Precise activity measurements of medical radionuclides using an ionization chamber: a case study with Terbium-161.

<sup>161</sup> Tb draws an increasing interest in nuclear medicine for therapeutic applications. More than 99% of the emitted gamma and X-rays of <sup>161</sup> Tb have an energy below 100 keV. Consequently, precise activity measurement of <sup>161</sup> Tb becomes inaccurate with radionuclide dose calibrators when using inappropriate containers or calibration factors to account for the attenuation of this low energy radiation. To evaluate the ionization chamber response, the sample activity must be well known. This can be performed using standards traceable to the Système International de Référence, which is briefly described as well as the method to standardize the radionuclides. In this study, the response of an ionization chamber using different container types and volumes was assessed using <sup>161</sup> Tb. The containers were filled with a standardized activity solution of <sup>161</sup> Tb and measured with a dedicated ionization chamber, providing an accurate response. The results were compared with standardized solutions of high-energy gamma-emitting radionuclides such as <sup>137</sup> Cs, <sup>60</sup> Co, <sup>133</sup> Ba and <sup>57</sup> Co. For the glass vial type with an irregular glass thickness, the <sup>161</sup> Tb measurements gave a deviation of 4.5% between two vials of the same type. The other glass vial types have a much more regular thickness and no discrepancy was observed in the response of the ionization chamber for these type of vials. Measurements with a plastic Eppendorf tube showed stable response, with greater sensitivity than the glass vials. Ionization chamber measurements for low-energy gamma emitters (< 100 keV), show deviation depending on the container type used. Therefore, a careful selection of the container type must be done for activity assessment of <sup>161</sup> Tb using radionuclide dose calibrators. In conclusion, it was highlighted that appropriate calibration factors must be used for each container geometry when measuring <sup>161</sup> Tb and, more generally, for low-energy gamma emitters