Direct Measurement of Absorbed Dose from a Pre-Clinical Am-241 Alpha Particle Irradiator
Abstract
Purpose
Accurate absorbed dose determination for alpha emitting sources remains challenging due to short particle ranges and sensitivity to source geometry. This work presents a practical framework for direct absorbed dose to water measurements from a plated 241Am source using a parallel plate ionization chamber.
Methods
Source homogeneity and effective dimensions were evaluated using unlaminated Gafchromic EBT3 film. Alpha spectrometry measurements were performed using a custom vacuum chamber to characterize energy loss in the aluminized mylar entrance window and to acquire in air alpha spectra for Monte Carlo (MC) source inputs. A custom fixed gap parallel plate ionization chamber was constructed and characterized. MC simulations were used to determine entrance window and volume averaging correction factors, as well as fluence-weighted stopping power ratios. Ionization chamber measurements were performed with repeated charge integrations and appropriate correction factors applied.
Results
Film measurements demonstrated spatial uniformity of the plated 241Am source and showed no increase in effective source dimensions with distance, indicating quasi-parallel alpha emission produced by a microcapillary array. MC correction factors reflected evolving alpha energy distributions. Fluence-weighted stopping power ratios decreased across the sensitive volume, with greater variability at larger distances due to increased low energy alpha contributions. The absorbed dose to water was determined at the two source to detector distances.
Conclusion
This work demonstrates a repeatable and experimentally constrained methodology for direct absorbed dose to water measurements from alpha emitting sources using an ionization chamber. By combining film-based uniformity assessment, alpha spectrometry, and spectrum weighted MC corrections, the framework enables accurate alpha particle dosimetry without explicit source geometry modeling. The approach is extensible to other alpha emitting radionuclides and applications requiring traceable absorbed dose measurements.