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 ) = {mnucl(A, Z) − mnucl(A, Z − 1) − me} × c and, neglecting the electron binding energies, we can use atomic masses, 2 = [M(A, Z) − Zme − {M(A, Z − 1) − (Z − 1)me} − 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
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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