Abstract:
An x-ray fluorescence instrument for measuring the presence of lead in a sample. The lead is bombarded with gamma rays from a source thereof and in turn, emit x-rays. The x-rays are detected by a scintillation detector which emits photons that are directed to a photomultiplier tube which provides signals to an electronic processor including multichannel analyzers to provide output signals representative of the lead present in the sample. The instrument includes a cone plug disposed between the gamma ray source and the detector to prevent gamma rays from directly striking the detector.

Description:
This Application is based upon Provisional Application No. 60/097,042 filed Aug. 19, 1998 for Nuclear/X-Ray Fluorescence Determination of Lead and Other Elements by Karl Bernstein and Charles Chapman Kidd. 
    
    
     BACKGROUND OF THE INVENTION 
     It has become apparent in recent years that the incidence of lead in the atmosphere, the soil and other surrounding environmental areas occupied by human beings is greater than previously anticipated. It is now recognized that lead can be airborne and may contaminate areas in which workers are housed while performing their day-to-day employment tasks. It has also become recognized that as a result of the use prior to the 1978 of paint containing lead, substantial soil contamination around older school buildings and older houses has resulted. In addition, upon the demolition of older buildings which were painted with lead containing paint, particles of lead may become airborne thus creating a toxic environment for the demolition workers and other individuals within the general demolition area. 
     There are many industries and activities in which lead in one form or another is commonly used. For example, in shooting galleries where live ammunition is utilized lead projectiles are propelled from the weapons toward the targets. The propelling of the lead projectiles and their impact with the target and with the projectile restraining walls beyond the targets creates a situation in which particles of lead enter the ambient atmosphere and remain airborne for a substantial period of time. 
     It is also known that in many industries where radiation of one form or another occurs, lead has been used as a common material for shields to protect personnel from bombardment by nuclear radiation. The presence of such lead shields in any significant amount generates a situation where lead particles can contaminate the ambient atmosphere. A similar type of situation occurs in industries in which lead is an integral or component part of the activity or the resulting product. Typical examples of such instances are where soldering occurs such as in the manufacture of electronic devices, the mining of lead, the smelting of ores containing lead, the manufacture of batteries which contain lead therein, the refining of petroleum products and many other industries which those skilled in the art will recognize. 
     As a result of the extensive lead contamination of the ambient atmosphere, it becomes extremely desirable to monitor the lead content of the atmosphere to determine whether or not personnel present may be subjected to a sufficient amount of lead to constitute a toxic danger. 
     Typically monitoring the ambient under such circumstances is accomplished by passing the ambient air, or a portion thereof, through an appropriate filter having sufficiently small pores to capture the airborne lead particles or at least a substantial number thereof. The filter containing the lead particles can then be subjected to appropriate measurement to determine the lead content in the ambient atmosphere to determine whether or not sufficient lead particles exist to constitute a potential danger to personnel in that area. This measuring process can be accomplished on a periodic basis which is determined by the surrounding circumstances and the application involved. For example the measuring can be done on a daily basis with appropriate records kept to track changes in the airborne lead content of the ambient atmosphere on a day-to-day basis. Under certain circumstances, the activity involved may be such that the measuring should be conducted on an hour-by-hour type basis or even more frequently in order to detect the existence of an immediate problem which may have occurred as a result of a malfunction of manufacturing processes or equipment thus constituting a sufficient danger that the immediate area should be cleared of all personnel for a time sufficient to recirculate the ambient atmosphere and remove the excess airborne lead particles therefrom. 
     To accomplish the measurement of the lead particles which may be present either in the air, the soil or other portions of the environment, it is recognized that x-ray fluorescence (xrf) is a very useful elemental analysis technique. Xrf uses common nuclear instrumentation to provide extra-nuclear indications of elemental composition both qualitatively and quantitatively. Xrf works best with medium high to high atomic number elements which includes lead. 
     The method usually uses gamma ray bombardment of the lead particles to excite x-ray emission from them and then uses scintillation or semiconductor detectors and computer-based multi-channel analyzers (MCA) to complete the elemental analysis. Small isotopic sources of gamma rays generally are preferred to x-ray sources of excitation. 
     One of the problems associated with xrf utilizing gamma ray excitation is that comparatively few of the gamma rays will be observed by traces of lead in the field of view of the xrf instrument head. A substantially larger fraction of the gamma rays emitted by the excitation source will be scattered and ultimately detected by the MCA as lower energy gamma rays. 
     The scatter gamma rays create a background noise and as a result will interfere with the desired x-ray signal. This results because gamma rays and x-rays are substantially the same in all ways except for their motive creation and the x-rays of value and the scattered gamma rays are combined in their distribution throughout the spectrum. Their combined presence in the spectrum presents data processing and/or instrument hardware requirements. One resolution of the difficulty of this combined scattered gamma ray and desired x-ray information is to use semi-conductor detectors which greatly reduce the interference relative to that of scintillation detectors. However, to use the semi-conductor detectors substantially increases the cost of the instrument. As a result, scintillation detection systems are highly desirable in an instrument of the type with which the present invention is concerned. 
     It is thus desirable that an instrument utilizing a scintillation detection system in which appropriate controls are effected to substantially mitigate the interference of scattered gamma rays emitted by the excitation source with the x-ray signals received from the excited particles of lead which are to be detected is used. Without effective control the gamma rays will enter the ambient in all directions from the isotopic source and even if some containment mechanism is utilized, these gamma rays will reflect one or more times and still interfere with the operation of the instrument by providing improper activation of the scintillation detector. 
     SUMMARY OF THE INVENTION 
     An x-ray fluorescence instrument for detecting radiation emanating from a sample to be measured which instrument includes a detector, means for positioning a sample adjacent the detector means, a source of radiation contained within a mounting head which includes a shield substantially surrounding said radiation source and defining an opening therethrough for passage of radiation from the source toward the sample. The opening is defined by a conical surface diametrically diverging away from the opening. The instrument further comprises a plug disposed between the shield opening and the sample to preclude radiation from the source directly striking the detector. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The single FIGURE represents a schematic diagram of an x-ray fluorescence instrument constructed in accordance with the principles of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawing, there is illustrated an x-ray fluorescence instrument constructed in accordance with the principles of the present invention. As is therein shown, a scintillation detector  10  such as a sodium iodide crystal is mounted to a housing  12  constructed of material which functions as a shield or gamma and x-rays. The scintillation detector  10  emanates photons  14  therefrom in response to x-ray radiation striking the surface  16  and passing through the window  18 . The photons  14  strike the surface of a photomultiplier tube  20  which generates a signal in response thereto as is well known in the art. The signal is transmitted through an appropriate cable  22  to an electronic data processor  24  which will include multi-channel analyzers adapted to process the signals received from the photo-multiplier tube  20 . The output signals from the electronic data processor  24  are then passed to an output device  28  by the appropriate cabling  26  all as well known to those skilled in the art. 
     Positioned internally of the housing  12  is a sample  30  which is supported by appropriate means as shown by the dashed lines  32 . The sample  30  will contain the materials which are to be analyzed by the x-ray Fluorescence instrument of the present invention. As above discussed, the sample  30  will typically be a filter through which ambient air has been passed so as to trap the airborne particles of lead. The filter  30  will be removed from the filtering apparatus on the desired periodic basis and inserted into the housing  12  of the x-ray Fluorescence instrument. 
     Although the specific example of a filter is used it should be recognized that other apparatus may be employed. For example, if continuous monitoring is to be accomplished the sample  30  may be a baffle device through which ambient air is continuously passed as shown by inlet arrow  31  and the outlet arrow  33 . A simple fan or compressor may be operated to create a sufficient pressure to flow ambient air in a controlled amount through a baffle having a predetermined configuration to provide a desired volume of ambient air for measurement of airborne lead particles contained therein. A typical flow rate for such purposes would be on the order of 1 to 3 cubic feet per minute. The baffle would be designed to assure that any single lead particle would not generate an undue number of x-ray signals as it passes through the instrument. 
     Also positioned within the housing  12  is a head  34  which is adapted to receive a source  36  of gamma rays. Typically, the source  36  will be a relatively small isotopic source of such gamma rays and for example, may be Cobalt-57. The source  36  is mounted within an appropriate holder  38  which in turn is positioned internally of a shield  40 . The shield  40  is preferably constructed of a non-radioactive high atomic number material which will effectively trap the gamma rays not allowed to directly pass from the head  34  or at least require them to negotiate multi-scattering around the inner surfaces of the shield  40 . Preferably the shield  40  is constructed of tungsten material. 
     An important feature of the present invention is that the tungsten shield  40  defines an opening  42  adjacent the source  36  through which gamma rays may pass. The opening is further defined by an outwardly diverging conical surface  44  along which the gamma rays are directed toward the sample  30 . As above referred to the gamma rays will leave the source  36  in all directions and in substantially straight lines. By providing the tungsten shield  40 , only those gamma rays exiting the source  36  through the opening  42  will be directed toward the sample  30 . The remainder of the gamma rays which would otherwise be scattered throughout the internal portion of the housing  12  and would interfere with the signal being generated by the detector  10  are effectively trapped within the shield  40  and absorbed by the tungsten. Through utilization of the outwardly diverging conical surface  44 , the gamma rays are directed upwardly as viewed in the figure and toward the sample  30 . 
     It should also be recognized by the those skilled in the art that it is not desirable for gamma rays to directly impact the surface of the detector  10 . Therefore, there is positioned a plug  46  between the opening  42  and the detector  10 . The plug  46  is constructed in such a manner that the exterior surface  48  is also conically shaped to allow the passage of gamma rays thereby but to such a degree that any gamma rays passing thereby if they were to continue in a straight line toward the detector  10  would just miss the detector. That is the plug  46  effectively shields the detector  10  from radiation emanating directly from the source so that it cannot directly strike the detector  10 . The plug  46  must have a density such that it can shield the source gamma rays insofar as the detector  10  is concerned. It is also desirable that the atomic number of the material from which the plug  46  is made be different by a significant amount from that of the material which is contained in the sample  30  and is to be measured. The further the separation, the less likely the x-ray energy generated by the plug  46  will interfere with the signal which it is desired to detect with the detector  10 . The diameter of the plug  46  must be such that it will effectively shadow or shield the detector from the gamma rays. The thickness of the plug  46  must be such that it is sufficient to be able to stop the gamma rays; that is gamma rays will not penetrate the plug  46  and travel directly onto the detector  10 . Obviously, the thickness of the cone plug  46  will depend upon the source of the gamma rays. If the source is, as above suggested, Cobalt 57 then the thickness of plug  47  should be one to two centimeters. In a preferred embodiment of the present invention, the plug  46  is constructed of tungsten material. 
     With the construction of the shield and the plug as above described, a conical beam of gamma rays having the center thereof removed, as shown by the arrows  50 - 52 , will emanate from the source  36  and strike the sample  30 , that is, an annulus of gamma rays will impinge upon the sample  30 . As the gamma rays strike the sample  30 , as illustrated at  54  and  56 , the particles of lead trapped within the sample  30  have the energy content thereof increased by absorption of the gamma rays  54 - 56 . The release of this absorbed energy then results in the lead particles radiating x-rays as shown at  58  and  60 . These x-rays then will strike the detector  10  by passing through the window  18  causing the photons  14  to be radiated therefrom. Obviously, there will be a great deal more radiation activity than is illustrated in the figure and the x-rays emanating from the sample will radiate in directions other than directly at the detector  10 . However, for simplicity of illustration and discussion, only the two gamma rays  54 - 56  resulting in the radiation of the x-rays  58  and  60  have been illustrated and described. 
     As above indicate, the photons will be processed by the multi channel analyzers in the electronic data processor  24  and provide a signal to the output device  28  which is representative of the elements in the sample. For example, lead will provide a peak at approximately 75 Kev and 73 Kev with other elements at different positions on the spectrum. The MCA will be programmed to separate the signals representative of the desired element being monitored, such as lead. 
     As above indicated and as known to those skilled in the art, there will be gamma rays as well as x-rays within the instrument which are not absorbed by cone plug  46 , the sample  30 , nor by the detector  10 . The housing  12  will be constructed of a material which will either absorb or deflect the gamma rays and the x-rays which strike the interior surface thereof. As a result personnel resident in the general area in which the instrument is being used will be protected from the resultant gamma rays and x-rays. 
     There has thus been described a simple x-ray fluorescence instrument for detecting radiation emanating from a sample containing material which is to be measured and which utilizes gamma rays as a source of radiation to generate the x-rays to be detected. The structure of the present invention concentrates or focuses the gamma rays so that they impinge upon the sample and do not directly impinge upon the detector while at the same time controlling the emanation of the gamma rays from the source.