Towards implementing new isotopes for environmental research:Redetermination of the 32Si half-life

Abstract

The predicted climate changes on Earth will significantly impact the environment and human society. The climate patterns observed during the past centuries led to a better understanding of the driving forces for such changes. However, more studies using new, innovative techniques are necessary to broaden the knowledge on the involved processes and accomplish a reliable scientific basis for decision-makers. On this subject, radiometric dating is a well-established contemporary technique whereby various time periods (from a few years to millions of years) can be covered with this method. Notably, a dating gap exists so that the period from 100 to 1000 years (before today) is currently not covered. However, this period is of great interest, as it allows for studying and understanding environmental processes such as glacier dynamics, ocean, and atmospheric circulation. Interestingly, a potential nuclide that could fill this dating gap is available: Silicon-32 (Si-32). The problem, however, is the currently very inaccurately determined T1/2 of around 150 years so that a use for radiometric dating is hindered. Hence, the aim of the SNSF-funded SINCHRON (Si-32: a new chronometer) project, and therefore the primary goal of this thesis, concerns the re-determination of the Si-32 half-life. Previous half-life determinations were limited to using samples with low activities. To overcome the issue with the small amounts, Si-32 was artificially produced at the Paul Scherrer Institut (PSI). Subsequently, an innovative wet chemical separation system was developed to allow for the selective removal of Si-32 from an irradiated vanadium matrix. As a result, 20 mL of an ultra-pure Si-32 solution could be produced, fulfilling the desired parameters related to the half-life re-determination. Within the framework of this work, the determination of the half-life via the direct method was applied; i.e., both the determination of the number of atoms (N), in combination with the activity (A), are required. Within the SINCHRON-collaboration, several independent measurements were performed between various multinational metrological institutes. Based on the different requirements, the Si-32 solution was manufactured accordingly: (I) the solution's activity concentration was confirmed to be greater than 100 kBq/g, (II) a chemically very stable Si species was chosen, and (III) ultra-traces of S-32 were removed. Besides, (IV) solid samples for accelerator mass spectrometry (AMS) could be prepared from the stock solution, too. As a result, we are getting close to providing a new, recommended value with low uncertainty (less than 5%). Within the scope of this work, a preliminary T1/2 for Si-32 of 125 +/- 5 years has been determined. Additionally, we studied vanadium as a target material and determined the production cross-sections of Ti-44, Ca-41, and Al-26. These radionuclides are produced as by-products during the irradiation process. For the latter two nuclides, the presented data describe the very first experimental determination of the cross-section for vanadium as a target. In this context, two independent gamma spectrometric measurement systems were used for the activity determination of Ti-44, and no prior chemical separation of Ti-44 from the matrix was required. Contrarily, Ca-41 and Al-26 were successfully separated using a selective and robust chromatographic wet chemical separation scheme. The activity of these nuclides could then be determined using AMS.