EP2483909B1 - Radiation detectors and autoradiographic imaging devices comprising such detectors - Google Patents

Description

The present invention relates to a radiation detector and an auto radiographic imaging device comprising such a detector.

The invention more particularly relates to a β radiation detector.

The interest of pharmacologists, physicians or biologists in the use of β-autoradiography is to obtain, in a relatively short time, a precise and well-quantified image of the distribution of radioactivity in an entire organism for biodistribution. or in an organ for receptor or in situ hybridization. The use of a wide range of β-emitting radioelements makes it possible today to perform autoradiographies of virtually all existing molecules or drugs. Biologists can also go back to the volume activity in three dimensions by superimposing the different images of sections obtained.

In the state of the art, film slides and phosphor screens are available for performing autoradiographic β studies.

These two techniques are said to blind exposure, that is to say that the image will be obtained after revelation of the support. This revelation is done either by a laser for phosphor screens or by a chemical bath for photographic films. These techniques have the disadvantage of not making it possible to carry out studies in real time.

• une enceinte contenant un mélange gazeux de Néon et d'isobutane pour générer des électrons sous l'effet de radiations,
• une cathode par laquelle pénètrent les radiations à détecter,
• une anode pour générer des signaux en fonction d'un courant généré par le déplacement de charges au voisinage de cette anode, ces charges correspondent à des électrons dont les radiations sont directement ou indirectement à l'origine,
• des moyens de polarisation générant un champ électrique adapté pour entraîner des électrons dans une direction allant de la cathode vers l'anode,
• une structure amplificatrice, située entre la cathode et l'anode, comprenant un espace d'amplification des électrons entre une électrode d'entrée et une électrode de sortie, les électrodes d'entrée et de sortie étant configurées pour que dans l'espace d'amplification règne un premier champ électrique adapté à ce que des électrons soient générés par avalanche dans l'espace d'amplification, l'espace d'amplification débouchant sur au moins une ouverture de l'électrode de sortie, pour laisser passer au moins une partie des électrons générés par avalanche,
• un espace de dérive situé entre l'électrode de sortie et l'anode, dans lequel règne un deuxième champ électrique adapté à une diffusion, dans des directions perpendiculaires à ce champ, des électrons par diffusion sur les atomes et molécules du milieu contenu dans l'enceinte.

Requirement FR 2,837,000 describes a β radiation detector for real-time study. This detector includes:

• an enclosure containing a gaseous mixture of neon and isobutane to generate electrons under the effect of radiation,
• a cathode through which the radiation to be detected penetrates,
• an anode for generating signals as a function of a current generated by the displacement of charges in the vicinity of this anode, these charges correspond to electrons whose radiation is directly or indirectly at the origin,
• polarization means generating an electric field adapted to drive electrons in a direction from the cathode to the anode,
• an amplifying structure, located between the cathode and the anode, comprising an electron amplification gap between an input electrode and an output electrode, the input and output electrodes being configured so that in the space of amplification reigns a first electric field adapted so that electrons are generated by avalanche in the amplification space, the amplification space opening on at least one opening of the output electrode, to let through at least one part of the electrons generated by avalanche,
• a drift space located between the output electrode and the anode, in which there is a second electric field suitable for diffusion, in directions perpendicular to this field, of electrons by diffusion on the atoms and molecules of the medium contained in the 'pregnant.

The detector described in FR 2,837,000 can detect low energy emitters such as tritium ³ H corresponding to a β radiation of about 18 keV with a good resolution. However, such a device has signals that are unsatisfactory for high energy transmitters.

There is therefore a need for a β radiation detector which does not have the drawbacks of the prior art, in particular which allows a satisfactory detection of the radiation of high energy emitters in real time.

We know from the document US-6,011,265 , a radiation detector which notably comprises an enclosure, an input electrode and an output electrode and wherein the distance between these two electrodes is between 25 μm and 500 μm. The document " Parallel ionization multiplier: A gaseous detector dedicated to the tracking of minimum ionization particles " by Beucher et al discloses a detector whose amplification space has a thickness of between 50 .mu.m and 125 .mu.m.

An object of the present invention is to provide a new radiation detector offering expanded viewing possibilities and an auto radiographic imaging device comprising such a detector.

• une enceinte contenant un milieu adapté pour générer des électrons sous l'effet de radiations,
• une cathode par laquelle pénètrent les radiations à détecter,
• une anode pour générer des signaux en fonction d'un courant généré par le déplacement de charges au voisinage de cette anode, ces charges correspondent à des électrons dont les radiations sont directement ou indirectement à l'origine,
• des moyens de polarisation générant un champ électrique adapté pour entraîner des électrons dans une direction allant de la cathode vers l'anode,
• une structure amplificatrice, située entre la cathode et l'anode, comprenant un espace d'amplification des électrons délimité entre une électrode d'entrée et une électrode de sortie, les électrodes d'entrée et de sortie étant configurées pour que dans l'espace d'amplification règne un premier champ électrique E1 adapté à ce que des électrons soient générés par avalanche dans l'espace d'amplification, l'espace d'amplification débouchant sur au moins une ouverture de l'électrode de sortie, pour laisser passer au moins une partie des électrons générés par avalanche,
• un espace de dérive D situé entre l'électrode de sortie et l'anode, dans lequel règne un deuxième champ électrique E2 adapté à une diffusion, dans des directions perpendiculaires à ce champ E2, des électrons par diffusion sur les atomes et molécules du milieu contenu dans l'enceinte,

où le milieu adapté pour générer des électrons sous l'effet de radiations comprend un gaz comprenant un mélange d'au moins 50% d'un gaz rare et de dioxyde de carbone, par exemple ininflammable, l'espace d'amplification est constitué d'au moins 90% en volume par ledit gaz, et que la distance (e) séparant les électrodes d'entrée (8) et de sortie (9) est supérieure ou égale à 500 µm et inférieure ou égale à 1,5 mm.The invention thus proposes a radiation detector comprising:

• an enclosure containing a medium adapted to generate electrons under the effect of radiation,
• a cathode through which the radiation to be detected penetrates,
• an anode for generating signals as a function of a current generated by the displacement of charges in the vicinity of this anode, these charges correspond to electrons whose radiation is directly or indirectly at the origin,
• polarization means generating an electric field adapted to drive electrons in a direction from the cathode to the anode,
• an amplifying structure, located between the cathode and the anode, comprising an electron amplification gap delimited between an input electrode and an output electrode, the input and output electrodes being configured so that in the space of amplification reigns a first electric field E1 adapted so that electrons are generated by avalanche in the amplification space, the amplification space opening on at least one opening of the output electrode, to let pass to least part of the electrons generated by avalanche,
• a drift space D located between the output electrode and the anode, in which there is a second electric field E2 adapted for diffusion, in directions perpendicular to this field E2, electrons by diffusion on the atoms and molecules of the medium contained in the enclosure,

wherein the medium adapted to generate electrons under the effect of radiation comprises a gas comprising a mixture of at least 50% of a rare gas and carbon dioxide, for example non-flammable, the amplification space consists of at least 90% by volume by said gas, and that the distance (e) separating the input (8) and output (9) electrodes is greater than or equal to 500 μm and less than or equal to 1.5 mm.

Advantageously, the use of a gaseous mixture of neon and carbon dioxide combined with an amplification space of at least 500 microns makes it possible to obtain a detector adapted to high energy emitters.

• l'électrode d'entrée de la structure amplificatrice correspond à la cathode ;
• l'électrode d'entrée est formée d'une face au moins partiellement conductrice d'un échantillon S émetteur de radiations ;
• le milieu adapté pour générer des électrons sous l'effet de radiations comprend un gaz comprenant un mélange Néon et de dioxyde de carbone, le Néon représentant au moins 85% et au plus 95% en volume du mélange ;
• la structure amplificatrice est configurée de sorte qu'entre l'électrode d'entrée et de sortie règne un champ électrique d'au moins 2,5 kV/cm ;
• une deuxième structure amplificatrice, située entre l'espace de dérive et l'anode, comprenant une électrode d'entrée et une seconde électrode, comportant au moins un espace d'amplification des électrons, l'électrode d'entrée et la seconde électrode étant configurées pour que des électrons soient générés par avalanche dans l'espace d'amplification, l'espace de dérive débouchant sur au moins une ouverture de l'électrode d'entrée, la deuxième structure amplificatrice étant configurée de sorte que son gain soit supérieur ou égal à 5000 ;
• la seconde électrode correspond à l'anode ;
• l'espace d'amplification situé entre l'électrode d'entrée et l'électrode de sortie est constitué d'au moins 99% en volume par ledit gaz, par exemple 99.97% ;
• les électrodes d'entrée (8) et de sortie (9) sont constituées de microgrilles ayant une résolution comprise entre 500 et 2000 lpi ; et
• les microgrilles ont un paramètre de maille compris entre 30 et 50 µm.

A detector according to the invention may further comprise one or more of the following optional features, considered individually or according to all possible combinations:

• the input electrode of the amplifying structure corresponds to the cathode;
• the input electrode is formed of an at least partially conductive face of a radiation emitting sample S;
• the medium adapted to generate electrons under the effect of radiation comprises a gas comprising a Neon mixture and carbon dioxide, the neon representing at least 85% and at most 95% by volume of the mixture;
• the amplifying structure is configured such that between the input and output electrode an electric field of at least 2.5 kV / cm prevails;
• a second amplifying structure, located between the drift space and the anode, comprising an input electrode and a second electrode, having at least one electron amplification gap, the input electrode and the second electrode being configured so that electrons are generated by avalanche in the amplification space, the drift space opening on at least one opening of the input electrode, the second amplifying structure being configured so that its gain is greater or equal to 5000;
• the second electrode corresponds to the anode;
• the amplification gap between the input electrode and the output electrode is at least 99% by volume by said gas, for example 99.97%;
• the input (8) and output (9) electrodes consist of microgrits having a resolution between 500 and 2000 lpi; and
• the microgrids have a mesh parameter of between 30 and 50 μm.

The invention also relates to a device self-radiographic imaging system comprising a detector and a sample holder, wherein the cathode is constituted by an at least partially conductive sample disposed on the sample holder.

• détermination des coordonnées d'un point A correspondant aux interactions moyennes des électrons dans l'espace de dérive,
• détermination des coordonnées d'un point B correspondant aux interactions moyennes des électrons dans l'espace d'amplification de la structure amplificatrice,
• détermination du point d'émission comme étant le point représentant l'intersection de la droite D passant par les points A et B et le plan d'altitude de référence.

The invention also relates to a method for determining the emission position of the electrons detected by the anode of a detector which comprises the following steps:

• determination of the coordinates of a point A corresponding to the average interactions of the electrons in the drift space,
• determination of the coordinates of a point B corresponding to the average interactions of the electrons in the amplification space of the amplifying structure,
• determining the point of emission as being the point representing the intersection of line D passing through points A and B and the reference altitude plane.

• la figure 1 est une coupe schématique perpendiculaire à ses faces principales, d'un premier mode de réalisation d'un détecteur selon l'invention ;
• la figure 2 est une coupe schématique, dans un plan analogue à celui de la figure 1, d'une partie de l'anode du détecteur représenté sur la figure 1 ;
• la figure 3 représente schématiquement en perspective les pavés constitutifs de l'anode représentée sur la figure 2 ;
• la figure 4 représente schématiquement une vue de dessus, l'agencement des pistes croisées de l'anode représentée sur les figures 2 et 3 ;
• la figure 5 représente schématiquement le mode de connexion des pavés aux pistes de l'anode représentée sur les figures 2, 3 et 4 ;
• la figure 6 représente schématiquement le détail du multiplexage des pistes d'une anode d'un détecteur selon un mode de réalisation de l'invention,
• la figure 7 illustre le principe de reconstruction de la position d'émission avec extrapolation de trajectoire,
• la figure 8 est une coupe schématique, perpendiculaire à ses faces principales, d'un mode de réalisation d'un détecteur selon l'invention.

The invention will be better understood on reading the description which follows, given solely by way of example and with reference to the appended drawings in which:

• the figure 1 is a schematic section perpendicular to its main faces, a first embodiment of a detector according to the invention;
• the figure 2 is a schematic section, in a plane similar to that of the figure 1 of a part of the anode of the detector represented on the figure 1 ;
• the figure 3 schematically represents in perspective the constituent blocks of the anode shown in FIG. figure 2 ;
• the figure 4 schematically represents a view of above, the arrangement of the crossed tracks of the anode shown on the Figures 2 and 3 ;
• the figure 5 schematically represents the connection mode of the blocks to the tracks of the anode shown on the Figures 2, 3 and 4 ;
• the figure 6 schematically represents the detail of the multiplexing of the tracks of an anode of a detector according to one embodiment of the invention,
• the figure 7 illustrates the principle of reconstruction of the emission position with trajectory extrapolation,
• the figure 8 is a schematic section, perpendicular to its main faces, of an embodiment of a detector according to the invention.

For the sake of clarity, the different elements shown in the figures are not necessarily to scale.

The term "high energy emitters" in the sense of the invention means a β radiation emitter whose average energy is greater than or equal to 100 keV.

According to the first embodiment, shown in the figure 1 , the detector 1 comprises a flattened enclosure 2 with two main faces 2a and 2b opposite and parallel to each other. This chamber 2 contains a medium adapted to emit primary electrons under the effect of ionizing radiation emitted by a sample S disposed near one of the main faces 2a of the enclosure 2. Advantageously, the medium consists of a gas mixture circulating in the chamber 2 between an inlet 3 and an outlet 4.

This gaseous mixture comprises at least 50% of a rare gas and at least 5 to 15% of carbon dioxide. The carbon dioxide molecules are intended for control the avalanche amplification process.

According to one embodiment of the invention, the gaseous mixture is chosen so as to be non-flammable. Advantageously, the manipulation of the detector according to the invention is simplified.

In the particular case of the detection of β particles, this gaseous mixture is advantageously at a pressure of between 0.5 and 2 bar, for example between 0.9 and 1.1 bar, and comprises a rare gas whose average electron density is close to 10 electrons per atom, such as neon.

The chamber 2 encloses a cathode 5, an anode 6 and an amplifying structure 7.

In the embodiment shown at figure 1 , the cathode 5, the anode 6 and the amplifying structure 7 are parallel to each other and parallel to the two main faces 2a, 2b of the enclosure 2.

The anode 6 is located near the face 2b of the chamber 2 opposite that 2a near which is the sample S.

The amplifying structure 7 is situated between the cathode 5 and the anode 6. The space of the enclosure 2 situated between the cathode 5 and the amplifying structure 6 constitutes a conversion space C. The ionizing radiations emitted by the sample S enter the conversion space C through the cathode 5.

The space of the chamber 2 located between the amplifying structure 7 and the anode 6 constitutes a diffusion space D.

The amplifying structure 7 comprises an input electrode 8 and an output electrode 9 substantially parallel to the cathode 5 and the anode 6 and delimiting an amplification space A.

According to one embodiment of the invention, the amplification gap between the input electrode and the output electrode is at least 99% by volume by said gas, for example 99.97%.

The document US 6,011,265 has a hole counter. This type of microstructure suffers from an intrinsic limitation arising from the limited density of amplification channels. Thus, these types of microstructures have an amplification stage consisting mainly of insulating and solid materials for delimiting the holes in which the avalanches are confined.

Such radiation detectors can in no way be used to generate electrons by another principle than that of avalanches. In doing so, they can not be used for example to generate the electrons resulting from the interaction of the ionizing particle with the gas as according to the present invention.

Polarization means 10 are connected to the cathode 5, to the anode 6 and to the input and output electrodes 8 and 8. They enable the cathode 5 to be carried at a potential V1, the anode 6 at a potential V2, the input electrode 8 at a potential V3 and the output electrode 9 at a potential V4.

According to one embodiment of the invention, these potentials satisfy V2> V4> V3> V1.

In one embodiment of the invention, the input and output electrodes 8 and 8 are spaced apart by a distance greater than or equal to 500 μm and less than or equal to 1.5 mm.

The diffusion space D has a dimension perpendicular to the input and output electrodes 8 of between 2 mm and 3 cm, for example equal to 2 cm.

According to one embodiment of the invention, the anode 6 is connected to ground. The cathode 5, the input electrode 8 and the output electrode 9 are brought to negative potentials.

The polarization means 10 thus make it possible to create electric fields E1, E2, E3 respectively in the conversion space C in the amplification space A and in the diffusion space D. The polarization means 10 drive the electrons from the cathode 5 to the anode 6.

According to one embodiment, the input electrode 8 of the amplifying structure 7 coincides with the cathode 5. The input electrode 8 is formed of an at least partially conductive face of the sample S.

According to one embodiment, the cathode 5 may consist of an electrically conductive thin plate with a thickness substantially equal to 5 microns.

According to one embodiment of the invention, the cathode 5 consists of a conductive adhesive, for example a copper adhesive, bonded to one side of a microscope glass slide. Sample S being disposed on the opposite side of the microscope slide.

The input electrodes 8 and output 9 may consist of microgrids MICROMEGAS type.

Advantageously, the microgrids have a thickness less than or equal to 10 microns, for example equal to 5 microns allowing very fast electric field transitions.

There are two main types of microgrids: electroformed microgrids and chemically etched microgrits.

Electroformed microgrids are originally used as high precision filters but their good characteristics, in particular their thin, regular and thin patterns, make them very attractive candidates for use in detectors according to the invention. They are usually in nickel with a thickness of about 5 microns and have square holes of 39 microns apart separated by bars of 11 microns which give them a pitch of 51 microns for a grid of 500 lpi (Line per Inch). However, there are all kinds of geometries and sizes ranging from 100 to 2000 lpi with sizes ranging from 7 to 11 inches. The electroforming method, however, does not allow precise control of the thickness of the microgrid, in particular on the edges where it can vary from single to double. However, for the center of the microgrid, the thickness is well controlled.

The microgrits etched by a chemical process have a surface of 25x25 cm ² and a thickness of 5 μm, they have circular holes 30 μm in diameter arranged in equilateral triangular mesh at a pitch of 60 μm. The manufacturing process allows direct incorporation of the insulating spacer to the grid in the form of Kapton ™ pads of 80 μm in diameter arranged every 3 mm. Due to their small surface area, they make it possible to drastically reduce the dead zone between the grid and the support to approximately 0.05% of the surface of the grid. The advantage of this chemical etching process is the control of the thickness of the grid over its entire surface unlike the electroformed grids.

According to one embodiment, the input and output electrodes 8 and 8 respectively consist of an electrically conductive thin plate, of small thickness and pierced with small openings. For example, the apertures have a square shape of 35 μm spaced from each other with a pitch of 50 microns which corresponds substantially to an opening number per linear inch of 500 lpi. It is also possible to use the input and output electrodes 8 of 2500 lpi, which corresponds substantially to 8 μm openings spaced 10 μm apart. Such input electrodes 8 and output 9 each form a gate, which given the small size of the openings, can be called "microgrid". Such microgrids have been described for example in the document EP855086 .

The distance e separating the input electrode 8 and output electrode 9 is greater than or equal to 500 μm and less than or equal to 1.5 mm.

Indeed, the constraints associated with high energy electrons are different from those associated with low energy electrons. High energy electrons travel much larger distances in the gas. Their practical path R _p is relatively important since it is of the order of 20 cm. An electron of 300 keV will travel nearly a meter in gas before being shut down.

The inventors measured the altitude of the first interaction and the energy deposited during the first interaction with the gas for ⁴⁶ SC (E _max = 356 keV) and ³² P (E _max = 1710 keV). The emission is supposed to be isotropic in space. The first interaction generates an average energy deposit of 49.83 eV for phosphorus and 61.93 eV for scandium at a respective altitude of 589 and 157 μm. These interactions take place at significant distances from the emission point which results in the creation of few electron-ion pairs in the amplification space 7 in contact with the source S.

To gain efficiency and statistics of pair creation, the inventors propose using a relatively large amplification space.

The energy loss of an electron of 100 keV crossing 1 cm of gas is 3.5 keV and that of a 300 keV electron is 1.9 keV. This results in the respective creation of 97 and 52 electron-ion pairs along their path. The creation of primary ionization charges along the trajectory can then be used to characterize the trace of the electron in the detector by a trajectory tracking method according to the invention.

The potentials V3 and V4 of the input and output electrodes 8 and 9 are chosen so that in the amplification space A an electric field E2 is present, adapted to the fact that electrons are generated by avalanche in the space d amplification A.

The phenomenon of electronic avalanche occurs when electrons arrive in an area of strong electric field typically of a few tens of kV / cm. The energy they will acquire between two collisions will become important enough that they in turn produce ionizing interactions. The ionization electrons thus created will then be accelerated and give rise to other ionizations.

où :

• n : nombre d'électrons à une position donnée,
• α_T : premier coefficient de Townsend défini comme étant la probabilité de faire une interaction ionisante par unité de longueur : α_T = 1/λ,
• λ : libre parcours moyen (distance moyenne que doit parcourir un électron avant de faire une interaction ionisante).

En intégrant sur la distance x parcourue, on obtient : $$n=n_0{e}^{{\alpha }_{T}x}$$

où :

• n ₀ : nombre initial d'électrons créés par l'ionisation,
• x : distance parcourue.

Le gain G du détecteur est alors défini comme le rapport entre le nombre d'électrons présents après une longueur x sur le nombre d'électrons initial. Ce qui se traduit par : $$G=\frac{n}{n_0}=e^{{\alpha }_{T}x}$$

Pour un mélange gazeux donné α_T dépend uniquement de la valeur du champ électrique et de la pression du gaz. Il est nécessaire que α_T soit alors plus grand que le coefficient d'attachement électronique η_e pour constater l'amplification. α_T et η_e représentent respectivement le nombre de paires électron-ion créées et le nombre d'électrons recapturés par unité de longueur. Les inventeurs ont observé que le champ électrique doit être supérieur à 2,5 kV/cm dans un mélange gazeux de type Néon+10% CO₂ pour amorcer le phénomène d'amplification.In the case of microgrids of the MICROMEGAS type, it is possible to consider that the field lines are parallel. Therefore, the increase in the number of electrons after a dx path results in: $$d\mathrm{not}=\mathrm{not}{\alpha }_{T}dx$$

or :

• n : number of electrons at a given position,
• α _T : Townsend's first coefficient defined as the probability of doing an ionizing interaction per unit length: α _T = 1 / λ,
• λ : average free path (average distance traveled by an electron before making an ionizing interaction).

By integrating on the x distance traveled, we obtain: $$\mathrm{not}={\mathrm{not}}_0{e}^{{\alpha }_{T}x}$$

or :

• n ₀ : initial number of electrons created by the ionization,
• x : distance traveled.

The gain G of the detector is then defined as the ratio between the number of electrons present after a length x on the initial number of electrons. Which results in: $$\mathrm{BOY\; WUT}=\frac{\mathrm{not}}{{\mathrm{not}}_0}=e^{{\alpha }_{T}x}$$

For a given gas mixture α _T depends only on the value of the electric field and the pressure of the gas. It is necessary that α _T is then greater than the electronic attachment coefficient η _e to observe the amplification. α _T and η _e represent respectively the number of electron-ion pairs created and the number of electrons recaptured per unit length. The inventors have observed that the electric field must be greater than 2.5 kV / cm in a Neon-type gas mixture + 10% CO ₂ to initiate the amplification phenomenon.

As shown on the figure 2 , the anode 6 has a planar multilayer structure. It comprises an outer layer 15 and two inner layers 16, and a ground plane 17, all resting on an insulating substrate 28.

As shown on the figure 3 the outer layer 16 is segmented into elementary anodes or blocks 15 forming a two-dimensional checkerboard network whose rows are aligned along X and Y coordinate axes. Each block 15 forms a square of less than one millimeter on a side, for example of 650 μm. The blocks 15 are alternately assigned to reading one or the other X and Y coordinates. Two neighboring tiles do not measure the position according to the same coordinate. The space between the pavers 15 is as small as possible, but must allow to maintain a perfect insulation between them. Advantageously, this space is less than or equal to 100 μm.

As shown on the figure 4 the inner layers of the anode 6 are formed of cross-conducting tracks 18. On one of the inner layers 16, the tracks 18 extend parallel to the first rows of blocks 15. On the other of the inner layers 16, the tracks 18 extend parallel to second rows of blocks 15, perpendicular to the first. According to this example, the tiles 15 of a row associated with the X coordinate are located on an inner layer different from that connected to the pavers arranged on a row corresponding to the Y coordinate. The tracks 18 are separated from the pavers 15 by an insulator. through which are drilled connecting holes 19 (known to those skilled in the art under the term "Via Hole"), lined with an electrically conductive material to ensure the electrical connection of the pavers 15 with the tracks 18 of one or the other of the inner layers 16 (see figure 2 ). The connecting holes 19 have for example a diameter of 100 microns.

The tracks 18 are separated from each other by as little distance as possible while maintaining perfect insulation between them. Laying the tracks in overlapping layers isolated from each other allows to gain integration while maintaining the required quality of insulation.

The blocks 15, thanks to the tracks 18, are connected to fast amplifiers 20 themselves connected, via electronic reading channels, to electronic processing means 21 (see FIG. figure 4 ).

To limit the number of electronic reading channels, and therefore the cost of the detector 1, several tiles belonging to the same row can be connected to the same track 18. The number of tiles 15 separating two blocks connected to each other depends on their size and the technology used to achieve them.

For example, as shown on the figure 5 each track 18 periodically connects, in a row, one of four pavers. As two neighboring blocks are respectively connected to tracks 18 extending along the X and Y axes, a track X1 connects two blocks spaced from three blocks, these three blocks comprising two adjacent blocks of the two blocks connected to the track X1, themselves. they are respectively connected to the tracks Y1 and Y7, separated by a block connected to a track X2, this arrangement being reproduced on the whole of the checkerboard consisting of the blocks 15 (on the figure 5 two blocks 15 connected to each other are represented by identical patterns).

According to one embodiment, the anode 9 can be divided into 32 400 elementary pixels of 170 × 170 μm ² made by laser cutting in a copper plane. This technology makes it possible to reduce the interpolation insulating distance to 30 μm unlike chemical etching whose minimum insulating distance is 75 μm. The pitch of the pixels is thus 200 μm in the two directions X and Y. The reading tracks to which the pixels are connected are in two different planes. The floor has 128 tracks in X and 128 tracks in Y. They are placed diagonally with respect to the pixels as the figure 6 and geometrically multiplexed like a chessboard. Every second pixel is connected to one X track and the other one to a Y track. For a given playback direction, 64 tracks are played on one side and the other 64 are played on the other side. Each pixel is connected to its track by a metallized hole made by laser drilling. With the placement of the tracks diagonally to the pixels, the playback pitch of the tracks is thus 282.84 microns. This pixel pitch is one of the best granularities realized to date in view of the surface for this type of gas detector.

The flatness of the anode is ensured by gluing on an 8 mm thick aluminum reference plane.

With reference to the figure 1 when an ionizing particle I is emitted by the sample S and it enters a detector 1 by the face 2a thereof opposite to that adjacent to the anode 6, it passes through the space of C conversion in which it interacts with the gas mixture and generates primary electrons. These primary electrons, under the effect of the electric field E1, gain the amplification space A in which they are multiplied by the electronic avalanche phenomenon to form an electron cloud 23.

Part of this cloud of electrons 23 then passes through the output electrode 9 and enters the diffusion space D. The electric field E3 prevailing in the diffusion space D is moderate, for example less than or equal to 10 kV / cm and conducive to a lateral spread of the electron cloud 23 by diffusion of the electrons which constitutes it on the atoms and molecules of the gas.

The tracking of a particle β consists, starting from the path of the electron in the gas, to go up by a geometric reconstruction at the emission point in the sample S. This tracking method assumes that the trajectory of the electrons of high energy is made in a straight line without significant angle deviation and that the coordinates of the line extrapolating the path of the electron can be determined from two points characteristic of the measured trajectory. The first point lying in the amplification space A and the second in the diffusion space D.

As explained above, the energy loss of an electron of 100 keV crossing 1 cm of gas is about 3.5 keV. This loss of energy is negligible compared to the incident energy of the β particle. Therefore, it can be considered that the particle will not be or very little deflected through 1 cm of gas and that its trajectory will be a straight line.

As explained previously, an electron that arrives on the anode will have crossed the amplification space A and the diffusion space D. The two different spaces crossed by the electron make it possible to go up to the two characteristic points of the straight line constituting the trajectory of the electron.

The first point corresponding to the mean point of the interactions in the amplification space Λ, is determined from the interactions of the electron in the amplification space A. The low energy loss associated with the small thickness of gas traversed makes it possible to determine the average position in this space using a so-called barycenter method.

The second point of the line extrapolating the path of the electron is determined from the interactions of the electron in the diffusion space D. The fact that the particle deposits its energy almost continuously in the medium it through, allows to consider that the average altitude of the interactions is in the middle of the height of the space of diffusion.

This then makes it possible to determine the second characteristic point of the line.

The figure 7 illustrates the principle of reconstruction of emission by the method of extrapolation of the trajectory. The curve TR represented on the figure 7 represents the real trajectory of an electron projected in the X direction, and the line TC the line representing the calculated trajectory of the projected electron in the X direction.

The average of the spatial distribution of the charge in the diffusion space D gives us the point A corresponding to the average altitude of the interactions in this diffusion space D. The same method applied for the amplification space A leads us in point B. The geometric extrapolation reconstruction method then consists of assimilating the trajectory of the β particle to a line TD passing through these two points. The extrapolated emission position is then given as the point representing the intersection of this line with the line corresponding to a zero altitude. In the same way it is possible to evaluate the emission position in the Y direction.

et $$Y_{em}=Y_{mA}-Z_{cA}\frac{Y_{mA}-Y_{mD}}{Z_{cA}-Z_{cD}}$$

• avec (X_em, Y_em, 0) les coordonnées du point d'émission, et (X_mA, Y_mA, Z_cA) les coordonnées du point correspondant à la moyenne de la distribution spatiale de charge dans l'espace d'amplification A,
• (X_mD, Y_mD, Z_cD) les coordonnées du point correspondant à la moyenne de la distribution spatiale de charge dans l'espace de diffusion D,

avec dans le cas du ⁴⁶Sc, d'un espace d'amplification de 50 µm et d'un espace de dérive de 1 cm :

• Z_cA -123µm,
• Z_cD =5 500µm

We then get: $$X_{em}=X_{m\mathrm{AT}}-Z_{\mathrm{vs}\mathrm{AT}}\frac{X_{m\mathrm{AT}}-X_{mD}}{Z_{\mathrm{vs}\mathrm{AT}}-Z_{\mathrm{vs}D}}$$

and $$Y_{em}=Y_{m\mathrm{AT}}-Z_{\mathrm{vs}\mathrm{AT}}\frac{Y_{m\mathrm{AT}}-Y_{mD}}{Z_{\mathrm{vs}\mathrm{AT}}-Z_{\mathrm{vs}D}}$$

• with (X _em , Y _em , 0) the coordinates of the point of emission, and (X _mA , Y _mA , Z _cA ) the coordinates of the point corresponding to the mean of the spatial distribution of charge in the amplification space AT,
• (X _MD , Y _MD , Z _CD ) the coordinates of the point corresponding to the mean of the spatial distribution of charge in the diffusion space D,

with in the case of ⁴⁶ Sc, an amplification space of 50 μm and a drift space of 1 cm:

• Z _cA -123 μm ,
• Z _cD = 5,500 μm

These coefficients are defined according to the emitter in particular for the component of the amplification space, and the geometry.

According to an embodiment shown in figure 8 , the detector further comprises a second amplifying structure 30 located between the diffusion space D and the anode 6. This second amplifying structure 30 comprising an input electrode 31 and an output electrode 32, for example merged with the anode 6, comprising at least one A2 amplification gap of the electrons. The input electrode 31 and the output electrode 32 are configured so that the electrons are generated by avalanche in the amplification space A2, the diffusion space D opening on at least one opening of the electrode. input 31, the second amplifier structure 30 being configured such that its gain is greater than or equal to 5000.

The distance e2 separating the input and output electrodes 32 from the second amplifying structure 30 is greater than or equal to 100 μm and less than or equal to 200 μm, for example equal to 125 μm.

$$gai{n}_{\min }=\frac{nombre\mathrm{}de\mathrm{}pistes}{nombre\mathrm{}de\mathrm{}paires}\times 2\times le\mathrm{}seuil$$

The signal from the primary ionization in the diffusion space must be sufficiently amplified to be extracted from the electronic noise. The inventors have observed that it is advantageous to have a gain of the second amplifying structure at least equal to 4,400, preferably at least 5,000, in order to be able to extract the signal due to the ionizations in the diffusion space, whatever it may be. the direction of the incident particle and whatever the projection plane considered for an electron of 100 keV. In the same way, the inventors have observed that in the case of a particle β , leaving with an energy of 300 keV, the minimum gain to be applied in the second amplifying structure in order not to favor the directions relative to the others is 8,000, preferably 9,500. By way of example, the inventors have considered an electron of 100 keV crossing the diffusion space with an angle of 45 ° in the X direction. Assuming its straight trajectory, the electron then passes 5.7 mm along the X reading axis, which corresponds to a loss of energy of 2 keV. This creates 55 pairs of electro-ions. Assuming a floor reading pitch of 282.84 micrometers, the signal then spreads on 20 tracks assuming perfect transparency of the microgrids, the load collected per track and the minimum gain to be applied in this case are given by: $$q_{piste}\left(e^{-}\right)=\mathrm{boy\; Wut}\mathrm{at}i\mathrm{not}\times \frac{\mathrm{not}ombre\mathrm{}dep\mathrm{at}ires}{\mathrm{not}ombre\mathrm{}de\mathrm{}pistes}$$

$$\mathrm{boy\; Wut}\mathrm{at}i{\mathrm{not}}_{\min }=\frac{\mathrm{not}ombre\mathrm{}de\mathrm{}pistes}{\mathrm{not}ombre\mathrm{}de\mathrm{}p\mathrm{at}ires}\times 2\times le\mathrm{}seuil$$

The factor 2 comes from the fact that the electronic cloud spreads on a multiplexed pixel floor. Thus, half of the load is collected by the X reading tracks and the other half by the Y reading tracks.

For a threshold per track of 6,000 electrons, this results in a gain of at least 4,400 in the amplification space in contact with the anode to extract the signal from the electronic noise. In the case of an electron of 300 keV the minimum gain will be 8000 and in according to an electron of 1 MeV, the minimum gain will be 32 000.

The invention is not limited to the embodiments described and will not be interpreted in a limiting manner, and encompasses any equivalent embodiment. In particular a detector according to the invention may comprise more than two amplifying structures.

Claims (11)

1. Radiation detector comprising:

   - a chamber (2) containing an environment adapted for generating electrons under the effect of radiations,

   - a cathode (5) by which radiations to be detected enter,

   - an anode (6) for generating signals according to a current generated by the movement of charges near this anode (6), these charges correspond to electrons from which the radiations directly or indirectly originate,

   - polarisation means (10) generating an electrical field adapted for leading electrons in a direction going from the cathode (5) to the anode (6),

   - an amplifying structure (7), situated between the cathode (5) and the anode (6), comprising a space for amplifying (A) electrons, delimited between an input electrode (8) and an output electrode (9), the input electrodes (8) and output electrodes (9) being configured such that in the amplification space, a first electrical field (E2) prevails, adapted such that electrons are generated by flooding into the amplification space, the amplification space leading to at least one opening (12) of the output electrode (9), to let at least some of the electrons generated by flooding pass,

   - a diffusion space (D) situated between the output electrode (9) and the anode (6), wherein a second electrical field (E3) prevails, adapted to a diffusion, in directions perpendicular to this field (E3), of electrons by diffusion on atoms and molecules of the environment contained in the chamber (2),

   the environment adapted for generating electrons under the effect of radiations comprising a gas, comprising a mixture of at least 50% of a rare gas and of carbon dioxide, the amplification space being constituted of at least 90% by volume by said gas, characterised in that the distance (e) separating the input electrodes (8) and output electrodes (9) is greater than 500µm and less than or equal to 1.5mm.
2. Detector according to claim 1, wherein the input electrode of the amplifying structure (7) corresponds to the cathode (5).
3. Detector according to one of claims 1 or 2, wherein the input electrode (8) is formed of a face, at least partially conductive, of a radiation-emitting sample (S).
4. Detector according to one of the preceding claims, wherein the environment adapted for generating electrons under the effect of radiations comprises a gas, comprising a Neon and carbon dioxide mixture, the Neon representing at least 85% and at the most, 95% by volume of the mixture.
5. Detector according to one of the preceding claims, wherein the amplifying structure is configured such that between the input electrode and output electrode, an electrical field of at least 2.5kV/cm prevails.
6. Detector according to one of the preceding claims, further comprising a second amplifying structure (30), situated between the diffusion space (D) and the anode (6), comprising a second input electrode (31) and a second output electrode (32), comprising at least one space for amplifying (A2) electrons, the second input electrode (31) and the second output electrode (32) being configured such that electrons are generated by flooding in the amplification space (A2), the diffusion space (D) leading to at least one opening of the input electrode (31), the second amplifying structure being configured such that the gain thereof is greater than or equal to 5000.
7. Detector according to claim 6, wherein the second output electrode (32) corresponds to the anode (6).
8. Detector according to one of the preceding claims, wherein the input electrodes (8) and output electrodes (9) are constituted of microgrids having a resolution of between 500 and 2000 lpi (between 19.7 lines per mm and 78.7 lines per mm).
9. Detector according to claim 8, wherein the microgrids have a mesh parameter of between 30 and 50µm.
10. Autoradiographic imaging device comprising a detector according to one of the preceding claims and a sample holder, wherein the cathode is constituted by a sample, at least partially conductive, disposed on the sample holder.
11. Method for determining the position for emitting electrons detected by the anode of a detector according to any one of claims 1 to 9, comprising the following steps:

    - determining the coordinates of a point A corresponding to the average interactions of the electrons in the diffusion space (D),

    - determining the coordinates of a point B corresponding to the average interactions of the electrons in the amplification space (A) of the amplifying structure (7),

    - determining the emission point as being the point representing the intersection of the straight line (TC) passing through the points A and B and the reference altitude plane.

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