WO2005071440A1

WO2005071440A1 - Converter foil containing 6li for neutron detectors and method for their production

Info

Publication number: WO2005071440A1
Application number: PCT/DE2004/002690
Authority: WO
Inventors: Jakob Schelten; Josef Zillikens
Original assignee: Forschungszentrum Jülich GmbH
Priority date: 2004-01-23
Filing date: 2004-12-08
Publication date: 2005-08-04
Also published as: DE102004003398A1; EP1718991A1

Abstract

The invention relates to a converter foil for a neutron detector. Said converter foil consists of a mechanically stabilised foil containing 6Li. The mechanical stabilisation is achieved e.g. by a framework device, e.g. by a stainless steel screen. This allows the formation of large detector surfaces. The invention also relates to a method for producing the converter foil.

Description

Description ⁶ Li-containing converter film for neutron detector and process for its production

The invention relates to a converter film for a neutron detector and a method for its production.

Spatially resolving neutron detectors are known as multi-wire chambers with He gas filling or as scintillation detectors based on the Anger camera principle. With such neutron detectors, an acceptable detection probability and an appropriate spatial resolution are achieved.

Thermal neutrons are indirectly detected by absorption, producing ionizing reaction products. Possible absorbers are the isotopes ³ He, ⁶ Li, ¹⁰ B, ²³⁵ U and ¹⁵⁷ Gd, in which ions of the light isotopes ³ H, ⁴ He, ⁷ Li or of fission products of uranium are released as reaction products or a conversion electron is formed. Energy in the MeV area is deposited here.

For this purpose, a gap chamber has a wall thinly coated with the ²³⁵ U isotope. The isotope absorbs the thermal neutrons. The fission products generated deposit part of their high released energy of around 200 MeV in the gas, where they generate ion pairs drift to existing electrodes in the applied electrical field and thereby influence charges on the electrodes.

It is disadvantageous that thermal neutrons are only absorbed with very little probability.

In ³ He and ¹⁰ BF ₃ counter tubes, the detection is most likely to be made directly by the formation of electrons and ions in the gas with subsequent electron avalanche on the anode wire. High demands are placed on the purity of the counting chamber and on safety, since the detection takes place at up to 10 atm overpressure.

The detection of thermal neutrons is indirect through the emission of scintillation light, which is generated in ⁶ Li or ¹⁰ B-containing scintillators by neutron absorption. The light is then detected using photomultipliers.

For special requirements, the neutrons are detected by depositing part of the energy of a reaction nucleon in the blocking zone of a p-n semiconductor and thereby detecting it.

The detection principles mentioned have been used for surface detectors with one- or two-dimensional spatial resolution.

From DE 195 32 415 AI is a neutron detector with a ⁶ Li-containing converter film for converting neutrons into ionizing radiation and downstream Detection of thermal or sub-thermal neutrons is known. Neutrons are absorbed in ⁶ LiF converter foils in an ionization chamber. The neutron absorption by the ⁶ Li isotope follows the reaction equation: ^x ₀ n + ⁶ ₃ Li -_ ³ ιH + ₂ He + 4.78 MeV.

The lighter reaction nucleon receives the greater energy share of 2.73 MeV. The heavier helium isotope 2.05 MeV. The lighter ³ _x H nucleon thus has a much greater range.

A plurality of consecutively arranged, approximately 30 microns thinner ^s LiF converter foils this disadvantage are needed to efficiently thermal neutrons, that is to stop with a probability of about 70%.

The object of the invention is to provide a converter film which is highly likely to absorb thermal neutrons and which can be used to form large detector areas with a simple structure.

The object is achieved by a converter film according to the main claim and by a method for its production according to the secondary claim. Advantageous configurations result from the patent claims referring to them.

The starting material of the converter foil consists of mechanically stabilized ⁶ Li-containing foil. The converter film can e.g. B. be stabilized by a scaffold-like device, in particular by a suitable sieve-like device.

This measure makes it possible to form large detector areas. The proportion of the ⁶ Li isotope can be chosen to be high. Pure, metallic ⁶ Li can even be used as the starting material in order to absorb thermal neutrons particularly effectively in detector operation. The production of a converter film from pure, approximately 96% metallic ⁶ Li was previously not possible, since this is too soft for large-area processing.

A converter foil made of metallic ⁶ Li can advantageously be made so thin due to the stabilization that the ³ H reaction products can deposit at least 1 MeV in the gas space.

In the case of a converter film made of pure ^δ Li, a cover layer that provides chemical protection is necessary. The protective layer is then conductive on its surface. When converting from pure ⁶ Li, the reactivity is carried out in a protective gas atmosphere.

In the case of ⁶ LiF as the starting material for the converter film, these precautionary measures are not necessary. But since ^s has an insulating LiF, has at its

Use as a converter film in the manufacture of a conductive coating that ensures the electrical function of the converter film as a cathode. The surface of the ⁶ Li-containing material of the converter film according to the invention advantageously has a thin metal layer for the purposes mentioned. The metal layer can comprise Al, Cu, Ti, Cr, Ag and / or Au. It has a protective effect against chemical reaction with air and the ionization gases and / or, if necessary, ensures the conductivity of the film. The metal layer is so thin that no more than 0.1 MeV of the ³ E is deposited in it during operation of the neutron detector.

On the surface of a ⁶ Li-containing converter film z. B. a thin with a metal coated PET film. PET films coated on both sides with aluminum are advantageously already commercially available.

A mechanically stabilized ⁶ LiF film is self-supporting and easy to manufacture.

After production, the mechanically stabilized converter foils do not react with the surrounding gases or the ionization chamber. As a result, one is particularly advantageously not restricted in further handling of the film.

The converter film according to the invention is mechanically so stable that even larger areas of, for example, 100 cm ² can be spanned. However, the size is by no means limited to this specification. Rather, even larger areas can be spanned. The converter film has an optimal thickness, which means that it absorbs as many neutrons as possible and the ³ ιH nucleon (Triton) released during absorption can penetrate the converter and deposit at least 1 MeV energy in the adjacent gas space.

In the metallic ⁶ Li, the resulting ³ ιH nucleon with 2.73 MeV has a very large range of about 140 μm. In all other substances the range is significantly shorter. It deposits ³ H nucleons that fly at 45 ° to the film normal and through the entire film still the energy of 1 MeV in the adjacent gas space of the detector. Similar conditions exist for ⁶ LiF when the layer thickness of the converter film is approximately 30 μm.

The converter film is mechanically stable on the one hand and also chemically stable on the other hand. This means that the converter film is inert to air and air humidity, which makes it easier to install the detector. Due to the protective layer, the converter film does not react with the detector gases such as Ar, Xe, CF ₄ and ₂ F ₆ .

A converter foil consisting of metallic ^s Li has an optimal thickness of 0.10 mm and is protected by a thin cover layer of less than 1 μm thickness due to the chemical reactivity of the lithium with air.

An existing from the chemically inert ^s LiF converter film has an optimal thickness of 0.03 mm, WO with a metallic conductive thin coating on the surface of the converter film is required. The more efficient material is metallic ⁶ Li.

The converter film is always electrically conductive on both sides of its surface because it forms the cathode of an ionization chamber of a neutron detector. In a neutron detector, opposite the cathode, electrically conductive collecting electrodes are arranged on both sides as the anode of the ionization chamber of the neutron detector.

Several converter foils can be arranged in series in a neutron detector with collecting electrodes in order to increase the probability of absorption of the neutrons to be detected. Three converter foils made of pure ⁶ Li with a thickness of 100 μm are required to stop thermal neutrons with a 70% probability.

About six ⁶ LiF converter foils with a thickness of 30 μm are required in order to absorb thermal neutrons with a 70% probability.

In Table 1 are for ⁶ and Li ⁶ LiF the values p to the mass density, the atomic density n i _L, length to l / e Absorption L ^{1 / e} for neutrons of wavelength λ = 0.2 nm at 100% ⁶ Li Enrichment and to the track length Ri for the 2.73 MeV ³ ιH nucleon formed during neutron absorption, at which the residual energy of 1 MeV is reached.

The table shows that ⁶ LiF absorbs neutrons more effectively than metallic ^s Li, but that the specified range of the ³ H nucleon in ⁶ LiF is significantly shorter than in metallic ^s Li. Therefore, ⁶ Li is ultimately the more efficient absorption material for thermal neutrons.

The process for producing the converter film provides to choose than ⁶ Li-containing starting material metallic Li ^s or powdered ⁶ LiF and contribute in a scaffold-like device. A stainless steel sieve can in particular be provided as the device. The starting material can, for example, be rolled or coated into the device.

The framework-like device has a hole percentage of approximately 90%. The device accordingly reduces the neutron absorption by a maximum of 10% compared to a converter film made of pure starting material and the proportion of disturbed traces of ³ H nucleons is approximately 5%. In the case of metallic ⁶ Li, the metal is rolled into the device, in particular the close-meshed metal screen, under a protective gas atmosphere. This keeps the contamination of the converter film with oxygen and water vapor in the ppm range. Argon is advantageously used as the protective gas.

In the case of metallic ⁶ Li as the starting material for the converter film, talc and / or paraffin or another suitable one, ie reducing the adhesion of the ⁶ Li to the roll, can be used for the first rolling step

Lubricant can be used to lubricate the roller. This prevents the roller from sticking.

Due to the optimal layer thickness of the absorber foil made of metallic ⁶ Li of only 100 μm, corresponding converter foils of this thickness require mechanical stabilization, which both disturb the absorption of neutrons and the traces of high-energy ³ H nucleons formed in lithium. Since metallic lithium reacts violently with oxygen and water vapor as well as with the possible detector gases CF ₄ and C ₂ F ₆ , the surface of the converter film is protected with a cover, in particular a metal layer. The cover is gas-tight, sufficiently thin and electrically conductive. As a result, it has a permanent effect and little energy of the ³ H nucleon is deposited in it. The converter film forms an electrode of an ionization chamber. With such converter foils, one is flexible in the manufacture of the neutron detector, since protection is also provided outside the neutron detector.

The invention is described in more detail below on the basis of exemplary embodiments and the attached figures.

Fig. 1 shows a stainless steel screen as a device for stabilization for metallic ^s Li or ^s LiF. To stabilize ^s Li foils, the metal sieve has a thickness of 100 μm and ⁶ LiF powder pastes has a thickness of 30 μm.

Fig. 2 shows stopping power values S in the unit MeV per mgcπf ² as a function of the material occupancy and the ^* energy of the ³ χH nucleon (Triton), calculated with the TRIMM code for some materials of the converter film.

Production of converter foil from pure ⁶ Li: 1. The mechanical stabilization is achieved by a close-meshed metal sieve with a thickness of 100 μm. The sieve consists of a perforated grid with 2mm x 2mm holes and 0.2mm narrow webs (Fig. 1). Traces of ³ H nucleons that are formed at a distance of less than 100 μm from the edge of the hole can be disturbed, so that less than 1 MeV may be deposited in the adjacent gas. With the dimensions indicated in FIG. 1, the sieve reduces the neutron absorption by approximately 10% and the proportion of disturbed traces of ³ H nucleons is approximately 5%. A metal roller, such as that used by goldsmiths, was introduced into a glove box in order to roll lithium blocks into the sieve under protective gas and thereby produce the 100 μm film.

The adhesion of the rolled ⁶ Li foils to the rolling rolls was prevented with talcum powder as a lubricant during rolling. The amount of talcum powder pressed into the rolled converter foils from ^s Li proved to be low and did not impair the stopping power of the ³ H nucleons in the lithium. The use of liquid paraffin as a lubricant was also successful.

The last rolling step is to roll the ⁶ Li into the screen. The embedding is also permanent due to the acute-angled edge profile of the holes, which is formed during the etching of the holes.

Two variants were tested to cover the surface of the 100 μm film.

A commercially available, 6 μm thin PET film, which is coated on both sides with -20 nm aluminum and has a surface resistance of 1.5Ω, was glued to the ⁶ Li with a two-component adhesive. The converter film is thereby against reactions with 0 ₂ and H ₂ 0 during the detector assembly and against reactions with the detector gases z. B. CF ₄ or C ₂ F ₆ protected in detector operation.

For this purpose, the PET films are stretched on glass plates, coated with an adhesive and on both sides of the sieve ^s stabilized Li converter foil overall pressed. A vice provides the contact pressure. To further reduce the remaining amount of adhesive, the film package is heated to about 60 ° C.

The PET film has a weight of 0.5 mg / cm ² , the two-sided Al coating has a weight of 0.01 mg / cm ² . The ³ 1 H nucleon according to FIG. 2 can deposit a maximum of 0.2 MeV in this protective layer. Weights of less than 1 mg / cm ^{2 could be} achieved for the adhesive layers, in which a maximum of 0.4 MeV is deposited by the ³ ιH nucleon.

2. As an alternative, in the glove box a sputtering chamber can be introduced, in which the stabilized mechanically ^s Li converter film is used as the substrate. The sputter cathodes can consist of copper on both sides of the substrate. A plasma is ignited in the chamber and a -25 nm thin copper layer is generated on the converter film by sputter deposition. In such thin metallic protective layers of about 0.02 mg / cm ² , the maximum possible energy deposition of a ³ ιH nucleon is negligible and is only 0.004 MeV.

Production of converter foil from powdery ⁶ LiF:

A mixture of ⁶ LiF powder and epoxy resin and hardener is first mixed. For stabilization, the paste is spread into a close-meshed 30 μm thick metal sieve with lateral dimensions as indicated in FIG. 1.

Aluminum-coated 6μm PET films are placed on both sides of the converter film and pressed in a vice until the curing process is complete. In this way, the powder layer is mechanically stabilized without disturbing the trace of the ³ ιH nucleons. A converter film with a thickness of approximately 30 μm is produced. The

Layer surfaces of the converter film are covered on both sides with thin, electrically conductive layers, so that the ³ ] _. H nucleons deposit little energy in the cover layer and the converter can act as an electrode of ionization chambers. The manufacturing process is suitable for areas of 100 cm ² and larger.

Such converter foils are particularly suitable for the absorption of thermal neutrons, and release ³ ιH nucleons which always deposit an energy of at least 1 MeV in the adjacent gas space of a neutron detector.

Claims

Patent claims

1. Converter film for absorbing thermal neutrons in a neutron detector, characterized in that the converter film comprises mechanically stabilized ⁵ Li or ⁶ LiF.

2. Converter film according to claim 1, characterized in that the converter film has a framework-like device for stabilization.

3. Converter film according to one of the preceding claims, characterized in that the frame-like device has a hole percentage of about 90%.

4. Converter film according to one of the preceding claims, characterized in that the converter film has a stainless steel sieve for stabilization.

5. Converter film according to one of the preceding claims 1 to 4, characterized in that the ⁶ LiF converter film has a layer thickness of 30 microns.

6. converter film according to one of the preceding claims 1 to 4, characterized in that the ⁶ Li converter film has a layer thickness of 100 μm.

7. Converter film according to one of the preceding claims, characterized in that a metal layer is arranged on the surface of the ⁶ Li-containing converter film.

8. Converter film according to one of the preceding claims, characterized in that a PET film coated on both sides with a metal is arranged on the surface of the ⁶ Li-containing converter film.

9. Converter film according to one of the preceding claims 7 or 8, characterized in that the metal layer comprises Al, Cu, Cr, Ti, Ag and / or Au.

10. Converter film according to one of claims 7 to 9, characterized in that the metal layer is so thin that no more than 0.1 MeV of ³ _X H is deposited in it during operation of the neutron detector.

11. Neutron detector with a converter film according to one of the preceding claims 1 to 10.

12. A method for producing a converter film according to one of the preceding claims 1 to 10, characterized in that a ⁶ Li-containing starting material for mechanical stabilization is introduced into a framework-like device.

13. The method according to the preceding claim 12, characterized by the choice of a stainless steel screen as a scaffold-like device.

14. The method according to any one of the preceding claims 12 or 13, wherein the ⁶ Li-containing starting material is introduced into the device under a protective gas atmosphere.

15. The method according to the preceding claim 14, in which argon is selected as the protective gas.

16. The method according to any one of the preceding claims 12 to 14, in which ⁶ Li is rolled into the device.

17. The method according to the preceding claim 16, wherein the roller is treated with a lubricant.

18. The method according to the preceding claim 17, characterized in that talc and / or paraffin are selected as lubricants.

19. The method according to claim 12 to 18, characterized in that powdered ⁶ LiF before being introduced into the Device is mixed with epoxy resin and hardener.

20. The method according to any one of the preceding claims 12 to 19, characterized in that a thin metal layer is applied to the ⁶ Li-containing converter film.

21. The method according to the preceding claim 20, characterized in that a metal layer comprising Al, Cu, Cr, Ti, Au or Ag is selected.

22. The method according to any one of the preceding claims 12 to 21, characterized in that on the surface of the stabilized ⁶ Li-containing converter film, an Al-laminated PET film z. B. is applied by gluing.