Initial characterization of the ASPIRE ⁴ He ⁺ ion beam using EBT4 radiochromic film and Monte Carlo simulation

Abstract Experimental radiochromic film dosimetry of a helium-4 ion beam at subclinical energies (0.5–1.5 MeV/u) was performed at the newly commissioned ASPIRE facility at TRIUMF. Peeled GafChromic EBT4 films were irradiated with low-energy 4 He + ions, then scanned and analyzed to assess their suitability for spatial and quantitative dosimetry at these energies. To account for the film under-response caused by the high linear energy transfer (LET) of the beam, a correction factor was applied based on the mean LET calculated across the film's active layer using simulations. Monte Carlo simulations were performed with TOPAS to model dose deposition in the EBT4 film. Agreement between film measurements and Monte Carlo results was variable, largely limited by the lack of accurate delivered beam charge measurements. The experiments also revealed beam asymmetries and smaller-than-expected beam widths, attributed to suboptimal tuning of the accelerator chain. Overall, peeled EBT4 films were shown to be a practical tool for spatial beam characterization of helium-ion beams at subclinical energies. However, large uncertainties in delivered beam charge and measured doses beyond the range of EBT4 and the calibration curve used made accurate dosimetry impossible. This work motivates future studies at higher, more clinically relevant energies, where improved charge measurement and beam stability will be critical.