9. Beta Decays Flashcards
What does beta decay fundamentally do to the nucleus?
It changes the proton neutron asymmetry
n -> p + e + neutrino
p -> n + e+ + anti-neutrino
What is electron capture?
When an electron from the atomic shell is captured by a proton, which turns into a neutron
e- + p -> n + neutrino
What does the BE Z graph look like for an even odd nucleus?
A parabola (Beta- decay going down, Beta+ decay going up) Note BE axis goes down not up
When do we specifically get beta + or beta - decays?
Beta + for proton excess
Beta - for neutron excess
State the BE equation for beta minus decay
Can occur if M(Z, A) > M(Z+1, A)
- No additional electron as it was included in the second term
- Z electrons pre decay, Z+1 after
State the BE equation for beta plus decay
Can occur if M(Z, A) > M(Z-1, A) + 2m_e
- Additional electron from positron emission and one wasn’t included in the M(Z-1, A)
Describe the competition between positron emission and electron capture
Positron itself competes with electron capture where an electron from the atomic shell is captured by the nucleus. Possible if
M(Z, A) > M(Z-1, A) + ε
ε - excitation energy of atomic shell of daughter
What type of nuclei does electron capture typically occur in, and what follows?
Heavier nuclei
- Light emission from moving an electron from the higher shell to the hole after capture
What does the BE Z graph look like for an even even/ odd odd nucleus?
Two parabolas stacked on each other, odd on top of even
- Get movements between the two curves and some odd-odd have been observed to undergo double beta decay
What are Fermi and Gamow transitions?
Fermi: ΔJ = | J_i - J_f | = 0
Gamow: ΔJ = 0 or 1
i and f are the parent and daughter state
What are the first and second forbidden transitions
When L = 1 and L = 2
- First forbidden transition changes the parity (-1)^L = -1
- Second won’t
- Parity doesn’t change in allowed transitions
Describe the graph for J(p) and p (electron momentum distribution) for Z = 0, Beta minus decay and Beta plus decays
Beta minus is a negative parabola skewed left (electron emission)
Z = 0 in the middle
Beta plus right (positron emission)
