Earlier works have suggested that the energy correlations in a spallation source may influence the neutron noise measurements in an ADS. For the calculation of this effect not only the generally known and used one-particle spectrum is needed but also the so-called two particle spectrum, which describes also the energy correlations. Since measured data are not available for the energy correlation of the neutrons from a single spallation event, the physical models of the MCNPX code have been used to investigate the effect. The calculational model has been successfully validated with measurements of the number distribution of spallation neutrons. The simulated one- and two-particle energy distributions and spectra proved that the energy correlations exist and have an important effect in low multiplicity spallation events and in thin targets. On the other hand, for thick targets this effect appears negligible and the factorization of the two-particle spectrum seems an acceptable approximation. Further investigations are in hand to quantify the actual effect of the energy correlations on the neutron noise measurements. 1. Introduction Spallation sources are considered as neutron sources for Accelerator-Driven Subcritical Systems (ADSs). In an ADS an accelerator is supposed to provide a high-energy (~1？GeV) proton beam, which produces neutrons in a heavy metal (e.g., lead) target through the spallation process. Besides the high neutron energies (up to the energy of the proton) the spallation neutron source is also distinguished from other neutron sources by its high multiplicity: one proton can produce up to 40–50 neutrons. The high multiplicity increases the importance of the source in a subcritical system, especially in case of deeper subcriticality. Therefore, the precise description of the spallation source is inevitable for the modeling of an ADS. A special case is the modeling of the neutron fluctuations in an ADS (e.g., neutron noise measurements for reactivity determination) as this requires also the description of the higher moments of the probability distributions. While one can easily find measured data about spallation sources for the average values (the first moment) in the literature, the higher moments are not available due to the smaller interest and the difficulties of such measurements. This paper makes an attempt to reproduce the higher moments and correlations needed for the accurate simulation of the neutron fluctuations in an ADS with the help of the physical models of the spallation process implemented in the MCNPX [1] high-energy Monte
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