[en] The plasma focus device PF-1000 at IPPLM Warsaw with neutron yields of 10⁹-10¹¹ neutrons per shot is convenient for the determination of the axial neutron energy distribution in the fusion D-D reaction due to the horizontal position of a discharge axis and the distance of scintillation detectors up to 85 m from the neutron source in both axial (downstream and upstream) directions. In this paper the determination of the axial energy component of the fast deuterons producing neutrons is presented for the shot with a relative small neutron yield and nearly isotropic neutron energy distribution and the total energy of fast deuterons producing neutrons is estimated. We calculated, if the dense structure with the length of 2 cm and the density of 2xl0²⁵ m^-3 could be the source of neutrons. We estimated the upper limit of energy of deuterons producing neutrons as 10-20 keV and their total number as 10¹⁸. The deuterons with energies below 10-20 keV can't produce high number of neutrons due to the small cross-section of D-D reaction but they can be principally confined by magnetic field. We discuss the influence of magnetic field on the motion of deuterons