Patent Application
20250102610
The technical field of the invention relates to estimation of a direction between a detecting device and a detected radioactive source. The detecting device forms a compass, pointing toward the radioactive source.
Determining a position of a radiant radioactive source in an environment can be a tedious operation. It is an operation liable to need to be carried out in nuclear installations, for radiation-protection purposes, or to control suspicious goods or waste, at borders for example.
A first option consists in using a radiation-measuring device such as those usually used in the field of radiation protection. This type of device allows an ambient radiation level to be measured, but provides no information on the position of the sources generating the radiation. Locating a radiation source requires a spatial scan while regular measurements are performed, in the hope of gradually getting closer to the source depending on the successively measured radiation levels. Such an operation is time-consuming, and therefore disadvantageous from the point of view of dose. The main advantage of this approach is that it employs measuring means that are inexpensive, simple to use and widely available.
Another option is to use an imager, such as a gamma camera. A gamma camera is a device that allows an image of an observed scene to be formed, in which image the main radioactive sources may be located. This type of device has been undergoing improvement since the 1990s and has reached industrial maturity. Compact and relatively light gamma cameras are currently commercially available. However, gamma cameras are expensive devices, requiring a certain level of know-how to use.
Patent U.S. Pat. No. 10,024,981B2 describes a measuring device borne by a drone, and intended to locate a radioactive source using a plurality of detectors. The source is located by comparing signals detected simultaneously by a plurality of detectors. However, the location is inexact, because it results from a simple comparison of the intensities of the measured signals.
The inventor has designed a device that operates like a compass, allowing a direction pointing toward a radioactive source to be indicated. The device is of simple design, and allows directional information to be obtained. It is also a device that is inexpensive, and simple to use.
A first subject of the invention is a device for estimating a direction pointing toward a radioactive source, the radioactive source emitting radiation composed of gamma particles or X-rays or neutrons, the device comprising:
According to one embodiment, the device comprises at least three non-aligned detectors.
According to one embodiment, the processing unit is configured to:
The device may comprise an orienting unit, configured to estimate a change in the position of the detectors between the first time and the second time.
The device may comprise a display unit, connected to the processing unit, and configured to display the direction pointing toward the radioactive source.
At least one detector or each detector may comprise a detection scintillator. At least one detector or each detector may comprise an inorganic detection scintillator.
According to one embodiment, the processing unit is configured to:
The device may comprise a geolocating unit, configured to measure the distance of movement of the device between the two successive measurement times.
A second subject of the invention is a method for determining a direction pointing toward a radioactive source using a device according to the first subject of the invention, the method comprising the following steps:
According to one embodiment:
Between the first time and the second time the device may rotate about the iso-centroid, the angle of rotation being predetermined and/or measured.
The method may comprise a step of displaying the direction pointing toward the radioactive source.
According to one embodiment, the method comprises steps of:
The method may comprise steps of:
A third subject of the invention is a device for estimating a distance with respect to a radioactive source, the radioactive source emitting radiation composed of gamma particles or X-rays or neutrons, the device comprising:
A fourth subject of the invention is a method for estimating a distance between a radioactive source and a device according to the third subject of the invention, comprising:
The invention will be better understood on reading the description of the examples of embodiment that are presented, in the rest of the description, with reference to the figures listed below.
Use of the device presupposes that the radioactive source is placed at a distance such that, in view of its activity, the radiation incident on the device is sufficient to be detected.
In the examples shown, the device is intended to be borne by a user, so as to indicate the direction pointing toward the source. It is therefore a device intended to be used nomadically. Alternatively, as described below, the device may be mounted on a vehicle, in particular in the context of deployment in hostile environments.
The device 1 comprises a holder 2 on which a plurality of detectors 101, 102 are placed. Each detector is configured to detect ionizing radiation, in particular neutrons or X-ray or gamma photons.
Each detector is formed from a detection material connected to a detecting circuit. Under the effect of an interaction in the detection material, a particle forming the ionizing radiation (neutron or X-ray or gamma photon) generates an electrical pulse, the latter being detected by the detecting circuit. When the detection material is a scintillator, the electrical pulse is generated by a photodetector coupled to the detection material. When the detection material is a semiconductor, the electrical impulse results from collection of charge carriers created in the material under the effect of the interaction. The detecting circuit allows a count rate to be established, which corresponds to a number of pulses detected per unit of time. For example, a number of pulses per second, usually referred to as the “counts per second”, is established. Alternatively, the detecting circuit allows a count to be established, which corresponds to a number of pulses detected during a given length of measurement time.
Generally, the detecting circuit allows a detection signal to be formed, the latter corresponding either to a count rate or to a count. In the examples described below, the detection signal is a count rate.
Preferably, the detection material of each detector is solid. It may in particular be a scintillator. When the particles forming the radiation are photons, inorganic scintillator materials are preferred, due to their high atomic number, which means they attenuate well. Among inorganic scintillators, mention may be made, non-limitingly, of NaI(TI), CsI(TI), GBO (bismuth germanate), LaBr3, YSO(Ce) (yttrium orthosilicate: cerium—Y2SiO5:Ce), and Srl2(Eu). When the particles forming the radiation are neutrons, organic scintillator materials are preferred, because of their high light-atom content.
In the example illustrated in
The device also comprises:
Based on the position of each detector on the holder 2, an iso-centroid O may be established. The iso-centroid O is a geometrical point located equidistant from each detector. To compute the iso-centroid, each detector 10i may be assigned a discrete position Pi, which corresponds to the center of the detector. i is a natural number designating each detector.
The processing unit 20 is configured to compute a centroid B of the position Pi of each detector 10i, weighted by the count rate measured by each detector, respectively. If Pi designates a point on the holder associated with each detector 10i, which may for example be the center of each detector, the centroid B is such that:
Ni corresponds to the count rate measured by the detector 10i.
The vector {right arrow over (OB)} points toward the source S. Thus, computation of the vector {right arrow over (OB)} makes it possible to estimate a direction, pointing toward the source, from the device. The direction pointing toward the source may be displayed on the display unit 22. The device then operates in a manner equivalent to a compass. The display unit may comprise an arrow pointing toward the source.
Using only two detectors, a single measurement is not enough, because the direction obtained is necessarily parallel to the direction in which the detectors are aligned.
However, by rotating the device about an axis perpendicular to the holder and passing through the iso-centroid O, the vector {right arrow over (OB)} may be computed a number of times and the orientation of the device for which the norm ∥{right arrow over (OB)}∥ is maximum may be determined. In such an orientation, the detectors are aligned in the direction pointing toward the source.
When the detectors 101, 102 and the source S are aligned, the detector 101 forms a front detector and the detector 102 forms a rear detector. The front detector is interposed between the source and the rear detector. In this arrangement, the radiation emitted by the source and reaching the rear detector is attenuated by the front detector. Since the detection materials of the detectors are absorbent, in particular when they are inorganic detectors, the attenuation of the radiation by the front detector is significant. Thus, in this configuration, the discrepancy between the respective count rates measured by each detector is maximum. This maximizes the norm ∥{right arrow over (OB)} ∥.
The display unit 22 may display an arrow the size of which is correlated to the norm ∥{right arrow over (OB)}∥. The user may thus visually determine the orientation of the device maximizing the norm ∥{right arrow over (OB)}∥. Alternatively, a numerical value, corresponding to the norm, is displayed on the display unit.
The processing unit 20 is configured to store the respective count rates at each measurement time and to compute a position of the centroid B.
where Ni(tj) is the count rate of detector 10i, occupying position Pi(tj), at measurement time tj, with j varying between 1 and 2 in this example.
According to one possibility, the device comprises an orienting unit 21 such as described above. In this case, the orienting unit 21 may be configured to detect, during the rotation of the device, when an angle of rotation of 90° with respect to the first time t1 has been reached, and to carry out the measurements directly.
According to another possibility, the orienting unit 21 determines an angle of rotation between the first time and the second time. This allows a more precise computation of the position of the centroid B and avoids the need for the user to rotate through an angle close to 90°. The user may actuate a switch, so as to signal the first and second times to the processing unit. The orienting unit determines the angle between the first time and the second time and delivers it to the processing unit. The actual value of the angle is taken into account to compute the centroid. The rotation between the two times may thus be different from 90°. It may be 45° or 110°. However, it is preferable for the angle of rotation to be close to 90°.
Whatever the configuration, when the radiation to be detected is gamma radiation, use of detectors based on inorganic materials maximizes absorption of some of the radiation emitted by the source by the detectors closest the source. This increases the contrast between the count rates measured by each detector. The centroid B is thus further away from the iso-centroid O, this increasing the accuracy of the direction estimated by the device. In order for the absorption resulting from one detector to be able to significantly influence the count rate of another detector, the distance between detectors is preferably less than 30 or 40 cm. In order for the effect of absorption to be more significant, it is preferable for the distance between the detectors to be less than 20 cm.
The inventor has carried out an experimental trial in which four detectors were arranged in the same horizontal plane, in a configuration similar to the one shown in
The difference between the various count rates made it possible to determine, without difficulty, the direction pointing toward the source.
A configuration such as shown in
It may be seen that the contrast observed between the front (101) and rear (102) detectors is high whatever the distance.
In another model, a 137Cs source such as described above placed 5 meters from the device was modeled. The material forming each detector was varied. Regardless of the material, each detector was cylindrical in size, and had a diameter and height equal to 25.4 mm. Table 2 shows the count rates estimated for each detector as a function of the detection materials, the latter being NaI(TI), BGO, LaBr3(Ce), and Srl2(Eu).
It may be seen that the contrast between the front (101) and rear (102) detectors is particularly high for BGO or, to a lesser extent, LaBr3(Ce), because of their high atomic number, which is conducive to a strong attenuation of the photons.
Operation of the device according to the embodiment described with reference to
The embodiments described above make it possible to determine a direction pointing toward a radioactive source. The directional information may then be used to estimate a distance between the device and the source.
Assuming a point source, the intensity of the radiation decreases proportionally to the inverse of the square of the distance. If r designates a distance between a radioactive source and the device, the intensity of the radiation, between the device and the source, varies as 1/r2. The intensity may correspond to a flux (number of particles per unit time) or to a fluence rate (number of particles per unit time and area). The count rate of each detector varies linearly as a function of the intensity of the radiation emitted by the source. Knowing the direction in which the source is located, by implementing one of the embodiments described above, it is possible to estimate a distance between the device and the source, via movement, by a known distance, with respect to the source, and via measurement of the variation in the count rate of at least one detector. The movement with respect to the source is made in the direction pointing toward the source. Knowing the distance traveled, combined with measurement of the count rates before and after the movement, makes it possible to evaluate the distance between the device and the source.
Thus, when the device is moved, in a rectilinear direction, pointing toward the source, between a position r1 and a position r2, see
where N1 and N2 are the count rates measured, by a detector (and preferably the same detector) when the device occupies the positions r1 and r2 with respect to the source, respectively. If d is a distance between r1 and r2, then r2=r1+d.
d is a signed distance, which is negative if r2<r1 and positive if r2>r1.
Development of (3) leads to:
N2 and N1 are measured. d may be predetermined or measured. When d is predetermined, the user bearing the device moves, along the direction pointing toward the source, by a predetermined distance, for example 50 or 60 cm, this corresponding to one increment. Alternatively, the device is equipped with a geolocating unit 23, allowing the distance d traveled between the positions r2 and r1 to be measured.
The processing unit 20 is programmed to estimate r1 by solving (4).
Count rates follow a Poisson distribution. Expression (4) may be employed provided that the difference between N1 and N2 is greater than a threshold, so as to be representative from a statistical point of view. Assuming that N1 and N2 are high enough to follow a Gaussian distribution, and a risk of false positive of 2.5%, the difference between N1 and N2 is considered statistically significant when
with 1−α=1.96.
Beyond the statistical threshold (1−α)√{square root over (2N2)}, the difference between N1 and N2 has a 97.5% probability of being statistically representative and of not being due to statistical fluctuations.
A feasibility trial has been carried out using a BGO inorganic scintillator detector of 25.4 mm×25.4 mm (1 inch by 1 inch). The scintillator was moved, in increments of 20 cm, in a straight line, with respect to a point 137Cs source. The 137Cs source generated an equivalent dose rate of 10 μSv.h−1, and was placed 5 meters from the detector. The distance between the detector and the source varied between 10 m and 2 m.
The relative difference
was also computed, the scale of the latter being plotted on the right-hand y-axis−unit %. Curve b) corresponds to the variation in the relative difference as a function of distance.
At each measurement point, the statistical threshold as defined in (5) was computed. Curve c) shows the variation in the statistical threshold as a function of distance.
The difference between the count rates N2−N1 may be considered to be statistically representative when N2−N1>(1−α)√{square root over (2N2)}. In this example, N2−N1 crosses the statistical threshold at a threshold distance ds of 6.5 m from the source. The differential measurement N2−N1 may thus be considered to allow an estimation of the distance between the detector and source over a distance range of between 0 and 6.5 m, i.e. up to the threshold distance. This corresponds to a relative difference of about 6%.
(grayscale levels) as a function of the variation in distance r1−r2 (x-axis−unit meter−r2<r1), for various distances r1 with respect to the source (y-axis−unit meter). The more r1 increases, the greater the distance variation r1−r2 must be to observe a relative difference greater than statistical fluctuations. This figure shows that the distance r1−r2 is preferably greater than 20 cm, to obtain a relative difference greater than 6%, the latter corresponding to the statistical threshold: see
To determine the distance between the source and the device, the detector closest to the source will preferably be used, since said detector is not subject to attenuation of the radiation emitted by the source by the other detectors.
Step 100: measuring the count rates of the detectors forming the device.
When the device comprises more than 2 detectors, step 120 may be implemented directly.
Step 110: this step is implemented in particular when the device comprises only 2 detectors. The device is rotated around iso-centroid O and perpendicular to the holder. The count rate of each detector is measured.
Step 120: Depending on the measurements performed in step 100 and the optional step 110, computing the centroid of the positions of each detector weighted by the count rates measured by each respective detector.
Step 130: Depending on the position of the centroid resulting from step 120, determining a direction pointing toward the source.
Step 140: moving the device in the direction resulting from step 130 by a predetermined distance or any distance.
Step 150: when the distance of movement of step 140 is arbitrarily chosen, estimating the distance by means of the geolocating unit or by means of the inertial measurement unit, beginning from a starting position.
Step 160: measuring the count rate of a detector of the device.
Step 170: based on the measurement resulting from step 160 and on the measurement made by the detector before the movement, for example in step 100 (or 110), estimating a distance between the source and the device.
Step 180: comparing the estimated distance with the threshold distance ds, so as to validate or invalidate the measurement. The threshold distance was determined beforehand.
The device described above may be used while being carried by a user. It may also be mounted on an autonomous vehicle. In this case, the direction determined by the device is transmitted to a control unit of the vehicle, so that the vehicle may be steered depending on this direction. Use of a vehicle is recommendable when operating in hostile environments, for example in the presence of contamination or high levels of radiation.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2114564 | Dec 2021 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/087838 | 12/25/2022 | WO |
