Radio-Photoluminescence Dosimeter Glass of Low Energy Dependence and Method of Manufacturing Same

1. Field of the Invention

The present invention relates to a radio-photoluminescence glass useful in dosimetry, hereinafter designated as "RPL dosimeter glass", of low energy dependence, and to a method of manufacturing same.

2. Description of the Prior Art

The dose indication of the RPL glasses possesses within the .gamma.-energy range between 40 keV and 60 keV a maximum of the energy dependence. This is rather disturbing when using the glass in dosimetry. Its suppression necessitates considerable additional expenditures for the encapsulation and mounting of such glasses, leading to a great increase in cost, an undesired increase in weight, and difficulties in cleaning the glasses. For this reason attempts have been made to reduce the quota of photo-electrons causing said maximum of the energy dependence by developing dosimeter glasses of a low mean nuclear charge number or atomic number.

Thus replacing barium metaphosphate Ba(PO.sub.3).sub.2 which has a high nuclear charge number by magnesium metaphosphate Mg(PO.sub.3).sub.2 is described in the journal "Nucleonics" vol. 18 (1960), pages 94 and 95. U.S. Pat. No. 3,554,920 discloses special glasses based on lithium metaphosphate LiPO.sub.3 and a lithium borate of the formula Li.sub.2 O. 4 B.sub.2 O.sub.3. However, the use of said special glasses in practice is prevented by their having too low a resistance to weathering.

Despite the small proportion of activating silver in the overall composition of RPL dosimeter glasses, this low silver content, because of its high nuclear charge number, decisively affects the energy dependence in basic glass compositions consisting otherwise of light chemical elements or compounds.

Thus it is known from "Health Physics" vol. 20 (1971), pages 662 and 663, to decrease the silver content of a dosimeter glass bearing the designation "Toshiba FD-3" while otherwise the basic glass composition in only slightly modified. Such a decrease in the amount of activator, however, results in a low sensitivity.

It is to be pointed out in particular that with the exception of the above-mentioned lithium borate glass of the formula Li.sub.2 O.4 B.sub.2 O.sub.3 the basic glass composition of all RPL glasses known at present is based on metaphosphates. Examples of such dosimeter glasses with metaphosphates as glass former are described, for instance, in German Application No. 1,596,750 published for opposition, German provisionally Published Application No. 1,621,004, and Austrian Pat. No. 257,858.

In such glasses the metaphosphate can partly be replaced by oxides, at least to a limited extent as this is described in the journal "Glastechnische Berichte" vol. 45, (1972), No. 6, pages 234 to 238. However, such a replacement results also in a reduction in sensitivity. Development of RPL glasses useful in dosimetry and being based on silica was also unsuccessful due to the low solubility of silver in silicates -- as follows from the "Journal of the Electrochemical Society" vol. 95 (1949), pages 70 to 79.

Silver-activated borate glasses have too low a weathering resistance and -- as compared with metaphosphate glass compositions -- too low a sensitivity.

Thus it appears not very promising further to develop RPL dosimeter glasses based on borate or silicate.

The aforesaid publications show that the efforts of those skilled in the art to produce RPL dosimeter glasses having a low energy dependence and at the same time further desirable properties such as high sensitivity, absence of fading at room temperature, and good resistance to weathering, up to now have not found any fully satisfactory solution, although such efforts have been continued extensively.

It is one object of the present invention to provide RPL dosimeter glasses of low energy dependence, high sensitivity, and good resistance to weathering, which glasses are free of the disadvantages of the known dosimeter glasses.

Another object of the present invention is to provide a simple and effective process of making such RPL dosimeter glasses.

Still another object of the present invention is to use such glasses in radio-photoluminescence dosimetry.

Other objects of the present invention and advantageous features thereof will become apparent as the description proceeds.

In principle these objects are achieved in accordance with the present invention by replacing the metaphosphate in metaphosphate glasses by fluorine. As a result thereof, with the same cations, the mean nuclear charge number and thus the energy dependence of the glass decreases. Thus by suitable selection of cations, it was possible to achieve glass formation for predominantly metaphosphate glasses, for fluorophosphate glasses and for pure fluoride glasses.

The glasses of the present invention are characterized by the feature that they consist of a silver compound and at least one fluoride.

The fluorides may be simple and/or complex inorganic fluorides of metals of the first to fourth groups of the Periodic System. The glass may contain, in addition thereto, at least one other oxide of a metal of the first to third groups of the Periodic System, furthermore, additionally phosphorus pentoxide and also nitrogen pentoxide. A dosimeter glass in accordance with the present invention may preferably consist of 0.0001 to 15.0 %, by weight, of silver oxide Ag.sub.2 O and a corresponding weight equivalent proportion of phosphorus pentoxide P.sub.2 O.sub.5 calculated for the chemical compound AgPO.sub.3, as well as of one or more fluorides in an amount sufficient to make up the glass composition to 100 %, by weight.

According to preferred embodiments of the present invention the glass may consist advantageously of 0.001 to 10.0 %, by weight, of silver oxide Ag.sub.2 O or of 0.01 to 2.0 %, by weight, of silver oxide Ag.sub.2 O and corresponding weight equivalent amounts of phosphorus pentoxide P.sub.2 O.sub.5 calculated to yield the chemical compound silver metaphosphate AgPO.sub.3, as well as amounts of one or more fluorides to make up the glass composition to 100 %, by weight.

According to a further embodiment of the present invention, the glasses may contain 2.0 to 25.0 %, by weight, of one or more metal oxides and 15.0 to 80.0 %, by weight, of phosphorus pentoxide P.sub.2 O.sub.5. In addition thereto, 0.5 to 5.0 %, by weight, of nitrogen pentoxide N.sub.2 O.sub.5 may be present.

Other advantageous glasses in accordance with the present invention may consist of 0.02 to 2.0 %, by weight, of silver oxide Ag.sub.2 O, 0.01 to 1.0 %, by weight, of phosphorus pentoxide P.sub.2 O.sub.5, and 5.0 to 99.5 %, by weight, of at least one fluoride, whereby there may be present in addition to the above components 5.0 to 25.0 %, by weight, of at least one metal oxide and up to 77.0 %, by weight, of phosphorus pentoxide P.sub.2 O.sub.5 and furthermore 1.0 to 2.0 %, by weight of nitrogen pentoxide N.sub.2 O.sub.5. Glasses of the present invention may further contain up to 10.0 %, by weight, of at least one monovalent metal oxide and/or up to 22.0 %, by weight, of at least one bivalent metal oxide and/or up to 10.0 %, by weight, of at least one trivalent metal oxide. Thus, for instance, up to 1.0 %, by weight, of sodium oxide Na.sub.2 O and/or up to 9.0 %, by weight, of lithium oxide Li.sub.2 O, up to 3.0 %, by weight, of magnesium oxide MgO, and up to 4.0 %, by weight, of calcium oxide CaO, or up to 22.0 %, by weight, of barium oxide BaO and up to 10.0 %, by weight, of aluminum oxide Al.sub.2 O.sub.3 may be present.

The glasses of the present invention may contain, furthermore, up to 50.0 %, by weight, of at least one monovalent metal fluoride and/or up to 65.0 %, by weight, of at least one bivalent metal fluoride and/or up to 35.0 %, by weight, of at least one trivalent metal fluoride. Thus, for instance, up to 49.0 %, by weight, of lithium fluoride LiF and/or up to 5.0 %, by weight, of potassium fluoride KF, up to 10.0 %, by weight, of beryllium fluoride BeF.sub.2 or up to 10.0 %, by weight, of magnesium fluoride MgF.sub.2 and/or up to 32.0 %, by weight, of calcium fluoride CaF.sub.2 and/or up to 12.0 %, by weight, of strontium fluoride SrF.sub.2 and/or up to 15.0 %, by weight, of barium fluoride BaF.sub.2 and up to 6.0 %, by weight, of lanthanum fluoride LaF.sub.3 and/or up to 31.0 %, by weight, of aluminum fluoride AlF.sub.3 may be present.

The process of producing an RPL glass according to the present invention is characterized by the feature that the glass is melted down from a mixture which consists of at least one fluoride and a silver compound.

The silver compound may be silver metaphosphate (AgPO.sub.3), silver nitrate (AgNO.sub.3), silver carbonate (Ag.sub.2 CO.sub.3), or silver lithium metaphosphate (LiAg(PO.sub.3).sub.2).

Glasses according to the present invention may advantageously be melted down also from a mixture which consists of 5.0 to 99.5 %, by weight, of at least one fluoride and 95.0 to 0.5 %, by weight, of at least one metaphosphate. In this connection the fluorides can be replaced partially or entirely by at least one hydrogen fluoride or a complex inorganic fluoride. In addition, this mixture may also contain up to 3.0 %, by weight, of sodium nitrate NaNO.sub.3.

Another advantageous process of producing a glass according to the present invention is characterized by melting down a mixture consisting of 0.0001 to 15.0 % by weight, of silver metaphosphate AgPO.sub.3 and an amount of at least one fluoride which makes up the mixture to 100 %, by weight. The mixture may also consist of 0.001 % to 10.0 %, by weight, of silver metaphosphate AgPO.sub.3 and an amount of at least one fluoride sufficient to make up the mixture to 100 %, by weight.

It is furthermore of advantage to provide a mixture which consists of 0.1 to 1.0 %, by weight, of silver metaphosphate AgPO.sub.3 and 99.0 to 99.9 %, by weight, of at least one fluoride. Such a mixture may additionally contain mono- and/or bi- and/or trivalent metal metaphosphates.

One particular advantageous embodiment of the process of producing a dosimeter glass in accordance with the present invention is characterized by melting together a mixture which contains 0.05 to 3.0 %, by weight, of silver metaphosphate AgPO.sub.3 ; up to 49.0 %, by weight, of lithium metaphosphate LiPO.sub.3 ; up to 57.0 %, by weight, of the alkaline earth metal metaphosphates of magnesium Mg and/or calcium Ca and/or barium Ba, the amount of magnesium metaphosphate Mg(PO.sub.3).sub.2 being up to 21.0 %, by weight, of amount of calcium metaphosphate Ca(PO.sub.3).sub.2 up to 14.0 %, by weight, and the amount of barium metaphosphate Ba(PO.sub.3).sub.2 up to 36.0 %, by weight; up to 49.0 %, by weight, of aluminum metaphosphate Al(PO.sub.3).sub.3 ; up to 5.0 %, by weight, of potassium hydrogen fluoride KHF.sub.2 ; up to 48.0 %, by weight, of lithium fluoride LiF; up to 65.0 %, by weight, of alkaline earth metal fluorides such as the fluorides of beryllium Be and/or magnesium Mg and/or calcium Ca and/or strontium Sr and/or barium Ba, whereby the amount of beryllium fluoride BeF.sub.2 is up to 10.0, by weight, the amount of magnesium fluoride MgF.sub.2 up to 10.0%, by weight, the amount of calcium fluoride CaF.sub.2 up to 31.0 %, by weight, the amount of strontium fluoride SrF.sub.2 up to 11.0 %, by weight, and the amount of barium fluoride BaF.sub.2 up to 15.0 %, by weight; up to 30.0 %, by weight, of aluminum fluoride AlF.sub.3, and up to 5.0 %, by weight, of lanthanum fluoride LaF.sub.3.

On a more detailed investigation of glass formation in the metaphosphate-fluoride system the following results are obtained: The glassiness, i.e. the reciprocal of the crystal growth rate, decreases only slightly with fluoride contents up to 20 mol%, but it decreases more strongly with higher fluoride contents. It passes through a minimum at 50 mol% of fluoride content, increases again in the region of 70 mol%, and finally decreases very strongly for fluoride contents of more than 90 mol%. This pattern of the degree of glassiness is due to the superimposition or interference of the tendency towards crystallization which increases with increasing fluoride content, and of the maximum of the tendency towards segregation at a fluoride content of 50 mol%. It has been found that glass formation can be facilitated by providing readily polarizable cations and the greatest possible number of components.

The invention will be described more in detail hereinafter with reference to the attached drawings in which:

FIG. 1 shows predose and sensitivity as a function of the fluoride content;

FIG. 2 shows band position and half-width value of the luminescence bands produced by irradiation, at half of its maximum intensity as a function of the fluoride content;

FIG. 3 shows build-up behavior after irradiation with 300 R of a cobalt-60 source for different fluoride contents as parameter at 20.degree. C.;

FIG. 4 shows fading behavior of the glass 2025 at different storage temperatures;

FIG. 5 shows maximum energy dependence as a function of the fluoride content;

FIG. 6 shows energy dependence of different known RPL dosimeter glasses as a function of the average nuclear charge number.

FIG. 7 shows position and half-width value of the luminescence bands produced upon irradiation with cobalt-60 as a function of the silver content;

FIG. 8 shows predose and sensitivity as a function of the silver content;

FIG. 9 shows build-up behavior at room temperature for different silver contents;

FIG. 10 shows build-up behavior at 100.degree. C. for different silver contents;

FIG. 11 shows maximum energy dependence and mean nuclear charge number as a function of the silver content.

As predose indicated in Roentgen (R), there is designated the .gamma.-dose of a cobalt-60 source with which a glass must be irradiated in order to double the luminescence intensity present prior to irradiation.

The absolute sensitivity depends on the overall evaluation system. Therefore, it is indicated in relative units for the following investigations. From the examples of these Tables it will be seen that it is possible to replace, within wide limits, the metaphosphate group by fluorides. Even fluorine glasses which contain only a very minor amount of metaphosphates still show the effect of radio-photoluminescence.

In order to develop RPL glasses of low energy dependence it is advisable to use preferably fluorides of a low nuclear charge number and to select the composition in such a manner as to obtain glasses which are resistant to weathering.

In Table 2 there are given other examples of glass mixture compositions in which certain dosimeter properties such as the predose, the sensitivity, and in addition the energy dependence, i.e. the ratio of the luminescence intensity at 40 keV to the luminescence intensity at 1 MeV, are given.

As is evident from these Tables, the theoretically optimum value 1 for the energy dependence is practically achieved, within the limits of error, by Melt No. 2026 in Table 2.

In attached Tables 3 and 4, the glass mixture components set forth in Tables 1 and 2 --namely the metal metaphosphates and the sodium nitrate -- have been split up into oxidic components and phosphorus pentoxide P.sub.2 O.sub.5 and nitrogen pentoxide N.sub.2 O.sub.5.

In addition thereto there have also been indicated the molar ratio of the fluoride components to the phosphate components and the weight ratio of the fluoride components to the phosphate components.

FIG. 1 of the attached drawings shows in its upper part the sensitivity of an RPL glass according to the present invention as a function of the fluoride content. The sensitivity increases slightly up to about 50 mol% of fluoride and then decreases slightly from 50 mol% to 80 mol%; for higher fluoride contents, it decreases much more strongly.

In the lower part of FIG. 1, the predose is shown as a function of the fluoride content. At a content of about 35 mol% of fluoride, the predose shows a first maximum, and at 75 mol% a minimum. This course of the curve is explained by the maximum of the tendency towards segregation at 50 mol% and by the strongly decreasing solubility of the silver with high fluoride contents. The black circles indicate melts of Table 1 and the unfilled circles melts of Table 2.

In the upper part of FIG. 2 there is shown the band position of the bands in nm. as a function of the fluoride content. It passes through a minimum at about 45 mol% of fluoride. In the lower part of FIG. 2 there is indicated the halfwidth of the luminescence band produced by irradiation with cobalt-60 as a function of the fluoride content. FIG. 2 illustrates melts of Tables 1 and 2.

The half-width depends on the environmental symmetry and the possible energy positions of the center. An increase in the fluoride content creates new possibilities of incorporation of the center, leads to new energy positions, and thus to an increase in the half-width of the band. In the region of 45 mol% of fluoride, structures of a higher order are formed due to the higher tendency towards segregation, and as a result thereof the environmental symmetry of the center increases, while due to said increase the band half-width decreases. The superimposition of these opposite effects leads to the formation of a minimum in the lower part of FIG. 2. Displacement of the band position towards smaller wave lengths means, in accordance with the ligand field theory, a decrease in the interaction of the center with its environment. The possibility of radiation-free energy transfer decreases. In agreement therewith, FIG. 1 shows that the luminescence yield -- i.e., the sensitivity -- has a maximum at about 50 mol% of fluoride.

The reason for this is the more self-contained glass structure of higher order at 50 mol% of fluoride, as a result of which the centers are isolated in the glass structure.

In FIG. 3 there is shown the build-up behavior of the RPL dosimeter glasses of the present invention containing a percentage of silver phosphate AgPO.sub.3 of 0.5 mol% for different fluoride contents between 0 mol% and 86.1 mol%. It is clearly evident that the RPL kinetics slows down with an increase in the fluoride content. The fact that an increasing fluoride content leads to a more rigid glass network is a known fact in glass technology. When recalling that diffusion processes are necessary for the development of the RPL centers, it explains the laws governing the RPL kinetics. This represents another advantage of the RPL glasses according to the present invention with respect to integrating dosimetry. The slow RPL kinetics prevents fading phenomena even at higher temperatures. FIG. 3 illustrates melts of Table. 1.

This becomes clearly evident from FIG. 4 in which the fading behavior at different storage temperatures between 20.degree. C. and 250.degree. C. is shown for a glass with melt No. 2025 as given in Table 2. Up to a temperature of about 100.degree. C., substantially no fading occurs. In contrast thereto, the known metaphosphate glasses show a considerably higher fading when stored at the same temperatures.

FIG. 5 shows the energy dependence, i.e., the ratio of the dose indication at 40 keV to the dose indication at 1 MeV, for a silver phosphate AgPO.sub.3 concentration of 0.5 mol% as parameter as a function of the fluoride content. The energy dependence decreases with increase in the fluoride content, since with the same cation upon replacement of the metaphosphate group by fluorine, the means nuclear charge number decreases. As long as an upper limit of the fluoride content which is dependent on the composition of the cations is not exceeded, the RPL glasses of the present invention have, for the same silver concentration, a higher sensitivity and a lower energy dependence than a pure metaphosphate glass. A decrease in the activator concentration leads -- as in all previously known dosimeter glasses -- to a lower sensitivity. In the RPL dosimeter glasses of the present invention it is possible, however, -- as shown by Melt No. 2026 in Table 2 -- to obtain, with relatively high sensitivity, practically complete absence of dependence of the dose indication on the .gamma.-energy.

In FIG. 6 there is shown the energy dependence of previously known RPL dosimeter glasses as a function of the mean nuclear charge number Z. For Z the following mathematical relationship applies: ##EQU1## In said formula: Z indicates the mean nuclear charge number;

Z.sub.i indicates the nuclear charge number of the element i;

x.sub.i indicates the molar fraction of the element i;

In FIG. 6, the letters a to k stand for the following RPL dosimeter glasses:

The composition of the tested glasses is given in Table 5 on page 26.

As can be seen, the RPL dosimeter glasses a to c of the present invention are distinguished by their low mean nuclear charge numbers which lie between 10 and 12 and their lower dependence on energy over the known glasses indicated by letters d to k. For low silver contents, the energy dependence become negligible small.

In order to compare the weathering resistance of the RPL dosimeter glasses of the present invention with that of known glasses, experiments were carried out in accordance with a weathering test usually employed in the optical industry. Such as test is described in the "Glass Catalog 3050" of the firm Schott & Gen. of Mainz, Germany, dated October 1966 on page III-3. The tested glasses are subjected in accordance therewith to a periodically varying steam atmosphere. The following three glasses were examined:

a. Glass of Becker (U.S. Pat. No. 3,554,920) having a base of lithium borate of the formula Li.sub.2 O.4 B.sub.2 O.sub.3 ;

b. "Toshiba FD-7" glass corresponding to the glass indicated by the letter d in FIG. 6;

c. Glass No. 2025 in accordance with Table 2 of the present invention.

The surfaces of said three glasses of low energy dependence exhibited after weathering the following changes:

Glass (a): Very strong surface attack.

Glass (b): Pronounced corrosion.

Glass (c): Scarcely detectable traces of an attack of its surface.

The glass of the present invention, therefore, has a surprisingly superior resistance to corrosion. Further tests show an increasing improvement in the resistance to weathering with an increase in the aluminum fluoride/lithium fluoride AlF.sub.3 /LiF ratio.

In addition to the glass base, the activator concentration is also of controlling influence on the dosimetric properties. To prove this, the silver concentration was varied in an RPL glass of the present invention. The basic glass mixture had the following composition:

26.0 %, by weight, of lithium fluoride, LiF,

9.0 %, by weight, of aluminum flouride, AlF.sub.3,

40.0 %, by weight, of aluminum metaphosphate, Al(PO.sub.3).sub.3, and

25.0 %, by weight, of lithium metaphosphate, LiPO.sub.3. This glass has a specific gravity of 2.42 g./cc., a coefficient of expansion of 17.8 . 10.sup..sup.-6 .degree.C..sup..sup.-1 , a transformation temperature of 321.degree. C., and an Abbe number of 73.3. As a result of the small index of refraction n.sub.e = 1.4993, only slight losses in reflection occur on evaluating the dose indication. The resistance to weathering is equivalent to that of optical glasses of the FK type (fluor crown glass type).

The upper part of FIG. 7 shows the position and its lower part the half-width of the luminescence band as it is obtained after irradiation with cobalt-60 as a function of the silver content. UV excitation was effected at 385 nm. With an increase in the silver content, the band maximum shifts towards shorter wave lengths and the half-width increases.

The predose is plotted in the lower part of FIG. 8 and the sensitivity in its upper part, in each case as a function of the silver content. The sensitivity passes through a maximum, while the predose increases initially only slightly, but subsequently very substantially.

FIG. 9 shows the build-up behavior at a temperature of 25.degree. C. and FIG. 10 to the build-up behavior at a temperature of 100.degree. C., in each case for different silver contents. In this connection the glasses represented by curves I to V have the following silver contents:

The upper part of FIG. 11 shows the maximum energy dependence and the lower part the mean nuclear charge number Z as a function of the silver content. In this connection there have also been entered in horizontal dashed lines the mean nuclear charge numbers of non-activated lithium fluoride LiF and of non-activated metaphosphate glass as compared with a non-activated fluoride glass.

The fading was also determined for different silver contents at different temperatures, namely at 25.degree. C., 100.degree. C., and 200.degree. C., as a function of the storage time. Thereby a high stability of the dose indication was found.

The glasses of the present invention are produced by melting down the mixture of the respective components which should be as pure as possible in a covered quartz crucible in a weakly oxidizing atmosphere in an electric furnace. Melting time and melting temperature are reduced with increasing fluoride content. While pure metaphosphate glass requires 40 minutes to melt at 1380.degree. C., 5 minutes at 1300.degree. C. is sufficient for a glass consisting of 30 mol% of fluoride components and 5 minutes at 1100.degree. C. is sufficient for a glass consisting of 70 mol% of fluoride components. The low viscosity and the low surface tension of fluoride-containing melts facilitate homogenizing of the glass melt. This is a great advantage in the production of the fluoride-containing RPL dosimeter glasses over the known glasses. The costs of manufacture are comparatively low due to the short melting times and the relatively low melting temperature; due to the good homogeneity of the melt, continuous agitation is also unnecessary, so that the possible introduction of additional impurities is excluded. Due to the short melting time and the low melting temperature of glasses of high fluorine content, contamination by the material of the melting crucible and by the environment causing increase in the predose, is relatively small. The casting or molding plate temperature also decreases with an increase in the fluorine content.

If the glasses are not cast in the cubic shapes, they are cut into cubic shape after pre-annealing them at 280.degree. C. to 450.degree. C. in a similar manner as this is done in the manufacture of optical glasses. The surfaces of the glasses are then ground and polished. The dosimetric properties of the RPL dosimeter glasses thus obtained can then be determined, after irradiation with a cobalt-60 source, by means of a dosimeter evaluator.

It is within the scope of the present invention to add oxidic additives, for instance, lithium oxide Li.sub.2 O, beryllium oxide BeO, and boron trioxide B.sub.2 O.sub.3 in an amount of a few percent to the melt mixture for glass technological reasons. The use of other crucible materials in place of quartz and the use of other types of furnaces in place of an electric furnace is also within the scope of the present invention.

The novel improved RPL dosimeter glasses according to the present invention have proved to be useful, for instance, in high-dose-level emergency, such as military or Civil Defense dosimetry, in routine low-level and accident high-level dosimetry in nuclear installations, for the measurement of low energy X-rays in medical installations, and for other purposes for which the known RPL dosimeter glasses are used. Such uses are described, for instance, by Klaus Becker in "Health Physics" vol. 14, pages 29-31 (1968).