# Ce- and B-Doped Silica Fibers for Monitoring Low-Energy Proton Beams on a Medical Cyclotron
## 1 Introduction  
Many isotopes for diagnostic and therapeutic applications can be produced on a medical cyclotron. To maximize the production of medical radioisotopes, the system's parameters must be understood. Therefore, the shape, trajectory, and the location of the proton beam relative to the production target must be quantified to optimize production yields. Currently, for low energy cyclotrons, no techniques are available that allow for real-time monitoring of proton beams that also enable operators to evaluate the beam position and angle relative to the target being irradiated, or the beam shape in real time and in situ. High energy particle beams can be profiled in real time by intercepting the beam with sensors. Unfortunately, low energy cyclotrons cannot easily be profiled in this manner as these materials would absorb a large fraction of the beam's energy; thus, reducing the cyclotron's production efficiency [1]. Concomitantly, these devices are at risk of thermal failure due to their propensity to accumulate excess power at production currents. Alternative methods of beam profiling include periodically traversing the beam with a wire [2], radiographically analyzing irradiated foils using radiosensitive films [1; 3], optimizing the beam while irradiating a scintillating material and quantifying the results [4],using electrically isolated materials at the front of the target such as collimators [5], or tuning the beam at the beginning of a production run using a beam profile monitor [1]. Nonetheless, none of these methods provide an online, rapid feedback with sharp spatial resolution for measuring the beam profile and trajectory simultaneously onto a target. Overall prompt radiation has also been measured to monitor target conditions [5].  
One possible solution that has shown promise for profiling proton, photon, and electron beams are Ce-doped silica-fiber optics [6; 7; 8; 9]. In fact, these fibers have previously been tested at TRIUMF to measure dose response and steering of proton beams for proton therapy [6; 10], and at Bern University Hospital to characterize an 18 MeV proton beam used for medical isotope production [11] where it was used as a beam profile monitor incorporated into a beam line.  
When placed near a proton beam, ionizing radiation such as gamma rays or neutrons deposit energy in doped optical fibers and excite or ionize electrons, and consequently leave the atoms of the dopant in an excited state. These excited states are short-lived, and thus the atoms decay fluorescently to return to their ground state. The fluorescence can be transmitted via an optical transport fiber and detected by a photodetector, converting the radiance reading into a current related to the energy imparted on the fiber. The combination of fast fluorescence with small fiber optics allows for near real-time data acquisition and high spatial resolution [9].  
Overall, being extremely thin and potentially radiation resistant in comparison to organic fibers, inorganic fiber optics offer an elegant solution to profile low energy cyclotron proton beams [10; 12; 13]. However, as the proton beam profile is only measured in the plane in which the fiber(s) probe the beam, the direction and divergence of the beam is only accessible if it is measured in several locations along the beam axis. Many isotope production cyclotrons are very compact and do not have room for one or several profile monitors in front of the production target. To still give information about the beam current, beam direction and beam divergence, we explored the possibility of attaching the fibers onto the outside walls of targets to provide information about the beam movement and its position by comparing the differing fluorescence intensities of the fiber array. The objective of this research was to evaluate the potential of fiber optics to give information about the current, contour, and trajectory of low energy proton beams.