Abstract:
An on-line material analyzer which utilizes pulsed neutron generation in order to determine the composition of material flowing through the apparatus. The on-line elemental material analyzer is based on a pulsed neutron generator. The elements in the material interact with the fast and thermal neutrons produced from the pulsed generator. Spectra of gamma-rays produced from fast neutrons interacting with elements of the material are analyzed and stored separately from spectra produced from thermal neutron reactions. Measurements of neutron activation takes place separately from the above reactions and at a distance from the neutron generator. A primary passageway allows the material to flow through at a constant rate of speed and operators to provide data corresponding to fast and thermal neutron reactions. A secondary passageway meters the material to allow for neutron activation analysis. The apparatus also has the capability to determine the density of the flowed material. Finally, the apparatus continually utilizes a neutron detector in order to normalize the yield of the gamma ray detectors and thereby automatically calibrates and adjusts the spectra data for fluctuations in neutron generation.

Description:
[0001]    This application claims priority to provisional application No. 60/092,194 filed Jul. 7, 1998. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a system which is used to determine the elemental content of materials such as coal and cement, by neutron irradiation. More particularly, the present invention is directed towards an apparatus which divides the flow of material into two paths, subjects the material to neutron bursts, and by utilizing the gamma rays emitted from the interrogated object, provides the ability to calculate the concentration of major and minor chemical elements in the material.  
           [0004]    2. Discussion of the Prior Art  
           [0005]    Neutron-based elemental on-line analyzers have been used by several industries such as the coal and cement industry over several decades. These analyzers provide a quantitative analysis of the major chemical elements contained in a material. Using neutron producing radioisotopic sources such as californium-252, these on-line analyzers determine the amount of H, N, C, Al, Si, Cl, Ca, S, Fe, etc. contained in the examined materials. Through these measurements, bulk properties such as density, volatile matter, calorific value etc. can be determined using algorithms such as:  
           Calorific value (BTU/lb)= a (%C)+ b (%H)+ c (%N)+ d (%S)+ e (%O)+ f (%ash)  
           [0006]    However, there are limitations with the apparatus&#39; used to determine these values.  
           [0007]    With the currently available on-line elemental analyzers, the material moves on a conveyor belt or falls continuously through a chute. A neutron emitting radioisotopic source is placed on one side of the chute or conveyor belt, and a set of gamma-ray detectors is placed on the other side or at a position adjacent to the neutron source. The gamma-ray detectors analyze the gamma rays emitted from the coal sample after it is irradiated from the neutrons. These gamma rays act as fingerprints of the elements contained in the material. For example, a 4.43 MeV gamma ray is emitted from carbon, while a 5.42 MeV gamma ray is emitted from sulfur. The gamma-ray signals are transferred to the nuclear electronic modules where they are analyzed, and using the appropriate software, the elemental content of the material in the chute is determined.  
           [0008]    The radioisotopic-source based systems, although they measure quite well most of the chemical elements, can be improved by replacing the low energy continuous neutrons emitted from the neutron source with higher energy pulsed neutrons emitted from a pulsed neutron generator. Some of the major advantages from such replacement are:  
           [0009]    Direct measurement of carbon (the main element in coal) without interference from other elements.  
           [0010]    Measurement of elements such as oxygen and sodium that cannot be measured with a californium-based system.  
           [0011]    Major reduction in the radiation hazard during removal and transportation of the source of neutrons.  
           [0012]    Reduction of the background in the gamma-ray spectrum.  
           [0013]    Ability to utilize a large number of nuclear reactions (fast neutron induced reactions, thermal neutron induced reactions, and neutron activation reactions) which allow improvement of the sensitivity and precision in the measurement of a specific chemical element.  
           [0014]    Prior art devices which are disclosed in U.S. Pat. Nos. 5,396,071, 4,841,153, and 4,582,992 utilize isotopic neutron sources and do not allow separation of the measured spectra in spectra produced from fast neutron reactions, thermal neutron reactions, and neutron activation reactions thus limiting their accuracy and calculations of material content. Further prior art disclosed in U.S. Pat. No. 5,162,095 utilizes a neutron generator to examine the flow of bulk material. However, it only uses one pathway for the flow of the bulk material, acquiring at the same location with the same gamma-ray detector spectra from thermal neutron reactions and from neutron activation reactions. In this configuration, all materials (including metal structures, containers, belts, etc.) in the vicinity of the bulk material are also activated. This activation appears as background, obscuring the results from the activation of the bulk material. Furthermore, the prior art devices measure either a thermal neutron spectrum or a neutron activation spectrum, but not both simultaneously. Additionally, in this prior art configuration, approximately 50% of the time either spectrum is not measured.  
         SUMMARY OF THE INVENTION  
         [0015]    The present invention is for an elemental on-line material analyzer which utilizes a pulsed neutron generator in combination with a specially designed material flow pathway in order to determine the composition of material flowing through the apparatus. The elements in the material interact with the fast and thermal neutrons produced from the pulsed generator while the material is resident in a first pathway. Spectra of gamma-rays produced from fast neutrons interacting with elements of the material are analyzed and stored separately from spectra produced from thermal neutron reactions. Measurements of neutron activation takes place separately from the above reactions and at a distance from the neutron generator in a second pathway designed particularly for activation reaction analysis.  
           [0016]    The present invention is for an elemental on-line material analyzer based on a pulsed neutron generator which analyzes three spectra of data for composition determination. The apparatus utilizes a system which is composed of two pathways for the movement of the material to be analyzed. The main pathway is a chute or alternatively, a conveyor belt that is surrounded by a neutron generator and a system of gamma-ray and neutron detectors. The gamma-ray detectors in the main pathway are triggered by the neutron pulses and they accumulate gamma rays produced from fast neutron and thermal neutron reactions. The neutron detectors monitor the neutron production from the neutron generator and also measure the transmission of neutrons through the bulk of the material. The secondary pathway is a chute or conveyor belt used to measure the gamma rays from elements excited from neutron activation reactions. Elements such as sodium, silicon, aluminum and oxygen are some of the elements more accurately measured through neutron activation reactions. The material flows continuously through the main chute or conveyor, and the gamma ray detectors surrounding the main chute measure continuously the gamma-ray yield on the neutron pulse and off the neutron pulse. The material through the secondary chute flows in batches of a few kilograms each. Each batch is stopped next to the neutron generator where it is irradiated by the pulsed neutrons for a predetermined time. At the end of the irradiation period, the material is quickly moved to another location and a separate gamma-ray detector in the secondary pathway measures for a predetermined time the spectrum produced from the neutron activation. At the end of the measurement period, the material is weighed with an automatic electronic scale, and then is returned to the material flow exiting the main chute or conveyor. The analyzer also contains appropriate high voltage and low voltage power supplies for the production of neutrons, associated data collection electronics, and electrical devices that control the flow of the material samples through the main and the secondary chute. A computer-based data acquisition system receives the appropriate signals generated by the gamma-ray detectors and the various neutron detectors and converts them to digital values. These values are then analyzed by elemental characterization software which determines in real time the amount of the major and minor constituent elements present in the material under interrogation. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    A better understanding of the invention will be had upon reference to the following description in conjunction with the accompanying drawings in which like numerals refer to like parts and wherein:  
         [0018]    [0018]FIG. 1 a  is a schematic perspective view of the pulsed neutron-based material analyzer of the present invention;  
         [0019]    [0019]FIG. 1 b  is a cutaway view indicating positions of the gates on the analyzer shown in FIG. 1 a;    
         [0020]    [0020]FIG. 2 is a time diagram generated by the pulsed neutron output of the apparatus of FIG. 1 a;    
         [0021]    [0021]FIG. 3 is a graph of the gamma-ray spectrum for fast neutron reactions generated by the apparatus of FIG. 1 a;    
         [0022]    [0022]FIG. 4 is a graph of the gamma-ray spectrum for thermal neutron reactions generated by the apparatus of FIG. 1 a;  and,  
         [0023]    [0023]FIG. 5 is a graph of the gamma-ray spectrum for neutron activation reactions. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0024]    The apparatus  30  for a dual pathway pulsed neutron-based material analyzer  30  is shown schematically in FIG. 1 a  wherein each of the separate elements of the analyzer are clearly depicted. FIG. 1 b  shows a cutaway view of the pulsed neutron-based material analyzer  30  shown in FIG. 1 a.  The apparatus  30  is a chute-based elemental material analyzer. The material to be analyzed flows continuously through the main chute  1 . The main chute  1  is maintained full of material at all times. A height control gauge  2  monitors the flow of the material through the main chute  1 . Gauge  2  is an infrared transmitter-receiver. When gauge  2  detects absence of material, it sends a signal to the conveyor belt (not shown) that removes the material at the bottom of the chute  1  to slow down the material removal. This ensures that the main chute is always full of material to be analyzed and controls the speed of material through the chute. A neutron generator  3  emits pulses of fast neutrons that impinge on the material flowing through the main chute  1 . The time sequence of the neutron pulses is shown in FIG. 2. Referring to FIG. 2, fast neutrons are emitted during the pulsed time period  21  which is around 10-15 μs followed by a measuring of gamma rays for about 80-100 μs which is required in order to properly generate the thermal neutron reactions. Both of these reactions generate measurements which are stored by the data acquisition system  23  for proper determination of material composition in chute  1 .  
         [0025]    The pulsed neutron generator  3  is computer  23  controlled. The analyzer contains a high voltage (larger than 100,000 Volts) power supply for the acceleration of deuterons, a 2,000-4,000 Volt supply for the pulsing of the deuteron beam and a low voltage supply for the deuteron ion source. The neutrons are generated by the impinging of the accelerated deuterons on a tritium target. The energy of the neutrons emitted from the generator must be larger than 6 MeV in order to properly cause the reactions necessary for elemental determination.  
         [0026]    Electrical switching circuits are used to control gates  10  and  12  and thus the flow of material through main pathway  1  and secondary pathway  8 . Each gamma-ray detector requires a high voltage (of the order of 1,000 volts) for the photomultiplier tube attached to it, and a low voltage supply (12 to 24 volts) for the preamplifier.  
         [0027]    The apparatus  30  contains approximately thirty-five points which are monitored to determine its reliable operation. These points include the opening and closing of gates  10  and  12 , the temperature stabilization of each gamma-ray detector, the high voltage for each gamma-ray and neutron detector, the adequate production of neutrons from the neutron generator and the radiation safety controls for the operators. These signals are fed to computer  23  and either alerts the operator of the analyzer or the computer  23  attempts to rectify the situation by altering a predetermined voltage.  
         [0028]    The neutron generator  3  is a sealed tube deuterium-tritium neutron generator, providing pulsed neutrons with energy of at least 6 MeV and up to about 14 MeV. As represented in FIG. 2, each pulse  21  of neutrons is several microseconds wide and has a frequency of between 10 kHz and 14 kHz. These pulsed neutrons initiate the fast neutron reactions within the object being interrogated through main chute  1 . Outgoing gamma rays from the material are detected by the gamma-ray detector  4  in chute  1  for a specified time period corresponding to the actual pulse time of the neutron pulse  21  from the neutron generator  3 . The gamma rays representing fast neutron reactions are measured and recorded as a single spectrum of data during the time period  21  in the data acquisition system that resides in the computer  23 . FIG. 3 shows a spectrum of gamma-rays produced from nuclear reactions induced from fast neutrons reactions. At the end of the neutron pulse  21 , the fast neutrons are thermalized for a time period  22 , shown in the FIG. 2 timing diagram.  
         [0029]    During the time thermalization period  22 , the thermalized neutrons initiate capture reactions with the material in main chute  1  and those capture reactions are recorded and translated into an appropriate spectrum of data similar to that of FIG. 3. The gamma rays from the thermal neutron or capture reactions are detected by the detectors  5  and  6 , and they are stored in a separate spectrum in computer  23 . FIG. 4 shows a spectrum of gamma-rays produced from nuclear reactions induced from thermal neutrons.  
         [0030]    Unfortunately, information from fast neutron reactions and thermal neutron or capture reactions only provide a partial framework for analysis of element composition. Neutron activation reactions must be detected and used in the modeling software in computer  33 . Thus, spectra from all three can be used to accurately pinpoint the true composition of the material. However, these activation reactions take significantly longer to initiate as compared to thermal and fast reactions. A secondary chute  8  is thus provided to give a means for generating and detecting activation reactions in an environment separate and distinct from that generating thermal and fast reactions.  
         [0031]    While the material flows through the main chute  1 , a gate  7 , shown in both FIG. 1 a  and  1   b  diverts a pre-determined amount of material (usually a few kilograms) to the secondary chute  8 . This pre-determined time period is dependant upon the half life of the element of interest and varies between seconds to minutes. The amount of diverted material is controlled by the duration of time gate  7  remains open or raised. The diverted material is stopped at position  9  with a gate  10 , and the material in the secondary chute  8  is irradiated from the pulsed neutron generator  3  for a pre-determined time period. This time period depends on the particular chemical element under investigation. For example, if the element sodium is to be measured, the sample will be irradiated at position  9  for approximately 30 seconds. At the end of the irradiation period, gate  10  opens and the sample falls under gravity to position  11  where another gate  12  stops it. At this position  11 , a gamma-ray detector  13  measures for approximately the same duration as the irradiation time (e.g. for sodium it will measure for approximately 30 seconds) the gamma rays from the radioactivity built in the sample. The gamma rays detected from detector  13  are also stored in computer  23 . FIG. 5 shows a spectrum of gamma-rays produced from nuclear reactions induced from neutron activation. At the end of the measuring period, gate  12  opens and the material falls onto a scale  14  where it is weighed. Following the weighing of the irradiated sample, the material in the secondary chute returns to a conveyor system which also has the material that flows through the main chute.  
         [0032]    The gamma-ray spectra measured by the detectors  4 ,  5 ,  6 , and  13 , are composed of several gamma rays produced from chemical elements contained in the interrogated object. The spectra are stored in computer  23 . The analysis of the spectra and the extraction of the gamma-ray yield for each element of interest is accomplished through a data de-convolution process which analyzes the response from the detectors and determines the chemical content and makeup of the object being interrogated. Through utilization of the apparatus of the present invention, this de-convolution process is made possible due to the ability of obtaining reactions from fast neutron reactions, thermal neutron reactions and neutron activation. This data more accurately determines the content of the material being analyzed.  
         [0033]    Along with the gamma-ray detectors  4 ,  5 ,  6  and  13  which measure the gamma rays emitted from the irradiation of the material, there is also a gamma-ray detector  17  which measures the density of the material that flows through the chute. The number and size of the detectors  4 ,  5  and  6  as well as their location are dictated by the number of neutrons/pulse emitted by the neutron generator and also the maximum number of counts per second that a given detector can detect without being saturated and thereby becoming ineffective.  
         [0034]    A cesium gamma-ray source  18  is shielded and collimated so that the emitted gamma-rays from the source are directed towards the material that lies between the source  18  and the detector  17 . The gamma ray source  18  in combination with detector  17  is calibrated and the number of gamma rays from the source  18  that are seen by the detector  17  give an accurate measure of the density of the flowing material within main chute  1 .  
         [0035]    Detectors  15  and  16  are neutron detectors. Neutron detector  15  measures the fraction of the fast neutrons emitted from the neutron generator  3  that traverse the material in the main chute  1  without losing any energy, as well as the fraction of the fast neutrons that have lost nearly all of their energy (thermal neutrons) as they traverse the material in main chute  1 . The measurement of the combination of fast and thermal neutrons is used for the establishment of the total hydrogen content of the material sample, and subsequently the moisture of the flowing material. Neutron detector  16  is a monitor of the fast neutron yield from the neutron generator. By measuring the number of fast neutrons emitted per unit time (e.g. per minute) the neutron generator yield is monitored, and any fluctuations or normal diminishing of the generator output are used to normalize the yield of the gamma ray detectors  4 ,  5 ,  6  and  13  and of the neutron detector  15 .  
         [0036]    To avoid any further irradiation of the material from neutrons emitted from the neutron generator  3  when the sample in the secondary chute  8  is measured at position  11 , radiation shielding  19  is provided to separate gamma ray detector  13  from the generator  3 .  
         [0037]    Radiation shielding  19  is composed of neutron absorbing materials such as boric acid or borated polyethylene, and is placed between the neutron generator  3  and gate  12 . Radiation shielding is also provided around the gamma-ray detectors  4 ,  5 , and  6 . Shielding  20  directly below neutron generator  3  adjacent to main chute  1  is composed of high atomic number material such as lead or tungsten, and shields the gamma-ray detectors  4 ,  5  and  6  from gamma rays generated by the neutron generator  3 . To minimize the irradiation of the detectors themselves from neutrons, detectors  4 ,  5 , and  6 , have a concentric jacket composed of materials such as boron nitride or lithiated compounds that have a high thermal neutron absorption cross section.  
         [0038]    The gamma-ray detectors  4 ,  5 ,  6 ,  13 , and  17  are scintillator detectors such as sodium iodide or bismuth germinate. The neutron detectors  15  and  16  are helium-3 detectors, liquid scintillators, or a combination of lithium glass-liquid scintillator detectors.  
         [0039]    With this emitter and detector arrangement, the pulsed neutron generator  3  irradiates simultaneously the material in the main chute  1 , along with the material in the secondary chute  8 . The material in the main chute  1  is flowing continuously, while the material in the secondary chute  8  is flowing in batches of preselected weight. The use of the pulsed neutron generator  3  instead of an isotopic source such as californium-252, allows the accumulation of gamma rays in three distinct spectra: one spectrum from fast neutron reactions, another spectrum from thermal neutron reactions, and a third one from neutron activation. The gamma-ray spectra from fast neutron reactions are accumulated only during the time the neutron pulse is on. The gamma-ray spectra from thermal neutron reactions are accumulated only during the time between the neutron pulses. The distinctly different spectra acquired from the three sets of nuclear reactions are shown in FIGS. 3, 4, and  5 .  
         [0040]    The arrangement shown in FIG. 1 a  and the discussion concerning the arrangement of the detectors can also be accommodated in a horizontal arrangement with the material moving on a conveyor belt or other similar horizontal material transport configuration. Modification into this horizontal arrangement is well within one of ordinary skill in the art.  
         [0041]    The foregoing detailed description is given primarily for clearness of understanding and no unnecessary limitations are to be understood therefrom for modifications will become obvious to those skilled in the art upon reading this disclosure and may be made without departing from the spirit of the invention or the scope of the appended claims.