Measurement of the specific activity of 39Ar in natural argon

We report on the measurement of the specific activity of Ar-39 in natural argon. The measurement was performed with a 2.3-1 two-phase (liquid and gas) argon drift chamber. The detector was developed by the WARP Collaboration as a prototype detector for WIMP Dark Matter searches with argon as a target. The detector was operated for more than two years at Laboratori Nazionali del Gran Sasso, Italy, at a depth of 3400 m w.e. The specific activity measured for Ar-39 is 1.01 +/- 0.02(stat) +/- 0.08(syst) Bq per kg of Ar-nat. (c) 2007 Elsevier B.V. All rights reserved.