[en] A Monte-Carlo code was created and used for modellization of experimental spectra and of neutron registration efficiency by means of a large volume neutron detector (V=24.1). As is shown, n many cases simple estimations and calculations (also when threshold is taken into account) agree within a few %. Pulse height spectra at E_n=2.45, 4 and 5.2 MeV (at E_n=24.5 MeV also for deuterized scintillator) are presented. Results for a neutron detector with diameter 310 mm and length 160 mm used at JINR for investigations of muon catalysed fusion process d+d are demonstrated. 11 refs.; 8 figs.; 2 tabs