have there been any instances of suicide by radiation poisoning?

There was a russian guy who was killed in london in 2006 when someone put polonium-210 in his food. His name was Alexander Litvinenko, you can look him up and read all about it.

I never heard of anyone committing suicide that way, and given it took Litvinenko about 3 weeks to die it would seem a very unpleasant way to go.

Oh and the discoverer of radioactive materials - Marie Curie eventually died from exposure but that was over years, not a single dose like the Russian guy.

The only examples I can think of would be at the Chernobyl accident where twenty-eight firemen and emergency clean-up workers died in the first three months after the explosion from Acute Radiation Sickness. I am presuming of course that they were aware of the dangers that they were exposing themselves to and continued to work to prevent the accident being far more of a disaster.