# A Two-Dimensional Non-Destructive Beam Monitoring Detector for Ion Beams
## 2 Materials and Methods  
The \(\pi^{2}\) detector was developed to enhance the beam monitoring capabilities of the medical cyclotron laboratory at the Bern University Hospital (Inselspital) [4]. The cyclotron, a Cyclone 18/18 model from the Belgian company IBA, provides proton beams with a nominal energy of 18 MeV and a maximum beam current of 150 \(\upmu\)A. The facility is used for both radioisotope production and a range of multidisciplinary research studies, and has been specifically designed to accommodate both types of use. To enable the research activities, a Beam Transfer Line (BTL) has been installed, which transports the proton beam to a separate bulker located adjacent to the main cyclotron bunker [20], with independent access. The BTL is 6.5 m in length and features two steering dipole magnets and two quadrupole magnets to allow for displacing and focusing of the beam on both horizontal and vertical planes. The BTL is also equipped with two destructive beam viewers located at the beginning and end of the transfer line, which are used to visually monitor the beam and to measure its current. The cyclotron can be operated using a specialised method that allows for stable beams down to the picoampere range, making it possible to support a wide range of research activities that require very low beam currents. This method was developed by our research group and has been documented in a separate publication [21]. The \(\pi^{2}\) consists of a cerium- and terbium-doped yttrium orthosilicate (Y\({}_{2}\)SiO\({}_{5}\):Ce,Tb) phosphor scintillating material (P47) deposited on an aluminium foil. When traversed by the beam, it emits light in the blue region (\(\lambda\simeq 400\) nm), so the beam footprint can be observed on the scintillating screen. The light emission is characterised by a fast luminescence decay, reaching 10% of its initial intensity within just 100 ns. The thickness of the deposition and of the foil can be chosen according to the specific use, taking into account, however, that while a very thin foil is less intercepting, it is also more fragile and could brake, especially when the primary vacuum pump of the beam line is switched on. We used \(\sim\)15 \(\upmu\)m-thick aluminium foils coated with a layer of P47. The thickness of the P47 coating was measured to be less than 1 \(\upmu\)m using an Ortec Alpha Duo Spectrometer, based on the measurement of energy loss of alpha particles emitted by a \({}^{241}\)Am source. The working principle of the device and its technical drawing are depicted in Figure 1. The aluminium foil is securely attached to an elliptical support using SikaPower 1548 adhesive for optimal heat dissipation. The scintillating screen is positioned in a vacuum environment at the center of a DN-40 ISO-KF 4-way cross and tilted at a 45\({}^{\circ}\) angle relative to the beam path. The scintillating foil covers an elliptical area, which corresponds to a 20-mm diameter circle when projected in the transverse plane of the beam and covers the full beam pipe cross-section. One of the two cross ports perpendicular to the beam path is used to mount the scintillating screen on a pneumatic actuator, which enables it to be easily inserted and removed from the beam path. The opposite port, on the coated side of the scintillating screen, is fitted with a transparent glass flange to allow for the scintillating light to escape and be captured by a CMOS camera, which is located inside a Poly-Oxy-Methylene (POM) cylinder for neutron shielding. The device we realised is depicted in Figure 2, as installed in the BTL (left). In an effort to make the device cost-effective, we chose to use a Raspberry Pi micro-controller in conjunction with a Raspberry Camera Module V2 to record the beam profile on the scintillating screen. This combination of components offers a solution that is both affordable and reliable. Both the camera and the micro-controller can be easily replaced in case of degradation or failure due to radiation. The camera features an 8 megapixel CMOS sensor.