Patent Number: 
Section: description

FIG. 1 shows an embodiment of a sterilising device comprising an irradiation arrangement according to the present invention. The irradiation arrangement comprises a particle accelerator. The particle accelerator comprises a particle source 1, a buncher 2 and linear accelerator 3. The particle source 1, in this embodiment an electron gun, which is built in a conventional manner, emits the particles to be used for the irradiation. In this embodiment, the electron gun 1 is of a Pierce type. The buncher 2 pushes the original continuous electron beam together in bunches and introduces the bunches as electron pulses into a linear accelerator 3. Thereby, fewer electrons end up outside any accelerable phase window in the linear accelerator 3, at the same time as the energy dispersion of the electron beam output from the accelerator reduces. In this embodiment, the buncher gives particle pulses of a frequency of 3 GHz. The linear accelerator accelerates the particles by means of electrical fields and sends in the particles towards a radiation chamber 4. In this embodiment, the particles are accelerated to an end energy of 1.5 to 2.5 MeV, and with a particle pulse current in the order of magnitude of 800 mA, which gives an average particle current of 2.4 mA. The pulse lengths are about 5 xcexcs and the pulse repetition frequency is 600 Hz. The particle beam is focussed by a quadropole magnet 6 at the exit of the linear accelerator, before it enters into a scanning magnet 7. The total arrangement comprising particle accelerator and radiation chamber is enclosed by radiation shields 5, consisting of lead, with an approximate thickness of 250 mm. In FIG. 2, an enlarged drawing of the radiation chamber is shown. When the particle beam leaves the accelerator, it is bent by the scanning magnet 7 in an angle xcex2 with respect to the original radiation axis. The angle xcex2 may in this embodiment vary between about 15 and 45 degrees, in both positive and negative directions. The scanning magnet is an electromagnet operated by a bipolar current supply, the output current of which may be programmed. The particle beam is incident in a direction of redeflection magnets 8, 9, positioned at each side. The redeflection magnet 8, 9 is in this embodiment a permanent magnet with a particular shape, which is described more in detail here below. When the particle beam enters between the pole pieces 9 of the redeflection magnets, it is bent into a path determined by the magnet flux in the pole gap which is adapted for a deflection angle of xcex2+90xc2x0, which implies that the particle beam leaves the field of the redeflection magnet in a path perpendicular to the original radiation axis. The particle beam passes a vacuum window 11, which normally consists of a thin metal foil of titanium or aluminum. In the central area of the radiation chamber 4, hereinafter referred to as the radiation sector 13, the products to be irradiated pass on a conveyor belt 14 (see FIG. 3). The radiation beam will therefore impinge on the products with a radiation angle of essentially 90 degrees with respect of the original radiation axis. Beam optics calculations have been performed to determine the size of the radiation spot. In this embodiment, the radiation spot at the position of the irradiated products is approximately 20 mm. The part of the radiation passing the radiation sector 13 and the two vacuum windows 11 without being absorbed, e.g. as a result of that there are no products in the radiation sector at the moment, continues a rectilinear path until it enters between the pole pieces 9 of the opposite redeflection magnet, is bent and absorbed by a cooled particle stopper 10, preferably made by copper or aluminum. FIG. 2 further shows a radiation chamber boundary 12. In FIG. 2, six of the innumerable possible particle paths a-f in the radiation chamber are shown. Each particle path is characterised by its exit angle xcex2 from the scanning magnet 7. By changing the output current from the current supply of the scanning magnet, the exit angle xcex2 may be changed. This exit angle uniquely determines the position where the particle beam enters into the magnet field of the redeflection magnets and is started to be bent. The bending of the particle beam is performed along a circular arch, the radius of which is determined by the mass and velocity of the particle and the strength of the magnetic field. This embodiment is based on a perpendicular irradiation of the products, which gives rise to a demand that the particle beam should leave the influence of the redeflection magnet under a right angle with respect of the original beam axis. If one starts from the position, where the particle beam enters into the magnet field of the redeflection magnet, and the bending radius is known, a position, where the beam has a perpendicular direction, is uniquely defined. This position has to coincide with the position where the particle beam leaves the magnet field, whereby the local appearance of the redeflection magnet uniquely is determined. Since the particle beams enter into the field of the redeflection magnet in different angles at different positions, the design of each small part of the redeflection magnet is determined by the demand of the perpendicular irradiation angle. A shape of the redeflection magnet 9 may thereby, mainly by pure geometrical considerations, easily be calculated. The same considerations are of course valid for the opposite redeflection magnet, when the angle xcex2 is negative. In the shown embodiment, the geometry of the redeflection magnet has been approximated to a circular arch. The centre of the circle is placed 0.77 cm from the entrance to the scanning magnet as measured along the entering beam and 97.2 cm from the axis of the entering beam. The circle has a radius of 139.8 cm. The angle of irradiation will in this case deviate from 90 degrees by less than 1 degree for all scanning positions. By changing the output current from the current supply of the scanning magnet, the particle beam may thus irradiate the products 13 perpendicularly at different positions, and by changing the polarity of the current, the products may also be irradiated from the other side. If the current through the scanning magnet has a high positive value, the particle beam is bent by a large angle and follows e.g. the particle path a and impinges on the irradiated product close to its inner, towards the accelerator facing, end. When the current then gradually is reduced, the exit angle from the scanning magnet will decrease, which in turn leads to that the particle beam hits the irradiated product increasingly further out, away from the accelerator. The particle beam c, with a rather small exit angle, impinges on the product at its farther end, and its exit angle is so small that it starts to become disturbed by the mechanical parts of the vacuum enclosure. A particle beam with an exit angle with lower absolute value, is thus not to any real use and forms only radiation losses, why the current supply of the scanning magnet rapidly changes its polarity to give rise to a particle beam d, with corresponding negative exit angle instead. This particle beam irradiates the outer part of the product, but now from the other side. By now gradually, in absolute figures, increase the current through the scanning magnet, a particle beam with a gradually larger negative exit angle is achieved, whereby the beam irradiates the product closer to the accelerator end. In order to return to the original state, it is advantageously to scan back in a similar manner, since one otherwise easily would get problems with rapid current changes in the scanning magnet. The particle beam during a complete scan, thus starts e.g. from the path a, scans over to the path c, then rapidly changes to the path d and scans over to the path f, after which it turns and scans back to the path d, rapidly changes over to path c and scans back to the original path a. In this embodiment, the largest exit angle is approximately 45xc2x0, while the angle of the smallest absolute value is approximately 15xc2x0. At the occasions, when the radiation sector is not fully covered by the products to be irradiated, e.g. for irregularly formed products or for interspaces between the products when they are transported past the radiation sector, a part of particle beam will pass the radiation sector 13 and the vacuum windows 11 without being absorbed. This radiation continues in a rectilinear path towards the opposite redeflection magnet 8, 9. When the beam enters between the pole pieces 9 of the redeflection magnet, it is bent into a curved path. Due to the direction of the magnetic field, this curvature will be directed away from the accelerator. Examples of such a path is indicated by g in FIG. 2. These paths will impinge on the particle stopper 10 positioned at either side, where the particles are absorbed and the heat generation thereby occurring is collected by the cooling medium of the particle stopper. In this way it is avoided that the radiation which is not absorbed by the products will destroy the irradiation arrangement from the inside or will cause incorrect dose distribution. In the present embodiment, the particle stopper is made of aluminium or copper and the cooling medium in the particle stopper is circulating water. Aluminium has the advantage to have a low cross section for X-ray emission, while copper has the advantage of conducting the heat very efficiently. Both materials may advantageously be used in vacuum applications. Each beam leaving the particle accelerator 1-3 has a certain emittance and energy dispersion. In the shown embodiment, the emittance has been assumed to be 5 mm mrad and the energy dispersion xc2x13%. This means that along the path of the beam, the cross section of the beam will vary slowly. Each element along the path of the beam has its characteristic manner to influence the properties of the particle beam. This means, that if one compares the size of the radiation spot at the radiation sector, with identical settings for the quadropole lens, between two different deflections in the scanning magnet, these will differ. Such a variation may give rise to an inhomogeneous irradiation of the product. To compensate for this effect, the quadropole lens 6 may in this embodiment of the invention be used to change the focusing properties of the particle beam at different deflection angles. It is important that the products at the conveyor belt are irradiated with an even dose over the entire irradiated area. Since the relation between the current of the scanning magnet and the radiation position on the product generally does not follow a linear relation, the scanning of the current has to be adapted in such a way that the irradiation of the product becomes even. An example of a typical current diagram for a scanning cycle is illustrated in FIG. 4. The scanning starts at the time t0, where the current I0 is sent through the scanning magnet, and the current then varies along a curve, whereby the scanning magnet scans the particle beam evenly over the surface of the product up to the time t1, where the current I1 is sent through the scanning magnet. The polarity of the current is rapidly turned and the product is irradiated from the other side. The negative current is increased from I2 to I3 according to a corresponding curve until the time t2, when the cycle turns and scans back in a corresponding manner. The cycle is completed at the time t4. The scanning of the current may be performed continuously or in the form of discrete steps in pace with the pulse frequency. Independently of used method, each new particle pulse will impinge on the product at a new position. In this embodiment, this step between successive particle beam pulses is about 15 mm at the product position, which means that two successive irradiation areas do overlap somewhat to ensure that all surfaces are irradiated. The total scanning width is about 400 mm, which sets the maximum width of the products to be irradiated. A total scanning cycle as described above is repeated with a frequency of 5.6 Hz. To achieve an absolutely homogeneous radiation dose over the entire surface of the product, a fine adjustment of the current profile may be performed after measurement of the radiation dose along the plane of irradiation. In the shown embodiment, the relation between the field strength of the scanning magnet and the scanning position in very close to linear. The deviation is calculated to be maximum 3%. This does not have such a big principal importance, but may simplify the practical use. The scanning magnet has in the shown embodiment a pole gap of 4 cm. The maximum magnetic field needed in the scanning magnet is 33 mT. By a bipolar current supply of 72 V and 6 A, 174 turns are required in the magnet coils. Change of irradiation side at the lowest used field as described above performed, is in this case from +10 mT toxe2x88x9210 mT. This should be done as fast as possible, without making the inductive voltage too large, and in the shown embodiment, this is performed during the duration of two pulses. The currents I0 and I3, respectively, thus correspond to the largest used deflection in the scanning magnet, in positive and negative direction, respectively, which in turn correspond to a radiation position in the radiation sector positioned at the end closest to the accelerator. In the same manner, the currents I1, and I2, respectively, correspond to a radiation position at the farther, away from the accelerator facing, end. If products with a size which do not occupy the entire width of the radiation sector are to be irradiated, the currents I0, I1, I2, and I3 may easily be adapted so as to not radiate the area outside the products. Such a control possibility of the radiation width, makes the use of the arrangement for different types of products very flexible. The products are brought through the radiation sector in the radiation chamber on a conveyor belt, which is more closely described below. The feeding velocity is adapted so as to give the products the necessary radiation dose. Requested feeding velocity is given by the radiation power, the scanning width and the required dose and for this embodiment it is 0.76 m/min at 6 kW radiation power, 30 cm scanning width and 25 kGy dose. The geometry of the irradiation arrangement is important. A system using directly impacting particles inevitable obtains different angle of incidence against the products since the beam is deflected from one and the same point. Either the distance between the scanning magnet and the product has to be large, or the angle of incidence will vary substantially for products of reasonable dimensions. For instance, an angle of incidence of 45 degrees against the product reduces the penetration depth by 30%. A system where all beams are redeflected before the irradiation may be constructed compact and give homogeneous angles of incidence. Furthermore, if the system is symmetric, the control of the scanning is facilitated even if this does not imply any fundamental difference. In the irradiation arrangement, the products are normally irradiated when they are placed in a horizontal position. At use of direct impact, it is required that the accelerator arrangement is directed substantially vertically, which gives the arrangement a large height and may be impossible to install in premises with normal roof height. By using redeflected beams, one may easily create a configuration where both the accelerator device and the products may be placed substantially horizontally. By using a relatively low particle energy, a particle accelerator of a relatively small size may be used, and the lower energy reduces the need for radiation shielding. The total size of the arrangement may, due to this and the geometrical arrangements described above, be reduced significantly and the described embodiment has a total volume of 8 m3 and covers an area of 4, 2 m2. The total mass is approximately 16,000 kg. This, together with the fact that the transport needs and the internal logistic problems at the irradiation arrangements are set aside by the double-sided irradiation, implies that the arrangement advantageously is used directly in a production line, which sets aside many problems in connection with transport and storage. In FIG. 3, a vertical section of an embodiment according to the present invention is shown, perpendicularly to the one shown in FIGS. 1 and 2, and which substantially shows the operation of the conveyor belt. The products, often in the form of tubes or other small details, packed in flabby bags, are to be transported past the particle beam with an even velocity to achieve a homogeneous radiation dose. By efficiency point of view, these products should be able to be positioned closely without risk for being moved or shadowing each other during the transport through the irradiation arrangement. This transport is performed by means of a conveyor belt, which comprises two webs 14, 15 of flexible net or the like, with a width larger than the one of the products, but which may pass through the radiation chamber 4. The products to be irradiated are transported jammed in between the two net webs. Wires or chains are arranged along the edges of the net webs, which are used to drive the net webs forward and to stay the net webs. The net webs are driven separately by one motor 17 each, but in a co-ordinated manner with each other, in a closed travelling path each. These travelling paths are connected to each other during the path at which the webs are passing through the particle radiation device, from the position 19 where the products are brought to the webs to the position 20 where the products are leaving the webs. Along this length, the wires or chains are jammed together at regular intervals by rolls 16, and in this manner the net webs jam the products to be irradiated between each other. In order for the conveyor belt to allow for irradiation from both sides and not obstruct the particles in any substantial amount, the net of the net webs 14, 15 are made of thin metal wire, with a diameter less than 1 mm, and with a distance between the wires of approximately 20 mm. The conveyor belt is in this manner very flexible and may easily be driven along a narrow and curved tunnel by the rolls 16 through a so-called labyrinth 18. The labyrinth is necessary for stopping the secondary X-ray radiation to penetrate out from the radiation chamber. Since the products are fixed by the net webs, this passage may be performed without risk for displacements of the products along the conveyor belt. It is thus guaranteed that the velocity of the products past the radiation corresponds to the velocity of the net webs, which may be measured and regulated from the outside. This velocity is regulated to give the products the right radiation dose. The entire irradiation device is supposed to be included in a production line and the products brought in at 19 are assumed to originate directly from a production device for the products. At the output side 20, a packaging machine may e.g. be disposed to take care of the radiation treated products. The previous detailed description of an embodiment has only been given to facilitate the understanding of the basic idea of the invention and no additional limitations beyond what is stated by the patent claims should be understood from this, since alterations are obvious for someone skilled in the art. All numerical examples given above are tied to the specific exemplified embodiment and are not generally true for the invention as such. Someone skilled in the art easily understands that e.g. the type of particle accelerator may be varied. The exact design of the particle extraction and acceleration is not of importance for the basic features of the invention. Given numerical examples are related to the exemplified embodiment and have generally no direct influence of the basic features of the invention, but will of course effect the design of other parts of the irradiation arrangement. The required particle energy is thus important for the design of the construction of the accelerator as well as the extent of the radiation shields. The pulse repetition frequency, the particle pulse current and the beam size effect e.g. the maximum scanning velocity. In the same manner, it is understood that many parts of the equipment may be changed for other types with a corresponding effect. One may as one example mention that, instead of the permanent magnets used in the embodiment above, one may use electromagnets as redeflection magnets. However, these are more sensitive to radiation damages and are generally more space consuming, why permanent magnets are to prefer. However, this choice does not influence the basic feature of the invention. The scanning magnet may in a similar way also be designed in alternative ways, where the exit angle of the beam from the magnet is easily controllable. Alternative solutions are also that the particle accelerator is designed with a controllable focusing action, or that this function is integrated with the scanning magnet. The vacuum window may in an analogue manner be designed in different ways and with different materials, but have the same basic properties, i.e. to isolate vacuum but to let the particle beam through with as small losses as possible. The particle stopper in the described embodiment consists of a separate means disposed at the redeflection magnets. Other imaginable solutions are e.g. that they are disposed with another geometry, but still acting in the manner stated in the claims. The absorption means do not even have to consist of a separate particle stopper, but its function may e.g. be integrated in other parts of the chamber, e.g. directly in the walls of the radiation chamber. The range of angles, within which the scanning magnet operates is of course dependent of the design of the magnet and its function, and by the geometrical configuration of the redeflection magnets and the radiation sector. Given numerical examples are related solely to the described embodiment. It is also understood that even if the above described embodiment operates with perpendicular irradiation of the products, other geometrical configurations may be thinkable. Such changes will thereby have repercussions on the exact geometrical design of the redeflection magnets and the control of the scanning magnet. The perpendicular irradiation is, however, considered as the most favourable, since it gives the largest penetration depth for a certain particle energy for substantially planar products. For arrangements dedicated for a product with a certain geometrical shape, the optimal geometrical configuration may be different, e.g. with other irradiation directions or positioning of the radiation sector. The details and in particular the given numerical indications of the control of the scanning magnet are also related only to the described embodiment. The same is of course valid for the design and the stated dimensions of the total size of the arrangement, which only serves to emphasize the advantage with the compact shape of the irradiation arrangement of the embodiment. The transport system described is also solely exemplifying. The detailed design of the net webs may and should be determined by which products are to be transported. The transport webs are here described as net webs, but fully covering webs of any thin radiation durable material with low electron absorption would also be imaginable, as well as webs which only covers parts of the products.