For some isotopes of some very heavy nuclei, including nuclei of thorium, uranium, and plutonium, the nucleus will fission (split apart) when it absorbs a slow-moving neutron. for example, plutonium-239, with 94 protons and 145 neutrons, can fission when it absorbs a neutron and becomes plutonium-240. the two fission fragments, called "daughter" nuclei, can be almost any two nuclei whose charges q1 and q2 add up to 94e (where e is the charge on a proton), and whose nucleons add up to 240 protons and neutrons (pu-240, formed from pu-239 plus a neutron).one of the possible fission modes involves nearly equal fragments, silver nuclei (ag-120) each with electric charge q1 = q2 = 47e. the rest masses of the two daughter nuclei add up to less than the rest mass of the original parent nucleus. (in addition to the two main fission fragments there are typically one or more free neutrons in the final state; in your analysis make the simplifying assumption that there are no free neutrons, just two daughter nuclei.)