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Determination of the activity and half-life of zirconium-89 – a radionuclide for medical diagnosis

20.01.2022

Zirconium‑89 is a radionuclide that can be used for diagnostic applications in medicine. At PTB, activity determinations have now been carried out by means of liquid scintillation counting in order to enable reliable calibrations of zirconium‑89 solutions in the future. In addition, ionization chamber measurements have been used to very accurately determine the half‑life of zirconium‑89.

Positron emission tomography (PET) is an important diagnostic imaging technique in medicine which normally relies on very short‑lived radionuclides that undergo beta‑plus decay. With the zirconium‑89 radionuclide, the application range of PET can be expanded. For one thing, the slightly longer half‑life makes it possible to use this radionuclide even in places where no cyclotron is available for producing conventional PET isotopes. Moreover, zirconium‑89 can be applied in the form of labeled antibodies to aid in the targeted diagnosis of tumors.

In 2021, PTB researchers accurately determined the activity of a zirconium‑89 solution. Primary standard measurements of this radionuclide are considered especially challenging due to the decay characteristics involved. Zirconium‑89 decays primarily via electron capture and about 23 % by beta‑plus decay. In addition, there is an isomeric transition to yttrium‑89 (909 keV) that practically excludes any possibility of determining absolute activity using the typical methods of coincidence counting. PTB therefore applied liquid scintillation counting to determine the activity, a procedure that involved the use of two devices, one equipped with two and one with three photomultipliers, to allow analysis by both the CIEMAT/NIST and the TDCR methods. A detailed description of the experiments and in particular of the highly elaborate methods for computing the counting efficiencies has now been published [1].

In contrast to earlier work [2,3], current PTB research takes account of a number of additional key factors, such as the photon backscattering effect, which significantly influences the determined activity. The in‑depth examination of the uncertainties, which are dominated in particular by model assumptions and by the underlying radionuclide and atomic data, likewise led to a reassessment at PTB.

Furthermore, aliquots of two zirconium‑89 solutions were measured by means of ionization chambers. These systems represent important secondary standard measurement facilities that will reduce the time and effort required for future calibrations at PTB. In one of these ionization chambers, the individual measurements were carried out over longer durations in order to determine the half‑life of zirconium experimentally. Readings from the chamber were taken with a new current measurement system developed at PTB [4] that significantly improves the linearity when measuring the small ionization currents. During the analysis of the measurement data, minor corrections had to be made to account for very slight impurities from the yttrium‑88 and zirconium‑88 radionuclides. The radioactive impurities were measured by means of gamma spectrometry, with a quantitative determination only possible after substantial parts of the zirconium‑89 had already decayed. The newly determined half‑life is 78.373(23) hours [1], which is in very good agreement with other measurements carried out in recent years [2,3].

Literature

[1]   Kossert, K., Nähle, O.J., Honig, A., Röttger, S.: Activity standardization by means of liquid scintillation counting and determination of the half‑life of 89Zr. Applied Radiation and Isotopes 181 (2022) 11078.

[2]   García-Toraño, E., Peyrés, V., Roteta, M., Mejuto, M., Sánchez-Cabezudo, A., Romero, E.: Standardisation and half‑life of 89Zr. Applied Radiation and Isotopes 134 (2018) 421‑425.

[3]   Fenwick, A.J., Collins, S.M., Evans, W.D., Ferreira, K.M., Paisey, S.J., Robinson, A.P., Marshall, Ch.: Absolute standardisation and determination of the half‑life and gamma emission intensities of 89Zr. Applied Radiation and Isotopes 166 (2020) 109294.

[4]   Drung, D., Krause, C., Becker, U., Scherer, H., Ahlers, F.J.: Ultrastable low‑noise current amplifier: a novel device for measuring small electric currents with high accuracy. Review of Scientific Instruments 86 (2015) 024703.

Contact:

Opens local program for sending emailK. Kossert, Department 6.1, Working Group 6.14