The application of mass-separators for the production of high-purity radionuclides used for diagnostics and therapy is a promising and extensively developed method. Among the most widely used medical radionuclides are isotopes with relatively low ionization potential, for example, isotope-generator 82Sr which is utilized for PET diagnostics of heart and brain diseases, alpha-ecaying 223,224Ra used for the therapy of different malignant tumors at a very early stage of their formation and some Tl and In isotopes. The simplest way to ionize these isotopes for subsequent delivering by a mass-separator to its collector is the surface ionization technique. As a rule, the ionizer is made of a tungsten or tantalum tube at the temperature of about 2300 ℃ to produce the surface-ionized species. However, the ionization efficiency of such a type of ionizer for non-alkalis is low (for example, about 1% for Tl). On the other hand, surface ionization is the simplest and cheapest ionization method.
To increase the surface ionization efficiency, a new type of surface ion source has been proposed and put into operation at the mass-separator IRIS working on-line with the 1-GeV proton synchrocyclotron of Petersburg Nuclear Physics Institute. The main part of this ion source is the tube made from a mono crystal of tungsten with the orientation of the tube axis along the crystallographic  direction. The work function of the tube internal surface is about 5 eV.
The ionization efficiency is determined as the ratio of the yield of the chosen long-lived isotope at the exit of the mass-separator to its in-target production rate. The latter is readily calculated with the known target and proton beam parameters along with the cross-sections of the reaction (p 1GeV, 238U)A, where A is the isotope in question . The yields have been deduced by the measurement of the characteristic gamma or alpha-line intensities. The life-lives of the chosen isotopes were sufficiently long (larger than several hours) to neglect the decay losses due to the release time. For elements with the moderate ionization potentials, IP,the ionization efficiencies prove to be sufficiently high to consider the new ion source as the part of the future installation for the medical isotope production. Values of the ionization efficiencies for Tl (IP = 6.1 eV), In (IP = 5.8 eV) and Ra (IP = 5.3 eV) are 21(8), 33(8) and 38(10)% correspondingly.
This ion source tube was also successfully used as a hot cavity of a laser ion source during the experiments on in-source laser spectroscopy of the short-lived Bi isotopes . The increase of the laser ionization efficiency in comparison with the efficiency obtained with the conventional tungsten ion-source tube at the same temperature was observed.
1. M. Bernas et al., Nucl. Phys. A 765, 197 (2006).
2. A. Barzakh et al., Phys. Rev. C 95, 044324 (2017).