Fig. 1. Scintillating fiber imaging spectrometer (SciFi stack) concept. (a) A parallel fiber array forms a single-axis beam-profile monitor. (b) A layer is formed from parallel fiber array panels rotated at a number of angles. This layer samples the 2D beam profile such that it can be reconstructed with tomography methods. (c) Stacking many layers enables reconstruction of many energy bins, maintaining the ability to introduce filtering between the layers to extend the range to high energies in a compact manner.

Fig. 2. Modeled performance of a SciFi stack imaging spectrometer. (a) Estimated spatial resolution as a function of the number of projection angles, m, for fiber diameters, μm. Resolutions for designs with angles are labeled. The resolutions of HDV2 and EBT3 RCFs and a plane scintillator instrument with a filter array^[25] are shown for comparison. (b) Energy resolution of protons for SciFi stack designs for each fiber diameter and RCF active layers. (c) Detector sensitive range as a function of the numerical aperture, NA, for μm and the imaging system described in Section 3. The effects of altering the instrument via the magnification, M, or optical density, OD, of the imaging system and scintillation yield, , or fiber coupling efficiency, , are indicated with black arrows.

Fig. 3. Computer-aided design model render of the SciFi BPM detector head construction, excluding optical transport fibers. Scintillating fibers and fiber clamps are highlighted in green and yellow, respectively. The top and right-hand side fiber clamps have been removed to show the grooves machined in the Al to set the fiber positions.

Fig. 4. (a) Experimental setup for determination of SciFi BPM sensitivity (not to scale), illustrating the incoming laser path, target and SciFi BPM geometry. A PTFE block is in the path between the target and the SciFi BPM, introducing an edge to the beam, and five layers of RCF are used to absolutely characterize the proton spectrum. Scintillation light from the SciFi BPM travels to the ends of optical transport fibers that are imaged through a window with a camera outside the chamber. (b) Raw image on the SciFi BPM camera. (c) Calibrated SciFi BPM profiles after processing the raw image data.

Fig. 5. Calibrated RCF and SciFi BPM data for measuring sensitivity and verifying the spatial capability of the detector. (a) Scanned RCF from the front of the SciFi BPM. Green lines indicate positions of scintillating fibers in the BPM behind the RCF layers, used as ROIs for comparison to fiber signals. The yellow line is the ROI for fiber . (b) Proton spectrum for fiber . Blue markers are the RCF summed signals in the fiber ROI, at the Bragg peak energies of each RCF layer found with Monte Carlo simulations and labeled in the lower right-hand legend. The dashed red line is the proton spectrum from Equation (4) fit to the RCF data, with MeV. Solid blue and green lines are the simulation proton deposition for RCF layers and 0.5 mm scintillating fibers, respectively, scaled by the fitted spectrum. Green markers are the predicted deposition in the scintillating fibers. Error bars are the full width at half maximum (FWHM) of the Bragg peaks. (c) Horizontal and (d) vertical profiles from RCF fiber ROIs (blue) and calibrated SciFi BPM signals (green). See the main text for discussion of uncertainty limits. The darker grey shaded region in (c) shows positions where the whole length of fibers is blocked by PTFE, and the lighter grey region indicates fibers that are partially blocked due to the angle of the filter from vertical. (e) SciFi horizontal (dark green) and vertical (light green) profiles on a linear y-scale. Black solid and dashed lines are idealized proton x- and y-profiles, respectively, with the filter modeled as a binary mask rotated at ° to the vertical.

Fig. 6. (a) Proton beam profile used for Geant4 simulations. (b) Sinograms generated from selected layers of the SciFi stack. (c) MLEM reconstructions of the energy deposited in the selected SciFi stack layers. Regions of interest with a radius of are used for evaluating the reconstructed energy deposited by each beamlet, and are shown with white dashed lines.

Fig. 7. Energy deposition as a function of depth of plastic. Black symbols show the total energy deposition in each layer, where gold circles highlight the layers whose spatial reconstructions are plotted in Figure 6(c). Blue, purple and red symbols are the reconstructed energy deposition within the regions of interest for beamlets I–III, respectively. The grey shaded regions indicate the 2 mm thick filters between the scintillating fiber layers.