Ionizing radiation detector incorporating a modulator to modulate the radiation intensity

Abstract

A device for detecting ionizing radiation comprises a detector of the ionizing radiation, configured to interact with particles constituting the ionizing radiation and to generate a detection signal dependent on the intensity of the ionizing radiation incident on the detector. The device also includes a modulator, comprising at least one absorbing portion, configured to absorb a portion of the ionizing radiation. The device includes a control unit for periodically moving the absorbing portion in front of the collimator, and a processing unit for estimating the intensity of the radiation incident on the detection device. Figure 1A

Description

Title of the invention: Ionizing radiation detector comprising a modulator for modulating the intensity of the radiation Technical field

[0001] The technical field of the invention is the measurement of ionizing radiation. PREVIOUS ART

[0002] Instrumentation for nuclear measurement is now widely industrialized. Most detectors used in nuclear measurement include a detection material, in which ionizing radiation is likely to form interactions. Each interaction allows a generation of charge carriers, the latter being collected by a shaping circuit to form a pulse. The detection material can be a gas, a liquid or a solid.

[0003] Sensitive, compact detectors are currently available which can be used for monitoring nuclear waste or for carrying out in-situ measurements, for example for the purposes of radiological characterization of nuclear equipment or installations.

[0004] However, a frequently encountered problem is background noise. Background noise is formed by radiation detected by the detector, and not coming from the object that one wishes to characterize. It can in particular be radiation emitted in the vicinity of the detector. It is known that the higher the background noise, the more the detection limit increases. The detection limit quantifies the minimum intensity of radiation detectable with a sufficient level of confidence. In the presence of high background noise, it is necessary to increase the measurement duration, so as to lower the detection limit. One solution to limit the influence of background noise is to place shielding around the detector, so as to protect the detector from background noise and delimit the observation field in which the detector makes a measurement.When the detector is a photon detector (X or gamma), which is a common case, the shielding is achieved using a dense material, for example a metal. If it is to be sufficiently effective, the thickness must be relatively significant, which significantly increases the weight of the equipment.

[0005] The influence of background noise is harmful when it is desired to detect low irradiation intensities, in particular in the monitoring of low-irradiating nuclear waste or measurements carried out on glove boxes. The influence of background noise is also problematic in in-situ nuclear measurements, due to the presence of different objects, the irradiation of some forming a background noise for the measurement of others.

[0006] The invention addresses this problem, and provides a compact solution, of design simple and easy to implement to measure the intensity of ionizing radiation in the presence of background noise. The objective is to lower the detection limit. Description of the invention

[0007] A first object of the invention is a device for detecting ionizing radiation, formed of photons or neutrons, the device comprising:

[0008] - a detector of the ionizing radiation, the detector being configured to interact with particles forming the ionizing radiation and to generate a detection signal dependent on an intensity of the ionizing radiation incident on the detector; - a modulator, the modulator comprising at least one absorbing portion, the absorbing portion being configured to absorb a portion of the ionizing radiation; - a control unit, configured to move the modulator, so that the absorbing portion is periodically arranged facing the detector, so as to periodically absorb the radiation incident on the detector, so that the detection signal oscillates periodically between • a maximum signal, when the absorbing portion is not positioned facing the detector; • a minimal signal, when the absorbent portion is placed facing the detector; - a processing unit, programmed to: • i) storing detection signals generated by the detector, at different detection times, the detection times being distributed according to at least one period of the detection signal; • ii) calculating an autocorrelation function of the detection signal, the autocorrelation function being a periodic function whose period is the period of the detection signal; • iii) from an oscillation amplitude of the autocorrelation function according to at least one period, estimation of the intensity of the radiation incident on the detection device.

[0009] The autocorrelation function can in particular be defined for different time shifts, the autocorrelation function, at each time shift being calculated from a sum, according to a duration of at least half a period, and preferably at least one period, of each detection signal measured during said duration multiplied by said detection signal shifted by the time shift.

[0010] According to one possibility, the modulator comprises:

[0011] - a first material, of a first thickness, forming the ab- portion outgoing; - a second material, of a second thickness, forming a support for the absorbent portion;

[0012] the modulator being such that the absorbing portion has an absorption of the incident radiation at the detector greater than the absorption of the support.

[0013] Preferably, the absorption of the absorbing portion is at least 10% greater than the absorption of the support. By absorption of the support is meant the absorption of the incident radiation by the support.

[0014] According to one possibility, the modulator is formed from a first material, of variable thickness between a minimum thickness and a maximum thickness, the absorbent portion being formed by a part of the material having the maximum thickness.

[0015] According to one possibility, the modulator comprises:

[0016] - a first material, forming the absorbent portion; - an opening, formed through the first material.

[0017] Preferably, whatever the embodiment, the absorbing portion is configured to absorb at least 10% of the intensity of the radiation incident on the detection device.

[0018] The modulator may be movable in translation or in rotation relative to the detector. The control unit advantageously comprises a motor for moving the modulator relative to the detector.

[0019] According to one embodiment, the detector is connected to an electronic circuit configured to select an amplitude range of pulses formed following each interaction of a particle in the detector, such that the detection signal corresponds to an intensity of the radiation detected in a predetermined energy window, corresponding to the selected amplitude range. The selected amplitude range is preferably adjustable.

[0020] Another object of the invention is a method for detecting an intensity of ionizing radiation comprising the following steps:

[0021] - a) arrangement of a device according to the first object of the invention facing an object emitting radiation; - b) using the control unit, periodic movement of the modulator relative to the detector; - c) during step b), acquisition of detection signals, generated by the detector at different times during at least one period of the movement of the modulator; - d) using the processing unit, • i) storing the detection signals generated by the detector during step c) • ii) calculation of an autocorrelation function of the detection signal, the autocorrelation function being periodic; • iii) determination of an oscillation amplitude of the autocorrelation function during at least one period and estimation of the intensity of the incident radiation from the detection device.

[0022] Sub-step iii) preferably comprises the application of a calibration function to the oscillation amplitude of the autocorrelation function. The calibration function can be established during an experimental calibration phase or by modeling.

[0023] The ionizing radiation can be neutron radiation or X- or gamma-type photon radiation.

[0024] The invention will be better understood upon reading the description of the exemplary embodiments presented in the remainder of the description, in conjunction with the figures listed below. FIGURES

[0025] [Fig.lA]

[0026] [Fig. 1B] Figures 1A and 1B represent an example of a device allowing an implementation of the invention.

[0027] [Fig.2] [Fig.2] shows a schematic of a rotating mobile modulator used in the device described in connection with figures IA and IB.

[0028] [Fig.3] [Fig.3] illustrates the main steps of implementing the invention.

[0029] [Fig.4A] [Fig.4A] represents a detection signal detected at different times.

[0030] [Fig.4B] [Fig.4B] represents a detection signal detected at different times, without time shift, as well as the detection signal to which a time shift is applied.

[0031] [Fig.4C] [Fig.4C] shows an autocorrelation signal obtained from the signal shown in [Fig.4B].

[0032] [Fig.4D] [Fig.4D] illustrates a calibration function.

[0033] [Fig.5A] [Fig.5A] is a variant of the rotating mobile modulator.

[0034] [Fig.5B] [Fig.5B] is another variant of the rotating mobile modulator.

[0035] [Fig.5C]

[0036] [Fig.5D] Figures 5C and 5D show another variant of the rotating mobile modulator.

[0037] [Fig.5E]

[0038] [Fig.5F] Figures 5E and 5F show a variant of the modulator in which the modulator performs a translational movement.

[0039] [Fig.6A] [Fig.6A] shows a simulation of a detection signal.

[0040] [Fig.6B] [Fig.6B] represents an autocorrelation signal obtained from the signal detection shown in [Fig.6A]. PRESENTATION OF SPECIAL EMBODIMENTS

[0041] Figures 1A and 1B represent an example of a detection device 1 allowing an implementation of the invention. The detection device comprises a detector 10, configured to detect incident radiation 2. In the example described, the incident radiation is ionizing photonic radiation, in particular X or y type radiation. It may also be another type of ionizing radiation, for example neutron.

[0042] The detector is configured to interact with the particles forming the ionizing radiation. During each interaction of a particle in the detector, charge carriers are formed, the latter inducing an electrical pulse. The detector generates a detection signal S(t) at each measurement instant. The measurement signal is a number of pulses detected during a determined time interval or a counting rate, representative of a number of pulses detected per unit of time, for example per second. In practice, the term "counts per second" is usually used. The detection signal is correlated with the intensity of the incident radiation. The intensity of the incident radiation / corresponds to a number of photons reaching the detector per unit of time, generally per second, or a number of photons reaching the detector per unit of time or area.

[0043] The detector may be of the gas meter type, scintillator detector or semiconductor detector. These detectors are well known in the field of nuclear measurement. In this example, the detector comprises a scintillator detector material of the NaI(Tl) type (Sodium Iodide doped with Thallium) with a volume of 2 inches by 2 inches, or approximately 5 cm by 5 cm. The detector material is coupled to an electronic circuit allowing generation of the detection signal S(t). The detector material comprises a detection surface 11, through which the radiation incident on the detector propagates. The detector extends around a central axis X, the latter being perpendicular to the detection surface.

[0044] The detection device 1 comprises a modulator 20, intended to modulate the intensity of the radiation incident on the detector. The modulator 20 is arranged facing the detector. In the example shown in Figures 1A and 1B, the modulator comprises a first portion 21, or absorbent portion, intended to absorb part of the incident radiation. The modulator comprises a second portion 22, here serving as a support for the first portion 21. [Fig. 2] shows the modulator 20 schematically. In this example, the second portion 22 is thinner than the first portion 21, the thickness being considered along the central axis X. The absorbent portion 21 preferably comprises a first material having a high absorbance with respect to the particles forming the radiation. When the particles are photons, the portion absorbent 21 may comprise or be made of a dense material, for example a metal or a metal alloy, for example iron, copper, lead, tungsten, etc. The support 22 is formed of a second material whose absorbance is considered low with respect to the particles forming the radiation. It may for example be a plastic or aluminum. According to one possibility, the first material and the second material are identical. The absorbent portion 21 corresponds to an excess thickness of the modulator. For example, the absorbent portion 21 is at least 1.5 times thicker, or even 2 or 3 times thicker than the support 22.

[0045] The detection device 1 comprises a control unit 30, configured to set the modulator 20 in motion relative to the detector 10. In the example shown, the control unit 30 allows the modulator 20 to rotate relative to the detector 10. The rotation of the modulator is carried out along an axis of rotation A parallel to the central axis X.

[0046] This allows rotation of the absorbent portion 21 relative to the detector 10. Thus, the absorbent portion is, periodically:

[0047] - arranged facing the detector 10, so as to absorb part of the radiation incident on the detector: the detector is then in an absorption configuration; - arranged next to the detector 10, without absorbing, or negligibly, the radiation incident on the detector: the detector is then in an open configuration.

[0048] By radiation incident on the detector is meant the radiation propagating towards the detector, through the detection surface 11.

[0049] The movement of the modulator relative to the detector is periodic, so that the detector is periodically in the absorption configuration and in the open configuration. The period of the movement, or modulation period, may be of the order of 1 second. Preferably, the modulation period is a few seconds: it extends for example between 1 second and 30 seconds. The modulation period is defined to allow detection of a sufficient number of detection signals during the same period. It therefore depends on the sampling frequency of the detector. In the example described, the modulation period is 20 seconds.

[0050] The modulator 22 allows a temporal modulation of the intensity of the radiation incident on the detector. Preferably, the modulation is greater than 5%, or even 10%. This means that the absorbing portion 21 induces an additional absorption of the incident radiation of at least 5%, or at least 10%, and preferably at least 50% compared to the absorption produced by the modulator when the detection device is in the open configuration.

[0051] Preferably, the absorption induced by the absorbing portion 21 is between 10% and 90%, or even more, of the intensity of the radiation incident on the detection device. By intensity of the radiation incident on the detection device, we mean the intensity of the radiation propagating towards the detection device, the intensity being determined upstream of the modulator.

[0052] The term absorption is known to those skilled in the art. The absorption abs2i of the absorbent portion 21 can be expressed according to the expression: [°° 53 1 «ta 21 = -z n [^]-

[0054] where:

[0055] - / 21 denotes the intensity of the incident radiation at the detector in the confi absorption figure, that is to say the intensity of the radiation passing through the absorbing portion 21 and propagating towards the detector 10. - / denotes the intensity of the radiation at the detector in the absence of a screen facing the detector.

[0056] The absorption abs22 of the support 22 can also be expressed as follows: [°° 57 1 abs 22 = -Zn[^]'

[0058] where / 22 denotes the intensity of the radiation incident on the detector in the open configuration, i.e. the intensity of the radiation passing through the support 22 and propagating towards the detector 10.

[0059] Taking into account the above, the modulator is such that abs^ S abs22 and preferably abs2 { > 1.05 abs22 and even more preferably abs21 4 1.1 abs22 .or even abs2ï > 1.5 abs22 or abs2i 4 2 abs22. The ratio between abs^ and abs22 is an absorption contrast. It is preferable that the latter is as high as possible.

[0060] Other configurations of the modulator are possible, as described below in connection with Figures 5A to 5E.

[0061] The device comprises a processing unit 40, configured to receive the detection signal S(t) at different times of the same modulation period. The processing unit is programmed to implement calculation steps described in connection with [Fig.3]. The processing unit may in particular comprise a microprocessor.

[0062] [Fig.3] shows schematically the main stages of the implementation of the invention.

[0063] Step 100 corresponds to the step of detecting the incident radiation at different detection instants '* extending over at least half a modulation period and preferably according to one or more modulation periods. Preferably, the detection signal is generated at a frequency such that several consecutive detection signals are produced during each modulation period. Preferably, at least 10 detection signals, or even several tens of detection signals, are

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[0076] generated during each modulation period. The intensity I of the incident radiation at the detection device 1 is assumed to be constant, apart from statistical fluctuations, during the modulation periods, apart from statistical fluctuations. The intensity of the incident radiation is generally considered to be distributed according to a Poisson law. In parallel with step 100, the control unit 30 sets the modulator 20 in motion, so as to obtain a periodic modulation of the intensity of the radiation incident on the detector: this corresponds to step 110. During step 120, the processing unit 40 performs processing of the detection signals S(t) acquired during step 100. The processing consists of taking into account different time shifts T, each time shift being between 0 and nT or n is real, preferably greater than or equal to 1 and T is the modulation period. Thus, the processing aims to generate an autocorrelation signal S"(r) such that: S (r) = J (1) - n is a real number, preferably greater than or equal to 1; n can be an integer. - T is the modulation period. Expression (1) is established assuming that the detection signals are acquired at an acquisition frequency such that the function S(f) can be considered continuous. Taking into account the time discretization of the detection signal, expression (1) can be equivalent to: s (T) « L 0 S(l)S(tT) (2) Figure 4A represents an example of detection signals, resulting from the exposure of a device as previously described to an irradiating source of 152Eu. In Figure 4A, the abscissa axis corresponds to time and the ordinate axis corresponds to the detection signal S(t), in this case a count lasting 330 seconds. We observe that during each period, the detection signal oscillates between: • a maximum signal, when the absorbing portion is not positioned facing the detector; • a minimal signal, when the absorbent portion is placed facing the detector; Figure 4B represents a detection signal 5(0) (in black) as well as a time-shifted signal S(t - t) (in gray) with r = 12. Figure 4C represents the autocorrelation signal S (t) tC' 9uexPrimé in (1) established from the detection signal shown in Figure 4B. In Figure 4C, the axis of the abscissa corresponds to the time shift T.

[0077] It is observed that the autocorrelation signal $ follows a periodic variation, the period of which corresponds to the modulation period T. The autocorrelation signal forms, during each period, an oscillation of amplitude A. During step 130, the amplitude A of at least one oscillation, or an average amplitude of several oscillations, is determined. Step 130 may comprise smoothing of the autocorrelation signal $ by applying a low-pass filter, so as to smooth out any statistical fluctuations.

[0078] During step 140, the intensity of the ionizing radiation / is determined. The intensity I of the ionizing radiation incident on the detection device 1 is correlated to the amplitude A of the autocorrelation signal. Generally speaking, a calibration function / can be used, such that I — f ( A ). The intensity can be expressed per unit of time (for example photons per second), in which case it is a flux, or in units of time and area, in which case it is a fluence rate (for example number of photons per cm2 per second).

[0079] The calibration function / can be determined during a calibration step, which corresponds to step 90 shown diagrammatically in FIG. 3. The calibration consists of implementing steps 100 to 130 while the intensity I of the incident radiation at the device 1 is known. The amplitude A of the autocorrelation signal is measured for different intensity values, which allows an empirical determination of the calibration function. Figure 4D represents an example of a calibration function. In Figure 4D, the abscissa axis corresponds to the intensity Z of the incident radiation (arbitrary unit) while the ordinate axis corresponds to the amplitude A of the autocorrelation signal. The calibration function can also be determined on the basis of models.

[0080] Figures 5A to 5E diagram other possible configurations of the modulator 20. In the example of [Fig.5A], the thickness of the absorbent portion 21 is similar to the thickness of the support 22. The absorbent portion is formed of a more absorbent material than the support. For example, the support is formed of plastic while the absorbent portion comprises a metal chosen from Fe, Cu, Pb, W.

[0081] In the example of [Fig.5B], the modulator comprises an absorbent portion 21 delimiting an opening 23. The absorption of the opening is zero. According to this configuration, the second part of the modulator 20 is not a support 22, as previously described, but the opening 23.

[0082] In the example of Figures 5C and 5D, the modulator is formed from a single material, of variable thickness. The absorbent portion 21 corresponds to a portion of the modulator whose thickness is beyond a threshold thickness e. [Fig.5D] is a sectional view of the modulator shown in [Fig.5C], the section being made along the dotted line shown in [Fig.5C].

[0083] Figures 5E and 5F diagram a configuration according to which the modulator 20 is movable in translation, perpendicular to the central axis X. As described in connection with [Fig.2], the modulator comprises an absorbent portion 21, which is periodically brought in front of the detector 10, so as to modulate the intensity of the radiation incident on the detector. [Fig.5E] represents the device according to an open configuration. [Fig.5F] represents the device according to a closed configuration.

[0084] An advantage of the invention is to be able to estimate an intensity of radiation in the presence of strong background noise. Figure 6A represents a simulation of a detection signal S(f) (y-axis) as a function of time (x-axis - arbitrary unit) in the presence of incident radiation but also strong background noise. The modulation of the intensity of the incident radiation is barely perceptible due to the background noise.

[0085] Figure 6B is a simulation of an autocorrelation signal S'(t) (y-axis) established from the detection signal shown in [Fig.6A]. In [Fig.6B], the autocorrelation signal is shown respectively without low-pass filtering (dark curve a) and after application of a low-pass filter (light curve b). Despite the background noise observed in [Fig.6A], it can be seen that the invention makes it possible to obtain, by means of smoothing, a usable autocorrelation signal. The amplitude of the oscillations of the autocorrelation signal can be determined without difficulty.

[0086] According to one possibility, the invention can be applied by performing a count in a predetermined energy window. The detector is then connected to an electronic circuit making it possible to classify the detected pulses according to their amplitude, each amplitude corresponding to an energy released in the detector. Thus, only the pulses whose amplitude is located in a predetermined amplitude range form the detection signal. The amplitude range is selected according to the energy window that one wishes to address. The relationship between the amplitude of the pulses and the energy released during the interaction giving rise to the interaction is assumed to be known.

[0087] The use of such an electronic circuit is conventional in the field of gamma spectrometry. The energy window is advantageously adjustable. For example, when the detector is placed facing an object likely to contain 137Cs, the energy window can be centered on 662 keV, and of width equal to a few keV or a few tens of keV, depending on the energy resolution of the detector. The energy window can be a few keV when the detector is a detector with good resolution, typically a hyperpure Germanium (HPGe) detector, or a few tens of keV when the resolution is average, ty sting with a scintillator detector of the NaI(Tl) type. The energy window is advantageously centered on an emission energy of a radionuclide that one wishes to characterize.

[0088] Such an embodiment makes it possible to significantly improve the detection sensitivity in the selected energy window. The energy window can be successively modified, so as to successively address different emission energies of a radionuclide or different radionuclides.

[0089] The invention can be applied for the radiological characterization of nuclear waste or equipment in the presence of background noise. The use of a mobile, and preferably motorized, modulator makes it possible to carry out measurements using a compact device of simple design. Although very little sensitive to background noise, the invention can also be combined with the use of shielding forming a sheath around the detector, and delimiting an observation field. In this case, the modulator is arranged in the observation field, between the detector and the object to be characterized. The use of shielding is not necessary.

[0090] Although described in connection with a NaI(Tl) scintillator, the invention can be implemented with gaseous detectors, for example an ionization chamber, or a solid detector: scintillator, for example of the CsI, LaBr3 type, or semiconductor, for example Si, GeHP...

Claims

Claims

1. Device for detecting ionizing radiation (1), formed of photons or neutrons, the device comprising: - an ionizing radiation detector (10), the detector being configured to interact with particles forming the ionizing radiation and to generate a detection signal dependent on an intensity of the ionizing radiation incident on the detector; - a modulator (20), the modulator comprising at least one absorbing portion (21), the absorbing portion being configured to absorb a portion of the ionizing radiation; - a control unit (30), configured to set the modulator in motion, so that the absorbing portion is periodically arranged facing the detector, so as to periodically absorb the radiation incident on the detector, so that the detection signal oscillates periodically between • a maximum signal, when the absorbing portion is not positioned facing the detector; • a minimal signal, when the absorbent portion is placed facing the detector; - a processing unit (40), programmed to: • i) storing detection signals (S(t)) generated by the detector, at different detection times, the detection times being distributed according to at least one period of the detection signal; • ii) calculate an autocorrelation function (S'(t)) of the detection signal, the autocorrelation function being a periodic function whose period is the period of the detection signal; • iii) from an oscillation amplitude (A) of the autocorrelation function according to at least one period, estimation of the intensity ( / ) of the radiation incident on the detection device.

2. A device according to claim 1, wherein the function autocorrelation function is defined for different time shifts, the autocorrelation function, at each time shift (r), being calculated from a sum, over a duration of at least half a period, of each detection signal (S(t)) measured during said duration multiplied by said detection signal (S(t - r)) shifted by the time shift.

3. Device according to any one of the preceding claims, in which the modulator comprises: - a first material, of a first thickness, forming the absorbent portion (21); - a second material, of a second thickness, forming a support for the absorbent portion (22); the modulator being such that the absorbent portion has an absorption of the incident radiation at the detector greater than the absorption of the support.

4. A device according to claim 3, wherein the absorption of the absorbent portion is at least 10% greater than the absorption of the support.

5. Device according to any one of claims 1 or 2, in which the modulator is formed from a first material, of variable thickness between a minimum thickness and a maximum thickness, the absorbent portion being formed by a part of the material having the maximum thickness.

6. Device according to any one of claims 1 or 2, in which the modulator comprises: - a first material, forming the absorbent portion (21); - an opening (23), formed through the first material.

7. A device according to any preceding claim, wherein the absorbing portion is configured to absorb at least 10% of the intensity of the radiation incident on the detection device.

8. Device according to any one of the preceding claims, in which the modulator (20) is movable in translation or in rotation relative to the detector.

9. A device according to any preceding claim, in which the control unit has a motor to set the modulator in motion relative to the detector.

10. Device according to any one of the preceding claims, in which the detector is connected to an electronic circuit configured to select an amplitude range of pulses formed following each interaction of a particle in the detector, such that the detection signal corresponds to an intensity of the radiation detected in a predetermined energy window, corresponding to the selected amplitude range.

11. A device according to claim 10, wherein the selected amplitude range is adjustable.

12. Method for detecting an intensity of ionizing radiation comprising the following steps: - a) arranging a device (1) according to any one of the preceding claims facing an object emitting the radiation; - b) using the control unit, periodically moving the modulator (20) relative to the detector; - c) during step b), acquiring detection signals, generated by the detector at different times during at least one period of the movement of the modulator; - d) using the processing unit, • i) storing the detection signals (S(0) generated by the detector during step c) • ii) calculating an autocorrelation function (S'(r)) of the detection signal, the autocorrelation function being periodic;• iii) determination of an oscillation amplitude (A) of the autocorrelation function during at least one period and estimation of the intensity of the incident radiation of the detection device.;

13. A method according to claim 12, wherein sub-step iii) comprises applying a calibration function ( / ) to the oscillation amplitude (A) of the autocorrelation function.

14. A method according to claim 12 or claim 13, wherein the ionizing radiation is neutron radiation or a X-ray or gamma-type photon radiation.

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