Brittle fracture of rocks in compression: Neutron emissions and geomechanics evidences of piezonuclear reactions

Neutron emission measurements by means of helium-3 neutron detectors were performed on solid test specimens during crushing failure. The tests were carried out at the Laboratory of Fracture Mechanics of the Politecnico di Torino. The materials used were marble and granite, selected in that they present a different behaviour in compression failure (i.e., a different brittleness index) and a different iron content. All the test specimens were of the same size and shape. Neutron emissions from the granite test specimens were found to be of about one order of magnitude higher than the natural background level at the time of failure. These neutron emissions should be caused by nucleolysis or piezonuclear “fissions” that occurred in the granite, but did not occur in the marble: Fe (30,26) = 2Al (14,13) + 2 neutrons. The present natural abundance of aluminum (7-8% in the Earth crust), which is less favoured than iron from a nuclear point of view, is possibly due to the above piezonuclear fission reaction.