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Investigation of the 230Th(p,2n)229Pa Reaction as a Route to 225Ac


Actinium-225 has important medical applications as an agent for targeted alpha therapy, but existing supplies of this isotope are limited. The 230Th(p,2n)229Pa reaction has been considered as a route to 225Ac, however, there is no available cross section data for this reaction in the literature. The purpose of this work is to measure the 230Th(p,2n)229Pa reaction cross section in the energy range where this reaction has been calculated to peak to determine the feasibility of using this reaction for 225Ac production.

As there is not a commercial source of 230Th, for this work, thorium, naturally enriched in 230Th, was separated from uranium ore. A novel procedure was developed for this separation consisting of leaching, liquid-liquid extraction, precipitations and ion exchange chromatography. The thorium was fabricated into accelerator targets using electrodeposition. The final targets were deposited onto a thin (10 μm) titanium backing and were highly uniform and stable. Target thicknesses ranged from ~900 to 1900 μg Th/cm2, which was sufficient to make the relevant cross section measurements.

Chemical processing of the irradiated targets was necessary to determine the activity of the protactinium activation products. The target processing was optimized for rapid, high yield and high radiopurity chemical separations. Column chromatography was used for the separation and the final chemical processing procedure is based on a Dowex 1x8 column in HCl media. The chemical studies showed that titanium was an ideal target backing material due to its chemical behavior and dissolution properties. CL resin (TrisKem International) was considered for the target processing, and, while ultimately not used, it was characterized further and batch studies are presented for radium, actinium and thorium in HCl and HNO3 as well as radium, thorium and protactinium in HF. Six column separation studies were also performed with thorium, radium, actinium and protactinium in a variety of solution conditions. Further extraction studies were done with radium and actinium using Pb resin (Eichrom) and Rose Bengal. Both radium and actinium show a strong affinity (k’ >10,000) for the resin at intermediate pHs from solutions with Rose Bengal with the pH range and magnitude of uptake significantly increased by the presence of Rose Bengal compared to basic solutions without this large counter ion.

Thorium and protactinium tracer isotopes were necessary for all of the chemical development studies and were produced with isotope generators. A 237Np/233Pa isotope generator was made and extensively characterized. It was in continual use for over a year with an average yield of ~75% before breakthrough occurred. A 227Ac/227Th isotope generator was also made based on an existing procedure described in the literature.

Nuclear data measurements were done for several relevant protactinium isotopes, including half-life measurements for 228Pa and 229Pa, and modern measurements of the major gamma-ray emissions from the decay of 231Pa. For many of these measurements, the error envelopes have been significantly decreased compared to previous measurements, while remaining in good agreement with the existing literature values.

The 230Th enriched targets were irradiated at the Center for Accelerator Mass Spectrometry at Lawrence Livermore National Laboratory with proton energies ranging from 14.1 to 16.9 MeV. Excitation functions in this energy range are reported for the first time in the literature for the 230Th(p,2n)229Pa and 230Th(p,3n)228Pa reactions. The peak measured value of the 230Th(p,2n)229Pa reaction was 182 土 12 mb at 14.4 土 0.1 MeV. Based on the measured 230Th(p,2n)229Pa reaction cross section, this reaction could reasonably be used for 225Ac production, although significant amounts of relatively isotopically pure 230Th would be needed for significant production.

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