EP0732705A1 - Particle microcollimation device, particle detector and detection method, manufacturing method and use of said microcollimation device - Google Patents

Abstract

A device for micro-collimation of incident particles consists of a random arrangement of micro-holes in an insulating sheet of thickness between a few microns and several millimetres. Also claimed are (i) a particle detector including the above device; (ii) a particle detection method employing the above device; and (iii) a method of fabricating the above device. Pref. the sheet consists of a material in which latent tracks can be created by heavy ion bombardment, pref. a plastics such as polycarbonate, kapton or polyimide, or consists of cleaved mica and the hole density is <10<8>/cm<2>.

Description

Technical area

The present invention relates to a particle microcollimation device, a detector and a method for detecting particles, a method of manufacturing and using said collimation device.

State of the art

Neutrons are neutral particles. They cannot be detected directly with conventional detectors because they operate by collection of charges created during the passage of the particle to be detected. The detection of neutrons requires a converter which will sign the presence of a neutron by the formation of one or more charged particles. In detectors operating on the principle of a charge collection, it is these charged particles which will make it possible to detect the presence of a neutron.

The present invention relates to pulse-by-pulse detection of thermal neutrons using semiconductor or gas detectors. The detection of thermal neutrons is an important problem, particularly for monitoring the operation of nuclear reactors. This pulse-by-pulse detection results in difficulties related to energy losses in the converter and the angle of arrival of charged particles in the detector.

The conversion of a thermal neutron into charged particles can be done by several nuclear reactions having a large cross section. In the following description, the most used reactions will be cited, but the invention relates to any nuclear reaction creating charged particles from, for example, a thermal neutron or the like: $${}^{\text{10}}\mathit{\text{B}}\text{+}\mathit{\text{not}}{\text{→}}^{\text{4}}\mathit{\text{Hey}}{\text{+}}^{\text{7}}\mathit{\text{Li}}\text{+ 2310 keV}$$

The cross section of this reaction for thermal neutrons is 3900 barns. $${}^{\mathit{\text{3}}}\mathit{\text{Hey}}\text{+}\mathit{\text{n →}}{}^{\text{1}}\mathit{\text{H}}{\text{+}}^{\text{3}}\mathit{\text{H}}\text{+ 764 keV}$$

The cross section of this reaction for thermal neutrons is high: 5400 barns. Since helium is a gas, the converter must be confined between two thin sheets supported by wires if the pressure is high. Helium must be enriched in ³ He, because the proportion of this isotope in the natural isotopic composition is only one per thousand. $${}^{\text{235}}\mathit{\text{U}}\text{+}{\mathit{\text{n → F}}}_{\text{1}}\text{+}{\mathit{\text{F}}}_{\text{2}}\text{+}\mathit{\text{xn}}\text{+ ≈194 MeV}$$

The cross section with respect to thermal neutrons is smaller (580 barns) but the energy released is very large and the fragments are heavy. This means that they can be easily stopped in 10 to 20 µm of plastic. It may be recalled that natural uranium contains only 0.7% of ²³⁵ U.

In the following description, we consider the first reaction ( ¹⁰ B + n → ⁴ he + ⁷ Li) to illustrate the invention, but the invention of course applies to all other reactions, not explicitly indicated here.

The device, shown diagrammatically in FIG. 1, is a semiconductor detector 10, in crystalline silicon or in amorphous silicon for example, on which a thin layer of boron ¹⁰ B has been deposited (converter 11). The large cross section of thermal neutron capture by boron ¹⁰ B makes it possible to convert a neutron flux into two charged fragments: a ⁴ He of 1.47 MeV and a ⁷ Li of 0.84 MeV emitted at 180 ° on the other (fragment F1 and fragment F2 in the figure). The path of ⁴ He (helium) and ⁷ Li (lithium) in ¹⁰ B does not exceed 3.6 µm. Consequently, there is no point in increasing the thickness of the layer beyond 3.6 μm because the fragments can no longer arrive in the detector and remain in the boron deposit.

The capture of a thermal neutron is a random process governed by a large cross section. The two fragments F1 and F2 are emitted at 180 ° from each other, which means that only one of them is emitted in the half-space containing the semiconductor detector. Therefore, at best, the detector can detect only one of the two fragments emitted. The angular distribution of emission of the two fragments is isotropic in the reference frame of the center of mass of the system made up of ¹⁰ B and the neutron. Given the low kinetic energy of the thermal neutron (1/40 eV) this frame of reference coincides with that of the laboratory and this is the reason for which the two fragments are emitted 180 ° from each other. The angle of emission of the fragment arriving in the detector can be arbitrary (from 0 to 180 °, where 90 ° corresponds to a normal incidence on the detector). The position of the emission of the fragment in the converter can also be arbitrary. This is schematically shown in Figure 2.

In the case of operation in pulses, a thermal neutron gives, in the semiconductor detector, a signal whose amplitude varies from a very small value (emission of the fragment close to 0 or 180 °) up to a maximum value corresponding to a 90 ° emission close to the input face of the detector. This variation of the pulse height is continuous and it is difficult, for low values, to separate the signals due to the neutrons from those due to the background noise of the detector. This can also be important if this detector consists of a thin layer such as amorphous silicon, for example.

To illustrate quantitatively what has just been said on the angle of emission of the fragment emitted in the half space (we neglect to do this the problems of energy loss), we show in Figure 3 the proportion of the fragments emitted with an angle θ relative to the vertical to the detector (θ = 0 corresponds to an emission perpendicular to the entry face of the detector, while θ = 90 ° corresponds to an emission parallel to this entry face). It can be seen in this figure that few fragments emitted in the converter give a sufficient signal in the detector. On the other hand, the resulting energy spectrum varies from 0 to a maximum value defined above. If we take into account the energy loss in the converter, this effect is amplified and the spectrum observed in the schematic form indicated in FIG. 4. Any quantitative measurement is therefore greatly hampered by the effects described above. In particular, for the low energy part, it is difficult to separate the contribution to the spectrum coming from fragments of low energy from that coming from the background noise of the detector or of the electronics. When operating with current, ie for high neutron fluxes, this effect can on average be taken into account after careful calibration of the detector. In this case, we can measure an average neutron flux. For pulse operation this is no longer possible. Indeed, as shown in Figure 4, the counting rate (dn / dE) decreases sharply and continuously when the kinetic energy of the detected product increases. An electronic threshold then leads to a strong error because it depends on external conditions: a small variation of the threshold leads to a large variation of the counting rate. On the other hand, it is difficult to envisage a separation of the signals by an advanced method of signal processing because they are all of the same type.

The object of the present invention is to overcome these various drawbacks.

Statement of the invention

The invention relates to a device for microcollimation of incident particles, consisting of a set of micro-holes, of size on the order of a micrometer, randomly drilled, but oriented in parallel, in an insulating sheet of thickness between a few micrometers and several millimeters.

Advantageously, the insulating sheet is made of plastic, for example polycarbonate, kapton, or polyimide. It can also be made of cleaved mica. More generally, it can be made of a material in which latent traces can be created by bombardment of heavy ions. The density of the holes is less than 10 ⁸ / cm ² .

• un convertisseur de particules permettant de générer des particules chargées ;
• un ensemble de microcollimateurs ayant chacun une taille de l'ordre du micromètre percés aléatoirement, mais orientés, dans une feuille isolante d'épaisseur comprise entre quelques micromètres et plusieurs millimètres ;
• un détecteur de particules chargées.

The invention also relates to a particle detector comprising:

• a particle converter for generating charged particles;
• a set of microcollimators each having a size of the order of a micrometer pierced randomly, but oriented, in an insulating sheet of thickness between a few micrometers and several millimeters;
• a charged particle detector.

Advantageously, the effective capture or conversion section in the converter is much greater than that of the sheet. The converter comprises, in the illustrative example, a layer of boron. The charged particle detector is a crystalline, polycrystalline or amorphous semiconductor, or a gas detector. The particles can be thermal neutrons, neutrons or photons.

The invention also relates to a particle detection method which consists in placing the device in a particle detector, between a layer for converting the particle into electrically charged fragments and a charged particle detector. The particles to be detected can be thermal neutrons, neutrons or photons. The invention can also be used for other neutral particles, atoms or aggregates for example. This method, in a pulse-by-pulse counting mode, consists of the implementation of the aforementioned microcollimation device, without processing the signals collected in the charged particle detector.

The proposed invention can also be used to detect other particles if these are emitted in a large solid angle of space. For this, their kinetic energy must be such that they can be stopped by the microcollimation assembly if they do not pass through one of the holes. In this sense, the device of the invention acts as a directional filter: it lets through only the particles which arrive almost perpendicular to the surface of the device. This filtering is also accompanied by a significant reduction in the counting rate since only a small proportion of particles are "filtered". In this sense, this device can also serve as a counting rate attenuator.

The invention also relates to a method of manufacturing a microcollimation device which comprises a step of bombarding a plastic sheet with a beam of heavy ions. Advantageously, the heavy ions are projectiles having at least the mass of krypton. The particle flux is approximately 5 × 10 ⁷ particles / cm ² . In a variant, this manufacturing process comprises a manufacturing step by lithographic technique.

Advantageously, collective production is carried out (by bombardment of heavy ions or by lithography) of a set of microcollimators making it possible to collimate particles whether or not they are charged (ions, atoms, etc.).

Brief description of the drawings

• Figure 1 illustrates a semiconductor detector of the prior art;
• FIG. 2 illustrates the position of the emission of a fragment in the converter illustrated in FIG. 1;
• FIG. 3 illustrates the proportion of fragments emitted with an angle θ relative to the vertical to the detector of FIG. 1;
• FIG. 4 schematically illustrates the spectrum observed with the detector illustrated in FIG. 1;
• FIG. 5 illustrates an exploded view of a detector according to the invention.

Detailed description of embodiments

• . les microtrous permettent de collimater des particules incidentes. Ne passent à travers des trous que les particules qui sont émises presque perpendiculairement au détecteur. La profondeur du trou permet de faire varier, dans une certaine gamme, cette ouverture angulaire ;
• . le second rôle de la feuille percée de trous est d'absorber les particules qui ne passent pas exactement dans les microtrous. Cela permet d'éliminer les fragments émis avec un angle d'incidence supérieur à celui défini par les microtrous. Le résultat de l'interposition de la feuille est d'extraire du spectre en énergie continu de la figure 5 la partie de grande énergie et donc de mesurer et d'identifier précisément le flux de neutrons thermiques.

The invention proposes to use holes, drilled randomly but oriented in the same direction, in an insulating sheet 15, made of plastic or cleaved mica for example, to collimate the fragments from the neutron converter 16. To do this, this sheet is placed between the converter depot 16 and the input face of the detector 17, as shown in FIG. 5 for an exploded view. The holes 18 drilled in this sheet have a dimension of the order of a micrometer (μm). The sheet has a thickness which can vary from a few micrometers to several millimeters depending on the nature and the energy of the fragment emitted by the converter. Indeed, the process, proposed in particular for thermal neutrons, can also have applications for any particle converter provided that the cross section of capture or conversion in the converter is much higher than that of the plastic sheet. The role of the plastic sheet pierced with holes on the micron scale is twofold:

• . micro holes allow collimating incident particles. Only particles which are emitted almost perpendicular to the detector pass through holes. The depth of the hole makes it possible to vary, within a certain range, this angular opening;
• . the second role of the sheet pierced with holes is to absorb particles which do not pass exactly through the microholes. This eliminates the fragments emitted with an angle of incidence greater than that defined by the microholes. The result of the interposition of the sheet is to extract from the continuous energy spectrum of FIG. 5 the high energy part and therefore to measure and identify precisely the flux of thermal neutrons.

This collimation device therefore plays the role of a direction selector for the incident charged particles. As a result, the number of particles passing through the microholes is a small proportion of the incident particles. The device therefore also plays the role of counting rate attenuator.

The use of a collimator or collimators to select the direction of an incident particle is of course not new. A collimator is usually produced by drilling or machining. This process is perfect for making collimators with macroscopic dimensions. On the other hand, it cannot be extrapolated to dimensions on the order of a micron. The invention proposes the production of these collimators by a process which is not usually not used in the detection field. It is a question of realizing them thanks to a beam of heavy ions of suitable kinetic energy. Each heavy ion plays the role of a drill and creates a defect in the material which can be transformed into a hole of micronic dimensions by chemical development.

To make the microholes randomly placed in a plastic sheet (polycarbonate, kapton, polyimides, etc.), the simplest method is to irradiate it with a beam of heavy ions coming from an accelerator or from a source of fission fragments such as ²⁵² Cf. The slowdown of a heavy ion in matter begins with an electronic slowdown which generates charges, followed by a nuclear slowdown when the kinetic energy of the ion incident is less than about 0.1 MeV per nucleon. When slowing in an insulating material, and possibly a semiconductor, the ion creates a latent trace whose diameter is of the order of 10 nanometers. This latent trace is surrounded by a halo coming from the ejection of electrons torn off during the deceleration of the heavy ion (so-called delta electrons). The diameter of the halo is of the order of a micrometer. By chemical revelation of the latent trace, holes are obtained having a diameter of the order of a micrometer.

Compared to conventional lithography techniques, the advantage of heavy ions is that each of them creates a latent trace. This is well defined geometrically and allows, after revelation, to obtain holes of the order of a micrometer. The heavier the ion, the more the trajectory of the ion in the matter is straight and well defined. In practice, it is necessary to create holes with projectiles having at least the mass of krypton. The use of heavy ions in etching is very different from that of photons or electrons. Indeed, for the latter, the formation of a latent trace requires the participation of several electrons or particles. A mask is therefore necessary in the case of photons (visible, ultraviolet, X or γ rays). For electrons, we can consider controlling them because they are charged. For small thicknesses, conventional lithography makes it possible to make ordered holes. However, as soon as large thicknesses are desired and the distribution of the holes can be random, heavy ions are best suited.

The number of holes that can be created in the sheet depends on the incident flow. Typically, a density of 10 ⁸ holes / cm ² represents a maximum not to be exceeded. This is well below the capabilities of a particle accelerator. With such a density of holes, the porosity, defined as the number of holes multiplied by the surface of one of them, is 0.785. This large value implies that the probability of having overlapping holes is not zero. This is however only a minor drawback since even if several holes overlap, they still define an angle for the fragments which is close to the vertical. A lower flux, such as 5 x 10 ⁷ particles / cm ² , greatly reduces this probability of overlapping while keeping a porosity of 0.4.

The depth of the hole depends on the energy and the size of the incident ion. For kinetic energies of the order of 1 MeV per nucleon, the depth is of the order of 10 micrometers. The advantage of using heavy ions is the possibility of having a large energy dynamic allowing thus to control the depth of the hole while remaining within reasonable costs.

If we then consider the angular opening of these microcollimators and their effectiveness in terms of detection. We can take, to fix ideas, a hole of 1 micrometer in diameter and 10 micrometers deep. The angular opening is 5.7 °. This represents a solid angle of 0.03 sr, or 0.25% of the total space. This small opening will greatly reduce the counting rate compared to the case where the converter is not separated from the detector by the microcollimators. However, the particles detected are now perfectly identified and separated from the background noise. On the other hand, this low angular opening also has the advantage of making it possible to measure, in pulse mode, much larger fluxes than in the absence of microcollimators. This can be an advantage for the measurement of neutron fluxes in intermediate regime (10 ^-6 - 10 ⁹ neutrons / cm ² / s). In this case, the collimation device also plays the role of attenuator.

Claims (21)

Device for microcollimation of incident particles, characterized in that it consists of a set of micro-holes, of size on the order of a micrometer, randomly drilled, but oriented in parallel, in an insulating sheet of thickness between a few micrometers and several millimeters. Microcollimation device according to claim 1, characterized in that the insulating sheet is made of a material in which latent traces can be created by bombardment of heavy ions. Device according to claim 2, characterized in that the insulating sheet is made of plastic. Device according to claim 3, characterized in that the sheet is made of polycarbonate, kapton, or polyimide. Device according to claim 1, characterized in that the insulating sheet is made of cleaved mica. Device according to claim 1, characterized in that the density of the holes is less than 10 ⁸ / cm ² . Particle detector, characterized in that it comprises: - a particle converter (16) for generating charged particles; - A set of microcollimators (15) each having a size of the order of a micrometer, pierced randomly, but oriented parallel, in an insulating sheet of thickness between a few micrometers and several millimeters; - a detector (17) of charged particles. Detector according to claim 7, characterized in that the effective cross-section of capture or conversion in the converter is much greater than that of the insulating sheet. Detector according to claim 7, characterized in that the converter comprises a layer of boron. Detector according to claim 7, characterized in that the charged particle detector is a crystalline, polycrystalline or amorphous semiconductor, or a gas detector. Detector according to claim 7, characterized in that the particles are thermal neutrons, neutrons or photons. Particle detection method, characterized in that it consists in placing the microcollimation device according to any one of Claims 1 to 6, between a layer for converting the particle into electrically charged fragments and a detector of charged particles. Method according to claim 12, characterized in that the particles are thermal neutrons, neutrons or photons. Method according to claim 12, in a pulse-by-pulse counting mode, characterized in that it consists of the use of the microcollimation device, without processing the signals collected in the charged particle detector. Method of manufacturing an incident particle microcollimation device according to any one of Claims 1 to 6, characterized in that it comprises a step of bombarding a plastic sheet with a beam of heavy ions. Method according to claim 15, characterized in that the heavy ions are projectiles having at least the mass of krypton. Method according to claim 15, characterized in that the particle flux is approximately 5 x 10 ⁷ particles / cm ² . A method of manufacturing a device for microcollimation of incident particles according to any one of claims 1 to 6, characterized in that it comprises a manufacturing step by lithographic technique. Process according to any one of Claims 15 to 18, characterized in that the collective production is carried out, by bombardment of heavy ions or by lithography, of a set of microcollimators making it possible to collimate particles whether they are charged or no. Use of a set of microcollimators as defined according to any one of Claims 1 to 6 for separating particles having different incidences (direction filter). Use of a set of microcollimators as defined according to any one of claims 1 to 6 for attenuating an incident beam.

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