Patent Application
20230290532
This application claims priority of European patent application no. 22161257.5 filed on Mar. 10, 2022, which is incorporated herein by reference in its entirety.
The present disclosure relates to a device for the production of radioisotopes by irradiating a target with X-rays formed by Bremsstrahlung upon bombarding a converter with a high energy electron beam. In particular, the present disclosure relates to a specific geometry of the converter reducing the heat generated by the electron beam and allowing conventional cooling systems to be used to maintain the temperature of the converter within acceptable boundaries.
Radioisotopes can be produced by different reactions using charged particles or using of photonuclear reactions (e.g., X-rays). For example, 225Ac can be prepared by decay of 225Ra formed by photonuclear reactions caused by irradiation with X-rays of an 226Ra-target. The energy of the X-ray, which is directly dependent on the energy of the electron beam, must be controlled accurately to form the desired isotope. For example, irradiating an 226Ra-target can yield 223Ra, 224Ra, and 225Ra depending on the energy of the photoirradiation. Other examples of radioisotopes commonly used in medical applications include 99mTc,
X-ray can be produced by irradiating a converter with a high energy electron beam. The converter is positioned between a source of high energy electron beam including electron accelerator such as a rhodotron or a linear accelerator; and the target (in the example, 226Ra). The converter is formed by foils of a high-Z metal, such as Ti, or Ta. As the converter is struck by the electron beam, the latter is decelerated, and the released energy is converted into X-ray radiation which reaches the target to form the desired radioisotope. This mechanism is referred to as “Bremsstrahlung”.
As only a fraction of the energy of the electron beam is converted in Bremsstrahlung, the remaining fraction being converted into heat, thermal degradation of the converter is a serious issue. For this reason, the converter must be cooled. Conventional coolers use a gas such as He, or liquid such as water.
In order to enhance cooling of the converter and to enable a wider geometric spread of the resulting photon beam by the converter, WO1999052587 proposed to scan the electron beam over a scanned area of the converter using magnetic scanning coils. US20120025105 combines the scanning of the electron beam with the translation of the target synchronized with the scanning of the electron beam such that the target is constantly exposed to the full intensity of the Bremsstrahlung produced by the converter.
WO2017076961 describes a focusing lens used to collimate or focus an electron beam. Collimation of the electron beam is useful because a diverging electron beam would increase the divergence of photons generated. This would in turn require larger targets in order to collect the photons. The focusing lens can be formed from magnets, and may be a multipole lens such as quadrupole, hexapole, octupole lenses.
In spite of the foregoing improvements, it remains a problem to sufficiently cool the converter with conventional cooling systems to prevent the converter from thermally degrading prematurely. The present disclosure solves the dual problem of preventing premature thermal degradation of the converter using conventional cooling means, while at the same time maintaining a focused high intensity electron beam, and therefore a highly focused X-ray radiation. The solution proposed by the present disclosure to achieve this dual goal is explained below.
The present disclosure is defined in the appended independent claims. Preferred embodiments are defined in the dependent claims. In particular, the present disclosure provides a system for the production of radioisotopes comprising,
The electron accelerator, the scanning unit, the focusing unit, the converting unit, and the target holder, are all aligned along the irradiation axis (Z) and arranged downstream of one another in that sequence, wherein “downstream” is defined relative to the electron beam direction.
In a first embodiment, the scanning unit is configured for deviating the electron beam along the predefined scanning pattern extending along the first transverse axis (X) and a second transverse axis (Y), wherein X⊥Y⊥Z. The focusing unit is configured for focusing the scanned beam also over a second irradiation plane (Y, Z) towards a second focusing point (Fy) located on the irradiation axis (Z). The second focusing point (Fy) can be same as, or different from the first focusing point (Fx). The one or more bremsstrahlung converters are in the shape of an ovoid cap, preferably a spherical cap, defined by a first curved cross-section in the first irradiation plane (X, Z) and by a second curved cross-section in the second irradiation plane (Y, Z).
Each of the one or more bremsstrahlung converters has a first curved cross-section in the first irradiation plane (X, Z) which is preferably defined by a substantially circular arc of radius (d1-dn) centered on the first focusing point (Fx). A “substantially circular arc” is defined herein as a curved segment having a radius of curvature which varies by not more than 10% over the length of the curved cross-section. Alternatively, or concomitantly, each of the one or more bremsstrahlung converters has a second curved cross-section in the second irradiation plane (Y, Z) which is preferably defined by a substantially circular arc of radius (d1-dn) centered on the second focusing point (Fy). It is preferred that the second focusing point (Fy) be the same as the first focusing point (Fx) (i.e., Fx=Fy).
In a second embodiment, the scanning unit is configured for deviating the electron beam along the predefined scanning pattern extending along the first transverse axis (X) only. The one or more bremsstrahlung converters are in the shape of a section of cylinder, defined by a curved cross-section in the first transverse plane (X, Z), and generatrixes extending along a second transverse axis (Y), wherein X⊥Y⊥Z. Each of the one or more bremsstrahlung converters has a first curved cross-section in the first irradiation plane (X, Z) which is preferably defined by a substantially circular arc of radius (d1-dn) centered on the first focusing point (Fx).
The focusing unit can be configured for forming the focused beam with a focusing half-angle (β) formed at the first focusing point (Fx) with the irradiation axis (Z) on the first irradiation plane (X, Z) comprised between 20 and 55°, preferably between 30 and 45°.
The one or more bremsstrahlung converters can be made of tantalum (Ta) or tungsten (W) or titanium (Ti). Each of the one or more bremsstrahlung converters has a thickness (L90) measured along a radius of curvature which is preferably not more than 3 mm, preferably the thickness (L90) is comprised between 0.2 and 2.5 mm, more preferably between 0.5 and 1.5 mm. It is further preferred that a nth bremsstrahlung converter located nearest the target holder) has a larger thickness (L90) than a first bremsstrahlung converter located nearest the focusing unit.
The converting unit can comprise between 1 and n bremsstrahlung converters, wherein n is comprised between 2 and 8, preferably between 3 and 5, separated from one another by cooling channels. The converter cooling system can comprise gas or liquid forced cooling flowing through the channels.
The present disclosure also provides a process for producing a radioisotope by X-ray irradiation of a target comprising,
The target can be selected from one of 226Ra for producing 225Ac, or 199Mo for forming 99mTc, or 186W for producing 187Re, or 134Xe to form 131I, or 68Zn for producing 67Cu.
For a fuller understanding of the nature of the present disclosure, reference is made to the following detailed description taken in conjunction with the accompanying drawings in which:
As shown in
A converter provided as a converting unit (4) is located between the focusing unit (3) and the first focusing point (Fx). The converting unit comprises one or more bremsstrahlung converters (4.1-4.n), configured for converting the focused beam (10f) into a photon beam (11x). The converting unit is equipped with a converter cooling system (4c) configured for cooling the one or more bremsstrahlung converters (4.1-4.n).
A target holder (5h) configured for holding a target (5) exposed at the first focusing point (Fx). The target holder is equipped with a target cooling unit (5c) configured for cooling the target (5) when held in the target holder (5h).
The electron accelerator (1), the scanning unit (2), the focusing unit (3), the converting unit (4), and the target holder (5h), are all aligned along the irradiation axis (Z) and arranged downstream of one another in that sequence, wherein “downstream” is defined relative to the electron beam direction.
The gist of the present disclosure is that the one or more bremsstrahlung converters (4.1-4.n) are curved such that the focused beam (10f) intersects each of the one or more bremsstrahlung converters (4.1-4.n) with an intersecting angle (α) comprised between 65° and 115° at all points, preferably between 75° and 105° at all points, more preferably the intersecting angle (α) is equal to 90°±5°.
Electron accelerators are well known in the art. The present disclosure is not restricted to any particular type of electron accelerator, as long as it is capable of producing an electron beam (10) of energy of between 10 and 40 MeV, preferably between 15 and 30 MeV, preferably between 20 and 25 MeV. The diameter of the electron beam (10) can be less than 10 mm. The electron accelerator (1) can be for example a linear particle accelerator (e.g., linac) or a petal-like accelerator (e.g., rhodotron).
Scanning units are well known in the art. The present disclosure is not restricted to any particular type of scanning unit, as long as it is capable of scanning the electron beam (10) along the predefined scanning pattern to form the scanned beam (10s). Upon impinging with the bremsstrahlung converters, only a fraction of the energy of the electron beam is converted into X-ray energy. The rest is dissipated in heat. Scanning the electron beam on the converter yields a flat beam distribution over the whole surface of the converter and reduces the concentration of the beam power and heating in a small, scanned area of the converter.
The scanning unit (2) can be equipped with scanning magnetic coils (2m) (shown in
In a first embodiment, the scanning unit (2) is configured for deviating the electron beam (10) along the predefined scanning pattern extending along the first transverse axis (X) only. Alternatively, in a second embodiment, the scanning unit (2) is configured for deviating the electron beam (10) along the predefined scanning pattern extending along the first transverse axis (X) and a second transverse axis (Y), wherein X⊥Y⊥Z.
As discussed supra, scanning the electron beam over a first and optionally a second transverse direction onto the converter facilitates the cooling of the converter. It yields, however, a wider geometric spread of the photon beam thus formed. In some cases, where large targets are available, this can be an advantage. When the target material is scarce, however, and targets of small dimensions must be used, such as with 226Ra, a wide geometric spread of the X-rays can become an inconvenience. For this reason, it has been proposed in the art to use a focusing unit to converge the scanned beam (10s) to focus the beam onto the converter via focusing magnetic coils (3m).
With targets of smaller dimensions, a scanned beam (10s) cannot be used efficiently as such. because the photons beam (11x) formed by the interaction of the scanned electron beam with the converting unit (4) is also spread out. Refocusing of either the scanned beam (10s) or the photon beam (11x) is required for targets of small dimensions. Focusing of the photon beam (11x) is described, e.g., in WO2012022491. In the present disclosure, the system comprises a focusing unit (3) located upstream of the converting unit (4) for focusing the scanned beam (10s) to form the focused beam (10f).
The focusing unit (3) is configured for focusing the scanned beam (10s) over a first irradiation plane (X, Z) towards the first focusing point (Fx) located on the irradiation axis (Z), to form the focused beam (10f). The first irradiation plane (X, Z) is defined by the irradiation axis (Z) and the first transverse axis (X), with X⊥Z. Focusing units of this type are well known in the art. The present disclosure is not restricted to any particular type of focusing unit (3), as long as it is capable of focusing the scanned beam (10s) towards the first focusing point (Fx) as it is being scanned to form the focused beam (10f). With targets of smaller dimensions, focusing points (Fx) of correspondingly smaller dimensions are required.
As illustrated in
In the embodiment, wherein the scanning unit (2) is configured for deviating the electron beam (10) along the predefined scanning pattern extending along the first transverse axis (X) and a second transverse axis (Y), the focusing unit (3) can be configured for focusing the scanned beam (10s) also over a second irradiation plane (Y, Z) towards a second focusing point (Fy) located on the irradiation axis (Z). The second focusing point (Fy) can be same as, or different from the first focusing point (Fx), as shown in
The focusing half-angle (β) shown in
As shown in
First, as represented in
Second, as illustrated in
In
The present disclosure proposes to replace the bremsstrahlung converters in the form of flat sheets used up to now in the art by curved bremsstrahlung converters (4.1-4.n) in the form of curved sheets, shown in
As shown in
By contrast, a focused beam (10f) traversing a bremsstrahlung flat sheet as shown in
The use of bremsstrahlung converters (4.1-4.n) which are curved such that the focused beam (10f) intersects each of the one or more bremsstrahlung converters (4.1-4.n) with an intersecting angle (α) comprised between 65° and 115° at all points, clearly contributes to homogenizing over the scanned area of the bremsstrahlung converter the heat generated by the interaction with the focused beam. This renders the cooling of the converting unit easier than for flat sheets, and conventional cooling systems (4c) can be used with success.
Referring to
The one or more bremsstrahlung converters (4.1-4.n) can be in the shape of a section of cylinder, defined by a curved cross-section in the first transverse plane (X, Z), and generatrixes extending along a second transverse axis (Y), wherein X⊥Y⊥Z. This geometry is preferred in case the scanning unit (2) is configured for deviating the electron beam (10) along the predefined scanning pattern extending along the first transverse axis (X) only. It could also be preferred in case the target (5) has a length defining an elongated shape, and the scanned beam needs not be focused over a plane including the length of the elongated target. A converting unit (4) of this type is illustrated in
In an alternative embodiment, the one or more bremsstrahlung converters (4.1-4.n) are in the shape of an ovoid cap, preferably a spherical cap, defined by a first curved cross-section in the first irradiation plane (X, Z) and by a second curved cross-section in the second irradiation plane (Y, Z). This type of converting unit is illustrated in
In both embodiments (i.e., single or double curvature) it is preferred that the radius of curvature of the curved sections be constant, i.e., defining an arc of circle, or a spherical cap, respectively. The radius of curvature is preferably close to the distance (di) separating a bremsstrahlung converter (4.1-4.n) to the first focusing point (Fx).
In a preferred embodiment, each of the one or more bremsstrahlung converters (4.1-4.n) has a first curved cross-section in the first irradiation plane (X, Z) defined by a substantially circular arc of radius (d1-dn) centered on the first focusing point (Fx). A “substantially circular arc” is defined herein as a curved segment having a radius of curvature which varies by not more than 10% over a length of the curved arc. With this geometry, the focused beam (10h) reaches the bremsstrahlung converters with an intersecting angle α=90° along the first irradiation plane (X, Z).
In yet another preferred embodiment, each of the one or more bremsstrahlung converters (4.1-4.n) has a second curved cross-section in the second irradiation plane (Y, Z) defined by a substantially circular arc of radius (d1-dn) centered on the second focusing point (Fy). It is preferred that the second focusing point (Fy) be the same as the first focusing point (Fx) (i.e., Fx=Fy), defining a geometry of spherical cap centered on the single focusing point (Fx=Fy).
As shown in
Each of the one or more bremsstrahlung converters (4.1-4.n) has a thickness (L90) measured along a radius of curvature, of not more than 3 mm, preferably the thickness (L90) is comprised between 0.2 and 2.5 mm, more preferably between 0.5 and 1.5 mm. The radius of curvature at one point of a bremsstrahlung converter is defined as the radius of a circle which touches the bremsstrahlung converter at that point and has the same tangent and curvature at that point. The radius of curvature is therefore normal to the tangent of the bremsstrahlung converter at that point. This is illustrated in
In a preferred embodiment, the nth bremsstrahlung converter (4.n) in the sequence of n bremsstrahlung converters, which is located nearest the target holder (5h) has a larger thickness (L90) than the first bremsstrahlung converter (4.1) located nearest the focusing unit (3). Preferably, each bremsstrahlung converter (4.i) in the sequence is thicker than the adjacent bremsstrahlung converter (4.(i-1)) located upstream, i.e., L90(41)>L90(4.(i-1)). Since the scanned areas of the bremsstrahlung converters decreases as the bremsstrahlung converters are nearer the first focusing point (Fx), increasing the thicknesses of the bremsstrahlung converters located downstream in the sequence allows homogenizing the volume of bremsstrahlung converter material interacting with the focused beam (10f). This way all bremsstrahlung converters contribute equally to the production of X-rays. The heating generated by the interaction which must be evacuated is also more homogeneously distributed between the various bremsstrahlung converters of the converting unit (4), thus facilitating the cooling thereof.
The 1 to n bremsstrahlung converters (4.1-4.n) can be made of tantalum (Ta) or tungsten (W), or titanium (Ti).
Because of the use of a focusing unit, the system of the present disclosure is particularly suitable for targets (5) of small dimensions. The target (5) can be 226Ra for producing 225Ac commonly used for diagnostic imaging. Other examples of targets which can be used with the system of the present disclosure to form diagnostic imaging isotopes include 100Mo-target for forming 99mTc, or 186W-target for producing 187Re, or 134Xe to form 131I, or 68Zn for producing 67Cu, and the like.
As the transmutation reaction caused by the interaction of the X-ray (11x) with the target generates heat, a target cooling system (5c) is provided, which is configured for cooling the target (5) when held in the target holder (5h). Like the converter cooling system (4c) discussed supra, the target cooling system (5c) can comprise gas or liquid forced cooling, with a refrigerating fluid flowing through cooling channels in thermal contact with the target (5). Keeping the temperature of the target (5) below a degradation temperature is of course important.
If the first and second focusing points are the same (i.e., Fx=Fy) and the X-ray thus produced by the converting unit (4) converge towards a small converging area around the focusing point (Fx), the sample holder can be configured for moving the target (5) such that a larger area of the target is scanned by the focusing point (which is static). This is particularly interesting in case of targets of larger dimensions, whose exposed area is larger than the converging area of the X-ray, so that transmutation occurs over a larger area/volume of the target than if it remained static.
The system of the present disclosure can be used in a process for producing a radioisotope by X-ray irradiation of a target. The process comprises providing a system as described supra. After loading a target (5) onto the target holder (5h), scanning and focusing an accelerated electron beam onto the converting unit (4) to produce X-ray, to irradiate the target with the thus produced X-ray.
The target can be for example, 226Ra for producing 225Ac, or 100Mo-target for forming 99mTc, or 186W-target for producing 187Re, or 134Xe to form 131I, or 68Zn for producing 67Cu, and the like.
| Number | Date | Country | Kind |
|---|---|---|---|
| 22161257.5 | Mar 2022 | EP | regional |
