A long term change in the cosmic ray composition: studies on fossil cosmic ray tracks in lunar samples

The etching techniques for the identification of very heavy cosmic ray ions from their etchable tracks in mineral track detectors are described and the results so far obtained for the ancient galactic cosmic ray Cr group (V + Cr + Mn) to Fe abundance ratio are presented. It was found that the etchable radiation damage of fossil cosmic ray tracks has probably only been slightly affected by annealing processes. The track data obtained on pyroxenes of different lunar rocks and on pyroxenes and feldspars, i.e. detectors of different track retaining characteristics, yielded consistent results. From this measurement, an ancient Cr group to Fe ratio of approximately 0.7 to 0.8 was deduced. In comparison with the present day galactic cosmic ray composition, this ratio is enhanced by a factor of about two. From the track data obtained in different lunar soil samples it was concluded that a variation in the Cr group to Fe ratio between 0.4 to 0.8 exists. Both results indicate, that either a long term change in the cosmic ray composition has taken place or the interpretation of track data is much more complicated than assumed. (author)