Activity standardisation of ¹⁶¹Tb

Highlights

• ¹⁶¹Tb has aroused increased interest recently for medical imaging and therapy.

• ¹⁶¹Tb was standardised using the β(PS,LS)-γ(CeBr₃) coincidence techniques.

• ¹⁶¹Tb was also standardised with the TDCR method using MC electron spectra.

• Coincidence results are consistent with TDCR measurements.

• An ampoule of the solution was submitted to the International Reference System.

Abstract

¹⁶¹Tb, which emits low-energy β⁻- and γ-particles in addition to conversion and Auger electrons, has aroused increased interest for medical imaging and therapy. To support the use of this radionuclide, a¹⁶¹Tb solution was standardised using the β-γ coincidence technique, as well as the TDCR method. The solution had 4.5·10⁻³% of ¹⁶⁰Tb impurities. Primary coincidence measurements, with plastic or liquid scintillators for beta detection, were carried out using both analogue and digital electronics. TDCR measurements using defocusing, grey filtering and quenching for varying the efficiency were also made. Monte Carlo calculations were used to compute the detection efficiency. The coincidence measurements with analogue electronics and the TDCR show a good consistency, and are compatible with the digital coincidence results within uncertainties. An ampoule of this solution was submitted to the BIPM as a contribution to the international reference system.

Introduction

¹⁶¹ Tb decays to the ground and excited states of ¹⁶¹Dy via nine β branches, with endpoint energies ranging between 43 and 593 keV (Reich, 2011). Its decay scheme is shown in (Durán et al., 2020bb). ¹⁶¹Tb emits a considerable number of conversion and Auger electrons per decay, typically about ten times more than those emitted in ¹⁷⁷Lu decay for example, which gives it a high radiotherapeutic efficiency (Champion et al., 2016). Its most intense γ-ray emissions, which are below 75-keV energy, can be used for SPECT imaging (Müller et al., 2012, 2014). All these features make ¹⁶¹Tb attractive for targeted radionuclide therapy (Hindié et al., 2016; Lehenberger et al., 2011).

As an initial step towards standardising the activity of ¹⁶¹Tb, we recently re-measured its half-life using the decay method with two ionisation chambers and a CeBr₃ gamma spectrometer (Durán et al., 2020b). We determined a value of 6.953 (2) days in which the uncertainty was significantly improved while remaining compatible with both the power moderated weighted mean of all earlier measurements (Reich, 2011) and the Nucleide-LARA and ENSDF reference value (Nucléide-LARA, 2011, and NNDC).

In this work, we report the first activity standardisation of ¹⁶¹Tb by IRA-METAS, the designated national metrology institute in Switzerland. The primary standard was realized by β-γ coincidence counting, involving liquid and plastic scintillation for beta detection, using both analogue and digital electronics. Back-up measurements with the liquid scintillation triple to double coincidence ratio (TDCR) technique and integral gamma counting with Monte Carlo computed efficiencies were also carried out. This standardisation was taxing, due to the rather dense low-intensity low-energy gamma spectrum of ¹⁶¹Tb and its strong electron conversion contribution that introduces marked nonlinearities in the efficiency extrapolation. When estimating the Monte Carlo detection efficiencies for the gamma integral counting method, significant discrepancies questioned the accuracy of the published γ-ray emission probabilities. The solution and activity standardisation were, therefore, used to measure them again; this work will be published elsewhere.

¹⁶¹Tb's half-life is fairly long for nuclear medicine applications and, thus, smoothens the shipping of standards to the International Bureau of Weights and Measures (BIPM) in Paris. Aliquots of the standardised solution were sent to the BIPM as part of a contribution to the Système International de Référence.