  Ultracold neutrons (UCNs) are used in experiments investigating fundamental
interactions, testing the Standard Model of particle physics and searching for
phenomena beyond it. Knowledge of the energy spectrum of UCNs is very often a
key ingredient to determine the systematic effects in precision measurements
utilizing UCNs. The proposed novel method allows for the in-situ measurements
of the UCN velocity distribution and its time evolution. In addition, the
proposed UCN spectrometer can be a handy diagnostic tool for monitoring the
spectrum in critical places in the UCN transport system connecting an UCN
source with experiments. In this paper, we show the results from the
measurements performed at the Paul Scherrer Institute's UCN source.
