Direct measurement of cold neutron moderator spectra

The neutron flux distribution with respect to energy/wavelength of the cold source of the Budapest Neutron Centre has been measured using the energy resolved pinhole imaging method. A 2D position sensitive multiwire proportional counter has been used in time of flight regime together with a chopper placed at the position of the pinhole. Gas absorption efficiency and detector dead time corrections have been applied and the accuracy of the results analyzed. Three Maxwell–Boltzmann distributions required for a proper fit to the resulted spectrum yield the multispectral characteristic of the cold source due to incomplete moderation in the finite-sized liquid hydrogen volume. The measurement has been performed during the reconstruction of the out-of-pile guide system of a beamline, offering direct view of the cold source.