We describe in this lecture neutron irradiation experiments performed on superluid He-3-B at ultralow temperatures. The measurement of the thermal energy released by the neutron-He-3 nuclear reaction reveals the existence of an energy deficit. We propose that this deficit can be ascribed to the formation energy of vortices, these topological defects being created by the Kibble-Zurek mechanism as the region heated to the normal state by the neutron irradiation recrosses the superfluid transition. A quantitative agreement is found with Zurek's predictions for the density of topological defects created during a rapid quench through a symmetry-breaking phase transition.