Abstract
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Objectives 69Ge (t1/2=39 h, 21% β+, Emax=1205 keV) is an attractive PET isotope due to its low positron energy and low abundance of gamma emissions. It is produced from the (p,n) reaction on natural Ga (69Ga, 60%; 71Ga, 40%). The co-produced 71Ge (t1/2=11.43 d) decays by pure electron capture, making it a potential therapeutic isotope with its subsequent Auger electron emission. In this work, we have developed three different target systems (A,B,C) for Ga irradiation, as well as the separation chemistry based on extraction chromatography using a commercially available resin.
Methods (A) 16 MeV protons were directed downward on an external beamline (-30°) onto 300 mg of molten elemental natGa pooled on a water-cooled niobium support. (B) A second target was fabricated by pressing ~100 mg of Ga2O3 into a water-cooled aluminum target holder, covered with a molybdenum foil (250 µm) to degrade the proton energy to 11 MeV to avoid the 69Ga(p,2n)68Ge reaction. (C) A third target was a Ga-Ni alloy with a melting point >500°C made by electrodeposition from a mixed salt bath onto a gold substrate. The separation of 69/71Ge from the bulk target material and other radionuclidic impurities was carried out in HNO3 using 200 mg of an extraction resin packed in a 5 mm diameter column. Radionuclidic assay employed a high purity germanium (HPGe) detector. The Ga-Ni alloy composition was verified by x-ray fluorescence (Ga = 9.26 keV; Ni =7.48 keV) using a 109Cd excitation source.
Results 69Ge yields for the three target systems were as follows: A=0.33; B=0.18+/-0.02 (n=2); C=0.19+/-0.03 mCi/μA-h (n=3). Electrodeposited Ga-Ni targets had a thickness of 42+/-6 mg/cm2. Yields of 62+/-7% (n=4) of 69Ge activity were eluted in 600 μL of de-ionized water. All the contaminating radionuclides were effectively separated from the 69/71Ge final product, as quantitatively measured with the HPGe detector.
Conclusions We have developed the targetry and separation chemistry for 69/71Ge, a potentially useful radioisotope pair for PET and targeted radionuclide therapy.