Abstract:
Described is an element analysis device, which can be used to obtain precise measurements even under unfavorable environmental conditions. For this, the device is provided with a transporting means with a measuring region ( 14 ) for transporting the substance (S) to be measured, an excitation source with an exit window located in a first case ( 22 ) and an X-ray fluorescence detector ( 30 ) that is directed toward the measuring region ( 14 ), as well as an entrance window ( 34 ) that is located in a second case ( 32 ). To minimize the air absorption and prevent dust and dirt from being deposited, a tube ( 40, 50 ) extends from the entrance window ( 34 ) and/or the exit window ( 24 ) in the direction of the measuring region, which tube is essentially tightly connected to the respective case ( 22, 32 ) and is open at the end facing the measuring region and is provided with a connection ( 44, 54 ) for feeding a flushing gas into the tube (FIG.  1 ).

Description:
TECHNICAL FIELD OF INVENTION 
   The invention relates to an element analysis device, as defined in the preamble to claim  1 . 
   For example in the field of mining and metallurgy, it is frequently necessary to measure continuously (online) the share of specific elements in a substance. For this, the substance is transported past a respective measuring device while positioned on a conveyor belt or the like. A frequently used measuring method in that case is the X-ray fluorescence spectroscopy. The device for realizing this measuring method comprises an excitation source, which irradiates a measuring region on the conveyor belt with fluorescence-exciting radiation. The fluorescence-exciting radiation for the most part involves gamma rays or X-rays. The measuring device is furthermore provided with an X-ray fluorescence detector for spectrally measuring the X-ray fluorescence radiation emitted by the substance, wherein the measuring result is then used to compute the share of specific elements with the aid of known methods. 
   However, various measuring-technical problems occur when using this measuring method, especially in the industrial sector. The first problem is that lightweight elements such as aluminum must frequently be measured. However, the X-ray fluorescence radiation of these elements is relatively long-wave and is therefore absorbed not only by detector windows, but is absorbed strongly by a few centimeters of air already. Connected to this problem is the second problem that the measuring environment is frequently dirty and dusty, thus further reducing the radiation transmission in air, so that the entrance window and the exit window are quickly covered with an absorbent layer. A third problem is that for some application cases, the substances to be measured are relatively hot and that the heat radiation reduces the function of the X-ray fluorescence detector. 
   PRIOR ART 
   A measuring device of the generic type is known from Reference WO 00/16078, which proposes arranging the X-ray fluorescence detector near—within 5 cm of—the surface of the substance to be measured to solve the problem of strong air absorption. To be sure, this measure allows minimizing the air absorption, but the problem of radiation-dampening dust deposits still remains. Furthermore, hot substances cannot be measured at all or only with great difficulty with a measuring device as proposed in this document, because the close proximity of the X-ray fluorescence detector to the surface of the substance does not allow the detector to reach the required low operating temperature. 
   SUBJECT MATTER OF THE INVENTION 
   Starting from this point, it is the object of the present invention to improve a generic device of this type, such that it can also be used under unfavorable environmental conditions. In particular, the measuring device must also be suitable for use in a dusty and dirty environment and for measuring hot substances. 
   This object is solved with a device having the features as defined in claim  1 . 
   The basic idea behind the invention is to guide the exciting and/or fluorescent radiation to the area near the surface of the substance to be measured inside a tube, which tube is open on the end facing the measuring region. The tube is flushed with gas, which exits the tube in the direction of the measuring region, thereby preventing any dust deposits. 
   In particular for the measuring of lightweight elements, helium is preferably used as flushing gas since the absorption in helium is considerably lower than in air because of its lower density. The tube must be open toward the bottom in that case and should preferably be arranged substantially vertically. This arrangement has the further advantage that owing to the low absorption of the helium, the X-ray fluorescence detector can be arranged relatively far from the substance to be measured, so that even a relatively high temperature of the substance to be measured does not present a problem. Arranging the X-ray fluorescence detector at a distance that is relatively far from the surface of the substance to be measured furthermore has the advantage that certain fluctuations in the surface height have only a slight effect on the intensity of the radiation impinging on the X-ray fluorescence detector because of the square law. 
   Preferred embodiments of the invention follow from the dependent claims and the exemplary embodiments, shown in further detail in the Figures, wherein these show in: 

   
     SHORT DESCRIPTION OF THE DRAWINGS 
       FIG. 1  A schematic representation of a first embodiment of the invention; 
       FIG. 2  A schematic representation of a second embodiment of the invention; 
       FIG. 3  A schematic representation of a third embodiment of the invention; 
       FIG. 4  The schematic representation of a fourth embodiment of the invention; 
       FIG. 5  The embodiment shown in  FIG. 2  with an added material sensor. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  shows a first exemplary embodiment, for which an outer case  10  is arranged above the conveyor belt  12 , which serves as a transporting means for this embodiment and conveys the substance S to be measured. Inside this outer case  10 , an X-ray tube  20  that functions as an excitation source is installed inside a first case  22 . An X-ray beam y R  that is directed so as to impinge on a measuring region  14  passes through an exit window  24  in the first case  22 . Since the outer case  10  as a rule has relatively thick walls, for example is composed of stainless steel, the case underside is provided with an exit opening  16  for the X-ray radiation. To prevent dust and other particles from entering the inside of the outer case  10 , the exit opening is covered with a window, for example a beryllium film. 
   As a result of this irradiation, X-ray fluorescence radiation y F  is generated in the measuring region  14 , which is then measured by the X-ray fluorescence detector  30  that is also located inside the outer case  10 . From the second case  32  of the X-ray fluorescence detector  30 , a first tube  40  extends vertically downward and is open at its lower end. The first tube  40  is connected via a rubber bellows  41  to the second case  32  of the X-ray fluorescence detector  30 , wherein this connection is substantially gastight. The first tube  40  is composed totally or in part of zirconium. The use of an elastic rubber bellows serves to mechanically uncouple the first tube  40  from the X-ray fluorescence detector  30 . This is necessary since most X-ray fluorescence detectors are mechanically relatively sensitive. At its lower end, the first tube  40  is tightly connected to the outer case  10 . 
   The X-ray fluorescence radiation generated in the measuring region  14  travels through the tube opening  42  into the first tube  40  and then travels through this tube to the entrance window  34 , through which it enters the X-ray fluorescence detector  30 . The entrance window  34  in that case can be open or can be covered with a film, for example, depending on the detector used. 
   To prevent dust, ashes, or similar material from absorbing the X-ray fluorescence radiation along its path from the measuring region  14  to the X-ray fluorescence detector  30 , the first tube  40  is flushed with helium. For this, the first tube  40  is provided with a connection  44 , which connects the tube  40  to a helium source  46  that is usefully located outside of the outer case  10 . The connection  44  is preferably located in an upper section of the tube  40 , near the entrance window  34 . The helium flows from the connection  44  into the tube  40  where it flows in downward direction and subsequently leaves the tube  40  through the tube opening  42 . To achieve maximum intensity, the tube diameter should at least match the diameter of the measuring sensitive surface of the detector that is used. The tube preferably has an inside diameter of 10 to 50 mm. A relatively large tube diameter is furthermore important, so that the flushing gas connection can be attached. To reach high flow speeds for the flushing gas at the tube opening, it may be useful to taper the tube toward the tube opening ( FIG. 1   a ). 
   It is furthermore possible to arrange the exit opening  16  immediately adjacent to the tube opening  42 , so that the exit opening  16  can also be flushed with helium. It may be useful in that case to expand the tube  40  toward the tube opening  42 . In particular, it may be favorable in that case to embody the region around the tube opening  42  asymmetrical, such that flushing gas exiting the tube opening  42  is guided in the direction of the exit opening  16 . 
   Since helium is considerably lighter than air, a helium column is always present in the tube  40  to prevent air from entering. The use of a helium flushing operation thus has several effects. On the one hand, no window film or only an extremely thin window film is needed at the entrance window  34  of the X-ray fluorescence detector  30 , thereby resulting in a very low absorption by the measuring device itself. On the other hand, it prevents the depositing of dust, ashes, and the like on the window film, as well as the presence of dust, ashes, and the like in a large portion of the beam path for the X-ray fluorescence radiation. This not only reduces the total absorption, but also prevents absorption fluctuations, which could hinder the measuring operation in a manner that it is hard to reproduce. Finally, a large portion of the radiation path is free of air, which also contributes to a strong reduction in the total absorption. Furthermore, the spectral absorption of some air constituents, for example argon, is strongly reduced in some application cases. This effect can be important, particularly when measuring elements that are located adjacent to the absorbing constituent in the periodic system. 
   With the exemplary embodiment shown in  FIG. 1 , it is possible for particles to be deposited on the cover for the exit opening  16 . Because of the at least relatively high energy of the X-rays used, this is not very critical with regard to the required intensity, but can still lead to a distortion of the measuring result. In the exemplary embodiment shown in  FIG. 2 , the beam path of the exciting X-ray radiation therefore passes through the walls of the first tube  40 . In that case, the first tube  40  can be produced from a material that is pervious to the exciting X-ray radiation. Rubber is one example for such a material, wherein rubber furthermore has the advantage of not transmitting vibrations to the X-ray fluorescence detector  30  even without installing a special rubber bellows in-between. A different option is shown in  FIG. 2  and consists of providing an opening  48  in the wall of the first tube  40 , through which the exciting X-ray radiation enters the first tube  40 . The opening  48  can be covered with a window, for example a beryllium film, to prevent leakage of the helium through this opening  48 . To minimize the measuring background, the window can also be replaced with a filter, through which only photons above a cutoff energy can travel. To further minimize scattering, an aperture  49  is preferably arranged above the opening  48 , for example an aperture of zirconium. As alternative to the embodiment shown in  FIG. 2 , the opening  48  can also be arranged directly adjacent to the tube end, so that it extends in the form of a recess from the open tube end into the tube wall. It is furthermore possible to connect the opening  48  and the exit window  24  of the X-ray tube with an X-ray conductor. The aperture  49  is located directly at the recess or opening  48  and is aligned parallel to the direction of irradiation for the excitation source. 
   With the exemplary embodiment shown in  FIG. 3 , not only the largest portion of the beam path for the X-ray fluorescence radiation, but also the largest portion of the exciting X-ray radiation is located inside a helium-flushed tube. According to the exemplary embodiment shown in  FIG. 2 , the X-ray radiation coming from the X-ray tube  20  enters through the opening  48  into the first tube  40 . The second tube  50 , which is also connected via a different connection  54  to the helium source  46 , is located between this opening  48  and the exit window  24  in the X-ray tube  20 . As a result, the interfering absorption by air can be prevented in some cases. As an alternative to the embodiment shown in  FIG. 3 , it is also conceivable to keep the first and second tube completely separate, as shown in  FIG. 4 . 
   To keep the operating costs as low as possible, the tube or tubes should be flushed with helium only during the active measuring operation. According to  FIG. 5 , a material sensor  60  that is installed upstream of the measuring region  14  is therefore proposed, which sensor detects whether or not the conveyor belt contains a substance to be measured. The signals from this material sensor  60  are transmitted to the control unit  64 . If no substance is detected on the conveyor belt, then the measurement is ended, if necessary with a time delay, and the valve  47  is closed. To prevent dirt from entering the first tube  40  during the time when no helium flushing takes place, a shutter  62  that is operated by a motor  66  is provided at the tube opening  42 . The shutter closes off the tube end once the valve  47  is closed. 
   If a substance is again detected on the conveyor belt  12 , then the shutter  62  and the valve  47  are opened. As a result, the first tube  40  is pre-flushed, until the substance reaches the measuring region  14  and the measuring operation starts or is continued. 
   The main object of the above-described exemplary embodiments is to reduce the absorption of X-ray fluorescence radiation. To be sure, in most application cases this will be the most important point, but the herein described principle of the “open gas flushing” can also be used, for example, exclusively for the beam path of the exciting radiation. 
   REFERENCE NUMBER LIST 
   
       
         10  outer case 
         12  conveyor belt 
         14  measuring region 
         16  exit opening 
         20  X-ray tube 
         22  first case 
         24  exit window 
         30  X-ray fluorescence detector 
         32  second case 
         34  entrance window 
         40  first tube 
         41  rubber bellows 
         42  tube opening 
         44  connection 
         46  helium source 
         47  valve 
         48  opening 
         49  aperture 
         50  second tube 
         54  additional connection 
         60  material sensor 
         62  shutter 
         64  control unit 
         66  motor 
       S substance