Quantification of trace 227Ac and other radionuclidic impurities in mass-separated 225Ac samples produced at CERN-MEDICIS

Abstract

²²⁵Ac is a promising candidate medical radionuclide for targeted alpha therapy of advanced stage cancers. One of the main production pathways is the high-energy proton spallation of thorium-based targets, that requires an efficient, nuclide-selective separation method to recover ²²⁵Ac from hundreds of co-produced spallation and fission products. The main radioactive contaminant of concern is ²²⁷Ac (T_1/2 = 21.8 years), that could preclude extensive medical use if not significantly suppressed. In this work, ²²⁵Ac samples were produced by mass separation of radioactive ion beams extracted from proton-irradiated thorium-based targets. The activity of ²²⁵Ac and other possible contaminants of the samples were measured using complementary gamma- and alpha-decay spectrometry methods, while ²²⁷Ac activity was calculated by performing alpha-decay spectrometry of recoiled progeny from the sample. Using this novel method, accurate measurement of trace ²²⁷Ac activity in ²²⁵Ac samples was performed much faster than with conventional spectrometry techniques, thanks to its 10,000-fold increase in relative sensitivity. The end of collection activity ratio of ²²⁷Ac to ²²⁵Ac in two samples from irradiated targets were determined to be [Formula: see text] and [Formula: see text] respectively, three orders of magnitude below the ²²⁷Ac activity in ²²⁵Ac products obtained through radiochemical separation. The high separation factor of ²²⁵Ac over ²²⁷Ac suggests the suitability of mass-separated accelerator-based ²²⁵Ac for medical use.

Fig. 1

Schematic overview of the stages of alpha recoil decay spectrometry. In each stage, the dynamics of recoiling daughters are illustrated using the colors described in the key. The decay schematic for both ²²⁷Ac and ²²⁵Ac is shown at the bottom of each panel, indicating the possible transitions between states composed of the product of isotope generation and location. The dashed box in the ‘measurement’ panel shows the nuclide chains that are measured through this method.

Fig. 2

Measured alpha-decay recoil spectra over the experimental campaigns for samples A (above), B (middle) and C (below). After one day of measurement, the spectra are dominated by the ²²¹Fr and daughter alpha decays (blue). In the remainder of the measurement time, few further counts of ²²¹Fr and daughter peaks were recorded, while ²²³Ra and daughter alpha-decay peaks appeared (black). ²²³Ra and daughter alpha-decay peaks are identified in the spectra once the data of the first day are removed (red).

Fig. 3

Nuclear-decay energy spectra for each of the samples. analyzed peaks in the ²²⁵Ac decay chain are highlighted in blue. -decay spectrometry: 6341 keV, 6126.3 keV and 6241.8 keV for ²²¹Fr, 7066.9 keV for ²¹⁷At and 8376 keV for ²¹³Po. -decay spectrometry: 218 keV for ²²¹Fr, 440.45 keV for ²¹³Bi and 1567.08 keV and 465.14 keV for ²⁰⁹Tl. -coincidence spectrometry: 1567.08 keV and 465.14 keV for ²⁰⁹Tl. Several peaks of ²¹⁴Pb and ²¹⁴Bi daughters of ²²⁶Ra are due to background radiation (yellow). ²⁰⁶Po and ²⁰⁶Bi peaks for sample C were partially used to confirm presence of PoO (red). See supplementary material for details.

Fig. 4

Purity of ²²⁵Ac samples.