def get_ground_state_j(mass, nshell):
    """Get the j for the ground state for the given mass number and shell.
    :param mass: mass number
    :param nshell: (0=s, 1=p, 2=sd, ...)
    """
    if mass % 2 == 0:
        return 0.0
    elif nshell == 1:
        return 1.5
    elif nshell == 2:
        return 2.5
    else:
        return None