Photoluminescent phosphors and x-ray imaging systems utilizing such phosphors

- 1 ~ RD-14,315 PHOTOLUMI~JESCENT PHOSPHORS ANO X-RAY IMAGIt~G SYSTEMS UTILIZING SUCH PHOSPHORS 8ackground of the Invention ~he present inv~ntion relates to radiographic X-ray imaging systems and, more particularly, to photoluminescent phosphors especially suitable for use in X-ray imaging systems at temperatures in excess of about 77 kelvin.

X-ray imaging systems are useful for producing im3ges of, for example, internal organs of a human body.

The basic principle of their operation involves the passing of X-rays through an object of inquiry (e.g., a human body) 9 which X-rays then impinge ~n a photoluminescent layer.

The internal parts of the object of inquiry absorb some of the X-rays as a function of their structure, whereby the X-ray pattern impinging on the photoluminescent layer is representative of the structure of such ` internal parts.

`i 15 The X-ray pattern on the photoluminescent layer stim~ ulates a luminescent 'or light) image in the photoluminescent ;, layer.

The material of the photoluminescent layer that luminescesis kno~n as a phosphor.

It is desirable that the phosphor exhibit a "fast-acting" response to stimulation by X-rays; that i5, as used helein, a rapid response both in luminescing upon excitation by X-rays and ceasing to luminesce upon cessation of X-ray excitation.

This is because typical X-ray imaging systems incorporate sophist~cated and expensive digital computer apparatus that are most economically operated at high speed.

~ypical known phosphors th~t are composed of ceramic exhibit a relatively slow-acting response to X-ray stimulation.

This increases the expense of operation of a typical imagina system including complex digital computer apparatus that is costly to operate. 1~ would thus be desirable to provide a phosphor that e~hibits a fast-acting response to X-ray stimulation.

A further class of known phosphors are effective onlv at temp~ratures below about 77 kelvin and are doped with luminescent .:

~ '~ .~ ~ '''' ~Z~S2~;~ - 2 - R~-14,315 activators.

These phosphors comprise, for example: Cs2ZrC16 doped with one of Ir, Os, Re.

Pt, Mo, Ru or U; Cs2HfC16 doped with Os, Mo or Re; and Cs2SnC16 doped with Ir, U, Re or Os.

These doped phosphors are known to exhib~ luminescence at about 4.4 kelvin (liquid helium temperature), but not at about 77 kelvin ~liquid nitrogen temperature) or higher temperatures due to a parasitic effect within the host material.

Accordingly, they are not suitable for use in typical X-ray imaging systems, at least without employing expensive cooling apparatus to maintain the required cold temperature.

Objects and Summary of the Invention An object of the present invention is to provide an X-ray imaging system utilizing a phosphor with a fast-acting response to X-ray stimulation.

A further object of the invention is to provide an X-ray imaging system utilizing an undoped phosphor tha~ luminesces at temperatures above about 71 kelvin.

The foregoing objects are achieved in an X-ray imaging syst~m, which, in a preferred form, comprises a photoluminescent layer including a phosphor of the general formula:

2MX6 wherein A is cesium ~Cs~ Rubidium (Rb),potassium (K~ ~- or sudium (Na3;

;: M is titanium (Ti, zirconium (Zr), hafnium (Hf), tellurium (Te) or tin (Sn); and X is chlorine (Cl) or bromine (~r).

The phosphor preferably has a purity with respect to naturally- occurring impurities of at least about 98.0 percent by weight and is sufficiently deficient of luminescent activators effective at ; 30 temperatures below about 77 kelvin that the phosphor exhibits luminescence at temperatures greater than about 77 kelvin.

An X-ray source is included and is adapted to expose the photoluminescent (PL) layer to X-rays passing through an ; object and thereby to stimulate emission of a light image from the 3S PL layer.

The light image is then read by a photodetection mealls, ' ~,'.

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5 2'~ Z ~ 3 ~ RD-14,315 such as a photodi ode device.

Brief Descriptio, of the Drawin~s While the specification concludes with claims set~ing forth the particular features of the invention which are deemed to be novel, it is believed that the organization and method of using the invention will be best understood from considering the follGwing description in connection with the drawings~ in which:

Figure 1 is a schematic illustration partially in block diagram form of a radiographic lmaging systPm in accordance with the present invention;

Figure 2 is a cross-sectional view illustrating an X-ray screen construction utilizing the phosphor materials of the present inventionj Figure 3 is a graph of luminescent output versus purity of a particular phosphor of the present invention;

, Figure 4 is a graphical illustration of the luminescent spectral output of a phosphor of the present invention as well as a phosphor according to the prior art that includes a luminescent activator;

Figure 5 is a schematic depiction of a zone refining apparatus useful in purification of the phosphor of the present '. invention; and :j Figure 6 is an illustration of luminescent output ., intensity of a phosphor of the present invention in response to X-ray stimulation.

Description of_the Preferred Embodiments !

Referring to the drawings, Figure 1 schematically illustrates a radiographic imaging system 10 suitable for real time use and which utilizes a photoluminescent layer (PL) 12 incorporating one or more of the phosphors of the present invention, whish are described in detail below. PL layer 12 is held in position by a physical support 14 that is transparent to X-rays,and is adapted to receiYe X-rays 16 from an X-ray source 18. X-rays ~, ., f ''''~' "'.

'''' `' 5 Z ~ ~ ~ 4 ~ RD-l4,315 16 are transmitted through an object of investigation 20, such as a human body. PL layer 12 is stimulated into photoluminescence by X-rays 16 that have passed through object 12 at different levels and exhibits a light image representative of the internal parts of object 20~ A photodetection means 22 is provided adjacent PL layer 12 so as to convert the light image information on PL layer 12 to a digitized electrical image, which is then transmitted to digital processor 24 in order to enhance the "quality" (i.e. clarity) of the digi~ized electrical image.

The resulting digitized electrical image from digital processor 24 is then transmitted to a digital imager 26 in order to provide a final digitized electrical image correcting certain deficiencies found in the original light image exhibited on PL layer l2.

Photodetection means 22 may comprise, by way of example, a photodiode device, a photomultiplier device, or a charge-coupled device, each of which are known in the art.

Digital processor 24 and digital imager 26 each may suitably comprise conventional devices.

PL layer 12 preferably comprises a monocrystalline layer of one or more of the inventive phosphors that are described in detail below.

Alternatively, PL layer 12 may comprise a polycrystalline layer of one of more of the inventive phosphors in particle form that are bound within an optically isotropic plastic layer, such as a polyvinyl napthalene toluene copolymer. A further alternative embodiment of PL layer l2 comprises one or more of the present inventive phosphors in polycrystalline, particle form, suitably adhered to support 14 cf Mylar plastic filmr for example, by an adhesive such as polymethylmethacrylate.

In Figure 2 there is depicted a self-supporting representative X-ray screen construction according to the present invention, that is useful to permanently record a photoluminescent light image on photographic film.

Specifically, the X-ray screen construction of Figure 2 utilizes a pair OT flexible backing layers 30 and 32 along with a pair of optical reflecting layers 34 and 36 and a further pair of photoluminescent layers 38 and 40, :;' t ~, ~Z~52~Z 5 - RD-14,315 as illustrated, to expose an in~ermediate, double-emulsion photographic film ~ember 42.

As used herein, "photodetection means" is intended to embrace such photographic film member 42.

~` In accordance with the present invention, th~ photoluminescent layers used in the above-described radiographic imaging system (Figure 1) and X-ray screen construction (Figure 2 include at least one phosphor of the general formula:

.

A2MX6 wherein, A is selected from the sroup consisting I0 of cesium, rubidium, potassium, or sodium; M is selected from the group consisting of titanium, zirconium, nafnium, tellurium or tini and X is selected from the group consisting of chlorine or bromine.

) In a more preferred embodiment of the invention A can be cesium, rubidium, or potassium; M can be zirconium, ; hafnium, or tellurium; and X is again selected from the group ' consisting of chlorine and bromine.

In a still more preferred embodiment of the invention, the phosphor composition is one in which A is cesium or rubidium; M is zirconium, hafnium, or tellurium; and X is chlorine or bromine.

In a particularly preferred aspect of the invention the phosphor composition of the photoluminescent layer can be characterized by the general formula:

A2M 1-xM xX6 wherein Z, M and X are defined above. Illustrative compositions include:

Cs2Hf1 xTexC16 and Cs2Zrl xTexC16 where it is especially preferred that .

.005' x< 0.1 ~` .

~, ~` ,, ,:, ~. `' :`.

., , ~ ' ~Z~5212 - 6 - RD-14,315 Phosphors of the foregoing formulas constitute stoichiometrically different materials that are related in that they crystallize with the cubie K2PtC16 type structure.

Each phosphor preferably has a puri~y with respect to naturally-occurring impurities of at least ahout 98.0 percent by weight.

By "naturally-occurring impurities" is meant impurities typically found in the reagents used to produce a phosphor of the present invention.

The importance of attaining the foregoing, high degree of purity in the present phosphor (or phosphors) is made apparent from the graph of Figure 3, illustrating the luminescent output of a phosphor of the present invention for different degrees of naturally-occurring impurities (by weight).

From Figure 3 it can be appreciated that a phosphor of the present invention exhibits a luminescent output of about 5 percent of the maximum possible when its purity with respect to naturally-occurring impurities is about 98.0 percentO The higher the purity o~ the phosphor, the higher its luminescent output.

It is particularly preferred that the purity of the foregoing phosphor be about 99.99 percent, which advantangeously results in the attainment of about 99.9 percent of the maximum luminescen-t output.

The data of Figure 3 has been verified for the following phosphors: Cs2ZrC16, Cs~HfCl~ and Cs2Zr 99Te olC16.

It i~ thus reasonaDle to expect similar performance from the other phosphors of the foregoing formula.

In order for the phosphors of the present invention to luminesce at temperatures above about 77 kelvin, the phosphcrs must additionally be sufficiently deficient, or free of~ the luminescent activators discussed above with respect to one class of prior art phosphors.

Such luminescent activators do not occur naturally in the reagents used to produce the present phosphors.

As noted above, these prior art luminescent ~ctivators are effectiYe at about 4.4 kelvin (liquid helium temperature) but not at about 77 ~ kelvin (liquid nitrogen temperature). A tolerable impurity level ; of a luminescert activator is one that is sufficientl~ low whereb~ ~', , :' ~z~sz~z - 7 - RD-14,315 the phosphor exhibits luminescence at temperatures of about 77 kelvin or above.

The maximum tolerable impurity level of a prior art luminescent activator will be apparent to those skilled in the art based upon (1) the above-discussed, distinct temperature range wherein a sufficiently pure phosphor luminesces (i.e., at temperatures greater than about 77 kelvin) and (2) the unique luminescent spectrum exhibited by a sufficiently pure sample of ~he present phosphor Cs2HfC16, by way of example.

This particu1ar phosp'nor, when sufficiently deficient of luminescent activators effective only at temperatures below about 77 kelvin (and sufficiently pure with respect to naturally-occurring impurities), exhibits the luminescent spectrum 50, shown in Figure 4, whereas a phosphor with the same formula (i.e.

Cs2HfC16), when doped with the prior art luminescent activator Os exhibits the distinctly different luminescent spectra 52, shown in Figure 4.

The spectrum 50 comprises a single continuous broad emission, while the spectra 52 comprises a multiplicity of individual narrow band emissions.

A preferred method of preparing the present phosphor of the general formula A2MX6, as discussed in detail above, comprises thorou~hly mixing two moles of alkali meta' halide ~AX~ such as CsC1 with one mole of a non-alkali metal halide (MX4) such as HfC14 in a sealed evacuated silica, tube, and then raising this mixture at least momentarily to about 85QC, preferably at a rate limited to about 25C per hour.

This procedure produces one mole of Cs2HfC16.

The mixture of alkali metal halide and non-alkali metal halide can be raised in temperature at rates other than the preferred rate of 25C per hour; a faster rate results in a more volatile, possibly explosive, reaction between the alkali metal halide and the non-alkali metal halide, and a slower rate requires more time to produce the phosphor.

Lesser ~ maximum temperatures can be used, although the mixture would ; need t~ be maintained at the lesser maximum temperature for a longer time to permit the phosphor to be produced.

For example, the alkali metal halide and non-alkali metal halide mixture could be held at about 805C for a period ol about 24 hours to produce ,, S2~ - 8 - RD-14,315 the desired phosphor~ The Eoregoing processing temperature are considerably lower than those required to produce prior art ceramic phosphors and, as such, the phosphors of the present invention are more economical to fabricate that ceramic phosphors.

The present invention comprises the mixing of one mole of alkali metal halide with two moles of non-alkali metal halide in an aqueous solu-tion, with the desired phosphor precipitating from the aqueous solution. This procedure, however, typically results in a phosphor having a higher impurity level than a phosphor produced according to the foregoing procedure involving heating of the alkali metal halide and non-alkali metal halide mixture.

Phosphors made by the foregoing phosphor-producing procedures are polycrystalline in form and typically contain naturally-occurring impurities. A preferred technique for purifying the resulting polycrystalline mass is the technique of zone refining, which may be carried out in the apparatus schematically illustrated in Figure 5. According to the technique of zone refining, a polycrystalline mass of phosphor 60 is placed within a silica (or other suitable refractory , material) tube 62. A heater 64 surrounds tube 62 and is ; effective to transform the portion of polycrystalline mass 60 in the vicinity of heater 64 into molten metal 66. Impurities from polycrystalline mass 60 then diffuse into molten metal 66. Heater 64 is moved in the direction of arrow 68 from one end of tube 62 to the other. Further heaters, such as heater 70, can be employed so as to reduce the overall purification time, with six heaters being preferred. Alternatively, three heaters could be passed from one end of tube 62 to the other, by way of example.

By using the foregoing technique of zone refining, ; phosphors with a purity of about 99.99 percent (by weight) with respect to naturally-occurring impurities have been obtained.

Further details of the technique of zone refining are contained in Pfann Zone Melting, 2nd Ed., New York: Wiley Interscience (1965).

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., ~~5Z~l~ - 9 - RD 14,315 After purification, the polycrystalline mass of phosphor can be used in its existing particle form. Alternatively, the polycrystalline mass can be formed into a single crystal in accordance with the Bridgman crystalliza-tion technique, by way of example. This technique (not illustrated herein) involves placement of the polycrystalline mass into a specially shaped tube and then inserting the tube into a furnace at a temperature preferably in the range from about 810C to about 830C. The Bridgman techni~ue is described lG more fully in W.O.

Lawson and S.

Nielsen, Preparation of Single Crysta]s, London: Butterworths Scientific Publications (1958) (see especially discussion commencing on page 14).

After crystallization, the resulting crystal is cut into a desired shape for use in a radiographic imaging system such as described above.

Phosphors of the present invention have been found to exhibi-t highly desirable characteristics for use in radiographic imaging systems, as explained presently with reference to Figure 6. Intervals of X-ray emission from an X-ray source (not shown) are depicted in Figure 6 along with the luminescent output of a phosphor that is stimulated into photoluminescence by these X-rays.

Important characteristics of the luminescent output are denoted on the luminescent output waveform and discussed below.

Of particular importance in determining the speed at which X-ray imaging may occur is the "primary decay" of the luminescent output/ which is the time it takes for the luminescent output to decay to l/e (about 37 percent) of its maximum value after cessation of X-ray stimulation.

Of considerable importance also is the "afterglow" of the phosphor, which is a spurious or unwanted continuation of luminescence of the phosphor after a relatively long interval after cessation of an X-ray stimulation signal. Of importance in resolution ~uality of a radiographic image is the "hysteresis", or change in maximum luminescent output, betweensuccessive periods of photoluminescence of a phosphor.

The foregoing characteristics of the present phosphors ~' .

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521Z - 10 - RD-14,315 are tabulated as follows for various, preferred phosphors of the present invention, along with the characteristic of phosphor efficiency in terms o~ percent of the industrial standard phosphor CsI:T1.

PrimaryAfterglow Phosphor Decay@ 10 msec. Hysteresis Efficiency Cs2HfC16 <O.lmsec.1~ <1% 40~ Cs2~rC16 <0.1msecØ5% ~i% 40-55% Cs2HfBr6 <0.1msec.1% 2% 45~ Example 1: Preparation of Cs2 ~ in Single ~stal form A single crystal of 11.38 9 Cs2ZrC16 was prepared by first placing 4.66g of ZrC14 of 9g.99 percent purity into a clear, dry 1/2-inch diameter fused silica tube with 6.729 of CsCl of 99.9999 purity.

The purity levels in these examp~es are in terms of weight and concern naturally-occurring impurities.

The tube and contents were evacuated to lO~Hg, at which point the tube was sealed off from the vacuum line at an overall tube length of 8 inches.

The tube was placed in a tubular resistance furnace equipped with explosion shields.

The temperature was raised at 250 per hour to 8300 and maintained at this temperature for two hours.

Then the furnace was cooled at 100~C per hour to room temperature (21C) and the tube remov~d from the furnace.

The tube was opened carefully to allow any HCl gas present to safely escape and then the polycrystalline Cs2ZrC16 was mechanically extracted.

This polycrystalline mass of Cs2~rC16 was ground in an agate mortar and then placed in a clean,dry 1/2-inch diameter silica tube for purification in accordance with the zone refining technique.

The tube was evacuated to 1~Hg then sealed off from ; 30 the vacuum source.

The tube was raised at 1/2-inch per hour through two heated zones each being 9/lo-incn in diameter ~ 1/2inch length and spaced 3 inches from each other.

The temperature at the center of the zones was 830C.

This purification procedure was repeated for tWO more pas es.

The tube was again carefully ,.~ . . .

' ':, ,'.,.

~., :, lZ~5Z~Z - 1l - RD-14,3l5 opened and the purified polycrystalline Cs2ZrC16 extracted.

For crystal growth~ the pure Cs2ZrC16 was placed in a cleang dry tapered silica tube (Bridgeman type~ and evacuated to 1 Hg.

The tube was sea1ed and then lowered at 1/4 inch per hour ~hrough a furnace at 830~C, the furnace having a uniform temperature zone at least lO inches length by l-inch diameter.

After this heating step, a single crystal of Cs2ZrCl6 was mechanically extracted after opening the tube.

Example ?.

Preparation of Cs~HfCl~ in Single Cr~_tal_Fonm .~ 10 A single crystal of l3.l2g Cs2HfCl6 was prepared in accordance with the procedure of Example l,above, as modified by using 6.409 of HfCl4 of 99.99 percen~ purity in place of 4.669 of ZrCl 4 .

Example 3: Preparation of Cs2HfBr in ~ar-icle '~rm The phosphor CszHfBr~ in particle form was prepared by first mixing 4.989 of HfBr4 of 99.999 percent purity with ; 4.269 of CsBr and placingthe resulting mixture into a clean, dry silica tube 1 inch in length.

The tube was evacuated to 10~Hg and then sealed.

The tube and its sontents were heated to 830C at 25C per hour in a furnace equipped with explosion shields.

After maintaining the tube and contents at 830C for 1 hour, the tube was cooled at 100C per hour to room temperature (21~C).

The tube was then carefully opened and the polycrystalline Cs2HfBr6 extracted.

' Example 4.

Preparation of Cs~ ~ The phosphor Cs2TeC16 in particle form was prepared ~ by first mixing 6.72g of CsCl and 5.~89 of TeCl4 and placing the : resulting mixture into a clean, dry silica tube, which was then evacuated, heated and cooled as in Fxample 3, above.

Example 5: Preparation of Cs~Zr Te Cl in Particle Form --.~.01~ The phosphor Cs2Zr ggTe o1Cl6 in particle form was prepared by first mixing 6.72g of CsCl, .0549 TeC14 and 4.61g ZrCl4 and placing the resultir,g mixture into a clean, dry silica :

:

~,~ , '~' 3L2~5ZlZ - 12 - RD-14,315 tube, which was then evacuated, heated and cooled as in Ex~mple 3, above.

Example 6: Preparation of Cs~Hf Te Cl in Particle Form . 9~ . 01~ The phosphor C52Hf g9Te olC16 in particle form was prepared by first mixing 6.729 of CsCl, .054gf TeC14 and 6.349 HfC14 and placing the resulting mixture into a clean, dry silica tube, which was then evacuated, heated and cocled as in Example 3~ above.

In each of the preceding examples, all steps were preformed in a dry N2 atmosphere.

The foregoing describes phosphors that are particularly suitable for use in radiographic imaging systems because they can be fabricated more economically than present ceramic phosphors due to lower fabrication temperatures and are fast-acting, thoreby penmitting increased operating speed of a radiographic imaging system in which a phosphor (or phosphors) is utilized.

While the invention has been described with respect to specific embodiments, many modifications and substitutions of the invention will be apparent to those skilled in the art.

It is, therefore, to be understood that the appended claims are intended to cover all such modifications and substitutions as fall within the true spirit and scope of the inv ntion.