Search for double beta-decay modes of Zn-64 using purified zinc

The production of ultra-pure raw material is a crucial step to ensure the required background level in rare event searches. In this work, we establish an innovative technique developed to produce high-purity (99.999%) granular zinc. We demonstrate the effectiveness of the refining procedure by measuring the internal contaminations of the purified zinc with a high-purity germanium detector at the Laboratori Nazionali del Gran Sasso. The total activity of cosmogenic activated nuclides is measured at the level of a few mBq/kg, as well as limits on naturally occurring radionuclides are set to less than mBq/kg. The excellent radiopurity of the zinc sample allows us to search for electron capture with positron emission and neutrinoless double electron capture of Zn-64, setting the currently most stringent lower limits on their half-lives, T-1/2(epsilon beta+) >2.7 x 10(21) year (90% CI), and T-1/2(0 nu 2 epsilon) >2.6 x 10(21) year (90% CI), respectively.