**4. Conclusions**

Targets made of natNi and <sup>60</sup>Ni were irradiated by bremsstrahlung photons with energy up to 55 MeV; the radionuclide composition and yields of nuclear reactions on natNi, <sup>60</sup>Ni, and <sup>58</sup>Ni were determined. It was established that in every case, the activities of produced 56,57,58Co were higher than the activity of <sup>55</sup>Co, and therefore enough for preclinical research of radiopharmaceuticals based on cobalt. It was also demonstrated that the radionuclide purity of <sup>55</sup>Co produced by the photonuclear method at 55 MeV is not sufficient for PET. A fast, simple, and effective method of cobalt isotopes separation without a carrier from irradiated targets by extraction chromatography was developed; it was demonstrated that the separation of Co(II) is possible in wide range of HCl concentrations (from 0.01 to 3 M). The separation factor of Ni/Co was 2.8·10<sup>5</sup> , the yield of Co(II) was close to quantitative, and separation lasted for no longer than 0.5 h.

**Author Contributions:** Conceptualization, A.G.K., A.A.K., S.E.V. and B.F.M.; methodology, A.G.K., S.S.B., V.V.K. and A.A.K.; software, S.S.B. and A.A.K.; validation, A.G.K., A.A.K., S.E.V. and B.F.M.; formal analysis, A.G.K., S.S.B. and A.A.K.; investigation, A.G.K., J.S.B., T.Y.E., S.S.B., A.A.K. and V.V.K.; resources, V.V.K., S.E.V. and B.F.M.; data curation, S.S.B.; writing—original draft preparation, A.G.K., J.S.B. and T.Y.E.; writing—review and editing, S.S.B., A.A.K., S.E.V. and B.F.M.; visualization, A.G.K., J.S.B., T.Y.E., S.S.B. and A.A.K.; supervision, B.F.M.; project administration, B.F.M.; funding acquisition, A.G.K., S.E.V. and B.F.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** The study was supported by the Russian Science Foundation (project no. 21-13-00449).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
