Neutron spectra in heterogeneous assemblies, a summary of the work of the Karlsruhe Group Submitted to the Conf. on Neutron Thermalization, Brookhaven National Laboratory, Apr. 30 - May 2, 1962 - Häfele, Wolf 1927-2013 (Kernreaktor Bau- u. Betriebs-Ges., 1962)