Neutron cross-section data are fundamental for the design of nuclear interrogation systems, the maintenance of nuclear materials and waste, and the understanding the consequences of nuclear catastrophe. Although a large body of nuclear data exists, it is often old, unreliable, or poorly determined. For several years we have collaborated, as part of an IAEA Coordinated Research Project, to precisely measure the partial thermal neutron gamma ray cross sections for all elements from hydrogen to uranium at the Budapest Reactor. These data will replace the unreliable tables of Lone et al [1], still widely in use, and will be published as an IAEA TECDOC.

Precise gamma-ray thermal neutron capture cross sections have been measured at the Budapest Reactor for all elements with Z=1-83,92 except for He and Pm. These measurements and additional data from the literature been compiled to generate the Evaluated Gamma-ray Activation File (EGAF), which is disseminated by LBNL and the IAEA. These data are nearly complete for most isotopes with Z<20 so the total radiative thermal neutron capture cross sections can be determined directly from the decay scheme. For light isotopes agreement with the recommended values is generally satisfactory although large discrepancies exist for 11B, 12,13C, 15N, 28,30Si, 34S, 37Cl, and 40,41K. Neutron capture decay data for heavier isotopes are typically incomplete due to the contribution of unresolved continuum transitions so only partial radiative thermal neutron capture cross sections can be determined. The contribution of the continuum to theneutron capture decay scheme arises from a large number of unresolved levels and transitions and can be calculated by assuming that the fluctuations in level densities and transition probabilities are statistical. We have calculated the continuum contribution to neutron capture decay for the palladium isotopes with the Monte Carlo code DICEBOX. These calculations were normalized to the experimental cross sections deexciting low excitation levels to determine the total radiative thermal neutron capture cross section. The resulting palladium cross sections values were determined with a precision comparable to the recommended values even when only one gamma-ray cross section was measured. The calculated and experimental level feedings could also be compared to determine spin and parity assignments for low-lying levels.

The Evaluated Gamma-ray Activation File (EGAF), a new database of prompt and delayed neutron capture g-ray cross sections, has been prepared as part of an International Atomic Energy Agency (IAEA) Coordinated Research Project to develop a "Database of Prompt Gamma-rays from Slow Neutron Capture for Elemental Analysis." Recent elemental g-ray cross-section measurements performed with the guided neutron beam at the Budapest Reactor have been combined with data from the literature to produce the EGAF database. EGAF contains thermal cross sections for ~;35,000 prompt and delayed g-rays from 262 isotopes. New precise total thermal radiative cross sections have been derived for many isotopes from the primary and secondary gamma-ray cross sections and additional level scheme data. An IAEA TECDOC describing the EGAF evaluation and tabulating the most prominent g-rays will be published in 2004. The TECDOC will include a CD-ROM containing the EGAF database in both ENSDF and tabular formats with an interactive viewer for searching and displaying the data. The Isotopes Project, Lawrence Berkeley National Laboratory continues to maintain and update the EGAF file. These data are available on the Internet from both the IAEA and Isotopes Project websites.