Precise Measurement of Strontium-82 Radioactivity in the Sr-Rb PET Generator

Application/instrumentation: Precise measurement of 82Sr radioactivity in the Sr-Rb PET generatorm
Developed at: Oak Ridge National Laboratory, Holifield Radioactive Ion Beam Facility (HRIBF)
Developed in: 2004-present (multiple developments, some continuing
Result of NP research: Spectroscopy of very neutron-rich nuclei near the N=50 and N=82 closed neutron shells. Results provide input to the study of the evolution of structure with neutron excess and are relevant to the rapid-neutron capture process (r-process).
Application currently being supported by: Developments continue with support of base funding and DOE 09-13 ARRA funding. The accumulation of data in support of the nuclear energy program is being funded by DOE-Office of Nuclear Energy, beginning in FY2011.
Impact/benefit to spin-off field: Strontium-Rubidium positron emission tomography (PET) generator

The radionuclide 82Rb is the most widely used radionuclide for positron emission tomography (PET) assessment of the heart muscle. Due to its 1.26 min half-life, 82Rb must be obtained locally at the imaging facility by extracting it from an 82Sr-82Rb generator. Much as a battery is used to store electricity, the radioactive 82Sr with its long 25.35 days half-life serves as an “82Rb battery” by β decaying to 82Rb (see Fig. 1), which is easily extracted from the generator. Although in use for many years, a question has arisen concerning the amount of 82Sr necessary to extract a given amount of 82Rb. The amount of 82Sr in the generator, a crucial quantity for evaluating its supply, is determined by measuring the number of the 776.5 keV γ-rays arising from the β-decay of 82Rb to stable 82Kr per 100 atoms of 82Sr. This number was initially measured to be 13.41(47) [1]. The weighted average of two later measurements is 15.08(16) [2, 3]. These measurements were all performed under similar conditions and using the same assumption that all relevant energy levels are known. The difference between these previous measurements has resulted in uncertainty in the estimate of 82Sr needed by the medical community.

82sr-82Rb-82Kr decay process

To clarify the situation, the Holifield Radioactive Ion Beam Facility has undertaken the challenging task of measuring the branching ratio using a new method which is independent of knowledge of decay scheme (except for small γ summing corrections). A beam of 82Sr, after acceleration in the tandem electrostatic accelerator to 215 MeV, was delivered to an ionization chamber that counted the 82Sr ions and measured their energy loss. The strontium ions were then implanted in thin aluminum foils. The long half-life of 82Sr allowed the implantation samples - four were produced - to be measured offline with a germanium detector. This new method has the advantage that it unambiguously identifies the 82Sr ions and provides a count of the 82Sr ions implanted. Although analysis is continuing, we have the preliminary result that the medical isotope community should expect 14.93(37) γ-rays of 776.5 keV for every 100 atoms of 82Sr in the 82Rb generator.

[1] H.-W. Muller, Nuclear Data Sheets, 50, 1 (1987)
[2] S.M. Judge et al., Appl. Radiat. Isot. 38, 185 (1987).
[3] D. D. Hoppes et al., Appl. Radiat. Isot. 38, 195 (1987).