MXPA06013565A - Forced convection target assembly - Google Patents

Abstract

Provided is a modified target assembly in which the target fluid is moved within the target assembly in a manner that increases the effective density of the target fluid within the beam path, thereby increasing beam yield utilizing forced convection. The target may also include optional structures, such as nozzles, diverters and deflectors for guiding and/or accelerating the flow of the target fluid. The target assembly directs the target fluid along an inner sleeve in a direction opposite the direction of the beam current to produce a counter current flow and may also direct the flow of the target fluid away from the inner surface of the inner sleeve and toward a central region in the target cavity. This countercurrent flow suppresses natural convection that tends to reduce the density of the target fluid in the beam path and tends to increase the heat transfer from the target.

Description

FORCED CONVECTION OBJECTIVE ASSEMBLY DECLARATION OF PRIORITY This application claims the priority of the Patent Application Provisional Notice of the United States of America No. 60 / 583,433, filed on June 29, 2004, the contents of which are hereby incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION The production of radioisotopes typically involves irradiating a target fluid (gas or liquid) held within a target assembly with an energy-charged particle beam. The energetically charged particle beam can be characterized by one or more parameters such as particles per second, beam current, (typically measured in microamperes (μA) or milliamperes (mA)), particle velocity, beam energy (measured in a way common in kilo electron volts (KeV) or mega electron volts (MeV)), and beam power (common mode measurement in watts (W)). The interaction of one of the energetic particles from particle beams with a card core in the target fluid will tend, under appropriate conditions, to produce a nuclear reaction that transforms the target core into a different element. These nuclear reactions can be written as an abbreviated expression X (a, b) Y in which X represents the target nuclei, a is the incoming or beam particle, b is the particle emitted by the nuclei, and Y represents the nuclei resulting or product. An example of such an expression is 18O (p, n) 18F, which indicates a nuclear reaction in which the oxygen isotope 18O is bombarded by a proton, which enters the nucleus and causes a neutron to be ejected, resulting in a change in the nuclear structure for the 18F fluorine isotope. Another example of said expression of said expression is 14N (pa) 11C, which indicates that the isotope of nitrogen 14N is bombarded with proton, which enters the nucleus and causes a particle to be emitted, resulting in a change in the nuclear structure for the carbon isotope 11C. The probability of a nuclear reaction occurring is referred to as the cross section and is a function of the incoming particular energy and differs for each combination of target nuclei, incoming particle, and outgoing particle. For the production of a particular radioisotope, particle type, beam current, beam energy, target cores and target density can be selected to increase the probability of the preferred nuclear reaction and the production of the desired product. The systems used to generate bundles of charged energy particles, such as cyclotrons, electrostatic accelerators and radiofrequency quadrupoles, are usually expensive (usually more than US $ 1,000,000) for their acquisition, expensive to maintain and to operate and require highly experienced technical staff. In some cases, the preferred target material can also be expensive to purchase, such as 18O enriched gas (commonly more than US $ 500 per liter) and 18O enriched water (commonly more than US $ 100 per milliliter). However, these enriched 18O materials are commonly used target materials for the production of the fluorine isotope F. F, in turn, is used frequently in the production of radiolabelled materials, such as radiopharmaceutical 18F-fluorodeoxyglucose ( FDG), which can be used in positron emission tomography (PET) for the diagnosis of cancer and other conditions. As mentioned previously, the cross-sectional parameter reflects the probability that the desired nuclear reaction will occur. Therefore, the yield of the desired product can be increased by increasing the number of incoming energetic particles, i.e., the beam current. Increasing the number of incoming energetic particles, while maintaining the same beam energy, will tend to increase the number of product cores generated. The range, or distance displaced through a medium, of a particular charged is a function of the energy of the charged particle and the properties of the medium or medium through which it will move. Range values for a wide range of particles, energies and media are generally known or readily available to those skilled in the art. There is a phenomenon in fluid targets, particularly gaseous targets, which tend to reduce the energy deposited in the target material even as the total power applied to the target assembly increases if the beam energy remains substantially constant. This phenomenon is referred to as a density reduction. This phenomenon has been attributed to the interaction between the beam of charged particles and the target fluid during which most of the energy transfer results in ionization instead of nuclear reactions. This energy transfers heat to the target fluid, causing it to rise and consequently away from the region of the incoming particle beam. This phenomenon was mentioned for the first time in Bame SJ. Jr., Perry J.E. Jr., T (d, n) 4He Reaction, Physical Review, Vol. 107, pp.1616-20, 1957. Robertson et al. 'S 1961 article, ie, Robertson LP, White BL, Erdman KL, Beam Heating Effects in Gas Targets, Review of Scientific Instruments, Vol. 32, p. 1405, 1961, provides a study of beam warming. And, in 1982, Heselius et al. published photographs of the beam interaction in a gas target in Heselius SJ., Lindbolm P., and Solin O., Optical Studies of the Influence of An Intense Ion Beam On High-Pressure Gas Targets, Int'l J. of Applied Radiation, Vol. 33, pp. 653-659, 1982, which illustrated the spread of beam spread as the beam current was increased for a fixed energy. Each of the referred articles is incorporated by reference, in its entirety. This movement of the target cores from the beam region reduces the number of cores in the beam path (density) and therefore increases the range of the beam, or in the case of a fixed distance, the ratio of the power of the beam decreases. beam transferred to the target cores. This in turn decreases the number of nuclear reactions that will occur and reduces the number of product cores that are produced. One factor that affects the density reduction in a target of g s is the ability of the target assembly to maintain the gas at a uniform temperature. One approach is intended to suppress the convective movement of the target gas heated away from the incident particle beam by configuring the target assembly to provide a lens cover that is tightly coupled to the configuration of the incoming charged particle beam, substantially forcing This way all the target cores stay in the path of the beam. Other approaches include increasing the length of the target and / or increasing the loading pressure to increase the number of target cores that will be exposed to the incident particle beam substantially above those values required when little heat is generated in the target assembly. These approaches can compensate to some degree the pressure differential that will be generated within the target fluid within the target sheath and the resulting localized density reduction. An additional factor that affects the process performance is that the incoming charged particle beam tends to lack spatial uniformity with respect to particle distribution. In fact, a typical distribution of particles within the beam will exhibit a substantially Gaussian radial distribution perpendicular to the direction of the beam. This means that the particular distribution within the beam is shifted to a central portion of the beam and the convective movement of the target gas will tend to displace the target cores towards areas within the target assembly that are exposed to smaller beam particles., thus tending to decrease the production of the desired product (s) isotope (s). As a result, even narrower coupling of the configuration of the objective camera to the beam shape will generally not counteract the density reduction induced by heating of the target gas in regions of higher beam density. In addition, target assemblies in which the target chamber includes little or no volume that is not within the beam's impact region tend to experience much greater pressure increases than targets that include substantial target chamber volume that does not is within the beam impact region. In order to accommodate the greatest pressure increases experienced within the reduced volume objective chamber, the camera beam windows and chamber walls must be made more resilient which, in the case of the camera beam window, can reduce the percentage of the beam energy and / or the beam current that can be applied to the target gas.

BRIEF SUMMARY OF THE INVENTION The invention provides a modified target assembly in which the target fluid is moved within the target assembly in a manner that increases the effective density of the target fluid within the beam path, thereby increasing the beam efficiency. As detailed below, the invention uses forced convection, and optional structures placed within the objective sleeve, to direct the target fluid within an inner sleeve in a direction opposite to the direction of the beam current, i.e. producing a countercurrent flow of the target fluid, and optionally directing the fluid of the target fluid towards a central region. This countercurrent flow of the target fluid suppresses, to some degree, the natural convective effects which tend to reduce the effective density of the target fluid within the beam path as a result of fluid heating and tends to increase the thermal transfer from the objective, allowing the operation at lower temperatures and / or pressures.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other features and advantages of the present invention will become apparent upon describing in detail the illustrative embodiments thereof with reference to the accompanying drawings, in which: FIGURE 1 illustrates a first example objective configuration; FIGURE 2 illustrates a second example target configuration; FIGURE 3 illustrates a third example objective configuration; FIGURE 4 illustrates a fourth example objective configuration; FIGURE 5 illustrates a fifth example objective configuration; and FIGURE 6 illustrates a sixth example objective configuration. These drawings have been provided to aid in understanding the illustrative embodiments of the invention as described below in greater detail and will not be considered as unduly limiting the invention. In particular, the relative spacing, dimensioning and dimensions of the different elements illustrated in the drawings are not drawn to scale and may have been exaggerated, reduced or otherwise modified in order to improve clarity. Those of ordinary skill in the art will also appreciate that certain structures that can be used commonly in the construction of such couplings, such as tool alignment structures or accessories, have been omitted simply to improve clarity and reduce the number of drawings.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE MODALITIES The particle beam must enter the target, preferably with as little energy loss as possible. The generation of the particle beam (in the accelerator) and the transport to the target must occur in a vacuum to minimize the loss of particles. The high-pressure environment of the target must be isolated from this vacuum although the particle beam is allowed to enter the target chamber. A method for forming a beam or port window uses a pair of thin metal sheets between which helium or other cooling gas passes to remove the heat produced in the thin sheets by the passage of the particle beam. Another method for forming a beam window or port employs a single thin metal sheet supported by the water-cooled structure referred to as a grid as described in U.S. Patent No. 5,917,874, the contents of which are incorporated In its whole. However, this grid will partially intercept the particle beam, thereby reducing the number of beam particles that actually enter the target and reach the target cores. The advantages provided by thinner entry foils, for example less energy loss in the passage through the foil, are directly in disagreement with the advantages provided by the thicker entrance foils, for example, the increased mechanical resistance that will allow the containment of higher pressure. An improved target assembly as described herein uses forced convection to increase the heat transfer from the target gas to the target body which, in turn, cooled, reduces local heating to which the target gas will be subjected. during the irradiation and in this way will reduce the corresponding density reduction. The movement of the fluid is generated by a fan or blower apparatus incorporated within the fluid chamber. Illustrative embodiments of the improved objective assembly are shown in FIGURES 1-6. Because the gas velocities generated by forced convection in the objective assembly of the invention are many times greater than those resulting from the natural convection produced as the beam heats the target fluid, higher speeds can be obtained. Cooling. These higher cooling rates will result in reduced temperature variations within the target gas distributed through the target chamber and correspondingly increased density of target cores within the beam path, a combination that tends to improve performance of the desired isotope product (s) on a conventional target for a given beam current and target fluid charge and / or increased beam currents and target fluid charges. Similarly, the advantages provided by convection likewise allow an increase in beam current or a reduction in the target fluid volume while maintaining or even increasing the production of the desired isotope. The selection of the appropriate regime in which the objectives according to the invention operate will depend on which advantages are most desirable to the user. The improved target assembly includes a blower assembly that is mounted within or adjacent to the objective sleeve and is rotated by an external motor through a direct or magnetic coupling. The blower assembly forces the gas from the central region towards the target walls where the gas advances towards the back of the target. The walls of the lens cover can be configured for improved thermal transfer through, for example, modification of the surface finish, the addition of fins in order to increase the thermal transfer surface area, or by the addition of metal foam bonded to the target wall to increase the surface area. Metal foam suitable for use in the invention is commercially available from distributors such as ERG Materials and Aerospace Corporation (Oakland CA, USA). Although such modifications to the configuration of the walls of the lens cover can improve the cooling performance of the lens assembly, the benefits of the present invention do not depend on such modifications. A nozzle assembly can be provided towards the back of the objective sleeve to direct the target gas towards the anterior portion of the objective cover where the particle beam is entering the objective cover. The nozzle can be positioned and configured so that the target gas is directed through the objective sleeve in an opposite direction and generally coaxial with the particle beam entering the objective sleeve. This flow of target gas has sufficient volume and velocity to at least partially suppress the density reduction of the target gas associated with gas heating and maintain an increased average target gas density within the particle beam and compensate at least partially the loss of density associated with beam heating. Additionally, the thermal transfer from the target gas to the surrounding target assembly structure will be commonly improved through increased gas movement and more turbulent flow and alteration of the gas barrier layer on the objective sleeve surfaces, thus suppressing in addition the density reduction of the target gas. FIGURE 1 illustrates a first exemplary embodiment of the invention 100 which includes an inner sleeve 102, which may be configured as an open cylinder, surrounding a target cavity 110. The inner sleeve 102 is surrounded by an outer sleeve 106 which defines the objective cover. A portion of the outer jacket 106 is replaced with a target sheet 104 or objective window through which the particle beam can enter the objective sleeve in a beam direction B. As illustrated in FIGURE 1, A motor can be disposed outside the objective sleeve and connected through a shaft 114 extending through seals 116 to a fan blade or impeller 118 positioned within the objective sleeve. When actuated, the fan or impeller 118 will tend to produce a flow of the target fluid through the target cavity in a flow direction F that is in a direction generally opposite that of the beam direction B. The fluid of The objective will tend to flow through the target cavity in a countercurrent direction relative to the particle beam, thus counteracting the natural convection resulting from the heating of the target fluid by the particle beam and increasing the effective density of the target fluid. As the target fluid reaches the end of the beam of the target cavity, it will tend to assume a radial flow direction and flow into a space 108 defined between an outer surface of the inner sleeve 102 and a corresponding internal surface of the sleeve external 106. When the opposing surfaces of both the inner sleeve and the outer sleeve are generally cylindrical, the space 108 will have a generally annular configuration. FIGURE 2 illustrates a second embodiment of the invention 200 in which outer jacket 106 includes integral coolant channels 122 through which the coolant injected into inlet 120 will flow through the coolant channels and out through a coolant channel. coolant outlet 124, thereby cooling both the outer jacket and that portion of the target fluid within the space 108. As also illustrated in FIGURE 2, the inner surface of the inner sleeve may be provided with one or more baffles 126 which they will tend to redirect the flow of target fluid induced by the fan or impeller 118 to a more central region of the objective cavity 110. FIGURE 3 illustrates a third exemplary embodiment of the invention 300 in which a nozzle structure 128 is provided in the inner sleeve 102 adjacent to the fan or impeller 118. The nozzle structure will tend to accelerate the flow of the fluid or target as it passes into the rest of the target cavity and can be used to focus the flow of the target fluid more precisely into the particle beam. FIGURE 4 illustrates a fourth illustrative embodiment of the invention 400 in which the inner sleeve 102 has a trunco-conical configuration with a smaller end, or bundle end, 102a towards the bundle and a larger end 102b adjacent to the fan or pusher 118. The frustoconical configuration will tend to confine the target fluid and accelerate the flow of the target fluid in the region of the target cavity 110 more closely adjacent to the objective sheet through which the particle beam enters the objective sleeve . As illustrated, the truncoconical shape tapers along the entire length of the inner sleeve 102, as will be appreciated, the tapered region can be substantially confined to the end of the bundle 102a with the remaining length which is substantially cylindrical. FIGURE 5 illustrates a fifth illustrative embodiment of the invention of the invention 500 in which the fluid propellant assembly is positioned is within the space defined between the external surface of the internal sleeve 102 and a corresponding internal surface of the external jacket 106. As it was indicated earlier, the coupling between the motor and the impeller or other blade 132 to compress and / or accelerate the target fluid may not be direct, but instead rely on the magnetic coupling in order to reduce the likelihood of dripping reduce and / or contamination inside the objective cover. FIGURE 6 illustrates a sixth example embodiment of the invention 600 in which the fluid propellant assembly 112, 114, 116, 118 is positioned generally perpendicular to the longitudinal axis of the lens cavity 110. Accordingly, structures of additional deflector and baffle 134, 136 at or adjacent to the inner sleeve 102 for redirecting the initial radial flow within an axial flow along the objective cavity 110.

In all modes, the deposition of energy from the particle beam within the target fluid causes an increase in pressure in the target assembly. The mechanical strength of the target assembly structure thus limits the total beam power that can be deposited on the target. The pressure increase observed in the target assembly for a given power deposition is a measure of the heat transfer properties of the target assembly with a lower pressure increase that indicates better heat transfer. A thermal transfer parameter can be determined for a given target assembly when a known power in the target is deposited from Equation 1. An apparatus of this type and the thermal transfer parameters measured for a target with and without a blower assembly producing the forced convection fluid flow described above have been constructed. The results of these tests are shown in Tables 1 A (natural convection) and 1 B (forced convection).

TABLE 1 A TABLE 1B Equation 1 hc = SL AT (P2 / P1 - 1) where: hc = heat transfer coefficient Q = heat input (watts) A = target internal surface area (m2) T = target surface wall temperature (K) P2 = pressure with applied heat (psia) Pt = initial pressure (psia) Table 1 clearly shows the improved performance of the lens assembly to increase heat transfer properties and reduce the pressure increase in the target fluid. At the same power levels this is rather a simple and non-optimized embodiment of a forced convection target assembly according to the invention that produced a reduced pressure increase of about 45% (143 psig up to 94 psig) and a Increased thermal transfer parameter of approximately 70% (180 watts / m2K vs. 105 watts / m2K). Accordingly, the present invention will allow the isotope generation process to be operated at higher beam currents, with higher lens fluid loads, with a thinner lens sheet and / or with improved performance. While the present invention has been shown and described in particular with reference to the illustrative embodiments thereof, the invention will not be considered as a limitation to the particular embodiments set forth herein; instead, these modalities are provided to more fully transfer the concept of the invention to those skilled in the art. In particular, those of ordinary skill in the art will appreciate that several of the structures illustrated and described in relation to the various embodiments may be combined separately to form additional embodiments that also provide the advantages of the present invention. It will therefore be apparent to those skilled in the art that various changes in shape and details can be made without departing from the spirit and scope of the present invention as defined by the following claims.

Claims (17)

1. An objective assembly comprising: an external sheath positioned and configured to contain a target fluid during irradiation; an internal sleeve positioned within the outer sheath and configured to encompass a portion of the objective fluid within a target cavity; a beam window provided through the outer sheath through which a beam of energetic particles can enter the target space in a beam direction; and a fluid propellant assembly positioned and configured to induce the target fluid to move in a direction of flow within the target cavity during irradiation, the direction of flow that is opposite to that of the beam direction.

2. The objective assembly in accordance with the claim 1, further comprising: the outer sheath which further includes a thermal transfer structure configured to remove heat from the target fluid and transfer the heating to a cooling fluid.

3. The objective assembly in accordance with the claim 2, characterized in that: the outer sheath includes structures for increasing the target assembly in accordance with the claimed heat transfer rate from the target fluid to the cooling fluid. The lens assembly according to claim 3, characterized in that: the outer sheath includes structures for increasing the rate of heat transfer from the target fluid to the outer sheath. The objective assembly according to claim 3, characterized in that: the outer sheath includes structures for increasing the heat transfer rate from the outer sheath to the cooling fluid. The lens assembly according to claim 3, characterized in that: the outer sheath includes a coolant channel disposed between an inner sheath surface and an outer sheath surface through which the cooling fluid flows. The objective assembly according to claim 1, characterized in that: the fluid propellant assembly includes a motor placed outside the outer sheath and an impeller placed inside the external sheath, the motor and the impeller which are mechanically coupled or magnetically. The lens assembly according to claim 7, characterized in that: the fluid propellant assembly includes a motor positioned outside the outer sheath, an axle extending from the engine and passing through an opening in the outer sheath . The objective assembly according to claim 7, characterized in that: an impeller shaft is generally coaxial with a longitudinal axis extending through the objective cavity. The objective assembly according to claim 7, characterized in that: an impeller shaft is generally perpendicular to a longitudinal axis extending through the objective cavity. The objective assembly according to claim 9, characterized in that: the impeller is placed in a return space defined between an external surface of the inner sleeve and a corresponding internal surface of the outer sheath. The lens assembly according to claim 9, characterized in that: the impeller is positioned in a generally annular space defined between an outer surface of the inner sleeve and a corresponding inner surface of the outer sheath. The objective assembly according to claim 1, characterized in that: the fluid propellant assembly is positioned and configured to force the target fluid into the objective cavity through a nozzle. The objective assembly according to claim 1, characterized in that: the inner sleeve includes a deflector assembly positioned on an internal surface to deflect the objective fluid that moves in a direction of flow toward a central region of the body cavity. objective. The objective assembly according to claim 1, characterized in that: the fluid propellant assembly is positioned and configured to cause the objective fluid to flow in an initial direction; and a structure provided within the objective assembly redirects the objective fluid in a flow direction generally parallel to a longitudinal axis of the objective cavity and in a direction opposite to the beam direction. 16. A method for the preparation of a radioisotope product comprising: introducing a target fluid into a target cavity; irradiating the target fluid within the target cavity with a beam of energetic particles to form the radioisotope product; and inducing movement within the target fluid as it is irradiated, the induced motion that is at least an order of magnitude greater than the movement resulting from natural convection. 17. A method for the preparation of a radioisotope product according to claim 16, characterized in that: The induced movement of the target fluid is in a direction that is generally coaxial with and in the opposite direction to the beam direction of energetic particles. .