Non-Destructive Beam Monitoring via Secondary Radiation Detection with Ce-Doped Silica Fibers

TL;DR

The paper addresses the need for non-destructive beam diagnostics in low-energy medical cyclotrons to avoid perturbing the beam. It introduces and tests an external fiber monitor (EFM) based on Ce-doped silica fibers that detect secondary radiation around existing beamline components, evaluated on the Bern Medical Cyclotron across three scenarios: intensity monitoring, beam-loss monitoring, and beam-position monitoring. The results show a linear EFM response to beam current over about three orders of magnitude, a monotonic relationship between EFM signal and beam losses proximate to a collimator, and decoupled position sensitivity using opposing-fiber ratios; the approach demonstrates high signal-to-noise and insensitivity to direct beam interception. The study highlights the EFM as a practical, retrofit-ready diagnostic that can supplement interceptive devices, with prospects for improvement through higher-light-yield scintillators and neutron/gamma discrimination to enhance accuracy and spatial resolution.

Abstract

Non-destructive beam diagnostics are essential for low-energy medical cyclotrons, where even thin interceptive devices can severely degrade beam quality. We investigate an external fiber monitor (EFM) based on Ce-doped silica scintillating fibers that detects secondary radiation generated at existing beamline components of the 18 MeV Bern Medical Cyclotron beam transfer line (BTL). Three use cases were studied: (i) beam intensity monitoring around an electrically isolated, water-cooled beam dump; (ii) beam-loss monitoring around a 10 mm collimator under varying the beam focusing; and (iii) by steering a 6.5 mm $\times$ 6.5 mm beam spot on a beam dump. For case (i), the summed EFM signal exhibits a linear dependence on the current on target over nearly three orders of magnitude. In case (ii), a normalized EFM-based beam-loss proxy scales monotonically with an electrical loss proxy across several focusing settings. Furthermore, opposing-fiber signal ratios provide decoupled, monotonic sensitivity to horizontal and vertical beam displacements.

Figures (7)

• Figure 1: Schematic of the measurement setup. The proton beam (in blue) strikes an aluminium beam dump, producing secondary radiation that excites the Ce-doped fibers; the resulting scintillation light is read out with a single-photon counter, connected to a DAQ.
• Figure 2: On the left, a photograph of the water-cooled target installed on the BTL is shown. The four EFM sensing fibers (1) are mounted around the beam dump (2). The four transport fibers with the SMA connectors are labelled with (3) in the picture on the left. They are routed away from the beam dump to avoid signal pickup. The proton beam's direction is indicated by the blue arrow. On the right, a simplified schematic of the set-up is shown, showing the internal collimating structure of the target and the axial positions of the fibers around it; schematic not to scale.
• Figure 3: The BTL setup used for the Beam Loss Monitoring measurement. The FWHM of the proton beam is measured with the UniBEaM (1) right before the beam collimator (2) (10 aperture), where the EFM was installed (not shown in picture). After 1.4 of drift space (3) the Pi2 detector (4) is installed before the beam dump (5).
• Figure 4: (a) The BTL setup for the beam steering measurement. The double-slit collimator (1) allows controlled horizontal and vertical steering of a beam spot. An additional collimator (2) is installed to prevent the beam from hitting the beam dump's (4) edge. The Pi2 detector (3) is installed to have an additional reference for the beam spot's position. The EFM fibers are installed around the beam dump at (4). (b) The collar holding the four EFM fibers around the aluminium beam dump.
• Figure 5: Recorded EFM signal strength is plotted as a function the proton current measured on the water-cooled beam dump. The background subtracted data are fitted with a linear fit on the logarithmic scale (power law fit). The top-right panel shows the relative residuals between the data and the fit, with the $\pm3\%$ interval highlighted in red. The bottom-right panel displays the background-corrected signal-to-noise ratio for each data point.