Beta Particle

I

Introduction

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.

II

Beta Decay

Atoms emit beta particles through a process known as beta decay. Beta decay occurs when an atom has either too many protons (positively charged particles) or too many neutrons (electrically neutral particles) in its nucleus. When this occurs, a force called the weak nuclear force causes the unstable atom to change an extra proton into a neutron, or neutron into a proton, and become stable.

Beta decay can produce positive or negative particles. In positive beta decay, a proton in an unstable nucleus turns into a neutron by emitting a positively charged beta particle (positron) and an electron neutrino. Neutrinos are high-energy elementary particles with little or no mass. In negative beta decay, a neutron in an unstable nucleus turns into a proton by emitting a negatively charged beta particle (negatron, or electron) and an electron antineutrino, the antimatter counterpart of the electron neutrino. Negative beta decay, which occurs in atoms having too many neutrons, is far more common than positive beta decay.

---

---

III

Characteristics and Behavior

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.

IV

History

British physicist Ernest Rutherford discovered beta radiation in 1899. While studying the element uranium, Rutherford observed two different types of emission, which he called alpha and beta rays. Alpha radiation had a positive electric charge and did not penetrate deeply into metal foil (see Alpha Particle). The beta radiation Rutherford observed was negatively charged and penetrated farther into foil. Based on his observations, Rutherford concluded that the decay of atoms produced radioactivity. British physicist Sir Joseph John Thomson had already discovered the electron in 1897, but neither Rutherford nor Thompson connected the discoveries. French physicist Antoine Becquerel identified the negative beta particle with the electron in 1899, shortly after Rutherford’s discovery.

Many naturally occurring elements emit beta particles, so the particles are relatively easy to study. In some of the most interesting work on beta particles, scientists study the range of energies beta particles can have and the effects of beta particles on human tissue.

	More from Encarta
	•  Presidential Myths Quiz •  Coffee break: Recharge your brain