Decompositions and extremal type II codes over Z/sub 4/

In previous work by Huffman and by Yorgov (1983), a decomposition theory of self-dual linear codes C over a finite field F/sub q/ was given when C has a permutation automorphism of prime order r relatively prime to q. We extend these results to linear codes over the Galois ring Z/sub 4/ and apply the theory to Z/sub 4/-codes of length 24. In particular we obtain 42 inequivalent [24,12] Z/sub 4/-codes of minimum Euclidean weight 16 which lead to 42 constructions of the Leech lattice.