Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

come the nuclear binding forces. This energy is often provided by the absorption ofneutrons. Neutrons can induce fission only in certain very heavy nuclei. Whereashigh-energy neutrons can induce fission in several such nuclei, there are only threenuclei of reasonably long half-life that are fissionable by neutrons of all energies;these are called fissile nuclides.The most widely used fissile nuclide is uranium 235 (U-235). Elemental uraniumexists in nature primarily as U-238 (99.3%) with a small fraction of U-235(0.7%). U-235 has a high fission cross-section (i.e., high fission probability); therefore,its concentration in uranium ore is usually enriched (typically to 3% to 5%)to make the fuel used in nuclear reactors.When a U-235 nucleus absorbs a neutron, the resulting nucleus (U-236) is inan extremely unstable excited energy state that usually promptly fissions into twosmaller nuclei called fission fragments. The fission fragments fly apart with very highkinetic energies, with the simultaneous emission of gamma radiation and the ejectionof two to five neutrons per fission (Equation 19-4).1?85n23 1 23 7' 19211 + Onthetmal ~ [92\J] +3 onfast+ 'Y+ ~200 MeV'1~Mo235U + Onthetmal 1 ~ 99M0 +1345 n + 3 1on = 236SiOn .The fission of uranium creates fission fragment nuclei having a wide range ofmass numbers. More than 200 radionuclides with mass numbers between 70 and160 are produced by the fission process (Fig. 19-4). These fission products are neutronrich and therefore almost all of them decay by beta-minus (~-) particle emis-..-';!l.~"CCD>= 10-1C0'iij.r!U.10-2FIGURE 19-4, Fission yield as a percentage oftotal fission products from uranium 235.