  Cross section data are fundamental quantities which affect the accuracy of
all calculations in nuclear applications. A new dosimetry library IRDFF-II that
contains cross section evaluations with full uncertainty quantification was
developed by the International Atomic Energy Agency and released in January
2020 (https://www-nds.iea.org/IRDFF). A previous version, IRDFF-1.05, was
released in 2014 and experimental validation of the newly released cross
section by spectrum averaged cross section (SACS) measurements is a high
priority task. For such purpose, a neutron dosimeter set containing 5 target
foils was activated in 2 independent experiments at the VR-1 reactor of the
Czech Technical University in Prague. Care was taken to derive SACS with low
uncertainties. New experimental evaluation method is in good agreement with
previous approaches based on relative measurements using monitor foils. Good
agreement of measured SACS and evaluated IRDFF-II cross sections is observed.
Slight overestimation of evaluated ENDF/B-VIII.0 235U(nth,f) PFNS above 10 MeV
is discussed.
