The greatest need, however, is for a viscosity curve of great accuracy that can be accepted as standard to take the place of Table I. Next in importance, there should be available standard samples of glass representing a wide range of softness temperatures, annealing points, and strain points. Some parts of such a program are under way. It is hoped that the practical importance of the program will be recognized and that special effort will be made to complete the work. In the meantime, it will be necessary to make use of artificial standards such as that presented in Table I.
LITERATURE CITED
(1) Boow, J., and Turner, W. E. S., J. SOC.Glass Technol., 26, 216 (1942). (2) Fuloher, G.S., J. Am. Ceram. Soc., 8,339 (1925). (3) Lillie, H.R.,Ibid., 14, 602 (1931). (4) Loomia, W.E., and Sharp, D. E., unpublished report. ( 5 ) Othmer, D. F., and Conwell, J. W., IND. ENG.CHEM., 37, 1112 (1945). (6) Reynolds, O., Phil. Trans. London, 177A, 157 (1886). (7)Vaughan, T. C., unpublished report. RECEIVED for review March 23, 1953.
ACCEPTEDOctober 14, 1953.
n
Some Effects of High Energy Radiation on Glass NORBERT J. KREIDL
U
Bausch & Lomb Opbical Co., Rochester, N . Y .
As atomic energy uses increase, glass remaining transparent under conditions of radiation becomes necessary for optical instrumentation. NIost optical glass when exposed to radiation becomes colored in a manner which may be considered from the viewpoint of well-known effects in crystals. Cerium inhibits coloration due to irradiation; this mechanism is being studied. Glasses are developed to optical specifications incorporating cerium to make them insensitive to stated levels of radiation. In addition, glass is used for absorbing radiation and for dosimetry.
A
S USES of atomic energy expand through the nation,
objects and reactions must be observed through glass under more and more powerful radiation of high enegy. Therefore, windows and optical instruments-microscopes and periscopes-are required for detailed observation under exposure to radiation with the specification that, in some caEes, they absorb this radiation and, in all cases, they retain reasonably permanent transparency. The rather limited purpose of this presentation is to summarize broadly work that has been done and that is under way in the laboratory of the author, partly under the sponsorship of the Atomic Energy Commission and the Bureau of Ships, as well as in other laboratories. A more factual paper, on some phases of research on high energy radiation, with documentation of experimental data and literature, will be forthcoming a t a later stage of the current investigation of these phases. For other phases, an attempt is made to refer in this summary to some of the most pertinent sources of information published t o date. DISCO LORATION
Almost all ordinary and optical glasses were found to discolor, starting a t certain intensities of gamma radiation, and a t other intensities the discoloration became severe. An optical borosilicate crown, for instance, retained only 4% transmittance a t 450 mp, a t 10-mm. thickness, when irradiated with 1 X 106 roentgen units of gamma rays. One roentgen unit is that amount of radiation passing through pure air, under standard conditions, which will produce 1 electrostatic unit of ions, of either sign, per cubic centimeter. At still greater intensities of radiation, say 1 X 1010 roentgens, physical damage-shattering-may occur.
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There is evidence that this discoloration is related in kind to the beautiful colors that were observed in rock salt crystals in the early days of radioactivity research. Since then the coloration of crystals by gamma radiation has been the subject of much experimentation and speculation, and the results can be applied, with some caution, to glass technology. An extensive review of this subject, including a bibliography, was published as part of a study under the sponsorship of the Bureau of Ships (15’). SfOSt Of COLOR CENTER FORMATION I N DEFECTLOCAWIONS. these effects are believed due to the trapping of electrons in placee where a negatively charged ion is missing-for instance, by some “error” in the growth process of the crystal (vacancy). Electrons so located seem to behave like those situated in certain levels of coloring ions and form what are known a8 “color centers.” Color centers formed by electrons knocked off by gamma radiation and trapped in sites somewhat analogous to such vacancies may be the cause of a large portion of the discoloration of glass so irradiated. The detailed relations are still fairly obscure, as many of the absorption bands which could offer explanation are in the ultrcG violet range where the glass proper absorbs, and, therefore, remain hidden, Recent French work on pure silica glass ( 4 , 6 ) ,which is very transparent in this ultraviolet region, has brought forward detailed information on color oenter formation and liquidation which may be cautiously applied to more complex glasses. INHIBITION BY CERIUM. Cerium was found to inhibit (8, 15’1, to a considerable extent, discoloration due to gamma radiation up to the range of 106 roentgens, if added in amounts of the order of 1%. The study of this mechanism is under way (a). Phos-
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Ceramics and Glass phate glasses were found suitable because of their transparency in the ultraviolet. They reveal very sharp bands that may be associated with the inhibition mechanism. Cerium(II1) also fluoresces, and this fluorescence may serve as an indicator in the observation of the inhibition process. Atomic energy research has coined the term “protection” for this inhibition, and glasses reasonably stable against discoloration are termed “protected glasses.” The term “nonbrowning” is also used, particularly where electron beams are under consideration. PROTECTED (NONBROWNING) GLASSES
The next task was the development of a series of protected glasses that would permit a minimum of periscope, and other instrument design. First, the fine effect of cerium on refractive index and dispersion was established, and glasses were synthesized that allowed duplication of existing types to =tO.OOl in index (TLD) and +0.3 in reciprocal relative dispersion [V = ( n ~ - 1 ) / ( n -~nc)]. Next, the series of minimum design elements was arbitrarily specified as one borosilicate crown (%D = 1.517, V = 64.5), one extra dense flint (TLD = 1.649, V = 33.8), one dense flint (TID = 1.617, V = 36.6), and two light barium cromns ( n =~ 1.541, V = 59.9 and TLD = 1.573, V = 57.4). The minimum cerium levels to obtain sufficient protection were established, and usually found of the order of 1% (0.5 to 2%), when protection was defined as a change in absorbance below 0.2 when subjected to 1 X 106 roentgens of gamma radiation (1.2 m.e.v.). I n a glass of average reflection this would mean a residual transmittance better than about 75y0 for 1-em. thickness a t any wave length. These definitions were arrived a t by some compromise which involves the fact that, depending upon the glass type, additions of the order of 1%Ce20a cause color in itself. This color is a rather pure yellow that does not affect total transmittance too much. It is noi5- thought that some such color should be accepted if protection against 1 x 106 roentgens is obtained; alternative types of more intrinsic color might be developed for higher radiation resistance. ABSORPTION OF RADIATION
The present work, sponsored by the Atomic Energy Commission, is not concerned with this problem. However, a summary of the author’s previous work and that of many others would have to state: Light nuclei (beryllium, lithium, boron) as sole constituents of glass provide x-ray windows. Present work, mostly abroad, is limited to details of composition fields (7). Heavy nuclei (lead, barium) as major constituents of glass generally absorb x-rays and gamma rays best, although there are specific absorption peaks for certain energy ranges (1). High lead (“extra dense”) optical flint is usually suitable. The lead tungsten phosphate glass developed by Rothermel,
January 1954
Sun, and Silverman ( 9 ) , that has a superior absorption, althou b F h a p s not enough for general use a t present cost, has recen8y een perfected for limited production (IS). Specific nuclei, cadmium in particular, combined with the most suitable glass former, boron stops slow neutrons. Glasses based on cadmium borates (2, 6 ) have been developed. Some combinations with lead give fair absorptions for mixed radiation (Q DOSIMETRY
The reactions of glass with high energy radiation can be used t o detect these radiations quantitatively. The best known dosimeter glass was invented a t the Naval Research Laboratory by Schulman (11)in cooperation with others on the basis of composition researches by Weyl (14). This silver-containing phosphate glass was investigated for improvements and developed to full production scale in the author’s laboratories. The silver in the glass becomes fluorescent in proportion to gamma radiation received, and a black light with a photocell represents the instrument that evaluates glass samples carried in a housing on a dog tag. Research also goes on in these laboratories on using color changes the same way; the most promising type, however, leads from glass to plastics in which sensitive powderse.g., strontium sulfate: sodium (lO)-are properly embedded, as also proposed by Schulman. LITERATURE CITED
(1) Brewster, G. F., J . Am. Ceram. S O C 35, , 194-7 (August 1952). (2) Brewster, G. F., and Kreidl, N. J., Ibid., 35, 259-64 (1952). (3) Kreidl, N. J., “Irradiation Damage to Glass,” Atomic Energy Commisaion, AEC Rept. NYO-3777 (1953). (4) Kreidl, N. J., Glass Id., 34, 127-31, 158-9 (March 1953). (5) hlayer, G., and Gueron, J., J. chim. phys., 49, 204-12 (April 1952). (6) Melnick, L. M., Safford, H. W., Sun, K. H., and Silverman, A,, J . Am. Ceram. SOC.,34,82-6 (March 1951). (7) Menzel, H., and Adam, J., Glastech. Ber., 25 (ll), 354-61 (1952). (8) Monk, G. S., Nucleonics, 10, 53 (November 1952). (9) Rothermel, J. J., Sun, K. H., and Silverman, A., J . Am. Ceram SOC..32. 153-62 (1949). (10) Schulman,’J.H., Ginthe;, R. J., and Kirk, R. D., J. Chem.Phys., 20 (12), 1966-7 (1952). (11) Schulman, J. H., Ginther, R. J., and Klick, C. C., J . Appl. Phya., 22, 1479-87 (1951). (12) Sun, K. H., and Kreidl, N. J., Class Ind., 33, 511-14, 589-94, 651-2 (October, November, December 1952). (13) Weidel, R. A., J . Opt. SOC.Amer., 43, 540-1 (June 1953). (14) Weyl, W. A., Schulman, J. H., Ginther, R. J., and Evans, L. W., J. Electrochem. ~ o c . 95, , 70-9 (1949). RECEIVED for review April 14, 1953. A C C ~ P T ~Ootober D 15, 1853. Important phases of this work are sponsored by the Atomic Energy C o m b sion. Experiments were carried out by the staff a t the Argonne National Laboratory under K. R. Ferguaon and a t the B a u d & Lomb Optical Co. by Tyler Pett and Raymond Hensler.
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