Fluorescent Screen

This invention relates to luminescent screens, and particularly to a fluorescent screen which emits visible electromagnetic radiation in response to incident radiation invisible to the human eye, particularly X-rays.

Fluorescent screens are resorted to for investigating the internal structure of objects in X-ray topography, in medical radiology, and in other fields in which it is desired to convert a pattern of invisible X-rays into a visible image. Fluorescent screens are similarly employed for visualizing patterns of cathode rays.

The fluorescent materials employed in known screens are generally in the form of powders of small grain size, a known, typical material being granular manganese-doped zinc orthosilicate deposited in a thin layer on an optically transparent sheet material, such as glass. Best results are achieved with known screens if the particle size of the fluorescent material is in the range of a few .mu.m.

The particle size limits the attainable resolution in the visible image. The resolution could be improved greatly if it were possible to synthesize a screen in the form of a homogeneous glassy or monocrystalline layer instead of particulate conventional material. Fluorescent screens forming monocrystalline layers, however, were not known heretofore.

The luminescent effect of fluorescent screens is due to electronic excitation of active ions in the lattice of the fluorescent material, such as the manganese ions, in the afore-mentioned zinc orthosilicate. In order to produce a useful image, the energy of excitation must be released by the emission of visible radiation, and not by internal decay. A useful fluorescent material thus must contain ions having a gap in their spectrum of electronic excitation which is sufficiently great not to be capable of being bridged by lattice oscillations.

Another necessary property of good fluorescent screen material is a high luminous intensity, that is, a high density of active ions. Yet, despite this high density, the ions must be well shielded from each other to prevent rapid spreading of the exciting energy among the ions and the resulting loss of resolution and intensity.

The primary object of the invention is the provision of a fluorescent screen whose active material satisfies the requirements outlined above, and thus produces superior visible images.

According to our invention such superior images are generally produced by screens consisting essentially of any pure non-metallic rare earth compound provided the rare earth ions are spaced more than 5A apart. Specifically, such images are produced, particularly in response to X-rays, by a screen whose fluorescent material consists essentially of at least one compound of the formulas

(A.sub.2 O).sub.a (Ln.sub.2 O.sub.3).sub.b (P.sub.2 O.sub.5).sub.c (I)

(ln.sub.2 O.sub.3).sub.p (Al.sub.2 O.sub.3).sub.q (B.sub.2 O.sub.3).sub.r (II)

or

LnZrF.sub.7 (III)

wherein a is zero or a positive integer, b, c, p, q, and r are positive integers, each integer being preferably not greater than 7, A is alkali metal or a mixture of alkali metals, and Ln is a rare earth metal or a mixture of rare earth metals.

The fluorescent materials deposited on a carrier in a fluorescent screen of the invention are distinguished by forming very stable, stoichiometric single crystals of high density of active ions, good shielding, and high quantum yields. Therefore, they are particularly suitable for excitation by X-rays, but may also be employed to advantage for visualizing patterns of other ratiation, such as electron beams. Some of the compounds which are employed as luminescent materials in the screens of the invention are known in themselves. The known compounds are LiNdP.sub.4 O.sub.12, NdP.sub.5 O.sub.14, and NdAl.sub.3 B.sub.4 O.sub.12. Other compounds of Formula (I) are prepared by the method described in the published German Patent Application 2342182 and also published by Yamada et al [J. Appl. Phys. 45 (1974) 5096]. Compounds of Formula (II) are prepared by the known method of A. Bobman [Ann. Mineralog. 47 (1962) 1380].

The fluorescent materials employed in the screens of the invention are suitable crystalline laser materials. We discovered that the basic requirements to be met by a material suitable for laser crystals are also basic for a good fluorescent material according to the invention.

In the fluorescent screens of the invention, the fluorescent materials are preferably present in the form of a single crystal layer. The thickness of the layer may be up to 20 microns. No improvement is achieved at greater thickness because of the limited penetrating power of X-rays and electron beams. All kinds of fluorescent screens which permit the use of a layer of fluorescent material having a thickness up to 20 .mu. may be made according to the invention, such as screens for medical radioscopy and amplifying foils. The fluorescent materials may also be employed in the form of amorphous glasses, particularly when the values of a, b, c, q, r, or p are greater than 7 and up to about 100.

The following Table lists light yields and luminescent colors of screens consisting of pure rare earth compounds in the form of single crystals. The light yield data are expressed in percent of the light yield of our best conventional screen whose fluorescent material is granular, manganese-doped zinc orthosilicate deposited on a glass carrier.

The decay time of the luminescence from these screens ranged from 10.sup.-8 seconds (PrAl.sub.3 B.sub.4 O.sub.12) to 10.sup.-3 seconds (TbP.sub.5 O.sub.14). The image resolution, limited by the diffusion of excited states through the lattice, is better than 1000 A. The characteristic graininess of images produced by conventional screens is absent. Although the materials in our table represent only a first try their efficiency is comparable to that of the best conventional screen. This is certainly a consequence of the large density of rare earth ions and the stoichiometry, i.e., the fact that the active ions are not doped into the material but regularly distributed in the lattice.

Pure rare earth compounds, including the ones listed in our table, can be used directly for fluorescent screens. It may be advantageous, however, to use thin layers of said compounds deposited on a substrate. One possibility for deposition is to sputter the compound onto quartz glass or another suitable substrate. Another possibility is to prepare the compounds in a glassy form according to the method described for NdP.sub.5 O.sub.14 in the German Patent Application No. P2342182.1-41 filed Aug. 21, 1973, and deposit them on quartz glass or another suitable substrate according to the method described for NdP.sub.5 O.sub.14 in the German Patent 2417963.

Alternatively, a layer of the pure rare earth compound can be grown epitaxially onto a substrate. Suitable substrates for TbP.sub.5 O.sub.14, for instance, are La.sub.x Y.sub.1-x P.sub.5 O.sub.14 with 0.1 < .times. <0.2 and Gd.sub.x Y.sub.1-x P.sub.5 O.sub.14 with 0.5 < .times. < 0.6. They were prepared with the method basically known from the German Patent Application P 2342182.1-41 filed Aug. 21, 1973. Their lattice constants closely match those of TbP.sub.5 O.sub.14 within the range given for x. Lattice constants depend on the average of the ionic radii of the rare earth ions in the crystal according to the expitaxy-section p.271 of H. G. Danielmeyer in: Festkorperprobleme (Advances in Solid State Physics), Vol. XV, pp.253-275, H. J. Queisser (ed., Pergamon/Vieweg, Braunschweig (1975). When a saturated solution of terbium oxide in polyphosphoric acid was poured at 550.degree. C. over these substrates, the desired layer of TbP.sub.5 O.sub.14 grew epitaxially within a few minutes. It was found to be advantageous to add 10% by weight of Gd.sub.2 O.sub.3 or Y.sub.2 O.sub.3 to terbium oxide before heating said solution: the epitaxial layer can match its lattice constant to that of the substrate by automatically incorporating the required portion of Gd or Y into the lattice of the layer.