Half-life of 221Fr in Si and Au at 4 K and at millikelvin temperatures

The half-life of the α-decaying nucleus 221Fr was determined in different environments, that is, embedded in Si at 4 K, and embedded in Au at 4 K and about 20 mK. No differences in half-life for these different conditions were observed within 0.1%. Furthermore, we quote a value for the absolute half-life of 221Fr of t1/2=286.1(10) s that is of comparable precision to the most precise value available in the literature