Summary of changes:

The remarks of the referee were all to the point and have been taken care of:
1) the text below Eq.(2) has been changed according to the suggestion given by the referee. The old reference [10] has been deleted.
2) The formula for sigma has been corrected.
3) The error in the expressions for characteristic times has been corrected.

At the conference the question of a “muon induced effect” has been discussed, sometimes feverously. Since Pr compounds are candidates in that direction we have added a few lines of  text at the end of the paper pointing out briefly the situation for the present case

Somme misprints in the text have been corrected

G. M. Kalvius for the authors.