Rotational Alignment and Bandcrossings

In the year 1971, in 160Dy [1] nucleus, a discontinuity in the pattern of gamma-ray spectra normally observed in the decay of rotational states, was discovered at angular momentum, I ~ 14ħ. This was interpreted in [2] as due to the decoupling of a pair of high-j nucleons from the rotating nuclear core and subsequent alignment of their angular momenta along the rotation axis at particular rotational frequency. A few years later, in 1977, a second discontinuity was observed in the gamma-ray spectra at angular momentum, I ~ 28ħ in 158Er [3]. These two were the most significant discoveries in the initial phases of high spin physics. Most of the present day knowledge and the huge amount of information both experimental and theoretical, in a very large number of nuclei at high spins, owe its existence to the interest developed as a result of these discoveries. As will be explained later in this chapter, the rotation alignment of high-j low-Ω pairs of nucleons manifests as bandcrossing (popularly known as ‘backending’) between rotational bands. This mode of quasiparticle excitation is imposed on the collective nuclear rotational motion.