Some neutron probes, which measure water content in soils by sending neutrons into them and detecting the reflection, use an actinium/beryllium source; the beryllium captures alpha particles from actinium's decay products and releases the neutrons. The isotope used for this application is actinium-227, as indicated by the counter opposite Applejack.
Americium-241 is more often used for this purpose as it's slightly cheaper, but there is a more prominent application for that.
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