Method and apparatus for identifying attributes of a radiation source

Abstract

An embodiment of the present invention provides a computer-implemented method for locating a radiation source (210) within a medium (200), the method including: determining, at each of a plurality of positions of a scintillation detector (320) within the medium, a representation of an energy distribution of ionizing radiation incident on the detector; calibrating (630) one of a plurality of first models to the representations of the energy distribution of ionizing radiation determined at each of the plurality of positions within the medium (200), each first model including a first portion representing a photopeak of the ionizing radiation from the first radiation source; performing the calibrating (630) to identify a photopeak portion of the energy distribution at each respective position; and calibrating (920) a second model including an asymmetric function to the photopeak portions of the energy distribution at the plurality of positions to identify the location of the radiation source (210) within the medium (200).

Description

[Technical Field]

[0001] FIELD OF THE INVENTION Embodiments of the present invention relate to methods and apparatus for determining one or more attributes of a radiation source. In particular, some embodiments of the present invention relate to methods and apparatus for determining the location of a radiation source. [Background technology]

[0002] It may be desirable to determine one or more attributes of a radiation source, such as its location within a medium. The radiation source may be naturally occurring, such as natural uranium in exploration and mining, or may be for assessing radioactive contamination acting as a radiation source. The medium may be soil, in which case a borehole is created and a radiation detector is inserted into the borehole to measure ionizing radiation within the borehole, for example, to assess soil contamination at a site contaminated with radioactive materials. However, accurate measurement of such ionizing radiation is difficult for a variety of reasons. Due to environmental conditions, such as water, ice, etc., using a radiation detector in these environments for extended periods of time can be problematic. Furthermore, it will be appreciated that while known radiation detection systems may output a dose value indicative of ionizing radiation, determining a dose estimate can be difficult. Furthermore, determining the location of the radiation source, such as its depth within the soil, from such results is difficult.

[0003] It is desirable to more accurately identify the location of radiation sources within a medium such as soil. Summary of the Invention [Problem to be solved by the invention]

[0004] It is an object of embodiments of the present invention to at least alleviate one or more of the problems of the prior art. [Means for solving the problem]

[0005] According to one aspect of the present invention, there is provided a computer-implemented method for locating a radiation source within a medium, the method including receiving data indicating a representation of the energy distribution of ionizing radiation incident on a scintillation detector at each of a plurality of positions of the detector within the medium, and calibrating one of a plurality of models to the representation of the energy distribution of the ionizing radiation identified at each of the plurality of positions within the medium, each model including a first portion representing a photopeak of the ionizing radiation.

[0006] According to one aspect of the present invention, there is provided a computer-implemented method for locating a radiation source within a medium, the method including: determining, at each of a plurality of positions of a scintillation detector within the medium, a representation of an energy distribution of ionizing radiation incident on the detector; calibrating one of a plurality of models to the representations of the energy distribution of ionizing radiation determined at each of the plurality of positions within the medium, each model including a first portion representing a photopeak of the ionizing radiation from a first radiation source; and locating the radiation source within the medium dependent on an asymmetric function representing one or more attributes of the first portion of each of the plurality of models.

[0007] According to another aspect of the present invention, there is provided an apparatus for locating a radiation source within a medium, the apparatus including a detector for detecting ionizing radiation and a signal processing module co-located with the detector, the detector and signal processing module may be configured to be inserted into the medium, and the signal processing module may be configured to output a digital signal indicative of a count of radiation detections made by the detector.

[0008] According to another aspect of the present invention, there is provided computer software configured to, when executed by a computer, perform a method according to an embodiment of the present invention. The computer software may be stored on a computer-readable medium. The computer software may be tangibly stored on a computer-readable medium.

[0009] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0010] [Figure 1] A diagram of the medium is shown. [Figure 2] A diagram of the soil borehole and radiation source is shown. [Figure 3] 1 shows a diagram of a system according to one embodiment of the present invention. [Figure 4] 1 shows a schematic diagram of a system according to one embodiment of the present invention; [Figure 5] 1 shows a representation of the energy distribution of ionizing radiation incident on a detector according to one embodiment of the present invention. [Figure 6] 1 illustrates a method according to one embodiment of the present invention. [Figure 7] 1 illustrates components of a model and recorded data according to one embodiment of the present invention. [Figure 8] 1 shows the display of ionizing radiation at the detector versus depth. [Figure 9] 1 illustrates a method according to one embodiment of the present invention. [Figure 10] 1 shows a plot of a model according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] Embodiments of the present invention relate to locating a radiation source within a medium 100. FIG. 1 illustrates medium 100, which may be a substantially solid material in which a radiation source resides. To locate the radiation source within medium 100, a borehole 110 or blind aperture 110 is formed that extends into medium 100. In the illustrated example, borehole 110 extends horizontally a distance into medium 100. In embodiments of the present invention, borehole 110 is used to determine a distance 130 into medium 100 where the radiation source resides, as indicated by arrow 120. Thus, the location of the radiation source may, in some embodiments, be determined as a distance from a reference location, such as the outer surface of medium 100. Borehole 110 allows a detector to be inserted into medium 100 to detect ionizing radiation emitted by or attributed to the radiation source, as will be described.

[0012] In some embodiments, the medium may be soil 200, as shown in FIG. 2. A radiation source 210 is located at a depth in the soil 200. To locate the radiation source 210, a borehole 220 is formed, e.g., drilled, into the soil 200. The borehole 220 is generally a vertical blind aperture that extends downward into the soil 220. The borehole 220 may also be known as a well or a monitoring well 220. The borehole 220 allows a detector to be inserted or lowered into the soil 200, as will be described. The location of the radiation source 210 may be determined as the depth of the radiation source 210 into the soil 200. As shown in FIG. 3, in some embodiments, a sleeve or blind pipe 230 may be inserted into the borehole 220. The blind pipe 230 may be made of metal and may extend above the top surface of the soil a distance, as shown. The blind pipe 230 may be a pipe sealed to the blind end of the borehole 220 or may itself have a blind end. The blind pipe may provide structural strength to the walls of the borehole 220.

[0013] FIG. 3 shows a diagram of a system 300 according to one embodiment of the present invention configured for use in connection with the soil 200 and borehole 220 of FIG.

[0014] System 300 includes a control unit 310 and a detector 320 communicatively coupled to control unit 310. Detector 320 may communicate with control unit 310 wirelessly or via a wired connection. In some embodiments, detector 320 communicates with control unit 310 via a wired interface, which advantageously provides improved communication, particularly when detector 320 is located deep within borehole 220 where wireless signals may be obstructed. Furthermore, in some embodiments, the wired interface may be a network communication protocol such as an IP-based communication protocol. In one embodiment, the wired interface may be Ethernet. Such a network communication protocol may provide improved long-distance communication over, for example, USB communication. As described, detector 320 may have signal processing capabilities that may locally process and analyze signals from detector 320. Detector 320 may be connected to a flexible support member, such as a cable 345, e.g., a metal cable or chain, from which detector 320 may be suspended within borehole 220. System 300 is configured to selectively raise and lower detector 320 within borehole 220 as will be described.

[0015] System 300 includes an apparatus 330 for controllably inserting detector 320 into medium 200. In the illustrated example, the apparatus is configured to controllably suspend detector 320 at a determinable or known depth within borehole 220. Apparatus 330 may include a winch 340 configured to winch or unwind a flexible support member 345 from which detector 320 is suspended. In the illustrated example, winch 340 is generally mounted to a support frame 350 such that it is positioned above borehole 220 during use. However, it will be understood that other configurations are possible. For example, winch 340 need not be positioned on support frame 350. In another embodiment, winch 340 may be positioned at a lower elevation, such as at ground level, and a pulley or guide may be positioned on support frame 350 over which flexible support member 345 extends to allow for direction changes. Winch 340 is controlled by control unit 310 to controllably raise or lower detector 320 within borehole 220. Control unit 310 may instruct winch 340 to raise or lower detector 220 a commanded distance, for example, 0.1 m, 0.2 m, etc. In other embodiments, control unit 310 may instruct winch 340 to raise or lower detector 320 until commanded to stop. In such embodiments, system 300 may include a measurement unit (not shown) to determine the amount or distance that detector 320 has raised or lowered, such as by recording the passage of flexible support member 345 and reporting the amount or distance to control unit 310. In this manner, control unit 310 is configured to raise or lower detector 320 a known amount or distance within borehole 220. Control unit 310 controls the system so that ionizing radiation can be recorded at multiple known depths within borehole 220. A calibration step may be used to identify when the detector 320 is initially located at a reference position. The reference position may correspond to the top of the blind pipe 230, the height at which the top surface of the soil is located, i.e., ground level, or another selected reference position. The position of the radiation source 210 may be identified as a distance or depth 305 from the reference position, which in the illustration of FIG. 3 is ground level, although it will be understood that this is not a limitation.

[0016] FIG. 4 schematically illustrates a portion of system 300, including control unit 310 and detector 320, according to one embodiment of the present invention. Detector 320 includes a radiation detector 321 for detecting ionizing radiation. The detector may be a scintillator 321, such as in the form of a scintillation crystal. In some embodiments, the scintillation crystal may be cerium bromide (CeBr). Detector 320 may include a signal processing module 322 for processing signals from scintillator 321. Detector 320 may record counts corresponding to interactions of ionizing radiation with the detector in one of multiple channels, each corresponding to an energy. In this manner, the energy distribution of ionizing radiation interacting with the detector may be determined. As previously mentioned, in some embodiments, detector 320 and control unit 310 communicate via a wired interface 330, such as Ethernet. Detector 321 may be obtained from Scionix® of the Netherlands. Detector 320 may include a photomultiplier tube and preamplifier for processing signals from the scintillation crystal. The signal processing module 322 may be configured to perform pulse height analysis on the signal output by the detector 321. The signal processing module 322 may be configured to output data indicative of the number of counts of ionizing radiation detected by the detector over time in each of a plurality of channels corresponding to each energy band. An example is shown in FIG. 5. FIG. 5 shows the detector 320 in the form of a cascade of channels, designated by the numeral 510. 137 3 shows the number of counts recorded during one hour when exposed to a Cs source and in the absence of a radiation source, indicated by numeral 520, i.e., no background radiation detection. The number of counts in each channel can therefore be understood to indicate the number of ionizing radiation detections within each energy band or range. Signal processing module 322 can be configured to receive the analog electrical signal output by detector 320 and output a digital signal representing data indicative of the number of ionizing radiation counts detected by detector 321, thereby digitizing the analog data. By co-locating signal processing module 322 with detector 321, i.e., physically nearby, degradation of the signal output by detector 321 is avoided.

[0017] The control unit 310 may include a processor 311 and a memory 312 for storing data. The memory 312 may store computer-readable code or instructions for execution by the processor 311. The control unit 310 may further include a communications module 313 for receiving data from the detector 320, such as via a wired interface 330. The communications module 313 may be a network interface, such as an Ethernet network interface. The received data may be stored in the memory 312 of the control unit 310. The communications module 313 may be configured to output signals to control the position of the detector 320 within the borehole 220, such as outputting signals to control the winch 340, and / or receive data indicative of the position of the detector 320, such as indicating a distance the detector 320 has been raised or lowered within the borehole 320.

[0018] In some circumstances, the radiation source 210 may include multiple radionuclides, i.e., the radiation source may include multiple respective radiation sources of different types. For example, the radiation source may include: 137 Cs and 90 Sr or both. 137 Cs emits gamma (γ) rays with energies around 0.6617 MeV, which are capable of propagating a sufficient distance within the medium 200 and, in some embodiments, penetrating the walls of the blind tube 230 and the body of the detector 321, so that they can be detected directly by the detector 321. 90 Sr emits beta particles with an energy of 0.546 MeV, 90 Y, which also emits a beta particle with an energy of 2.28 MeV. Due to the short range of the beta particles, they are unlikely to be directly detected by detector 321. However, the generated bremsstrahlung photons resulting from the beta particles may be detected by detector 321. Thus, detector 321 may receive ionizing radiation produced by multiple radiation sources, particularly the first and second radiation sources. Embodiments of the present invention utilize a model that includes components of the first and second radiation sources, as described.

[0019] 6 illustrates a method 600 according to one embodiment of the present invention. Computer-readable instructions representing the method 600 may be stored in the memory 312 and executed by the processor 311. Thus, the system 300 may implement one embodiment of the method 600. The method 600 is a method of calibrating a model to a representation of the energy distribution of ionizing radiation identified at a position in the medium 200. The model f1 includes a first portion or part representing a photopeak of ionizing radiation from a first radiation source. The first radiation source is 137 Cs and ionizing radiation such as gamma rays emitted therefrom. The model f1 includes a second portion or part representing ionizing radiation resulting from a second radiation source.

[0020] Prior to performing method 600, system 300 of FIGS. 3 and 4 is used to record data indicative of the energy distribution of ionizing radiation at one or more locations within medium 200. For example, in connection with borehole 220 shown in FIG. 3 , system 300 is used to store data using detector 320 at one or more depths within soil 200, i.e., below borehole 220. Each depth may be indicated by a respective indicator i, where i=1, 2, 3, etc. For example, i=1 may be 0.1 m, i=2 may be 0.2 m, etc., and distances may be measured from a reference location, such as the ground surface or the top of blind pipe 230. Thus, in some embodiments, increasing values ​​of i indicate increasing depth into soil 200. Detector 320 is positioned at each depth for a period of time, such as one hour, although other durations may be used. The data at each depth indicates the number of counts of ionizing radiation recorded in each of multiple channels, each channel corresponding to an energy band and thus indicating an energy distribution. An example of data recorded at a depth is shown in Figure 5. When system 300 is used to record data at multiple depths, at each of a plurality of i depths, data is recorded that indicates the energy distribution at each particular depth or location within the medium. Thus, a data set such as that shown in Figure 5 is recorded at each depth. The data for each of the multiple depths may be stored in memory 312 of control unit 310. The memory may store multiple data sets, each corresponding to a respective depth down borehole 220, as shown in Figure 5.

[0021] 6 calibrates a model for each representation of the energy distribution of ionizing radiation identified at each location within the medium. That is, at each depth, the model is calibrated to the data at that respective depth. If system 300 is used to record three data sets, for example, at depths i=1, 2, and 3, method 600 may be run three times to identify one or more attributes of the data at each depth. As illustrated, in block 620, depth i is selected by acquiring appropriate data recorded with detector 320 at that particular depth.

[0022] Referring to FIG. 6, the method 600 includes a block 610 that initializes one or more variables used in the method 600 .

[0023] In block 610, a region of interest (ROI) of the energy spectrum is selected. The ROI is defined between a lower energy bound L and an upper energy bound U. The ROI is defined as 、137 The ROI is intended to define or capture an energy region corresponding to the photopeak of ionizing radiation from a first radiation source, such as Cs. The lower and upper energy limits may be defined by channel numbers in some embodiments. The lower energy limit may be defined by L, and the upper energy limit may be defined by U, where L and U may be channel numbers or energy values. Thus, ROI[L,U] defines a portion of the energy spectrum as a region of interest. In block 610, one or more other variables may be initialized, such as one or more of A0, B0, C0, T0, σ0, μ0, b0, where a subscript 0 indicates an initial value. A is the amplitude of the Gaussian peak, B is a coefficient such that AB is the amplitude of the step component, C is a coefficient such that AC is the amplitude of the tail component, T is a dimensionless parameter that scales the rate at which the tail component decays, σ is the standard deviation of the Gaussian peak, μ is the centroid of the Gaussian peak, b is a constant that may be small and therefore may be omitted in some embodiments.

[0024] The initial values ​​of U and L were chosen by observing typical spectra acquired. L can be the channel number to the right of the Compton edge shoulder (before the detected counts start to curve against the channel number). U can be the channel number to the right of the photopeak where the detected counts are comparable or similar to the background noise.

[0025] As mentioned above, in block 620, data at depth I, such as i=1 as an example, is selected. In further iterations of the method, other depths may be selected, i=2, 3, ..., until all depths have been considered by method 600.

[0026] In block 630, the model f1 is calibrated or fitted to data representing a representation of the energy distribution of ionizing radiation identified at the selected depth. As described below, the model is 137 In some embodiments, the model f1 includes a first portion representing a photopeak of ionizing radiation from a first radiation source, such as Cs. 90 It includes a second portion representing ionizing radiation produced by a second source, such as Sr.

[0027] The model f1 is given by Equation 1: f1=G(x)+S(x)+T(x)+b Equation 1 where x represents the channel number or energy. The first portion G(x) represents the photopeak of the ionizing radiation as defined in Equation 2.

number

[0028] It will be understood that the photopeak is the region of the energy distribution spectrum resulting from complete photoelectric absorption of gamma rays by the scintillator 321 of the detector 320. G(x) is a Gaussian peak function and may represent all energy rays, A is the amplitude of the Gaussian function, μ is its centroid, and σ is the standard deviation.

[0029] 7 is a plot of detector data (dots) along a curve 710 representing the photopeak G(x). FIG. 8 is a plot of the total number of counts versus the depth of the detector 320 in the medium 200.

[0030] In some embodiments, model f1 includes a second portion that may represent a second radiation source. The second portion may include one or both of the second and third functions defined by Equation 3 and Equation 4, respectively.

number

number

[0031] S(x) is a step function that represents the step discontinuity that may be seen in the continuum under the Gaussian peak toward the low energy side. S(x) may be generated by the detection of Compton scattered photons from the surrounding material to the detector 320, convolved with Gaussian noise. B is the step function amplitude expressed as a fraction of the A amplitude of G(x), and erfc(x) is the complementary error function.

[0032] T(x) is a tail function representing the exponential discontinuity that may be seen in the continuum under the Gaussian peak toward low energy. T(x) may represent the effect of insufficient charge collection in the detector volume, modeled by an exponentially decaying distribution of counts below the peak, convolved with Gaussian noise. C is the amplitude of T(x) expressed as a percentage of A, and T is the slope of the exponential function. In some embodiments, b is a constant representing residual background counts.

[0033] Plot 720 in Figure 7 represents S(x) + T(x) + b. The second part of the model formed by Equations 3 and 4 represents the secondary radiation that gives rise to a detection event at detector 320, 137 The events from Cs are considered primary emissions. Secondary emissions are 90 Sr and 90 Bremsstrahlung from Y, 137 These are scattered photons from Cs, spontaneous emission, etc. The components S(x), T(x) and b in Equation 1 represent secondary emissions as shown at 720 in FIG.

[0034] generated that may reach the detector 320 90Sr and / or 90 The Y bremsstrahlung photons contribute to the measured spectrum, whose individual spectra extend up to an energy equal to the beta particle energy. Significant yields are limited to low energies (below the ROI), but the bremsstrahlung spectrum continues up to a given maximum, so some counts add up at higher energies (ROI) as well.

[0035] In FIG. 7, model f1, which corresponds to the combination of the first and second portions discussed above, is shown by line 730, and the 1 sigma range is shown by dashed line 740.

[0036] The model f1 is fitted to the data at the selected depth, i.e., depth i, in block 630. The model may be fitted using an appropriate fitting algorithm, such as a least-squares minimization algorithm. The least-squares minimization algorithm may be the Levenberg-Marquardt algorithm for finding a local minimum, although other algorithms may be used. The fitting algorithm aims to find values ​​of the unconstrained parameters that fit the function f1 to the data at the selected depth i, which in some embodiments may include one or more of A, B, C, T, σ, μ, and b. The fitting algorithm may begin at an initial iteration using the initial values ​​determined in block 610, i.e., A0, etc. In some embodiments, a fitting algorithm, i.e., scipy.optimize.curve_fit() in Python, may be used, although other algorithms are available. A fitting algorithm, such as scipy.optimize.curve_fit(), determines the optimal unconstrained parameter values ​​to fit the data, i.e., the dots in FIG. 7.

[0037] In block 640, it is determined or checked whether any errors occurred with the fitting process. If one or more errors occurred, such as the fitting being unable to find optimal parameters, one or more adjustments or changes are made in block 650. After one or more adjustments, block 630 is repeated. In block 650, in one embodiment, the ROI may be adjusted, such as by changing its position in the energy spectrum or its size, or both. In one embodiment, in block 650, the value of U is changed. The value of U may be changed by lowering the value of U, i.e., to a lower energy. For example, block 650 may include U=U−1, i.e., lowering the value of U, the upper limit of the ROI, by one channel.

[0038] In block 650, some embodiments may check whether U<μ, i.e., whether the upper limit is less than the value of the centroid of the Gaussian peak. If so, method 600 may end. However, if there are no errors and optimized parameters are identified in block 630, the method proceeds to block 660.

[0039] At block 660, one or more attributes associated with the photopeak are calculated. As discussed above, the photopeak is defined in the above model f1 by G(x). In some embodiments, the one or more attributes are calculated based on the area under the Gaussian peak N at a selected depth i, as defined by Equation 5 below: i N i may be the number of counts corresponding to the photopeak.

number

[0040] The uncertainty u associated with the area under the Gaussian peak can be defined by Equation 6:

number

[0041] In some embodiments,

number

[0042] Method 600 may be repeated for each depth i for which a data set has been identified. For example, method 600 may be repeated at each of the four depths in this example to identify a respective value of N at each i. N is determined as the total number of counts corresponding to photopeaks.

[0043] FIG. 10 shows the N as a value 1010 determined by the method 600 described above at each of several depths (only two of which are shown) matched with a radiation source at a depth of approximately 80 cm. i Illustrated is a plot of: Given the count values ​​determined at each of a plurality of depths, it is desirable to determine the location of the radiation source 210, such as the depth of the radiation source within the medium 200.

[0044] 9 illustrates a method 900 according to a further embodiment of the present invention. Computer-readable instructions representing the method 900 may be stored in the memory 312 and executed by the processor 311. Thus, the system 300 may implement one embodiment of the method 900. The method 900 is a method for locating a radiation source 210 within the medium 200. The method 900 utilizes the output of the method 600 described above. The method 900 receives an indication of the output of the detector 320 corresponding to a photopeak of a first radiation source at each of a plurality of depths within the medium 200. As described above, the photopeaks may be 137 It is caused by ionizing radiation from a primary source such as Cs.

[0045] It has been found that the location of the radiation source 210 within the medium 200, such as the depth of the radiation source 210 within the soil, can be identified using a model f2 that includes an asymmetric function. The asymmetric function can represent one or more attributes of the first portion of each of the multiple models. The asymmetric function can be an asymmetric point spread function. Similar functions have been used in astrophysical problems. However, the inventors surprisingly recognized that such functions can be used in problems within the field of the present invention. The asymmetric function can be a Moffatt function with a skew component. Such asymmetric functions are believed to be useful in a borehole environment due to the blind ends of the borehole causing skew in the distribution.

[0046] In one embodiment of the present invention, model f2 is

number

[0047] 9, block 910 includes initializing values ​​of unconstrained variables used in method 900. Block 910 may include initializing values ​​of one or more of A, β, μ, w, ​​and γ, where the subscript 0 indicates an initial value, which may be, for example, a predetermined or random initial value.

[0048] At block 910, the uncertainty of multiple values ​​of N is calculated as:

number

[0049] In block 920, model f2 is fitted to the data at multiple depths, i.e., the data received from method 600. Model f2 may be fitted using an appropriate fitting algorithm, such as a least-squares minimization algorithm. The least-squares fitting algorithm may be the Levenberg-Marquardt algorithm for finding a local minimum, although other algorithms may be used. The fitting algorithm aims to find values ​​for one or more of the unconstrained parameters discussed above that fit function f2 to the data at multiple depths. The fitting algorithm may begin at an initial iteration using the initial values ​​determined in block 910, i.e., A0, etc. In some embodiments, a fitting algorithm may be used, i.e., scipy.optimize.curve_fit() in Python, although other algorithms are available. A fitting algorithm, such as scipy.optimize.curve_fit(), determines optimal values ​​for the unconstrained parameters.

[0050] In block 920, a test of independence is

number

number

number

[0051] In block 940,

number

[0052] The above provides a return to further iterations of block 920 to improve the fitting of the model.

[0053] FIG. 10 illustrates the fitting of model 1000 to data 1010 from method 600. As a result of blocks 920-940, attributes indicative of radiation source 210 are identified. The attributes include μ, which is the estimated depth of radiation source 210 within medium 200. Thus, an asymmetric function is used to identify the depth of the radiation source within medium 200. The depth may be output to a user, such as on a display screen. However, some embodiments of the method include an additional block (not shown) that excavates or excavates a distance to or beyond the depth of radiation source 210 to remove material that forms the radiation source. System 300 may control excavation to the identified depth. For example, data indicative of the depth may be provided to a robotic excavator. The removed material is removed and stored with appropriate safety precautions, such as encasing it in a shielding material, such as concrete.

[0054] It will be appreciated that embodiments of the present invention can be used to identify the location of a radiation source within a medium, such as the depth of a radiation source in soil, using multiple measurements of ionizing radiation at each location. In this manner, a convenient and safe method of locating a radiation source is provided. For example, embodiments of the present invention can be used to locate and remove radioactive contamination in soil.

[0055] It will be understood that embodiments of the present invention can be realized in the form of hardware, software, or a combination of hardware and software. Any such software can be stored in the form of volatile or non-volatile storage, such as a storage device, e.g., ROM, whether erasable or rewritable, or in the form of memory, e.g., RAM, memory chips, devices, or integrated circuits, or on an optically or magnetically readable medium, e.g., CD, DVD, magnetic disk, or magnetic tape. It will be understood that a storage device or medium is an embodiment of machine-readable storage suitable for storing one or more programs that, when executed, implement embodiments of the present invention. Thus, embodiments provide a program including code implementing the system or method according to any preceding claim, and machine-readable storage having stored such a program. Furthermore, embodiments of the present invention can be transmitted electronically via any medium, such as a communication signal carried via a wired or wireless connection, and the embodiments encompass this, as appropriate.

[0056] All of the features disclosed in this specification (including any accompanying claims, abstract and drawings) and / or all of the steps of any disclosed method or process may be combined in any combination, except combinations in which at least some of such features and / or steps are mutually exclusive.

[0057] Each feature disclosed in this specification (including any accompanying claims, abstract, and drawings), unless expressly stated otherwise, may be replaced by alternative features serving the same, equivalent, or similar purpose. Thus, unless expressly stated otherwise, each feature disclosed is only an example of a generic series of equivalent or similar features.

[0058] The invention is not limited to the details of any of the above embodiments. The invention extends to any novel one or any novel combination of features disclosed in this specification (including any accompanying claims, abstract, and drawings), or to any novel one or any novel combination of steps of any method or process disclosed. The claims should not be construed to cover merely the above embodiments, but rather to cover all embodiments that fall within the scope of the claims.

Claims

1. 1. A computer-implemented method for locating a radiation source within a medium, comprising: determining, at each of a plurality of locations of a scintillation detector within the medium, a representation of the energy distribution of ionizing radiation incident on the detector; calibrating one of a plurality of first models to the representation of the energy distribution of ionizing radiation identified at each of the plurality of locations within the medium, each first model including a first portion representing a photopeak of ionizing radiation from a first radiation source, to identify a photopeak portion of the energy distribution at each respective location; calibrating a second model including an asymmetric function to the photopeak portion of the energy distribution at the plurality of locations to identify the location of the radiation source within the medium; 10. A computer-implemented method comprising:

2. The method of claim 1 , wherein each of the plurality of first models includes a second portion representing ionizing radiation incident on the scintillation detector caused by a second radiation source.

3. The method of claim 2 , wherein the second portion further represents background radiation incident on the scintillation detector.

4. 4. The method of claim 2 or 3, wherein the second portion further represents scattered radiation from the first radiation source incident on the scintillation detector.

5. The method of any one of claims 2 to 4, wherein the second portion represents bremsstrahlung photons incident on the scintillation detector, which are produced by the second radiation source.

6. The method of any one of claims 1 to 5, wherein the first portion comprises a Gaussian distribution representative of the photopeak of ionizing radiation from the first radiation source.

7. The first portion G(x) is [Equation 1] 7. The method of claim 6, wherein A is the amplitude of a Gaussian peak, x denotes energy, μ is the centroid of the Gaussian peak, and σ is the standard deviation of the Gaussian peak.

8. A method according to any one of claims 2 to 5 or any claim dependent thereon, wherein the second part comprises a step function.

9. The method of any one of claims 2 to 5 or any claim dependent thereon, wherein the second part comprises a tail function.

10. 10. The method of claim 1, wherein calibrating one of the plurality of first models comprises calibrating one or more variables associated with a Gaussian peak of the model to data indicative of the energy distribution of ionizing radiation at each of the positions in the medium in dependence on a fitting algorithm, optionally wherein the fitting algorithm is a least-squares minimization algorithm.

11. The method of any one of claims 1 to 10, wherein the asymmetric function is skewed towards greater detection of incident radiation at lower depths.

12. The method of any one of claims 1 to 11, wherein the asymmetric function comprises a Moffatt distribution.

13. the asymmetric function represents a peak; and [Equation 2] The method according to any one of claims 1 to 12, in the form of

14. where A is the amplitude of the peak, β is the width of the tail of the peak, x is the depth of the detector, μ is the position of the radiation source within the medium, γ is a value indicating the skewness of the peak, and w is the half-width at half maximum of the peak.

15. 15. The method of any one of claims 1 to 14, wherein calibrating the second model comprises calibrating one or more variables associated with the asymmetry function to the photopeak portion of the energy distribution at the plurality of locations in dependence on a fitting algorithm, optionally wherein the fitting algorithm is a least-squares minimization algorithm.

16. 16. The method of any one of claims 1 to 15, comprising calibrating a plurality of first models to respective representations of the identified energy distribution of ionizing radiation to identify a photopeak portion of the energy distribution at each respective location within the medium.

17. The first radiation source comprises: １３７ 17. The method of any one of claims 1 to 16, comprising Cs.

18. The second radiation source comprises: ９０ The method of any one of claims 1 to 17, comprising Sr.

19. A method according to any preceding claim, comprising outputting an indication of the position of the radiation source within the medium, optionally the indication being output to a display device.

20. A method according to any preceding claim, comprising controlling an excavator to excavate to the location of the radiation source.

21. 1. An apparatus for locating a radiation source within a medium, comprising: an interface for receiving data indicative of a representation of the energy distribution of ionizing radiation incident on a scintillation detector at each of a plurality of locations of the detector within the medium; a processor; A memory containing instructions; the instructions, when executed by the processor, cause the processor to: calibrating one of a plurality of first models to the representation of the energy distribution of ionizing radiation identified at each of the plurality of locations within the medium, each model including a first portion representing a photopeak of ionizing radiation from a first radiation source, to identify a photopeak portion of the energy distribution at each respective location; calibrating a second model including an asymmetric function to the photopeak portion of the energy distribution at the plurality of locations to identify the location of the radiation source within the medium; A device that performs the following.

22. 22. A system comprising an apparatus according to claim 21 and a scintillation detector configured, in use, to provide an indication of the energy distribution of ionising radiation incident thereon.

23. 23. The system of claim 22, comprising an actuator configured to control a position of the scintillation detector within the medium in dependence on a control signal from the device, optionally the actuator comprising a winch.

24. Computer software arranged to carry out the method of any one of claims 1 to 20 when executed by a computer.

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