FIG. 7-1 The Thomson model of the atom. Experiment shows it to be incorrect.

FIG. 7-2 Principle of the Rutherford experiment. Nearly all the alpha particles pass through the foil with little or no deflection, but a few of the particles are scattered through large angles, even in the backward direction. This result means that strong electric fields must act on the particles, and such fields can arise only if atoms have very small nuclei in which their positive charge is concentrated.

FIG. 7-3 In the Rutherford model of the atom, the positive charge is concentrated in a central nucleus with the electrons some distance away. This model correctly predicts that some alpha particles striking a thin metal foil will be scattered through large angles by the strong electric fields of the nuclei.

FIG. 7-4 The elements that correspond to the atomic numbers 1, 2, 3, and 4 are hydrogen, helium, lithium, and beryllium. The various particles are actually far too small to be seen even on this scale.

FIG. 7-5 The isotopes of hydrogen.

FIG. 7-6 The radiations from a radium sample may be analyzed with the help of a magnetic field. Alpha particles are deflected to the left, hence they are positively charged; beta particles are deflected to the right, hence they are negatively charged; and gamma rays are not affected, hence they are uncharged.

FIG. 7-7 Alpha particles from radioactive materials are stopped by a piece of cardboard. Beta particles penetrate the cardboard but are stopped by a sheet of aluminum. Even a thick slab of lead may not stop all the gamma rays.

FIG. 7-9 The decay of the radium isotope 226/88 Ra. The number of undecayed radium atoms in a sample decreases by one-half in each 1600-year period. This time span is accordingly known as the "half-life" of radium. The radium alpha decays into the radon isotope 222/86 Rn, whose own half-life is 3.8 days.

FIG. 7-10 Sources of radiation dosage for an average person in the United States. The total is about equivalent to 25 chest x-rays. Actual dosages vary widely. For instance, radon concentrations are not the same everywhere, some people receive more medical x-rays than others; cosmic rays are more intense at high altitudes; and so on. Nuclear power stations are responsible for 0.08 percent of the total, although accidents can raise the amount in affected areas to dangerous levels.

FIG. 7-11 The mass of a deuterium atom (2/1 H) is less than the sum of the masses of a hydrogen atom (1/1 H) and a neutron. The energy equivalent of the missing mass is called the binding energy of the nucleus.

FIG. 7-12 The binding energy of the deuterium nucleus is 2.2 MeV. A gamma ray whose energy is 2.2 MeV or more can split a deuterium nucleus into a proton and neutron. A gamma ray whose energy is less than 2.2 MeV cannot do this.

FIG. 7-13 The binding energy per nucleon is a maximum for nuclei of mass number A = 56. Such nuclei are the most stable. When two light nuclei join to form a heavier one, a process called fusion, the greater binding energy of the product nucleus causes energy to be given off. When a heavy nucleus is split into two lighter ones, a process called fission, the greater binding energy of the product nuclei also causes energy to be given off.

FIG. 7-14 In nuclear fission an absorbed neutron causes a heavy nucleus to split into two parts. Several neutrons and gamma rays are emitted in the process. The smaller nuclei shown here are typical of those produced in the fission of 235/92 U.

FIG. 7-15 Sketch of a chain reaction. The reaction continues if at least one neutron from each fission event on the average induces another fission event. If more than one neutron per fission of the average induces another fission, the reaction is explosive.

FIG. 7-16 Basic design of a typical nuclear power plant.

FIG. 7-17 The nonfissionable uranium isotope 238U, which makes up 99.3 percent of natural uranium, becomes the fissionable plutonium isotope 239Pu by absorbing a neutron and beta-decaying twice. This transformation is the basis of the breeder reactor, which produces many times more nuclear fuel in the form of plutonium than it uses up in the form of 235U.

FIG. 7-18 The search for truly elementary particles has led to the discovery of particles within particles. Today all ordinary matter seems to be made up of electrons and quarks. Shown are the various levels of organization of a lithium 7/3 Li atom.

FIG. 7-19 The mutual annihilation of an electron and a positron results in a pair of gamma rays whose total energy is equal to mc², where m is the total mass of the electron and positron.

FIG. 7-20 Pair production. (The presence of a nucleus is required in order that both momentum and energy be conserved.) Proton-antiproton and neutron-antineutron pairs can also be produced if the gamma ray has enough energy.

FIG. 7-21 The four fundamental interactions determine how matter comes together to form the characteristic structures of the universe.

FIG. 7-22 One of the goals of physics is a single theoretical picture that unites all the ways in which particles of matter interact with each other. Much progress has been made, but the task is not finished.

FIG. 7-23 Quark models of the proton and neutron. Electric charges are given in units of e.