Magnetic dipole γ-ray strength functions in neodymium isotopes in the configuration-interaction shell model

The accurate description of neutron capture rates is important for understanding r-process nucleosynthesis. These rates can be calculated using the Hauser-Feshbach theory, in which the gamma-ray strength function (γSF) is a key input. A low-energy enhancement (LEE) was observed in the deexcitation γSF of compound nuclei, attracting intense experimental and theoretical interest since its discovery. If the LEE persists in heavy neutron-rich nuclei, it would significantly impact r-process nucleosynthesis calculations by enhancing the neutron capture rates near the neutron drip line. A LEE was identified in the magnetic dipole (M1) γSF of medium-mass nuclei using conventional configuration-interaction (CI) shell model methods but such calculations are prohibited in heavy open-shell nuclei.

We present results for the M1 γSF for a chain of even-mass neodymium isotopes computed using the CI shell model. To manage the large CI model spaces, we employ the shell-model Monte Carlo method. The γSFs are inferred from imaginary-time response functions by analytic continuation using the maximum entropy method. We identify a LEE in the M1 γSF of the neodymium isotopes and also observe the spin-flip and scissors modes.