# Ce- and B-Doped Silica Fibers for Monitoring Low-Energy Proton Beams on a Medical Cyclotron
## 3 Results and Discussion
### Beam-Alignment Effect
To further explore this effect, the left collimator current (displaying correlation with the fiber signal) and top collimator current (displaying no clear correlation with the fiber signal) were graphed against the fiber signal, the beam current, and the target's pressure. The pressure \(p\) was normalized to match the range of the current data for comparison using the transformation formula  
\[\overline{p} = \frac{I_{max}-I_{min}}{p_{max}-p_{min}}\times(p-p_{max})+I_{max}\]  
where \(I_{max}\) and \(I_{min}\) are the maximum and minimum currents for the given data set and \(p_{max}\) and \(p_{min}\) are the maximum and minimum pressures for the time period the data corresponds to. The data was collected during an irradiation with maximum beam steering, where the Ce-200 and B-200 fibers were attached to the bottom collimator's side of the target, see Figure 2. The results comparing the left collimator's current with the fiber signal are displayed in Figure 7a,b, while the top collimator's current is compared in Figure 7c,d. Note that the negative collimator current outputs are believed to be the result of a minor calibration error in the TR13 current read-out module. In all four conditions, the target current, the target pressure and the fiber signal increase with increasing collimator current before decreasing again. This is indicative of the beam being slightly misaligned with the target center. For instance, a peak in the target pressure implies that the beam is irradiating the largest target-area cross section and depositing the most power. Since the pressure increases when the beam is steered slightly towards the left and top collimators, the beam must not have been optimally aligned despite balanced collimator currents. The only exception is when the beam is steered onto the left collimator at high currents, in which case the fiber signal continues to increase slightly after 1 \(\mu\)A. Again, this can be explained by the beam angle in relationship to the  
Figure 6: (**a**) Correlation coefficients as a function of beam current on target for regular steering up to 3 \(\mu\)A on the right and left collimator and (**b**) on the top and bottom collimator. (**c**) Correlation coefficients as a function of beam current on target for intense steering up to 5 \(\mu\)A on the right and left collimator and (**d**) on the top and bottom collimator. target axis as sketched in Figure 5. In addition, the maximum for the pressure and fiber signal for the left/right plane is at a left-collimator current of \(\sim\)1.25 \(\upmu\)A, while in the top/bottom plane, it is at top-collimator current of 0.75 \(\upmu\)A. This can be explained by a larger beam size in the left-right direction than in the top-bottom direction, as confirmed by beam profile measurements [1]. The results in this section imply that our fiber-sensing technique holds information about the centering of the beam on the target material and the alignment with the target axis. In addition, it may be possible to extract information about the size of the beam as a larger beam results into potentially more beam deposition into the target wall, which in turn results in a higher fiber signal.