Cosmic-ray gradient measured by the argon 37/argon 39 ratio in the Lost City meteorite

The gradient in the cosmic-ray intensity was deduced from measurements of the argon 37 (35-day half-life) and argon 39 (269-year half-life) activities in the metallic phase of the Lost City meteorite. A gradient of 60±25%/AU, adjusted to solar minimum, was deduced from these activities and the known orbit of the meteorite. This high value of the gradient is about twice as high as that observed by the O'Gallagher-Simpson experiment on the Mariner 4 spacecraft in 1964–1965.