Shell Model with residual interactions – mostly 2-particle systems. Simple forces, simple physical interpretation. Lecture 2. Independent Particle Model. Some great successes (for nuclei that are “doubly magic plus or minus 1”).
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Shell Model with residual interactions – mostly 2-particle systems
Simple forces, simple physical interpretation
So, we will have a Hamiltonian H = H0 + Hresid.where H0 is that of the Ind. Part. ModelWe need to figure out what Hresid. does.Since we are dealing with more than one particle outside a doubly magic core we first need to consider what the total angular momenta are when the individual ang. Mon. of the particles are vector-coupled.
j1+ j2 All values from: j1 – j2 to j1+ j2 (j1 =j2)
Example: j1 = 3, j2 = 5: J = 2, 3, 4, 5, 6, 7, 8
BUT: For j1 = j2: J = 0, 2, 4, 6, … ( 2j – 1) (Why these?)
How can we know which total J values are obtained for the coupling of two identical nucleons in the same orbit with total angular momentum j? Several methods: easiest is the “m-scheme”.
Can we obtain such simple results by considering residual interactions?
Separate radial and angular coordinates
How can we understand the energy patterns
that we have seen for two – particle spectra
with residual interactions? Easy – involves
a very beautiful application of the Pauli
This is the most important slide: understand this and all the key ideas about residual interactions will be clear !!!!!
Think of the three particles as 2 + 1. How do the 2 behave?
We have now seen that they prefer to form a J = 0 state.
Treat as 20 protons and 20 neutrons forming a doubly magic core with angular momentum J = 0. The lowest energy for the 3-particle configuration is therefore
J = 7/2.
Note that the key to this is the result for the 2-particle system !!
Multipole Decomposition of Residual Interactions
We have seen that the relative energies of 2-particle systems affected by a residual interaction depend SOLELY on the angles between the two angular momentum vectors, not on the radial properties of the interaction (which just give the scale).
We learn a lot by expanding the angular part of the residual interaction,
Hresidual = V(q,f)
in spherical harmonics or Legendre polynomials.