  The density of accessible levels at low spin in the (3-He,alpha gamma)
reaction has been extracted for the 161,162-Dy and 171,172-Yb nuclei. The
energy shift between the level densities of the even-odd and even-even isotopes
is measured as a function of excitation energy. The results are compared with
predictions from various semi-empirical models. The energy shift procedure
works well for excitation energies between 3.5 and 7 MeV in the even-even
nucleus, provided that a proper level density function is used. The
experimental energy shift is close to the pairing gap parameter Delta.
