Three neutron reactions leading to production of the rare gases Ar41 and Xe135 have been in vestigated. Single crystal samples of KF, CaF2 , and BaF2 were irradiated in the Berlin reactor and the target-free absolute activities of the rare gas isotopes measured by a /^-/-coincidence method. The calculated cross sections are K41 (n,p)Ar41: o = ( 2 ,73±0,41) mb; o0= (98,6± 14,8) mb; Ca44(n,a)Ar4...