# An Active Irradiation System with Automatic Beam Positioning and Focusing for a Medical Cyclotron
## 2 Materials and Methods  
The AFS was developed to enhance the irradiation performance of the Bern medical cyclotron laboratory. This facility is based on an IBA 18/18 HC cyclotron providing proton beams with a nominal energy of 18 MeV and beam currents in the range from a few pA to 150 \(\mu\)A [13]. The cyclotron features eight out-ports: four are used for routine production of \({}^{18}\)F for PET imaging, 1 is connected with a commercial IBA Nirta solid target station, two are reserved for future developments and one is connected to the BTL. The BTL is 6 m long and its optical elements consist of two horizontal-vertical (H-V) quadrupole doublets, the former located in the cyclotron bunker and the latter in a second bunker, where experiments are conducted [12]. The two hardware components of the AFS are--the Mini-PET Beamline (MBL) [14], and a two-dimensional beam profiler (UniBeM). The MBL is an innovative compact (about 50 cm long) magnet embedding two quadrupole and two steering magnets within the same structure, produced by the Canadian company D-Pace. In order to maximize its compactness, the two quadrupoles are located next to each other without any drift space in between. The two bending dipole coils are enclosed within the quadrupoles, leading to interference effects. This unusual design allows focusing and steering the beam in a very limited space, as in a standard cyclotron bunker. The UniBeaM detector [15] was developed by our group to measure the beam profile in a wide range of currents. It is based on scintillating silica doped fibers passing through the beam. The position of the fiber is recorded together with its light output to obtain the beam profile at a given location along the beam line. Several commercial versions of the UniBeaM are produced under licence by D-Pace [16]. For the tests described in this paper, the AFS was installedat the end of the BTL, as shown in Figure 1. According to the specific measurement to be performed, three different instruments were used to assess the beam characteristics in the exact position where the IBA Nitta production solid target station is foreseen to be located--a two-dimensional UniBEaM profiler (dual axis UniBEaM by D-Pace), a two-dimensional non-destructive beam monitor (named \(\pi^{2}\), indicated with (4) in Figure 1) followed by a beam dump or a specific target station with current read-out. The \(\pi^{2}\) detector [17] is based on a thin (about 1 \(\upmu\)m) aluminum foil coated by P47 scintillating material read-out by a Charge-Coupled Device (CCD) camera allowing to assess the beam characteristics at the position of the target in two dimensions. The foil is tilted by 45\({}^{\circ}\) with respect to the horizontal plane. To measure the real dimensions of the beam, images are corrected by taking into account the effects due to the tilt and to distortions of the optics of the CCD camera. The target station with current read-out was developed by our group to measure nuclear reaction cross sections [5]. It has a 6 mm diameter collimator behind which a specifically designed target coin [8] can be placed to perform production tests at low current (below 1 \(\upmu\)A) since this device is not water-cooled. This coin has a pocket for the target material that is covered by a thin aluminum foil that produces a negligible degradation of the beam energy. Aluminum disks can be located in front of the coin to reduce the impinging energy if necessary. To suppress secondary electrons that can increase and therefore alter the current reading, an electrode was used as in Faraday cups (50 V bias voltage). For the AFS, a specific two-dimensional UniBEaM detector ((3) in Figure 1) was developed, as shown in Figure 2. It features two silica fibers moving vertically and horizontally whose planes are only 5 mm apart. This is realized by a specific vacuum chamber, named Magic-Box, in which the KF-50 flanges are two-by-two appropriately staggered. The Magic-Box allows measuring the horizontal and the vertical profiles in two very close locations along the beam path.