beta rays

from The Century Dictionary.

• See ray.

from the GNU version of the Collaborative International Dictionary of English.

• (Physics) a form of ionizing radiation emitted by radioactive substances (such as radium), more penetrating than alpha rays, and consisting of negatively charged electrons. The electrons are the same kind of particle as those of cathode rays, but have much higher velocities (about 35,000 to 180,000 miles per second). They are readily deflected by a magnetic or electric field.