# An Active Irradiation System with Automatic Beam Positioning and Focusing for a Medical Cyclotron
## 1 Introduction  
Compact medical cyclotrons for the production of radioisotopes for Positron Emission Tomography (PET) are common instruments in modern medicine. They accelerate H\({}^{-}\) ions at energies in the range 10-25 MeV with maximum currents in the range 100-300 \(\mu\)A [1]. Radioisotopes are usually produced using liquid targets, as in the case of \({}^{18}\)F, which is the most common radionuclide for PET imaging. Research is nowadays concentrating on the production of novel medical radionuclides which can be used to label bio-molecules. In particular, pairs of radiometals, one positron emitter for PET imaging and one \(\beta^{-}\) emitter for therapy, can be used to label proteins and peptides paving the way to the new field of theranostics in nuclear medicine [2, 3]. This is the case of the \({}^{43}\)Sc/\({}^{47}\)Sc and \({}^{44}\)Sc/\({}^{47}\)Sc pairs [4, 5, 6, 7]. The production of novel radioisotopes with adequate purity and quantity using compact medical cyclotrons is challenging. A very promising method is the use of solid targets. The bombardment of isotope enriched materials in solid form allows production of activities in the 1-100 GBq range. Since the target can be made thin, an accurate knowledge of the beam energy and of the production cross section allows optimization of the radionuclide purity. Furthermore, targets are often necessarily small (about 5 mm diameter) as materials are expensive and available only in small quantities and in form of powder. In this framework, two key issues have to be addressed--the preparation of the target and the optimization of the irradiation process. As far as thetarget is concerned, our group developed a specific capsule that can fit a commercial Solid Target Station (STS) [8]. As far as the irradiation is concerned, STSs are usually mounted in out-ports of compact medical cyclotrons right after the stripper without any optical device for beam steering and focalization. This results in a beam of 10-15 mm Full Width at Half Maximum (FWHM) on target, which implies that only a small fraction (25% or less) of the beam hits the about 5 mm target material. The remaining part of the beam causes unwanted activation of the materials (mostly the capsule) and unnecessary stress for the accelerator. Furthermore, beam tuning procedures are very limited since the centering of the beam on target can only be performed in a coarse way by varying the angle of the stripper. The irradiation procedures for the production of non-conventional radioisotopes using commercial STSs at compact medical cyclotrons need therefore to be optimized by the development of new equipment and methodologies, as reported for example in References [9; 10]. However, it has to be considered that hospital-based cyclotron facilities consist of a single bunker and that very limited space is available to insert beam optics elements to focus and steer the beam. For this purpose, we conceived and developed an Automatic Focusing System (AFS) that steers and focuses the beam on target, controls its size and position and corrects its characteristics if necessary. It is based on a compact (about 1 m long) apparatus combining a magnet with both focusing and steering capabilities, a non-destructive two-dimensional beam monitoring detector and a specific feedback software control system. The development of such an apparatus is complex and requires easy access to the beam area, which is not possible in the compact medical cyclotrons used for routine radioisotope production due to radiation protection issues. The medical cyclotron facility in operation at the Bern University Hospital (Inselspital) [11] was conceived to perform radioisotope production together with multi-disciplinary research by means of a 6 m long Beam Transfer Line (BTL) ending in a bunker with an independent access [12]. In this paper, we present the conception and realization of the AFS together with the first beam tests performed using the BTL.