Method for estimating dose rate based on spectral image

Abstract

An embodiment of the invention relates to a method for estimating a dose rate based on measurements performed by a gamma camera (2), the gamma camera defining an observation field (Ω), the estimated dose rate originating from a radiation source (10 a , 10 b ), these radiation sources emit ionizing electromagnetic radiation; the observation field is discretized into a grid; the gamma camera (2) includes pixels (2 j ), each pixel being configured to detect ionizing electromagnetic radiation during an acquisition time and thereby forming an energy spectrum, each pixel being associated with at least one point of a grid, said pixels together allowing to obtain the position of a radiation source in an energy band or in a plurality of energy bands in the observation field; the method comprising estimating a dose rate produced on a gamma camera by a point of the grid.

Description

Technical Field

[0001] The technical field of the invention is the characterization of radiation sources present in the environment, in particular in nuclear facilities or facilities comprising radiation sources. Background Art

[0002] Gamma cameras are devices that can generate images that map radiation sources in a given environment, particularly within nuclear facilities. These devices were developed in the 1990s and are increasingly used in nuclear facilities for radiological characterization. The goal is to identify the primary radiation sources within the facility. Radiation sources are not uniformly distributed. They are often concentrated in localized areas, forming "hot spots," as the term is commonly used in the radiation protection field. The advantage of gamma cameras is that they can locate these hot spots at a distance.

[0003] The development and use of gamma cameras has been extensively described in the literature. Spectral gamma cameras have been under development since the early 2000s. These cameras are based on pixelated imagers, where each pixel can derive a spectrum from the radiation it detects. This makes it easier to localize the radiation source. In particular, the spectral measurement function can select an energy band of interest corresponding to non-scattered photons—that is, photons that have not been deflected since being emitted by the radiation source. The path of non-scattered photons is straight. By selecting within a predetermined energy band, noise corresponding to scattered photons can be eliminated. Since these photons have been deflected since being emitted, they do not provide useful information about the location of the illumination source. Scattering is therefore a source of noise that can significantly limit spectroscopic methods.

[0004] Another advantage of spectral gamma cameras is that knowledge of the photon energy allows identification of the isotope responsible for the radiation. This is important information in the field of radiation protection, in the management of radioactive waste, and even when dismantling nuclear facilities or performing radiological characterization after accidents.

[0005] Once a radiation source is detected, the problem arises of determining the level of radiation it produces. Radiation levels are usually expressed as a dose rate, conventionally in Gy / h, or as an equivalent dose rate, conventionally in Sv / h. The dose rate corresponds to the amount of energy released per unit volume, while the equivalent dose rate is a unit used for radiation protection purposes and quantifies the biological damage caused by radiation. When the radiation is caused by photons, the equivalent dose rate corresponds to the dose rate.

[0006] Estimation of the dose rate produced by a radiation source first requires knowledge of the emission energy of that source. This information can be obtained using a spectral gamma camera.

[0007] Furthermore, the dose rate produced by a radiation source varies with distance from the source. When the source is point-like, the radiation is known to vary as the inverse of the square of the distance. Therefore, by estimating the dose rate at a point at a known distance from the source, it is possible to estimate the dose rate produced by the source at any point in the observation field, provided that the source can be considered point-like. However, when attempting to estimate radiation near a source, the assumption of point-like nature is no longer necessary.

[0008] Difficulties arise when the radiation source is planar. Specifically, in this case, the spatial distribution of the source must be considered. Models can be generated using computational codes, but this is time-consuming and still subject to the validity of the adopted model, particularly the geometric assumptions regarding the extent and homogeneity of the source.

[0009] The inventors have provided a method for generating a radiation map in the form of dose rate or equivalent dose rate for a facility comprising planar radiation sources, which are not necessarily uniform. A planar source is a source that cannot be considered as a point. Summary of the Invention

[0010] A first subject of the invention is a method for estimating the dose rate based on measurements made by a gamma camera defining an observation field, the method being as follows:

[0011] - the estimated dose rate originating from radiation sources emitting ionizing electromagnetic radiation located in the observation field;

[0012] - Discretize the observation field into a grid;

[0013] - the gamma camera comprises pixels, each pixel being configured to detect ionizing electromagnetic radiation during an acquisition time and to form an energy spectrum therefrom, each pixel being associated with at least one point of the grid such that the pixels together can obtain a spatial distribution of radiation sources in the observation field in one or more energy bands;

[0014] The method comprises the following steps:

[0015] a) using pixels to collect spectra;

[0016] b) considering the emission spectrum, which is defined as one or more energy bands;

[0017] c) Select the points of the grid;

[0018] d) estimating, for the points of the selected grid, the spatial distribution of the activity corresponding to the emission spectrum considered in step b) based on the spectrum acquired by each pixel;

[0019] e) in each energy band of the emission spectrum, taking into account a pre-established transfer function relating the dose rate to the photon flux detected by the gamma camera;

[0020] f) estimating the dose rate generated in the gamma camera by the set of points selected in step c) based on the transfer function resulting from step e) and the spatial distribution of the activity determined in step d).

[0021] The photon flux detected by the gamma camera refers to the photon flux detected by the pixels of the gamma camera.In step f), the estimated dose rate corresponds to the dose rate corresponding to the emission spectrum considered in step b).

[0022] Dose rate refers to the dose rate or equivalent dose rate.

[0023] According to one embodiment:

[0024] - step b) taking into account an isotope or a group of isotopes that may be present in the observation field, the emission spectrum corresponding to the emission spectrum of this isotope or the emission spectrum of this group of isotopes;

[0025] - Step d) consists in estimating the spatial distribution of the activity of the isotope or the group of isotopes in the observation field.

[0026] The dose rate estimated in step f) then corresponds to the dose rate generated by the isotope or group of isotopes under consideration.

[0027] The method may include any of the following features, employed individually or in any technically feasible combination:

[0028] - In step b), the emission spectrum considered comprises one or more energy bands.

[0029] - Step d) takes into account a spatial model associated with each pixel of the image, this spatial model defining the probability that a photon emitted by each point of the grid is detected by the pixel associated with this spatial model.

[0030] - In step d), the activity spatial distribution corresponds to the distribution of the activity on the surface of the object.

[0031] - estimating the transfer function by simulation or by exposing at least one pixel of the gamma camera to a calibration radiation source, whereby the dose rate to which the pixel is exposed is known.

[0032] In each energy band of the emission spectrum, and for each pixel, step d) comprises:

[0033] Estimate the photon flux detected by a pixel in an energy band based on the spatial distribution of activity at selected points in the grid;

[0034] Determining the photon flux detected by the pixel in the energy band based on the spectrum detected by the pixel;

[0035] Step d) further comprises minimizing, in each energy band and for each pixel, the difference between the estimated photon flux and the measured photon flux.

[0036] The gamma camera is associated with a rangefinder for measuring the distance between the gamma camera and the observed scene. For example, after step f), the method may further comprise:

[0037] g) measuring the distance between the gamma camera and at least one point of the observation field;

[0038] h) using the distance measured in step g) and the activity space distribution estimated in step d), estimating a dose rate generated by the activity space distribution at a position different from the position occupied by the gamma camera.

[0039] - Step h) may take into account the dose rate estimated in step f).

[0040] - The dose rate estimated in step f) is an equivalent dose rate.

[0041] - Step c) consists in selecting all or some of the points of the grid of the observation field.

[0042] A second subject of the invention is a measuring device comprising:

[0043] a gamma camera comprising pixels, each pixel being configured to detect, during an acquisition time, ionizing electromagnetic radiation emitted by at least one radiation source located in the field of observation of the gamma camera and to form an energy spectrum therefrom, each pixel being associated with at least one point of a grid of the field of observation, such that the pixels together make it possible to obtain a spatial distribution of each radiation source in the field of observation in one energy band or in a plurality of energy bands;

[0044] - a processing unit configured to:

[0045] Receive spectra collected by multiple pixels of a gamma camera;

[0046] Based on the acquired spectrum, at least steps b) to f) of the method according to the first subject matter of the invention are performed. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] The present invention will be better understood by reading the description of exemplary embodiments, which is described below with reference to the accompanying drawings listed below.

[0048] Figure 1A A gamma camera is shown schematically.

[0049] Figure 1BSchematically shown are the pixels of a gamma camera.

[0050] Figure 1C Schematically shows an image in one energy band acquired by a gamma camera.

[0051] Figure 2 The main steps of the method according to the invention are shown.

[0052] Figure 3 The position of the radiation source on the surface of a planar object is shown.

[0053] Figure 4A An example of the spatial response function of a gamma camera including a coded aperture collimator is shown.

[0054] Figure 4B An example of the spatial response function of a Compton gamma camera is shown.

[0055] Figure 4C An example of the spectral response matrix of the pixels of a gamma camera is shown.

[0056] Figure 5 The dose rate-to-energy conversion function is shown as a function of energy.

[0057] Figure 6A 、 6B and 6C are embodiments of the present invention. DETAILED DESCRIPTION

[0058] Figure 1A A measurement device 1 in which the present invention may be implemented is shown. The measurement device includes a gamma imager 2 or a gamma camera. The gamma imager is configured to detect ionizing electromagnetic radiation, such as X-rays or gamma rays, with energies typically ranging from 10 keV to 10 MeV, in an observation field Ω. The observation field extends around a central axis Δ. The device may include a rangefinder 3 coupled to the gamma camera, as described below.

[0059] Gamma camera included with Pixel 2 j , each pixel corresponds to a basic spatial region of the observation field. Figure 1B When the elementary spatial region corresponding to a pixel includes a radiation source that emits X-rays or gamma rays, some of the radiation emitted by the source reaches the pixel and is detected by the pixel. Therefore, as the elementary spatial region associated with it becomes more radioactive (i.e., emits more X-rays or gamma rays), the amplitude of the signal of the pixel of the gamma image increases. In the remainder of the description, examples related to gamma radiation sources are given, which correspond to the most common application cases. This can be directly converted into an X-ray radiation source.

[0060] Typically, the Pixel 2 jare coplanar and preferably regularly distributed in a two-dimensional matrix array. The matrix array may, for example, comprise 512×512 pixels, or even more. Each pixel 2 j It is a basic radiation detector.

[0061] The gamma imager can be a Compton gamma camera, a pinhole collimator gamma camera, or a coded aperture gamma camera. Non-exhaustively, collimation can also be a problem for gamma cameras with parallel, converging, or diverging channels. Therefore, the term "gamma camera" corresponds to an imager having an observation field and configured to form an image that allows a radiation source to be located in the radiation field. Regardless of the type of gamma imager, it allows the formation of a gamma image comprising pixels, each pixel corresponding to an elementary spatial region of the observation field. The observation field Ω can be discretized into a grid in coordinates (x, y). Each pixel can therefore be associated with one or more points of the grid. When using a Compton gamma camera, the correspondence between pixels and points of the grid changes depending on the detected interaction.

[0062] Preferably, each pixel 2 j Performs a spectroscopic function, in the sense that it can spectrally separate the radiation detected during the acquisition time into different spectral bands or energy bands. When using this type of pixel, different gamma images of a given observation field can be formed, each corresponding to an energy band (denoted as E i ). Each energy band E i The width dE i The energy resolution of each energy band can be variable and depends on the performance of the pixel in terms of energy resolution. The width of each energy band can be about 1 keV, or a few keV, or even tens of keV.

[0063] The acquisition time T of a spectrum by each pixel depends on the photon flux to which the pixel is exposed. It can be tens of milliseconds or seconds, and can last for minutes or even hours. The gamma spectrum acquired by each pixel can then include intensity peaks corresponding to the emission intensity of the known isotope.

[0064] Known emission spectrum S k Associated with each isotope k. Such an emission spectrum corresponds to a histogram of the emission rate as a function of energy. The emission rate is the number of photons emitted per unit activity of the isotope. Typically, the unit activity is 1 Bq. Therefore, in each energy band, the emission spectrum corresponds to the number of photons emitted for the unit activity in question, in this case 1 Bq.

[0065] This can be done by considering the emission spectrum S corresponding to the isotope kThe gamma image is created by combining the spectral bands of the isotope in question. This combination can be a weighted sum. This image then represents the spatial distribution of the activity of the isotope in question.

[0066] exist Figure 1C In the example schematically shown in FIG, two radiation sources 10 have been shown. a , 10 b , which emits in a spectral band centered at 661.66 keV, which corresponds to the isotope shown 137 Cs. In this energy band, the higher the photon flux detected by a pixel, the brighter the pixel appears in the image.

[0067] For some gamma imagers, in particular Compton gamma cameras or coded aperture gamma cameras, the images acquired by the imager do not allow direct observation of the radiation sources in the observation field. Taking into account the response function of the camera, the acquired images are processed so as to obtain a gamma image in each energy band, in which the intensity of each pixel corresponds to the flux of detected photons originating from each point of the grid.

[0068] The processing unit 4 receives the image of each pixel 2 of the gamma camera 2. j The image processing unit is particularly configured to perform Figure 2 The described operation.

[0069] The observation field Ω is gridded in such a way that it is discretized into points. Since the observation field is not known a priori, it can be compared to the surface of a virtual object P O On the virtual object surface, each observation point has coordinates (x, y). An important element of the present invention is to regard the points of the object reference system as belonging to the same object surface P O .

[0070] According to the first method which is easy to implement, the surface P of the object O is a flat surface. The angular field of view Ω of the gamma camera extending around the optical axis Δ describes a portion of the sphere S (see Figure 3 ). Object surface P O corresponds to a plane tangential to the sphere S and perpendicular to the optical axis Δ. Radiation sources present in the observation field are considered coplanar and belong to the object plane. The distance between the detection plane and the object surface is arbitrary and may be unknown. When the device includes a rangefinder 3, the distance between the object surface and the camera is established by at least one distance measurement performed by the rangefinder. The rangefinder may be a LIDAR, allowing the acquisition of distances at multiple points of the observation field. The observation field can then be considered a non-planar surface defined by the distances measured at multiple points.

[0071] Each pixel of a gamma camera is characterized by a spatial response function and a spectral response function.

[0072] For each pixel 2 j Establish spatial response function B j (x,y). The spatial response function corresponds to the pixel 2 j The probability of detecting a photon emitted by a point (x,y) in the observation field. Therefore, each pixel 2 j With a spatial response function B established for all or some points (x, y) of the observation field Ω j (x,y). The spatial model can be constructed analytically or by modeling. A spatial response function can be constructed for the isotope k, in this case it is expressed as B j，k (x,y). The photon it emits to pixel k is reflected by pixel 2 at point (x,y) in the observation field. j The probability of detection was quantified.

[0073] Figure 4A Figure 2 shows the spatial response model of a gamma camera using a coded aperture. The spatial model shown corresponds to an angular field of view of 180°×180°. Figure 4B The spatial response model is shown when the gamma camera is a Compton gamma camera. In this configuration, the spatial response model changes according to the detection of the interaction. Figure 4B In the model shown, the dashed lines represent the directions defined by the detection of two interactions, the latter represented by the black dots. The spatial model is defined by the scattering angle θ, which is defined by measuring the energy released at each interaction. Figure 4A and 4B In the image, the lower the gray level (i.e., the darker the tone), the higher the probability of photon emission.

[0074] A spatial model can also be determined for a given isotope. In this case, the spatial model takes into account the emission energy and its respective branching ratio. The spatial model can then determine the probability of the isotope being present.

[0075] Figure 4C For each pixel 2 j The established spectral response function A j Spectral response function A j Corresponds to the following probability: with energy E P The incident photon is captured by pixel 2 j After detection, it is considered to have energy E i In other words, the spectral response function A j When measuring energy E i With the incident energy E p A connection was established between them. Figure 4CThe spectral response function shown is the response matrix. The size of the response matrix is ​​P×I, where I is the number of pixels 2 j The number of channels (energy bins) for each spectrum formed, and P is the number of channels in which the incident spectrum is discretized. Each entry of the response matrix A j (E p , E i ) represents pixel 2 j The energy incident on the detector is considered to be E p The photon has energy E i probability.

[0076] In the following, the response matrix is ​​considered to be the same for every pixel and is denoted as A.

[0077] like Figure 4C As shown, each row of the matrix A(E p ,.) corresponds to when the photon incident on the detector has energy E p The energy E is detected by the detector i The probability distribution of . Figure 4C As shown, each column of the matrix A(.,E i ) corresponds to when the photon detected by the pixel has energy E i The incident energy E p The probability distribution of . In the following, each column A(., E i ) is designated as A i .exist Figure 4C In the example, the darker the gray level, the higher the probability. j In the case of an ideal detector, the matrix A is the identity matrix.

[0078] Now refer to Figure 2 The main steps of a method for estimating the dose rate produced by one or more radiation sources located in the observation field of a gamma camera are described below.

[0079] Step 100: Acquire a spectral image M.

[0080] In this step, the pixel size is sufficient to allow the camera to j The spectrum image M is acquired by acquiring the available spectrum acquisition time. j Each spectrum is composed of one pixel 2 during the acquisition period. j Each spectrum M j Included in the energy band E i The photon flux M detected in i，j . Flux M i，j is the energy band E per unit time i Pixel 2 jThe number of photons detected.

[0081] Step 110: Select one or more energy bands to form an emission spectrum. In this step, for different pixels 2 j , select one or more energy bands. This selection can be made in advance. This is especially the case when one or more isotopes that may be present in the observation field are known in advance. Typically, the selection will be based on a list of about ten or dozens of possible gamma-ray emitting isotopes, the respective emission spectra of which are known. In some nuclear facilities, the list may contain only a few isotopes that are considered to be dominant, or even a single isotope. In the following, each isotope is represented by an integer k included in 1 to K. K is the number of potential isotopes. As mentioned above, each isotope k is associated with an emission spectrum S k The emission spectrum of an isotope consists of the emission energies (which are discrete) and the branching ratios associated with each energy. The branching ratios correspond to the emission probabilities.

[0082] According to an alternative, a plurality of isotopes can be selected and the emission spectrum formed by combining the emission spectra of each isotope. The combination can be, for example, a weighted sum. Thus, an emission spectrum comprising a predetermined isotope mixture can be formed.

[0083] According to one possibility, the emission spectrum comprises only a single energy (e.g. when the 137 661.66keV for Cs) or multiple discrete energy bands (e.g. when the 60 1173keV and 1332keV for Co).

[0084] Step 120: Determine the flux detected in each energy band.

[0085] In this step, for each pixel 2 j , determine each energy band E selected in step 110 i The flux M detected in i，j . Flux M i，j Corresponding to the energy band E per unit time i The number of photons detected in .

[0086] Step 130: Modeling the detected flux

[0087] In this step, when the object surface P O Each isotope k has an apparent activity O k (x,y) is represented by 2 for each pixel j Detected flux When each point in the observation field is considered to belong to the object surface P O When the apparent activity Ok (x,y) corresponds to the activity of the isotope. Remember that the activity of an isotope corresponds to the number of decays per second. It depends on the apparent activity O k (x,y), by each pixel 2 j In the energy band E i The flux detected in for:

[0088]

[0089] in:

[0090] × is the element-wise product (Hadamard product);

[0091] ·A i (E) is a vector of size (1, I) corresponding to the column of matrix A for energy channel i;

[0092] ·S k (E) is the emission spectrum of isotope k, which is discretized into I energy bands; it takes the form of a vector of size (1, I);

[0093] ·B j、k (x, y) is the pixel 2 with isotope k j The associated spatial response function; it is a matrix of size (X,Y), where X and Y are the observation surface P discretized into coordinates (x,y) O size;

[0094] ·O k (x,y) is the spatial distribution of the apparent surface activity. It is a matrix of size (X, Y);

[0095] It should be noted that expression (1) includes the sum for each isotope k in question.

[0096] Pixel 2 j The inner energy band E i The contribution of isotope k in for:

[0097]

[0098] as well as

[0099]

[0100] Step 140: Determine the apparent activity

[0101] In step 140, in each energy band E i and for each pixel 2 j , the flux M detected in step 120i，j The flux modeled in step 130 For each isotope k in question, the problem is to find the i，j and The matrix O that minimizes the error (e.g., square error) between k (x,y).

[0102] therefore,

[0103]

[0104] According to a preferred embodiment, the minimization may be of Poisson type such that:

[0105]

[0106] This minimization can be performed using the MLEM algorithm (MLEM stands for Maximum Likelihood Expectation Maximization), which is known to those skilled in the art.

[0107] At the end of step 140, as many images as there are isotopes in question will have been obtained. k (x,y).

[0108] When isotopes (e.g. 60 Co) When there are different emission lines in different energy bands, the image O k (x,y) corresponds to the spatial distribution of the isotope activity, taking into account the emission spectrum S of the isotope k .

[0109] Step 150: Estimate Dose Rate

[0110] In step 150, an estimate is made of the dose rate of at least one isotope k, or even of the isotope k for which a significant apparent activity k has been detected at at least one point (x, y) of the grid. k The dose rate of each isotope k at (x,y) is estimated.

[0111] The dose rate generated by the isotope K on the pixel of the gamma camera is:

[0112]

[0113] It can also be written as:

[0114]

[0115] Scalar D i Yes, it can bring E i The value of the conversion function D in the energy band E is used to convert the photon flux into a dose rate. iEstablish the conversion function D in .

[0116] Corresponding to the emission spectrum considered, D k It is the dose rate usually expressed in Gy / h, or the equivalent dose rate usually expressed in Sv / h. It corresponds to different isotopes k or different emission spectra S k , different dose rates D k An estimate is made and the sum is taken for each of these dose rates.

[0117] Step 150 allows the dose rate generated by all or some of the isotopes k in the observation field to be estimated. This function allows the distribution of the different isotopes in the observation field to be evaluated.

[0118] Steps 110 to 150 may be performed for the entire observation field or for certain points of the observation field. For example, this may be a question of a point selected by the operator based on the spectral image acquired in step 100. For example, it may be a question of a specific area of ​​the observation field that includes a specific radiation source.

[0119] The method further comprises the following steps.

[0120] Step 160: Estimating the dose rate based on the distance

[0121] The gamma camera can be associated with a rangefinder 3 in order to estimate the distance between the gamma camera and different points of the observation field. The rangefinder can be optical, acoustic or electromagnetic. The distance d O Corresponding to the camera and object surface P O The distance between them.

[0122] This allows for a simple estimation of the dose rate at different distances. k (x,y)(i.e. the surface of the object P O If x' and y' represent the activity of the object parallel to the surface P O The dose rate produced by isotope k at this time can be estimated using the following expression:

[0123]

[0124] where c k is a factor that allows distance to be taken into account, and this factor will be described below.

[0125] In x' and y' coordinates, any point on the camera's optical axis has coordinates (0, 0).

[0126] The factor that allows taking distance into account is the gamma camera and the object surface P O Distance dO The following is then obtained:

[0127]

[0128] Among them D k (0, 0, d o )=D k , where D k is the dose rate obtained from step 150: see one of expressions (6) and (6').

[0129] It will be appreciated that as a result of step 140, the apparent activity O k The availability of (x,y) allows the estimation of the dose rate at different points of the observation field at different distances relative to the object plane. However, this assumes that the distance d between the gamma camera and the object surface is known. O , so that the distance factor c can be considered k Perform calculations.

[0130] In the above-described embodiments, the object surface is considered to be a planar surface. Given that the radiation sources are distributed over such a surface, the need for a three-dimensional reconstruction of each radiation source can be avoided. This is therefore a simplifying assumption that avoids the need for complex computational methods. According to a variant, the distance between the camera and multiple points of the observation field can be measured. This measurement can be achieved using a ranging sensor (e.g., a LIDAR sensor) that scans along the observation field. In this case, the object surface is a non-planar surface. It is defined based on the distance between the camera and different points in a grid, for which the distance to the camera is determined.

[0131] The above method assumes a priori calibration in order to determine the conversion coefficients D in multiple energy bands i . This is the subject of step 90.

[0132] Step 90: Determine the dose rate-photon flux conversion function

[0133] The dose rate at energy E can be obtained using the empirically derived conversion function:

[0134]

[0135] The conversion function D(E) allows conversion between dose rate and photon flux detected at energy E by a pixel of the gamma camera.

[0136] The parameters β, α and E0 can be determined by simulation.

[0137] β=3.5×10 -11 (Sv / h) / (keV / s);

[0138] α = 0.45;

[0139] E0=400keV.

[0140] Since the parameters β, α and E0 are known, expression (9) allows to obtain the conversion function for different energies.

[0141] The parameters of the transfer function D(E) can also be determined experimentally. This is done by exposing the gamma camera to a calibration source that produces a known emission spectrum. The radiation source can, for example, be monoenergetic, although this condition is not absolutely necessary. The energy band E is well characterized. i The dose rate D generated by the source at the camera is i .

[0142] The dose rate to which each pixel of a gamma camera is exposed can be shown:

[0143]

[0144] where T is the acquisition time, and S(E) is the spectrum acquired by the pixel during the acquisition time. θ is the set of parameters of the transfer function, namely β, α, and E0.

[0145] During calibration, a spectral S is performed using one or more pixels of the gamma camera. q (E) The number of acquisitions Q. Since the exposure source is known and the distance between the calibration source and the gamma camera is also known, during each acquisition the camera is exposed to a known dose rate D q Therefore, for each collection, it can be written as:

[0146]

[0147] Where T q is the acquisition time of each spectrum, and D θ，q (E) is the transfer function given by expression (9) during each acquisition.

[0148] By minimizing D q And each point The difference between θ and θ is used to estimate the parameters θ of the transfer function, which is parameterized by the set of parameters θ. Therefore, the error between the dose rate actually exposed by the camera and the dose rate estimated using the transfer function according to expression (9) is minimized.

[0149] therefore,

[0150]

[0151] is an estimate of the optimal parameters of the transfer function.

[0152] In expressions (6) and (6'), the coefficient D i for:

[0153]

[0154] These correspond to the parameters of the transfer function, which are estimated through modeling or experimental calibration.

[0155] Figure 5 An example of a conversion function is shown. A conversion function can be established for one pixel and applied to all pixels.

[0156] Gamma cameras comprising CdZnTe pixels have been used as well as using two point 57 Co source was tested. Figure 6A An image obtained at a distance of 1 meter is shown. The equivalent dose rates produced by each source upon contact were 91.7 μSv / h and 81.2 μSv / h, respectively.

[0157] based on Figure 6A The measurements shown are for 5 cm (see Figure 6B ) and 1m (see Figure 6C The equivalent dose rates at the two sources are estimated to be 50 μSv / h and 0.4 μSv / h. These are the dose rates produced by the two sources.

[0158] The present invention is applicable to different nuclear facilities, or more generally to operations for finding and characterizing radioactive sources.

Claims

1. A method for estimating a dose rate based on measurements performed by a gamma camera (2) defining an observation field (Ω), wherein: - Estimated dose rates from radiation sources located in the observation field (10 a , 10 b ), the radiation source emits ionizing electromagnetic radiation; - Discretize the observation field into a grid; - the gamma camera (2) comprises pixels (2 j ), each pixel being configured to detect ionizing electromagnetic radiation during an acquisition time and thereby forming an energy spectrum, each pixel being associated with at least one point of the grid such that said pixel allows obtaining the position of the radiation source in the observation field in one energy band or in a plurality of energy bands; The method comprises: a) Using pixel to collect spectrum (M j ); b) Considering the emission spectrum (S k ), the emission spectrum is defined as one or more energy bands (E i ); c) selecting a point (x, y) of the grid; d) For the selected grid point, according to each pixel (2 j ) collected spectra (M j ), estimate the activity (O corresponding to the emission spectrum considered in b). k (x,y)) spatial distribution; e) In each energy band (E i ), consider the pre-established conversion function (D i ), said transfer function relating the dose rate to the photon flux detected by the gamma camera; f) According to the conversion function (D) obtained from e) i ) and the activity determined in d) (O k (x,y)) to estimate the dose rate (D) produced by the point selected in c) on the gamma camera. k ); Wherein, in each energy band (E i ), and for each pixel (2 j ), d) include: - Estimate the photon flux detected by said pixel in said energy band based on the spatial distribution of the activity of the points of the chosen grid - According to the spectrum (M j ), in which the photon flux detected by the pixel is determined (M i，j ); in, d) further comprising: in each energy band and for each pixel, minimizing the difference between the estimated photon flux and the measured photon flux.

2. The method according to claim 1, wherein: -b) comprising: taking into account one or a group of isotopes that may be present in the observation field, said emission spectrum corresponding to the emission spectrum of this isotope or to the emission spectrum of this group of isotopes; -d) comprising: estimating the spatial distribution of the activity of the isotope or the group of isotopes in the observation field.

3. The method according to claim 1, wherein In b), the emission spectrum considered consists of a single energy band.

4. The method according to claim 1, wherein d) consists in taking into account a spatial model associated with each pixel, said spatial model defining the probability that a photon emitted by each point of said grid is detected by the pixel associated with said spatial model.

5. The method according to claim 1, wherein In d), the spatial distribution of the activity corresponds to the distribution of the activity on the surface of the object.

6. The method according to claim 1, wherein The transfer function is estimated by simulation.

7. The method according to claim 1, wherein The transfer function is estimated by exposing at least one pixel of the gamma camera to a calibration radiation source such that the dose rate to which the pixel is exposed is known.

8. The method according to claim 1, wherein The gamma camera is associated with a rangefinder (3) for measuring the distance between the gamma camera and the observed scene, and wherein the method comprises, after f): -g) measuring the distance between the gamma camera and at least one point of the observation field; - h) using said distance measured in g) and the spatial distribution of activity estimated in d), estimating the dose rate produced at a position different from the position occupied by the gamma camera by means of the spatial distribution of activity estimated in d).

9. The method according to claim 8, wherein h) takes into account the dose rate estimated in f).

10. The method according to claim 1, wherein The base rate estimated in f) is the equivalent dose rate.

11. The method according to claim 1, wherein c) comprising: selecting all or some points of the grid of the observation field.

12. A measuring device comprising: a gamma camera (2) comprising pixels, each pixel being configured to detect, during an acquisition time, ionizing electromagnetic radiation emitted by at least one radiation source located in the field of observation of said gamma camera and to form an energy spectrum therefrom, each pixel being associated with at least one point of a grid of said field of observation such that said pixel allows obtaining the spatial distribution of each radiation source in the field of observation in one energy band or in a plurality of energy bands; - a processing unit (4) configured to: Receiving a spectrum collected by a plurality of pixels of the gamma camera; Based on the acquired spectrum, at least methods b) to f) according to claim 1 are performed.

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