Scaling laws and effective-range approach for electromagnetic strength in single-particle halo nuclei

Electromagnetic strength functions of single-particle halo nuclei exhibit universal features that can be described in terms of characteristic scale parameters [1, 2]. These exotic nuclei are often well described assuming a simple nucleon+core structure. They have been studied extensively in recent years by electromagnetic excitation with the help of the Coulomb breakup method. A large electromagnetic strength is observed at low excitation energies [3, 4]. Neutron halo nuclei are characterized by a small separation energy Sn = h̄ q2/(2μ) of the least bound neutron in a s or p state with reduced mass μ. For larger orbital angular momenta the strong centrifugal barrier prevents the formation of a halo. The relevant matrix elements for electric transitions to continuum states with energy E are essentially determined by the asymptotics of the bound and scattering wave functions with orbital angular momenta li and lf . The reduced transition probability for an Eλ excitation can be expressed as