Beta particle

[Rennhack]

A charged particle emitted from a nucleus during radioactive decay, with a mass equal to 1/1837 that of a proton. A negatively charged beta particle is identical to an electron. A positively charged beta particle is called a positron. Large amounts of beta radiation may cause skin burns, and beta emitters are harmful if they enter the body. Beta particles may be stopped by thin sheets of metal or plastic.

[Rennhack]

An electron or positron emitted by certain radioactive nuclei. Beta  particles can be stopped by aluminum.  They can pose a serious direct or external radiation threat and can be lethal depending on the amount received.  They also pose a serious internal radiation threat if inhaled or ingested.

[Rennhack]

http://www.epa.gov/radiation/understand/beta.htm