Photomultiplier for well-logging tool

A photomultiplier includes a tube and plurality of dynodes within the tube and including at least one first dynode and at least one second dynode. A respective insulator is between adjacent pairs of dynodes. The at least one first dynode includes a conductive outer ring and a medial conductive member coupled to the conductive outer ring in spaced relation therefrom. The at least one second dynode includes a conductive outer ring and a conductive inner ring supported within the conductive outer ring.

BACKGROUND

A photomultiplier is often optically coupled to a scintillator and incorporated into a well-logging tool to measure radiation within the wellbore. A scintillator crystal emits visible or near-visible light in response to the detected radiation. The photomultiplier receives the light from the scintillator crystal and transforms that light into electrical pulses. The frequency and amplitude of the pulses are related to the radiation measured during well-logging. In the photomultiplier, the electrons are amplified by a linear arrangement of electrodes called dynodes that accelerate and focus the electrons. This linear arrangement of electrodes is termed a venetian blind dynode structure and it is considered to resist the vibration and high heat in well-logging better than other dynode structures, such as a box-type dynode structure.

In the venetian blind dynode structure, the first dynode intercepts about 80% of the incident electrons. Many of the intercepted electrons, however, produce secondary electrons that may be poorly collected by subsequent dynodes.

SUMMARY

A photomultiplier includes a tube and plurality of dynodes within the tube and includes at least one first dynode and at least one second dynode. A respective insulator is positioned between adjacent pairs of dynodes. The at least one first dynode includes a conductive outer ring and a medial conductive member coupled to the conductive outer ring in spaced relation. The at least one second dynode includes a conductive outer ring and a conductive inner ring supported within the conductive outer ring.

In another example, a well-logging tool to be positioned in a wellbore of a subterranean formation includes a tool housing and a photomultiplier carried by the tool housing. The photomultiplier includes a tube and a plurality of dynodes within the tube and including at least one first dynode and at lease one second dynode. A respective insulator is positioned between adjacent pairs of dynodes. The at least one first dynode includes a conductive outer ring and a medial conductive member coupled to the conductive outer ring in spaced relation. The at least one second dynode includes a conductive outer ring and a conductive inner ring supported within the conductive inner ring.

A method of multiplying a signal in a photomultiplier is disclosed and the photomultiplier includes a tube containing a plurality of dynodes and including at least one first dynode and at least one second dynode and a respective insulator is positioned between adjacent pairs of dynodes. The method includes collecting electrons at the first dynode, which includes a conductive outer ring and a medial conductive member coupled to the conductive outer ring in spaced relation. The method further includes conducting electrons at the second dynode that includes a conductive outer ring and a conductive inner ring supported within the conductive outer ring.

DETAILED DESCRIPTION

Different embodiments will now be described more fully hereinafter with reference to the accompanying drawings. Many different forms can be set forth and described embodiments should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope to those skilled in the art.

FIG. 1illustrates a well site system40in which various embodiments of a photomultiplier76coupled to a scintillator74and as described below may be implemented. In the illustrated example, the well site is a land-based site, but the techniques described herein may also be used with a water or offshore-based well site as well. In this example system, a borehole41is formed in a subsurface or geological formation42by rotary drilling, for example. Some embodiments may also use directional drilling, as will be described below.

Although this description proceeds with the description of a Drilling and Measurement (D&M) system that includes a drill string, it should be understood that a wireline drilling and logging system may be used and slickline or other tubing conveyance. The radiation detector as described below may be used with either system.

A drill string43is suspended within the wellbore, also termed borehole41, and has a bottom hole assembly (“BHA”)44which includes a drill bit45at its lower end. The system40further includes a platform and derrick assembly46positioned over the borehole41. The assembly46illustratively includes a rotary table47, kelly48, hook50and rotary swivel51. The drill string43in this example may be rotated by the rotary table47, which engages the kelly48at the upper end of the drill string. The drill string43is illustratively suspended from the hook50, which is attached to a traveling block (not shown). The kelly48and the rotary swivel51permits rotation of the drill string relative to the hook. A top drive system (not shown) may also be used to rotate and axially move the drill string43, for example.

In the present example, the system40may further include drilling fluid or mud52stored in a pit53formed at the well site (or a tank) for such purpose. A pump54delivers the drilling fluid52to the interior of the drill string43via a port in the swivel51, causing the drilling fluid to flow downwardly through the drill string as indicated by the directional arrow55. The drilling fluid exits the drill string43via ports or nozzles (not shown) in the drill bit45, and then circulates upwardly through an annular space (“annulus”) between the outside of the drill string and the wall of the borehole, as indicated by the directional arrows56. The drilling fluid lubricates the drill bit45and carries formation cuttings up to the surface as it is cleaned and returned to the pit53for recirculation.

The BHA44of the illustrated embodiment may include a logging-while-drilling (“LWD”) module57, a measuring-while-drilling (“MWD”) module58, a rotary steerable directional drilling system and motor60, and the drill bit45. These modules are part of downhole tubulars formed from respective housings as illustrated.

The LWD module57may be housed in a special type of drill collar, as is known in the art, and may include one or more types of well-logging instruments, including example radiation detectors. It will also be understood that optional LWD and/or MWD modules may also be used in some embodiments, including a well-logging tool indicated generally at58that includes a tool housing70used to contain the photomultiplier76. (References, throughout, to a module at the position of57may mean a module at the position of61as well). The LWD module57may include capabilities for measuring, processing, and storing information, as well as for communicating the information with the surface equipment, e.g., to a logging and control unit62, which may include a computer and/or other processors for decoding information transmitted from the MWD and LWD modules57,58and recording and calculating parameters therefrom. The information provided by the MWD and LWD modules57,58may be provided to a processor64(which may be off site, or in some embodiments may be on-site as part of the logging and control unit62, etc.) for determining volumetric and other information regarding constituents within the geological formation42and process sensor data collected from sensors located in different modules.

A wireline cable may be used instead that includes a standard cable head connected at its lower end to a logging tool with a wireline cable extending to the surface of the borehole. During a logging operation, data may be transmitted from the logging tool to the wireline cable through the cable head and into the logging and control system62such as shown inFIG. 1. The downhole tubular may include one or more pressure bulkheads that enclose a protected area as an enclosure for a module and contain the electronic devices such as the photomultiplier in accordance with a non-limiting example and connected to the scintillator74and photomultiplier76to form a radiation detector, including sensors for downhole logging and processors and other electronics. The bulkhead may form a pressure housing as part of the downhole tubular.

FIG. 2shows a portion of a well-logging tool68to be positioned in a wellbore of a subterranean formation and showing a tool housing70that may be part of a LWD and/or MWD module as described above and used for well-logging and other applications. A scintillator74is optically coupled to a photomultiplier76. A scintillation crystal (SC)80detects radiation and generates light pulses that are collected by the photomultiplier76formed as a tube in this example. The scintillator74and photomultiplier76are optically coupled together to form a scintillator package82. Radiation interacts with the scintillation crystal80and emits photons in the visible or near visible region of the electromagnetic spectrum. The scintillator crystal80may be formed from organic crystals, inorganic crystals or plastic phosphors. The scintillator74and photomultiplier76as a package82are held together using a separate mechanical support84that could be formed as a cylindrical sleeve or a larger housing enclosing both the scintillator74and photomultiplier76. In one example, the photomultiplier76is formed by a vacuum tube with a glass envelope containing a photocathode86and a series of electrodes called dynodes88. In another example, the vacuum tube is composed of a brazed ceramic and metal structure.

An optical window90couples the scintillator with the photomultiplier (FIG. 2). Light from the scintillator crystal80liberates electrons from the photocathode86by the photoelectric effect. In the photomultiplier, the electrons are attracted by a voltage drop to the nearest dynode shown at88ato which the photoelectrons strike and liberate new electrons for each photoelectron. These subsequently released electrons are attracted to a second dynode shown at88bwhere a larger third-generation group of electrons is emitted. This continues through a number of stages within this photomultiplier inFIG. 2that shows successive stages of the dynodes. At the final dynode88n, sufficient electrons produce a pulse of sufficient magnitude for further amplification and electronic processing.

The photomultiplier76may be formed from an evacuated glass housing with a high vacuum to house the photocathode86and the multiple dynodes88. The photons from the scintillator crystal80strike the photocathode86, which may be formed separate as shown inFIG. 2or as a thin deposit on the optical window90producing electrons that strike the first dynode. Each dynode is at a more positive voltage than the previous dynode. As electrons move towards the first dynode88a, they are accelerated by the electric field and arrive with greater energy at the next, subsequent dynode. When the electrons strike the first dynode88a, lower energy electrons are emitted, which in turn are accelerated towards the second dynode88b. The serial arrangement of the dynodes allows cascading of an ever-increasing number of electrons at each stage. The accumulated charge results in a sharp current pulse output from the photomultiplier that can be electronically processed. In one example, the optical window90may be braised or mechanically fastened to a flange on the mount84formed as a sleeve or larger housing. The scintillator74may be inserted through one end of the sleeve84or housing. The scintillator crystal80may be formed from a hydroscopic crystal composition as known to those skilled in the art. This scintillator74is not limited to the use of hydroscopic scintillator crystals, but non-hydroscopic scintillator crystals may be used.

As explained in greater detail with reference toFIG. 5below, an insulator is positioned between adjacent pairs of dynodes. This is also known as a washer/insulator dynode structure because the insulator is formed similar to a washer or ring. A grid associated with each dynode increases the electric field to enhance collection of secondary electrons from the previous dynode and reduce a potential barrier on the dynode to which it is associated. Each multiplication stage in the photomultiplier76includes a dynode and a grid with the dynode providing the multiplication through the secondary electron emission and the grid providing a low electric field region upstream of the dynode. The grid from the next stage provides a high electric field region downstream of the dynode. These low and high electric fields on each side of a dynode provide the extracting force for the secondary electrons emitted at the dynode surface so that they may leave the dynode and reach the next dynode stage.

As noted before, the linear structure of the dynodes allows cascading with an ever-increasing number of electrons produced at each multiplication stage. This linear structure as shown inFIG. 2has a structure that is rugged and appropriate to a well-logging environment. This type of design intercepts about 80% of the incident photoelectrons and the intercepted electrons produce secondary electrons that are inefficiently collected by the subsequent dynode because the dynode intercepts 80% of the secondary electrons in dynode geometry.

A dynode structure in accordance with a non-limiting example is shown in the photomultiplier100ofFIG. 5and forces secondary electrons to move back and forth radially from one dynode to the next.FIGS. 3 and 4show a respective dynode pair that is repeated in the photomultiplexer.FIG. 3shows an example of the odd-numbered dynodes103starting with the first dynode that intercepts the electrons. The even-numbered dynodes120starting with the second dynode are shown inFIG. 4. This first dynode103of each dynode pair includes a conductive outer ring105and a medial conductive member107coupled to the conductive outer ring105and in spaced relation therefrom. This first dynode103includes at least one conductive radial support109coupled between the medial conductive member and an adjacent portion of the conductive outer ring105. Three radial supports109are illustrated in the example ofFIG. 3. The medial conductive member107in this example has a conical shape.

The second dynode120of each dynode pair is formed as a conductive outer ring122and a conductive inner ring124supported within the conductive outer ring122. This second dynode120of each dynode pair as shown inFIG. 4includes a radially outermost sloped ring segment126extending in a first longitudinal direction, a radially innermost sloped ring segment128extending in a second longitudinal direction, and an intermediate flat ring segment130extending between the radially outermost and radially inner most ring segments.

As shown inFIG. 5, a plurality of the odd numbered ones of the first dynode103are alternated with a plurality of even-numbered ones of the second dynode120. Although the base material of dynodes103and120is conductive, in practice, the surface of these dynodes is coated with a high secondary emissive material, such as BeO. Examples of the base material include copper beryllium alloy or nickel beryllium alloy. The surface of these may be oxidized to form a BeO secondary emissive layer. An insulator140as a washer or ring is positioned between adjacent pairs of dynodes. A conductive grid142is between adjacent ones of the plurality of dynodes as shown inFIG. 5. This configuration is also schematically illustrated in the graph ofFIG. 6, showing the photocathode102at zero volts, the insulator140followed by a grid142and followed by a first dynode103(FIG. 3), a second grid142and the second dynode120(FIG. 4). The grid material may be, for example, nickel or stainless steel mesh that is formed to the shape shown.

The graph ofFIG. 6shows the trajectories for photoelectrons with an energy of 0.6 eV emission energy. Secondary electrons with 0.1 eV energy are shown at the graphs ofFIGS. 7 and 8with the secondary electrons from the first dynode103shown in the graph ofFIG. 7and the secondary electrons from the second dynode120shown in the graph ofFIG. 8. The structure is symmetrical azimuthally along its axis and plotted in the r-z space. With these energies of the electrons, the photoelectrons and secondary electrons are intercepted by the appropriate dynode. The pattern shown for the first and second dynodes103,120is repeated to construct as many stages of gain as desired.

The radial supports109configured as “arms” in the first dynode103ofFIG. 3do not intercept the secondary electrons and do not substantially reduce the collection efficiency because the secondary electrons are accelerated away from the arms, which are at the same potential as the surface from which the electrons are leaving towards the grid of the next dynode, which is at a positive potential with respect to its emitting surface.

The energy of the photoelectrons could be less than 1 eV and the electron trajectories for this energy from the photocathode are shown inFIG. 9. With the exception of a few photoelectrons emitted at divergent angles near the edge of the cathode102, trajectories are intercepted by the first dynode103. Secondary electrons from the dynodes, however, have a wide range of energies and the secondary electrons to be directed to the next dynode, or for the higher secondary electron energies, to be reflected back to the dynode surface enable the generation of additional secondary electrons. The graph inFIG. 10shows the trajectories of 10 eV secondary electrons from the first dynode103. With a few exceptions, the trajectories are either directed onto the second dynode120or reflected back to the first dynode103for re-emission. A similar effect is shown with the 100 eV secondary electrons from the first dynode103as shown in the graph ofFIG. 11.

The secondary electron trajectories for the 10 eV and 100 eV emission energies from the second dynode120are shown in respective graphs ofFIGS. 12 and 13. Relatively few of these trajectories reach subsequent dynodes and instead many of the trajectories are reflected back to the second dynode120where they have an additional chance to create low energy secondary electrons. As compared to a standard venetian blind dynode structure, the photomultiplier100shown inFIG. 5has excellent single photoelectron resolution and high detection efficiency due to the efficient use of secondary electrons of various energies. It is a symmetrical photomultiplier100and will not have the azimuthal gain variation present in the venetian blind structures. This photomultiplier100provides excellent gamma-ray resolution compared to a venetian blind structure and preserves the alternating washer/insulator dynode structure that is rugged and appropriate to the well-logging environment. In this type of dynode structure, the insulator140is configured and similar to a ring or “washer”), thus referring to the washer/insulator structure.