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The Effect of Spatiotemporal Variations in Thermal Neutron Transmission On Yb-169 Source Re-Activation Efficiency

RT Flynn1*, QE Adams1, KM Hopfensperger2, (1) Department of Radiation Oncology, University of Iowa, Iowa City, IA, (2) Department of Biomedical Engineering, The University of Iowa, Iowa City, IA


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: Commercializing intensity modulated brachytherapy techniques based on partially-shielded intrauterine or interstitial applicators necessitates the cost-effective production of intermediate energy sources. ¹6?Yb is a promising but expensive isotope for this purpose, with 93 keV average ?-ray energy. Re-activating a single ¹6?Yb source 10+ times in a nuclear reactor was theoretically shown to reduce cost by ~75% relative to conventional single-activation sources. Substantial spatiotemporal variation in isotopic source composition occurs between activations via ¹68Yb burnup and ¹6?Yb decay, altering neutron transmission and cost per re-activation. These effects are accounted for to maximize the efficiency of the ¹6?Yb re-activation process.

Methods: A computational framework to model time-dependent neutron transport, isotope transmutation, and decay was developed. Thermal neutron flux within each cylindrical sub-volume was calculated using ray-tracing through the spatiotemporal dependent isotopic composition throughout the source, accounting for thermal neutron attenuation along each ray. Re-activation of a ¹6?Yb source constructed of 82%-enriched ¹68Yb-Yb2O3 was modeled, with active dimensions of 0.6 mm diameter, 10.5 mm length, a volume of 3 mm³, and a density of 8.5 g/cm³.

Results: Average thermal neutron transmission throughout the active source more than doubled between the first activation and the twelfth (final) activation, increasing from 29.7% to 61.7%. The presented model predicts a 32% reduction in the reactor-time needed per clinic-year of ¹6?Yb activity produced at a reactor flux of 4 × 10¹4 n cm?² s?¹ relative to a model that does not account for spatiotemporal changes in thermal neutron transmission throughout the active source.

Conclusions: Accounting for spatiotemporal changes in thermal neutron attenuation within a re-activatable ¹6?Yb source demonstrates a 32% reduction in reactor time per clinic-year needed to generate the ¹6?Yb sources. This finding increases the feasibility of the ¹6?Yb approach to enabling intensity modulated brachytherapy approaches such as rotating shield brachytherapy or direction-modulated brachytherapy.

Funding Support, Disclosures, and Conflict of Interest: RTF is the founder of pxAlpha, LLC, which is developing rotating shield brachytherapy technologies.


Nuclear Interactions, HDR, Brachytherapy


TH- Brachytherapy: Development (new technology and techniques)

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