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The ultracold neutron (UCN) source at the Paul Scherrer Institute (PSI)provides high intensities of storable neutrons for fundamental physicsexperiments. The neutron velocity spectrum parallel to the beamline axis wasdetermined by time-of-flight spectroscopy using a neutron chopper. Inparticular, the temporal evolution of the spectrum during neutron productionand UCN storage in the source storage volume was investigated and compared toMonte Carlo simulation results. A softening of the measured spectrum from amean velocity of 7.7(1) m s$^{-1}$ to 5.1(1) m s$^{-1}$ occurred within thefirst 30 s after the proton beam pulse had impinged on the spallation target. Aspectral hardening was observed over longer time scales of one measurement day,consistent with the effect of surface degradation of the solid deuteriummoderator.

Time-of-flight spectroscopy of ultracold neutrons at the PSI UCN source