# What is the sd-shell in the nuclear shell model?

I understand that the energy levels in the nuclear shell model go as:

$$1S_{1/2}, 1P_{3/2}, 1P_{1/2}, 1D_{5/2}, 2S_{1/2}, 1D_{3/2}$$

But what is meant by the "sd-shell"? Is it something to do with the 2S and 1D shells overlapping? Thanks in advance.

• Can you please provide your present education level? I'll try to answer accordingly. – Yajnya Sapkota Jun 29 at 12:11
• I am at Masters level. ZeroTheHero's answer was enough for me to figure out what I needed but a more complete answer may prove useful for other users of the site. – Tech Jun 30 at 14:45

Please look up the angular momentum of the 3D harmonic oscillator, which often approximates the nuclear potential. The states are exactly degenerate in the h.o. approximation. The $$n=2$$ states have $$\ell=0$$ (i.e. $$s$$ states) and $$\ell=2$$ ($$d$$ states). See also this energy diagram for the shell model, which illustrates the near degeneracy and labelling of the states in a shell.