![]() ![]() It bounces off the nucleus by elastic scattering, which occurs 5 times more often than the next process.There are three possibilities for further reaction for such neutron (in descending possibility): Cross Section - The neutron released immediately (about 10 -14 sec) in the fission process is called prompt neutron. ![]() An isotope like uranium-235 that can be split by both slow and fast neutrons, is called "fissile", while uranium-238 which can be split only by fast neutrons, is called "fissionable".įigure 14-11c Neutron Induced Fission and Chain reaction įigure 14-11d Symmetric Fission Anyway, the following equation is an example of the neutron induced reaction :ĩ2U 235 + n 92U 236 56Ba 144 + 36Kr 89 + 3n + 166 Mevįor uranium-238, the excitation energy is about 1 Mev less, so fission is not possible with slow neutrons it can take place only for neutrons with 1 Mev energy or more. Another dangerous by product is cesium-137, which has a half life of 30 years, but the biological half-life (elimination from the body) is much shorter at about 70 days. They are supposed to prevent further intake of radioactive iodine by the thyroid. The most probable and radioactive product is iodine-131 (half life 8 days), that's why the run on iodine pills and iodized salts during the Fukushima incident. There are about 90 different isotopes that can be produced by the fission. Actually, the products in the fission process can be any two in each of the humps shown by Figure 14-11d as long as charge conservation is observed (thus called symmetric fission). Neutron Induced Fission - When an uranium-235 nucleus captures a neutron, the newly formed isotope uranium-236 turns into an excited state with excitation energy about 6 Mev, which is a bit more than the barrier height and splits into two fragments as shown by the equation below (also see illustration on top of Figure 14-11a, and schematic in Figure 14-11c).Alpha decay is a process of asymmetric fission because it usually involves a larger nuclear fragment and the much smaller alpha particle.įigure 14-11a Nuclear Potential įigure 14-11b Quantum Tunneling This formula provides a crude approximation in estimating the probability of alpha decay, which depends inversely on the height "V" and width "a" of the barrier. The transmission coefficient T for a one dimensional rectangular barrier is given by the formula:įor p 2a/ > 1, where p 1 = (2mE) 1/2, and p 2 = 1/2. In quantum theory there is a certain probability to tunnel out under the barrier as shown in Figure 14-11b. Classical theory dictates that the system can break apart only when it is excited with energy beyond the barrier height. Theory of Fission - A stable nucleus is in a state of minimum energy in a potential well with a barrier further out as shown in Figure 14-11a.For elements lighter than iron it is the process of fusion that releases the binding energy. If the mass of each fragment is equal to or greater than that of iron at the peak of the binding energy curve (see Figure 14-01), then the products of the decay will be bound more tightly than they were in the uranium nucleus, and that decrease in mass comes off in the form of energy according to the Einstein's equation E = mc 2, where m is the loss of mass in the reaction. Release of the Binding Energy - If a massive nucleus like uranium-235 breaks apart (nuclear fission), then there will be a net yield of energy because the sum of the masses of the fragments will be less than the mass of the uranium nucleus.Nuclear Fission : Release of Binding Energy, ![]()
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