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(11) | EP 0 980 077 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Procedure for the incorporation of hazardous or costly onto aluminium supports with very low thickness of anodising |
| (57) It comprises, in general, the following stages: preparation of a solvent-additive
mixture which complies to the conditions: surface tension less than 25 din cm-1, steam pressure greater than 40 mm of Hg at 20°C and solid residue below 4.5 10-3 g/l, preparation of a 10-10 M solution of the substance to be incorporated into the solvent - additive mixture,
removal of the excess and rubbing with paper soaked in the solvent - additive mixture;
using a water - ethanol mixture for the platting of 241Am on an anodising thickness of 4 µm in the manufacture of extensive radioactive sources. |
[0001] The subject matter of the present invention is a procedure to deposit hazardous or costly substances onto the anodised layer of an aluminium support.
[0002] The applications of the subject matter of the invention are, mainly the preparation of extensive radioactive sources and in general the deposit onto anodised aluminium supports of hazardous (radioactive, toxic, medical, etc.) or costly products (silver salts for photography, noble elements for decoration, antithermal ceramic elements, etc.)
[0003] The Spanish standard UNE 21-72-90 and international one ISO 8769 specify the characteristics that radioactive sources used for the calibration of superficial contamination monitors must fulfil. According to this standard, the area of the radioactive source must have a surface of at least 100 cm2 and, as to the homogeneity or distribution of the activity of the radionuclide on the source, there should not be a variation above 10% in the specific surface activity measured at different parts of the source.
[0004] The preparation of radioactive sources of large superficial area cannot be done with the electroplatting methods used in the preparation of small sources (less than 5 cm2) due to the problems that appear on the homogeneity of the electrical fields applied which give rise to an irregular distribution of the activity of the radionuclide on the surface, which causes a series of dark spots that deteriorate the appearance of the mentioned source.
[0005] An alternative to the electroplatting methods are the methods of direct platting of the solution of the radionuclide onto the support of the source as described in Lally et al. Nucl. Instr. Meth. 223 (1984)
[0006] 259. Among these methods, the simplest one is the one consisting in the evaporation of the solution deposited on the support. The homogeneity of the activity on the surface of the source is quite acceptable, but has the drawback that the deposit is unprotected and may readily come off.
[0007] The procedure of the invention is based on the direct platting of the radionuclide solution onto an anodised aluminium support. The radionuclide, being absorbed onto the aluminium oxide porous layer, is platted in a homogeneous way and protected.
[0008] Conventional procedures for the incorporation of stable elements (Co, Mn, Cr, etc.) onto an anodised aluminium layer are well known, and are based on the use of aqueous solutions of the elements to be incorporated under diverse conditions of temperature, pH, concentration, etc.
[0009] The application of the above mentioned procedures to the preparation of extensive radioactive sources has not produced good results, obtaining very low yields (around 2%) for the deposit of the radionuclides. Besides, as a consequence of other stable elements being incorporated to the deposit, causes an increase in the thickness of the latter which degrades the radiation energy, making more difficult the calibration of the source.
[0010] The possible causes that may produce these poor results, are thought to be:
a) The thickness of the anodised aluminium layer of the supports of the extensive radioactive sources must be at most 4 µm, so that there is no important degradation in the radiation energy, much thinner, therefore, than the minimum 10 µm used in conventional anodising procedures.
b) Conventional procedures use stable element solutions, whose concentrations are in the 10-1 M range, much above those of the radionuclide which are 10-10 M. The concentrations of the radionuclide cannot be in the same range as those of the stable elements due to the problems that working with very high radioactive activities would originate.
[0011] The process for the incorporation of the radionuclide to the aluminium oxide layer is ruled by the superficial liquid-solid interaction. It is this interaction that controls the solution volume absorbed and, therefore, the amount of radionuclide that deposits onto the porous aluminium oxide layer.
[0012] The procedure subject matter of the invention manages to increase this volume by modifying the surface tension between the liquid-solid phases; i.e., by varying the composition of the radionuclide solution it is possible to obtain a better absorption onto the anodised porous layer.
[0013] In order not to hinder the calibration of the source, it is necessary to limit to the maximum the contamination of the anodised layer, and thus it is essential to operate with a low solid residue rate.
[0014] On the other hand, a high steam pressure will allow to operate at room temperature which, besides providing easiness and economy, makes possible the platting of substances which would not endure high temperatures.
[0015] A series of tests were carried out with solutions of a radionuclide with different composition and surface tension values, and it was found that when the surface tension value was 72 din cm-1 (versus air and at 20°C) very low yields were obtained (around 2%), whilst when the surface tension value was around 20 din cm-1 (versus air and at 20°C) the yield was excellent (around 80%) .
[0016] The procedure for the incorporation of hazardous or costly substances onto an anodised aluminium support subject matter of the invention consists in the use of solutions with small surface tension values (around 20 din cm-1 versus air and at 20°C) in solutions which must present determined characteristics.
[0017] Components
- The substance to be incorporated; which must be soluble in the solvent which accompanies it.
- A solvent which has a low residue rate.
- Various additives; in order to obtain the adequate surface tension and steam pressure values, in such a way that it is not necessary to use high temperatures.
[0018] Parameters
- Surface tension; less than 25 din cm-1 versus air and at 20°C
- Steam pressure; greater than 40 mm of Hg at 20°C
- Solid residue; less than 4.5 10-3 g/l
[0019] In order to help to a better understanding of the invention, a detailed description of a preferred embodiment will follow, with an orienting and not limiting character, supported by the figure attached, being an integral part of this descriptive report and which represents the incorporation yield of a 10-10 M solution of 241Am onto an aluminium plate with an anodised thickness of 4 µm as a function of surface tension of the water-ethanol mixture used in the solution.
[0020] The reckoning of the deposit yield of the 10-10 M solution of 241Am was done in the following way: Preparation of a sample with a solution volume which is a function of the plate surface, as will be stated below. Measurement of the radioactive activity.
[0021] Preparation of a solution volume identical to the preceding one and deposition of the same onto the anodised aluminium plate in the conditions that will be described below. Measurement of the radioactive activity of the anodised aluminium plate impregnated in this manner.
[0022] The procedure for the preparation of extensive radioactive sources comprises the following stages:
A solvent - additive mixture is prepared that complies to the following conditions: surface tension below 25 din cm-1, steam pressure greater than 40 mm of Hg at 20°C and solid residue below 4.5 10-3 g/l which in this case will be made up of a water ethanol mixture with a considerable larger rate of this latter product in a proportion at least of approximately 20/80.
The 10-10 M solution of the radionuclide is prepared with the previously prepared mixture from a solution of 241Am in 4M nitric acid, by drying and change of medium.
The solution thus prepared is deposited by successive drops onto an aluminium support with a thickness of anodising of 4 µm. A volume of 1 ml is sufficient for one plate of 100 cm2.
The excess is removed after one minute, by means of blotting paper.
The deposit which may have remained on the anodised layer is removed by rubbing with paper soaked in the solvent - additive mixture used for the dissolution of the radionuclide.
[0023] Once the example of a preferred embodiment of the invention has been described, the generalisation of the procedure for the incorporation of other substances onto anodised plates will be evident for any person skilled in the art.
[0024] The solvent shall be chosen as a function of the substance that is to be incorporated. Given that the solution thus prepared will not comply, in general, to the limits established for the surface tension, steam pressure and solid residue parameters which ensure the success of the procedure, it will be necessary to include surfactant additives which must also be compatible, in terms of solubility, with the solvent used.
[0025] The last removal stage, by rubbing, of the deposit which could have remained on the anodised layer, is essential in the application of the procedure to the preparation of extensive radioactive sources, in order to ensure the homogeneity of the radioactive activity and its endurance against mechanical aggressions, but may not be essential in other applications.
[0026] As regards the industrial performance of the procedure subject matter of the invention, it will be evident for any person skilled in the art that the removal of the excess solution by means of blotting paper is an operation whose purpose is the same as the high steam pressure requirement. Thus, for the performance of the method by hand, it will be possible to operate with a lower steam pressure as a function of the ability of the operator with the blotting paper, whilst in a mechanised production installation, it will be mandatory to respect the steam pressure values specified and simplify or even leave out the stage of removal of the excess by means of blotting paper.
preparation of a solvent-additive mixture which complies to the conditions: surface tension below 25 din cm-1, steam pressure greater than 40 mm of Hg at 20°C and solid residue below 4.5 10-3 g/l,
preparation of a 10-10 M solution of the substance to be incorporated with the mentioned solvent - additive mixture,
deposition of the solution thus prepared by successive drops onto an aluminium support with an thickness of anodising of 4 µm,
removal of the excess, after one minute, by means of blotting paper,
removal of the deposit which may have remained on the anodised layer by rubbing with paper soaked in the solvent - additive mixture mentioned before.