US3464932A - X-ray absorbing glass compositions - Google Patents

Description

lfl- 1969 J. H. CONNELLY ETAL ,93

X-RAY ABSORBING GLASS COMPOSITIONS Filed Sept. 6. 1968 o a 6 4 2 4 2 l.

WAVE LENGTH IN K UNITS 5m mw MM EH I, W e 0 ho A 0 2% United States Patent M US. Cl. 252-478 4 Claims ABSTRACT OF THE DISCLOSURE This invention relates to the production of alkali metal silicate glasses especially suitable for face panels of television picture tubes operating at extremely high voltages in that these glasses exhibit a high absorption of X-radiation within the range of wavelengths of 0.330.77 A., thereby inhibiting the penetration of X-rays therethrough into the surrounding environment. Such glasses contain up to about 20% by weight SrO'.

X-radiation is always produced when moving electrons are decelerated or stopped due to collisions with the atoms of a substance. The intensity of this X-radiation is a function of the accelerating voltage, the electron current, and the atomic number of the material bombarded.

Commercial X-ray tubes are designed for maximum elficiency. Electrons from a hot cathode are focussed into a small spot and accelerated to the anode or target. Most of the energy is converted to heat and the target must be able to resist high temperatures. The tungsten employed for the target satisfies this requirement, as well as having a high atomic number. A television picture tube contains the same basic elements as an X-ray tube, viz., a focussed beam of electrons and a high D.C. accelerating voltage. The target from which the X-radiation is emitted is the aluminum film, the phosphor screen, and the glass walls of the bulb. In a color television picture tube, the shadow mask is the principal target.

In contrast to the X-ray tube, the X-ray source in a television picture tube is a very large one, being equal to the scanned area of the screen. Because of the large size of the source and because the measurements of X-ray intensities are undertaken close to the picture tube, the observed dose rates are a complicated function of the distance between the tube and the detector and of the location of the detector with respect to the tube. In any event, when the electron beam in a television picture tube operating at 25,000 volts (25 kv.) strikes the phosphor screen, the kinetic energy of the electrons has been determined to be transformed into other forms of energy as follows:

Percent Heat 0 80 Light c 20 X-rays c 0.25

It has been demonstrated that the intensity of X-radiation at the target varies directly with the square of the voltage, other factors being constant. However, because of the heavy filtering character of the bulb glass, the X-ray exposure rate external to the tube actually varies at about the 20th power of the accelerating voltage. Therefore, upon the advent of color television with the accompanying need for higher operating voltages and with the prospect of higher operating voltages in black and white television receivers, the matter of X-ray protection i of growing concern.

The absorption of X-rays by an elementary material is dependent upon the wavelength of the radiation, the density and thickness of the material, and the mass absorption 3,464,932 Patented Sept. 2, 1969 coefiicient thereof. The mass absorption coefiicient is the unit used to express the absorbing characteristics of material. In the case of X-rays, the mass absorption coefiicient is independent of the physical state of the material and can be applied to gases, liquids, and solids. In a compound, or a mixture such as glass, each component element absorbs independently of the others. The total absorption, then, is the summation of these separate independent absorptions. Thus, the mass absorption coefiicient of a mixture is determined through summation of the contribution of the components as follows:

mixture 2 (i e X e) where f =weight fraction of each component element mixture and w =mass absorption coefiicient of each component element.

In general, the mass absorption coefficients of the elements increase with increasing atomic number, so that to increase the mass absorption coefficient of a compound of mixture, an element having a higher atomic number will commonly be substituted for an element with a lower atomic number. Further, the mass absorption coefficient of a given element normally increases with increasing wave length of the incident X-rays. In view of these factors, the reasonable approach to enhance the resistance of a glass to X-radiation transmission would be to incorporate a heavy element therein. The relative impenetrability of lead to X-rays is well known in the art. However, a reaction occurs between the lead in a glass and the impinging high voltage electrons such that a brown-to-black discoloration may appear when sufficient lead is present to inhibit the transmission of X-rays to a desired level. Thus, the bombardment of high voltage electrons upon the glass develops this discoloration which may be due to impingement of the electrons on the glass and/or the exposure of the glass to the resulting X-radiation. The same situation occurs where other readily reducible heavy metal oxides have been included in the glass compositions.

It can be appreciated that, while discoloration in the funnel portion of a television picture tube is of essentially no importance since that area is not viewed and the discoloration does not affect the operation of the picture tube, such coloring is undesirable in the face plates of black-and-white television receivers and even less tolerable in color television picture tube face panels. Thus, the use of lead in the glass comprising the funnel portion of television picture tubes is conventional today and, in black-and-white television receivers, a small amount has been included in the face panels with the resultant browning being masked with various ingredients, such as MnO, to produce a neutral color in the glass. Nevertheless, as the operating voltages of television receivers have been increased to exceed 20 kv., more lead has been required to absorb the resultant X- radiation and the concomitant browning has become more severe. This has resulted in greater masking coloration being necessary to yield a neutral color which, in turn, has led to some decrease in the brightness of the black-and-white picture. And, of course, such browning is even less acceptable in color television tube face plates. This factor has led to the use of the heavy barium in the glass composition employed in the manufacture of television picture tube face plates, especially in color television where even slight browning is intolerable. Such glasses are disclosed in United States Patent No. 2,527,- 693. However, the efiiciency of barium in reducing the transmission of X-radiation is not as great as would be desirable such that with higher and higher operating voltages being employed in television picture tubes, X-ray transmission through the glass becomes a problem of considerable concern. Two apparent, but commercially unattractive, solutions to the problem would be to increase the amount of barium in the glass composition and/or increase the wall thickness of the glass tube. The first proposed solution results in melting problems whereas the second leads to a heavier unit and a more expensive tube since the quantity of glass is greater.

Therefore, the primary object of this invention is to provide a glass suitable for a television picture tube which exhibits very high X-ray absorption and which will not become unsightly discolored when subjected to the impingement of high voltage electrons.

We have discovered that this object can be achieved by including strontium in the glass composition up to about 20% by weight, reported as SrO.

For each element, there are a number of characteristic wave lengths at which the mass absorption coeflicient undergoes a marked decrease for a slight increase in wave length. This wave length is called a critical absorption wave length or absorption edge for the element. These absorption edges are related to the characteristic X-ray emission lines of the elements. The wave lengths of the absorption edge corresponds to the smallest quantum required to excite the characteristic line associated with the edge.

The characteristic emission lines are related to electron energy transferse within the atom. The spectra are designated by the letters K, L, M, N and 0. There is one absorption edge for the K energy level whereas there are-three L edges, five M edges, seven N edges, and nine edges. The K energy level is the most vital and for strontium the K absorption edge is at 0.77 A., while for barium the K absorption edge is at 0.33 A. This results in the mass absorption coefficient of strontium being greater than barium between these two wave lengths. This range of X-ray wavelengths happens to encompass the principal part of that emitted by television picture tubes operating at voltages greater than about 20 kv.

The appended drawing graphically depicts the mass absorption coefficients of BaO and SrO at various wave lengths and illustrates the K absorption edges thereof.

Hence, whereas barium has a higher atomic number than strontium with an accompanying higher atomic weight such that it would be assumed that barium would be a better absorber of X-radiation than strontium, at the high voltages at which a television picture tube operates, particularly in the case of color television, the absorption edge of barium occurs in the lower range of wave lengths of the X-rays being emitted such that the lower atomic number, lower weight strontium is a more eflicient absorber of X-rays than barium. It is this circumstance, then, which forms the basis for our invention.

Thus, glasses suitable for our invention reside in the R O--SiO field, wherein R 0 consists of Na O and/or K 0, to which up to about 20% SrO is added. Particularly useful glasses consist essentially, by weight on the oxide basis, of about 420% R 0, 0-l0% A1 0 40-70% SiO and 1-20% SrO, wherein R 0 consists of 0-10% Na O and 013% K 0, the sum of R 0, SiO and SrO constituting at least 70% by weight of the total composition. Various compatible metal oxides in the indicated amounts may be included to aid in melting or working the glass as well as modifying the physical and chemical properties thereof. MgO, CaO, and ZnO may be present in amounts totalling 15% by weight and BaO may be present up to about 20% by weight. However, since SrO is a much more efficient absorber of X-radiation at the wave lengths in question, the presence of BaO for that purpose is superfluous. Rb O and Cs O may be substituted for K 0 but at present are too costly for general commercial use. Li O should not be included in amounts over about by weight. ZrO and A1 0 may be present in amounts less than 10% to raise the annealing point of the glass and improves the chemical durability thereof. Fluorine, in amounts less than about 2% by weight fluoride, may be added as a melting aid. Various fluxes such as B 0 and P 0 may be present but, preferably, in amounts less than about 5% by weight each. PbO, while desirably absent, may be included in amounts up to about 3% by weight. And, in accordance with conventional practice in the manufacture of glass for television receiver tubes, various coloring agents, e.g., C0 0 Cr O V 0 CuO, and NiO, may be present in very small amounts to impart a neutral shade to the glass. Finally, conventional fining agents such as AS203 and Sb O may be added, where desired.

Whereas the addition of even a very small amount of SrO to the glass composition will be effective in improving the X-radiation absorption qualities thereof, we have found that at least about 1% by weight is required to demonstrate a truly significant effect. Where quantities greater than about 20% by weight are employed, the glass tends to become unstable. Therefore, we prefer to utilize SrO in amounts ranging between about 5-15 by weight.

The following table reports glass compositions, expressed in weight percent on the oxide basis, illustrating the effectiveness of SrO in providing glasses having excellent X-ray absorption qualities. The batch ingredients may comprise any materials, either the oxides or other compounds, which, on being melted together, are converted to the desired oxide compositions in the proper proportions. Since it is not known with which cation fluorine is combined in the glass structure, it is reported separately as fluoride in accordance with conventional glass analytical practice.

In the specific examples recorded in the table, the batch ingredients were compounded, mixed together to aid in obtaining homogeneous melt, and then melted in open platinum crucible at 1450-1500 C. for about four hours, the molten batch being stirred to insure a homogeneous melt. The melts were thereafter poured into steel molds, 6" x 6" x 1", and transferred to an annealer operating at about 480500 C. The glass shapes were removed from the molds, ground and polished, and then tested for the transmission of X-radiation.

TABLE I Percent Percent The mass absorption coeflicients of the oxides conventionally used in television picture tube glasses and those of S10 are recorded in Table II over the wave length range of 0.3 A. to 1.0 A. The kilovoltage equivalent to these wave lengths is 41 kv.-12.4 kv. which more than covers the ranges of operation of direct view black-andwhite and color television receivers. From a practical viewpoint, the wave length range of about 0.35-0.7 A. is of prime concern for glasses utilized in present day TABLE II 0.6 A. 0.7 A. 0.8 A. 1.0 A

Table 11 clearly illustrates the greater effectiveness of SrO than BaO in inhibiting the transmission of X-radiation over the particularly vital 0.350.7 A. range of wave lengths. This means, then, that to obtain the equivalent absorption of X-radiation within that range of wave lengths, the BaO-containing glass would have to be about one and onehalf times as thick as a glass containing an equal weight percent of SrO.

As has been explained above, the intensity of X-radiation at the target varies directly with the square of the voltage. Nevertheless, from the above-recited values of mass absorption coefficients, it can be observed that higher voltage not only produces a higher intensity but also shifts the radiation to shorter wave lengths where the glass is more transparent. This situation give rise to a very high exponential dependence underscoring the criticality in enhancing the X-ray absorption behavior of glass employed in television picture tube face panels to meet the present day trend of increasing operating voltages.

A log-log plot of the mass absorption coefiicient of each of the elements reported in Table II between 0.35- 0.7 A. results in a straight line. This feature allows a given glass to be characterized by specifying the absorption coefficient at a single wave length. For purposes of control, we have arbitrarily selected 0.6 A. Hence, for control of X-ray absorption, the linear coefficient i calculated from the total chemical analysis. A minimum value is established which is based upon the making of X-ray dose rate measurements in the conventional manner on tubes with parts of known thickness and composition.

Thus, Table III records the linear absorption coefficient measured at 0.6 A. for each of the glasses listed in Table I.

TABLE III Example No.: Linear absorption coefiicient at 06 A.

From the linear absorption coefiicient, the transmittance of a narrow, parallel monochromatic beam of X-rays incident perpendicularly upon a material of uniform thickness can be calculated utilizing the following well-know Lambert equation:

T=I/I eor 111T: llt where T=fraction transmitted,

I =intensity of incident radiation,

I=intensity of transmitted radiation,

t=thickness in cm.,

u=linear absorption coeflicient,

u=wd, where w=mass absorption coefficient, and d=density.

Table III clearly demonstrates the high efficiency exhibited by SrO in reducing the transmission of X-radiation through glass where the wave length of each is between about 0.33-0.77 A. Thus, Example 13 represents a typical, commercially-available glass (without the conventional coloring agents) utilized in the production of television picture tube bulbs. Examples 2 and 3 demonstrate the marked improvement in the reduction of X-ray transmission which can be achieved through modest additions of SrO to the BaO-containing glass whereas Examples 6 and 7 illustrate the effectiveness of SrO alone. Examples 9 and 10 manifest the efficacy of PbO in reducing X-ray transmission but such glasses assume an undesirable brown-to-black discoloration after extended exposure to electrons at high voltages. But, as Examples 11 and 12 demonstrate, SrO additions to PbO-containing glasses further improve the X-ray absorbing qualities thereof.

While the present invention has been directed specifically toward television picture tubes, it 'will be appreciated that the disclosed glasses are equally useful in other electronic tubes operated under high voltages where X-rays may be emitted.

We claim:

1. A glass demonstrating excellent resistance to electron and/ or X-ray browning and exceptional absorption of X- radiation in the range of wave lengths between about 0.330.77 A. consisting essentially, by weight on the oxide basis, of about 010% Na O and/ or 013% K 0, the total of Na O plus K 0 constituting 4'20%, 40-70% SiO and an effective amount up to 20% SrO, Na O and/or K 0, SiO and SrO constituting at least 70% by weight of the total composition.

2. A glass according to claim 1 wherein the SrO content thereof ranges about 515% by weight.

3. A glass according to claim 1 wherein said composition contains up to 20% by weight BaO.

4.. A glass according to claim 1 wherein said composition contains up to 15% by weight total of CaO, MgO, and ZnO.

References Cited UNITED STATES PATENTS 1,633,534 6/1927 Long 252478 2,025,099 12/ 1935 Gelstharp 252478 2,747,105 5/1956 Fitzgerald et al 252478 3,138,561 6/1964 Labino 252478 3,356,579 12/1967 Harrington 252478 X 3,369,961 2/1968 Dalton et a1. 252478 X CARL D. QUARFORTH, Primary Examiner S. J. LECHERT, Assistant Examiner US. Cl. X.R. 106-47, 52

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 6 ,932 Dated September 2, 1969 Inventor) John H. Connelly and George B. Hares It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the Drawing, on the vertical axis change "LINEAR" to MASS Change "u" to w and delete "IN cm' Column 2, line 21, change "of" to or line 65, after "heavy" insert metal Signed and Scaled this Third Day of August 1976 [SEAL] Arresr:

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