Beta Decay Flashcards
Which force is responsible for beta decay?
The weak force.
Do neutron-rich nuclei decay by emission of electrons or positrons?
Electrons.
What are the Q values for beta-plus and beta-minus decay?
Don’t forget to minus 2 electron masses in beta-plus! (Otherwise v. similar to alpha Q value formula)
What was the evidence for neutrinos in beta decay?
The beta decay spectrum observed was continuous so the energy released in the decay must have been shared between 3 particles, not 2 as was previously supposed.
What is the beta end point?
The point at which the electron/positron’s kinetic energy is the same as the Q value.
From what other considerations were neutrinos required?
Conservation of angular momentum and parity conservation also required the presence of the neutrino.
Explain what parity invariance is, what consequences parity invariance would have in a decay process and how experimental evidence revealed the parity violation in beta decay.
Parity invariance means the process is symmetric under space inversion. If beta decay was parity invariant, the electrons emitted equally with orientations along and opposite to the axis of the spins of the initial nuclei, even if the nuclear spins were reversed. The experiments to test for parity violation were carried out with cobalt, and it was found that the electrons are emitted preferentially opposite to the nuclear spin - therefore the orientation of the spins did affect the outcome and hence beta decay is not symmetric under space inversion.
What correction has to be added to Fermi’s golden rule for transition rates?
A correction for the Coulomb force between the beta particle and the nucleus.
What is the expression for the transition rate? Explain the terms.
\lambda = C |M|^2 f(Z,E) C is a constant, the matrix element is M and f(Z,E) is the Fermi factor.
What is the expression for the transition rate? Explain the terms.
\lambda = C |M|^2 f(Z,E) C is a constant, the matrix element is M and f(Z,E) is the Fermi function.
What is a forbidden transition?
A transition for which the matrix element depends on L.
What is a superallowed transition?
It is both a Fermi transition and a Gamow-Teller transition.
What quantity can determine the type of transition? What are some typical values for L=0,1,2,3?
log(ft) where f is the Fermi function and t is the half life. ~3.5, ~5.5, ~7.5, ~12
What is a Fermi transition? What is a Gamow-Teller transition?
S=0 and S=1 where S is vector sum of the electron and neutrino spins (each 1/2)
What is the equation for the parity transformation?
\psi (r, \pi - \theta, \pi + \phi) -> (-1)^L \psi (r, \theta, \phi)
