  Understanding the evolution of level densities in the crossover from
spherical to well-deformed nuclei has been a long-standing problem in nuclear
physics. We measure nuclear level densities for a chain of neodymium isotopes
$^{142,144-151}$Nd which exhibit such a crossover. These results represent to
date the most complete data set of nuclear level densities for an isotopic
chain between neutron shell-closure and towards mid-shell. We observe a strong
increase of the level densities along the chain with an overall increase by a
factor of $\approx 170$ at an excitation energy of 7.5 MeV and saturation
around mass 150. Level densities calculated by the shell model Monte Carlo
(SMMC) are in excellent agreement with these experimental results. Based on our
experimental and theoretical findings, we offer an explanation of the observed
mass dependence of the level densities in terms of the intrinsic
single-particle level density and the collective enhancement.
