Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

Although the ~- particles emitted by a particular radio nuclide have a discretemaximal energy (EmaX>, most are emitted with energies lower than the maximum.The average energy of the ~- particles is approximately 1/3 Emax. The balance of theenergy is given to the anti neutrino (i.e., Emax = Ef + Ey ). Thus, beta-minus decayresults in a polyenergetic spectrum of ~- energies ranging from zero to Emax (Fig.18-3). Any excess energy in the nucleus after beta decay is emitted as gamma rays,internal conversion electrons, and other associated radiations.Just as beta-minus decay is driven by the nuclear instability caused by excess neutrons,"neutron-poor" radio nuclides (i.e., those with a low N/Z ratio) are also unstable.Many of these radionuclides decay by beta-plus (positron) emission, whichincreases the neutron number by one. Beta-plus decay can be described by the followingequation:~x~Z~lY + ~+ + V + energy(positron) (neutrino)The net result is the conversion of a proton into a neutron with the simultaneousejection of the positron (W) and a neutrino (V). Positron decay decreases thenumber of protons (atomic number) by 1 and thereby transforms the atom into adifferent element with an atomic number of Z-I. The number of neutrons isincreased by 1; therefore, the transformation is isobaric because the total number ofnucleons is unchanged. Accelerator-produced radionuclides, which are typicallyneutron deficient, often decay by positron emission. Positron decay increases theN/Z ratio, resulting in a daughter closer to the line of stability.The energy distribution between the positron and the neutrino is similar tothat between the negatron and the antineutrino in beta-minus decay; thus positronsare polyenergetic with an average energy equal to approximately 1/3 Emax. As with~- decay, excess energy following positron decay is released as gamma rays and otherassociated radiation.Although W decay has similarities to ~- decay, there are also important differences.The neutrino and antineutrino are antiparticles, as are the positron and negatron.The prefix anti- before the name of an elementary particle denotes anotherparticle with certain symmetry characteristics. In the case of charged particles suchas the positron, the antiparticle (i.e., the negatron) has a charge equal but oppositeto that of the positron and a magnetic moment that is oppositely directed withrespect to spin. In the case of neutral particles such as the neutrino and antineutrino,there is no charge; therefore, differentiation between the particles is madesolely on the basis of differences in magnetic moment. Other important differencesbetween the particle and antiparticle are their lifetimes and their eventual fates. Asmentioned earlier, negatrons are physically identical to ordinary electrons and assuch lose their kinetic energy as they traverse matter via excitationand ionization.