Intrinsic radioactivity of a Li6Eu(BO3)3Li6Eu(BO3)3 crystal and αα decays of Eu

A small Li6Eu(BO3)3Li6Eu(BO3)3 crystal (2.72 g) was measured deep underground at the Gran Sasso National Laboratories with a HP Ge detector of 408cm3 during 1500 h. While the crystal is not polluted by usual radioactive contaminants (only limits on the activity of U/Th chains, 40K, 60Co, 137Cs were set on the level of 0.01–1 Bq/kg), the presence of radioactive 152Eu and 154Eu is evident; these are created in capture of neutrons by naturally abundant 151Eu and 153Eu. The measured activities correspond to 0.95 and 0.21 Bq/kg for 152Eu and 154Eu, respectively, with a ratio far from the standard expectation. As by-products, limits on half lives for the αα decays Eu151→Pm147(Eexc=91.1keV) and Eu153→Pm149 are determined as T1/2>2.4×1016yr and T1/2>1.1×1016yr, respectively.