Title: Alpha rays and planned evacuation zone
Source: Koichi Oyama’s Blog (Minamisoma city council member)
Date: April 3, 2012

Google Translation: Measured the alpha rays seen in the railing of a bridge

More on Alpha Emitters -Wikipedia (Emphasis Added)

“When alpha particle emitting isotopes are ingested, they are far more dangerous than their half-life or decay rate would suggest, due to the high relative biological effectiveness of alpha radiation to cause biological damage, after alpha-emitting radioisotopes enter living cells. Ingested alpha emitter radioisotopes (such as transuranics or actinides) are an average of about 20 times more dangerous, and in some experiments up to 1000 times more dangerous, than an equivalent activity of beta emitting or gamma emitting radioisotopes. [...]

Because of this high mass and strong absorption, if alpha-emitting radionuclides do enter the body, alpha radiation is the most destructive form of ionizing radiation. It is the most strongly ionizing, and with large enough doses can cause any or all of the symptoms of radiation poisoning. It is estimated that chromosome damage from alpha particles is anywhere from 10 to 1000 times greater than that caused by an equivalent amount of gamma or beta radiation, with the average being set at 20 times.”

EPA: Which Radionuclides are Alpha Emitters?

There are many alpha emitting radioactive elements, both natural and manmade. You can find fact sheets for several key alpha emitters at the Radionuclides page:

• americium-241
• plutonium-236
• uranium-238 [...]