GB2444310A

GB2444310A - Surface Sterilisation

Info

Publication number: GB2444310A
Application number: GB0623718A
Authority: GB
Inventors: Sergei Wasiliewich Mitko
Original assignee: BRIXS Ltd
Current assignee: BRIXS Ltd
Priority date: 2006-11-28
Filing date: 2006-11-28
Publication date: 2008-06-04
Anticipated expiration: 2026-11-28
Also published as: GB0623718D0; GB2444310B

Abstract

Sterilisation of a surface and/or low thickness materials is performed by exposing the surface to a large area X-ray beam having an energy less than 50 KeV and ideally 8KeV. An X-ray source capable of producing soft X-rays over a large area with a substantially uniform dosage over the area is described which comprises an evacuated envelope 10 containing an electron source 18 that is at least partially transparent to X-rays. Electrons 24 emitted by the source 18 bombard a target 12 to generate X-rays 26. The X-rays 26 pass through the electron source and leave the evacuated envelope through a window 16 arranged on the opposite side of the electron source 18 from the target 12. The arrangement is particularly applicable for sterilizing product packaging in sheet form via production line or conveyor means. Two sources can be used to irradiate both sides of the packaging material.

Description

METHOD AND APPARATUS FOR SURFACE STERILISATION

Field of the invention

The present invention relates to a method and apparatus for sterilising surfaces arid/or low thickness materials in particular packaging materia]s.

Background of the invention C)

There is a current demand to sterilise packaging used to store foodstuffs. Conventionally, packaging is exposed to radiation produced by means of a radioactive source, such as radioactive cobalt. The radlat1on used for the sterilisation comprises "hard" X-rays, i.e. radiation having a high energy, measured in millions of electron volts, and current standards require a dosage of the order of 25 KG (kilo greys) to achieve efficient destruction of the bacteria, a million fold reduction being required in the active bacteria remaining after sterilisation.

Such a dosage requires exposure of the packaging to a radioactive source for a prolonged period of time, usually several hours. For this to be practicable, such sterilisation is carried out in batches. This is possible because the hard X-rays, by virtue of their high energy, have the ability to penetrate deep into a large stack of packages.

Summary of the invention

According to a firsL aspect of the invention, there is provided a method of sterilising a surface which comprises exposing the surface to a large area X-ray beam having an energy less than 50 KeV.

The X-rays preferably have an energy in the range of KeV to 25 KeV, and more preferably still an energy of 8 Key.

The present invention is predicated on the discovery that soft X-rays are better suited to sterilisation of surfaces, allowing dosages that are lower by an order of magnitude to achieve the same degree of sterilisation as hard X-rays.

Experiments have demonstrated that the sterilisation efficiency of X-rays is strongly dependent upon their energy. X-rays having an energy in the region of 50 to KeV are highly inefficient at sterilisation. Both below and above this range, the sterilisation efficiency improves and hitherto it has been normal to use much higher energy X-rays. One reason for this is that harder X-rays can be generated more efficiently and easily and their ability to penetrate was regarded as an advantage, by allowing batch processing.

In the present invention, however, the use of much softer X-rays is proposed to enable inline processing of packaging material avoid the need to stack and unstack the packaging material and transport it to a separate sterilisation facility.

In accordance with a second aspect of the invention, there is provided a production line for sterile packaging of a product in a sterile sheet material, wherein the packaging material is received in the production line in a non-sterile state and passes through a sterilisation station prior to being formed into a sealed package, the sterilisation station comprising a large area uniform source of X-rays having an energy of less than 50 KeV.

The sterilisation station may either comprise a single X-ray source, positioned to irradiate the inner surface only of the packaging material, or two radiation source to irradiato opposite surfaces of the packaging material. The latter is preferred, in particular when the material includes a metallic oxygen barrier layer.

The Applicants know of no X-ray source capable of producing a large area beam of soft X-rays of significant average power. X-rays are produced by bombarding a target with an electron beam in an evacuated tube. Often, the target has a surface disposed at an angle to an electron beam that is focused on it and the X-rays are emitted at right angles to the electron beam. Such a construction is not suitable for producing an even energy density over a large area as the X-ray originate effectively from a single point on the surface of the target. However, this configuration does allow the target to be cooled and to have sufficient mass to withstand bombardment by the electron beam.

Another possibility is to use a large area electron source to irradiate a target parallel to the electron source. The target in this case emits X-rays from its side facing away from the electron source. Such a construction has the problem that it is not possible to cool the target effectively. For effective generation of X-rays, the target has to be very thin and this limits the ability to coo' it.

If it is supported on a large substrate capable of acting as a heat sink, then that substrate would prevent soft X-rays from escaping from the evacuated tube.

In a further aspect of the invention, there is provided an X-ray source capable of producing soft X-rays over a large area with a substantially uniform dosage over the area, the source comprising an evacuated envelope containing an electron source that is at least partially transparent to X-rays and having an electron emitting surface and a target having a surface extending parallel to the electron emitting surface of the electron source, the envelope including an X-ray transparent window arranged on the opposite of the electron source from the target, X-rays generated on the surface of the target facing the electron source passing through the electron source before being emitted from the window.

Preferably the X-ray transparent window is formed of a polyimide plastics material such as commercially available under the trade name Kapton.

The electron source may suitably comprise a wire mesh but to achieve a greater degree of collimation in the generated X-ray beam the electron source may be formed of a honeycomb.

Advantageously, the electron source may be formed of aluminium and the target of copper.

In order for the electron source to emit electrons without arcing taking place between the electron source and the target, the electron source needs to be heated preferably to a temperature of the order of 10000 to 200 C.

No special steps need to be taken to achieve this once X-ray production has commenced. However, when starting with a cold cathode, one may either externally apply heat to the cathode or gradually ramp the voltage applied between the cathode and the target to heat the cathode gradually while avoiding arcing.

Brief description of the drawings

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which Figure 1 is a section through an X-ray source of the present invention, and Figure 2 is a schematic representation of a production line incorporating the X-ray source of Figure 2.

Detailed description of the preferred embodiment

The X-ray source of Figure 1 comprises an evacuated envelope 10 which may be of strip like constructions, i.e. of narrow width in the plane of the drawings but of any desired length in the plane normal to that of the drawing.

Preferably, the envelope is evacuated up to a pressure of 0.01 to 1 mbar in order to be able to sustain the glow discharge. The upper side of the evacuated envelope 10 is is formed by the anode and target 12 of the X-ray source, which is an electrically and thermally conductive material, preferably copper, that emits X-rays when bombarded with electrons. The side walls 14 of the source 10 are of an electrically insulating material, such as a glass or ceramic material. The lower side 16 of the source 10 is an X-ray transparent window, preferably made of Kapton. Within the envelope 10 there is supported on the side walls 14 a cathode 18 comprising an X-ray transparent wire mesh or honeycomb, made preferably of aluminium. The cathode 18 is shown as being supported on a mounting ring 20 which serves as a resistive heating element.

Under steady state operating conditions, a voltage of a few thousand volts is applied between the cathode 18 and the target. The current flow and bombardment keep the cathode at a temperature of around 100 C to 200 C. At this temperature, electrons represented by short arrows 24 in Figure 1, are emitted by the cathode 18 and are accelerated by the applied voltage towards the target 12. The collision of the electrons with the target 12 results in X-rays being emitted having an energy of between 5 and 25 Key. The X-rays, represented by the longer arrows 26 in Figure 1 pass through the cathode 18 and are emitted through the X-ray transparent window 16.

It is important that no arc should be struck between the cathode and the anode. To assist the cathode in reaching its steady state temperature, it is possible either to heat the cathode using the mounting ring 20 or else the anode voltage can be ramped up gradually until the steady state is reached without arcing taking place.

The rear surface of the target 12 lies outside the evacuated envelope and can be cooled in a conventional manner. In the illustrated embodiment, cooling fins 22 are formed on the target 12 to assist with air cooling but it is alternatively possible for it to be water cooled. The target 12 can also have a substantial thickness to be able to withstand the electron bombardment without suffering any damage or deformation.

The emitted X-ray beam can be in the form of a strip of any desired length while maintaining uniform energy distribution along its length. When used to sterilise a web in a continuous production, this allows the entire width of the web to be irradiated continuously.

In Figure 2, a web 50 of material yet to be rendered sterile is fed from a supply roll 52 through a sterilisation station 54 which contains two X-ray sources 10, as shown in Figure 1, irradiating the opposite sides of the web 50. The sterilised web is then introduced into a processing or packaging station 56 where it is formed into packaging for articles 58. The articles 58 being packaged could for example be foodstuffs or items of medical equipment.

Even though only the side of the web to form the inner side of the packaging needs to be sterile, both sides of the web are preferably sterilised as the entire environment within the packaging or processing station 56 may need to be maintained sterile. It is thus advantaqeous to irradiate both sides, more especially when the material includes a metallic oxygen barrier layer, The energy of the X-rays used is preferably in the range of 5 to 25 KeV, this being substantially below the energy of X-rays used in sterilisation. Because of their low energy, these X-rays have little penetrating power and most of their energy is absorbed by the surface being sterilised.

As a result, the efficiency of the X-rays in destroying bacteria on the surface is very high and the total exposure that is required is much lower that when using high energy X-rays, resulting in significant power saving. The lower X-ray dosage is also desirable in that it reduces the risk of damaging the material of the web and the softness of the X-rays also allows them to be used safely in a production line without risk personnel or the need for extensive lead shielding.

It has been found that if the web contains an internal metal film, aluminium being often incorporated in packaging material as an oxygen barrier, then because of photoelectrons escaping from the metal film back towards the surface, the X-ray dosage needed for efficient sterilisation can be reduced still further.

Claims

CLIMS

1. A method of sterilising a surface and/or low thickness materials which comprises exposing the surface to s a large area X-ray beam having an energy less than 50 KeV.

2. A method as claimed in claim 1, wherein the energy of the X-rays is in the range of 5 KeV to 25 KeV,

3. A method as claimed in claim 2, wherein the energy of the X-rays is substantially 8 KeV.

4. A production line for packaging of a product in a sterile sheet material, wherein the packaging material is received in the production line in a non-sterile state and passes through a sterilisation station prior to being formed into a sealed package, the sterilisation station comprising a large area uniform source of X-rays having an energy of less than 50 KeV.

5. A production line as claimed in claim 4, wherein the sterilisation station comprises two X-ray sources, positioned to irradiate the opposite sides of the packaging material.

6. An X-ray source capable of producing soft X-rays over a large area with a substantially uniform dosage over the area, the source comprising an evacuated envelope containing an electron source that is at least partially transparent to X-rays and having an electron emitting surface and a target having a surface extending parallel to the electron emitting surface of the electron source, the envelope including an X-ray transparent window arranged on the opposite of the electron source from the target, X-rays generated on the surface of the target facing the electron source passing through the electron source before being emitted from the window.

7. An X-ray source as claimed in claim 6, wherein the X-ray transparent window is formed of a polyimide plastics material.

8. An X-ray source as claimed in claim 6 or 7, wherein the electron source comprise a wire mesh.

9. An X-ray source as claimed in claim 6 or 7, wherein the electron source comprises a honeycomb.

10. An X-ray source as claimed in any of claims 6 to 9, wherein the electron source is formed of aluminium and the target of copper.

11. An X-ray source as claimed in any of claims 6 to 10, wherein a heating element is provided to heat the electron source during a warm up phase of the X-ray source.

12. An X-ray source as claimed in any of claims 6 to 10, wherein means are provided for gradually ramping the voltage applied between the electron source and the target during a warm-up phase of the X-ray source.