Types of decays b β decay b α decay b spontaneous fission b nucleon emission b γ decay c KS Krane, Figure 6.1 P. Teixeira-Dias PH2510 - Atomic and Nuclear Physics Royal Holloway Univ of London β decay and electron capture Electron number (Le) and charge are conserved: matter anti-matter + e− νe e ν¯e charge −1 0 +1 0 Le +1 +1 −1 −1 A A + + ZXN ! Z−1YN+1 + e + νe (β decay) A A − − ZXN ! Z+1YN−1 + e + ν¯e (β decay) Nuclear capture of a K-shell electron: A − A ZXN + eK ! Z−1YN+1 + νe P. Teixeira-Dias PH2510 - Atomic and Nuclear Physics Royal Holloway Univ of London β decay and electron capture The energy released in the decay process is the Q-value of the reaction. The reaction is energetically allowed if Q > 0: there is a decrease in the mass of the system, which is converted into the kinetic energy of the decay products. Let's work out Q for positron emission (β+): + 2 Q(β ) = fmnucl(A; Z) − mnucl(A; Z − 1) − meg × c and, neglecting the electron binding energies, we can use atomic masses, 2 = [M(A; Z) − Zme − fM(A; Z − 1) − (Z − 1)meg − me] × c = 2 = [M(A; Z) − M(A; Z − 1) − 2me] × c P. Teixeira-Dias PH2510 - Atomic and Nuclear Physics Royal Holloway Univ of London β-decay (cont'd) . The Q-values are: Q(β−) = [M(A; Z) − M(A; Z + 1)] × c2 + 2 Q(β ) = [M(A; Z) − M(A; Z − 1) − 2me] × c Q(EC) = [M(A; Z) − M(A; Z − 1)] × c2 Electron capture and β+ decay involve the same initial and final nuclides. + Nuclei for which β+ decay is possible can also undergo electron capture, but the reverse is not necessarily true, as it is possible to have Q(EC) > 0 and Q(β+) < 0 P. Teixeira-Dias PH2510 - Atomic and Nuclear Physics Royal Holloway Univ of London P . T eixeira-Dias range α neutrons, + nuclei. sp o The The r ontaneous α decay Some β of − fission fission deca middle and heavy A A−4 ZXN ! Z 2YN−2 + α y) − ma fragments fragments but w nuclei y eight 4 deca nuclea The α-particle (a 2He 2 nucleus) is a very tightly a re PH2510 bound nucleus ( B=A curve) and therefore maximizes fission nuclei. y statistically the kinetic energy available for the decay products. - do a A b tomic re y r neutron and generally not Energy release (Q−value) in various modes of sp Nuclea fission 232U decay: ontaneously r rigidly Physics distributed emission. Emitted Q (MeV) Emitted Q (MeV) have particle particle determined n −7.26 4He +5.41 1 −6.12 5 −2.59 an H He 2 6 into H −10.70 He −6.19 3 6 excess over H −10.24 Li −3.79 3He −9.92 7Li −1.94 t w (as the o of lighter in entire Ro + In the Lab we will study α−particles from the y al Hollo decay w a 241 237 y Univ Am ! Np + α of London P. Teixeira-Dias PH2510 - Atomic and Nuclear Physics Royal Holloway Univ of London nucleon emission A A−1 1 ZXN ! Z−1YN + 1p A A−1 1 ZXN ! Z XN−1 + 0n Far away from the \valley of stability" the mass energy differences between neighbouring isobars ( SEMF parabolas) increases. If eventually the mass differences exceed the nucleon binding energy (≈ 8 MeV) then it is possible to have radioactive decay by nucleon emission. This type of decay occurs most frequently in fission products, to rid them of their neutron excess. P. Teixeira-Dias PH2510 - Atomic and Nuclear Physics Royal Holloway Univ of London γ decay Monochromatic γ is emitted in the deexcitation of nuclear state to lower energy state. Analogous to atomic optical or X-ray deexcitation process. No change in nuclear species. + In the Lab we will use γ−ray spectrometers to study γ−rays from 60Co; 137Cs; etc. P. Teixeira-Dias PH2510 - Atomic and Nuclear Physics Royal Holloway Univ of London.