| Emissions from radioactive sources were initially called rays and included alpha, beta, and gamma rays. The properties of these rays were mysteriously different. Alpha rays were positively charged, could be bent by a magnetic field, and were easily stopped by a sheet of paper. Beta rays were negatively charged, could be bent in the opposite direction by a magnetic field, and could penetrate paper but be stopped by an aluminum shield. Gamma rays were neutral, were not deflected by a magnetic field, and penetrated both paper and aluminum, but could be stopped by a heavy shield of concrete or lead. Today alpha rays are generally called alpha particles and are known to be the slow-moving nuclei of helium atoms. Beta rays are generally called beta particles and are known to be electrons. Gamma rays are still called gamma rays and are typically included in charts of the electromagnetic spectrum. However, they also can be thought of as particles called photons. |
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