  High quality nuclear data is the most fundamental underpinning for all
neutron metrology applications. This paper describes the release of version II
of the International Reactor Dosimetry and Fusion File (IRDFF-II) that contains
a consistent set of nuclear data for fission and fusion neutron metrology
applications up to 60~MeV neutron energy. The library is intended to support:
a) applications in research reactors; b) safety and regulatory applications in
the nuclear power generation in commercial fission reactors; and c) material
damage studies in support of the research and development of advanced fusion
concepts. The paper describes the contents of the library, documents the
thorough verification process used in its preparation, and provides an
extensive set of validation data gathered from a wide range of neutron
benchmark fields. The new \mbox{IRDFF-II} library includes 119 metrology
reactions, four cover material reactions to support self-shielding corrections,
five metrology metrics used by the dosimetry community, and cumulative fission
products yields for seven fission products in three different neutron energy
regions. In support of characterizing the measurement of the residual nuclei
from the dosimetry reactions and the fission product decay modes, the present
document lists the recommended decay data, particle emission energies and
probabilities for 68 activation products. It also includes neutron spectral
characterization data for 29 neutron benchmark fields for the validation of the
library contents. The IRDFF-II library and comprehensive documentation is
available online at https://www-nds.iaea.org/IRDFF/. Evaluated cross sections
can be compared with experimental data and other evaluations at
https://www-nds.iaea.org/exfor/endf.htm. The new library is expected to become
the international reference in neutron metrology for multiple applications.
