On the periodic clustering of cosmic ray exposure ages of iron   meteorites
  Two recent papers claimed to have found a periodic variation of the galactic cosmic ray (CR) flux over the last 1-2 Gyr, using the CR exposure ages of iron meteorites. This was attributed to higher CR flux during the passage of the Earth through the spiral arms of the Milky Way, as suggested by models. The derived period was 143+-10 Myrs. We perform a more detailed analysis of the CR exposure ages on the same data set, using extensive simulation to estimate the influence of different error sources on the significance of the periodicity signal. We find no evidence for significant clustering of the CR exposure ages at a 143 Myr period nor for any other period between 100 and 250 Myrs. Rather, we find the data to be consistent with being drawn from a uniform distribution of CR exposure ages. The different conclusion of the original studies is due to their neglecting the influence of (i) data treatment on the statistics, (ii) several error sources, and (iii) number statistics. 
Knud Jahnke (Astrophysikalisches Institut Potsdam)