RESPONSE FUNCTION OF A SCINTILLATOR

Abstract

A method for generating a response function of a scintillator to incident gamma rays of energy within a range of energies of interest, the method including: obtaining the responses {S.sub.i} of the scintillator to a plurality of known radionuclides i (i=1, . . . N), each radionuclide i emitting gamma rays with known energetic properties (E.sub.ij, Y.sub.ij), decomposing, for each radionuclide i, response S.sub.i into primary responses of the scintillator S.sub.ij=ƒ(λ.sub.ij, Y.sub.ij, X.sub.ij), each primary response corresponding to the response of the scintillator to a received gamma ray of a known energy E.sub.ij for this radionuclide i, deriving from the primary responses {S.sub.ij} the response function ƒ(λ, X) of the scintillator to an incident gamma ray of any energy E within the range of energies of interest.

Claims

1. A method for generating a response function of a scintillator to incident gamma rays of energy within a range of energies of interest, the method comprising: (a) obtaining the responses {S.sub.i} of the scintillator to a plurality of known radionuclides i (i=1, . . . N), each radionuclide i emitting gamma rays with known energetic properties (E.sub.ij, Y.sub.ij), (b) decomposing, for each radionuclide i, the response S.sub.i into primary responses of the scintillator S.sub.ij=ƒ(λ.sub.ij, Y.sub.ij, X.sub.ij), each primary response corresponding to the response of the scintillator to a received gamma ray of a known energy E.sub.ij for this radionuclide i, (c) deriving from the primary responses {S.sub.ij} the response function ƒ(λ, X) of the scintillator to an incident gamma ray of any energy E within the range of energies of interest.

2. The method according to claim 1, wherein the known energetic properties of the gamma rays of a radionuclide i comprise possible values of energy E.sub.ij at which the gamma rays can be emitted, and for each energy value E.sub.ij, the corresponding yield Y.sub.ij.

3. The method according to claim 1, wherein the plurality of known radionuclides is chosen as to obtain after step (b) at least 15 different primary responses {S.sub.ij} of the scintillator.

4. The method according to claim 1, wherein the response {S.sub.i} being decomposed at step (b) through a factor analysis method.

5. The method according to claim 1, wherein the decomposition of the responses {S.sub.i} in step (b) comprising: via factor analysis, determining, for each monoenergetic or pseudo-monoenergetic radionuclide i of the plurality of radionuclides, at least one correlation coefficient λ.sub.i between the response S.sub.i of the scintillator to the radionuclide i and the factors, λ.sub.i depending on the energy E.sub.i at which gamma rays are emitted by the radionuclide i, computing, by interpolation of the coefficients {λ.sub.i} at least one correlation coefficient λ.sub.ij for each value of energy E.sub.ij at which gamma rays are emitted by a radionuclide i, where j=1, . . . , P with P being the number of possible values of energies for radionuclide i, determining, for each radionuclide i and energy j at least one model component X.sub.ij representing the response of the scintillator for each value of energy E.sub.ij at which gamma rays are emitted by a radionuclide i, X.sub.ij being preferentially determined by regression analysis of the responses of the scintillator to monoenergetic or pseudo-monoenergetic radionuclides and/or other primary responses with similar energy, determining, for each radionuclide i, from coefficients λ.sub.ij, the decomposition of the signal S.sub.i into primary responses S.sub.ij=ƒ(λ.sub.ij, Y.sub.ij, X.sub.ij).

6. The method according to claim 5, wherein M correlation coefficients {λ.sub.ij,k: k=1, . . . , M} being determined for each value of energy E.sub.ij at which gamma rays are emitted by a radionuclide i, M being greater than 1.

7. The method according to claim 1, wherein the response function ƒ being determined in step (c) using a factor analysis method.

8. The method according to claim 1, wherein at least one response S.sub.i being obtained in step (a) through direct experimental measurement.

9. The method according to claim 1, wherein the range of energies of interest is larger than [60 keV-1500 keV].

10. A method for determining the energy distribution of an incident gamma radiation, the method comprising: providing a plastic scintillator comprising a response function determined beforehand according to the method described claim 1, measuring the response of the scintillator when exposed to the incident gamma radiation, performing a deconvolution of the measured response using the response function of the scintillator, the deconvolution process generating an output energy spectrum representative of the energy distribution of the gamma radiation.

11. The method according to claim 10, comprising identifying at least one radionuclide present in the source of the incident gamma radiation based on at least one peak of the output energy spectrum representative of an energy value at which the incident gamma radiation can be emitted for the at least one radionuclide.

12. A radiation detector comprising: a sensor comprising a plastic scintillator producing photons when interacting with gamma radiation, at least one photodetector arranged to detect the photons produced by the plastic scintillator and generate a signal representative of the response of the detector to the gamma radiation, a memory storing data representative of a response function ƒ(λ, X) of the scintillator, response function resulting from the method defined claim 1, and, a processor using data to perform a deconvolution of the signal received by the photodetector and generate a deconvoluted signal representative of the energy distribution of the gamma radiation.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0051] For a more complete understanding of the present invention, a description will now be given of several examples, taken in conjunction with the accompanying drawings, in which.

[0052] FIG. 1 is a flowchart schematically illustrating an exemplary method for generating a response function of a scintillator in accordance with the invention,

[0053] FIG. 2 is a graph illustrating an example of responses {S.sub.ij} of the scintillator obtained in the first step of the method of FIG. 1,

[0054] FIG. 3 is a flowchart schematically illustrating further details of the second step of the method of FIG. 1,

[0055] FIG. 4 is a graph illustrating an example of primary responses {S.sub.ij} determined in the second step of the method of FIG. 1,

[0056] FIG. 5 is a graph illustrating an example of response function that can be obtained in the last step of the method of FIG. 1,

[0057] FIG. 6 shows a simplified schematic representation of a radiation detector in accordance with the invention,

[0058] FIG. 7 is a graph illustrating an example of the energy distribution of an incident gamma radiation determined by deconvolution of a measured response of a radiation detector,

[0059] FIG. 8 is a graph illustrating another example of the energy distribution of an incident gamma radiation obtained by deconvolution of a measured response of a radiation detector, and

[0060] FIG. 9 (already described) is a graph illustrating an example of response function of an inorganic NaI(TI) detector.

DETAILED DESCRIPTION

[0061] An exemplary method 1 for generating a response function of a scintillator is represented in a simplified manner in FIG. 1. The method 1 comprises three steps 10, 20 and 30 which correspond to the steps (a), (b) and (c) described above, respectively.

[0062] In the following, an example of application of the method 1 is given for generating the response function ƒ(λ, X) of a detector 5 comprising a plastic scintillator using the plurality of known radionuclides {i,i=1, . . . N} described in Table 1 below, which comprises 10 (N=10) different radionuclides.

[0063] The radionuclides used in this example can be monoenergetic (j=1), such as Am-241, Cs-137, Mn-54, Zn-65 and Hg-203, pseudo-monoenergetic, such as Co-57 and Co-60, or can emit gamma rays with distinct values of energy, such as Ba-133, Na-22 and Y-88.

TABLE-US-00001 TABLE 1 Nuclide i Energy E.sub.ij Yield Y.sub.ij /Energy line j Nuclide [keV] [%]  1/1 Am-241 59.5 36.3  2/1 Ba-133 79.6 2.5  2/2 Ba-133 81 33  2/3 Ba-133 276.4 6.9  2/4 Ba-133 302.8 17.8  2/5 Ba-133 356 60  2/6 Ba-133 383.9 8.7  3/1 Co-57 122.1 85.5  3/2 Co-57 136.5 10.6  4/1 Co-60 1173.2 100  4/2 Co-60 1332.5 100  5/1 Cs-137 661.7 85.1  6/1 Mn-54 834.8 100  7/1 Na-22 511 180.7  7/2 Na-22 1274.5 99.9  8/1 Y-88 898 93.4  8/2 Y-88 1836 99.4  9/1 Zn-65 1115.5 50.8 10/1 Hg-203 279.2 81

[0064] As illustrated in FIG. 1, in step 10, a detector 5 comprising a plastic scintillator is exposed to each individual radionuclide i of Table 1 emitting gamma rays of energies E.sub.ij. The response S.sub.i of the detector 5 to the gamma radiation, i.e. the absorbed energy spectrum, is measured from the scintillation generated by the scintillator.

[0065] All of the responses {S.sub.i} measured in step 10 are represented on the graph in FIG. 2.

[0066] In the case of a plastic scintillator, as in the example considered, the responses {S.sub.i} do not show full energy peaks but Compton edges more or less broadened, depending on the design of the detector.

[0067] In contrast to the responses obtained with a Na (TI) detector (as illustrated in FIG. 9), the responses {S.sub.i} of a plastic scintillator offer a relatively poor signature from which the particular gamma ray emission can be readily identified.

[0068] The present invention allows to generate a full response function which provides the necessary elements for such identification.

[0069] In step 20, the measured responses {S.sub.i} of FIG. 2 are used in combination with the known energetic properties (E.sub.ij, Y.sub.ij) or the radionuclides to compute the primary responses {S.sub.ij} of the scintillator.

[0070] Each primary response S.sub.ij represents the response of the scintillator to a received gamma ray of a known energy E.sub.ij for a radionuclide i.

[0071] Hence, the number of primary responses obtained in step 20 is the number of different values of energy at which gamma rays can be emitted for the plurality of radionuclides, which corresponds, in the example considered, to the number of lines in Table 1, that is, 19.

[0072] The 19 different values of energy are denoted “energy lines” in the following.

[0073] A method 25 to decompose the measured responses {S.sub.i} into primary responses {S.sub.ij} is illustrated in more details in FIG. 3.

[0074] In the example considered, Am-241, Cs-137, Mn-54, Zn-65, and Hg-203 are monoenergetic nuclides, so the measured responses {S.sub.i} of the scintillator to theses nuclides are primary responses per se and do not need to be decomposed.

[0075] The responses {S.sub.i} of the scintillator to Co-57, Co-60 Ba-133, Na-22 and Y-88 need to be decomposed into primary responses {S.sub.ij} since those nuclides present multiples energy lines.

[0076] The decomposition method comprises for instance the following steps: [0077] In step 250, for each monoenergetic or pseudo-monoenergetic nuclide, i.e., for Am-241, Cs-137, Mn-54, Zn-65, Hg-203, Co-57 and Co-60, a set of three correlation coefficients λ.sub.i=(λ.sub.i,1, λ.sub.i,2, λ.sub.i,3), also called “factor loadings”, are determined. [0078] The determination of λ.sub.i is done using a factor analysis method, a method known in the art, described for instance in the book from Esbensen et. al “Multivariate Data Analysis: In Practice: an Introduction to Multivariate Data Analysis and Experimental Design”, 5th ed. CAMO, 2002. [0079] In the present example, there are three factor loadings λ.sub.i=(λ.sub.i,1, λ.sub.i,2, λ.sub.i,3). [0080] In step 252, the sets of factors loadings {λ.sub.i} are interpolated to compute a set of factors loadings λ.sub.ij=(λ.sub.ij,1,λ.sub.ij,2,λ.sub.ij,3) for each energy line j of the nuclides Ba-133, Na-22 and Y-88. The factor loading λ.sub.i depends on E.sub.ij. [0081] In step 254, a set of three model components X.sub.ij=(X.sub.ij,1,X.sub.ij,2,X.sub.ij,3) is computed for each primary response {S.sub.ij} which is to be obtained. [0082] X.sub.ij may be determined by regression analysis using the responses of the scintillator to monoenergetic radionuclides which are the “closest” to the response to be decomposed. The same component X.sub.ij=X.sub.i can be used for the pseudo-monoenergetic nuclides. [0083] For instance, to compute X.sub.3=(X.sub.3,1,X.sub.3,2,X.sub.3,3), which is the set of model components for the response S.sub.3 of the scintillator to the nuclide .sup.57Co, the measured spectra S.sub.1 from .sup.241Am and S.sub.10 from .sup.203Hg are used. As it can be seen in FIG. 2, S.sub.1 and S.sub.10 are the spectra the closest to the spectrum S.sub.3. [0084] The model component X.sub.ij can also be computed using primary responses already decomposed. [0085] In step 256, the responses {S.sub.i} are decomposed into primary responses {S.sub.ij} for each energy E.sub.ij using the precomputed parameters λ.sub.ij and X.sub.i, and the yields Y.sub.ij. [0086] For instance, for the nuclide .sup.57Co, the spectrum S.sub.3 is decomposed into two sub spectra S.sub.31 and S.sub.32 corresponding to the energies E.sub.31=122.1 keV and E.sub.32=136.5 keV, respectively. [0087] These sub spectra are given by:

[00001]$S_{31}=S\left({ }^{57}{\mathrm{Co}}_{122.1}\right)=f\left({\lambda }_{31},Y_{31},X_3\right)=\underset{k=1,.Math.,3}{.Math.}{\lambda }_{32,k}{Y}_{32}{X}_{3,k}$$S_{32}=S\left({ }^{57}{\mathrm{Co}}_{136.5}\right)=f\left({\lambda }_{32},Y_{32},X_3\right)=\underset{k=1,.Math.,3}{.Math.}{\lambda }_{32,k}{Y}_{32}{X}_{3,k}$

[0088] The result of the decomposition 20 into primary responses {S.sub.ij} is shown in the graph of FIG. 4. The number of lines has been increased to 19, starting from 10 measured responses, which constitutes a rich base from which the full response function can be generated.

[0089] The response function ƒ(λ,X) is generated in step 30 to determine the response of the scintillator to any gamma ray emission of energy E.

[0090] In the example considered, the energies {E.sub.ij} for which the responses {S.sub.ij} have been obtained, range from 59 keV to 1836 keV.

[0091] The response function will thus be able to cover incident gamma radiation of any energy E within a range close to [59 keV; 1836 keV].

[0092] A method similar to the one described in step 20 can be used to generate ƒ(λ, X).

[0093] Factor loadings λ are for example estimated for energies ranging from 40 keV to 2048 keV with step 2 keV using factor analysis and interpolation, as described above in steps 250 and 252.

[0094] The model components X are then estimated for each value of energy E by regression analysis of the primary responses {S.sub.ij}, as described in step 254,

[0095] The spectrum representing the response of the scintillator to an incident radiation of energy E can then be constructed, as illustrated in FIG. 5 for various discrete energy values. Naturally, the invention is not limited to these values of energy, and the response to a gamma radiation of any energy E within the range considered can be computed using this method.

[0096] Each radiation detector has a specific response function ƒ(λ, X), which is generated before using the detector for radionuclide detection or identification.

[0097] A radiation detector 5 according to the invention is represented in a simplified and schematic manner in FIG. 6.

[0098] The radiation detector comprises a sensor 50 comprising a plastic scintillation material and a photodetector 52 connected to the sensor 50, arranged to detect the photons produced by the scintillation material when exposed to incident gamma radiation R.

[0099] The photodetector 51 generates a signal 520 representative of the response of the detector to the incident radiation which is received by a processor 56.

[0100] A response function ƒ(λ, X) of the detector 5 is for instance stored in a memory 54.

[0101] The processor 56 uses the response function to treat the signal 520, for example by a deconvolution process, and generate a deconvoluted signal 560 representative of the energy distribution of the incident gamma radiation.

[0102] The deconvolution process mentioned above is an algorithm-based process known from prior art. For instance, MLEM algorithm can be used to generate the deconvoluted signal, as described in the paper from Meng, Ling-Jian and David Ramsden. “An inter-comparison of three spectral-deconvolution algorithms for gamma-ray spectroscopy.” (IEEE Transactions on Nuclear Science 47.4 (2000): 1329-1336.).

[0103] The precision of the deconvolution process and whether the deconvoluted signal is accurate enough to help identifying isotopes in the source of radiation, depends on the accuracy of the response function.

[0104] Examples of deconvoluted signals 560 are shown in FIGS. 7 and 8 when the detector 5 is exposed to radiation emitted by radionuclides .sup.22Na and .sup.133Ba, respectively. As mentioned before and described in Table 1, these two radionuclides present multiple energy lines, meaning gamma rays with different values of energy are emitted following up a decay event.

[0105] The signal 560 from .sup.22Na shown in FIG. 7 has been deconvoluted from the initial response or measured spectrum S.sub.7 shown in FIG. 2 using the response function ƒ(λ, X) described above.

[0106] As it can been seen in FIG. 2, the measured spectrum S.sub.7 does not shown any significant energy peak that would help identify radionuclide .sup.22Na.

[0107] On the other hand, the deconvoluted signal 560 shown in FIG. 7 shows a distinct energy peak P1 centered around 510 keV, and a smaller but distinct variation of energy P2 between 1100 and 1500 keV.

[0108] The radiation source .sup.22Na, which has two energy lines at 511 keV and 1274.5 keV with yields of 180.7% and 99.9%, respectively, can thus be easily identified when looking at the deconvoluted signal 560.

[0109] Similarly, the signal 560 from .sup.133Ba shown in FIG. 8 has been deconvoluted from the initial response or measured spectrum S.sub.2 shown in FIG. 2 using the response function ƒ(λ, X) described above.

[0110] The deconvoluted signal 560 shows multiple peaks between 0 and 400 keV, centered around the expected values of energy for .sup.133Ba (energy lines at 79.6, 81, 276.4, 302.8, 356 and 383.9), the peaks height being proportional to the yield. The highest peak P3, for example, is centered around 356 keV which corresponds to the highest yield of 60%.

[0111] The above examples merely illustrate possible embodiments of various aspects of the present disclosure and are not intended to be limiting; other aspects and embodiments will be apparent to those skilled in the art.

[0112] For example, the method for generating the response function can be applied to other types of scintillators than just plastic scintillators.

[0113] Any other known radionuclide can be used in the plurality of radionuclides, not only the ones considered in the above examples.