US2853620A

US2853620A - Radiation detection phosphor

Info

Publication number: US2853620A
Application number: US238482A
Authority: US
Inventors: Roy C Fox; Walter S Lusby
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1951-07-25
Filing date: 1951-07-25
Publication date: 1958-09-23
Anticipated expiration: 1975-09-23

Description

Sept. Z3, 1958 R. c. Fox ETAL RADIATIQN DETEcTroN PHosFHoR Filed July 25. 1951 0 snm@ ROS Ovnu TOL NF WCS f myn. om

l ATTORNEY United States Patent RADIATION DETECTION PHOSPHOR Roy C. Fox, Pasadena, and Walter S. Lusby, Severua Park, Md., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvama Application July 25, 1951, Serial No. 238,482

7 Claims. (Cl. Z50-71) Our invention relates to the detection of radiations and, more particularly, to phosphors for the detection of radiations.

In accordance with the prior art of which we are aware, radiation detector phosphors have been used which constitute a plurality of small crystals of a material which is sensitive to high-frequency radiation, such as gamma rays, impinging thereon and which is capable of emitting light in response to the actionof the impinging radiation. However, it has been found that some radiations, such as gamma rays, have a high penetrating power, and a large volume of the crystals is required in order to absorb a substantial percentage `of the gamma rays impinging thereon. It has, accordingly, been the practice to build a large crystal of the material sensitive to the radiation to be detected, which is substantially transparent to the light emitted by that material in response to radiation impinging thereon. A large crystal built in this manner provides a larger volume of phosphor so as to absorb a large percent of the radiation impinging thereon while allowing a large percent of the light emitted by the material to escape from that material. However, it has been found to be highly diicult and expensive to build a large crystal of most of the desirable phosphors.

It is, accordingly, van object of our invention to build an improved radiation detector.

Another object of our invention is to provide an improved phosphor for a radiation detector.

An ancillary object of our invention is to provide a material which will absorb a large percent of the radiation impinging thereon, while being substantially transparent to the radiation emitted by that material in response to said radiation.

Still Vanother object of our invention is to provide a material having a large volume which has characteristics similar to those of a large single crystal phosphor but which is substantially less expensive.

ln accordance with our invention, we provide a plurality of small crystals sensitive to the radiation to be detected and capable of emitting light in response thereto. The crystals are embedded in a material having the same index of refraction as the crystal, and the materials are joined in such a manner that the crystals and the material around them form a translucent body. This material comprising the crystals and their supporting material is surrounded by a light-reflecting coating except NVice 2 embodying our invention employing 'a `liquid supporting material, and

Figure 2 is a showing in cross-section of a phosphor in accordance with our invention in which a solid supporting material is employed.

In accordance with one embodiment of our invention, we provide a container 2 having an outer layer or Wall 4 and an inner layer or wall 6. The layers 4 and 6 comprising the container wall are made of a material which is transparent to the radiation to be detected. The inner layer 6, in addition to being transparent to the radiation to be detected, is made of a light reflecting material. Thus, for example, if the radiation to be detected is gamma rays, an outer wall 4 of a thin layer of aluminum would be suitable. While the container is shown as having two layers, it is nevertheless understood that a wall of a single layer could be built which would be suitable for most purposes. Inside the container 2 is provided a plurality of small crystals 8 of a phosphor material such as calcium tungstate, which are responsive to gamma rays and are capable of emitting light in response to that radiation. By using the term light, we do not mean to restrict the materials to those emitting radiations in the visible range, but rather we mean to include those emitting radiations in the ultraviolet and infrared ranges inasmuch as most photosensitive devices are responsive to some radiations in these frequency ranges. Surrounding the crystals 8 in the container 2 is a supporting material 10 having an index of refraction substantially equal to the index of refraction of the crystals 8 for the light emitted by the crystals 8 in response to the action of the radiation to be detected impinging on those crystals 8. By Way of example, methylene iodide would be a suitable supporting material for use with a phosphor of calcium tungstate.

In the preferred embodiment of our invention, the supporting material 10 is a liquid which is capable of wetting the surfaces ofthe crystals 3 without being capable of dissolving the crystals 8. By saying that the liquid is not capable of dissolving the crystals 8, we do not mean that the liquid cannot contain therein a solvent of the crystals 8 because it is understood that the liquid 10 might comprise a solvent of the crystals 8 which is already saturated with the material of the crystals 8.

In accordance with the broader aspects of our invention, the supporting material l0 surrounding the crystals 8 need not be a liquid but could instead bea solid 11 as shown in Fig. 2. The solid has an index of refraction which is close to the index of refraction of the crystals 8 provided the crystals are immersed in the supporting material, i. e., provided the junction of the surfaces of the crystals S and the supporting solid 10 were such as to produce only a small amount of diffusion. This might be accomplished by mixing the crystals and the supporting material while the latter is a liquid and then causing the supporting material to solidify. An example of such a solid combination would be anthracine crystals in polyester resin. If the supporting material 1l is a solid, it is, of course, possible that in some situations the outer layer 4 of the wall could be omitted and the reflecting layer 6 could be coated on the supporting material. As shown in Fig. 2, the phosphor combination 8, 11 could be ernployed without any reflecting layer. Imbedding the crystals in a solid has the additional advantages of making the region moistureproof and dustproof and protecting the crystals from shock.

In some situations, it may be desirable to use a phosphor which is sensitive to the radiation to be detected but which does not emit light to which the light-responsive apparatus is responsive. In such a case, it would be desirable to employ a plurality of phosphors one of which is4 sensitive to the ,radiation to V,be detected and the other of which is sensitive to the radiation emitted by the rst and capable of emitting radiation to which the detecting apparatus istsensitive.

By using a plurality of phosphors, it is alsopossible to c ause -the response 4'of the detectorlto be proportional to energy, i. e., to cause the response to a given energy of short wavelength radiation to be Ymore nearly equal to the responseto the .same quantity ofenergy oflong Wavelengthi In accordance withfthe broader aspects of our invention, thesnpporting material `could be a gas. ndices ofnrefraction of a solid and a gas could be reasonably well matched `in many casesby placinggthe gas under several atmospheres pressure.

In 911, Wallof'the crystal container is a small opening 1,2 whichis'transparent to ythe radiation emitted by the phosphor crystals 8. The openingy 12 in the wall of the crystal container 2 is locatedl so as `to be opposite the photo-cathode 14 of afphoto-multiplier tube 16 or other light-responsive apparatus. Light emitted by the crystals 8 isthus reflected aroundV the inside of the crystal container by thereecting layer 6 until it reaches the transparentopening 12 inthe crystal container 2 and leaves the crystalrcontainer therethrough. Light leaving the crystal containerrthrough-the transparent opening therein impinges onthe photo-cathode 14 of the photo-multiplier tube 16, causing electrons to be emitted from the photocathode 14. The Velectrons are then employed in a manner well known in the art to give an indication of the light photons impinging on the photo-cathode.

We have thus described a radiation detector comprising a phosphor body wherein substantially all of the gamma rays impinging thereon may be absorbed therein, and substantially al1 of the light produced therein in respouseto gamma rays impinging thereon is emitted therefrom. This phosphor body is easy to construct and may readily be constructed in a much larger and more eicient size than was possible with the devices of the prior art.

Although We have shown and described specic embodiments of our invention, we are aware that` other modiiications thereof are possible. Our invention, therefore, is-not to be restricted except insofar as `is necessitated by the .prior art and the spirit of the invention.

We claim as our invention:

. 1. A scintillation counter comprising a unitary radiation-responsive device including a plurality of crystals adapted to translate the incoming radiation into light impulses, said crystals being substantially transparent to light impulses developed therewithin, a homogeneous substance surrounding said crystals, said substance being substantially transparent to light impulses developed withinsaid crystals and having a refractive index substantially equal to the refractive index of said crystals, and a detector positioned adjacent said device for translating lsaid light impulses into electrical current.

2..A counter for measuring radiation comprising, a container, a substantially transparent fluid within said container, a plurality of crystals submerged in and surrounded by said fluid, said crystals being substantially transparent to light impulses developed therewithin and being of a material to interact with incoming radiation v particles to produce a plurality of photons of light that emerge substantially simultaneously as a result of said interaction, said uidhaving a refractive index substantially the same as the refractive index of said crystals, and a photosensitive means responsive to said light emergence.

3. A phosphor for detecting radiation comprising: a plurality of small crystals of a material sensitive -to the radiationto be detected and capable of emitting light in response to said radiation and being substantially transparent to such emitted light, and a liquid which is substantially transparent to such emitted light and has an index of refraction which is substantially the same as the index of refraction of vsaid crystals, with said crystals being immersed in and surrounded by saidliquid.

4. A phosphor for detecting radiation comprising: a plurality of crystals sensitive to the radiation to Vbe detected and capable of emitting light in response thereto and being substantially transparent to such emitted light, and a solid material which is substantially transparent to such emitted light and has an index of refraction substantially equal to the index of refractionof saidy crystals, with said crystals being embedded in and surrounded by said solid.

5. A phosphor for detecting radiation comprising: a plurality of small crystals of a first material sensitive to radiation to be detected of a first Wavelength andy capable to emitting a secondary radiation in response thereto and being substantially transparent to said secondary radiation, a plurality of crystals of a second ymaterial sensitive to the secondary radiation emitted by said crystals of ,said rst material and capable of emitting light in Vresponse thereto and being substantially transparent to said emitted light, and a material which is substantially transparent to said emitted light and hasan index of refraction which is substantially the same as the index of refraction of said last-mentioned crystals, with said crystals being immersed in and surrounded by said material.

6. Apparatus as claimedv in claim 5 characterized in that the last-mentioned material is a liquid.

7. Apparatus asclaimed in claim 5 characterized in that the last-mentioned material is a solid.

. References Cited in the tile of this patent UNITED STATES PATENTS 2,534,932 Sun Dec. 19, 1950 2,559,219 Ludeman July, 1951 2,573,200 Husley i Oct. 30, 1951 OTHER REFERENCES Scintillation Counting With Solutions, by Hartmut Kallmann, from Physical Review, volume 78, 1950, pp. 621-22. Y