# A Two-Dimensional Non-Destructive Beam Monitoring Detector for Ion Beams
## 1 Introduction  
In the field of ion beam accelerators and their applications, the precise monitoring of the beam intensity, position, and shape is of utmost importance during both the commissioning and operational phases. This is particularly significant in research activities, where a comprehensive understanding of the beam distribution and intensity on the sample being irradiated is often a fundamental requirement to accurately quantify the radiation effects being studied. In the medical field, beam monitoring detectors play a crucial role in controlling the delivery of ion beams in various scenarios. For example, low-energy, high-intensity proton beams with energies around 20 MeV and intensities ranging from 10-100 \(\mu\)A are utilised for the production of radioisotopes in nuclear medicine [1; 2; 3]. On the other hand, high-energy, low-intensity beams, such as protons with energies around 250 MeV and intensities around 1 nA, and carbon ions with energies around 4800 MeV and intensities around 0.1 nA, are utilised in cancer particle therapy. Along this line, a research program focused ondeveloping advanced beam monitoring detectors for ion beams is underway at the Bern cyclotron laboratory [4]. A variety of options exist for beam monitoring in particle accelerators [5; 6]. One simple approach to measuring the beam intensity involves the use of Faraday cups [7]. In this kind of device, a metal body is placed in the path of the particle beam to absorb it. The electrical charge carried by the particles is transferred to the metal and can be measured using an ammeter in electrical contact with the cup. If the metal block is segmented into multiple electrodes with independent readout, additional spatial information can be gathered, and beam distribution measurements, typically called beam profile measurements, can be obtained [8]. Another method for beam monitoring involves the use of movable phosphor screens in combination with CCD cameras [9]. This solution is typically employed to monitor the beam along transfer lines. Since they are based on destructive measurements, Faraday cups and conventional phosphor screens do not allow, however, for real-time observation of the beam during irradiation. To achieve this, non-destructive detectors need to be used. These are solutions that aim to measure the beam properties with minimal disturbance, ensuring that monitoring can take place without disrupting the beam. Examples of non-destructive detectors include inductive pick-ups, which use coils located around the beam pipe to measure magnetic field variations and determine the beam's position and intensity [10], and Secondary-Emission Monitors (SEM's) [11]. SEM's rely on the emission of low-energy electrons that is produced when charged particles strike a metal surface. This so-called Secondary Electron Emission (SEE) is a surface effect, so very thin metal layers or even thin wires can be used [12]. Some examples of SEM's include the SLIM [13], developed by the TERA Foundation at CERN for particle therapy, and the BISE [14; 15], developed jointly by TERA and AEC-LHEP for low-energy cyclotrons used in the production of radioisotopes. One of the challenges in using SEM's for beam monitoring in accelerator environments is their sensitivity to magnetic fields, which makes it difficult to accurately reconstruct the beam profile. To overcome this limitation, our research group has focused on the development of 2D and 3D beam monitor detectors using scintillators [16; 17]. One of the detectors developed by our group is the UniBeMa, which uses moving scintillating fibers to measure the vertical and horizontal projections of the beam profile [18]. This device has been commercialised by the Canadian company D-Pace [19]. In this paper, we present the design, implementation, and characterisation of a two-dimensional non-destructive beam monitoring detector called \(\pi^{2}\). This detector is based on a very thin aluminium foil coated with a phosphor scintillating compound.