RU2769336C1 - Method for producing a nitrocellulose alpha particle detector - Google Patents

Abstract

FIELD: nuclear radiation detection.

SUBSTANCE: invention relates to the field of means for detecting nuclear radiation - solid-state track detectors (SSTD), specifically, it is a method for obtaining nitrocellulose SSTD of alpha particles, which is widely used in the Russian Federation (RF) and abroad for monitoring natural radioactivity, in particular in radon measurements. To prepare a solution of nitrocellulose, a mixture of butyl acetate, ethyl acetate, butanol, ethanol and toluene is used, the substrate is made of polyethylene terephthalate. It is vertically immersed in a nitrocellulose solution in a sealed chamber, after which the substrate with the solution film is removed from the solution and held vertically above the solution in saturated vapor. Next, the substrate with the solution film is vertically installed for exposure in another chamber and conditions are created there for the evaporation of the solvent from the solution film to ensure the formation of a solvent-depleted layer on the film surface and, based on the exposure results, the uniformity of the finished film in thickness and to eliminate the risk of delamination from the substrate after due to retention by adhesion forces, after which they begin pumping dust-free air through this chamber.

EFFECT: obtaining a nitrocellulose film with the required thickness characteristics suitable for track registration of alpha particles, while the heterogeneity in thickness of the resulting nitrocellulose film does not exceed 0.6 mcm, and the solvent consumption does not exceed 4.6 kg/kg NC, also, when using the claimed method, an adhesive connection of the NC film with the substrate is provided, which excludes spontaneous separation of the NC film from the substrate.

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Description

The invention relates to the field of detection of nuclear radiation using solid-state track detectors (TTD), specifically, it is a method for obtaining nitrocellulose TTD of alpha particles. Nitrocellulose TTD is widely used in Russia and abroad for monitoring natural radioactivity, in particular, in radon measurements.

A nitrocellulose track detector for alpha particles is a solid film (thickness in the range of 12-13 µm) of a composition of nitrocellulose plasticizer (for example, dibutyl phthalate, dioctyl phthalate or camphor) deposited on a flat polymer (for example, polyethylene terephthalate) base (substrate) and held on the substrate with an account adhesion forces. After exposure to the radiation field, the nitrocellulose-plasticizer film is subjected to chemical etching and the tracks formed under the action of alpha particles (hollow channels) in the film are counted. For the convenience of counting tracks by the optical method, it is allowed to introduce an organic dye compatible with nitrocellulose into the initial composition of the NC - plasticizer. The number of recorded tracks corresponds to the number of alpha particles that reached the nitrocellulose detector film.

The operational characteristics of nitrocellulose TTD are determined by the degree of polymerization and the nitrogen content in nitrocellulose (NC) and the conditions for obtaining a nitrocellulose film. The most important requirement is the uniformity of the nitrocellulose film in thickness, the absence of mechanical stresses in it. Spontaneous separation of the NC film from the substrate is not allowed.

NC belongs to the group of polymers, the pour point of which is unattainable, since it is above the decomposition temperature of the polymer. Therefore, the use of a NC solution is a necessary and indispensable step in the production of nitrocellulose TTD.

To obtain thin (from several nanometers to tens of microns) polymer films, methods of pouring a polymer solution onto a flat horizontal substrate with simultaneous centrifugation of the substrate are used (for example, Perunkov M.A., Shamin A.A. Obtaining a solution of dimethylformamide lead diiodide // Modern scientific research and Innovations 2017. No. 3 [Electronic resource] URL: https://web.snauka.ru/issues/2017/03/78293; SU No. 1805790; US patent No. 5055568; Ushakov, H. H. Computer production technology. - M .: Higher school, - 1991. S. 163-164).

The method for forming films based on polyaniline (SU No. 1805790) is based on the use of the centrifugation method and includes the following: a film-forming solution is applied to the center of a horizontal cylindrical substrate. The substrate is then driven into rotation at a speed of 5000-7000 rpm. Centrifugal acceleration leads to the fact that a drop of solution spreads in a thin layer over the surface of the substrate, part of the solution reaches the edge of the substrate and is thrown away under the action of centrifugal forces.

The method for producing nitrocellulose films (patent US 5055568) is also based on the principle of centrifugation of the NC solution on a flat surface and is rather complicated. The method uses such operations as preparing a solution of NC in an active solvent, precipitating high-molecular fractions of NC and gelling impurities from the solution with a "bad" NC solvent, separating the precipitate from the NC solution by decantation or centrifugation, distilling off the "bad" solvent from the NC solution by vacuum. The complexity of this method (patent US 5055568) complicates its practical implementation.

The general disadvantage of the methods (SU No. 1805790, US patent 5055568) and other methods for producing solid films by pouring a polymer solution onto a flat horizontal substrate with simultaneous centrifugation of the substrate is due to the fact that the thickness of the liquid film decreases from the center of rotation to the periphery of the substrate due to an increase in the separation factor, which makes it difficult to obtain a solid polymer film of uniform thickness. For example, in the manufacturing process of printed circuit boards, a method is used to apply a liquid photoresist to a board that receives rotation from an electric motor. To obtain thin films, a high rotational speed and a photoresist solution with a sufficiently low viscosity are used. The thickness of the finished photoresist film is from 2 microns to 4 microns, and the unevenness of the thickness is 0.5-1.0 microns (Ushakov, N.N. Computer production technology. - M .: Higher school, - 1991. S. 163- 164).

A method for producing a collodion (that is, nitrocellulose) film by the centrifugal method (RF patent No. 2458944) consists in spraying a mixture of NC and an organic solvent onto a rotating surface, and collodion (nitrocellulose solution) in ethyl alcohol and diethyl ether is used as a mixture in the following ratio of components, about. including: collodion 1; the organic solvent is ethyl alcohol, diethyl ether 2-200, and the mixture is sprayed onto a rotating cylindrical surface. Since a nitrocellulose film, when used as a TTD of alpha particles, must have a flat shape, when the NC film obtained according to the patent of the Russian Federation No. In addition, the use of an extremely volatile active solvent degrades the quality of the NC film due to residual stresses associated with the nonequilibrium structure of the film. All these shortcomings do not allow the use of the NC film obtained by the method (RF patent No. 2458944) as TTD alpha particles.

Closest to the proposed method for obtaining a nitrocellulose detector of alpha particles is a method for obtaining a nitrocellulose film in the thickness range of 10-20 μm (US patent No. 3928700), suitable for track registration of alpha particles. The method (US patent No. 3928700) includes the formation of a solution containing nitrocellulose and 7-15 wt. % dioctyl phthalate in terms of the mass of NC in tetrahydrofuran, pouring this solution (the concentration of NC in which is at the level of 1-1.5% wt.) onto a leveled flat surface (plate), evaporating the indicated solution on a plate, drying the resulting film, removing the film off the plate and then annealing the film to remove residual solvent. The thickness of the obtained NC film, which was determined in the middle of each quadrant of the film, varied within 15-16 μm, that is, the thickness inhomogeneity obtained according to US patent No. 3928700 NC film is 1 μm. The use of a dilute (1-1.5% wt.) solution of NC for casting NC-film in the method (US patent No. 3928700) leads to a large consumption of tetrahydrofuran (within 65-99 kg/kg NC) and the cost of recuperation of its vapors. The increased volatility of tetrahydrofuran makes it difficult to control the rate of its evaporation from the NC film, which leads to the appearance of residual stresses in the finished film. In addition, due to the edge effect at the point of contact of the NC solution with the edge of the casting tray, the thickness of the finished film in the area adjacent to the edge of the tray differs from the thickness of the film in the area adjacent to the center of the tray.

The technical objective of the proposed method is to obtain a nitrocellulose film in a given thickness range (10-30 μm), suitable for track registration of alpha particles, while the inhomogeneity in the thickness of the nitrocellulose film does not exceed 0.6 μm, the solvent consumption does not exceed 4.6 kg / kg of NC, and also creates adhesion of the NC film to the substrate, excluding spontaneous separation of the NC film from the substrate.

The problem is solved due to the fact that in the method for obtaining a nitrocellulose alpha-particle detector, which includes preparing a solution of nitrocellulose, forming a film of the resulting solution on a substrate and evaporating the specified solution on a substrate, according to the invention, a mixture of butyl acetate, ethyl acetate, butanol is used to prepare a nitrocellulose solution, ethanol and toluene, the substrate is used from polyethylene terephthalate, it is vertically immersed in a nitrocellulose solution in a sealed chamber, after which the substrate with the solution film is removed from the solution and vertically held above the solution in saturated solvent vapor in a sealed chamber, then the substrate with the solution film is vertically installed for holding in another chamber and create conditions there for the evaporation of the solvent from the solution film to ensure the formation of a solvent-depleted layer on the surface of the film, and, based on the results of exposure, the uniformity of the finished film in thickness and the exclusion of ri after which the pumping of dust-free air with a given optimum relative humidity and optimum temperature through this chamber is started and the rate of removal of solvent vapors from the film is controlled to ensure the completeness of the relaxation processes in the film as the solvent is removed from it for by establishing the optimally selected air pumping speed.

It is advisable to install the substrate with the solution film vertically in such another chamber, the volume of which is 12-20 cm ³ /(cm ^{2 of} the surface of the solution film), after which air can be pumped through this chamber at a speed of not more than 2 cm ³ /(min⋅cm ² solution film surface). These parameters are derived empirically, and then confirmed experimentally, which is confirmed by the information below.

The optimal parameters of the method operations are closely related to the microclimate of the working room in which the method is carried out - a room in which people are without street clothes for at least 2 hours continuously or 6 hours in total during the day in a standing or sitting position, being engaged in mental work. Therefore, the terminology used regarding the parameters of temperature and humidity, the speed of air movement, as well as the derived optimal values are made taking into account GOST 30494-2011, which specifies combinations of values for microclimate indicators, in which, with prolonged and systematic exposure to a person, a normal thermal state of the body is ensured at a minimum tension of thermoregulation mechanisms and a feeling of comfort for at least 80% of people in the room.

Parameters characterizing the microclimate in the working room where the method is carried out (working rooms):

- optimum air temperature 18-20°С;

- air speed 0.2 m/s;

- relative air humidity 30-45%;

At the same time, while ensuring microclimate indicators at various points of the working room, it is allowed:

- air temperature difference is not more than 2°C for optimal performance;

- change in air velocity - no more than 0.07 m/s for optimal performance;

- change in relative air humidity - no more than 7% for optimal performance.

The technical result of the claimed method is to obtain a nitrocellulose film with specified (required) thickness characteristics (in the range of 10-30 microns), suitable for track registration of alpha particles, while the heterogeneity in thickness of the resulting nitrocellulose film does not exceed 0.6 microns, solvent consumption does not exceed 4.6 kg/kg NC, and when using the claimed method, an adhesive connection of the NC film with the substrate is provided, which excludes spontaneous separation of the NC film from the substrate.

To prepare a solution of nitrocellulose (lacquer), it is preferable to use a solvent of the composition, wt %: butyl acetate 25, ethyl acetate 25, butanol 7, ethanol 18, toluene 25, as well as a flat substrate, which is used as a film of polyethylene terephthalate, which is vertically immersed in a solution (lacquer) in a closed (hermetic) chamber, then the substrate with the NC solution film is removed from the solution and held vertically above the solution in the chamber, then the substrate with the solution film is transferred to the chamber, the volume of which is from 12-20 cm ³ /(cm ^{2 of} the surface of the solution film) with air with a relative humidity of not more than 40%, vertically installed in the chamber, after which air with a relative humidity of not more than 40% and a speed of not more than 2 cm ³ /(min⋅cm ² of the solution film surface) is pumped through the chamber.

When volatile solvents active with respect to NC are used (such as acetone, tetrahydrofuran, ethyl acetate), the internal stresses arising during the formation of an NC film due to the evaporation of the solvent, associated with the nonequilibrium structure of the film, cause it to crack or peel off from the substrate. Unnecessarily rapid evaporation of the solvent lowers the temperature of the air adjacent to the NC film, which leads to rejection due to moisture condensation on the surface of the film that has not yet dried. In the proposed method, these undesirable phenomena are eliminated by introducing a non-volatile active solvent (butyl acetate) in combination with diluents (ethanol, butanol, toluene) into the composition of the solution (lacquer). Diluents and a non-volatile active solvent make it possible to control the pressure of the active solvent over the solution film and, as a result, the rate of removal of the active solvent from the system. The solvent used in the proposed method combines the possibility of obtaining a thin homogeneous transparent cellulose nitrate film (with the addition of a plasticizer and, if necessary, an organic dye), the availability and low cost of components with minimal toxicity.

After the substrate is immersed in the Hc-solution at room temperature (18-20°C), several minutes are enough to establish the equilibrium of the adhesive contact of the varnish with the substrate. solution, nitrocellulose and a plasticizer (for example, dibutyl phthalate, dioctyl phthalate or camphor), which may be part of the Hc solution. The finished NC-film is sufficiently firmly held on the surface of the PET-substrate and does not spontaneously separate from the substrate.

After removing the substrate from the solution, part of the solution (varnish) flows off the substrate. As shown in the proposed method, if conditions are created that prevent the solvent from evaporating from the solution film, then regardless of the rate of removal of the substrate from the solution, a solution layer remains on the substrate, the thickness of which is determined by the viscosity and density of the solution and the nature of the substrate. Evaporation of the solvent from the film NC-solution (NC-lacquer) in the proposed method is prevented by keeping the substrate with the solution in saturated vapor of the solvent in a sealed chamber with the solution; at this time, the excess solution drains from the substrate, and the film thickness over the entire surface of the substrate is leveled off due to surface tension forces, which makes it possible to obtain a finished NC film with a thickness inhomogeneity of no more than 0.6 μm. Next, conditions are created for the evaporation of the solvent from the solution film, which is achieved by transferring the PET substrate with the solution film into a chamber with air with a relative humidity of not more than 40% and a temperature of 18-20°C, the volume of which is from 12 to 20 cm ³ /(cm ² solution film surfaces). Due to this, an NC layer depleted in solvent is created on the surface of the NC solution on the substrate, after which they begin to pump dust-free air through the chamber with a relative humidity of not more than 40% and a speed of not more than 2 cm ³ / (min⋅cm ² of the solution film surface) . The controlled rate of removal of solvent vapors from the solution ensures the completeness of the relaxation processes in the NC film as the solvent is removed from it. The above technological methods make it possible, using NC varnishes with an experimentally selected concentration of NC, to obtain a monolithic NC film of a given thickness with a thickness inhomogeneity of not more than 0.6 μm, held on a PET substrate by physical adhesion forces that exclude spontaneous separation of the NC film from the substrate.

Thickness, uniformity of thickness and other performance properties of nitrocellulose TTD obtained by immersing a substrate in a NC solution is a complex function of the properties of nitrocellulose, the concentration of NC in the solution, the composition of the solvent, a, the nature of the substrate, the mode of removal of the solvent from the liquid film and can be selected experimentally .

The proposed method is illustrated by the following examples.

Example 1

Dry nitrocellulose with a nitrogen content of 12.5% and an average degree of polymerization of 440 and camphor (GOST 1123-72) were dissolved in a solvent composition, wt %. butyl acetate 25, ethyl acetate 25, butanol 7, ethanol 18, toluene 25. The resulting solution had a composition, wt %: nitrocellulose 18.0, camphor 5.5, solvent - the rest.

A film of polar polymer -polyethylene terephthalate (GOST 24234-80) was used as the substrate material, since preliminary experiments showed that the cellulose acetate substrate strongly "seizes" at the molecular level with a monolithic NC film due to swelling of cellulose acetate in active NC solvents. Films of non-polar polymers, such as polyethylene, polyisobutylene, when trying to use them as a substrate in the manufacture of nitrocellulose TTD, spontaneously separate from the NC film.

A PET substrate fixed on a steel plate, which ensured rapid immersion of the substrate into the viscous NC solution, was vertically immersed in the NC solution in a closed chamber. After the substrate is immersed in the NC solution, several minutes are sufficient to establish the equilibrium of the adhesive contact; therefore, in all examples, the residence time of the substrate in the NC solution was 10 min at room temperature.

After removing the substrate from the NC solution, part of the solution drained from the substrate. As shown experimentally, if conditions are created that prevent the evaporation of the solvent from the solution film, then a layer of the NC solution remains on the substrate, the thickness of which is determined by the viscosity and density of the solution and the energy factors of the interaction of the solution with the substrate surface. Evaporation of the solvent from the NC solution film in the proposed method is prevented by holding the substrate with the NC solution in saturated solvent vapor in a sealed chamber with the solution; at this time, the excess solution flows down from the substrate, and the film thickness over the entire surface of the substrate is equalized due to surface tension forces. The optimal time for the formation of an NC solution film on a PET substrate in the proposed method was about 180 min, after which conditions were created for the solvent to evaporate from the film, which is achieved by quickly transferring the PET substrate with the solution film into a chamber with air with a relative humidity of not more than 40% and room temperature 18-20°C, the volume of which is from 16 cm ³ /(cm ^{2 of} the surface of the film of the NC solution). Due to this, a solvent-depleted layer is created on the surface of the film on the substrate, after which fresh dust-free air was pumped through the chamber at a given speed of not more than 1.8 cm ³ /(min⋅cm ² of the solution film surface).

The controlled rate of removal of solvent vapors from the film ensures the completeness of the relaxation processes in the NC film as the solvent is removed from it. The monolithic nitrocellulose film obtained in this example, suitable for track detection of alpha particles, had a thickness of 14 μm, was held on the PET substrate by physical adhesion forces (that is, it did not spontaneously separate from the substrate), while the inhomogeneity in the thickness of the nitrocellulose film did not exceed 0, 6 µm

Example 2

Dry nitrocellulose with a nitrogen content of 11.5% and an average degree of polymerization of 260, dioctyl phthalate (GOST 8728-88), acetone-soluble Bordeaux dye was dissolved in a solvent of the same composition as in example 1. The resulting solution had a composition, wt %: nitrocellulose 39, 5, dioctyl phthalate 5.9, Bordeaux dye 4.5, the rest of the solvent.

The process of forming a film of the NC solution on the substrate and removing the solvent from the film was similar to example 1. film does not exceed 0.6 µm. The presence of the dye in the nitrocellulose TTD obtained in this example makes it possible to count the tracks of alpha particles in the TTD by an optical method.

Deviations from the composition of the solvent used in the proposed method are undesirable, since they cause a deterioration in the quality of the NC film and its adhesion to the substrate.

The volume of the chamber into which the substrate with the NC solution film is transferred should be maintained within the range of 12 to 20 cm ³ /(cm ² of the solution film surface). When the volume of the chamber is less than 12 cm ³ /(cm ² of the solution film surface), the thickness uniformity of the finished NC film deteriorates. When the volume of the chamber is more than 20 cm ³ /(cm ^{2 of} the surface of the solution film), the film of the NC solution becomes depleted of the solvent too quickly, which leads to mechanical stresses in the finished film and its delamination from the substrate. When the speed of pumping air through the chamber is more than 2 cm ³ /(min⋅cm ^{2 of} the surface of the solution film), the film of the NC solution is also depleted of the solvent too quickly.

The proposed method makes it possible to obtain nitrocellulose TTD with the required film thickness by changing the concentration of cellulose nitrate in the solution used to apply the film to the substrate.

An important advantage of the proposed method is due to the fact that the concentration of NC in the solution used for application to the substrate is, depending on the degree of polymerization of NC, from 18% wt. up to 39.5 wt%, which corresponds to the solvent consumption in the range of 1.6-4.6 kg/kg of NC, since it is technologically and economically expedient to work with concentrated solutions of film-forming polymers.

Claims (3)

1. A method for producing a nitrocellulose detector of alpha particles, including preparing a solution of nitrocellulose, forming a film of the resulting solution on a substrate and evaporating said solution on a substrate, characterized in that a mixture of butyl acetate, ethyl acetate, butanol, ethanol and toluene is used to prepare a solution of nitrocellulose, the substrate is used made of polyethylene terephthalate, it is vertically immersed in a nitrocellulose solution in a sealed chamber, after which the substrate with the solution film is removed from the solution and vertically held above the solution in saturated solvent vapor in the sealed chamber, then the substrate with the solution film is vertically installed for holding in another chamber and created there conditions for the evaporation of the solvent from the film of the solution to ensure the formation of a solvent-depleted layer on the surface of the film, and, based on the results of exposure, the uniformity of the finished film in thickness and the elimination of the risk of delamination from the substrate due to retention with adhesion silts, after which they start pumping dust-free air through this chamber with a given optimal relative humidity and optimal temperature and regulate the rate of removal of solvent vapors from the film to ensure the completeness of the relaxation processes in the film as the solvent is removed from it by establishing the optimally selected air pumping speed . 2. The method according to p. 1, characterized in that for the preparation of a solution of nitrocellulose, a solvent of the composition is used, wt %: butyl acetate 25, ethyl acetate 25, butanol 7, ethanol 18, toluene 25. 3. The method according to claim 1, characterized in that the substrate with the solution film is vertically installed in such another chamber, the volume of which is 12-20 cm ³ /(cm ^{2 of} the surface of the solution film), after which air is pumped through this chamber at a speed not more than 2 cm ³ /(min⋅cm ² of the solution film surface).