Production of Helium in Iron Meteorites by the Action of Cosmic Rays

The helium distribution in a slice from the iron meteorite, Grant, has been measured and plotted in the form of contour maps. The contours of constant helium show a minimum helium content and isotopic ratio, ${\mathrm{He}}^3$/${\mathrm{He}}^4$, near the center of the slice, the isotopic ratio varying from 0.26 near the center to 0.30 at the surface. A cosmogenic helium production rate equation was fitted to the data giving a ${\mathrm{He}}^3$/${\mathrm{He}}^4$ production ratio by primary cosmic rays of 0.50 and by secondary particles of 0.14. Primary and secondary particle interaction cross sections were found to be 540 mb and 720 mb, respectively. The ratio of the average postatmospheric radius to the pre-atmospheric radius of Grant was calculated to be 0.65.