Physics · Ch 13 — Nuclei
Nuclear Fission
Nuclear Fission
NUCLEAR FISSION is the process in which a heavy nucleus splits into two lighter nuclei of roughly comparable size, usually accompanied by the release of a few free neutrons and a very large amount of energy. Fission of nuclei such as is most readily triggered by the absorption of a slow ('thermal') neutron, which forms a highly excited, unstable compound nucleus that almost immediately breaks apart. A typical fission reaction of is
though many other pairs of fragment nuclei are also observed in practice, all with mass numbers falling in the medium- region well up the rising side of the curve of Section 8.7 -- which is precisely WHY the reaction releases so much energy: the two medium fragments together have a considerably HIGHER total binding energy than the original single heavy nucleus had, and this difference in binding energy is released, as calculated from the reaction's mass defect (Section 8.6), as roughly per fission event -- an enormous amount of energy released from a single nuclear reaction, millions of times more than any chemical reaction between a similarly small number of atoms could ever release. …
What this figure shows. A branching tree-like schematic diagram, growing left to right (or top to bottom) across several successive 'generations'. At the far left, a single neutron (a small filled dot with a short arrow) strikes one large circle labelled 'U-235', which then splits (drawn breaking apart into two smaller circles of noticeably different size, labelled generically as two fission fragments) and simultaneously releases three small dots representing three newly freed neutrons, each shown flying outward with its own short arrow. Two or three of these outgoing neutron arrows are then shown, a little further to the right, each striking its OWN separate fresh 'U-235' circle, and each of THOSE circles is shown splitting in exactly the same way, releasing its own set of new fragment pairs and new neutrons -- so that the number of splitting nuclei visibly multiplies at each successive stage moving rightward across the diagram (one splits, producing multiple neutrons, each of which can split another), giving the whole figure the branching, ever-widening look of a chain reaction. A caption beneath the diagram notes that each individual splitting event also releases a burst of energy (not drawn explicitly) and that the re …