METHODS AND SYSTEMS FOR THE MASS PRODUCTION OF RADIOACTIVE MATERIALS

PROCEDES ET SYSTEMES POUR LA PRODUCTION EN SERIE DE MATERIAUX RADIOACTIFS

English Abstract

Systems and methods mass produce radioactive materials and structures, such as
stents. A three-dimensional array of targets is disposed in a beam path of an
electron beam emitted from a source thereof (e.g., a linear accelerator). A
bremsstrahlung converter is interposed between the array of targets and the
source of the electron beam so as to convert the electron beam to an
irradiating beam which contains both electrons and photons. The electrons in
the irradiating beam may be divergently directed away from the beam path
(e.g., by magnetic sweepers) and along a divergent path so that the targets
are irradiated predominantly by photons. Furthermore, the electron beam can be
conditioned (focused) by means of magnetic stirring coils positioned upstream
of the converter. The bremsstrahlung converter is most preferably provided by
a plurality of individual converters which differ from one another in terms of
their thickness and/or high Z material. One or more of these individual
converters may thus be interposed in the beam path as may be desired in
dependence upon the targets to be irradiated.

French Abstract

Cette invention se rapporte à des systèmes et à des procédés permettant de produire en série des matériaux radioactifs et des structures, tels que des extenseurs médicaux. A cet effet, un réseau tridimensionnel de cibles est placé dans la trajectoire d'un faisceau d'électrons émis par une source (par exemple un accélérateur linéaire). Un convertisseur de Bremsstrahlung est placé entre le réseau de cibles et la source du faisceau d'électrons, afin de convertir ce dernier en un faisceau irradiant qui contient à la fois des électrons et des photons. Les électrons du faisceau irradiant peuvent être amenés à s'éloigner par divergence de la trajectoire du faisceau (par exemple par des moyens de balayage magnétique) et à suivre une trajectoire divergente, pour que les cibles soient irradiées essentiellement par des photons. Le faisceau d'électrons peut en outre être conditionné (focalisé), à l'aide de bobines d'agitation magnétique placées en amont du convertisseur. Le convertisseur de Bremsstrahlung est de préférence formé par plusieurs convertisseurs individuels différents les uns des autres en terme d'épaisseur et en terme de haute teneur en matériau Z. Un ou plusieurs de ces convertisseurs individuels peuvent ainsi être placés dans la trajectoire du faisceau, selon les besoins en fonction des cibles à irradier.

Claims

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WHAT IS CLAIMED IS:
1. A method for the mass production of radioactive materails comprising
the steps of:
(a) creating an electron beam along a predetermined beam
path;
(b) positioning a three dimensional array of radioactivatable
targets in the beam path; and
(c) interposing a bremsstrahlung converter in the beam path in
advance of said targets.
2. The method of claim 1, which includes varying at least one of the
thickness and material of the converter in dependence upon the targets to be
irradiated.
3. The method of claim 1, which includes translating the array of targets
relative to the beam path.
4. The method of claim 1, which wherein step (c) includes providing a
plurality of bremsstrahlung converters which differ from one another in at
least
one of thickness and material, and interposing at least one of the converters
in
the beam path in dependence upon the targets to be irradiated.
5. The method of claim 1, which includes (d) positioning a beam dump in
the beam path rearwardly of the array of targets.
6. The method of claim 1 or 3, which includes applying a negative
voltage to the array of targets.

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7. The method of claim 6, wherein the negative voltage applied to the
array of targets is between about -200 to about -5000 volts.
8. The method of claim 1, which includes positioning a sweeper magnet
downstream of the bremsstrahlung converter and laterally adjacent to the beam
path so as to cause a substantial number of electrons in the beam emitted from
the converter to be diverted from the beam path along a divergent path.
9. The method of claim 8, which includes positioning secondary beam
dump along the divergent path.
10. The method of claim 8, wherein a pair of sweeper magnets are
employed.
11. The method of claim 1, further comprising positioning a magnetic
scanning coil laterally of the beam path in advance of the converter.
12. The method of any one of the preceding claims 1-11, wherein the
targets include stents.
13. The method of any one of the preceding claims 1-11, wherein the
targets include capsules which contain radioactivatable material.
14. A system for the mass production of radioactive materials
comprising:
an electron beam generator for generating an electron beam along a
beam path;
a three dimensional array of radioactivatable targets positioned in the
beam path; and

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a bremsstrahlung converter interposed in the beam path between said
electron beam generator and said array of targets.
15. A system as in claim 14, further comprising a converter adjustment
assembly which includes a plurality of bremsstrahlung converters which differ
from one another in terms of thickness and/or material, wherein said converter
adjustment assembly is capable of selecting interposing at least one of said
converters between said electron beam generator and said array of targets.
16. The system of claim 14, which further comprises a translator coupled
operatively to said array of targets for translating said targets relative to
said
beam path.
17. The system of claim 14, which comprises a beam dump disposed in
the beam path rearwardly of said array of targets.
18. The system of claim 14, which includes a voltage generator coupled
operatively to the array of targets for applying a negative voltage to said
targets.
19. The system of claim 18, wherein said voltage generator applies a
voltage of between about -200 to about -5000 volts to said array of targets.
20. The system of claim 12, which includes a sweeper magnet
positioned downstream of said converter and laterally adjacent to the beam
path so as to cause a substantial number of electrons in the beam emitted from
the converter to be diverted from the beam path along a divergent path.
21. The system of claim 18, which includes a secondary beam dump
positioned along said divergent path.

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22. The system of claim 18, comprising a pair of said sweeper magnets.
23. The system of claim 12, further comprising a magnetic scanning coil
laterally positioned relative to said beam path in advance of said converter.
24. The system of any one of claims 14-23, wherein the targets include
stents.
25. The system of any one of claims 14-23, wherein the targets include
capsules which contain radioactivatable material.

Description

CA 02327824 2000-10-06
WO 99/52587 PCT/US99/07663
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_METHODS AND SYSTEMS FOR THE M9,SS PRODUCTION OF
RADIOACTIVE MATERIALS
FIELD OF THE INVENTION
The present invention relates generally to the production of radionuclides
s by irradiation with intense electron beams so as to form radioactive
materials
suitable for therapeutic andlor diagnostic medical purposes. In preferred
embodiments, the present invention relates to systems and methods whereby
radioactive structures, for example, stents, may be mass-produced by intense
electron beam irradiation.
BACKGROUND AND SUMMARY OF THE INVENTION
In my prior U.S. Patent Application Serial Nos. 08/963,068 and
08/962,834 each filed on November 3, 1997 (the entire content of each such
prior filed application being expressly incorporated hereinto by reference),
there
are disclosed techniques and methods for the production of point-of-use
~5 radionuclides by irradiation with intense electron beams so as to form
radioactive materials suitable for therapeutic and/or diagnostic medical
purposes andlor industrial applications.
Broadly, the systems and methods of the present invention involve the
mass production of radioactive structure, and especially the mass production
of
2o radioactive stets. More specifically, a three-dimensional array of targets
formed of, or containing, radioactivatable material, is disposed in a beam
path
of an electron beam emitted from a source thereof (e.g., a linear
accelerator). A
bremsstrahlung converter is interposed between the array of targets and the
source of the electron beam so as to convert the electron beam to an
irradiating
25 beam which contains both electrons and photons. The electrons in the
irradiating beam may be divergently directed away from the beam path (e.g., by
magnetic sweepers) and along a divergent path so that the targets are
irradiated predominantly by photons. Furthermore, the electron beam can be

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conditioned (focused) by means of magnetic stirring coils positioned upstream
of the converter.
The bremsstrahlung converter is provided by a plurality of individual
converters which differ from one another in terms of their thickness and/or
material. One or more of these individual converters may thus be interposed in
the beam path as may be desired in dependence upon the targets to be
irradiated.
The targets are most preferably translated relative to the beam path by a
driven translator assembly. In addition, the targets preferably have a high
1o negative voltage (e.g., between about -200 to about -5000 volts) applied
thereto.
These and other aspects and advantages of the present invention will
become more clear after careful consideration is given to the following
detailed
description thereof.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Reference will hereinafter be made to the accompanying drawing
FIGURE which depicts in schematic fashion one preferred system for the mass
production of radioactive stents in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
2o The invention will be discussed below in relation to the mass production
of radioactive stents. It should be understood, however, that reference to
stents
is to a particularly preferred embodiment of the invention and is nonlimiting
with
respect thereto. Thus, the present invention is well suited for the mass
production of virtually any structure which is formed of, or contains, a
radioactivatable material. The targets may therefore be in the form of stents
or
other radioactivatable structures, or may be capsules formed of a non-
radioactivatable material which contain material to be activated (e.g.,

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radioactivatable metal powders, radioactivatable liquid metals or liquid
suspensions of radioactivatable metals). Suffice it to say here that those of
ordinary skill in the art will appreciate the particular form that the
"targets" may
have and embraced by the scope of the present invention.
A particularly preferred system 10 which embodies the present invention
is depicted in the accompanying FIGURE. As shown, the system 10 includes a
standard linear accelerator 12 which emits an electron beam 12-1. The linear
accelerator 12 is most preferably is capable of generating an electron beam
energy of between about 15 MeV to about 50 MeV and an average electron
beam current of between about 0.05 to about 10 mA, most preferably about 1
mA.
The electron beam 12-1 impinges upon a beam converter assembly 13
which includes a bremsstrahlung converter 14 formed of a relatively high Z
(atomic number) material, such as tantalum or tungsten. The term "high Z" as
employed herein and in the accompanying claims is meant to refer to a material
having an atomic number of greater than about 30, more preferably greater than
about 70, for example between about 70 and about 92. As is well known the
converter 14 coverts a substantial amount of the electron energy of the
electron
beam 12-1 into a beam 15 comprised of a mixture of electrons (identified by
the
2o straight lines in the beam 15) and photons (identified the wavy lines in
the beam
15) which is directed toward a three-dimensional array of stent targets 16.
The converter 14 is connected operatively to a converter adjustment
assembly 18 which is capable of mechanically (e.g., via suitable mechanical
couplings (not shown) that may be manually or motor driven) inserting and
25 withdrawing the converter 14 into and out of the path of the electron beam
12-1,
respectively. In this regard, the adjustment assembly 18 is operatively
coupled
to a plurality of converters 14 of different thicknesses (as measured linearly
parallel to the path of the electron beam 12-1 ) and/or different high Z
materials.
Thus, by selectively moving (substituting) one or more of the converters 14
into
3o the path of the electron beam 12-1, the desired beam 15 characteristics may
be

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caused to emanate therefrom toward the stent targets 16. The adjustment
assembly 18 and the converters 14 coupled thereto are in thermal
communication with a cooling assembly 20 which is capable of circulating a
cooling fluid (e.g., water) through the adjustment assembly and thereby
providing necessary thermal cooling of the individual converters 14.
The use of a relatively thin bremsstrahlung converter (e.g., less than
about 2 mm thick) is most preferably employed for a single or a relatively few
number of scent targets 16. The use of relatively thicker bremsstrahlung
converters (e.g., from about 2 mm to about 6 mm thick formed of a tungsten
material) may be employed for multiple stent targets {e.g., numbering in the
hundreds) which may be directly exposed to the beam 15 or encapsulated by a
low Z material. Those skilled in the art will recognize that thin high Z
bremsstrahlung converts will position the high energy photon conversion into a
narrow cone and it is only within this cone that the effect desired - namely,
~5 activation - will occur. (See for example, Hubbell, J. of App. Phys., 30,
981-984
(1959), the entire content of which is incorporated expressly hereinto by
reference). It is noted that, according to Hubbell, the thicker bremsstrahlung
converters destroy the nature of the forward cone and redistribute the high
energy photons over a much wider angle. Thus, when the targets for activation
2o are selected, they should be disposed within this new distribution in an
optimum
manner. The optimum manner of such distribution can be designed by Monte
Carlo modeling methods (MCNP-4A, Los Alamos Laboratory, incorporated
hereinto by reference). The Monte Carlo computer code may be used to
calculate the distribution of energies over wider and wider cones. One may
25 then calculate the energy flux and cross section products using prior art
cross
sections and computer codes. (See, for example, Weeks et al, Med. Phys. 13,
762-764 (1986), the entire content of which is expressly incorporated hereinto
by reference.)
The stent targets 16 are disposed in a three dimensional array (it being
3o understood of course and only two dimensions of the array are visible in
the

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accompanying FIGURE) which, as noted above, is most usefully positioned
within the cone of energy flux which best matches or overlaps the nuclear
reaction resonance energy window as devised by the Monte Carlo modeling.
The array of stent targets 16 is also such that permutation of the scent
targets
s 16 is allowed. Thus, the stent targets 16 are preferably affixed to stent
support
brackets 16-1 which project from a translator system 22. In this regard, the
translator system 22 includes a motor drive (not shown) which selectively
moves (translates) the stent targets 16 as may be desired relative to the beam
15. The particular structure of the stents that may be radioactivated
according
to the present invention is not critical. Thus, the stents may be confgured to
suit the particular therapeutic need (e.g., see U.S. Patent No. 5,059,166 to
Fischell et al, the entire content of which is expressly incorporated hereinto
by
reference).
A beam dump 24 is positioned behind the array of stent targets 16 so as
15 to stop the beam 15. The beam dump is of a conventional variety and may
include a relatively large mass of material which stops the photon and
electron
beam. Most preferably, the beam dump most advantageously includes a
forward section formed of a relatively low Z material {e.g., aluminum) so as
to
be insubstantially affected by the beam 15. A suitable heat sump may be
2o positioned rearwardly of the forward section so as to transfer heat
generated
thereat. Alternatively, the beam dump 24 may be in the form of a water-holding
container having an ion chamber submersed therein to measure radiation by
recording ionization in the chamber with an electrometer. The thickness of the
beam dump depends on the electron energy and material. For example, the
2s thickness of a water beam dump may be on the order of about six (6) feet
for a
20 MeV beam, but may be on the order of only about one (1 ) foot for a lead
beam dump.
The system 10 also most preferably includes a pair of sweeper magnets
26 positioned laterally of the beam 15 emitted from the converter 14. The
3o sweeper magnets are of sufficient flux intensity so as to direct electrons
in the

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beam 15 out of the straight cone path and direct them to secondary beam
dumps 28 positioned below the plane of the FIGURE. The secondary beam
dumps 28 may be similar to the primary beam dump 24 described previously,
but may be of a greatly reduced size owing to the smaller penetrating ability
of
the electrons that will impinge thereon. For example, the thickness of water
for
a 20 MeV beam is about 15 cm, but is only about 6 cm for an aluminum beam
dump. In this manner, the sweeper magnets 26 and corresponding secondary
beam dumps 28 ensure that a high proportion of photons will travel on to the
stent targets 16 so as to activate the same.
In order to reduce radiation damage to the material forming the stent
targets 16, a high negative voltage (e.g., between about -200 to about -5000
volts) is applied to the stent targets 16. In order to accomplish this
function, the
system 10 may include a high voltage power supply 30 which is coupled
through the common stent support 32 to which the stent support brackets 16-1
~5 are physically attached. The support 32 is insulated electrically from the
translator 22.
Magnetic scanning coils 34 may optionally be provided laterally of the
electron beam 12-1. The coils 34 are provided in order to allow the electron
beam 12-1 to be scanned on the converter 14 so as to enable more efficient
2o cooling of the converter 20 and/or a wider geometric spread of the
resulting
photon beam 15 emitted therefrom. By providing a wider geometric spread of
the photon beam 15, a greater amount of stent targets 16 per unit time may be
activated thereby significantly reducing (or eliminating entirely) the
complexity of
movement required to be performed by the translator 22.
25 While the invention has been described in connection with what is
presently considered to be the most practical and preferred embodiment, it is
to
be understood that the invention is not to be limited to the disclosed
embodiment, but on the contrary, is intended to cover various modifications
and
equivalent arrangements included within the spirit and scope of the appended
so claims.

Representative Drawing