Cross-section Measurement and Thick Target Production of Terbium Radioisotopes by Enriched Gadolinium Targets
Introduction Short-lived radioisotopes of the terbium (Tb) family show great prospects in theranostics: the 149Tb can be used for alpha therapy, the 152Tb, as a positron emitter, can be applied for the positron emission tomography (PET), the 155Tb can be used for the single photon emission tomography (SPECT) and for Auger therapy, and finally, the 161Tb can be an alternative to 177Lu for β-therapy. Nevertheless, the applications of Terbium are limited at the moment due to its insufficient production and high cost: except for 161Tb, the other radionuclides are produced by nuclear spallation reactions. The use of enriched Gadolinium (Gd) targets can help to increase their availability according to the following production reactions: 152Gd(p,4n)149Tb 1, 152Gd(p,n)152Tb 1, 155Gd(p,n)155Tb 2 and 155Gd(d,2n)155Tb 3. In this work, the 155Tb is taken as a case study, and Gd2O3 enriched in 155Gd is used. Objectives of this work are on the one hand to measure the cross section of the 155Gd(d,2n)155Tb nuclear reaction induced by deuteron, and on the other hand to irradiate enriched Gd2O3 targets for thick target production with deuteron. Description of the Work or Project For the cross section measurement, thin targets (10-20 µm) are required while thicker targets are preferred for production. Therefore, two types of Gd targets with different thicknesses have been developed through two different techniques. Thin targets were manufactured via the electrochemical co-deposition technique. Uniform Ni/Gd2O3 composite targets with a thickness of 10-20 µm containing about 2 mg of enriched Gd were obtained after 35 min of deposition. These targets were irradiated at GIP ARRONAX cyclotron with deuteron beams. Cross sections of 155Tb and other Tb radionuclides (153Tb, 154Tb and 156Tb) were measured from 8 MeV to 30 MeV. These measurements give the first experimental results for the reaction 155Gd(d,x)Tb. From these results, the thick target yield and the purity of 155Tb were estimated. The irradiation parameters for thick target production were also determined from the simulation. Thicker targets were manufactured through the pelletizing technique. A uniform and compact target with a thickness of 390 µm was obtained using 0.6 g of enriched Gd2O3 powder. This target was irradiated by deuteron beams with an incident energy of 15.1 MeV and a beam intensity of 368 nA for 1 h. The production yield of 155Tb was 10.2 MBq/µAh and the purity was 89% after 14 days of decay. These results are consistent with the estimation obtained by the measured cross sections. Conclusions This work shows the possibility of using enriched gadolinium targets to produce terbium radioisotopes via biomedical cyclotrons. Cross sections of deuteron-induced reactions on enriched Gd were measured and a test of thick target production was carried out. As for large batch production, higher intensity and longer irradiation time will be necessary. To this end, specific encapsulation and cooling systems will also be designed and in addition, pure metal Gd targets with better thermal conductivity will be developed.
radioisotopes, Terbium, theranostics, cross-section measurement, radionuclide production, terbium, gadolinium, cyclotron, theranostics radionuclide