FIELD OF THE INVENTION

[0001] This disclosure generally relates to systems and methods for performing non-contact measurements of a volume of material in a container.

BACKGROUND OF THE INVENTION

[0002] Certain industrial processes involving high temperature, caustic, or otherwise hazardous materials pose unique challenges to common measurements. For example, measuring a level of material in a container may be difficult depending on the material and properties of the container. In large or thick-walled vessels, where the volume and/or density of a process material is the measurement of interest, radio-isotopes may be used to generate gamma rays that are absorbed by the process material and the container. The degree to which gamma rays are absorbed depends on the density and level of the material in the container as well as the container itself. A properly calibrated measurement of the gamma ray absorption yields a measurement of the amount of material in the container. A source such as Cs 137 or Co 60, for example, may be used to generate gamma rays for measurements in long, thick walled vessels that, for example, are large in diameter and may have steel walls that are 2 - 6 inches in thickness.

[0003] A radio-isotope source that is external to the container, however, may be unsuitable when the diameter of the vessel exceeds several meters. In such situations, the gamma radiation signal may be attenuated to an unacceptable degree as it penetrates the steel walls of the container. To overcome this problem, internal, or well-insertion, sources may be used. Such source solutions, however, have drawbacks in that they require modifications to the vessel and the sources must be designed to operate in hostile industrial environments.

[0004] The radio-isotope Cs 137 produces 660 keV gamma radiation and the radio-isotope Co 60 produces 1.25 MeV radiation. Such materials are only distributed to licensed users in limited quantities (e.g., less than 10 Ci per source). These materials are stored in heavily shielded containers. The penetration of gamma radiation generated by these sources, in a material to be measured, is determined by gamma ray energy, material absorption properties, radiation reduction due to distance from the source, as well as the amount of source material (e.g., smaller amount of source material leads to lower gamma radiation flux). Limited radio isotope sizes demand larger detectors to distinguish the gamma radiation produced by the radio isotope source from that of naturally occurring background radiation.

[0005] Considering the various drawbacks of using radio-isotope sources, alternative approaches to measurements in hostile environments are needed.

SUMMARY OF THE INVENTION

[0006] The disclosed embodiments fulfill a need by providing systems and methods of performing non-contact measurements of a volume of a process material in a container. The disclosed embodiments are especially useful for measurements in hostile environments of process materials in large containers.

[0007] A disclosed method determines an amount of material in a container. The method includes determining the flux of naturally-occurring charged particles, predominantly in the form of muons (which derive from energized protons originating from outer space) which enter the container and pass through the material therein. The method involves determining the flux of naturally-occurring charged particles entering and exiting the container. The method further includes determining a difference in the flux of naturally-occurring charged particles entering the container relative to the flux of charged particles exiting the container and determining the amount of material in the container based on the determined difference.

[0008] The method may further include determining the generally vertical flux of naturally- occurring muons absorbed by the material in the container based on a density of the material in the container and based on the determined difference in the generally vertical flux of muons at the general elevation of the container relative to the generally vertical flux of muons exiting the container. The method may further include determining the amount of material in the container based on the determined generally vertical flux of naturally-occurring muons absorbed by the material in the container. [0009] The method may further include using a first scintillation detector to detect muons primarily in an energy range of 100 MeV - 6000 MeV (main region of interest typically being below 4 GeV) to determine the generally vertical flux of muons entering the container and using a second scintillation detector to detect muons in an energy range of 100 MeV - 4000 MeV exiting the container.

[0010] The disclosed system determines an amount of material in a container using a first detector, a second detector, and a processor circuit. The first detector determines a flux of naturally-occurring charged particles, such as muons, entering the container (e.g., generally vertically), and the second detector determines a flux of those particles exiting the container.

The processor circuit is configured to use the relationship of the fluxes measured by the first and second detectors to determine a difference in the flux of particles entering the container relative to the flux of particles exiting the container, and to determine the amount of material in the container based on the determined difference.

[0011] Further embodiments, features, and advantages, as well as the structure and operation of the various embodiments, are described in detail below with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Embodiments are described with reference to the accompanying drawings. In the drawings, like reference numbers may indicate identical or functionally similar elements.

[0013] FIG. 1 is a schematic illustration of a configuration of detectors for measuring a process level based on detection of absorbed muons, according to an embodiment.

[0014] FIGS. 2 and 3 respectively illustrate a measured energy distribution of generally vertical and non-vertical muon flux, respectively, at sea level, in different energy ranges.

[0015] FIG. 4 is a flow chart illustrating a method of determining an amount of material in a container.

[0016] FIG.5 is an estimate of the measurable generally vertical muon flux at sea level for level process measurement up to lOOOg/cm ² .

[0017] The disclosed invention is described below with reference to the accompanying drawings. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the reference number.

DETAILED DESCRIPTION

[0018] This disclosure provides systems and methods that utilize high-energy charged particle radiation (e.g., naturally occurring muons), rather than conventional gamma radiation, for level measurements of materials in hostile environments. High-energy charged particle radiation may be generated with a high-energy particle accelerator; however, the present disclosure involves the use of high-energy muons from extra-terrestrial sources, according to embodiments described below.

[0019] Charged particles, like muons, lose energy as they travel through a material by ionizing atoms of the material. The energy loss is proportional to the amount of mass penetrated. The greater the energy of the particle, the greater the mass it can penetrate. Muons having energies in the range of 106 MeV - 4,000 MeV can penetrate large masses of matter before losing all of their energy and may penetrate much farther through materials than gamma radiation from conventional radio-isotope sources.

[0020] Gamma rays lose energy through the process of Compton scattering by which they give a part of their energy to electrons in a material resulting in ionization of atoms of the material. The ionization process induced by high energy gamma rays is limited to within an electron mean free path in the material, which is on the order of inches of material for electrons in an energy range of 100 keV - 500 keV. Thus, the penetration of gamma rays in a material is considerably limited in contrast to that of high-energy charged particles like muons which can travel greater distances before losing all their energy.

[0021] Muons therefore may be used as a radiation source for level measurements, according to an embodiment. Muons are generated in the Earth’s atmosphere by cosmic ray interactions and arrive at sea level at a rate of approximately 10,000/minute/m ² . Muons are positively and negatively charged heavy ions having a mass of roughly 221 times the mass of an electron.

Muons deposit roughly 2 MeV of energy in each g/cm ² of material. Muons that hit the Earth penetrate the Earth’s surface and continue to travel until they are completely absorbed. [0022] Muons arrive at the Earth’s surface with a mean energy of 4000 to 6000 MeV. As they pass through a material they lose energy and may be detected down to an energy of approximately 106 MeV, at which energy they decay into other particles. For example, if water is the process material, muons lose approximately 60 MeV per foot of water and 480Mev per foot of steel. If a muon penetrates enough process material to be reduced in energy down to 106 MeV, the muon then decays into other particles and is thus lost. If a 150 MeV muon penetrates 1 foot of water, for example, it will decay in the process and be lost before it can exit the process material and be detected. If a 1000 MeV muon penetrates 1 foot of water, it will exit at 840 MeV and may be detected. If any muon enters 10 feet of water, it will lose 600 MeV and only exit if it entered with energy exceeding 706 MeV. Likewise, when entering 1 foot of water, muons of energy less than 150 MeV will be lost, those with higher energies will exit.

[0023] Due to their large energy, muons may penetrate a combination of steel and process material totaling 2000g/cm ² and possibly 3000g/cm ² (6 GeV) may be detected using scintillation detectors, according to embodiments of the present invention.

[0024] Studies have indicated that the available muon flux at 78 meters above sea level is 1.6(1) x 10 ² s ^_1 cnT ² and generally vertical intensity of 1.0(1) x 10 ² s^cm^sr ¹ . At 25 meters (water equivalent) below ground the flux falls to 4.5(2) x 10 ³ s^cm ² sr ¹ and generally vertical intensity of 2.5(2) x 10 ³ s^cm^sr ¹ . See“Cosmic-Ray Muon Flux Measurements in Belgrade Low-Level Laboratory”, 30 ^th International Cosmic Ray Conference Proceedings 2007.

Underwater studies of muon flux reveal that detectible levels of muon flux remain at hundreds and even thousands of meters of underwater depth. See“Atmospheric Muon Flux at Sea Level, Underground and Underwater”, Phys. Rev. D58, 05401 (1998). Thus, the level of available muon flux in a generally vertical direction, even at sea level where it is most attenuated by passage through the atmosphere, is detectible and believed sufficient for level measurement.

[0025] Embodiments of the present invention, as described in greater detail below, provide several advantages. Unlike conventional approaches, radio-isotope sources requiring licensing are avoided, as are modifications to vessels. Further, the disclosed embodiments may be implemented using existing scintillation detector technology. The disclosed embodiments provide a complementary approach to conventional methods using external radio-isotope sources such as Cs 137 and Co 60. For example, while such radio-isotope sources may be applicable to small vessels, the use of charge particle sources (e.g., muons) has an advantage for large vessels for which external radio-isotope sources may not be applicable, as explained in the Background.

[0026] Muons have energies well above that of radiation generated by convention radioactive sources such as Cs 137, Co 60, uranium, radium, thorium, etc. The intensity of muons is approximately 1/min/cm ² , with an average energy of 4000 to 6000 MeV. Muons may penetrate vessels having walls made of thick (4 to 8 inches) steel and 30 - 100 feet of process material and escape with enough energy to be detected. According to an embodiment, using scintillation detectors having at least an area of 100 in ² (e.g., 2x50 in ² area), level measurements may be made in vessels where previously insertion sources of Co 60 or Cs 137 would have been used. In this embodiment, according to the invention, a first detector measures a generally vertical flux of muons entering the container and a second detector measures a generally vertical flux of muons leaving the container. The difference in the generally vertical flux of muons entering relative to the flux of muons exiting the container provides a measure of the flux of muons absorbed by the process material and the container. With a proper calibration, the measurement of the absorbed flux yields a measurement of the level of material in the container.

[0027] The detection of muons is not significantly affected by background radiation. The measurement of process level is a comparison of muon detection at different process levels. The muon detector configuration may be designed like that of a conventional radiation gauge where the source of radiation would be on the top or side of the vessel and the radiation detector would be opposite the source. According to an embodiment, the conventional radioactive source would be replaced by one or more muon entry detectors, which would measure the incoming muon flux that would be used for level measurement.

[0028] The disclosed embodiments may be contrasted with muon tomography. In known tomography applications, objects to be measured involved unknowns to be discriminated by the measurement. Such tomography measurements are therefore more complicated than the disclosed embodiments relating to level measurements of a process material in a vessel. In contrast, according to disclosed embodiments, vessels walls and other structural details are known to be constant and the only pertinent variable is the process level or density to be measured. Thus, muon-based level measurements are considerably simplified in contrast to muon tomography applications. [0029] Muons can penetrate the heaviest vessel walls found in most, if not all, industrial processes. Muons are especially attractive for use in applications where conventional external radio-isotope sources would not be suitable, such as for heavy walled, and/or large diameter vessels, that would otherwise require the use of internal radioactive sources. Muons also provide an attractive alternate to the use of licensed radioactive sources, where the issue of licensing and safety must be confronted.

[0030] Muons may easily be used to measure a level of process material in the 1500 - 2000 g/cm ² penetration range, which is roughly 60 feet of process material or walls of one foot of steel and 50 feet of process material. A longer or a larger range may also be possible.

[0031] FIG. 1 is a schematic illustration of a configuration of detectors 100 for measuring a process level based on detection of absorbed muons, according to an embodiment. Many other configurations are also possible. In this example, the upper detectors, 1 and 2 (there may be one or more than one), may measure a flux of incoming generally vertically-directed muons either above the vessel or near to the vessel, so long as the upper detectors effectively measure the muons expected to enter the vessel under the conditions to which the vessel is exposed. The upper detectors need not be placed above the vessel but may rather be near to the vessel and in the same general conditions as the vessel (e.g., at the same elevation above sea level and beneath the same or a similar roof structure and indoor atmosphere, if any). The lower detectors, 3, 4, 5 and 6 (there may be one or more than one) may measure the muons leaving the vessel and process material. The difference in flux measured by the upper detector(s) and the flux measured by the lower detector(s) provides a measure of the muon radiation absorbed by the vessel and process material. Such a measurement of absorbed flux may be calibrated (based on readings for known quantities of material in the vessel) to yield a measurement of the level of process material in the vessel.

[0032] It will be appreciated that this example is directed to the detection of generally vertically directed muons entering and exiting the vessel. The muon directions detectible by the detectors 1-5 will be randomly distributed and typically include muons arriving at angles divergent by several degrees from gravitationally vertical. The important criterion is that the upper detectors measure a muon flux that is the same as or is expected to be the same as the muon flux entering the vessel and process material, and that the lower detectors measure a muon flux that has passed through the process material. The lower detectors need not limit detection to generally vertically-travelling muons. For example, in FIG. 1, detectors 5 and 6 may measure muons traveling in non- vertical angles which are nevertheless passing through the vessel and process material. So long as the measurements by the detectors 1-5 are predominantly of muons which are at an angle that will pass through the vessel and process material in the vessel the principles of the present invention will be applicable.

[0033] Turning now to the way the device of FIG. 2 may provide a calibrated measurement of level, we must first discuss the energy levels at which detection is required. Muons at sea level have a generally vertical mean energy of 4,000Mev(4Gev) to 6,000 MeV (6 GeV). Muons do not exist below 106 MeV where they decay into other particles. Roughly approximately 90% of muons at sea level have energy in the range of 106 MeV—4,000 MeV. This range is where most opportunities of industrial gaging of level and density exist. The remaining 10% of muon have energies exceeding 4000 MeV. Such high-energy muons, however, may not be useful for level measurements of industrial processes even on very large scales since their relative abundance is too small and may not be suitable for these applications.

[0034] FIGS. 2 and 3 illustrate a measured energy distribution 26, 28 of muon fluxes at sea level. As may be seen, one half of all muons arriving at sea level have an energy between 100 MeV and 4 to 6Gev. In number, this energy range represents approximately 90% of incident muons, and may be used as a basis of predicting loss in the number of muons that penetrate a given g/cm ² of process material. As shown in FIG. 2, the lower energetic muons are more numerous than the higher energy muons. The scintillation process is essentially the same for all muons with energy in this range. According to an embodiment, a 2-inch-thick scintillation detector will absorb 10MeV/g/cm ² through the ionization process.

[0035] A range of level measurements may be theoretically derived from the measurements illustrated in FIGS. 1 and 2. Of the muons residing at sea level, 90% of the energy loss would be between lOOMev to 3900 MeV. For example, the water equivalent level range of measurement would be 50 - 60 feet of process material, including the vessel’s steel walls.

[0036] Just as with existing gamma ray detectors, process level is determined from changes in count rate of absorbed muons as the process level varies. At minimum process material level, the muon count rate is maximum, and at the largest process material level, the muon count rate is a minimum. The energy distribution and relative time stability of the muon flux makes reliable measurements possible. The use of first and second detectors to measure incoming and outgoing muon flux, respectively allows stability corrections to the level measurement to be generated. As such, fluctuations in muon flux would be corrected for because two measurements, incoming and outgoing, are subtracted.

[0037] According to an embodiment, large area scintillation detectors may be used in proof- of-concept tests and for obtaining a useful count rate as required by the measurement.

Coincidence, discrimination, and timing techniques may be used with the scintillation detectors to identify the generally vertical flux and separate muons from background radiation sources. According to an embodiment, a minimum of two detectors may be used, where one detector acts as a stability reference for the other detector, as described above.

[0038] In summary, muons easily penetrate thick walled vessels, and may be used for level measurement of process materials in hostile environments. The thickness of the vessel wall may be relatively unimportant as long as the process level range is at least several feet or more in diameter or height. Muon level devices may be applicable to vessels like coke drums, storage tanks, nuclear reactors, etc. The disclosed embodiments serve not necessarily to replace radioactive devices, but rather to provide: (1) an alternative to use of licensed sources, particularly those of large Ci content, (2) an alternative to internal source use, requiring vessel modifications, and, (3) any application where vessel modifications are undesirable.

[0039] FIG. 4 is a flow chart 400 illustrating a method of determining an amount of material in a container. The method includes (step 132) determining a flux 402 of naturally-occurring charged particles entering the container and determining (step 134) a flux 404 of naturally- occurring charged particles exiting the container. The method further includes determining (step 136) a difference 406 in the flux of naturally-occurring charged particles entering the container relative to the flux of charged particles exiting the container.

[0040] The method may further include determining (step 138) a flux 408 of naturally- occurring charged particles absorbed by the material in the container based on a density of the material in the container and based on the determined difference 406 in the flux 402 of naturally- occurring charged particles entering the container relative to the flux 404 of naturally-occurring charged particles exiting the container. The method may then further include determining (step 140) the amount (and/or density) 410 of material in the container based on the determined flux of naturally-occurring charged particles absorbed by the material in the container. [0041] FIG. 5 is an estimate of the measurable generally vertical muon flux at sea level for level process measurement up to lOOOg/cm ² , as a function of process level. As can be seen, a process level variation range of many feet within the vessel can be detected from a monotonic change in muon count, allowing calibration of the relationship between level and muon count and conversion of a measured muon count to a corresponding process level.

[0042] References in this specification to“one embodiment,”“an embodiment,” an“example embodiment,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but not every embodiment may necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic may be described in connection with an embodiment, it may be submitted that it may be within the knowledge of one of ordinary skill in the relevant art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

[0043] The above detailed description refers to the accompanying drawings that illustrate exemplary embodiments. Other embodiments are possible, and modifications may be made to the embodiments within the spirit and scope of this description. Those of ordinary skill in the relevant art with access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which embodiments would be of significant utility. Therefore, the detailed description is not meant to limit the embodiments described below.

[0044] Embodiments of the invention may include components that are implemented in hardware, firmware, software, or any combination thereof. Embodiments may also be implemented as instructions stored on a non-transitory machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; electrical optical, acoustical or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), and others. Further firmware, software routines, and instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc.

[0045] Embodiments may be implemented using software, hardware, and/or operating system implementations other than those described herein. Any software, hardware, and operating system implementations suitable for performing the functions described herein can be utilized. Embodiments are applicable to both a client and to a server or a combination of both.

[0046] Embodiments have been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined to the extent that the specified functions and relationships thereof are appropriately performed.

[0047] The foregoing description of specific embodiments will so fully reveal the general nature of embodiments of the invention that others can, by applying knowledge of those of ordinary skill in the art, readily modify and/or adapt for various applications such specific embodiments, without undue experimentation, without departing from the general concept of embodiments of the invention. Therefore, such adaptation and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the specification is to be interpreted by persons of ordinary skill in the relevant art in light of the teachings and guidance presented herein.

[0048] The breadth and scope of embodiments of the invention should not be limited by any of the above-described example embodiments but should be defined only in accordance with the following claims and their equivalents.