Experimental Investigation of Cold Neutron Spectra and Comparison with Monte-Carlo Simulations Kay Nuenighoff Forschungszentrum Juelich GmbH 52425 Juelich, Germany Email: k.nuenighoff@fz-juelich.de Phone: +492461-613232 Fax: +492461-61930 Abstract: In this contribution we like to present recent results of the JESSICA experiment (Jülich Experimental Spallation Target Set-up In COSY Area) located at the proton synchrotron COSY at Forschungszentrum Jülich. We were able to measure the time of flight spectra and determine the energy spectra of cold moderator materials at T=20 K (ice, liquid hydrogen, solid methane, mesitylene, methane-clathrate) and water at room temperature, which could or will be used in next generation high power spallation sources. Thus we are able to normalize the measured spectra to the number of incident neutrons our experiment is an ideal benchmark experiment to validate source terms of moderators. We compared our measured spectra with MCNPX calculations processing newly generated neutron scattering law data S(a,b,T). In case of water also a comparison with the multigroup transport code MORSE will be presented. We will discuss the observed discrepancies, which will have an impact on the calculation of the neutron induced activity of specific components, e.g. neutron beam line shutters or outer shielding, in high power spallation sources.