Initial Dosimetry and Characterization of the Aspire ⁴He⁺ Ion Beam Using EBT4 Radiochromic Film and Monte Carlo Simulation
Abstract
Purpose
To experimentally characterize low-energy helium-4 (⁴He⁺) ion beams at subclinical energies (0.5–1.5 MeV/u) at the newly commissioned ASPIRE facility at TRIUMF using radiochromic film dosimetry, and to validate the measured dose distributions through Monte Carlo simulations.
Methods
The base layer was peeled from one side of GafChromic EBT4 films, and films were irradiated with ⁴He⁺ ions at 0.5, 1.0, and 1.5 MeV/u, targeting a 10 mm FWHM beam size and 10 pC charge. To account for high linear energy transfer (LET) effects that cause film under-response, the mean LET across the active layer was calculated using TOPAS Monte Carlo, and LET-dependent correction factors were applied. Further simulations modelled dose deposition in the EBT4 active layer for comparison with experimental measurements, considering both 14 μm and 28 μm active layer thicknesses to account for potential thickness variations due to the peeling process.
Results
Mean ⁴He⁺ LET values across the active film layer were 115, 166, and 188 keV/μm for 1.5, 1.0, and 0.5 MeV/u beams, respectively. Measured beam profiles showed significant asymmetries: 1.5 MeV/u beams averaged 2.6mm (x) by 12.2mm (y), while 1.0 MeV/u beams were 2.5mm (x) by 12.7mm (y). Both energies deviated substantially from the intended 10 mm FWHM, while the 0.5 MeV/u beam was more symmetric at 5.48mm (x) by 4.34mm (y). Agreement between film and Monte Carlo simulations varied considerably, with peak dose differences of 56-69%, 13-24%, and 209-525% for 1.5, 1.0, and 0.5 MeV/u, respectively, depending on the assumed active layer thickness.
Conclusion
Peeled EBT4 films demonstrated useful spatial characterization of low-energy ⁴He⁺ beams. Quantitative dose agreement with Monte Carlo was limited by uncertainties in delivered charge and beam tuning rather than intrinsic film performance. Implementing reliable charge diagnostics and improved beam steering is expected to enable more accurate dosimetry, supporting future radiobiological studies.