*2.1. Radionuclide Composition of Irradiated Targets and Yields of Nuclear Reactions*

Gamma-spectra of irradiated targets made of natNi and <sup>60</sup>Ni are presented in Figure 2A,B; yields of photonuclear reactions leading to the formation of nickel and cobalt isotopes during the irradiation of natNi, <sup>60</sup>Ni, and <sup>58</sup>Ni are presented in Table 1. It was established that in each case, <sup>55</sup>Co was produced along with significant quantities of longlived impurities 56,57,58Co. Irradiation of natNi and <sup>58</sup>Ni resulted in the yield of 56,57,58Co being 1.2–1.5 times higher than the yield of <sup>55</sup>Co, and the irradiation of <sup>60</sup>Ni led to the yield of <sup>55</sup>Co being no more than 3% of the yield of all cobalt isotopes. Obviously, <sup>55</sup>Co with such low radionuclide purity is not suitable for PET. On the other hand, isotopes 56,57,58Co are gamma-emitters and can be used in preclinical research of radiopharmaceuticals based on cobalt, including in vivo experiments. For these purposes, we recommend irradiating natNi; in this case, the yield of <sup>57</sup>Co is 66 kBq/(µA·h·g/cm<sup>2</sup> ) on a thin plate, and this value can be increased by using massive target. <sup>58</sup>Ni can also be used as a target material increasing the yield of <sup>57</sup>Co up to 82 kBq/(µA·h·g/cm<sup>2</sup> ) on thin foil; however, targets made of <sup>58</sup>Ni are more expensive than ones made of natNi. Therefore, photonuclear method allows one to produce cobalt isotopes with sufficient activity for preclinical research.

**Table 1.** Yields of photonuclear reactions on natNi, <sup>60</sup>Ni (obtained experimentally), and <sup>58</sup>Ni (calculated using yields on natNi, <sup>60</sup>Ni nuclei) with maximum energy of bremsstrahlung photons being 55 MeV. Values obtained by TALYS [29] are presented in brackets.


Table 1 also compares the experimentally measured yields with theoretical calculations using the TALYS program, taking into account the bremsstrahlung spectrum. On the whole, we can see a satisfactory agreement between the experimental yields and the theoretical calculations. The difference in values can be due to two main factors: TALYS uses default photoabsorption cross-sections, and also does not take into account the isospin splitting of the giant dipole resonance, which has a significant effect on the yields of photo-proton reactions.
