AU2019250099B2 - Neutron generator - Google Patents

Abstract

5 NEUTRONGENERATOR A neutron generator with a neutron tube (10) comprising a reservoir (12) for absorbing and desorbing a gas, an ion source (14) producing ions by ionization of the gas, an ion acceleration system and a target (18) struck by the accelerated 10 ions to emit neutrons. The neutron generator includes an electronic control circuit (50) of the supply of electricity to the reservoir and ion source, and has a neutron emission mode and pause mode. The neutron generator (100) includes a regulation circuit (40) configured to determine, in pause mode, the electrical resistance of the reservoir (12) and regulate the electrical resistance of the 15 reservoir by varying the current (Ir) supplying the reservoir. Figure 1 1/4 60 40,50 13 12 14 Ir Ur % lions :HT THT R 16 1+ N 24 FG FIG. 1

Description

1/4

60 40,50

13

12 14

Ir Ur % lions

:HT THT

R 16 1+ N 24 FG

FIG. 1

NEUTRONGENERATOR

TECHNICAL FIELD The present disclosure relates to a neutron generator and a material analysis device by neutron interrogation comprising such a generator. This material analysis device allows to detect the presence of certain chemical elements in the material to be analysed by measuring the radiation emitted by these elements when the material is exposed to a neutron flux. It can be used, in particular, for mining analysis. TECHNOLOGICAL BACKGROUND There is now a growing interest in the mining industry for neutron interrogation material analysis equipment and, in particular, for neutron analysis probes intended to be introduced into a borehole. Examples of such probes are given in patent documents US 4288696 and US 4092545. These neutron analysis mining probes make it possible, in particular, to know the composition of the material surrounding a borehole several hundred metres deep, almost in real time. While the interest of these probes in estimating the resources of a deposit has already been demonstrated, the aim today is to use these probes at the production stage to analyse the material surrounding the blast holes before charging them with explosives. The firing holes are typically 5 to 15 metre deep holes (corresponding to a stratum of the mine), filled with explosives to break up the rock into pieces and then recover the material. For mining, the analysis time of the firing holes must be reduced to a minimum for technical (the holes are potentially unstable and collapse) and productivity reasons. This means that a mining probe must allow the holes to be analysed in a minimum of time. The speed of analysis is important but not only. Indeed, the operation is repetitive, the holes being spaced a few meters apart (typically about 6 meters), and the transfer of the probe from one hole to the other must also be as short as possible. The analysis cycle of a field of firing holes typically includes the following phases: [A] setting up, initializing and lowering the probe in the hole, [B] starting or restarting the neutron emission, [C] logging, [D] hole exiting and stopping the neutron emission, [E] movement to the next hole. Steps [B] to [E] are repeated (e.g. 50 times) until the last hole and the final stop of the probe. To protect people present at the site from radiation, it is necessary to stop neutron emission in steps

[D] and [E]. It appears that phases [B], [C] and [E] represent the largest part of the time required to analyse the field of firing holes. These phases must therefore be optimised. Step [C] depends essentially on the performance of the probe and step

[E] depends on the handling equipment and the vehicle used to operate the probe. Although they can be optimised, these points are not the object of this invention. Concerning step [B] of restarting the neutron emission, it is desirable to significantly reduce the time required to restart the neutron emission after it has been stopped when passing from one hole to another. It is the object of the present disclosure to substantially overcome or at least ameliorate one or more of the disadvantages of the prior art, to meet the above desire, or to provide a useful alternative. GENERAL OVERVIEW This presentation concerns a neutron generator comprising a neutron tube itself comprising a reservoir for absorbing and desorbing a gas, an ion source producing ions by ionization of the gas, an ion acceleration system and a target on which the accelerated ions are struck to emit neutrons. The neutron generator also includes an electronic circuit to control the supply of electricity to the reservoir and the ion source. The neutron generator has a first operating mode, or emission mode, in which neutrons are emitted by the neutron tube, and a second operating mode, or pause mode, in which neutron emission is temporarily interrupted. The electronic control circuit is configured to shut off the power to the ion source when switching from emission to pause mode, maintain this power off in pause mode, and restore the power to the ion source when switching from pause mode to emission mode. The neutron generator also includes a regulation circuit configured to, in pause mode, determine the electrical resistance of the reservoir and regulate the electrical resistance of the reservoir by varying the current supplying the reservoir. Such a neutron generator makes it possible to resume, or restart, neutron emission in a few seconds after a pause, in complete safety. During the pause, the electrical resistance of the reservoir is determined and regulated, i.e. maintained at a value as close as possible to a set point. This regulation ensures an almost constant gas pressure in the neutron tube when the power supply to the ion source is cut off. The thermodynamic balance of the tube is thus maintained during the pause, which allows the rapid resumption of neutron emission after the pause. The resumption of neutron emission may include not only the actual resumption but also the return to a stable and regulated neutron emission. In addition to the above-mentioned features, the proposed neutron generator may have one or more of the following features, considered separately or in technically possible combinations:

- the generator also includes a current measuring system supplying the reservoir and an electrical voltage measuring system at the reservoir terminals, the regulation circuit determining the electrical resistance of the reservoir from the measured voltage and current; - the set point for regulating the electrical resistance of the reservoir is calculated by the regulation circuit from resistance measurements over a certain period of time prior to switching from emission mode to pause mode; - the set point for regulating the electrical resistance of the reservoir is calculated by the regulation circuit by averaging the resistance measurements over a certain period of time before switching from emission mode to pause mode; - the regulation set point of the electrical resistance of the reservoir is calculated by adding a compensation value to the calculated average; - in emission mode, the regulation circuit regulates neutron emission by varying the current supplying the reservoir; - the regulation circuit regulates the target current, representative of neutron emission, by varying the current supplying the reservoir; - when switching from pause mode to emission mode, the regulation circuit regulates the target current again by varying the current supplying the reservoir. Alternatively, when switching from pause mode to emission mode, during a transient phase, the regulation circuit first regulates the target current, representative of neutron emission, by varying the voltage of the ion source, and regulates the average voltage of the ion source over a sliding time window by varying the current supplying the reservoir, before regulating the target current by varying the current supplying the reservoir. The present disclosure also relates to a material analysis device comprising a neutron generator as described above, and at least one gamma ray detector to measure the energy of gamma photons generated by the interaction of emitted neutrons with the material to be analysed. Such a device makes it possible, from the energy of the gamma photons measured, to detect the presence of at least one chemical element of interest in the material to be analysed. By looking at several chemical elements, it is possible to know, at least in part, the composition of the material to be analysed. Such a device can be used in many applications, including mining analysis or operations to detect suspicious civilian or military objects. In particular, the device can be a mining probe and can be used, in particular, for the analysis of a field of firing holes. Compared to known mining neutron analysis probes, the proposed probe allows the temporary interruption (i.e. pausing) of neutron emission when passing from one firing hole to another and significantly reduces the time required to resume neutron emission after the interruption. This results in a significant reduction in the total time required to analyse a firing holes field. The device may also be a material analyser through or around which the material to be analysed passes (e.g. raw cement, coal, copper, iron, nickel ore, etc.), this material being, for example, transported on a conveyor belt or flowing by gravity while the analyser remains fixed. The area around such a material analyser is often restricted from access by a physical barrier (grid or equivalent). Generally, for safety reasons and, in particular, to minimise the risk of radiation exposure, the neutron generator is automatically switched off in the event of intrusion into the area. This intrusion may be, for example, the act of a guard walking his way around. In this case, with a conventional material analyser, restarting the neutron emission may result in a loss of up to 10 to 15 minutes of measurement. The proposed solution makes it possible to limit this loss of time by pausing the neutron generator in the event of intrusion into the area around the analyser and by benefiting from the rapid restart of neutron emission when the intruder has left the area, while maintaining the same level of safety and absence of radiation when the intruder is present in the area. The material analysis device can also be used in the context of operations to detect suspicious civilian objects (intervention in stations, airports, etc.) or suspicious military objects, for which the speed of implementation is crucial for the safety of the intervention personnel and the public. In these operations, the threats considered may be mines (e.g. anti-tank or anti-personnel mines), improvised explosive devices or chemical threats (sarin, mustard gas, etc.). The proposed neutron generator makes it possible to initialise the analysis device before an intervention and thus save valuable minutes of analysis during the intervention. For mine-clearing missions in heavily mined areas, the principle of use follows the same scheme as for the analysis of a firing holes field in the mining sector, and has the same advantages: the proposed system makes it possible to significantly reduce the recovery time of neutron emission by moving from one analysis area to another. More generally, the proposed neutron generator can be useful for any application requiring rapid interruptions and restarts of neutron emission. The present disclosure also relates to a method in which the proposed neutron generator is used. The advantages of this method accrue from the advantages of the neutron generator. This method uses the neutron generator, the latter having a first operating mode, or emission mode, in which neutrons are emitted by the neutron tube, and a second operating mode, or pause mode, in which the neutron emission is temporarily interrupted. In this process, the power supply to the ion source is switched off when switching from emission to pause mode and kept off in pause mode. In pause mode, the electrical resistance of the reservoir is determined and regulated by varying the electrical current supplying the reservoir. In other words, the electrical resistance of the reservoir is regulated in pause mode, the electrical current supplying the reservoir being the regulating variable. The power supply to the ion source is restored when switching from pause mode to emission mode. In emission mode, neutron emission can be regulated by varying the electrical current supplying the reservoir. This process can be used for material analysis, especially on a hole field. In this case, the neutron generator is lowered into a hole and put into emission mode. The energy of gamma photons generated by the interaction of emitted neutrons with the material surrounding the hole is then measured and the material is analysed from this energy measurement. The neutron generator is then put into pause mode, removed from the hole and moved to another hole, and the above steps are repeated for that other hole. The aforementioned features and advantages, in addition to others, will appear on reading the following detailed description. This description refers to the appended drawings. BRIEF DESCRIPTION OF THE DRAWINGS The appended drawings are schematic and not to scale; their purpose is above all to illustrate the principles of the invention. In these drawings, from one figure (FIG) to another, identical elements (or parts of elements) are identified by the same reference signs. Figure 1 shows schematically an exemplary neutron generator. Figure 2 shows schematically the two operating modes of the generator. Figure 3 shows an example of transitions from one regulation mode to another. Figure 4 shows another example of transitions from one regulation mode to another. DETAILED DESCRIPTION OF EXAMPLES Exemplary embodiments are described in detail below, with reference to the appended drawings. These examples illustrate the features and advantages of the invention. It should be remembered however that the invention is not limited to these examples.

An example of a neutron generator is shown schematically on figure 1. This neutron generator 100 includes a neutron tube 10. This tube is sealed and includes a reservoir 12, or tank, to absorb and desorb a gas, an ion source 14 producing ions by gas ionization, an ion acceleration system and a target 18 on which the accelerated ions are struck to emit neutrons. On figure 1, the dotted arrow 1+ represents the accelerated ions while the dotted arrow N represents the emitted neutrons. The generator 100 also includes an electronic control circuit 50 for controlling the power supply to the neutron tube 10. The control circuit 50 may be combined with a control software 60, embedded on the control circuit 50 or on an external computer, including a set of instructions to ensure the desired control functions and the correct transitions between the different operating modes (emission mode or pause mode) of the neutron generator 100. Neutron tube 10 can be seen as a miniaturised particle accelerator producing neutrons. This accelerator is the site of a nuclear fusion reaction. This may be, for example, a D-D fusion, or a D-T fusion. These examples of fusion are symbolised by the following equations: - D-D fusion: D + D 4 n (2.5 MeV) + He3 (0.8 MeV) - D-T fusion: D + T 4 n (14.1 MeV) + He4 (3.5 MeV). Typically, neutron tube 10 consists of a mechanical assembly, based on ceramic-metal assemblies, ensuring the maintenance of the various components and the electrical insulation necessary for the proper functioning of the tube. The inside of tube 10 is under high vacuum (secondary vacuum). In the example shown, in addition to the metal casing and the various ceramics, the main components of the tube are reservoir 12, ion source 14, accelerator electrode 16 and target 18. The target 18 is the site of the nuclear reaction between the elements (deuterium and/or tritium) nuclei of which it is composed and a beam of incident ions. In order to cause the beam of ions 1+ to collide with the target 18, the beam of ions 1+ is accelerated by an intense electrical field resulting from application of a major voltage difference between the target (cathode) 18 and the accelerating electrode (anode) 16. The ions are obtained in advance from the ion source 14 in which a gas from reservoir 12 is ionised. The ions thus generated are extracted in the acceleration space, forming the beam of ions 1+. Reservoir 12 typically consists of a filament 13, e.g. tungsten, and a very porous metal (e.g. titanium sintering) to absorb and desorb a gas (e.g. deuterium or a deuterium-tritium mixture), thus becoming a metal hydride. Reservoir 12, more precisely its filament 13, is supplied with Ir power. This Ir current called

"reservoir current" allows to heat more or less the filament, heating in turn the metal hydride which releases more or less gas. As these aspects are well known, they are not described in more detail. The ion source 14 allows to ionise the gas and produce ions. There are many types of ion sources, one of the most traditional being the Penning ion source for ionizing the gas by electronic impact in a plasma maintained by applying an electric field (typically about 2 kV) and a magnetic field produced by permanent magnets. As these aspects are well known, they are not described in more detail. The accelerator electrode 16 produces a potential difference of several tens of kV (typically about 100 kV) to accelerate the ions created by the ion source 14 to target 18. Target 18 and accelerator electrode 16 are an example of an ion acceleration system according to the invention. However, other acceleration systems could be considered. As these aspects are well known, they are not described in more detail. Target 18, e.g. made of copper, is covered with a layer of metal, e.g. titanium or another hydrurable metal. Accelerated ions strike this target to produce nuclear fusion and give rise to neutrons. As these aspects are well known, they are not described in more detail. In the example shown, for the fusion reaction (and therefore the production of N neutrons) to take place, the following three conditions are required. (i) The gas must be in sufficient quantity, i.e. the partial pressure of gas in tube 10 must be sufficient. The higher the pressure, the higher the neutron emission (within the operating limits of the tube) (ii) The voltage of the ion source 14 must be sufficiently high, typically above 800 V. The higher the voltage of the ion source 14, the higher the neutron emission (within the nominal operating limits of the tube). In this application, the ion source 14 voltage is defined as the voltage at the terminals of the ion source. In the diagram of figure 1, the voltage of the ion source 14 corresponds to the voltage at the terminals of the generator 24 supplying the ion source 14. The voltage (high voltage) of the ion source 14 is noted HT. (iii) The voltage between target 18 and accelerator electrode 16, on the one hand, and ion source 14, on the other hand, must be sufficient, typically greater than several tens of kV. The higher this voltage is, the higher the neutron emission (within the operating limits of the tube). This voltage (very high voltage) is called the acceleration voltage and is noted THT. It should be noted that the power supply to neutron tube 10 generally operates in pulsed mode. Pulse mode means that the acceleration voltage THT is high during a pulse duration Ato and then zero during a duration Ati. In pulsed mode, this variation in the THT voltage enters a zero value (i.e. 0 V) and a non zero value is repeated periodically. The acceleration voltage THT causes the acceleration of the ion beam 1+ on target 18 and, thus, the emission of neutrons N during the pulse duration Ato. Then, no neutrons are emitted for the duration Ati. This is known as operation of the neutron generator 100 in pulsed mode. Neutron emission pulse can also be obtained by maintaining a continuous acceleration voltage THT between target 18 and accelerator electrode 16, on the one hand, and ion source 14, on the other hand, and by periodically varying the HT voltage of ion source 14 between a non-zero value (during a pulse duration Ato) and a null value (for a period of time Ati). In any case, the pulsed mode of the neutron generator should not be confused with the emission and pause modes described below. For neutron emission to start, the three conditions (i), (ii) and (iii) above must necessarily be met. However, if the high voltage HT of the ion source 14 and the very high voltage THT of the accelerator electrode 16 can be started up and reach their target values quickly (i.e. within a few micro seconds), the establishment of the appropriate gas pressure inside tube 10 (condition (i)) is significantly slower, i.e. in the order of a few minutes depending on the desired stability and, typically, of about 5 minutes for good stability. This slowness is due to the very design of the reservoir 12, detailed below. The metal hydride in reservoir 12 allows the gas (e.g. deuterium) to be trapped reversibly depending on its temperature. The hotter the metal hydride is, the higher the partial pressure of the gas inside tube 10 increases. The heating of the metal hydride is provided by filament 13: by circulating an electric current in filament 13, it heats up and, by conduction and radiation mainly, transmits the heat to the hydride. However, filament 13 is electrically isolated from the metal hydride to avoid a short circuit, which also creates a thermal resistance between filament 13 and hydride. The slowness of thermal phenomena therefore partly explains the slowness of gas release and, consequently, the time required to obtain an N neutron emission and, a fortiori, a stable and regulated neutron emission. The gas release kinetics also depends on the metal used (titanium, zirconium, etc.). During the research that led to the invention, the inventors found that simple solutions to counteract this disadvantage did not work: simply switching off the THT acceleration voltage or the HT voltage of the ion source 14 by leaving the reservoir current Ir on during the interruption of the neutron emission did not allow a fast and safe resumption of the neutron emission for tube 10. Indeed, leaving the HT voltage of the ion source 14 without the THT acceleration voltage quickly degraded the neutron tube 10 because the plasma from the ion source 14 metallised the walls of the ceramics, which eventually short-circuited the tube 10. The inventors therefore decided to cut off the power supply 24 of the ion source 14 during the pause. However, the inventors found that cutting off the power supply to the ion source (with or without the THT acceleration voltage) altered the thermodynamic balance inside tube 10: the gas pressure increased. This pressure drift did not allow the tube to be restarted quickly: the return to stable and regulated neutron emission was slow and typically took several minutes. In addition, there was a risk of damaging the tube if, at the time of recovery, the pressure was too high and led to potentially destructive slamming. On the contrary, the proposed solution makes it possible to maintain a good thermodynamic balance inside tube 10 during the temporary interruption of neutron emission. This solution exploits the link between the electrical resistance of reservoir 12 and the gas pressure inside tube 10, and regulates the gas pressure inside tube 10 during the shutdown of the ion source 14. This allows maintaining a good balance to restart neutron emission quickly. Typically, the return to a stable and regulated neutron emission can be achieved in less than one minute and, for example, in 10 to 30 seconds. In addition, the restart is done without risk of damaging the tube. According to this solution, the neutron generator has a first operating mode, or emission mode, in which neutrons are emitted, and a second operating mode, or pause mode, in which neutron emission is temporarily interrupted. The control electronic circuit 50 switches off the power to the ion source 14 when switching from emission to pause mode, keeps the power off in pause mode, and restores the power to the ion source 14 when switching from pause mode to emission mode. To disconnect the power supply to the ion source 14, it is possible, for example, to disconnect the generator 24 supplying the ion source 14. The neutron generator also includes a regulation circuit 40 to regulate the electrical resistance of reservoir 12 in pause mode by varying the reservoir current Ir. Regulation circuit 40 may be integrated into control circuit 50. Like control circuit 50, regulation circuit 40 may be combined with control software 60, the latter containing a set of instructions to perform the desired regulation functions. The control 50 and regulation 40 circuits control the various equipment including the power supplies (accelerator 16, ion source 14 and reservoir 12) and measuring equipment. For example, they send remote controls, receive remote measurements, (pre-)process measurements, regulations and ensure communications with the control software 60.

Regulation circuit 40 operates in at least two regulation modes: a first regulation mode called "standard regulation" corresponding to the emission mode of the neutron generator 100, and a second regulation mode called "reservoir resistance regulation" corresponding to the pause mode of the neutron generator 100. Figure 2 illustrates these two regulation modes: at the top, the standard regulation and at the bottom, the reservoir resistance regulation. Transitions from one regulation mode to another are also finely managed, as illustrated in figure 3. The standard regulation, shown above on figure 2, is the one used to monitor the neutron generator in emission mode: it allows, in particular, to have a substantially constant N neutron emission. The "Ions" current of target 18, which indirectly measures neutron emission at the voltage of the HT ion source and fixed acceleration voltage THT, is kept constant by adjusting Ir. The controlled variable is therefore the Ion current and the regulating variable is the current of the reservoir Ir. The regulation set point is a desired value of the ion current. On figure 2, the light lines represent fixed set points and the bold lines represent measurements and set points that vary due to their adjustment by regulation. When the neutron tube is to be paused, the regulation circuit 40 temporarily cuts off the neutron emission by cutting off the voltage of the HT ion source (the THT acceleration voltage is maintained, cut off or only lowered) and switches to the reservoir resistance regulation, shown below on figure 2. In this case, the regulated variable is the resistance of the reservoir Rr and the regulating variable is the current of the reservoir Ir. The regulation set point is a desired resistance value of the reservoir Rr. The transitions between the two modes are managed by the control software 60 which may be embedded on the control and regulation circuits 40, 50 or on an external computer (PC). The transitions ensure system safety between the two operating modes (emission mode/pause mode) of the generator and, therefore, between the two associated regulation modes (standard regulation/reservoir resistance regulation) and prepare the correct regulation instructions. Transitions include in particular: - Management of power supply shutdowns/restarts (voltage of the HT ion source and possibly THT acceleration voltage). - Preparation of the regulation set point for the reservoir resistance Rr, hereinafter referred to as "RR set point". This set point may be calculated from an average of the resistance measurements Rr over a certain period of time before the transition from emission to pause mode. In particular, the set point RR may be equal to said average, possibly added to a compensation value (or "offset") to anticipate the interruption and then the restart of the power supply to the ion source 14. This compensation value may typically be adjusted experimentally to optimise the restart time. - Good management of the resumption of standard regulation on restart and, in particular, the right timing between the resumption of power to the ion source 14 and the switch to standard regulation. Figure 3 shows an example of transitions from one operating mode (emission mode/pause mode) to another. In this figure, step SO corresponds to the initial start of the neutron tube. Step S1 corresponds to the "normal" operation of the generator according to its emission mode. The regulation implemented is the standard regulation. Step S2 is the transition from emission mode to pause mode. During this transition, the power supply to the ion source 14 is switched off (the power supply to the acceleration system can also be switched off) and the regulation set point for the reservoir resistance regulation is prepared. Step S3 corresponds to the pause mode of the generator. The regulation implemented is the reservoir resistance regulation. Step S4 is the transition from pause mode to emission mode. During this transition, the regulation set point for the standard regulation is prepared and the power supply to ion source 14 is restored (the power supply to the acceleration system can also be restored). Figure 4 shows another example of transitions from one regulation mode to another. In this example, the transition from pause mode to emission mode is more elaborate: during this transition, two overlapping regulations are implemented at the same time. This further reduces the time required to regain a stable neutron flux but makes the transition more complex than in figure 3. Steps SO, S1, S2 and S3 in figure 4 are the same as those in figure 3. The transition step S4 in figure 3, on the other hand, is replaced by steps S41 to S43. In step S41, the regulation instructions for the dual regulation are prepared and the power supply to ion source 14 is restored (the power supply to the acceleration system can also be restored, if it was switched off in step S2). Step S42 corresponds to the double regulation, i.e. the use of the following two regulations that operate at the same time. A first regulation of the ions current of target 18 by the voltage of the HT ion source. The regulated variable is therefore the ion current and the regulating variable is the voltage of the HT ion source. The regulation set point is a desired value of the ion current. A second regulation of the voltage of the HTm average ion source over a sliding time window, typically a few seconds, by the current of the reservoir Ir. The regulated variable is therefore the voltage of the average ion source HTm and the regulating variable is the reservoir current Ir. The regulation set point is the voltage of the average ion source HTm. The first regulation has the advantage of being very reactive because the influence of the voltage of the HT ion source on the target ion current is immediate. This regulation allows a stable neutron flux to be achieved in a few seconds and therefore to start the analyses a few seconds after the restoration of the power supply to the ion source 14. On the other hand, the pressure inside the tube 10 is not regulated, which can be dangerous for the tube over several minutes or hours. This problem is corrected by the second interlocking regulation. Indeed, the second regulation allows a state of equilibrium to be restored inside the tube 10. The equilibrium may be strictly identical to that of the standard regulation: for this, it is sufficient to match the regulation set point of the the voltage of the medium ion source HTm to the value of the voltage HT of the (fixed) ion source used in standard regulation. Thus, the regulation by the current of the Ir reservoir will slowly return the voltage of the HTm average ion source to its usual value. This regulation is similar in speed to the standard regulation (because it is limited by the thermal phenomena of the reservoir) and therefore the equilibrium can typically be restored in about 10 to 30 seconds. However, thanks to the first regulation (faster than the second), the neutron flux itself is indeed stable in a few seconds and allows the neutron analysis to start while tube 10 stabilises under pressure. In the example in figure 4, step S42 of dual regulation is followed by step S43, which corresponds to the preparation of the regulation set point for the standard regulation, i.e. the regulation of ions by Ir, and the switch to standard regulation. According to a variant, not shown, the standard regulation may be replaced by the double regulation mentioned above. The embodiments or examples described in the present disclosure are provided by way of illustration and are non-restrictive; a person skilled in the art can easily, in the light of this disclosure, modify these embodiments or examples, or contemplate others, while remaining within the scope of the invention defined by the attached claims. Furthermore, the different features of the embodiments or examples described in the present disclosure may be considered severally or combined with each other. When combined, these embodiments or examples may be combined as described above or differently, the invention not being limited to the specific combinations described above. In particular, unless otherwise specified or technically incompatible, a feature described in relation to one example or embodiment may be applied in a similar manner to another example or embodiment.

Claims (9)

The claims defining the invention are as follows:

1. Neutron generator comprising: a neutron tube comprising a reservoir for absorbing and desorbing a gas, an ion source producing ions by ionization of the gas, an ion acceleration system and a target struck by the accelerated ions to emit neutrons, and an electronic control circuit for controlling the supply of electricity to the reservoir and the ion source, in which, the neutron generator has a first operating mode, or emission mode, in which neutrons are emitted by the neutron tube, and a second operating mode, or pause mode, in which the neutron emission is temporarily interrupted, the electronic control circuit is configured to disconnect the power supply of the ion source when switching from emission mode to pause mode, maintain this power supply disconnected in pause mode, and restore the power supply of the ion source when switching from pause mode to emission mode, and the neutron generator comprises a regulation circuit configured to, in pause mode, determine the electrical resistance of the reservoir and regulate the electrical resistance of the reservoir by varying the current supplying the reservoir.

2. Neutron generator according to claim 1, further comprising a current measuring system supplying the reservoir and a system measuring the electrical voltage at the terminals of the reservoir, the regulation circuit determining the electrical resistance of the reservoir from the measured voltage and current.

3. Neutron generator according to claim 1 or 2, in which the set point for regulating the electrical resistance of the reservoir is calculated by the regulation circuit from the resistance measurements over a certain period of time prior to switching from emission to pause mode.

4. A neutron generator according to claim 3, wherein the set point for regulating the electrical resistance of the reservoir is calculated by the regulation circuit by averaging the resistance measurements over a certain period of time prior to switching from emission mode to pause mode, and adding a compensation value to the calculated average.

5. Neutron generator according to any one of claims 1 to 4 in which, in emission mode, the regulation circuit regulates the emission of neutrons by varying the current supplying the reservoir.

6. Neutron generator according to claim 5, in which the regulation circuit regulates the current of the target, representative of the neutron emission, by varying the current supplying the reservoir.

7. Neutron generator according to claim 6 in which, when switching from pause mode to emission mode, the regulation circuit regulates again the current of the target by varying the current supplying the reservoir.

8. Neutron generator according to any one of claims 1 to 6 in which, when switching from pause mode to emission mode, during a transient phase, the regulation circuit first regulates the current of the target, representative of the neutron emission, by varying the voltage of the ion source, and regulates the average voltage of the ion source over a sliding time window by varying the current supplying the reservoir, before regulating the current of the target by varying the current supplying the reservoir.

9. A material analysis device comprising a neutron generator according to any one of claims 1 to 8, and at least one gamma ray detector for measuring the energy of gamma photons generated by the interaction of emitted neutrons with the material to be analysed.

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