PhD Defense | Bernat Ballester | Photonuclear cross section measurements for the production of the medical radioisotope Actinium-225

Photonuclear cross section measurements for the production of the medical radioisotope Actinium-225

Actinium-225 (²²⁵Ac) is a high energy alpha-emitting radioisotope with considerable potential for therapeutic applications in targeted alpha therapy (TAT). This emerging technique is based on selectively targeting cancer cells by linking radioisotopes to specific carrier molecules. The relatively long half-life of ²²⁵Ac (9.92 days), combined with the high linear energy transfer (LET) and short range of its emitted alpha particles, makes it particularly effective for inducing irreparable double-strand DNA breaks while minimizing damage to surrounding healthy tissue. These properties position ²²⁵Ac as a highly promising candidate for the treatment of multiple types of cancers.

Interest in ²²⁵Ac has increased significantly in recent years, owing to positive results from preclinical studies and clinical trials. This rapid growth in demand has intensified the need for scalable and reliable production routes, as current supply, primarily based on decay of ²²⁹Th, is insufficient to meet projected clinical needs. One alternative production pathway under investigation is the photon-induced reaction ²²⁶Ra(γ,n)²²⁵Ra, followed by the β⁻ decay of ²²⁵Ra to ²²⁵Ac. However, the current knowledge of this reaction remains very limited. To date, no experimental cross-section data is available, and existing evaluated nuclear data rely exclusively on theoretical predictions from nuclear reaction. This lack of experimental validation introduces significant uncertainties in yield estimates and hinders the design and optimization of photonuclear production facilities.

The main goal of this PhD was to produce a first measurement of the ²²⁶Ra(γ,n) cross section, contributing to improve the quality of the photonuclear data available for the reaction. This has been achieved through the irradiation of ²²⁶Ra targets, produced at SCK CEN, with multiple bremsstrahlung photon beams in the giant dipole resonance energy region of ²²⁶Ra. The cross section has been derived from the measurement of the ²²⁵Ac activation yields produced in the irradiations, producing the first set of experimental data for this reaction reported in the literature. Beyond the production of novel cross-section data, this work also addressed several experimental and methodological challenges inherent to photonuclear measurements, such as the characterization of bremsstrahlung spectra, the handling of radioactive radium targets, and the unfolding of integral activation yields.

Promoters:

Maëlle Kerveno (Université de Strasbourg)

Jan Wagemans (UHasselt)

SCK CEN mentors:

Mirela Vasile

Alexey Stankovskiy

Stephan Heinitz