A search for the decay of Vanadium-50 with a low-level gamma-spectrometer

The spectrometer consists of a 4″×4″ NaJ-crystal with a double lead shield — the inner one being of selected lead — and a guard anticoincidence ring in between. The apparatus is situated in a cellar covered by 3 meters of soil. The total background above 0.03 MeV is 120 cpm, being extremely stable with an intrinsic r.m.s. variation below ±2 per mil. The crystal is surrounded by a sample container of volume 1.8 liters. An energy dependent gamma detection limit (2σ) ranging between 0.3 and 0.8 dpm has been obtained. This corresponds to a maximum detectable halflife of 1.0 to 2.7× 10¹⁸ years per mole of the radioactive nuclide in the sample container. In the case of vanadium-50 (isotopic abundance 0.25%) a lower limit of the half-life was found to be 9.0×10¹⁶ and 6.9×10¹⁶ years respectively for the two modes of decay V⁵⁰-EC-Ti^50*-γ(1.58 MeV) — Ti⁵⁰ and V⁵⁰-β^?-Cr^50*-γ(0.78 MeV)-Cr⁵⁰.