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Nuclear Excitation Functions for the Production of Novel Medical Radionuclides

  • Author(s): Voyles, Andrew Steven
  • Advisor(s): Bernstein, Lee A
  • et al.

The future of nuclear medicine would appear to be the paradigm of personalized medicine — targeted radionuclide therapy to spare healthy tissue, and theranostic medicine, which pairs an imaging isotope with a therapeutic isotope to provide simultaneous, real-time dose delivery and verification, leading to drastic reductions in prescribed patient dose. Candidate isotopes to meet these needs have been identified based on their chemical and radioactive decay properties. This dissertation represents two extremes of cross section measurement for isotope production, as part of a larger campaign to address deficiencies in cross-cutting nuclear data needs. These studies will serve to facilitate the production of pre-clinical quantities of radioactivity for emerging and novel medical radionuclides.

This dissertation details two experiments, focusing on production pathways for the radionuclides 90Mo, 64Cu, and 47Sc. Each of these experiments has been designed as part of efforts to measure production cross sections for emerging medical radionuclides and develop new methods for the monitoring of charged-particle beams. The discussion focuses on describing the experimental methods and analysis used for this measurement, and illustrates the importance of accurate knowledge of target composition. The experimental measurement of the 93Nb(p,4n) 90Mo reaction was motivated by its use as an intermediate-energy proton monitor, and was carried out through a stacked-target irradiation of thin niobium, copper, and aluminum foils at LANSCE-IPF. The work described herein is the first step in an effort to characterize this reaction as a robust and reliable, contamination-free monitor reaction channel for 40–200 MeV protons. In addition, it illustrates the potential for this 100 MeV proton beam as a production pathway for multiple novel radionuclides. The measurement of the 64Zn, 47Ti(n,p) cross sections was carried out at the recently-commissioned UC Berkeley High Flux Neutron Generator, a compact DD neutron generator designed originally for geochronology measurements. This work was motivated by the production of 64Cu and 47Sc, a pair of emerging medical radionuclides prized in particular for their capacity for theranostic applications. This measurement additionally provides context for the utility of neutron generators for isotope production, as well as introducing an efficiency quantification metric. Based on experience garnered from the LANSCE-IPF experiment, we have established a sister facility for measurements at the Lawrence Berkeley National Laboratory’s 88-Inch Cyclotron. First experiments have already been performed, and one such measurement is briefly described here. The work described here may help to enable exciting new campaigns of investigation in basic science, disease biology research, and nuclear medicine.

This dissertation serves as a pedagogical example to those who follow of how the assort- ment of unexpected difficulties in precision nuclear data measurements can make “simple” experiments not so simple, after all. One significant issue that was uncovered as a result of this work was the production of 22,24Na as the proton beam traversed the acrylic adhesive on the Kapton tape used to contain the individual stacked targets in these measurements. Reactions on the silicone adhesive would occur in any stacked-target activation experiments where adhesive-backed Kapton is used, and has the potential to systematically dampen the magnitude of reported cross sections by as much as 50%. This issue is discussed as a caution- ary note to future stacked-target cross section measurements. In addition, contributions to the slowing of charged particle beams due to the adhesive have often been neglected in much work performed to date. While this plays a limited role at high beam energies, it becomes increasingly important for proton energies below 25 MeV.

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