Establishment of a community-wide 252Cf source for enhancing production of confined track-lengths for apatite fission track thermochronology

• Applicant: Professor Edward Sobel, Ph.D.
• Subject Area: Palaeontology
• Term: from 2020 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 444986084

The apatite fission-track (AFT) method records the cooling of rocks below temperatures of ca. 110°C over geologic timescales. Fission-tracks form with a narrow length distribution at a constant rate due to radioactive decay of 238U and are subsequently shortened and may eventually disappear in response to elevated temperatures. This process is termed annealing. As a result, the track length distribution is a sensitive monitor of a crystal's thermal history, particularly in the temperature interval between ca. 60-110°C, the partial annealing zone; at significantly higher temperatures, annealing is geologically instantaneous. Measuring the lengths of a large number of confined tracks provides a proxy for the thermal history, which can be inverted by thermal modeling.Apatite fission tracks are only microscopically visible after etching with nitric acid. Confined tracks lie completely within the apatite crystal and can only be etched and hence seen when there is a pathway for the acid from the surface via another track. Typically, the operator seeks to measure ~100 confined tracks per sample. For apatite crystals with high amounts of U and which have remained at low temperatures for long geologic times, sufficient tracks are usually present to provide a robust dataset for inverse modeling. However, for younger samples and grains with low U abundance, there are very few spontaneous tracks which cross the surface and therefore have a chance to intersect the relatively small number of confined tracks. In these cases, it is extremely difficult or impossible to measure a large number of confined lengths using traditional sample preparation techniques, thus either precluding thermal modeling or drastically limiting the resolution of such models.In order to increase the number of measurable confined tracks, it is possible to bombard the sample with fission particles emitted from a 252Cf source, which creates a large number of additional pathways for the etchant to penetrate the apatite crystal, allowing numerous intersected confined tracks to be etched. As low-temperature thermal modeling is one of the greatest strengths of the AFT method, it is clearly desirable to improve the ability to measure confined tracks. Therefore, we seek funding to establish a German facility for apatite fission track irradiations by purchasing a 252Cf source. It is also necessary to build an apparatus to hold the fission track samples during irradiation. This must be properly permitted and shielded so that users are not exposed to unnecessary radioactive doses. This facility will be available to German fission track research groups.

• DFG Programme: Research Grants