particle beam using electrostatic forces as desired. Thus, by selecting or setting the potentials of the extractor electrode segments110a-100d, the point of impact between the ion beam and the target114can be changed both longitudinally and laterally.

By being able to select the point of impact between the ion beam and the target114, heat loading on the target114can be managed, depletion of embedded isotopes- in the target can be managed, the lifespan of the radiation generator100can be increased, and other desirable results as will be described below may also be accomplished.

Referring again toFIG. 2, in some applications, the target214can comprises a plurality of different target areas215,216. Although two different target areas215,216are shown, it should be appreciated that there can be any number of such target areas. These different target areas215,216may each have a different coating thereon, such as different metal hydride coatings that perform differently at different temperatures. Thus, these coatings can be selected so as to maintain an optimum neutron output at different operating temperatures of the radiation generator100. For example, the target area215may have a titanium coating, while the target area216may have a scandium coating. It is known to those skilled in the art that, as a metal hydride, titanium has a much lower desorption temperature than scandium. As a result, at temperature nearing the desorption temperature of titanium, scandium will retain a greater concentration of hydrogen isotopes, and hence be capable of producing more neutrons for a given reactant ion beam current.

Referring now toFIG. 3, in some applications, the target314can comprise a plurality of longitudinally spaced apart target portions315,316. This allows selection of different longitudinal points of neutron emission. Although the longitudinally spaced apart target portions315,316portions are shown as being part of a same target314, they may not be, and instead, they can be spaced apart longitudinally without being physically connected to each other. The longitudinally spaced apart target portions315,316may be electrically coupled in some applications.

Example circuitry such as may be used to control the independently selectable potentials of the extractor electrode segments310a-310dis now described. Each extractor electrode310a-310dis coupled to a power supply320via a respective adjustable voltage regulator324a-324d. A controller322is coupled to the adjustable voltage regulators324a-324dto control them, and thus, can adjust the potentials of each extractor electrode segment310a-310dseparately.

The controller322can be programmed to control the extractor electrode segments310a-310din a variety of ways. For example, the controller322can control the potentials of the extractor electrode segments310a-310dso as to direct the ion beam toward the different target areas315,316, and toward different subareas of the different target areas, to reduce the heat load on the target314. This can help extend the life of the radiation generator300. This may be done based upon the operating temperature of the radiation generator300, or based upon a condition of the target314. The condition of the target314can include the temperature of the target, and/or an amount of—erosion in the target, and/or a number of neutrons emitted from the target in a given period of time. The controller322may also control the potentials of the extractor electrode segments310a-310dso as to sweep the ion beam across the target314in a pattern designed to perform a certain function, for example to evenly distribute heat loading or wear patterning of the target.

The ion source301may be any form of ion source as will be appreciated by those skilled in the art. For example, the ion source301may be of a type employing an active cathode that accelerates electrons through ionizable gas to create ions, a type that ionizes gas through quantum tunneling of electrons from nanotips to gas molecules, a penning style ion generator, or a microwave or RF ion generator, for example.

An example where the ion source301includes an active cathode is now described with reference toFIG. 4. Here, there is a gas reservoir404within the sealed envelope402, with an ohmically heated cathode406downstream of the gas reservoir (although it should be noted that the gas reservoir404can be located anywhere within the radiation generator400). The ohmically heated cathode406emits electrons via thermionic emission. A cathode grid (or passive electrode having an aperture therein)408is downstream of the ohmically heated cathode406. A voltage difference between the ohmically heated cathode406and the cathode grid408accelerates the electrons away from the ohmically heated cathode in a downstream direction. An (optional) cylindrical electrode409is downstream of the cathode grid and helps to define the electric field in the ion source401. The extractor segments410a-410dare downstream of the cylindrical electrode410.

Another example where the ion source401includes a different type of active cathode is now described with reference toFIG. 5. Here, the ion source501is similar to the ion source401described above with reference toFIG. 4, but instead of an ohmically heated cathode contains a field emitter array (FEA) cathode506downstream of the gas reservoir504. The FEA cathode506can be a Spindt cathode, or instead may be a plurality of nanotips extending from a substrate or grid. If the FEA cathode506is a Spindt cathode, it emits electrons on its own (when internally biased properly), and the anode508downstream of the FEA cathode is optional, although still helpful in accelerating the electrons in a downstream direction due to the voltage difference between the anode and the FEA cathode. If the FEA cathode506is not a Spindt cathode, electrons are emitted from the FEA cathode due to the voltage difference between the FEA cathode and the anode508. The other portions of the ion source501are similar to those described above and need no further description.

Turning now to the radiation generator600ofFIG. 6, a penning type ion source601is employed. The penning type ion source601includes a back passive cathode electrode640, with a passive cylindrical anode electrode646downstream therefrom. A magnet644(permanent or electromagnet) is axially collocated with the passive cylindrical anode electrode646, and a front passive cathode electrode642is downstream of the passive cylindrical anode electrode. The extractor electrode segments610a-610dare downstream of the front passive cathode electrode62.

There is a large voltage difference between the back passive cathode electrode640and the passive cylindrical anode electrode646, as well as between the front passive cathode electrode642and the passive cylindrical anode electrode. This voltage difference is sufficient such that an electrical discharge between the back passive cathode electrode640and the passive cylindrical anode electrode646, and/or the front passive cathode electrode642and the passive cylindrical anode electrode, occurs.

Due to the voltage difference between the back passive cathode electrode640and the passive cylindrical anode electrode646, and between the passive cylindrical anode electrode and the front passive cathode electrode642, an electrostatic well is created between the back passive cathode electrode and the front passive cathode electrode. The electrostatic well electrically confines charged particles such that they remain between the back passive cathode electrode and front passive cathode electrode.

The charged particles as they travel from the back passive cathode electrode640to the front passive cathode electrode642are attracted toward the passive cylindrical anode electrode646. However, the magnet644generates a magnetic field pointing mostly downstream, such that the charged particles are prevented from traveling directly to the anode electrode, and instead are confined to orbits about lines of the magnetic field, travelling back and forth in the electrostatic potential well created by this Penning anode-cathode configuration. Thus, rather than following a relatively straight trajectory as they travel, the charged particles travel along a spiral or helical shaped trajectory, thereby greatly increasing the length of the path they follow. This increases the chances for ionization for the electrons present in the ionization region.

Moving along to the radiation generator700ofFIG. 7, a RF or microwave ion source701is employed. Here, a series of electrically conductive coils799are wrapped around the dielectric/electrical insulator702. The extractor electrode segments710a-710dare downstream of the series of electrically conductive coils799. During operation, the series of electrically conductive coils799cause radiofrequency energy to permeate and/or penetrate the insulator, and to ionize some of the ionizable gas contained in the envelope in proximity to the coil. Other configurations of coils or RF/Microwave generators are also applicable, as will be understood by those of skill in the art.

The above examples were applications where the radiation generator is a neutron generator. Now examples where the radiation generator is an x-ray generator are described with reference toFIGS. 10-12. The x-ray generators1100,1200,1300are similar to the neutron generators100,200,300(fromFIGS. 1, 2, 3, respectively) described above, except that instead of an ion source there is an electron source1101,1201,1301. The electron source1101,1201,1301may include any number of active cathodes, such as ohmically heated cathodes, Spindt cathodes, and field emitter array cathodes.

In addition, there is no gas reservoir, and no ionizable gas within the envelope1102,1202,1302. Indeed, there is instead a vacuum within the envelope1102,1202,1302. Due to the aforementioned similarities, lengthy descriptions of the structure or operation of the x-ray generators1100,1200,1300need not be given, and all descriptions about the structure and independently selectable potentials of the extractor electrode segments1110a-d,1210a-d,1310a-dfrom above apply. The x-ray generators1100,1200,1300operate by emitting electrons from the electron sources1101,1201,1301and accelerating them downstream toward the target1114,1214,1314. When the electrons strike the target1114,1214,1314, Bremsstrahlung x-rays are emitted. It should be noted that the operating voltages of the various electrodes may be different than those used in the ion sources100,200,300.

In addition, certain beam steering applications made possible by this disclosure may be of particular interest to x-ray generators, such as the application where there are different target portions1115-1116,1215-1216,1315-1316that could be longitudinally spaced apart, since there are no gas molecules in the x-ray generator1100,1200,1300to cause scattering of the electron beam, for example. This may be done to change the location of x-ray emission, as well as to allow the selection of emission of x-rays of different energies. The energy of emitted x-rays is based upon the potential drop between the extractor electrode segments1110a-d,1210a-d,1310a-dand the target1114,1214,1314; by having different target portions1115-1116,1215-1216,1315-1316at different potentials, different energies of x-rays can thus be emitted by selection of the appropriate target portion. The selection of emission of x-rays of different energies may be particularly useful in determining different formation properties, such as density, and probing different formation depths.

In addition, the different target portions1115-1116,1215-1216,1315-1316can have different densities and/or thicknesses, such that x-rays generated within one target portion are able to escape from that target portion, while x-rays generated within another target portion are unable to escape from that target portion (in which case, the emitted x-rays are just those emitted from interactions between the electron beam and the surface of that target portion). This can be done to select emission of x-rays of different energies in some cases. In addition, the materials for those different target portions1115-1116,1215-1216,1315-1316can be selected such that a notch is created in the emitted x-ray spectrum, which may be useful for some measurements. In either case, there may be a window1317in the insulator1302, to allow escape of some or all of the generated x-rays, as shown inFIG. 12. The material of the window1317may be selected such that x-rays of certain energies can pass through, while x-rays of other energies do not pass through, for example. This may also be useful for some measurements.

Turning now toFIG. 9, an example embodiment of a well logging instrument911is now described. A pair of radiation detectors930are positioned within a sonde housing918along with a radiation generator936(e.g., as described above as radiation generator100,200,300,400,500,600,700,1100,1200,1300inFIGS. 1-7, and 10-12) and associated high voltage electrical components (e.g., power supply). Although a pair of radiation detectors930are shown, it should be understood that there may be any number of radiation detectors, and that every radiation detector need not be of the same size, shape, area, or material. In addition, different radiation detectors930may be configured to detect different types of radiation, such as gamma rays, x-rays, and neutrons. The radiation detectors930may include scintillation crystals coupled to photomultipliers. Suitable scintillation crystals may include LaBr, YAP, LuAP, LuAG, NaI, GSO and others as known to those of skill in the art.

The radiation generator936may be a neutron generator, x-ray generator, or gamma ray generator. Supporting control circuitry914for the radiation generator936(e.g., low voltage control components) and other components, such as downhole telemetry circuitry912, may also be carried in the sonde housing918. The sonde housing918is to be moved through a borehole920. In the illustrated example, the borehole920is lined with a steel casing922and a surrounding cement annulus924, although the sonde housing918and radiation generator936may be used with other borehole configurations (e.g., open holes). By way of example, the sonde housing918may be suspended in the borehole920by a cable926, although a coiled tubing, etc., may also be used. Furthermore, other modes of conveyance of the sonde housing918within the borehole920may be used, such as wireline, slickline, and logging while drilling (LWD), for example. The sonde housing918may also be deployed for extended or permanent surface or subsurface monitoring in some applications.

A multi-conductor power supply cable may be carried by the cable926to provide electrical power from the surface (from power supply circuitry932) downhole to the sonde housing918and the electrical components therein (i.e., the downhole telemetry circuitry912, low-voltage radiation generator support circuitry914, and one or more of the above-described radiation detectors930). However, in other configurations power may be supplied by batteries and/or a downhole power generator, for example.

The radiation generator936is operated to emit neutrons, x-rays, or gamma rays to irradiate the geological formation adjacent the sonde housing918. Neutrons, gamma-rays, or x-rays that return from the formation are detected by the radiation detectors930. The outputs of the radiation detectors930are communicated to the surface via the downhole telemetry circuitry912and the surface telemetry circuitry932and may be analyzed by a signal analyzer934to obtain information regarding the geological formation. By way of example, the signal analyzer934may be implemented by a computer system executing signal analysis software for obtaining information regarding the formation. More particularly, oil, gas, water and other elements of the geological formation have distinctive radiation signatures that permit identification of these elements. Signal analysis can also be carried out downhole within the sonde housing918in some embodiments.