Discoveryofundergroundargonwithlowlevelofradioactive 39 Ar andpossibleapplicationstoWIMPdarkmatterdetectors

We report on the rst measurement of 39 Ar in argon from underground natural gas reservoirs. The gas stored in the US National Helium Reserve was found to contain a low level of 39 Ar. The ratio of 39 Ar to stable argon was found to be 4 10 17 (84% C.L.), less than 5% the value in atmospheric argon ( 39 Ar=Ar=8 10 16 ). The total quantity of argon currently stored in the National Helium Reserve is estimated at 1000 tons. 39 Ar represents one of the most important backgrounds in argon detectors for WIMP