Multipole modes in deformed nuclei within the finite amplitude method

Background: To access selected excited states of nuclei, within the framework of nuclear density functional theory, the quasiparticle random phase approximation (QRPA) is commonly used.Purpose: We present a computationally efficient, fully self-consistent framework to compute the QRPA transition strength function of an arbitrary multipole operator in axially deformed superfluid nuclei.Methods: The method is based on the finite amplitude method (FAM) QRPA, allowing fast iterative solution of QRPA equations. A numerical implementation of the FAM-QRPA solver module has been carried out for deformed nuclei.Results: The practical feasibility of the deformed FAM module has been demonstrated. In particular, we calculate the quadrupole and octupole strengths in a heavy deformed nucleus ²⁴⁰Pu, without any truncations in the quasiparticle space. To demonstrate the capability to calculate individual QRPA modes, we also compute low-lying negative-parity collective states in ¹⁵⁴Sm.Conclusions: The new FAM implementation enables calculations of the QRPA strength function throughout the nuclear landscape. This will facilitate global surveys of multipole modes and β decays and will open new avenues for constraining the nuclear energy density functional.