Spectrometry method and device for detecting ionising radiation for the implementation thereof

Abstract

Disclosed is a spectrometry method including: for at least one ionizing-radiation energy E.sub.i, obtaining, for each energy E.sub.i, a curve of the number of photons detected, during a measurement interval, as a function of time, by spectrometer; b) for each curve, computing a ratio of the number of photons detected defined and separate time periods to obtain, for each ionizing-radiation energy E.sub.i, a number a.sub.i, or for each curve, acquiring one or more fitting parameters PAJ.sub.i by making a fit to the corresponding curve with a fitting function; and comparing each number a.sub.i or each fitting parameter or set of fitting parameters PAJ.sub.i with reference constants a.sub.i or, respectively, with reference fitting parameters PAJ.sub.i associated with reference energies E.sub.i to determine, for each number a.sub.i or each fitting parameter or set of fitting parameters PAJ.sub.i, reference energy E.sub.i of the ionizing radiation for which the corresponding curve was measured.

Claims

1. A spectrometry method for detecting and characterizing ionising radiation, comprising: a) for at least one ionising-radiation energy E.sub.i detected by a spectrometer (15), obtaining, for each energy E.sub.i, a curve of a number of photons detected, during a measurement interval, as a function of time, by means of the spectrometer (15); b) for each curve thus obtained, computing a ratio of a total number of photons detected in at least two defined and separate time periods in order to obtain, for each ionising-radiation energy E.sub.i, a number a.sub.i, or for each curve thus obtained, acquiring one or more fitting parameters PAJ.sub.i by making a fit to said corresponding curve with a fitting function; and c) comparing each number a.sub.i or each fitting parameter or set of fitting parameters PAJ.sub.i thus obtained with reference constants a.sub.i(REF) or, respectively, with reference fitting parameters PAJ.sub.i(REF) associated with reference energies E.sub.i(REF) in order to determine, for each number a.sub.i or each fitting parameter or set of fitting parameters PAJ.sub.i, the reference energy E.sub.i(REF) of the ionising radiation for which said corresponding curve was measured.

2. The method according to claim 1, wherein, in step a), a curve is obtained for at least two different ionising-radiation energies E.sub.i.

3. The method according to claim 1, wherein, in step b), said fit is obtained via a multi-exponential fitting function.

4. The method according to claim 1, wherein said spectrometer (15) comprises a scintillator (16) coupled to a photodetector (17), and, in step b), said time periods are chosen so as to maximize the difference between the numbers a.sub.i for two different energies E.sub.i.

5. The method according to claim 1, wherein said spectrometer (15) uses a scintillator (16) wherein a yield of a scintillation light, as a function of ionising-radiation energy E.sub.i, has a non-proportionality higher than or equal to 2% of variation per order of magnitude of the energy of an incident radiation.

6. A method for calibrating a spectrometer (15), wherein the spectrometry method according to claim 1 is used, and wherein the following additional steps are carried out: d) comparing said reference ionising-radiation energies E.sub.i(REF) determined in step c) with the corresponding energies E.sub.i(mes) measured by said spectrometer (15) in an energy spectrum of the ionising radiation, in order to obtain an error value associated with each ionising-radiation energy measurement, said error value being a measurement of a discrepancy between the energy E.sub.i(mes) measured by said spectrometer (15) and the corresponding reference energy E.sub.i(REF) determined in step c); and e) when a plurality of error values are nonzero and in absolute value higher than threshold values, determining an energy correction to be made permanently to each energy E.sub.i(mes) measured by the spectrometer (15).

7. A method for increasing the spectral sensitivity of a spectrometer (15), wherein the spectrometry method according to claim 1 is used, and wherein, for each event measured by said spectrometer (15), the following additional steps are carried out: d) determining the difference between the energy E.sub.i(mes) measured by said spectrometer (15) and said corresponding reference energy E.sub.i(REF) determined in step c) in order to obtain an energy-discrepancy value for said event; e) comparing the absolute value of this energy discrepancy with a preset threshold value called the acceptability threshold value; and f) if this absolute value is lower than or equal to this threshold value, then counting this event in the creation of a spectrum of the number of events measured by the spectrometer (15) as a function of the energy E.sub.i(REF) of the ionising radiation, and if this absolute value is higher than this threshold value, then discarding the event in question, then obtaining a spectrum of the total number of events measured for each reference energy E.sub.i(REF).

8. A non-transitory computer-readable medium on which is stored a computer program comprising instructions that, when loaded into a memory of a computer and is executed by the computer, cause the computer to perform the spectrometry method according to claim 1.

9. A device for detecting ionising radiation comprising a processing unit including a microprocessor and a data-storage unit, said storage unit containing a data library containing data on ionising radiation, said detecting device including a spectrometer (15) including at least one scintillator (16) coupled to a photodetector (17), the device comprising a set of software instructions by virtue of which the device is able to control said spectrometer (15), and to detect and identify the ionising radiation interacting with said scintillator (16), said software instructions belonging to said computer program according to claim 8.

10. The device according to claim 9, wherein said photodetector (17) comprises a measuring module configured to convert an electrical signal emitted by said photodetector (17) following the detection of a photon and proportional to the number of visible photons detected by the photodetector (17), into a digital signal representative of the energy of the measured photon.

11. The method according to claim 2, wherein, in step b), said fit is obtained via a multi-exponential fitting function.

12. The method according to claim 2, wherein said spectrometer (15) comprises a scintillator (16) coupled to a photodetector (17), and, in step b), said time periods are chosen so as to maximize the difference between the numbers a.sub.i for two different energies E.sub.i.

13. The method according to claim 3, wherein said spectrometer (15) comprises a scintillator (16) coupled to a photodetector (17), and, in step b), said time periods are chosen so as to maximize the difference between the numbers a.sub.i for two different energies E.sub.i.

14. The method according to claim 2, wherein said spectrometer (15) uses a scintillator (16) wherein a yield of a scintillation light, as a function of ionising-radiation energy E.sub.i, has a non-proportionality higher than or equal to 2% of variation per order of magnitude of the energy of an incident radiation.

15. The method according to claim 3, wherein said spectrometer (15) uses a scintillator (16) wherein a yield of a scintillation light, as a function of ionising-radiation energy E.sub.i, has a non-proportionality higher than or equal to 2% of variation per order of magnitude of the energy of an incident radiation.

16. The method according to claim 4, wherein said spectrometer (15) uses a scintillator (16) wherein a yield of a scintillation light, as a function of ionising-radiation energy E.sub.i, has a non-proportionality higher than or equal to 2% of variation per order of magnitude of the energy of an incident radiation.

17. The method for calibrating a spectrometer (15), wherein a spectrometry method according to claim 2 is used, and wherein the following additional steps are carried out: d) comparing said reference ionising-radiation energies E.sub.i(REF) determined in step c) with the corresponding energies E.sub.i(mes) measured by said spectrometer (15) in an energy spectrum of the ionising radiation, in order to obtain an error value associated with each ionising-radiation energy measurement, said error value being a measurement of a discrepancy between the energy E.sub.i(mes) measured by said spectrometer (15) and the corresponding reference energy E.sub.i(REF) determined in step c), and e) when a plurality of error values are nonzero and in absolute value higher than threshold values, determining an energy correction to be made permanently to each energy E.sub.i(mes) measured by the spectrometer (15).

18. The method for calibrating a spectrometer (15), wherein a spectrometry method according to claim 3 is used, and wherein the following additional steps are carried out: d) comparing said reference ionising-radiation energies E.sub.i(REF) determined in step c) with the corresponding energies E.sub.i(mes) measured by said spectrometer (15) in an energy spectrum of the ionising radiation, in order to obtain an error value associated with each ionising-radiation energy measurement, said error value being a measurement of a discrepancy between the energy E.sub.i(mes) measured by said spectrometer (15) and the corresponding reference energy E.sub.i(REF) determined in step c), and e) when a plurality of error values are nonzero and in absolute value higher than threshold values, determining an energy correction to be made permanently to each energy E.sub.i(mes) measured by the spectrometer (15).

19. The method for calibrating a spectrometer (15), wherein a spectrometry method according to claim 4 is used, and wherein the following additional steps are carried out: d) comparing said reference ionising-radiation energies E.sub.i(REF) determined in step c) with the corresponding energies E.sub.i(mes) measured by said spectrometer (15) in an energy spectrum of the ionising radiation, in order to obtain an error value associated with each ionising-radiation energy measurement, said error value being a measurement of a discrepancy between the energy E.sub.i(mes) measured by said spectrometer (15) and the corresponding reference energy E.sub.i(REF) determined in step c), and e) when a plurality of error values are nonzero and in absolute value higher than threshold values, determining an energy correction to be made permanently to each energy E.sub.i(mes) measured by the spectrometer (15).

20. The method for calibrating a spectrometer (15), wherein a spectrometry method according to claim 5 is used, and wherein the following additional steps are carried out: d) comparing said reference ionising-radiation energies E.sub.i(REF) determined in step c) with the corresponding energies E.sub.i(mes) measured by said spectrometer (15) in an energy spectrum of the ionising radiation, in order to obtain an error value associated with each ionising-radiation energy measurement, said error value being a measurement of a discrepancy between the energy E.sub.i(mes) measured by said spectrometer (15) and the corresponding reference energy E.sub.i(REF) determined in step c), and e) when a plurality of error values are nonzero and in absolute value higher than threshold values, determining an energy correction to be made permanently to each energy E.sub.i(mes) measured by the spectrometer (15).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other advantages, aims and particular features of the present invention will become apparent from the following completely nonlimiting description that is given by way of explanation with reference to the appended drawings, in which:

(2) FIG. 1 is a perspective view of a device for detecting ionising radiation according to one particular embodiment of the present invention;

(3) FIG. 2 is a schematic representation of the components of the device for detecting ionising radiation of FIG. 1, which components are placed in the housing of said device;

(4) FIG. 3 is a so-called scintillation-decay curve obtained for an ionising-radiation energy E.sub.i by means of a spectrometer according to one embodiment of the method or the invention, two separate time periods being illustrated on this curve;

(5) FIG. 4 is a representation of scintillation-decay curves measured with a scintillation spectrometer for two different ionising-radiation energies: lower curve 17 keV, and upper curve 662 keV;

(6) FIG. 5 illustrates a scintillation-decay-curve section obtained with a scintillation spectrometer employing an inorganic scintillator of the alkali-halide type for various ionising-radiation energies: two beams (17 keV and 59 keV) generated with atoms of Americium-241 (.sup.241Am) and one beam generated with Cobalt-57 (.sup.57Co);

(7) FIG. 6 shows the variation in the fitting parameter t.sub.2[REF] as a function of the reference energy E.sub.i(REF) in keV; this parameter resulting from a multi-exponential fit of the curves illustrated in FIG. 5;

(8) FIG. 7 shows the sections of curve shown in FIG. 5, on which sections the time windows T.sub.1 and T.sub.2 employed to determine the constants a.sub.i have been shown;

(9) FIG. 8 is a representation of the variation in the reference constant a.sub.i(REF), which is equal to the ratio (Integral T.sub.1/Integral T.sub.2), as a function of the reference energy E.sub.i(REF).

DESCRIPTION OF THE EMBODIMENTS

(10) The drawings and description below contain, for the most part, elements of certain character. They will therefore possibly not only serve to better understand the present invention, but also contribute to the definition thereof, where appropriate.

(11) Firstly, it will be noted that the figures are not to scale.

(12) A detector of ionising radiation according to one particular embodiment of the present invention will now be described with reference to FIGS. 1 and 2.

(13) Because of its compactness and its ease of use, such a detector 10 is particularly advantageous for monitoring an environment and optionally a contaminated object in order to avert an immediate danger in case of measurement of high concentrations.

(14) This detector 10 includes a detector body provided with an entrance window 11 in its upper face, this window 11 being intended to pass particles to be detected. As known, this entrance window 11 may include a film that is opaque to light, such as a film of MYLAR or any other thin film that is opaque to the wavelengths of light detectable by the photodetector.

(15) On one of its lateral edges, it also comprises an indicator light allowing the operating state of the detector to be displayed (low battery, turned on or turned off).

(16) It also includes a connector 13 such as a USB port in order to allow the detector to be connected to a computer with a view to loading or downloading data and/or computer programs, and an on/off button 14.

(17) Of course, such a detector 10 could also include a communication unit for receiving and transmitting data.

(18) Advantageously, this communication unit could be a wireless communication module configured to communicate using one of the following protocols: Sigfox, LoRa or even ZigBee. In other words, it is a question of a low-power wireless communication module allowing the battery with which the detector is equipped to be economized. It could even comprise a wireless communication module such as a Bluetooth communication module in order to allow data or messages to be routed when a lack of Sigfox or LoRaWAN coverage is detected.

(19) Of course, it could also be a question of a wireless communication module based on one of the following protocols: IEEE 802.11 b/g/n (Wi-Fi), IEEE 802.15.1 (Bluetooth), or 2G, 3G, 4G, 5G or even GSM or GPRS.

(20) It could even include a wireless positioning means such as a GPS receiver in order to allow its position to be determined, and a storage unit for storing timestamped data.

(21) Each of these data could then comprise a position and a time at which this position was acquired by the GPS receiver, and whether or not ionising radiation was detected.

(22) The body of the detector is here made entirely of a fine wood such as ash, but it could also be made from another material such as a hard plastic.

(23) The interior walls of this detector body are opaque in order to block outside light.

(24) A spectrometer 15 allowing particles to be detected, the latter to be counted and energy spectra to be obtained is placed in the interior volume bounded by the detector body 10.

(25) This spectrometer thus comprises a scintillator 16 for simultaneously measuring and radiation. This scintillator 16 is here formed by a stack of layers of various inorganic and organic scintillator materials.

(26) This scintillator 16 is an elongate body, such as a strip, placed facing the window in order to receive the particles that pass through this entrance window 11. The dimensions of the scintillator are, purely by way of illustration, 6*6*50 mm. They could equally well be 6*6*100 mm or even 30*30*30 mm.

(27) A photodetector 17 is optically coupled to this scintillator 16 and includes a measuring module 18 configured to convert an electrical signal emitted by this photodetector 17, following the detection of a photon, and proportional to the number of visible photons detected by the photodetector, into a digital signal representative of the energy of the measured photon. This photodetector 17 therefore has a sensitivity appropriate for the emission wavelengths of the scintillator. It is here a question of a silicon-photomultiplier (or SiPM) sensor, but it could also be a question of conventional photomultiplier tubes or of photodiodes.

(28) It further comprises a processing unit 19 including a microprocessor, or field-programmable gate array (FPGA), and a data-storage unit, this storage unit containing a data library containing data on ionising radiation (energies, etc.).

(29) This detector 10 comprises a set of software instructions by virtue of which it is able to control the spectrometer, and to detect and identify the ionising radiation interacting with said scintillator, said software instructions belonging to a computer program allowing at least certain of the following steps of the method to be implemented:

(30) a) for at least one ionising-radiation energy E.sub.i, obtaining, for each energy E.sub.i, a curve of the number of photons detected, during a measurement interval, as a function of time, by means of a spectrometer,

(31) b) for each curve thus obtained, computing a ratio of the total number of photons detected in at least two defined and separate time periods in order to obtain, for each ionising-radiation energy E.sub.i, a number a.sub.i, or

(32) for each curve thus obtained, acquiring one or more fitting parameters PAJ.sub.i by making a fit to said corresponding curve with a fitting function,

(33) c) comparing each number a.sub.i or each fitting parameter or set of fitting parameters PAJ.sub.i thus obtained with reference constants a.sub.i(REF) or, respectively, with reference fitting parameters PAJ.sub.i(REF) associated with reference energies E.sub.i(REF) in order to determine, for each number a.sub.i or each fitting parameter or set of fitting parameters PAJ.sub.i, the reference energy E.sub.i(REF) of the ionising radiation for which said corresponding curve was measured.

(34) FIG. 3 shows an example of a curve of the type called a scintillation-decay curve, which example was obtained, in step a), with ionising radiation of given energy incident on the scintillator. From this scintillation-decay curve, the microprocessor, or FPGA, defines two separate time periods T.sub.1 and T.sub.2. For each of these time periods, it then integrates the number of photons emitted by the scintillator, i.e. what is also referred to as the amount of light, then computes the ratio of the total number of photons detected in the period T.sub.1 to the total number of photons detected in the period T.sub.1, which provides a number an associated with the excitation energy (step b).

(35) In step c), it compares the number a thus obtained with reference constants a(REF), b(REF), c(REF), etc. each associated with a reference energy E(REF) in order to identify the reference energy E(REF) for which the number a was obtained.

(36) This method thus allows the actual excitation energy for which the scintillation-decay curve was obtained in step a) to be very precisely identified.

(37) For one implementation of the present invention, FIG. 4 shows scintillation-decay curves measured by means of a scintillation spectrometer with a scintillator made of CsI-Tl (thallium-doped cesium iodide) for two different types of ionising radiation, namely exciting x-rays of 17 keV and exciting gamma rays of 662 keV.

(38) The upper scintillation-decay curve 20 was obtained for a gamma-radiation energy of 662 keV and the lower scintillation-decay curve 21 was obtained for an x-ray energy of 17 keV.

(39) It may clearly be seen that the energy of the exciting radiation has an effect on the so-called fast-to-tail ratio, which increases as the exciting energy decreases.

(40) This change in the dynamics of the light emission is related to the variation in the density of the electronic charge produced during the interaction of each gamma particle with the material.

(41) For a second implementation of the present invention, FIG. 5 thus illustrates a scintillation-decay-curve section obtained with a scintillation spectrometer employing an inorganic scintillator of the alkali-halide type for various ionising-radiation energies: two beams (17 keV and 59 keV) generated with atoms of Americium-241 (.sup.241Am) and one beam generated with Cobalt-57 (.sup.57Co).

(42) The amplitude of the measured signals has been normalized.

(43) With these three curve sections obtained with known ionising-radiation emitters, it is possible to determine the fitting parameter PAJ.sub.i(REF)/reference energy E.sub.i(REF) pairs that will subsequently be used in step c) of the method to determine the actual ionising-radiation energy for which any new scintillation-decay curve is obtained with the same scintillation spectrometer.

(44) To do this, a double-exponential fitting function is used to fit, or even take into account, the shape of each scintillation-decay-curve section illustrated in FIG. 5.

(45) This function is written:
A.sub.1e.sup.t/t.sup.1+A.sub.2e.sup.t/t.sup.2

(46) The reference fitting parameters obtained by fitting these various curves are thus: A.sub.1 [REF]=0.41092 t.sub.1 [REF]=1.7 A.sub.2 [REF]=0.68106
and the parameter t.sub.2 [REF], which is a variable dependent on the energy of the ionising radiation.

(47) Thus, by virtue of this reference parameter t.sub.2, it is therefore possible to determine the energy of the radiation E.sub.i(REF), on the x-axis of FIG. 6, for which any entirely new scintillation-decay curve has been produced with the scintillation spectrometer.

(48) In conclusion, since the curve shown in FIG. 6 is not subject to any calibration, it is a reference graph allowing a measurement obtained with the scintillation spectrometer to be calibrated.

(49) Once these reference parameters and associated reference energies have been determined, for any new measurement obtained with this scintillation spectrometer, after steps a) and b) of the present method have been carried out, in step (c) the fitting parameters (A.sub.i, t.sub.i), with i=1, 2, obtained in step b) of the method, for the measured scintillation-decay curve, are compared with the reference fitting parameters (A.sub.1[REF], A.sub.2[REF], t.sub.1[REF], t.sub.2[REF]) described above.

(50) Matching the fitting parameters obtained in step b), for an energy E.sub.i measured by the scintillation spectrometer, to reference fitting parameters (A.sub.1[REF], A.sub.2[REF], t.sub.1[REF], t.sub.2[REF]) obtained for a reference energy E.sub.i(REF), thus allows the actual radiation energy for which the scintillation-decay curve was measured to be determined.

(51) It is therefore possible to calibrate the scintillation spectrometer.

(52) Very advantageously, it will be clear from the above that the one or more reference fitting parameters and the associated reference energy may be determined at any time with at least two known sources of ionising radiation.

(53) Of course, such conclusions could also be reached not by fitting at least one section of the scintillation-decay curve with a fitting function, but by computing the ratio of the total number of photons detected in at least two defined and separate time periods of the scintillation-decay curve in order to obtain, for each ionising-radiation energy E.sub.i, a number a.sub.i.

(54) In step c), comparison of the number a.sub.i with a constant a.sub.i(REF) allows the actual energy, or indeed reference energy E.sub.i(REF), of the ionising radiation for which the curve was measured by the scintillation spectrometer to be determined.

(55) Thus, and such as shown in FIG. 7, in order to determine the pairs (a.sub.i(REF), E.sub.i(REF)), two time windows are defined beforehand for the previously obtained scintillation-decay curves.

(56) Generally, these time windows are typically defined by one tenth ( 1/10th) of the total integration duration.

(57) In the present case, one sixth (th) of the total duration of the integration has been used, which value corresponds to about three times the time constant of the first exponential computed in the preceding paragraph. This metric gives satisfactory results with this type of (alkali-halide) scintillator.

(58) It is then possible to compute the integral of the signals in these two windows, and to plot the ratio (Integral=T.sub.1/Integral T.sub.2) as a function of the reference energy E.sub.i(REF).

(59) Just as with the parameter t.sub.2[REF] of the multi-exponential fit, access is thus obtained, by virtue of the plot joining the points of FIG. 8, to information on the reference energy, or actual energy, measured by the scintillation spectrometer, via a measurement, of the ratio (Integral=T.sub.1/Integral T.sub.2), which does not depend on any calibration.