Isotope Production with Cyclotrons

Abstract

In the last two decades cyclotron produced isotopes have gained increasing importance in nuclear medicine diagnosis. The lecture will cover aspects on target technology with regard to production efficency, quality, reliability and automation. The production methods for the most commonly used radionuclides will be described. An example for two routine production systems based on gas target technology for I123 and Rb-81/Kr-81 are presented. 1 . INTRODUCTION Modern isotope production facilities consist of a compact Hcyclotron (Fig. 1) in the energy range between 10 MeV and 30 MeV with extracted currents up to 350 μA [1] and a highly sophisticated target technology and chemistry [2]. Fig. 1 Compact Hcyclotron built by Cyclotron Cooperation, USA The radioisotopes produced for medical applications are used in nuclear medicine for diagnosis. In contrary to X-ray studies where only static information can be obtained, nuclear medicine provides dynamic information and hence allows time-dependent studies of the function of organs. The labelled biomolecules and the radioisotopes involved should have the following ideal characteristics: – no βparticle – short effective half-life – γ-energy in the range between 100 and 300 keV. The above characteristics result in a maximum efficiency in the diagnosis and a minimum radiation dose to the patient. For example, Fig. 2 shows a comparison between I-131 and I-123. The physical properties show that the cyclotron produced I-123 (γ-energy 159 keV; no β-; short half-life) is more favourable than the reactor produced I-131 (γ-energy 364, 637; β-emission, long half-life) for use in nuclear medical diagnosis. Fig. 2 Comparison of the physical properties between I-131 and I-123 The production of radioisotopes used in nuclear medicine can be made with solid, gaseous and liquid targets. Figure 3 is an example for a solid target for 30 MeV protons and a current of 200 μA. The target material which is electrochemically plated on a solid copper backing is hit directly by the beam. The dissipated beam power of 6 kW can be removed by enforced cooling from the back-side of the target. To keep the power density low the beam strikes the target under an angle of 5 –7 . Thus the temperature on the target surface is kept below 150 C. Fig. 3 Layout of a solid target Figure 4 shows a target layout for gaseous or liquid target material. In contrary to the solid target the beam cannot hit the target material without entering the target vessel through a thin metal window. The target vessel itself has to be water-cooled as well to cool down the liquid or the gas. Fig. 4 Layout of liquid or gas target 2 . TARGET BODIES AND WINDOWS The choice of material for both the target body and window is dependent on the particular nuclide production process. Although a general rule does not exist, there are some aspects of the target bodies like activation, contamination, corrosion and cooling, which have to be considered [3, 4]. The parameters are influenced by the choice of bombarding particle, beam energy, beam current, material and solvent. The criteria for target-window materials should be the following: – thickness of 1 – 200 μm – pin-hole free – high mechanical strength – good thermal conductivity – high melting point – chemical resistance to oxidation The target windows can be sealed either by 0-ring or welding. This depends on the particular boundary conditions of cooling, temperature, pressure and radiation effects. 3 . ACTIVITY CALCULATIONS When irradiating a target material with charged particles from a cyclotron, the disintegration rate D of a produced radionuclide is: D = INσ 1 − e−λt ( )