Isotopes of protactinium

Protactinium (₉₁Pa) has no stable isotopes. As ²³¹Pa occurs in usable quantity, and comprises virtually all of the element, it defines the standard atomic weight.

Thirty radioisotopes of protactinium have been characterized, ranging from ²¹⁰Pa to ²³⁹Pa. The most stable isotopes are ²³¹Pa with a half-life of 32,700 years, ²³³Pa with a half-life of 26.975 days, and ²³⁰Pa with a half-life of 17.4 days. All of the remaining radioactive isotopes have half-lives less than 1.6 days, and the majority of these have half-lives less than 1.8 seconds. This element also has five meta states, ^217mPa (t_1/2 1.15 milliseconds), ^220m1Pa (t_1/2 = 308 nanoseconds), ^220m2Pa (t_1/2 = 69 nanoseconds), ^229mPa (t_1/2 = 420 nanoseconds), and ^234mPa (t_1/2 = 1.16 minutes).

The only naturally occurring isotopes are ²³¹Pa, ²³³Pa, ²³⁴Pa, and ^234mPa. The first occurs as an intermediate decay product of ²³⁵U, the second of (rare) ²³⁷Np, and the last two as intermediate decay products of ²³⁸U. ²³¹Pa dominates solely because of its longer life.

The primary decay mode for isotopes of Pa lighter than (and including) the most stable isotope ²³¹Pa is alpha decay to isotopes of actinium, except ²²⁸Pa to ²³⁰Pa, which primarily decay by electron capture to isotopes of thorium. The primary mode for the heavier isotopes is beta minus (β⁻) decay to isotopes of uranium.