Abstract
The cross section for B11(p,2p) Be10 has been measured to be 19 ± 6 and 25 ± 8 mb at 150 and 600 MeV, respectively. These values show that the tertiary production of Be10 in cosmic rays is very important. However, it still appears likely that an actual isotope abundance measurement in cosmic rays will be necessary to utilize this nuclide as a cosmic-ray "clock."