# Ce- and B-Doped Silica Fibers for Monitoring Low-Energy Proton Beams on a Medical Cyclotron
## 2 Materials and Methods
### TR13 Cyclotron  
The TR13 at TRIUMF is a 13 MeV negative hydrogen-ion cyclotron used for producing medical isotopes [15]. This machine has a design energy output of 13-19 MeV. Due to its location at TRIUMF and shielding restriction, the extracted beam energy is restricted to 13 MeV by a permanent beam blocker installed in the acceleration plane. The H\({}^{-}\) ions are transported through a short beam line from an external ion source. After they are accelerated to 13 MeV in the cyclotron, the ion flow is intercepted by a thin carbon foil, stripping the electrons from the H\({}^{-}\) ions, reversing the charge, and thus yielding protons with a curvature opposite to the original trajectory. This allows for the extraction of the proton beam. The TR13 cyclotron has two beam exits on opposite sides, each with a target selector equipped with four target ports, mounted on moveable bellows. Every target position has four electrically isolated collimators tracing the beam's position in relation to the target. A depiction of the target selector and a single target port is presented in Figure 2. The targets can be positioned into the axis of the beam using steering motors. The beam is centered by moving the target while monitoring the beam spill onto the collimators. The data points that were collected from the cyclotron during this analysis include the beam current, the left, right, top, and bottom collimator currents, and the gas pressure inside the target.  
During the experiments, a solid target with a thallium disk was installed in target position \(A\), a water target with de-ionized, natural water was installed in position \(B\), a target plug was installed in position \(C\), and the \({}^{18}\)F gas target was installed in position \(D\). The latter operates via a two-shot method, irradiating enriched \({}^{18}\)O gas and subsequently irradiating an Ar/F\({}_{2}\) mixture to recover the produced \({}^{18}\)F from the target body wall. The maximum currents on each target were 10 \(\mu\)A for the  
Figure 1: (a) 200 \(\mu\)m Ce-doped fiber and 200 \(\mu\)m B-doped fiber mounted to the \({}^{18}\)F gas target. (b) 200 \(\mu\)m borosilicate fiber in plastic housing without the black tape wrapping to prevent background from ambient light. (c) Schematic of the setup.  
water target, 15 \(\upmu\)A for the thallium target, 20 \(\upmu\)A for the Ar/F\({}_{2}\) gas mixture, and 25 \(\upmu\)A for the enriched \({}^{18}\)O gas target.  
The different irradiation conditions are summarized in Table 2. In general, the beam current was increased and decreased in 5 \(\upmu\)A increments. As the MPPC only has one channel, the fiber was switched at the MPPC connector at the end of each current step so that the signal from both fibers could be measured during the same irradiation subsequently. In experiments 1A to C, no excessive beam steering by moving the target, other than to center the beam, was performed. For runs 2A and B, \"regular\" steering refers to instances where the current on each collimator was typically only increased to approximately 3 \(\upmu\)A to stay under the trip level of the collimator. For experiment 3, \"intense\" steering refers to instances of steering the beam such that the collimator currents reached nearly 5 \(\upmu\)A.