The Nucleus and Radiation

The Nucleus and Radiation

Many scientists have contributed to concepts of radiation as produced by the particles making up the nuclei of atoms, and descriptions of some of those scientists and their work follow.

While experimenting with cathode ray tubes (similar to television screens of the 1950s), Wilhelm Konrad Röentgen (1845–1923) discovered an unfamiliar type of radiation. He named these “X-rays”, using “X” to signify the “unknown.”

Antoine Henri Becquerel (1852–1908) discovered alpha particles that are, in essence, the nuclei of the element helium ( He, or, as the ion, He ++ ). Compared to electrons, they are heavy, only travel short distances, and may be stopped by a sheet of cardboard. Ernest Rutherford is one of the discoverers of beta particles, which are somewhat like high-speed electrons that can travel at the speed of light over great distances. An example is electrons traveling over wires to our homes. Actually, an individual electron may not make the entire trip over the wire. Rather, electrons are “pushed” along the wire by a potential difference in voltage from their source to their destination. High-energy beta particles ( - e) stripped from the atom can penetrate several sheets of paper. An electron has a mass that is almost two thousand times less than the mass of a proton in the nucleus. Rutherford is also credited with the discovery of the fundamental positive particle of the nucleus known as the proton. Both Rutherford and James Chadwick (1891–1974) conducted experiments that led to the discovery of the neutron, the other fundamental particle of the nucleus. The neutron has approximately the same mass as the proton, but it is neutral with no electrical charge. In 1932, Chadwick was credited with the neutron’s discovery.

Marie Curie and Pierre Curie (1859–1906), who discovered radium by extracting it from uranium ore, experimented with gamma rays. Gamma rays have a short wavelength, travel at the speed of light, and can penetrate several inches of lead, depending on their source and energy level. There are several types of gamma rays. Some examples are the product of short bursts of protons in a cyclotron or “atom smasher,” cosmic radiation from outer space, and the natural disintegration of the nuclei of radioactive atoms in the Earth. Most of the heavier elements with an atomic number above 88 are radioactive to some degree See Table 2.1 for some information on the major sources and forms of radiation.

Previously mentioned was the classic experiment by Ernest Rutherford in the early 1900s that established the concept that most of the mass of an atom exists as a tiny center of posi- tively charged matter, called the nucleus. This nucleus is only about one millionth of the diam- eter of the whole atom. Rutherford’s experiment also established that the negative electrons surrounding the nucleus are at a relatively great distance from this very dense central mass. In addition, the electrons make up only an extremely small fraction of the atom’s weight (mass). Thus, most of the atom is empty space, consisting of a tiny, positively charged, relatively heavy, dense core surrounded by distant, and much lighter, negatively charged electrons.

Many other experiments also verified the concept that the nucleus consists of two major types of particles: protons, which carry a positive charge, and neutrons, which have a similar mass to the protons, but have no electrical charge. Thus, the total mass of a nucleus consists of the total number of both protons and neutrons. Together, they make up all but a tiny fraction of the weight of an atom. The protons, by themselves, are the source of the positive electrical charge of the nucleus, which is balanced by the negatively charged electrons.

Atomic Structure | 17

Table฀2.1:฀Major฀atomic฀particles฀and฀forms฀of฀radiation. Type

Charge Electron

1/1,835฀of฀proton

4฀(helium฀nuclei)

-1 Gamma

~0฀(hi฀energy฀electrons)

0 0 The science of particle physics continues to study electrons, protons, and neutrons, which