Ionisation enthalpies of Ce, Pr and Nd are higher than Th, Pa and U. Why?

Explanation

Ce, Pr and Nd are lanthanoids and have incomplete 4f shell while Th, Pa and U are actinoids and have 5f shell incomplete.
In the beginning, when 5f-orbitals begin to be occupied, they will penetrate less into the inner core of electrons. The 5f-electrons will therefore, be more effectively shielded from the nuclear charge than 4f electrons of the corresponding lanthanoids.
Therefore, outer electrons are less firmly held and they are available for bonding in the actinoids.