FR2720516A1 - Radiation detector for well logging appts. - Google Patents

Abstract

Appts. comprises: (a) a mechanism for detecting radiation including at least one dynode; and (b) a mechanism for electrically isolating the detecting mechanism from an electrical biasing mechanism which acts upon it and for physically surrounding the detecting mechanism. Also claimed are a further appts. and a process for detecting radiation.

Description

METHOD AND APPARATUS FOR DETECTING RADIATION
DESCRIPTION
The invention relates to a photomultiplier tube. More specifically, the invention relates to a photomultiplier tube fitted into an electrical shield and an electrical supply.

 Photomultiplier tubes are well known in themselves. Basically, a PMT is a tube that emits electrons when exposed to light or other radiation. A typical PMT includes a photocathode at one end of the tube, acceleration and focusing electrodes, a series of dynodes and an anode. The photocathode at one end of the tube includes a photo-emissive material which ejects electrons in response to photons hitting the material. An associated power supply polarizes the acceleration and focusing electrodes more positively than the photocathode to accelerate the electrons moving away from the photo-emissive material of the cathode, axially through the tube. The power supply also polarizes the series of dynodes, one more positively than the previous, to attract accelerated electrons.

Each electron colliding with an attractive dynode, the electron causes the dynode to eject one or more secondary electrons. Secondary electrons, in turn, are attracted to and collide with the next dynode, ejecting more electrons, creating a cascade effect. The cascade of electrons continues through the center of the tube towards the anode. The anode collects electrons at the other end of the tube and produces a signal indicating the amount of light or other radiation to which the photo-emissive material of the cathode has been exposed. The multiplier effect of a PMT is evident from this example: a single electron striking the first dynode can create a cascade of several thousand or even millions of secondary electrons at the anode. An example of PMT is described in U.S. Patent No. 4,639,638 to Purcell et al., For a
Photomultiplier Dynode Coating Materials and Process, deposited on January 27, 1987.

 Figure la is a schematic diagram of a photomultiplier tube (PMT) 10 having a conventional type of power supply.

The power supply 12 which polarizes the components of the PMT 10 may include an alternating current source 14 and a Cockcroft-Walton circuit 16. This circuit 16 is known in itself. The CW circuit 16 includes discrete elements, such as diodes 18 and capacitors 20, which are wired in a ladder circuit. A first stage of the CW 16 circuit multiplies the voltage of the voltage source 14. The successive stages of the CW 16 circuit multiply the voltage of the previous stage. Separate stages of the scale comprising the CW circuit 16 are connected to successive dynodes 22 of the PMT 10, for example, to polarize one dynode more positively than another in the direction of its anode 23. An example of a Cockcroft circuit Walton is described in the patent of
United States of America No. 5,191,517 assigned to
Stephenson, for Electrostatic Particle Accelerator
Having Linear Axial and Radial Fields, filed on 2
March 1993.

Figure 1b is a schematic diagram of a photomultiplier tube (PMT) having a conventional control circuit. In this case, the power supply includes a voltage source 25 and a chain of stabilizing resistors 24 (bleeder, in English) dividing the voltage. A chain of stabilizing resistors 24 comprising a series of resistors 26 connects to the
PMT 10. Each resistor 26 of the chain of stabilizing resistors 24 is connected to bias an adjacent accelerator stage of the PMT 10.

 Typically, the power supply, such as a CW circuit 16, is assembled to the PMT 10 in a linear arrangement. Specifically, one end of the ladder comprising the circuit CW 16 is fixed to the anode-end of a PMT so that the longitudinal axes of the circuit CW 16 and of the PMT 10 are aligned. This arrangement results in a rather narrow PMT assembly. However, this arrangement is relatively long. The resulting length takes up valuable space in a well logging tool, for example.

 Many companies market PMT photomultiplier tubes with miniature high-voltage bases, but, however, these tubes are not designed for the harsh environment conditions of logging in oil wells. This is because the high voltage power supply which controls the PMT is connected to the PMT using a multi-perforated arrangement of caps and contacts, or pins and bars, which is fragile at best. These high-voltage devices which are marketed use a linear end-to-end Cockcroft-Walton (CW) scale which delivers voltages to each stage of the PMT.

One application of a PMT is well logging. Typically, PMT and a radiation source are carried on the cradle of a logging tool. The high voltage power supplies, carried by the tool, are connected to the
PMT on the cradle. The PMTs are mounted on the cradle to be closer to the radiation source, which in turn produces better statistics and faster borehole logging. The power supplies are mounted in the tool itself to minimize the length of the cradle, which in turn reduces the movement of the cradle relative to the mud wall of the borehole during deviations or removal. After the logging tool has been lowered into a borehole, the cradle is extended from the tool to penetrate the mud wall of the borehole. The radiation source generates radiation energy in the earth formation surrounding the borehole, and the PMT detects return radiation when the logging tool is hoisted into the borehole. PMT produces an output signal in response to detected radiation, which is interpreted to indicate the characteristics of the earth formation.

The microchannel wafer detector (MCP) is used for the same general purposes as PMT, i.e. for the detection of optical radiation. However, MCP and PMT have different operating parameters and different applications. For example, MCP is limited in count rate relative to PMT, which is not acceptable in well logging applications. The MCP requires only 3 high voltage outputs. One tension is for the MCP pancakes. The wafers provide a fairly large current, about 30 micro-amps at 3-4 kV, typically using a Cockcroft scale
Walton (CW) three stories to get there. A second voltage polarizes the anode or the phosphor of the
MCP. If it is an anode, only a few hundred volts are required. If it is a phosphor, a few thousand volts (sometimes 5 kV) may usually be required with a three-stage CW scale. Typically, the energy required for this power is reduced. A third voltage polarizes the cathode of the MCP. This voltage is only intended to provide a few hundred volts to supplement the power of the pancakes. Typically, this power supply is a simple rectifier. The energy required for the third supply is even less than that required by the second.

 According to one aspect of the present invention, there is provided an apparatus comprising: a) means for detecting radiation having at least one dynode, b) means for electrically polarizing the means (a) for detecting radiation, and for substantially surrounding physically the means (a) for detecting radiation; and c) means for electrically isolating the means (a) for detecting radiation means (b) for electrically biasing; and to substantially physically surround the means (a) for detecting radiation.

 One embodiment of the invention relates to an apparatus comprising a structure and a multiplier circuit mounted on the structure. A voltage supply is connected to produce a first voltage to the multiplier circuit so that the multiplier circuit multiplies the first voltage to a second voltage. The apparatus also includes a discrete dynode vacuum tube (DVT) having a photocathode which emits electrons.

The DVT also has at least one dynode connected to the multiplier circuit so that the dynode is polarized to attract electrons from the photocathode. In addition, an electrical shield substantially surrounds the DVT so that the DVT is housed in the shield. The structure substantially surrounds the shield so that the shield is fitted into the structure. The shield electrically isolates the DVT from the multiplier circuit.

 The apparatus according to the invention finds a particular application in well logging tools.

 Another aspect of the invention provides a method for detecting radiation, the steps consisting in: a) supplying a first voltage from an electrical supply to a multiplier circuit mounted on a structure; b) multiplying the first voltage by a second voltage with the multiplier circuit; c) emitting electrons from a photocathode of a discrete dynode vacuum tube (DVT) in response to detected radiation, the DVT having at least one dynode connected to the multiplier circuit; d) polarizing the dynode with the second voltage of the multiplier circuit to attract the electrons coming from the photocathode; e) substantially surround the DVT with an electrical shield and install the DVT in the shield; f) substantially surround the shielding by the structure and install the shielding in the structure; g) electrically shield the DVT of the multiplying circuit thanks to the shielding; and h) generate a signal at the DVT anode to indicate the radiation detected.

 The technique of this invention provides low power consumption, high current output linearity, and reduced size for the power supply itself. These Benefits Benefit Well Logging and Other Applications by Producing Higher Count Rate Borehole Reports, Shorter Gap Between Detectors, Elimination of High Voltage Wiring in the Logging Tool and Tools shorter. Using the invention in a logging application results in a faster and more accurate survey report.

The invention will now be described by way of examples and with reference to the accompanying drawings, in which
Figures la and lb are schematic diagrams of a photomultiplier tube (PMT) having a conventional power supply;
Figures 2a and 2b illustrate side and end views of the enveloping PMT of this invention;
FIG. 3 illustrates the collapse of the voltage of the polarization network made up of the Cockcroft-Walton power supply and of the chain of stabilizing resistors; and
FIG. 4 illustrates the anode output as a function of the light input of the PMT of this invention and for a conventional PMT having a chain of stabilizing resistors.

An embodiment of this invention relates to a high voltage power supply
Cockcroft-Walton miniature envelope, which is connected by cable to a photomultiplier tube. The preferred embodiment of this invention provides 14 exits with a 14-story Cockcroft-Walton ladder.

The energy required for each output increases in the direction of the last dynode where the maximum energy is required. When there is no light coming into the detector, the power supply of this invention provides no current to the sensor, which provides energy savings. Conversely, powering the MCP (or similarly powered PMT) still requires powering its chain of stabilizing resistors, even when no light arrives at its detector.

 To produce the smallest possible detector, the invention uses the space available not only behind the PMT but also around the body of the PMT. The arrangement of this invention has an obvious advantage over the high voltage power supplies linearly arranged and marketed for PMT. The arrangement of this invention places the high voltage ladder around the body of the PMT in an enveloping configuration, and effectively utilizes the radial space.

 Figures 2a and 2b illustrate side and end views of a discrete and enveloping dynode vacuum tube, such as a PMT 30 of this invention. The PMT 30 includes a screen of photo-emissive material 32. In response to radiation, such as photons, striking the photo-emissive material 32, the material ejects electrons. The PMT 30 includes a cathode and focusing elements, polarized to guide the ejected electrons. The electrons are then attracted from the photo-emissive material to the dynodes 34 of the PMT accelerator 36.

Each dynode 34 of the accelerator 36 is connected through a corresponding conductor 38 to a corresponding stage of an electrical supply 40, described below. In this way, each dynode 34 is polarized successively with a voltage going crescendo to attract the electrons in an avalanche effect. An anode 42 collects the electrons and produces an output signal to the conductor of the anode 44. This output signal indicates the level of radiation detected by the PMT 30.

 However, the close proximity resulting from these electrically noisy elements vis-à-vis the anode 42 of the PMT 30, reveals a problem. Specifically, the anode 42 is subjected to the pickup of electrical noise from an oscillator and to the cutting of the field effect transistors (FET) (not shown) from the power supply 40. To solve this problem, a conductive chassis 46 is located between the PMT 30 and these high voltage power supply elements. The chassis 46 serves as electrical shielding to isolate the PMT, as a mold to support a high voltage power supply card and as a structural housing supporting the associated high voltage control elements. The chassis 46 may include aluminum, brass, or bronze, for example, or any other conductive material.

The generally cylindrical conductive frame 46 substantially surrounds the PMT 30. The frame 46 is open at one end 48 and closed at the other end 50. The end 50 comprises a disc of conductive material which is force-fitted into a tube of this type. material and welded in place to form the conductive frame 46. This shape allows the
PMT 30 to be fitted into the conductive frame 46, once the PMT 30 has been introduced into the open end 48 of the conductive frame 46.

 The conductive frame 46 comprises two flanges 52, 54 formed from the tube of conductive material. The flanges 52, 54 define a compartment for housing the electrical supply 40.

The conductive frame 46 includes an axial slot 56 which extends along the longitudinal direction of the frame 46 in its circumferential surface. The slot 56 extends longitudinally along the chassis 46 from the position of the first dynode 34a to the last dynode 34n. The interior and exterior of the conductive chassis are coated with polyimide 46a to prevent an arc from forming between the elements and the chassis. The polyimide is marketed by DuPont under the brand name "Kapton".

 Each dynode 34 is typically welded or brazed by joints across a copper or beryllium ring comprising a stage of the accelerator 36. When each electron strikes the dynode 34, more electrons are released by this dynode.

This results in a cascade process that multiplies the number of electrons in the PMT. The conductors 38 extend from their corresponding dynode 34 through the slot 56 of the conductive frame 46, and are connected to the stages of the power supply 40.

 The power supply 40 includes a transformer, an oscillator, a CW scale (not shown). The high voltage transformer, oscillator and scale are electrically similar to those of a linearly arranged PMT.

However, the oscillator and the transformer are mounted in the compartment of the conductive chassis 46, between the flanges 52, 54 of this chassis. The CW ladder is also mounted in the compartment between the flanges 52, 54 of this chassis. However, the CW ladder is mounted on a circuit board which curves to conform and mount inside the outer circumference 58 of the conductive chassis 46.

See figure 2b. In this way, the power supply 40 substantially surrounds the conductive frame 46. The CW scale, in this embodiment, is wrapped annularly around the PMT 30.

 The conductive chassis 46 also includes a section where the components of a preamplifier 60 are housed. The components of the preamplifier 60 are mounted on one side of the circular control card 62, for example. The components of the preamplifier 60 amplify the signal coming from the anode 42, representing the level of the detections detected by the PMT 30. The preamplifier is typical of a PMT although its assembly is unique. The preamplifier collects the electrons from the anode, amplifies them and generates a signal indicating the photons detected.

The control components 64 are mounted on the side opposite to the control card 62, and inside a recess of the conductive chassis 46.
Low voltage control components 64 are typical for controlling a high voltage power supply for use with a PMT, although their arrangement is unique for this invention. These low voltage control components 64 keep the voltage levels of the power supply 40, for example, constant.

 The circular control card 62 is held in place in the conductive chassis by a threaded retaining ring, not shown. The retaining ring is held on the chassis 46 by complementary threads located on the interior of the conductive chassis, for example. The connection between the control card 62 and the oscillator of the power supply 40 is made through an orifice 63 lower by 1/8 in diameter between the two compartments (see FIG. 2b). The resulting housing has an outside diameter of less than 25.4 mm (1 inch), if necessary.

 Figure 2b shows an end view of the conductive frame 46. The frame 46 is seen from the right end of Figure 2a. As this appears, the longitudinal axis of the conductive chassis 46 is not necessarily coaxial with the longitudinal axis of the PMT 30. In the preferred embodiment, the section of the conductive chassis 46 intended to receive the PMT 30 is offset towards the slot 56 of the conductive chassis 46, relative to the recess intended to receive the control components 64.

 FIG. 3 illustrates the collapse of the voltage of the polarization network of the Cockcroft-Walton power supply and of the chain of stabilizing resistors. To illustrate the operational advantage of this type of high voltage power supply compared to a conventional single output power supply having a chain of stabilizing resistors, a linearity test was carried out. A stabilizing resistor chain is a series of resistors, each connected to polarize the adjacent stages of an accelerator. Figure 3 shows that the conventional CW power supply with a 55.6 M-ohm stabilizing resistor chain, despite the fact that the primary high voltage has not varied, suffers from a voltage drop in the few last stages of dynode. This is due to the fact that the chain of stabilizing resistors cannot supply the required current to each of the lower stages, the last stages drawing higher currents. The result is that the voltage between each stage is redistributed between all the stages of the PMT, which significantly modifies the gain of the device. The voltage is redistributed between the other stages to compensate for the lowering of the voltage on the last stages. Thus, when each stage of PMT is efficiently controlled by its own separate power supply as in the case of the enveloping power supply of this invention, none of the stages suffers from a voltage drop. As a result, the device remains linear over a much larger interval of anode current.

This can be seen in Figure 4.

 FIG. 4 illustrates an anode output as a function of a light input of the PMT of this invention and of a conventional PMT having a chain of stabilizing resistors. As shown from Figure 4, the output signal from the stabilizing resistor chain loses linearity above 104 nA. The output signal of this invention continues linearly well beyond 105 nA.

 Due to the high voltage power supply 40 located near the anode 42 of the PMT 30, extreme care should be taken to minimize the hum of the last two stages of the power supply 40. The hum comes from the AC component of the DC signal from the power supply, and is caused by the components used in the device. The humming condition of the power supply must be strongly observed, especially near the anode. This is because the output near the PMT anode is capacitively coupled to the anode. Any excessive humming on these lines would alter the output signal.

The anode may be a last solid dynode collecting the electrons.

 Reducing the purring effect can be resolved in a number of ways. The easiest way is to use large capacitors or an RC filter in the last two stages of the CW power supply. Another method consists in injecting a complementary signal into the anode which is out of phase with respect to the hum of the power supply, canceling its effects. This can be accomplished using the same signal, but offset by 1800. The anode conductor is terminated in 50 ohms to match the preamplifier input termination of the detector preamplifier.

Claims (19)

 1. Apparatus characterized in that it comprises  a) means for detecting radiation having at least one dynode  b) means for electrically polarizing the means (a) for detecting radiation and for substantially physically surrounding the means (a) for detecting radiation; and  c) means for electrically isolating the means (a) for detecting radiation from the means (b) for electrically polarizing; and to substantially physically surround the means (a) for detecting radiation.  2. Apparatus according to claim 1, characterized in that the means (c) for electrically isolating are physically located between the means (a) for detecting radiation and the means of (b) for electrically polarizing.  3. Apparatus according to claim 1 or 2, characterized in that the means (a) for detecting radiation are fitted into the means (c) for electrically isolating; and the means (c) for electrically isolating are fitted into the means (b) for electrically polarizing.  4. Apparatus according to claim 1, 2 or 3, characterized in that the means (a) for detecting radiation comprises a vacuum tube with discrete dynodes, and the means (b) for electrically polarizing comprise an electrical supply circuit .  5. Apparatus according to claim 4, characterized in that the means (c) for electrically insulating comprise a conductive frame (46) in which the vacuum tube (30) with discrete dynodes (34) is re-fitted, and in that the power supply circuit (40) wraps around the conductive frame (46).  6. Apparatus according to claim 5, characterized in that the conductive frame (46) is substantially tubular and the power supply circuit (40) is wound annularly around the conductive frame (46).  7. Apparatus according to claim 6, characterized in that the vacuum tube (30) comprises a plurality of dynodes (34), the dynodes having corresponding conductors (38) extending from the vacuum tube (30), the conductive frame (46) having a longitudinal slot (56) through which the conductors (38) extend, the power supply circuit (40) comprising a ladder circuit having stages which connect to the extended conductors (38 ) dynodes (34).  8. Apparatus according to claim 6 or 7, characterized in that the conductive frame (46) has an open end (48) for accepting the vacuum tube (30) and a closed end (50), the apparatus comprising a circuit control device mounted at the closed end (50) of the conductive frame (46), characterized in that the conductive frame (46) comprises means for electrically isolating the vacuum tube (30) from the control circuit.  9. Apparatus according to claim 8, characterized in that the vacuum tube (30) comprises a photomultiplier tube and the conductive frame (46) has a polyamide coating.  10. Apparatus according to any one of the preceding claims, characterized in that it comprises  a structure ;  a multiplier circuit mounted on the structure;  a voltage supply connected to produce a first voltage to the multiplier circuit such that the multiplier circuit multiplies the first voltage by a second voltage;  a vacuum tube (30) with discrete dynodes (34) (DVT) having a photocathode which emits electrons and having at least one dynode (34) connected to the multiplier circuit so that the dynode (34) is polarized to attract the electrons from the photocathode; and  an electrical shield substantially surrounding the DVT in such a way that the DVT is fitted into the shield  wherein the structure substantially surrounds the shield in such a way that the shield is fitted into the structure, and the shield electrically isolates the DVT from the multiplier circuit.  11. Apparatus according to claim 10, characterized in that the multiplier circuit is wound annularly around the shield.  12. Apparatus according to claim 10 or 11, characterized in that the vacuum tube (30) comprises a photomultiplier tube and the shield comprises a conductive frame (46) having a polyimide coating.  13. Apparatus (30) according to. any one of the preceding claims, forming part of a well logging tool.  14. Method for detecting radiation, characterized in that it comprises the steps consisting in:  a) applying a first voltage from an electrical supply to a multiplier circuit mounted on a structure  b) multiplying the first voltage by a second voltage with the multiplier circuit;  c) emitting electrons from a photocathode of a vacuum tube (30) with discrete dynodes (34) (DVT) in response to the detected radiation, the DVT having at least one dynode (34) connected to the multiplier circuit  d) polarizing the dynode (34) with the second voltage of the multiplier circuit to attract the electrons coming from the photocathode;  e) substantially surround the DVT with an electrical shield and install the DVT in the shield;  f) substantially surrounding the shielding by the structure and fitting the shielding into the structure;  g) electrically shield the DVT of the multiplier circuit using the shielding; and  h) generate a signal at the DVT anode to indicate the radiation detected.  15. Method according to claim 14, characterized in that  step (e) consists in surrounding the DVT with a tubular electrical shielding; and  step (f) consists in annularly wrapping the multiplier circuit mounted on the structure around the electrical shielding.  16. The method of claim 14 or 15, characterized in that the DVT has a plurality of dynodes (34) and the multiplier circuit comprises a ladder circuit, the method being further characterized in that it comprises  the electrical connection of the plurality of dynodes (34) through a slot (56) in the electrical shield to the ladder circuit.  17. The method of claim 14, 15 or 16, characterized in that the electrical shielding having open and closed openings, said method further comprises  mounting the control circuit at the closed end of the electrical shielding; and  the electrical isolation of the DVT from the control circuit with the closed end.  18. Method according to any one of claims 14 to 17, characterized in that the DVT of step (c) comprises a photomultiplier tube and the electrical shielding is a conductive frame (46) having a polyimide coating.  19. Method according to any one of claims 14 to 18, - characterized in that it is used in well logging.