*2.2. Separation of Co(II) Isotopes without a Carrier Using Extraction Chromatography*

Obtained chromatograms of Ni(II) and Co(II) separation using DGA resin are presented in Figure 3. It was established that elution of Co(II) by HCl solutions with concentration varying from 0.01 to 3 M resulted in similar elution profiles; yield of Co(II) was close to quantitative in each case, and the process lasted for no longer than 0.5 h. To determine the separation factor of Ni(II) and Co(II), fractions marked in Figure 3 were selected for gamma-spectra registration for 24 h. Peaks of isotopes 56,57Ni were absent in registered gamma-spectra, and the separation factor of Ni/Co was calculated using detection limit of these radionuclides and was 2.8·10<sup>5</sup> , which is one order of magnitude higher than the

factor during the separation using sorbent with similar composition in work [28]. Thus, the method of carrier-free Co(II) separation was developed; it allows one to obtain necessary isotopes in different solutions of HCl, including the solutions with low concentration, which is preferable for nuclear medicine. *Molecules* **2022**, *27*, x FOR PEER REVIEW 4 of 12

**Figure 2.** Gamma-spectra of natNi irradiated by bremsstrahlung photons with energy up to 55 MeV during 1 h after EOB (**A**), and of irradiated 60Ni for 19 h after EOB (**B**). The most intense peaks of each isotope are labeled in the figures. **Figure 2.** Gamma-spectra of natNi irradiated by bremsstrahlung photons with energy up to 55 MeV during 1 h after EOB (**A**), and of irradiated <sup>60</sup>Ni for 19 h after EOB (**B**). The most intense peaks of each isotope are labeled in the figures.

**Table 1.** Yields of photonuclear reactions on natNi, 60Ni (obtained experimentally), and 58Ni (calculated using yields on natNi, 60Ni nuclei) with maximum energy of bremsstrahlung photons being 55

**Figure 3.** Elution curves of Ni(II) and Co(II) during separation on DGA resin column in HCl solutions.

**Figure 3.** Elution curves of Ni(II) and Co(II) during separation on DGA resin column in HCl solu-As stated above in Section 2.1, the highest yields of cobalt isotopes are achieved during the irradiation of enriched 58Ni targets. Obviously, the expensive target material should be regenerated after separation. For this purpose, it is possible to evaporate eluate-containing Ni(II) to dryness to irradiate NiCl2 to produce cobalt isotopes one more time. In this case, there will be no need to dissolve the target during prolonged heating, since NiCl2 is water-soluble. Another possible way is the irradiation of Ni(II) solution separated from the column, which can be eluted through the column again without any preparative treatment. In any case, the regeneration of enriched nickel is not complicated. It is also worth noting that formed isotopes of cobalt have T1/2 no longer than 272 d, allowing one to utilize samples with general waste after prolonged storage, i.e., no radioactive waste requiring special treatment and disposal is produced after the irradiation. As stated above in Section 2.1, the highest yields of cobalt isotopes are achieved during the irradiation of enriched <sup>58</sup>Ni targets. Obviously, the expensive target material should be regenerated after separation. For this purpose, it is possible to evaporate eluate-containing Ni(II) to dryness to irradiate NiCl<sup>2</sup> to produce cobalt isotopes one more time. In this case, there will be no need to dissolve the target during prolonged heating, since NiCl<sup>2</sup> is water-soluble. Another possible way is the irradiation of Ni(II) solution separated from the column, which can be eluted through the column again without any preparative treatment. In any case, the regeneration of enriched nickel is not complicated. It is also worth noting that formed isotopes of cobalt have T1/2 no longer than 272 d, allowing one to utilize samples with general waste after prolonged storage, i.e., no radioactive waste requiring special treatment and disposal is produced after the irradiation. Thus, this studied method of production and separation of cobalt isotopes is environmentally friendly due to the regeneration of the target and the absence of radioactive waste.

#### Thus, this studied method of production and separation of cobalt isotopes is environ-**3. Materials and Methods**

tions*.* 

#### mentally friendly due to the regeneration of the target and the absence of radioactive waste. *3.1. Theoretical Calculations of Cross-Sections and Yields of Photonuclear Reactions*

**3. Materials and Methods**  *3.1. Theoretical Calculations of Cross-Sections and Yields of Photonuclear Reactions*  The cross-sections were calculated using the TALYS program, while the total photoabsorption cross-section was calculated based on the parameters from the RIPL-2 experimental database [30]. To calculate the cross-sections for photonuclear reactions, The cross-sections were calculated using the TALYS program, while the total photoabsorption cross-section was calculated based on the parameters from the RIPL-2 experimental database [30]. To calculate the cross-sections for photonuclear reactions, TALYS uses a combination of the evaporative and exciton preequilibrium decay mechanism of a compound nucleus with the emission of nucleons and gamma-quanta. The obtained cross-sections of the main photonuclear reactions leading to the formation of 55,56,57,58Co and 55,56,57Ni isotopes are shown in Figure 4.

TALYS uses a combination of the evaporative and exciton preequilibrium decay mechanism of a compound nucleus with the emission of nucleons and gamma-quanta. The The theoretical yield of isotope formation, taking into account all possible reactions leading to the formation of the selected isotope, was calculated by Equation (1):

obtained cross-sections of the main photonuclear reactions leading to the formation of

$$Y = \frac{\lambda a}{\rho q\_\varepsilon} \sum\_i \eta\_i \int\_{E\_i}^{E\_m} \phi(E\_\gamma, E\_m) \sigma\_i(E\_\gamma) dE\_\gamma \tag{1}$$

where *λ* is the decay constant, *α* is the number of studied nuclei per 1 cm<sup>2</sup> of target, *ρ* is the surface density of the target, *q<sup>e</sup>* is the electron charge in µA·h, index *i* corresponds

to the number of the reaction contributing to the formation of studied isotope, *η<sup>i</sup>* is the percentage of the nickel isotope on which the reaction occurs in a natural mixture of isotopes, *E<sup>i</sup>* is the threshold of the corresponding reaction, *E<sup>m</sup>* is the maximum energy of the bremsstrahlung spectrum, *σ<sup>i</sup>* (*Eγ*) is the cross-section of the corresponding photonuclear reaction, and *φ*(*Eγ*,*Em*) is the bremsstrahlung spectrum on the target. *Molecules* **2022**, *27*, x FOR PEER REVIEW 7 of 12

**Figure 4.** Calculated cross-sections of the main photonuclear reactions leading to the formation of 55Co and 55Ni (**A**), 56Co and 56Ni (**B**), 57Co and 57Ni (**C**), and 58Co (**D**)*.*  **Figure 4.** Calculated cross-sections of the main photonuclear reactions leading to the formation of <sup>55</sup>Co and <sup>55</sup>Ni (**A**), <sup>56</sup>Co and <sup>56</sup>Ni (**B**), <sup>57</sup>Co and <sup>57</sup>Ni (**C**), and <sup>58</sup>Co (**D**).

The theoretical yield of isotope formation, taking into account all possible reactions leading to the formation of the selected isotope, was calculated by Equation (1): = ఒఈ ,ఊ ( ∑ ா ா )൫ఊ൯ఊ (1) The bremsstrahlung spectrum calculated using a full 3D simulation of the irradiation process using the Geant4 program, taking into account the formation of gamma-quanta both in the converter and in the target, is presented on Figure 5.

#### where *λ* is the decay constant, *α* is the number of studied nuclei per 1 cm2 of target, *ρ* is *3.2. Irradiation of Targets and Determination of Yields of Photonuclear Reactions*

ఘ

the surface density of the target, *qe* is the electron charge in μA·h, index *i* corresponds to the number of the reaction contributing to the formation of studied isotope, *ηi* is the percentage of the nickel isotope on which the reaction occurs in a natural mixture of isotopes, *Ei* is the threshold of the corresponding reaction, *Em* is the maximum energy of the bremsstrahlung spectrum, *σi*(*Eγ*) is the cross-section of the corresponding photonuclear reaction, and *ϕ*(*Eγ*,*Em*) is the bremsstrahlung spectrum on the target. The bremsstrahlung spectrum calculated using a full 3D simulation of the irradiation process using the Geant4 program, taking into account the formation of gamma-quanta both in the converter and in the target, is presented on Figure 5. To study the yields of photonuclear reactions on nickel nuclei, two targets were irradiated in RTM-55 microtron with maximum energy of electron beam being 55 MeV [31]. The first target was a plate made of natNi with the size of 1 cm <sup>×</sup> 1 cm, thickness of 500 <sup>µ</sup>m, and weight of 440 mg. Purity of natNi was determined in our work by atomic emission spectroscopy using Thermo Scientific ICAP-6500 Duo (Horsham and Loughborough, England) and was 99.78%. The second target made of <sup>60</sup>Ni purchased from Federal State Unitary Enterprise Combine "Elektrokhimpribor" (Lesnoy, Russia) was thin trapezoidal foil 67 µm thick and with a weight of 86.5 mg. Isotope composition of <sup>60</sup>Ni-target is presented in Table 2, purity of <sup>60</sup>Ni—99.9224% according to manufacturer's data. Tungsten plates 1 mm thick were used as convertors. Monitor targets were a 0.11 mm thick cobalt plates and were located directly after the targets for irradiation. Bremsstrahlung targets, nickel targets, and monitor targets were fully overlapping the beam. Current fluctuations during the irradiations were measured using Faraday cup. Normalization of current was carried out by the processing of bremsstrahlung spectrum and by comparing experimentally measured yield of <sup>59</sup>Co(γ,n)58Co reaction to the yield calculated using known cross-sections. The duration of irradiation of each target was 1 h, average currents were 73 and 48 nA for natNi and <sup>60</sup>Ni accordingly.

**Figure 5.** Bremsstrahlung spectrum per one beam electron for used convertors*.*  **Figure 5.** Bremsstrahlung spectrum per one beam electron for used convertors.



ezoidal foil 67 μm thick and with a weight of 86.5 mg. Isotope composition of 60Ni-target is presented in Table 2, purity of 60Ni—99.9224% according to manufacturer's data. Tungsten plates 1 mm thick were used as convertors. Monitor targets were a 0.11 mm thick cobalt plates and were located directly after the targets for irradiation. Bremsstrahlung targets, nickel targets, and monitor targets were fully overlapping the beam. Current fluctuations during the irradiations were measured using Faraday cup. Normalization of current was carried out by the processing of bremsstrahlung spectrum and by comparing experimentally measured yield of 59Co(γ,n)58Co reaction to the yield calculated using known cross-sections. The duration of irradiation of each target was 1 h, average currents were 73 and 48 nA for natNi and 60Ni accordingly. The residual activity of nickel targets after irradiation was registered using gamma-ray spectrometer with high-purity germanium detector GC3019 (Canberra Ind, Meridan, CT, USA). Relative efficiency of detector was 30%, energy resolution of detector was 0.9 keV for 122 keV and 1.9 keV for 1.33 MeV. Efficiency calibration of spectrometer was conducted using measurements of activity of certified point sources (152Eu, <sup>137</sup>Cs, <sup>60</sup>Co, <sup>241</sup>Am) in different location geometries of source and detector and was also modeled in GEANT4. The selection of the peak maximum in the spectra was carried out using an automatic system for registration and analysis of spectra specially designed for this purpose. Spectra with the duration of 3.5 s each were saved into the database, and the analysis system allowed us to summarize them and display the total spectrum with assigned duration [32]. Activity and yields of the produced isotopes were determined using areas of the most intensive peaks corresponding to the decay of the resulting isotope in spectra of residual activity, taking into account duration of irradiation, duration of transportation and registration of spectrum, and also efficiency of gamma-quanta registration and quantum yield of gamma-transition. Gamma-spectra of each irradiated target were registered three times for 2 days during the 2 months to exactly determine both short-lived and long-lived isotopes.

Yields of photonuclear reactions on natNi and <sup>60</sup>Ni in kBq/(µA·h·g/cm<sup>2</sup> ), normalized by electron beam charge and surface density of the target, were calculated using Equation (2):

$$Y = \frac{\lambda S}{Ck\rho \left(e^{-\lambda(t\_3 - t\_1)} - e^{-\lambda(t\_2 - t\_1)}\right)}\tag{2}$$

where *S* is the area of photopeak in spectra of residual activity, corresponding to the gammatransition during the decay of the resulting nucleus, occurring during the registration, *t*<sup>1</sup> is the irradiation time, *t*<sup>2</sup> is the starting time of the registration, *t*<sup>3</sup> is the ending time of the registration, *λ* is the exponential decay constant, *k* is the coefficient equal to the multiplication of detector efficiency, coefficient of cascade summation, and quantum yield of gamma-quant during gamma-transition, *ρ* is the target surface area, and C is the coefficient taking into account the change in accelerator current during the irradiation (Equation (3), Figure 6). *Molecules* **2022**, *27*, x FOR PEER REVIEW 10 of 12

$$\mathbf{C} = \int\_{0}^{t\_1} I(t)e^{-\lambda t}dt\tag{3}$$

**Figure 6.** Accelerator current during irradiation of natNi (**a**) and 60Ni (**b**)*.*  **Figure 6.** Accelerator current during irradiation of natNi (**a**) and <sup>60</sup>Ni (**b**).

Yields of reactions on 58Ni were calculated as a difference between the yields of isotope production on natural mix and on 60Ni, taking into account the percentage of 58Ni and60Ni in natural mix. The accuracy of the selected calculation method was confirmed by the ratio of formation yields after the irradiation of 60Ni and natNi (3.85 ± 0.05) coincided within the margin of error with the ratio of 60Ni content in targets—3.81. Yields of reactions on <sup>58</sup>Ni were calculated as a difference between the yields of isotope production on natural mix and on <sup>60</sup>Ni, taking into account the percentage of <sup>58</sup>Ni and <sup>60</sup>Ni in natural mix. The accuracy of the selected calculation method was confirmed by the ratio of formation yields after the irradiation of <sup>60</sup>Ni and natNi (3.85 <sup>±</sup> 0.05) coincided within the margin of error with the ratio of <sup>60</sup>Ni content in targets—3.81.

#### *3.3. Separation of Co(II) Isotopes from Irradiated Nickel Target 3.3. Separation of Co(II) Isotopes from Irradiated Nickel Target*

DGA resin Normal (base–N,N,N′,N′-tetraoctyl-1,5-diglycolamide, particle size 100– 150 μm, TrisKem Int., Bruz, France) was used for separation. This resin sorbs Co(II) in solutions of HCl with concentration more than 6 M [33]. In these solutions, the distribution coefficient (Kd) for Co(II) reaches 20, while Kd(Ni) does not exceed 1 in solutions of HCl with concentration less than 10 M, which allows one to separate Co(II) and Ni(II) using this sorbent. Kd(Co) decreases with quantity of HCl decreasing: in a more diluted solution of HCl, i.e., less than 4 M, Co(II) is not retained on the column. DGA resin Normal (base–N,N,N0 ,N0 -tetraoctyl-1,5-diglycolamide, particle size 100–150 µm, TrisKem Int., Bruz, France) was used for separation. This resin sorbs Co(II) in solutions of HCl with concentration more than 6 M [33]. In these solutions, the distribution coefficient (Kd) for Co(II) reaches 20, while Kd(Ni) does not exceed 1 in solutions of HCl with concentration less than 10 M, which allows one to separate Co(II) and Ni(II) using this sorbent. Kd(Co) decreases with quantity of HCl decreasing: in a more diluted solution of HCl, i.e., less than 4 M, Co(II) is not retained on the column.

To optimize the separation method of carrier-free Co(II) from irradiated nickel, ex-

then the solution was evaporated to dryness in 9 M HCl. The sorbent was preliminarily held in 0.001 M HCl for 1 h, then the column (height 4 cm, diameter 0.6 cm, volume 2 mL) was filled with it. After the dissolution of the irradiated target, 0.5–1 mL of obtained solution was placed in the column with DGA resin; Co(II), unlike Ni(II), was sorbed onto the column. The remaining Ni(II) was eluted with 5 mL of 9 M HCl, then Co(II) was eluted with 5 mL of HCl solution with the concentration varying from 0.01 M to 3 M. Fractions of 1 mL were collected during the separation, the content of Co(II) and Ni(II) was determined using gamma-spectroscopy with high-purity germanium detector GC1020 (Canberra Ind.). Co(II) was identified by peaks of 57Cо (122 keV, 85.6%), and

Ni(II) was identified by peaks of 57Ni (127 keV, 16.7%) and 56Ni (158 keV, 98.8%).

To optimize the separation method of carrier-free Co(II) from irradiated nickel, experiments with the plate made of natNi (1 cm <sup>×</sup> 1 cm), similar to the plate from Section 3.2 were carried out. The irradiated plate was dissolved in 12 M HCl by prolonged heating, then the solution was evaporated to dryness in 9 M HCl. The sorbent was preliminarily held in 0.001 M HCl for 1 h, then the column (height 4 cm, diameter 0.6 cm, volume 2 mL) was filled with it. After the dissolution of the irradiated target, 0.5–1 mL of obtained solution was placed in the column with DGA resin; Co(II), unlike Ni(II), was sorbed onto the column. The remaining Ni(II) was eluted with 5 mL of 9 M HCl, then Co(II) was eluted with 5 mL of HCl solution with the concentration varying from 0.01 M to 3 M. Fractions of 1 mL were collected during the separation, the content of Co(II) and Ni(II) was determined using gamma-spectroscopy with high-purity germanium detector GC1020 (Canberra Ind.). Co(II) was identified by peaks of <sup>57</sup>Co (122 keV, 85.6%), and Ni(II) was identified by peaks of <sup>57</sup>Ni (127 keV, 16.7%) and <sup>56</sup>Ni (158 keV, 98.8%).
