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Nuclear and Particle Physics SubAtomic Physics

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					SubAtomic Particle
Nuclear and Physics
      Physics
   Nuclear Physics

      Lecture 6


     3rd Year Junior
         Honours
          Course


    Dr Daniel Watts
                         Main points of lecture 7
     SEMF gives good description of masses and binding
     energy of nuclei

                                     But …See strong evidence for shell like
                                     structure in the nucleus

                                     Magic numbers – certain numbers of
                                     protons or neutrons produce very
                                     stable nuclei.

                                     Due to shell closure (e.g. as seen for
                                     noble gases in atomic physics)



      From analogy with atomic physics try a theory where
      nucleons move independently in a central potential
      → Shell model
                                    Coulomb repulsion
                                    adds to potential


                                                        Need to introduce spin-orbit
                                Proton                  coupling L.S
     Neutron
   potential well            potential well




                                                             valence
                                                             nucleon   17O
Fill nucleons into lowest available energy levels.
Predict ground state spin & parity from unpaired
nucleon                                              1d5/2
(Nuclei in the atoms around us usually in this
lowest energy                                        1p1/2
 “ground” state)                                     1p3/2

                                                     1s1/2
Notes
Notes
      What about excited states in the shell model?

 Ground state properties
     Given by unpaired nucleon with all nucleons in
    lowest possible energy states.

 Excited states
     Shell model can explain the spins and parities
     of some excited states through reconfiguration of
     nucleons to give different valence nucleon



            17
             8   O


                                                         2s1/2
                                                         1d5/2
                                                         1p1/2
                        1/2-                             1p3/2
                                                         1s1/2

                                                         2s1/2
                                                         1d5/2
                                                         1p1/2
                                                         1p3/2
                                                         1s1/2
                       1/2+
                                                           2s1/2
                     Jπ=5/2+                               1d5/2
                                                           1p1/2
                                                           1p3/2
                                                           1s1/2




Note : Mirror nuclei show similar orderings and energies in their
excited states (e.g. 17O and 17F)
Shell model predictions of magnetic moments of nuclei
Magnetic moments arise from:
   i) intrinsic magnetic moments of nucleons
  ii) magnetic moment from oribital angular momentum of nucleons

Magnetic moments of even-even nuclei are zero.

For nuclei with an odd neutron or proton shell model predicts two possible
values corresponding to the two possible orientations of the nucleon spin
with respect to the orbital vector for the valence nucleon




                                                     Measured values
                                                     tend to lie
                                                     between the shell
                                                     model predictions

                                                     Dipole moment more
                                                     sensitive test of model
                                                     than nuclear spins and
                                                     parities

				
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