Abstract:
A device for the online determination of the contents of a substance having a gamma-radiating isotope, which includes at least one detector, which measures the natural gamma radiation of said isotope. In order to be able to easily calibrate the device, a measurement is carried out at the same location for determining the surface dimensions of the substance within the detection region of the detector or of a representative partial region.

Description:
TECHNICAL FIELD OF THE INVENTION 
     In the field of industrial measuring technology, the natural gamma radiation of a substance is measured in order to determine different material parameters of this substance. 
     In particular, this method is used for determining the ash content of coal and for determining the potassium content of potassium salt. The potassium content is measured by determining the concentration of the radioactive isotope K-40 which is present worldwide in the same concentration in potassium. The ash content is measured by determining additional radioactive isotopes that are contained in the ash. 
     With a sufficiently large measuring volume, meaning if the measuring depth is sufficient so that saturation occurs, only a large-surface detector or a detector array is needed for detecting a statistically backed number of events during a suitable time period, so that the desired measuring variables can be determined following a corresponding calibration. This is generally the case if the detector is mounted on an industrial bunker for example. 
     In many cases, however, it is desirable to carry out a measurement on material conveyed on conveying belts or under different measuring conditions where the so-called saturation layer thickness is not reached. In those cases, either the layer thickness must be kept constant or a compensation of the layer thickness or in general of the available measuring volume is required. 
     PRIOR ART 
     Most widely used is the technique of carrying out a measurement with material on conveying belts. Belt scales are primarily used to compensate for the belt occupancy or bulk material level, wherein radiometric as well as mechanical belt scales are used. These units either exist already when the device for determining the concentration is installed, since the determination of the mass flow is of general interest, or they are installed at the same time as the measuring device. With radiometric belt scales the detector for determining the natural gamma radiation is installed at a far enough distance to the radiation sources on the belt scale that the measuring of the natural gamma radiation is not disturbed by these radioactive sources. With mechanical belt scales, the belt section determined for realizing the measurement in most cases is noticeably larger than the section determined by the detector for the natural gamma radiation. As a result, static calibrations (calibrations made while the belt is stopped and using mostly previously analyzed samples) are often very difficult and involved and frequently not very precise. Either large numbers of samples are required or the sample must be displaced without changing the mass per unit area and without changing the shape or the cross-sectional profile of the sample. In practical operations, this is achieved by manually moving the conveying belt on which the sample for calibration is positioned—oftentimes weighing more than a hundred kilograms—so that the sample is displaced between belt scale and radiation detector. With dynamic measurements, meaning for measurements taken while the belt is moving and under conditions similar to operational conditions, this problem does not occur, to be sure, but large amounts of sample material must be tested which is also extremely involved and subject to errors. 
     Subject Matter of the Invention 
     It is the object of the present invention to create a device which makes possible a static calibration, using a comparably small sample, wherein this sample is not displaced during the calibration. 
     This object is solved with a device having the features as disclosed in claim  1 . 
     According to the invention, the measuring of the natural gamma radiation and the measuring of the mass per unit area are realized at the same location. If a mechanical belt scale is used for determining the mass per unit area, then it must either be ensured that the radiation detector or detectors used detect the total region of the substance for which the weight is detected by the mechanical belt scale, or it must be ensured that the region detected by the detector is representative of the substance measured by the belt scale. 
     According to one preferred embodiment disclosed in claim  2 , the mass per unit area are determined with the aid of a gamma or X-ray radiator which irradiates the substance, wherein the radiation transmitted through the substance at least in part impinges on the detector for measuring the natural radioactivity, so that this detector functions to measure the natural radioactivity of the sample as well as to measure the weakening of the irradiated radiation and thus can be used to determine the mass per unit area. However, this requires means for distinguishing between the intensity of the gamma radiation of the isotope, measured by the detector, and the intensity of the transmitted radiation, wherein this can be achieved in two different ways: 
     On the one hand it is possible to use a detector having a sufficient energy resolution. The at least one additional radioactive radiator emits energy quanta with energies that differ strongly enough from the energies of the natural radioactive radiation, so that during a simultaneous detection the natural gamma radiation can be separated from the radiation emitted by the nuclide/X-ray tube or tubes. For the purpose of evaluation, a multi-channel analyzer is installed downstream of the detector which detects the spectrum of the occurring radiation. The natural radiation and the radiation caused by the nuclide/X-ray tube can thus be separated in a single evaluation unit and the concentration of the compensated for occupation level can be computed. Suitable nuclides are, in particular, cobalt, americium and cesium. 
     If the natural gamma radiation is limited to a few spectral lines, the multi-channel analyzer can also be replaced by a sufficient number of discriminators which make it possible to separate the spectral lines and the dragging of pulses to the low-energy range which is essentially caused by the Compton Effect. 
     According to claim  8 , separating the signals coming from the individual radiators and the natural gamma radiation can also be achieved through alternately fading out the radiation from the nuclide/X-ray tube, wherein a chopped operation is especially advantageous (claim  9 ). 
     According to claim  18 , the device according to the invention is particularly suitable for determining the potassium content in potassium salt and, according to claim  20 , for determining the ash content in coal. 
     Further advantageous embodiments are disclosed in the additional dependent claims. 
    
    
     
       SHORT DESCRIPTION OF THE DRAWINGS 
       Shown are in 
         FIG. 1  A schematic representation of a first embodiment of the invention; 
         FIG. 2  A cross section along the line I-I in  FIG. 1 , also shown as a schematic representation, wherein different substance levels are drawn in; 
         FIG. 3  Typical spectra for different substance amounts; 
         FIG. 4  The measuring range if two collimators are present, shown schematically; 
         FIG. 5  A second example for the first embodiment, shown in a schematic representation; and 
         FIG. 6  A second embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a first example of a first embodiment of the invention in the form of a strongly diagrammatic view. The device shown herein is used to determine the total amount of potassium conveyed on a conveying belt  10 , wherein potassium salt  50  is conveyed on the belt  10  in the direction T. The goal is to use the shown device for determining the total amount of potassium that is conveyed per time unit. The device is provided for this with a mechanical belt scale  12  and a corresponding evaluation unit  14 . The mechanical belt scale  12  with the evaluation unit  14  functions to determine the total tonnage of the potassium salt. Since the share of potassium in the potassium salt can vary, it is not possible to immediately draw a conclusion relating to the share of the potassium tonnage in the total tonnage. A measuring device is therefore provided which determines the percentage share of the potassium in the potassium salt. 
     This measuring device comprises a cesium 137 source  22  which for this example is arranged above the conveyor belt  10 , as well as a detector  20  that is arranged below the conveyor belt  10 . The detector can be a NaJ detector, for example. The measuring device furthermore comprises a multi-channel analyzer  24 , arranged downstream of the detector  20 , as well as a measuring computer  26  for evaluating the signals from the multi-channel analyzer  24 . The output of the evaluation unit for the belt scale  14  and the output of the measuring computer  26  are connected to a display unit  28  which displays and issues the desired information, for example the total tonnage per hour of potassium salt, as well as the total tonnage per hour of potassium. 
     The mode of operation of the measuring device is now explained in further detail with reference to  FIGS. 1 to 3 , wherein  FIG. 2  shows a section through the conveyor belt, at the location of the measuring device, and wherein different filling levels of the potassium salt are plotted on at the conveyor belt  10 . The dotted line in this case shows the maximum level of the bulk material, the drawn-out line shows the average level of the bulk material and the dash-dot line shows a low level. The cesium 137 source  22  irradiates at least a portion of the potassium salt flow transported on the conveyor belt  10 , wherein the geometry is arranged such that the total radiation transmitted through the potassium salt impinges on the detector  20 . The gamma quanta radiated by the cesium 137 source  22  have an energy of 660 eV, so that the absorption of this radiation depends in a manner known per se on the irradiated amount of the substance, namely on the mass per unit area. 
     In addition to the radiation from the cesium 137 source  22 —if applicable transmitted through the potassium salt—the natural radiation of the potassium isotope K-40 also impinges onto the detector  20  with a quantum energy of 1,461 MeV. The maximum filling level of the potassium salt is such that it does not result either in an almost complete absorption of the 660 KeV radiation or in a saturation of the natural gamma radiation of the K40. Spectra such as the one shown with the example in  FIG. 3  are consequently obtained for the various bulk material levels. 
     With a low bulk material filling level (dash-dot line), the absorption of Cs-137 radiation is low and the signal intensity measured by the detector  20  is high. The signal intensity of the natural K-40 radiation at 1,461 MeV is correspondingly low. If the level is increased with the same composition of the irradiated substance, then the peak at 660 keV is correspondingly reduced while the peak at 1,461 MeV is increased accordingly. The continuous line shows the spectrum for the average bulk material level while the dotted line shows the spectrum for the high level. 
     If we consider the dependence of the counting rate for the K-40 radiation in dependence on the layer thickness in a linear approximation, the following ratio is obtained: 
                 F     K   -   40   -   peak         ln   ⁢       F     Cs   -   137   -   peak         F   empty           =     const   ⁢           ⁢     (     concentration     K   -   40       )             
wherein:
 
F K-40-peak =area below the K-40-peak
 
F Cs-137-peak =area below the Cs-137-peak
 
F empty =area below the Cs-137 peak when the conveyor belt is empty
 
const(concentration K-40 )=constant which depends only on the K-40 concentration with the given geometry.
 
     This approach could be called a “semi-linear” approach, which can be used if a simple measuring geometry can be realized with the aid of strong collimation and if the maximum observed bulk material level is not very high. 
     The areas below the peaks and the corresponding quotients can be determined directly, so that with a change in the quotient, we can directly deduce a change in the K-40 and thus in the potassium content. The dependence of const(concentration K-40 ) on the K-40 concentration in many cases can be seen as linear. 
     To achieve a higher accuracy, it must for the most part be taken into consideration that the counting rate for the K-40 signal does not increase linear with the occupancy since the layer close to the detector absorbs photons from the layer at a distance to the detector. With extremely high layer thicknesses, saturation occurs and the above-described approximation method can no longer be used. It must furthermore be taken into consideration that with an increase in the layer thickness, the volume used for the potassium measurement does not increase linear, but increases faster because more radiation also impinges from the side onto the detector. The detected volume therefore does not take the form of a cylinder, but rather that of a cone. The opening angle can be defined by a collimator. The algorithms required for the evaluation are known and are used at the present time with potassium belt scales, for example, or with ash content measuring devices which are based on the measurement of natural gamma radiation. 
     In practical operations, corresponding calibration curves will be generated by taking measurements at differently high bulk material levels. The calibration is not very involved, however, since on the one hand only a relatively small amount of material is needed and, on the other hand, the conveyor belt need not be operational. The static calibration makes it possible to easily measure a sample at different layer thicknesses (bulk material levels). 
     The calibration and the mathematical treatment are simplified considerably if the irradiated region of the substance, which is “seen” by the detector  20 , is identical to the region of the substance, the K-40 radiation of which impinges on the detector  20 . An upper and a lower collimator  23 ,  21  can be provided ( FIG. 4 ) for this, wherein these collimators define the viewed spatial region. In particular the lower collimator  21  is of considerable importance since otherwise K-40 radiation of a substance that is not located in the region irradiated by the cesium 137 source  22  is also measured and the transmitted radiation of this substance also impinges on the detector  20 . The lower collimator should essentially be impenetrable for the 1,461 MeV radiation and preferably consists of lead. 
     A spectrum such as the one shown in  FIG. 3  is generated by the multi-channel analyzer  24  and is evaluated with the known algorithms by the measuring computer  26 . Since the two peaks are positioned at a long distance to each other, it would also be possible to replace the multi-channel analyzer  24  with a discriminator circuit. 
       FIG. 5  shows a second example for the first embodiment of the invention which can be operated without multi-channel analyzer or discriminator circuit. The measuring principle is basically the same as described in the above because the peaks are analyzed at 660 keV and 1,461 MeV. To determine in that case which pulses measured by the detector  20  come from which radiation source, so as to distinguish between them, a radiation chopper  30  is arranged above the conveyor belt  10  in the radiation path for the Cs-137 source  22 , wherein this chopper consists of a aperture wheel  32  and a motor  34 . The signals from the detector  20  are conducted to a pulse counter  25  which is synchronized by the radiation chopper  30 , so that the pulse counter  25  “knows” which counted pulses belong to the 660 keV peak and which pulses belong to the sum of the 660 keV and the 1,461 MeV peak. The difference must first be computed during the evaluation, so as to obtain separate “counting rates” for the two energies. The mathematical evaluation corresponds to the one described in the above. 
       FIG. 6  schematically shows a second embodiment of the invention. As explained in the above, the measuring of the absorption of the radiation coming from the cesium 137 source  22  is used to determine the weight of the potassium salt in the measuring region, meaning the belt occupancy over a specific belt section. Of course, this information can in principle also be obtained via the mechanical belt scale. However, it must be considered in that case that the viewed amount of the substance is relatively large. Ideally, the same region is used for measuring the weight with the belt scale and for measuring the natural K-40 radiation, as indicated schematically in  FIG. 6 . Here too, the measuring range is determined with the aid of a collimator  60 . One disadvantage of the second embodiment, as compared to the first embodiment, is that considerably larger amounts of the potassium salt are needed for the calibration. 
     A surface scanner can alternatively be used for compensating the bulk material level, wherein this scanner measures the profile of the conveyed potassium salt on the conveyor belt at the location where the K-40 radiation is determined. With the assumption of a constant density, the weight can be computed once the profile that is “seen” by the detector  20  is known, so that a corresponding compensation of the occupancy level is possible in that case as well. A surface scanner of this type, for example, can operate with lasers. 
     The present invention was shown with the aid of a determination of the amount of potassium in a potassium salt. However, this method can in principle also be used for other substances as long as these have isotopes emitting gamma rays with a suitable wavelength for which it is ensured that the isotope distribution is constant and known. 
     REFERENCE NUMBER LIST 
     
         
           10  conveyor belt 
           12  mechanical belt scale 
           14  evaluation unit for the belt scale 
           20  detector 
           21  lower collimator 
           22  Cs-137 source 
           23  upper collimator 
           24  multi-channel analyzer 
           25  pulse counter 
           26  measuring computer 
           28  output unit 
           30  chopper 
           32  aperture wheel 
           34  motor 
           50  potassium salt 
           60  collimator