WO2003092466A2

WO2003092466A2 - A source of penetrating electromagnetic radiation

Info

Publication number: WO2003092466A2
Application number: PCT/IB2003/001578
Authority: WO
Inventors: Gideon Jacobus Johannes Joubert
Original assignee: Csir
Priority date: 2002-05-02
Filing date: 2003-04-25
Publication date: 2003-11-13
Also published as: ZA200409240B; AU2003222622A8; WO2003092466A3; AU2003222622A1

Abstract

A source (10) of penetrating electromagnetic radiation includes a window or window layer (20) comprising graphite. Although the source may include an X-ray tube, it is expected that the invention will find particular application in radioisotope sources.

Description

A SOURCE OF PENETRATING ELECTROMAGNETIC RADIATION

THIS INVENTION relates to a source of penetrating electromagnetic radiation.

Some radioisotope materials, such as americium-241 , provide characteristic X- and/or gamma rays of energies particularly suitable for use in medical or veterinary diagnostic applications and security applications. It is desirable for the radioisotope material employed in a radioisotope source used in such applications to have a reasonably long half-life to reduce or eliminate the need for frequent replacement of the source. However, in general, a long half-life implies smaller photon fluency and thus that relatively long exposure times are required to obtain images or other useful data relating to objects exposed to the penetrating electromagnetic radiation emitted by such a source. It is thus desirable that the photon fluency or output of a radioisotope source be optimised.

For radioisotope sources providing penetrating electromagnetic radiation of energy E < 30 keV, the conventional choice for a window material is beryllium, which has a very high transmission at E = 18 keV of 97.78 % for a 0.5 mm thick window. For E > 30 keV, 0.2 mm thick stainless steel windows are often used, although the transmission of a 0.2 mm thick stainless steel window for E = 80 keV is only 91.23 %, and for E = 60 keV it is only 83.09 %. Other materials sometimes used as windows include aluminium and titanium. X-ray tubes usually employ beryllium windows.

Although beryllium has a high X-ray and gamma ray permeability, bombardment of beryllium with alpha particles produces neutrons, which is undesirable from a safety point of view. Beryllium is also very expensive, has a low ductility, a poor corrosion resistance, is extremely toxic when inhaled as a fume, and can cause dermatitis. According to the invention, there is provided a source of penetrating electromagnetic radiation which includes a window or window layer comprising graphite.

Although the source may include an X-ray tube, it is expected that the invention will find particular application in radioisotope sources.

The window or window layer may include a layer or layers of graphite with a thickness or combined thickness of between about 0.5 mm and about 3 mm. Typically, the layer or layers of graphite has/have a graphite thickness or combined thickness of between about 0.75 mm and about 2 mm.

Preferably, the graphite is synthetic graphite. The window or window layer may comprise at least 90 %, more preferably at least 99 % carbon.

The source may be a radioisotope source comprising a radioisotope material.

When the source is a radioisotope source comprising a radioisotope material, the radioisotope material may be encapsulated in a graphite capsule which includes the window or window layer comprising graphite. The graphite capsule may include a monolithic graphite body defining a socket and the window, and a graphite plug for the socket. In one embodiment of the invention, the socket has an internal thread and the plug has a complementary thread, allowing the plug to be screwed into the socket.

The radioisotope source may include a protective housing of a radiation shielding material, the housing having a radiation opening. The radiation shielding material may be selected from the group consisting of lead, a lead alloy, tungsten, a tungsten alloy, titanium, a titanium alloy, and stainless steel. Particularly preferred materials for the housing are a tungsten/copper alloy and a lead/antimony alloy.

In one embodiment of the invention, the housing includes an internally threaded cup, with the graphite body of the graphite capsule having a complementary external thread, allowing the graphite body to be screwed into the cup.

The radioisotope material may include as a major radioisotope constituent a radioisotope selected from the group consisting of gadolinium-153, iodine-125, cadmium-109, thulium-170 and americium-241. In a particularly promising embodiment of the invention for medical diagnostic and security or the like work, the radioisotope is americium-241.

The americium-241 may be present in the form of a hydroxide, i.e. Am(OH)₃, or an oxide. Instead, the americium-241 may be in metal form and may be electroplated on a substrate, which may be a reflector which includes a material providing K-X-rays and/or L-X-rays when exposed to radiation from the americium-241.

In one embodiment of the invention, the window is in the shape of a semi- spherical dome which has an even or uneven interior surface. In another embodiment of the invention, the end surface of the graphite plug is in the form of a semi-spherical dome, which may have an even or uneven surface facing the window.

The radioisotope material may include as a minor radioisotope constituent a material which produces K-X-rays and/or L-X-rays when exposed to radiation from the major radioisotope constituent. Instead, or in addition, the radioisotope source may include a backing layer or reflector for the radioisotope material, the reflector including a material which produces K-X-rays and/or L-X-rays when exposed to radiation from the radioisotope material. The minor radioisotope constituent and/or the reflector may include or may predominantly consist of a material having an atomic number between 40 and 69. In particular, the material may be selected from the group consisting of the elements, zirconium, silver, tin and caesium or a suitable lanthanide, e.g. europium or thulium.

The radioisotope material may be present in the form of a layer over an interior surface of the window. Instead, the radioisotope material may be present in the form of a layer over an end surface of the graphite plug. It is also possible for the radioisotope material to form part of a graphite body, with the radioisotope material being in admixture with the graphite and thus encapsulated by the graphite, with the graphite body having a surface layer, defining the window layer comprising graphite, which is free of the radioisotope material. The radioisotope material may thus define an electromagnetic radiation emission surface area which corresponds with the surface area of the radioisotope material layer. The radioisotope material layer may be arranged such that the emission surface area is larger than a projected area of the radioisotope material layer in a direction corresponding to a usual radiation direction for the source. Preferably, the emission surface area is at least 1.25 times greater than the projected area of the radioisotope material layer.

The emission surface area is thus typically non-planar and may be uneven. Instead, or in addition, the emission surface area may be arranged to follow a convexly or concavely curved surface.

The projected area of the radioisotope material may be circular or annular and may have a diameter of between about 3 mm and about 40 mm.

The invention will now be described, by way of example, with reference to the accompanying drawings in which

Figure 1 shows a longitudinal section through one embodiment of a radioisotope source in accordance with the invention; and Figure 2 shows a longitudinal section through another embodiment of a radioisotope source in accordance with the invention.

Referring to Figure 1 of the drawings, reference numeral 10 generally indicates a radioisotope source in accordance with the invention. The source 10 includes a layer of americium-241 , indicated by reference numeral 12, encapsulated in a graphite capsule generally indicated by reference numeral 14.

The graphite capsule 14 includes a monolithic circular cylindrical graphite body 16 which defines an internally threaded circular cylindrical passage or socket in which a complementary threaded graphite plug 18 with a circular cylindrical shank is screw-threadedly retained, and which includes or defines a graphite window 20.

The graphite capsule 14 is completely located inside a protective housing 22 of a radiation-shielding material, such as a tungsten/copper alloy or a lead/antimony alloy. The housing 22 is in the form of a cylindrical cup with an internal thread. The body 16 has a complementary external thread, allowing the body 16 to be screw- threadedly retained inside the housing 22.

An annular collimator 24 defining a radiation opening 26 is screwed over an open end 28 of the housing 22. The collimator 24 may be of any suitable collimator material, e.g. the same material as the housing 22.

In another embodiment of the invention (not shown) the housing 22 is not fixed or attached to the collimator 24. With this arrangement, the source 10 can be pivotally mounted to apparatus with which the source 10 is intended to be used and which includes the separate collimator. The housing 22 can then be rotated away from the radiation opening of the collimator, thereby ensuring that no radiation escapes during periods when the source 10 is not in use. In this embodiment, the housing 22 can be of semi-spherical or hemispherical or a like shape.

A metal O-ring 30 is located between a head 32 of the graphite plug 18 and the graphite body 16 in order to seal the graphite plug 18 to the graphite body 16.

Preferably, the O-ring 30 is of a metal having a similar coefficient of linear expansion as that of graphite, such as a titanium/zinc alloy, pure molybdenum, or pure tungsten.

Instead, the O-ring 30 may be of viton or a similar material.

Both the body 16 and the plug 18 are of pure synthetic graphite. The integral graphite window 20 is semi-spherical or hemispherical, bulges away from the plug 18 and has a thickness of about 1 mm. It is to be appreciated that the window 20 can also bulge inwards towards the plug 18. An end surface 34 of the graphite plug 18 is also semi-spherical or hemispherical and is more or less complementary to the shape of the window 20. The americium-241 layer 12, and a backing layer or reflector 36 are sandwiched between the end surface 34 and the window 20. The reflector 36 consists of silver, thus providing 3.2 keV L-X-rays and 21.99 keV K-X-rays when exposed to the electromagnetic radiation emanating from the americium-241. It is however to be appreciated that the reflector 36 can include any material with an atomic number between 40 and 71. The americium-241 in the layer 12 is in the form of americium hydroxide or an americium oxide. In the embodiment shown in Figure 1 of the drawings, the americium-241 layer 12 is about 0.15 microns thick and is coated over an interior surface of the window 20 and covered by the reflector 36. It is however to be appreciated that the reflector 36 and the americium-241 layer 12 may be attached to the end surface 34 of the plug 18. In another embodiment of the invention (not shown in the drawings), the americium-241 is in the form of americium metal electroplated onto a suitable substrate. This substrate can be graphite or a material with an atomic number between 40 and 71 which produces K-X-rays when exposed to electromagnetic and/or other radiation from the americium-241.

A space 38 between the plug 18 and the housing 22 may be filled with grease or a similar sealing material preventing ingress of moisture or egress of radioactive material.

As will be appreciated, the thickness of the walls of the housing 22 depends on the material from which the housing 22 is constructed and the actual radioisotope material used in the source 10. For example, for americium-241 , the wall thickness should be about 3 mm if the housing 22 is of lead, but must be thicker if the radioisotope material used is cadmium-109 or gadolinium-153. When the housing 22 is of tungsten, the wall thickness should be about 2.2 mm thick for americium-241.

The americium-241 layer 12 defines an electromagnetic radiation emission surface area which corresponds more or less with the interior surface area of the window 20. As will be appreciated, because of its curvature, this surface area is substantially larger than the projected area of the americium-241 layer in a direction corresponding to a usual radiation direction for the source, as indicated by arrow 40. This enhances the photon fluency of the radioisotope source 10. Although not shown in the drawings, the photon fluency can be even further enhanced by providing the americium-241 layer with an uneven surface, thereby increasing the difference between the surface area of the layer 12 and the projected area of the layer 12, as disclosed in WO 00/55866, the specification of which is incorporated herein by way of reference. For example, if the interior surface of the window 20 is corrugated, and on the assumption that the interior surface of the window 20 is hemispherical, the photon fluency of the radioisotope source 10 can be increased by a factor of more than three compared to a planar layer 12.

The following Table illustrates the transmission of pure graphite in comparison to those of stainless steel, aluminium, beryllium and titanium, and clearly shows that graphite is practically as good as beryllium at higher electromagnetic radiation energy values although, for soft tissue radiography, e.g. mammography, beryllium is the best window material if transmission is the only consideration. In some applications, the lower transmission of graphite compared to beryllium may be an advantage, as absorption of the lower energy electromagnetic radiation can significantly lower radiation dosage imparted to a subject, such as a human, exposed to radiation from the source 10. It is however to be noted that, although not shown in the Table, for a 0.5 mm thick graphite window, the transmission is 94.22 % at E = 18 keV, which compares favourably with the 97.78 % transmission of a 0.5 mm thick beryllium window at E = 18 keV.

Referring to Figure 2 of the drawings, reference numeral 50 generally indicates another embodiment of a radioisotope source in accordance with the invention. The source 50 is similar to the source 10 and, unless otherwise indicated, the same reference numerals are used to indicate the same or similar parts or features.

One difference between the source 50 and the source 10 is that the source 50 includes a second metal O-ring 52 further to improve the sealing between the plug 18 and the body 16.

Unlike the plug 18 of the source 10, the plug 18 of the source 50 does not have a semi-spherical or hemispherical end surface 34, but instead it has a planar end surface 54. A space 56 between the end surface 54 and the reflector 36 can be filled up with a solid semi-spherical or hemispherical graphite piece or loose graphite powder or the like. Naturally, if desired, the plug 18 of the source 50 can instead have a semi- spherical or hemispherical end surface 34. In another embodiment, the layer 12 and the reflector 36 may be absent, with the space 56 being tilled with a mixture of a radioisotope (e.g. americium-241 ), a K-X-ray producing material and graphite powder.

Americium-241 has prominent line energies just about optimum for both general X-ray and soft tissue radiography procedures and, with its half-life of 458 years, is practically an everlasting source of electromagnetic radiation. However, as a result of the long half-life, corrosion issues relating to radioisotope sources incorporating americium-241 should be carefully considered. Corrosion limits radioisotope sources having stainless steel or beryllium windows to a useful life of about 10 to 15 years. Instead of welds, conventional radioisotope sources may employ metal windows relying on knife-edge seals to prevent escape of the radioactive material from the source. Knife-edge seals however tend to develop leaks during temperature cycling, and are vulnerable to ingress of moisture which leads to corrosion in the vicinity of the knife- edge seals. The radioisotope sources 10, 50, as illustrated, do not suffer from any of these disadvantages and do not employ beryllium, thus avoiding the problems associated with the use of beryllium as set out hereinbefore. Graphite is abundant, cheap and easy to fabricate and has good strength properties up to very high temperatures. Graphite is also a good neutron moderator. Admittedly, the tensile strength of graphite is not nearly as good as that of beryllium. However, a machined graphite capsule with an integral window, survived drop tests up to heights of 2 m. Furthermore, for ambient conditions, graphite possesses impressive corrosion resistance to most materials. The self-lubrication characteristics of graphite ensure that matching graphite components do not stick, making it easy to recover the radioisotope material from sources using graphite encapsulation.

Claims

CLAIMS:

1. A source of penetrating electromagnetic radiation which includes a window or window layer comprising graphite.

2. The source as claimed in claim 1 , in which the window or window layer includes a layer or layers of graphite with a thickness or combined thickness of between about 0.5 mm and about 3 mm.

3. The source as claimed in claim 2, in which the layer or layers of graphite has/have a graphite thickness or combined thickness of between about 0.75 mm and about 2 mm.

4. The source as claimed in any one of the preceding claims, in which the graphite is synthetic graphite.

5. The source as claimed in any one of the preceding claims, which is a radioisotope source comprising a radioisotope material.

6. The source as claimed in claim 5, in which the radioisotope material is encapsulated in a graphite capsule which includes the window or window layer comprising graphite.

7. The source as claimed in claim 6, in which the graphite capsule includes a monolithic graphite body defining a socket and the window, and a graphite plug for the socket.

8. The source as claimed in any one of claims 5 to 7 inclusive, which includes a protective housing of a radiation shielding material, the housing having a radiation opening.

9. The source as claimed in claim 7 and claim 8, in which the housing includes an internally threaded cup, with the graphite body of the graphite capsule having a complementary external thread, allowing the graphite body to be screwed into the cup.

10. The source as claimed in any one of claims 5 to 9 inclusive, in which the radioisotope material includes as a major radioisotope constituent a radioisotope selected from the group consisting of gadolinium-153, iodine-125, cadmium-109, thulium-170 and americium-241.

11. The source as claimed in claim 10, in which the radioisotope is americium- 241.

12. The source as claimed in claim 11 , in which the americium-241 is present in the form of a hydroxide.

13. The source as claimed in claim 11 , in which the americium-241 is in metal form and is electroplated on a substrate.

14. The source as claimed in any one of claims 10 to 13 inclusive, in which the radioisotope material includes as a minor radioisotope constituent a material which produces K-X-rays and/or L-X-rays when exposed to radiation from the major radioisotope constituent.

15. The source as claimed in any one of claims 5 to 14 inclusive, which includes a backing layer or reflector for the radioisotope material, the reflector including a material which produces K-X-rays and/or L-X-rays when exposed to radiation from the radioisotope material.

16. A source of penetrating electromagnetic radiation as claimed in claim 1 , substantially as herein described and illustrated.

17. A new source of penetrating electromagnetic radiation, substantially as herein described.