It is useful to distinguish spontaneous fission SF which occurs in nuclei in their ground states from induced fission brought about by a reaction or decay process bringing in energy from the outside. SF is one of the main decay modes of superheavy nuclei and is therefore of great interest in the experimental search for them.
While SF primarily occurs from the nuclear ground state, it has also been observed from isomeric states. On the theory side, the relatively long lifetimes are due to the existence of a potential barrier that must be penetrated. Consequently SF is an inherently quantal process; see section 3. Fission of odd-odd nuclei is believed to be even more hindered, but credible data are scarce. In addition to a SF, fission can be induced by a variety of nuclear reactions. The fission-induced processes include: neutron capture responsible for energy production in fission reactors , electron capture and beta decay, photofission, and reactions involving charged particles and heavy ions.
In all these processes, the fissioning nucleus is created in an excited state, which may lie above or below the fission barrier. Theoretical descriptions of fission induced by fast probes often assume the creation of a compound nucleus at a given thermal excitation energy.
However, as discussed later, that assumption might be ill-founded for fast probes because the nuclear system may not have sufficient time to thermalise before undergoing fission.
This becomes increasingly important at higher energies where pre-equilibrium processes play an increasingly significant role and may lead to the emission of one or more nucleons before equilibrium is reached. Moreover, as the excitation energy of the compound nucleus is increased, neutron evaporation competes ever more favourably with fission and as a result, one or more neutrons may be evaporated before fission occurs multi-chance fission.
If stretched enough, the nucleus narrows in the middle. The number of nucleons in contact and the strength of the nuclear force binding the nucleus together are reduced. Coulomb repulsion between the two ends then succeeds in fissioning the nucleus, which pops like a water drop into two large pieces and a few neutrons. Neutron-induced fission can be written as. Most often, the masses of the fission fragments are not the same. Most of the released energy goes into the kinetic energy of the fission fragments, with the remainder going into the neutrons and excited states of the fragments.
This can also be seen in Figure 3. An example of a typical neutron-induced fission reaction is. This is not true when we consider the masses out to 6 or 7 significant places, as in the previous example. Figure 2. Neutron-induced fission is shown. First, energy is put into this large nucleus when it absorbs a neutron. Acting like a struck liquid drop, the nucleus deforms and begins to narrow in the middle.
Since fewer nucleons are in contact, the repulsive Coulomb force is able to break the nucleus into two parts with some neutrons also flying away.
Figure 3. A chain reaction can produce self-sustained fission if each fission produces enough neutrons to induce at least one more fission. This depends on several factors, including how many neutrons are produced in an average fission and how easy it is to make a particular type of nuclide fission. Not every neutron produced by fission induces fission. Some neutrons escape the fissionable material, while others interact with a nucleus without making it fission. We can enhance the number of fissions produced by neutrons by having a large amount of fissionable material.
The minimum amount necessary for self-sustained fission of a given nuclide is called its critical mass. Some nuclides, such as Pu , produce more neutrons per fission than others, such as U. Additionally, some nuclides are easier to make fission than others. In particular, U and Pu , are easier to fission than the much more abundant U. Both factors affect critical mass, which is smallest for Pu.
The reason U and Pu are easier to fission than U is that the nuclear force is more attractive for an even number of neutrons in a nucleus than for an odd number. When a neutron encounters a nucleus with an odd number of neutrons, the nuclear force is more attractive, because the additional neutron will make the number even. About 2-MeV more energy is deposited in the resulting nucleus than would be the case if the number of neutrons was already even.
This extra energy produces greater deformation, making fission more likely. Thus, U and Pu are superior fission fuels. The isotope U is only 0. This is followed by Kazakhstan and Canada. Most fission reactors utilize U , which is separated from U at some expense. This is called enrichment. The most common separation method is gaseous diffusion of uranium hexafluoride UF 6 through membranes.
Since U has less mass than U , its UF 6 molecules have higher average velocity at the same temperature and diffuse faster. Another interesting characteristic of U is that it preferentially absorbs very slow moving neutrons with energies a fraction of an eV , whereas fission reactions produce fast neutrons with energies in the order of an MeV. Water is very effective, since neutrons collide with protons in water molecules and lose energy. Figure 4 shows a schematic of a reactor design, called the pressurized water reactor.
Figure 4. A pressurized water reactor is cleverly designed to control the fission of large amounts of U , while using the heat produced in the fission reaction to create steam for generating electrical energy. Control rods adjust neutron flux so that criticality is obtained, but not exceeded. In case the reactor overheats and boils the water away, the chain reaction terminates, because water is needed to thermalize the neutrons. This inherent safety feature can be overwhelmed in extreme circumstances.
Control rods containing nuclides that very strongly absorb neutrons are used to adjust neutron flux. To produce large power, reactors contain hundreds to thousands of critical masses, and the chain reaction easily becomes self-sustaining, a condition called criticality. Neutron flux should be carefully regulated to avoid an exponential increase in fissions, a condition called supercriticality.
The purpose is to study in details the nuclear, atomic and chemical properties of very heavy and superheavy elements with considerably improved statistics. Among the decay properties of these nuclei, particular attention will be given to nuclear fission.
In the same period the SPES facility, under development at the Laboratori Nazionali di Legnaro, Italy, is expected to enter the production stage of neutron-rich radioactive beams. With these beams the study of fission in unexplored regions of the nuclear chart will be possible, in particular in the region of very heavy neutron rich nuclei.
The main goal of the workshop is to discuss the state of the art of our present knowledge of fission and quasifission in the region of the very heavy and superheavy nuclei and to identify possible physics cases to be explored with the above two new facilities under development. These two facilities can play an extremely important and unique role in accessing, with different capabilities, unexplored regions of the nuclear chart, in particular the unknown neutron rich side.
This area is particularly relevant because is of interest for the nucleosynthesis of the elements heavier that iron r-process and for the understanding of the existence and persistence of the shell closures.
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