Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

Beta-minus (~-) decay, or negatron decay, characteristically occurs with radionuclidesthat have an excess number of neutrons compared with the number of protons(i.e., a high N/Z ratio). Beta-minus decay can be described by the followingequation:A A A -zX ~Z+lY + 1-'- + V + energy(negarroo) (antineutrino)This mode of decay results in the conversion of a neutron into a proton with thesimultaneous ejection of a negatively charged beta particle (~-) and an anti neutrino(V). With the exception of their origin (the nucleus), beta particles are identical toordinary electrons. The antineutrino is an electrically neutral subatomic particlewhose mass is much smaller than that of an electron. The absence of charge and theinfinitesimal mass of antineutrinos make them very difficult to detect because theyrarely interact with matter. Beta decay increases the number of protons by 1 andthus transforms the atom into a different element with an atomic number Z + 1.However, the concomitant decrease in the neutron number means that the massnumber remains unchanged. Decay modes in which the mass number remains constantare called isobaric transitions. Radionuclides produced by nuclear fission are"neutron rich," and therefore most decay by ~- emission. Beta-minus decaydecreases the N/Z ratio, bringing the daughter closer to the line of stability (seeChapter 2):c0'iijUl0.6'Ew-0~ 0.4:0 asDE a.0.2