Experimental work on the dense-plasma focus device Jülich I is presented. The main objective of this program was the development of a neutron source for controlled thermonuclear reactor applications on blanket and material problems. Therefore, detailed studies of the neutron production mechanism and of the fusion plasma properties of the plasma focus experiment were necessary. The investigations have been performed using reaction neutrons as special tools of plasma diagnostics. The results of neutron spectrum measurements indicate the presence of high-energy deuterons of at least 300 keV moving predominantly parallel to the axis of the gun. Neutrons produced by fusion reactions of deuterium and 7Li have been observed, again showing the presence of ions with energies above 300 keV up to at least 1 MeV. By shadow bar techniques, it has been found that the origin of the neutrons is restricted to a cone that extends from the anode to the lid of the discharge vessel. This suggests an acceleration of deuterons near the anode within the plasma volume. The deuterons are extruded from the pinch, moving freely through the neutral gas. The fusion reactions have been detected all along the flight path of the deuterons up to a distance of 126 cm from the anode. Time-resolved measurements of the neutron production show dependence of the emission time on the axial dimensions of the vessel.