# Ce- and B-Doped Silica Fibers for Monitoring Low-Energy Proton Beams on a Medical Cyclotron
## 3 Results and Discussion
### Beam-Alignment Effect  
Experiments 2A and 2B were conducted at a target current of 20 \(\upmu\)A when the beam was aligned with the target. The target selector was moved to resemble regular steering at each of the 5 \(\upmu\)A current intervals. The purpose was to assess if the movement and position of the beam relative to the target could be detected using fiber optics. Figure 4a,b show an example from the Ce-200 fiber. Figure 4c summarizes all experiments with the Ce-200 and B-200 fiber, respectively. The fibers were mounted next to the bottom collimator, as shown in Figure 2.  
Specifically, in Figure 4a the fiber signal increases when the beam is steered into the left collimator at time 5 s and decreases when it is steered into the right collimator at time 12 s. On the other hand, in Figure 4b, the fiber signal decreases when the beam is steered into the top collimator and decreases even more when it is steered into the bottom collimator. Additionally, the fiber signal peaks when the top collimator current is slightly greater than the bottom collimator current. Overall, there is not a clear correlation between the current on the top and bottom collimators with the fiber signal as expected if the fiber signal strength is a measure of the alignment with the target material. When the beam is misaligned with the target, more beam is measured on the respective collimator and does not reach the target material as much. On the other hand, Figure 4a shows a correlation for increasing beam current on the left collimator. Figure 3: Response of the different fibers to changing beam current for different experiments. The R\({}^{2}\) values of the linear fits are (1B) 0.9968, (1A) 0.9661, (1C ascending) 0.9397, (1C descending) 0.9905, (1D Ce-200) 0.9929, and (1D B-200) 0.9905, respectively. We suspect that this correlation with the left collimator current occurs due to the beam entering the target already at an angle since it is extracted at 13 MeV, the lower end of the TR13's extraction energy range and the extreme position of the extraction foil. At the high energy the beam angel is directed into on direction, see Figure 5a, while at the lower extraction energy it is directed into the opposite direction, Therefore, when the beam is steered towards the left collimator by moving the target, the beam is actually steered into the wall of the target, thus increasing the prompt radiation. However, when the beam is moved in the opposite direction it cannot impinge as much on the opposite wall due to its angle and thereby stops in the collimator. This effect is not as strong in the top/bottom orientation as the beam does not have an extreme angle entering the target in this direction. An illustration of this hypothesis is shown in Figure 5.  
The relationships between the steering and the fiber signal were further explored at different beam currents and steering levels as shown in Figure 6, with regular steering in (a) and (b) and intense steering in (c) and (d) (experiment 3). While regular steering has a correlation with the fiber signal, especially in the right and left collimators, intense steering appears to have no clear correlation. Figure 4: In (**a**,**b**), the collimator currents are compared with the Ce-200 fiber’s signal while at a beam current on the target of 20 \(\mu\)A. Both fibers are mounted at the side of the target closest to the bottom collimator. The bar graph in (**c**) expresses the correlations between the collimator currents and the fiber signal when the target selector was moved in the plane that the collimators were positioned in, and the beam current’s correlation during these periods. Figure 5: Illustrations comparing the 19 MeV (**a**) and 13 MeV (**b**) extractions and the resulting beam steering effects. The arrows indicate target movement in the left-right direction.