Th-229 has has a half life of 7916 years, so has to be made artificially.
The process is based on irradiating natural thorium with neutrons to breed significant amounts of U-233, then allowing it to decay for some time and chemically separate out resulting Th-229.
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https://www.manchester.ac.uk/about/news/the-worlds-most-precise-nuclear-clock-ticks-closer-to-reality/Th-229 has has a half life of 7916 years, so has to be made artificially. The process is based on irradiating natural thorium with neutrons to breed significant amounts of U-233, then allowing it to decay for some time and chemically separate out resulting Th-229.