Since the nuclei that can readily undergo fission are particularly neutron-rich (e.g. 61% of the nucleons in uranium-235 are neutrons), the initial fission products are often more neutron-rich than stable nuclei of the same mass as the fission product (e.g. stable zirconium -90 is 56% neutrons compared to unstable strontium -90 at 58%). The initial fission products therefore may be unstable and typically undergo beta decay to move towards a stable configuration, converting a neutron to a proton with each beta emission. (Fission products do not decay via alpha decay.)