Atomic and Nuclear PhysicsDOE-HDBK-1019/1-93NUCLEAR FISSIONLiquidDropModelof a NucleusThe nucleus is held together by the attractive nuclear force between nucleons, which wasdiscussed in a previous chapter. The characteristics of the nuclear force are listed below. (a)very short range, with essentially no effect beyond nuclear dimensions(10-13 cm)(b)stronger than the repulsive electrostatic forces within the nucleus(c)independent of nucleon pairing, in that the attractive forces between pairs ofneutrons are no different than those between pairs of protons or a neutron and aproton(d)saturable, that is, a nucleon can attract only a few of its nearest neighborsOne theory of fission considers the fissioning of a nucleus similar in some respects to the splittingof a liquid drop. This analogy is justifiable to some extent by the fact that a liquid drop is heldtogether by molecular forces that tend to make the drop spherical in shape and that try to resistany deformation in the same manner as nuclear forces are assumed to hold the nucleus together.By considering the nucleus as a liquid drop, the fission process can be described.Referring to Figure 18(A), the nucleus in the ground state is undistorted, and its attractive nuclearforces are greater than the repulsive electrostatic forces between the protons within the nucleus.When an incident particle (in this instance a neutron) is absorbed by the target nucleus, acompound nucleus is formed. The compound nucleus temporarily contains all the charge andmass involved in the reaction and exists in an excited state. The excitation energy added to thecompound nucleus is equal to the binding energy contributed by the incident particle plus thekinetic energy possessed by that particle. Figure 18(B) illustrates the excitation energy thusimparted to the compound nucleus, which may cause it to oscillate and become distorted. If theexcitation energy is greater than a certain critical energy, the oscillations may cause thecompound nucleus to become dumbbell-shaped. When this happens, the attractive nuclear forces(short-range) in the neck area are small due to saturation, while the repulsive electrostatic forces(long-range) are only slightly less than before. When the repulsive electrostatic forces exceedthe attractive nuclear forces, nuclear fission occurs, as illustrated in Figure 18(C).Rev. 0Page 49NP-01
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