Abstract:
A radiation detector assembly comprises a radiation scintillator detector for generating a light signal as a function of radiation detected. A light detector is operatively connectable with the radiation scintillator detector for receiving a light signal from the radiation scintillation detector and generating an electrical signal as a function of the light signal received. A housing for the light detector is electrically connectable with the light detector. At least one of the housing and the light detector is electrically connectable with a pole of a power supply whereby the housing and the light detector are at substantially the same electrical potential when electrically connected.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates generally to radiation detectors. In particular, the invention relates to radiation scintillator detectors. 
     Radiation scintillator detectors are known in the well drilling industry for logging and measure while drilling (MWD) applications. Radiation scintillator detectors may also be used as security portal detectors or in medical applications. When the radiation scintillator detector is incorporated into a logging tool of a tool string used for the drilling of oil, gas and water wells, the logging tool identifies, locates and differentiates geologic formations along a well bore. Tool strings and logging tools for oil wells are often exposed to harsh operating environments including temperatures that can reach 185° C. and pressures up to 20,000 psi and can be exposed to severe shock and vibration. 
     A typical radiation detector assembly includes a scintillator coupled to a light detecting element such as a photomultiplier tube. Radiation, such as gamma rays emitted by geologic formations, is converted to light by the scintillator and conducted to the photomultiplier tube. The photomultiplier tube converts the light into electrons and produces an amplified electrical signal. The amplified electrical signal is then measured and used by monitoring electronics. It is desirable to have the amplified electrical signal produced by the photomultiplier tube, in the absence of noise, to be directly proportional to the gamma rays interacting in the scintillator that are converted to light. 
     Any “noise” component in the amplified electrical signal could lead to a misrepresentation of the gamma rays reacting in the scintillator. “Noise” or dark current, in the form of an electrons produced in the photomultiplier tube, can be created by thermal activity and not by a photoelectric effect. These electrons are known as thermionic electrons. The amplified electrical signal produced in the photomultiplier tube that include thermionic electrons, could misrepresent or distort the signal created by gamma rays deposited in, reacting or interacting with the scintillator. Operating the photomultiplier tube at high temperatures increases the emission thermionic electrons and noise levels. For certain applications, such as in downhole drilling, an accurate measure of a geologic formation&#39;s background radiation is required and spurious electron production from “noise” can affect the performance of the radiation detector assembly. It is, therefore, advantageous to eliminate or minimize the “noise” component in the radiation detector assembly. 
     BRIEF DESCRIPTION OF THE INVENTION 
     One aspect of the invention is a radiation detector assembly that produces less “noise” than previously known. The radiation detector assembly comprises a radiation scintillator detector for generating a light signal as a function of radiation detected. A light detector is operatively connectable with the radiation scintillator detector for receiving a light signal from the radiation scintillation detector and generating an electrical signal as a function of the light signal received. A housing for the light detector is electrically connectable with the light detector. At least one of the housing and the light detector is electrically connectable with a pole of a power supply whereby the housing and the light detector are at substantially the same electrical potential when electrically connected. 
     Another aspect of the invention is a radiation detector assembly comprising a crystal assembly for generating a signal indicative of a scintillation event. A photomultiplier tube assembly is operably and electrically connectable with the crystal assembly. The photomultiplier tube assembly is for receiving the signal from the crystal assembly and generating an electrical signal as a function of the signal received. The photomultiplier tube assembly comprises a housing for supporting a photomultiplier tube. The housing is electrically connectable with the photomultiplier tube. The housing and the photomultiplier tube are at substantially the same electrical potential when electrically connected to minimize the production of thermionic electrons in the photomultiplier tube. 
     Yet another aspect of the invention is a photomultiplier tube assembly comprising a photomultiplier tube for receiving a light signal from a source. The photomultiplier tube generates an electrical signal as a function of the light signal received. A housing supports the photomultiplier tube. The housing is electrically connectable with the photomultiplier tube. At least one of the housing and the photomultiplier tube is electrically connectable with a pole of a power supply. The housing and photomultiplier tube are at substantially the same electrical potential when electrically connected. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of the invention will be better understood when the following detailed description is read with reference to the accompanying drawings, in which: 
         FIG. 1  is a side elevation view illustrating a radiation detector assembly according to one aspect of the invention; 
         FIG. 2  is an exploded perspective view illustrating a portion of the radiation detector assembly shown in  FIG. 1 ; 
         FIG. 3  is an enlarged cross-sectional view of the radiation detector assembly illustrated in  FIG. 1 ; and 
         FIG. 4  is a view of an end portion of the radiation detector assembly, taken approximately along line  4 - 4  of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A radiation detector assembly  20  according to one aspect is illustrated in  FIGS. 1-3 . The radiation detector assembly  20  may be used for detecting and measuring levels or energies of gamma radiation from various sources and in various applications. The radiation detector assembly  20  includes two major operating devices, a radiation scintillator detector assembly  22  and a photomultiplier tube assembly  24 . The radiation detector assembly  20  also includes an outer housing  26  to protect the radiation scintillator detector assembly  22  and the photomultiplier tube assembly  24 . 
     The radiation scintillator detector  22  has a crystal (not shown) for generating a signal indicative of a scintillation event, such as when radiation of a certain level or energy is detected. For example, radiation, such as gamma rays, is converted to light by the crystal scintillator of the radiation scintillator detector  22  as a function of the radiation detected. The radiation scintillator detector  22  may include other devices capable of scintillation from radiation. For example, the crystal may be a cylindrical sodium iodide crystal doped with thallium (NaI(Tl)). Also by way of example, the crystal may have a diameter of one inch and may be up to five inches in length. 
     The crystal of the radiation scintillator detector  22  generates a light signal as a function of radiation detected by some of the radiation interacting with the crystal, as is known. For example, the light signal is generated as a function of the presence and amount of gamma radiation delivered to the radiation scintillator detector  22 . The radiation scintillator detector  22  further includes a housing  42  for supporting a crystal. The housing  42  may be made of any suitable material, such as titanium, prepared aluminum or stainless steel. The radiation scintillator detector  22  may also include support structure located between the housing  42  and the crystal. 
     The photomultiplier tube assembly  24  is operably and electrically connected to the crystal of the radiation scintillator detector  22 . The photomultiplier tube assembly  24  receives the light signal from the crystal and generates an electrical signal as a function of the light signal received. The photomultiplier tube assembly  24  includes a photomultiplier tube  40  ( FIG. 1 ), as is known, that includes a photo detector to receive the light signal from the crystal of the radiation scintillator detector  22 . 
     The photomultiplier tube assembly  24  may be any of several known photomultiplier tube assemblies. In the illustrated example, the photomultiplier tube assembly  24  has an outer diameter substantially identical to that of the radiation scintillator detector  22 . 
     The photomultiplier tube assembly  24  includes a housing  44  that supports the photomultiplier tube  40 . The housing  44  may be made of any suitable material, such as titanium, prepared aluminum or stainless steel. The photomultiplier tube assembly  24  may also include support structure  46  ( FIG. 2 ) in the form of spring material. The photomultiplier tube assembly  24  includes a resistive divider to bias the photomultiplier tube (not shown). The photomultiplier tube assembly  24  is attached to the radiation scintillator detector  22  at threaded connection  48  ( FIG. 3 ). 
     An optical window  60 , located between the photomultiplier tube assembly  24  and the radiation scintillator detector  22 , allows light from scintillation formed in the radiation scintillator detector to pass into the photomultiplier tube assembly. Amplified electrical pulses are produced at an anode of the photomultiplier tube  40 . The amplified pulses or electrical signals are then conducted through wires  64  to processing electronics. 
     The photomultiplier tube  40  is protected from the operating environment by the housing  44  that is rigid. The photomultiplier tube assembly  24  includes a cap  62  that is threaded into an axial end of the photomultiplier tube housing  44  to close one end of the housing. The cap  62  may be made of any suitable material, such as titanium, prepared aluminum or stainless steel. The other axial end of the photomultiplier tube housing  44  is threaded into the open end of the radiation scintillator detector  22  at threaded connection  48  to close the other axial end of the housing and the radiation scintillator detector. 
     A spring  82  engages the left axial end, as viewed in  FIG. 3 , of the photomultiplier tube  40  and the right axial interior surface of the cap  62 . The spring  82  is electrically conductive and made from a material such as metal. The spring  82  loads the photomultiplier tube  40  with a known biasing force and provides an electrical connection between the cap  62  and the photomultiplier tube. Potting material, such as RTV, may be placed around the wires  64  extending through the central opening in the cap  62 . Thus, the housing  44 , the photomultiplier tube  40  and the radiation scintillator detector  22  are electrically connected and exposed to the substantially the same electrical potential. The photomultiplier tube assembly  24  may, thus, be provided as a ready to use component or as a replacement part. 
     A second outer cap  100  ( FIG. 3 ) may be attached to the outer housing  26  to enclose and protect the photomultiplier tube assembly  24  and the radiation scintillator detector  22 . Potting material, such as RTV, may be placed around the wires  64  extending through the opening in the cap  100 . The outer cap  100  and the outer housing  26  are electrically insulated from the radiation scintillator detector  22  and the photomultiplier tube assembly  24 . Insulating material  140  is located between the outer housing  26  and the photomultiplier tube housing  44  and the radiation scintillator detector housing  42 . The insulating material  140  may completely encapsulate the photomultiplier tube housing  44  and the radiation scintillator detector housing  42  or may be discrete strips spaced circumferentially about housings. Insulating material  142  is also located between the cap  62  and the cap  100 . The insulating material  142  may also extend to prevent the screw  124  and terminal connector  122  from shorting against the caps  62  and  100 . Insulating material  144  is located between the right, as viewed in  FIG. 1 , axial inner end of the outer housing  26  and the radiation scintillator detector housing  42 . The insulating materials  140 ,  142  and  144  may be any suitable electrically insulating material. 
     Power is supplied to the photomultiplier tube  40  through the wires  64 . Each of the three wires  64  lead to a DC external power source (not shown). The three wires  64  are electrically connected to a ground, negative and positive terminal, respectively, of the power source. A wire end portion  120  is spliced to and extends from a wire  64   n  of the wires  64 . The wire  64   n  is electrically connected to the negative terminal of the power source. Preferably, the negative terminal of the power source is maintained at a negative voltage of at least 100 volts or more. A terminal connector  122  is attached to the wire end portion  120 . The terminal connector  122  is connected to the cap  62  by threaded fastener  124 . The wire  64   a  is electrically connected to the ground terminal of the power source. The wire  64   p  is electrically connected to the positive terminal of the power source. 
     The housing  44  for the photomultiplier tube  40  is electrically connected to the radiation scintillator detector  22  at the threaded connection  48 . Thus, the housing  44 , spring  82 , cap  62 , photomultiplier tube  40  and radiation scintillator detector  22  are electrically connected together and exposed to the substantially the same electrical potential. Preferably, the potential is at least a negative 100 volts. By electrically connecting the photomultiplier tube  40  to the surrounding metallic housing  44 , “noise” created by thermionic electrons is minimized or eliminated. The entire radiation detector assembly  20  will also be at the same polarity and potential as the photomultiplier tube  40 . 
     Thus, one of two components affecting the liberation rate of thermionic electrons in the photomultiplier tube  40 , the electric field, is eliminated or at least minimized. The radiation scintillation detection assembly  22  even operating at relatively high temperatures, such as 185° C., with a negative applied high voltage across components of the photomultiplier tube  40  will produce relatively few thermionic electrons. Without the generation of thermionic electrons the “noise” component in amplified electrical the signal produced in the photomultiplier tube  40  is at least decreased and preferably eliminated to improve the performance of the photomultiplier tube  40  and the radiation detector assembly  20 . The amplified electrical signal produced by the photomultiplier tube  40 , in the absence of “noise”, can then properly reflect the energy of the gamma rays deposited in, reacting or interacting with the radiation scintillator detector  22 . 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention extends beyond the specifically disclosed embodiments to other alternative embodiments and/or uses of the systems, techniques and obvious modifications and equivalents of those disclosed. It is intended that the scope of the invention disclosed should not be limited by the particular disclosed embodiments described above.