Atomic nuclei consist of protons and neutrons. The atomic nucleus consists of protons and neutrons. The gluon holds together quarks to form protons and neutrons. The nuclear force is nearly independent of whether the nucleons are neutrons or protons. Polonium is now usually obtained by irradiating bismuth with high-energy neutrons or protons. The total number of protons and neutrons determine the nuclide. So the residuum from the strong interaction within protons and neutrons also binds nuclei together. The description applies to matter composed of electrons, protons, neutrons or other fermions. Carbon-12 is composed of 6 protons, 6 neutrons, and 6 electrons. Neutron radiation is a form of ionizing radiation that presents as free neutrons. It has 8 protons, 7 neutrons, and 8 electrons. Boron atoms are also good absorbers of neutrons. Fission chain reactions occur because of interactions between neutrons and fissile isotopes (such as U). Th is an isotope of thorium whose nuclei contain 144 neutrons. Thermal neutrons are more likely than fast neutrons to cause fission.