32Si (T
1/2 = 153(19) y) is an extremely rare, naturally-occurring isotope that has been considered as a geochronometer suitable for radiometric dating over the time span from 100 to 1000 years ago – a time span that has proved rather difficult to explore in this manner. Past attempts to determine the 32Si half-life have resulted in a wide range of values possessing significant uncertainties because only low-activity samples could be made available for such measurements. Utilizing the 590 MeV ring cyclotron at PSI, megabecquerel quantities of 32Si have been produced by exposing metallic vanadium discs to high-energy protons in order to induce spallation. A radiochemical separation procedure has been successfully developed and applied to the irradiated discs as part of the SINCHRON project, based on a combination of ion-exchange and extraction resins. The process was shown to be reliable and robust with a high chemical yield. Radiochemically pure 32Si solutions with activity concentrations of up to several kBq/g can be produced to perform individual measurements (AMS, ICP-MS, LSC) for various studies. Thus, a careful redetermination of the 32Si half-life has become feasible to begin the first steps toward the confident implementation of this radionuclide for geochronological purposes.
Many useful and needed radionuclides for medicinal, astrophysical, and environmental research are produced naturally in inefficient quantities or not-at-all. In the method described here, rare cosmogenic isotopes were produced via spallation reactions in metallic vanadium and separated without adding any carriers. In the SINQ facility at the Paul Scherrer Institut, the vanadium targets were irradiated for two years with high-energy protons (≤590 MeV). After a cooling period of eight years, only relatively long-lived radionuclides such as 32Si, 44Ti, 41Ca, and 26Al remain present. After target dissolution, 32Si was first separated for a prospective half-life redetermination. The remaining 32Si-free solution was used for extracting 44Ti, 41Ca, and 26Al, three key isotopes which are scientifically interesting for nuclear astrophysics research as well as medical applications. Each separation scheme employed ion-exchange and extraction chromatography; developed and optimized using inactive model solutions analyzed with Inductively Coupled Plasma–Optical Emission Spectrometry (ICP–OES). The irradiated samples were tracked with γ-ray spectroscopy for γ-ray emitting impurities. As a result, radiochemically pure sample solutions of 44Ti, 41Ca, and 26Al were obtained as “ready for use” in different application fields.
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