Matter & Energy


Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.

Beta Particle

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Beta Particle, electrically charged particle emitted by certain radioactive elements, such as uranium and berkelium. Beta particles are produced when the nucleus of an unstable atom spontaneously decays, or transforms, changing the atom into an atom of a more stable element. In this process, the nucleus emits a beta particle and another elementary particle called a neutrino. Two types of beta decay occur. One type releases a positively charged beta particle called a positron; the other type releases a negatively charged beta particle called a negatron. The negatron is identical to the electron found in atoms.

The terms beta particle and beta radiation are used interchangeably. The term beta ray originally referred to a stream of beta particles, but it is no longer used. See also Radioactivity.

Beta particles have all the characteristics of electrons found in atoms, except the electric charge of positrons is opposite that of electrons. At the time of their emission, they travel at nearly the speed of light. The high energy of beta particles enables them to travel through matter to a depth of several millimeters. They lose their energy by interacting with other atoms and by releasing a form of radiation known as bremsstrahlung (German for “braking radiation”). Bremsstrahlung occurs when negative beta particles are attracted to the positively charged nuclei of atoms. The velocity of the beta particles increases as they approach the positive nuclei then decreases abruptly as they move away from the nuclei. The rapid acceleration and deceleration of the beta particles produces radiation. Bremsstrahlung radiation is strongest in radioactive elements, which have a high atomic number, or number of protons within the nucleus. Elements with a low atomic number, such as aluminum, are used in shields to block beta radiation. Lead blocks radiation types such as gamma rays and X rays but has high atomic number, so it cannot block beta radiation. When beta particles enter materials, they can cause chemical changes, such as darkening of glass. Most beta particles lack the energy to penetrate beneath the skin, but in high doses they can cause skin damage.