Abstract:
A system for measuring ionizing radiation with a scintillator incorporates a first assembly of at least two rod-shaped scintillator elements, which can be optically coupled to one another via their opposed face ends. The first assembly of scintillator elements is coaxially enclosed by a second assembly of interconnectable sheath elements. 
     According to the invention, one scintillator element ( 11 ), one sheath element ( 12 ) and two face-end window elements ( 131, 132 ) form a scintillator module ( 10 ) in each case, which is optically and mechanically connectable to additional scintillator modules. The individual scintillator elements can thus be produced as encapsulated scintillator modules by means of “suitable” sheath elements and window elements. This provides for a simple and reliable manipulation and assembly of the measuring assembly at the site of its application.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a system for measuring ionizing radiation, in particular for radiometric level measurements, with a scintillator incorporating a first assembly of at least two rod-shaped scintillator elements, which can be optically coupled to one another via their opposed face ends, a second assembly of interconnectable sheath elements wherein said first assembly of scintillator elements is coaxially enclosed by said second assembly of interconnectable sheath elements, and at least two face-end window elements. 
         [0002]    Radiometric level measurements are performed primarily by means of detectors with long organic scintillators, which are coupled at one end to a photomultiplier. The sensitive length of such detectors is limited by the manufacturer of the scintillator material and by practical aspects (production, transportation, assembly) to approximately 2 m. If measurement areas &gt;2 m are required in radiometric level measurements, multiple identical detectors are commonly distributed over the measurement area and are switched in series. The counts from the individual detectors are added together and the fill level is determined based on the calibration. One shortcoming of this approach lies in its high costs, as each detector must be provided with its own amplifier electronic units and evaluation units (e.g. photomultiplier, amplifier circuits, digital component, bus connection). Moreover, the detectors must be connected individually to supply and signal lines. 
         [0003]    In the German utility model DE 201 03 881 U1, a rod-type detector is described, in which the sensitive part consists of a plurality of scintillator rods that are placed on top of each other and are optically coupled to each other, and are read out at one end by a single photomultiplier. The advantage of such a detector lies, on the one hand, in the noticeable cost reduction by economies in measuring electronics and wiring and, on the other hand, in a lesser likelihood of a breakdown of the entire measuring system (FMEA number) due to a reduced number of components. In this manner large measurement areas can be covered while at the same time providing for a high degree of sensitivity and linearity. However, the scintillator rods can only be connected once they are on-site. The individual scintillator rods must be aligned relative to each other on-site, optically coupled and placed into a protecting tube. This method is problematic in light of the susceptibility of the scintillator material to soiling and mechanical damage and in light of the oftentimes difficult conditions on-site. 
       BRIEF SUMMARY OF THE INVENTION 
       [0004]    The present invention allows the simplest possible and reliable manipulation and assembly of the measuring system at the location of its application. 
         [0005]    To this end, the invention provides a system of the type described at the outset in which one of the scintillator elements, one of the sheath elements and two of the face-end window elements form a scintillator module in each case, which module is optically and mechanically connectable to additional scintillator modules. 
         [0006]    The underlying concept of the invention consists in combining the individual scintillator rods by means of “suitable” sheath elements and window elements as face end pieces to form encapsulated scintillator modules. 
         [0007]    A scintillator module thus consists of a scintillator rod that is closed off at both ends by windows through which the scintillator light can pass into the next module, as well as of a suitable sheath element. 
         [0008]    On-site, multiple scintillator modules of this type are connected to each other until the desired measuring length is reached, and connected at least on one end to a detector module consisting of a scintillator element, photomultiplier and evaluation electronics. 
         [0009]    The invention makes possible attainment of a high degree of detection sensitivity for ionizing radiation despite the large measurement area. 
         [0010]    This is achieved, in particular, by providing the photomultiplier with an evaluation circuit connected on the load side, which circuit has a measuring channel for detection, processing, and display of the measuring pulses, and a control channel for pulse formation and pulse buffering. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    A preferred exemplary embodiment of the invention will now be described in more detail based on the accompanying drawings, in which: 
           [0012]      FIGS. 1A and 1B  show a longitudinal section through a scintillator module in two states. 
           [0013]      FIG. 2  shows a longitudinal section through the coupling point of two scintillator modules according to  FIGS. 1A and 1B . 
           [0014]      FIG. 3  shows an exploded view of the coupling region of a scintillator module. 
           [0015]      FIG. 4  shows a longitudinal section through the first end region of the embodiment with a coupled-on detector module. 
           [0016]      FIG. 5  shows a longitudinal section through the second end region with a coupled-on end module, 
           [0017]      FIG. 6  shows a projection and cross section, respectively, of an overall assembly consisting of an end module, a scintillator module and a detector module. 
           [0018]      FIG. 7  shows an example of a measuring assembly with a block diagram of the evaluation circuit of the detector module. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    The scintillator module  10  shown in  FIGS. 1A ,  1 B,  2  and  3  forms the base module of a measuring system according to the invention. 
         [0020]    The function of a scintillator module is to provide for the secure encapsulation of the scintillator element and protection of the same from damage, soiling and scattered light. This is achieved with a robust sheath of stainless steel, which encompasses the scintillator element. Springs affixed at both ends of the scintillator module ensure a reliable optical coupling of the scintillator element to window elements ending at the face ends, and ensure that different dimensional changes of the sheath element and scintillator element, caused by thermal expansion and dimensional tolerances, are compensated for. The position of the scintillator element within the sheath element can be displaced by several centimeters in both directions without jeopardizing its coupling to the adjoining scintillator element or its sealing encapsulation. 
         [0021]    The design of a scintillator module will now be described in more detail based on  FIGS. 1 through 3 : 
         [0022]    A rod-shaped scintillator element  11 , which is centrally supported in the interior of the scintillator module  10 , is coaxially encompassed at its two ends in each case by an inside cylinder  161 ,  162 , an outside cylinder  171 ,  172  and a common sheath element  12  that is connected to the latter. 
         [0023]    Disposed on annular flanges  161 B,  162 B of the inside cylinders  161 ,  162  are face-end window elements  131 ,  132 , which seamlessly connect to the two face ends of the scintillator element  11 . The other end of each of the inside cylinders  161 ,  162  has a respective annular flange  161 A,  162 A that forms the first counter-bearing of a respective spring element  151 ,  152 . The other end of each spring element rests on a respective annular flange  171 A,  172 A of the associated outside cylinder  171 ,  172  that slides on the inside cylinder  161 ,  162  and forms the second counter bearing. The spring elements  151 ,  152  acting between the two annular flanges  161 B or  162 B and  171 B or  172 B permit a length change of the modular unit consisting of the scintillator element  11 /inside cylinder  161 ,  162  relative to the component consisting of the outside cylinder  171 ,  172 /sheath element  12 , which, through the pressure action of the spring elements  151 ,  152 , ensures that a relative displacement of the first modular unit relative to the second modular unit is made possible without diminishing, or interrupting, the contact between adjoining window elements  131 ,  132 . 
         [0024]      FIG. 1A  shows the positioning of the two modular units at a standard temperature;  FIG. 1B  shows the corresponding positioning of the two modular units at a temperature at which the longitudinal expansion of the sheath elements  12  of the assembly of the scintillator modules  10  is less than the longitudinal expansion of the scintillator elements  11 , so that an offset X 1 , X 2  of the outer face ends of the window elements  131 ,  132  occurs relative to the outer annular surface of the second annular flange of the outside cylinder  171 ,  172 , with x 2 &gt;x 1 , but the contact of the window elements  131 ,  132  with the window elements of the scintillator modules  10  (shown dashed) is maintained. 
         [0025]    In the presented exemplary embodiment, the upper end in  FIG. 1B  of the scintillator module faces the detector module  20  ( FIG. 4 ), at whose contact point with the first scintillator module  10  no displacement takes place (fixed coupling to its window element  231 ); the bottom end of the scintillator module faces the end module  30  ( FIG. 5 ). A total length difference N(x 2 −x 1 ) occurring with the use of N scintillator modules is compensated for by “insertion” of the “last” scintillator element  11  into the housing  31  of the end module  30  against the force of the compression spring  33 . 
         [0026]    As shown in  FIG. 2 , each pair of adjoining scintillator modules  10  that are equipped in this manner are connected to each other via a contoured ring  18  having a generally U-shaped connecting, or clamping, part. A coupling element  19 , for example in the form of a silicone plate, which maintains the optical contact between window elements, is inserted between opposed window elements  131 ,  132 . 
         [0027]    Sealing elements in flanges  171 B,  172 B ensure that no light or humidity can enter into the interior of sheath element  12 . 
         [0028]    The detector module  20  shown in  FIG. 4  is, at one end, identical to scintillator modules  10 , i.e., one scintillator element  21  is optically coupled to a window element  231  that is transparent to scintillation light and a window element  251  permits a length change of the scintillator element  21  relative to the sheath element  22 . Disposed on the side opposite to window element  231 , and converting the scintillation light into electronic pulses, are a photomultiplier  24 , which is coupled via a detector window  232  to the scintillator element  21 , and an evaluation circuit  25 . 
         [0029]    Referring to  FIG. 5 , the module facing away from detector module  10  forms an end module  30 . Its housing  31  is closed with a cover  32 . Disposed inside housing  31  is a compression spring  33 , one end of which is supported on cover  32  and the other end of which presses a reflector element  34  against the adjoining window element  131  of the contiguous scintillator module  10 , so that scintillation light is reflected from the reflecting end surface of the reflector element  34  back into scintillator element  11 . 
         [0030]    The scintillator modules, detector module, and end module are simply and quickly connected to each other on-site, mechanically via the contoured rings  18  and optically via the coupling elements  19 , as shown in  FIG. 6 . As explained above, the spring elements in the individual modules ensure that length changes that are caused by temperature fluctuations of at least ±40° C. can be compensated for, while at the same time maintaining the optical contact of the individual elements among each other. 
         [0031]      FIG. 7  shows an overall measuring system assembly being used for a fill level measurement in a container B with, for example, a 6-meter measurement area. 
         [0032]    A radioactive source Q sends out measuring radiation MS, which penetrates through container B and is attenuated in the process by the substance in the container that is being measured. The radiation is detected on the side across from the source by the inventive system. The same consists, in this case, of one detector module  20  of 2 m length and two scintillator modules  10  each having a length of 2 m, and hence of a total length of 6 m. The light flashes that are generated in the scintillator elements are detected by the photomultiplier  24  in the detector module  20 , converted into an electronic signal, and processed by the evaluation circuit  25  to determine the current fill level from the resulting count. 
         [0033]    The large, quasi-continuous length of the detector presents a challenge for the dynamics of the electronic amplifier circuit, unlike any that is known from detectors that have been used up to now. In order to attain a high degree of sensitivity of the detector, the goal is to detect even very weak “scintillation light flashes” on the one hand, like those that are generated by Compton scattering in the scintillator material. If this takes place at a large distance from the photomultiplier, the generated light experiences a noticeable weakening as a result of transmission in the scintillator elements and at the optical coupling locations, and therefore triggers only very few electrons at the photocathode of the photomultiplier. 
         [0034]    On the other hand, a great deal of scintillation light is generated by muons or cosmic radiation HS, which travel through up to 6 m scintillator length and have the effect that the resulting voltage pulses at the anode of the photomultiplier can reach very high levels. 
         [0035]      FIG. 7  also shows a circuit diagram of the evaluation circuit  25 , with which these problems are overcome. 
         [0036]    To prevent an overdriving of the electronics unit, but to also, on the other hand, still be able to detect even weak scintillation light, the signals are divided from the anode of the photomultiplier  24  onto two channels that are independent of each other: 
         [0037]    The first channel is a measuring channel  251  that is optimized for the signal processing of the pulses, which are crucial for the measurement, and has an electronic amplification of approximately 30. At the input of the measuring channel  251  the pulse height is limited by means of a suitable diode circuit, to prevent overdriving effects. 
         [0038]    The second channel is a control channel  252 , which is responsible for the automatic high voltage regulation. Channel  252  has a voltage divider, which decreases the pulse height at the input by a factor of approximately 5-10. The control channel  252  does not have any amplification but serves only for the purpose of pulse formation and buffering. 
         [0039]    This ensures that the required dynamic range is maintained under all operating conditions. The regulation of the high voltage HV for operating the photomultiplier to compensate for aging and temperature effects utilizes a method known per se, for example as disclosed in the patent document DE 41 14 030 C1. 
         [0040]    This application relates to subject matter disclosed in German Application number 10 2006 048 256.2, filed on Oct. 12, 2006, the disclosure of which is incorporated herein by reference. 
         [0041]    The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without undue experimentation and without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. The means, materials, and steps for carrying out various disclosed functions may take a variety of alternative forms without departing from the invention. 
         [0042]    Thus the expressions “means to . . . ” and “means for . . . ”, or any method step language, as may be found in the specification above and/or in the claims below, followed by a functional statement, are intended to define and cover whatever structural, physical, chemical or electrical element or structure, or whatever method step, which may now or in the future exist which carries out the recited function, whether or not precisely equivalent to the embodiment or embodiments disclosed in the specification above, i.e., other means or steps for carrying out the same functions can be used; and it is intended that such expressions be given their broadest interpretation.