Aim
In the recent years other -mostly metallic -radionuclides for PET gained more and more interest. However, for production of these alternative positron emitters the vast majority of them affords solid targets in form of metal foils, oxide or salt pellets [1, 2] which can not be operated by an automated processing. The disadvantages are: 1 st , manual cyclotron intervention is practically unsuited for daily routine radionuclide production and 2 nd the operating staff receives high radiation doses from the activated target. An alternative could be the irradiation of aqueous salts of target isotopes, allowing automated target operation. The major requirements are: I.) thermal stability of the dissolved compound, II.) avoidance of counter ions containing nuclides which produce long-lived radionuclides under irradiation and III.) high solubility of the salt in the aqueous matrix. Here we report the possibility of this alternative radionuclide production concept by production of 86 Y (cf. [3, 4]), generated by irradiation of strontium nitrate dissolved in water.
Method
A "Nitrogen-13 target" liquid target was filled with a Sr(NO 3 ) 2 solution (natural isotopic composition, 81 % of maximum solubility) and irradiated with 16 MeV protons for 60 minutes at 6 ยตA. After end of bombardment (EOB) the target content was delivered to a 25 mL glass vial containing phosphate buffer pH 7. Then, the target was flushed 5 times by a 25 mM HNO 3 solution collected in the same vial resulting in a total final volume of about 16 mL. Two aliquots (10 and 100 ยตL) were taken and measured several times applying gamma spectrometry (HPGe detector). Nuclear decay and emission data were taken from Ref. [5].
Results
Produced activities of Y isotopes at EOB were: 88 Y (1.2 MBq), 87 Y (4.5 MBq), 87m Y (16.8 MBq), 86 Y (21.6 MBq), 86m Y (46.7 MBq), 85 Y (<3.2 MBq), 85m Y (not detectable), 84m Y (<7.8 MBq). In case the beam upscaling for the described target system would work without physico-chemical complications, PET nuclide 86 Y would be produced in activities of more than 4 GBq by proton irradiation of aqueous dissolved 86 Sr(NO 3 ) 2 applying the parameters: 96.3 % isotopic enrichment of 86 Sr, 16 MeV protons, 30 ยตA beam current, 5 h irradiation time. Radioisotopic activity fraction (among Y radionuclides) will rise from 52 % (EOB) to 88 % (3 h later). Formed 13 N, 18 F, 11 C as well as Sr and Rb radionuclides can be chemically removed during workup of the irradiated target solution for 86 Y purification and 86 Sr recovery [4]. Since the used target was not tight enough regarding cooling water, we recommend to use the "high yield F-18 target" from GE-MS instead with differently designed loading.
Thus, demonstrated by the reported experiment, an opportunity is opened to produce radionuclides, which in the past were only accessible by solid target production technology.