A chemical element, Pa, atomic number 91. Isotopes of mass numbers 216, 217, and 222–238 are known, all of them radioactive. Only ²³¹Pa, the parent of actinium, ²³⁴Pa, and ²³³Pa occur in nature. The most important of these is ²³¹Pa, an α-emitter with a half-life of 32,500 years. The artificial isotope, ²³³Pa, is important as an intermediary in the production of fissile ²³³U. Both ²³¹Pa and ²³³Pa can be synthesized by neutron irradiation of thorium.