WO1996014593A1

WO1996014593A1 - High resolution scintillation detector

Info

Publication number: WO1996014593A1
Application number: PCT/US1995/014441
Authority: WO
Inventors: Richard A. Migliaccio
Original assignee: Ail Systems, Inc.
Priority date: 1994-11-07
Filing date: 1995-11-07
Publication date: 1996-05-17
Also published as: AU4148096A

Abstract

A particle or radiation detector in the form of a scintillation detector (30) detects high energy particles such as photons, x-rays and gamma rays (35). The scintillation detector (30) includes a scintillator (32) for converting said particles to light pulses, known as scintillations. A positive sensitive photomultiplier tube (PS-PMT) (38) includes a photosensitive surface (36) for receipt of the light pulses. A retro-reflector (34) is positioned over the scintillator (32) so as to reflect light from said light pulses back through the scintillator (32) and onto the photosensitive surface (36).

Description

HIGH RESOLUTION SCINTILLATION DETECTOR

FIELD OF THE INVENTION:

The present invention relates generally to a radiation detection apparatus. More particularly, the present invention relates to a radiation detection apparatus employing an improved scintillator.

BACKGROUND OF THE INVENTION:

Scintillation detectors are employed for detecting the presence of x-rays, gamma-rays or other high energy particles .

A scintillation detector functions as a particle or radiation detector by the emission of light flashes or pulses that are detected by a position sensitive light detection device. The position sensitive light detection device could be a position sensitive photomultiplier tube PS-PMT such as the Hammamatsu R2487, or a multiple number of discrete photomultipliers as found in ANGER Cameras. Solid state position sensitive light detectors such as charge coupled devices (CCDs) may also be employed. A scintillator can not only detect the presence of a particle, gamma-ray or x-ray, but can measure the energy or energy loss of the particle in the scintillator. A scintillator is inherently position sensitive due to the localization of the high energy photon or particle interaction and the resulting release of visible or ultra-violet light. The scintillating medium itself may be either solid or liquid. A scintillator is often placed in direct optical contact with a photosensitive surface of a photomultiplier.

The scintillator is a light emitted material.

High energy particles entering the scintillator are converted to light, typically light flashes or pulses, which are referred to as scintillations. In order to direct as much as possible of the light emitted by a scintillation to the photosensitive surface, a reflective surface is employed over the scintillator. The reflective surface reflects light emitted by the scintillation, which would ordinarily be stray light directed away from the photosensitive surface, back to the photosensitive surface so that the scintillation can be more accurately detected. While such reflective surfaces over the scintillator are commonly used in order to enhance the efficiency of the scintillator detector, it has been found that such reflected light has a tendency to reduce the spatial resolution of the scintillator detector. The reflected light has a tendency to be reflected over a wider extent of the photosensitive surface than would normally be achieved by the direct emission of light from the scintillation to the photosensitive surface. The reflective phenomena is depicted in Figure 1 where diagrammatically shown is a scintillator based gamma-ray detector 10. The gamma-ray scintillator detector 10 includes a scintillating medium or scintillator 12. A reflecting layer 14 through which a high energy particle or radiation ray may pass, is positioned over scintillator 12. The scintillator 12 is positioned over a photosensitive surface 16 of a (photomultiplier) PS- PMT 18. For clarity, the photosensitive surface 16 is shown as the top surface of PS-PMT 18. However it may be appreciated that more typically, the photosensitive surface is actually the back side of a thin front window where the active side is a vacuum. In the present illustration, the PS-PMT 18 may be schematically represented by many discrete elements such as 18a, 18b, 18c and 18d.

An incoming photon or particle 15 enters the scintillator 12 through reflecting layer 14. Scintillation 19a of the particle occurs within the scintillator. The light emitted by the scintillation 19a is directed outwardly therefrom (arrows A) . Some light (arrows A,) is directly detected by photosensitive surface 16. As can be seen in Figure 1, such light is detected by elements 18a and 18b. However, the scintillation 19a also emits light (arrows A₂) away from photosensitive surface 16. Such light is reflected off of reflecting layer 14 back onto photosensitive surface 16 over a wider expanse than is the light directly emitted by the scintillation 19a. For example, which direct light (arrows A.) from the scintillation 19a may be detected by elements 18b and 18c, the reflected light (arrows A₂) is detected by elements 18a and 18d. Thus, detectors 18 essentially detect a virtual image 19b of the scintillation of the incoming particle at a position different from the actual position of the particle at the time of scintillation. Such virtual image results in a reduction in the spatial resolution of the actual scintillation as the virtual image provides a false indication of the position of the scintillation and the light emitted therefrom.

It is therefore desirable to provide a scintillation detector which provides more accurate spatial resolution of the scintillation by eliminating the virtual image of the scintillation.

OBJECTS AND SUMMARY OF THE INVENTION:

It is an object of the present invention to provide an improved radiation detector for detecting radiation particles or photons.

It is a further object of the present invention to

* provide a scintillation detector which provides for scintillation of high energy particles for detection by a photomultiplier.

It is a still further object of the present invention to provide a scintillation detector having an improved retro-reflecting surface which reflects light emitted from the scintillation back through the scintillation itself. In the efficient attainment of these and other objects, the present invention provides a radiation detector for detecting radiation particles or photons. The detector includes a scintillator for converting the particles or photons to light pulses. The photomultiplier has a photosensitive surface which is positioned adjacent the scintillator. A reflecting surface is positioned over the scintillator for accepting the particle or photon and for directing light emitted from the light pulse to the photosensitive surface. The reflecting surface includes a retro-reflective member which reflects light directed away from the photosensitive surface back through the light pulse itself.

As more particularly described by way of the preferred embodiment herein, the present invention provides a retro-reflective surface over a scintillator where the retro-reflective, surface includes plural angular facets which reflect light emitted by the scintillation within the scintillator back through the scintillation thus reducing the footprint or spot size caused by the scintillation on the photosensitive surface of the photomultiplier. Thus, the reflecting layer of the present invention provides for the reflection of light from a scintillation without loss and spatial resolution.

BRIEF DESCRIPTION OF THE DRAWINGS:

Figure 1 is an enlarged schematic representation of a portion of a scintillator detector of prior art construction.

Figure 2 shows a scintillator detector of conventional construction which may be used in combination with the embodiment of the present invention.

Figure 3 is an enlarged schematic representation of a portion of a scintillator detector having an improved retro-reflective surface in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS:

Referring initially to Figure 2, the basic construction of a scintillation detector 20 of the type which may be used in accordance with the present invention may be described. Scintillator detector 20 includes an outer housing 22. Housing 22 supports a scintillating medium or scintillator 24 which is formed of solid or liquid. Scintillators of conventional construction may be formed of either crystal, plastic or glass. The scintillator may be a unified element or may be formed of multiple elements.

The scintillator 24 is placed in optical contact with a photosensitive surface 26 of a photomultiplier 28. A reflective surface 29 may be placed over the scintillator 24 in order to enhance its performance.

With additional reference to Figure 1, operation of a scintillator detector may be described. A high energy particle gamma-ray, x-ray or photon (hereinafter particle 15) enters through reflecting layer 14 into scintillator 12. Scintillator 12 exhibiting luminescent behavior, creates a scintillation 19a of particle 15. Light is emitted by scintillation 19a. Certain rays of light (arrows A.) are detected directly by the photosensitive surface 16 of photomultiplier 18. Other rays of light (arrows A₂) are directed away from photosensitive surface 16 but are reflected back thereto by reflective layer 14. The light detected by the photosensitive surface is measured using the photomultiplier 18 so as to provide an image map of radiation or particle emission sources. The operation of a scintillation detector is well known to those skilled in the art . While reflecting layer 14 is beneficial to the operation of a scintillation detector in that it directs light emitted by the scintillation back to the photosensitive surface enhancing the efficiency thereof, the 5 reflecting layer does have one significant drawback, such reflected light is scattered over a wider area of the photosensitive surface than the direct light. The optical resolution of the detected light is reduced by such scattering. This is evident in the depiction shown in

10 Figure 1 where reflected light evidenced by arrows A₂, is reflected over a wider surface of photosensitive surface 18 than is direct light indicated by arrows A_x. In essence, photosensitive surface 18 detects a virtual image 19b of the scintillation 19a of incoming particle 15. In an effort to

15 increase the spatial resolution, the present invention provides an improved reflecting surface in the form of a retro-reflector.

Referring now to Figure 3, schematically shown is a portion of an improved scintillator detector 30 employing

20 a retro-reflector 34. Scintillator detector 30 of the present invention is generally similar to that shown and described with respect to Figures 1 and 2. Scintillator 32 is placed in contact with or in close proximity to (such as is the case with a lens coupled CCD camera) a photosensitive

25 surface 36 of a photomultiplier 38. Photosensitive surface 36 may be a PMT window positioned adjacent a photocathode. An optically immersed interface 40a may be formed between photosensitive surface 36 and scintillator 32. An index matching epoxy or grease may be employed.

30 In the present illustrative embodiment, a retro- reflector 34 is provided. The interface 40b between retro- reflector 34 and scintillator 32 may also be optically immersed as described above. Retro-reflector 34 may be a plastic sheet with one flat side 34b and an opposed retro-

35. reflective surface 34a formed into an array of box corners. Generally these box corners have the shape of a triangular- based pyramid. These corners define - 3 < ° - ^ facets 34c. A commercially available retro-reflector which may be employed with the present invention is one sold by 3M as SCOTCHLITE Brand Diamond Grade Service 3330. Retro- reflective surface 34a provides for the reflection of light emitted by scintillation 39 of incoming particle 35 back to the scintillation 39 itself. The facets 34c are constructed and arranged in such geometry that light hitting the surface 34a will be reflected back at approximately 130°. Thus the light will be reflected back onto itself. Light so reflected will only be detected over an extent of the photosensitive surface 36 onto which light has been detected directly from scintillation 39. Thus, the direct image of the scintillation as well as the virtual image (represented by light reflected off of retro-reflecting surface 34a) will be approximately the same . In order to improve the range of angles through which retro-reflection occurs, the present invention further provides a metal backing 42 over the retro-reflective surface 34a. The metal backing which may be evaporated thereover, increases the range of angles over which retro-reflection may occur from about ± 30° for air to about ± 90° .

As shown by way of schematic example in Figure 3, light emitted by scintillation 39 directly onto photosensitive surface 36 (arrows B is decected by elements 38b and 38c. Reflected light from scintillation 39 indicated by arrow B₂, being reflected back through scintillation 39 is also detected only by elements 38b and 38c. This is in contrast to the examples shown in Figure 1 where reflected light is detected by elements 18a and 18d which are outboard of elements 18b and 13c. By providing the retro-reflective surface 34a of retro-reflector 34, increased spatial resolution is achieved by merging the actual image of the scintillation (direct light, arrows B-) with the virtual image (reflected light, arrows B_:) . In addition, the small size of the box corners of retrc- reflector 34 also advantageously effects spatial resolution. In the above-described embodiment, the distance between the corners of retro-reflector 34 range between 0.5 to 0.18 mm. The relatively small distance between adjacent box corners maximizes (increases the density of) the area available for retro-reflection. This results in less "dead space" and a reduction in light scattering. The retro-reflective surface 34a of retro-reflector 34 thereby provides an enhanced performance scintillation detector which may be used with radiation imaging systems such as gamma-ray imaging systems.

Various changes to the foregoing described and shown structures would now be evident to those skilled in the art. Accordingly, the particularly disclosed scope of the invention is set forth in the following claims.

Claims

WHAT IS CLAIMED IS:

1. A radiation detector for detecting radiation particles or photons comprising: a scintillator for converting said particle or photon to a light pulse; a photomultiplier having a photosensitive surface for receipt of said light pulse; and a reflecting element over said scintillator for accepting said particle or photon and for directing said light from said light pulse to said photosensitive surface; said reflecting element including a retro- reflective surface for reflecting said light fr:^*-¹. said light pulse back through said light pulse .

2. A radiation detector of claim 1 wherein said reflecting element includes a retro-reflectcr, having a plurality of closely spaced box corners defining said retro- reflective surface.

3. A radiation detector of claim 1 wherein said retro-reflective surface includes said box corners defining angularly disposed facets arranged so as to reflect said light back toward said light pulse.

4. A radiation detector of claim 3 wherein said angular facets extend from said reflective surface toward said photosensitive surface.

5. A radiation detector of claim 2 wherein said box corners define 90°-90°-90° facets.

6. A radiation detector of claim 4 further including a metallized layer deposited over said box corners.

7. A scintillator detector for detecting high energy particles such as photons, x-rays and gamma-rays comprising: a scintillating medium for scintillation of said high energy particles; a light detector having a light detecting surface for detecting light emitted by said scintillation of said high energy particles; and a retro-reflector for reflecting said light emitted by said scintillation onto said light detecting surface through said scintillation.

8. A scintillator detector of claim 7 wherein said retrc-reflector includes a plurality of angular reflectors thereon; said annular reflectors being constructed and arranged to reflect light back through the source of said ligh .

9. A scintillator detector of claim 8 wherein said reflectors form a 2-dimensional structure.

10. A scintillator detector of claim 7 wherein said retrc-reflector includes a surface having a plurality of faceted box corners.

11. A scintillator detector of claim 10 wherein said box corners include 90°-90°-90° facets.

12. A scintillator detector of claim 10 wherein said faceted box corners include a metallized layer deposited thereover.