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Novel Proton Range Probe: Multiple Monitor Reaction Variance Minimization in Activated Metallic Foils

S Graves

S Graves*, P Ellison, T Barnhart, R Nickles, J Engle, University of Wisconsin - Madison, Madison, WI


SU-H1-GePD-T-3 (Sunday, July 30, 2017) 3:00 PM - 3:30 PM Room: Therapy ePoster Lounge

Purpose: Conventional 1-D proton range measurement methods involve the use of costly multiple-layer ionization chambers (MLICs). Thin-foil activation quantification represents a simple alternative to these methods. Additionally, this technique offers the possibility of in vivo foil implantation near target margins. The purpose of this work was to evaluate the feasibility of employing multiple nuclear monitor reactions for residual proton range estimation.

Methods: A target stack was constructed containing thin Al (100 µm) and Cu (30-100 µm) foils encapsulated in 25 µm Kapton® polyimide tape between aluminum degrader layers. The target stack was irradiated by ~100 nAh of 100 MeV protons at the Los Alamos National Laboratory (LANL) Isotope Production Facility (IPF). The target stack was disassembled and characteristic gamma emissions were quantified by serial high-purity germanium (HPGe) gamma spectroscopy measurements. Proton transport through the target stack was simulated stochastically by MCNP6 and semi-emprically using Anderson & Ziegler (A&Z) formalisms. Literature cross section data for IAEA recommended monitor reactions (ⁿᵃᵗCu(p,x)⁵⁶Co, ⁿᵃᵗCu(p,x)⁶²Zn, ⁿᵃᵗCu(p,x)⁶⁵Zn, and ⁿᵃᵗAl(p,x)²²Na) were used to empirically determine proton fluence and energy in each target stack compartment. Proton energy distribution widths were determined by MCNP6 simulation prior to monitor reaction calculations.

Results: Average proton energy in the final stack compartment was predicted to be 43.1 MeV and 40.5 MeV by MCNP6 and A&Z calculations, respectively. Contrasting these results, an inter-monitor reaction fluence variance minimum was identified at 35.1 MeV, suggesting that predictions overestimated residual proton range in water by 3-5 mm. This discrepancy is likely due to target stack areal density mischaracterization, which illustrates the usefulness of this empirical proton energy determination tool.

Conclusion: Average proton energy has been empirically determined by thin-foil activation quantification and monitor reaction variance minimization. This technique shows promise for being a simple and inexpensive alternative to MLICs as a 1-D proton range probe.

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