Physics

Chapter 30

“Nuclear Physics”

The Composition and Structure of the Nucleus

• In your study of atomic structure you investigated experiments which revealed the general composition of the atomic nucleus.

• Recall that all nuclei except for ordinary hydrogen (protium) contain two fundamental particles-protons and neutrons.

• Nuclei of protium contain only a proton.

• The protons and neutrons are referred to as nucleons.

Strong Nuclear Force• The nucleons are packed tightly together. Because of

this close packing, positive charges repel each other.

• An attractive force is thus required to hold the positively charged protons in the tiny nucleus.

• The force that keeps the nucleons together is called the strong nuclear force. This force is the powerful short-range force of attraction between, respectively, proton and proton, proton and neutron, and neutron and neutron.

• The strong nuclear force affects only the nucleons, and is distinct from the electrostatic force or gravitational force.

Mass Defect and Nuclear Binding Energy

• On the atomic mass scale, the isotope of carbon with six protons and six neutrons in its nucleus is defined as having an atomic mass of exactly 12 u.

• On this scale, a He nucleus has a mass of 4.0015 u. The mass of a proton is 1.0073u and the mass of a neutron is 1.0087 u.

• A He nucleus contains two protons and two neutrons. You might expect its mass to be the combined mass of these four particles - 4.0320 u [2(l.0073 u) + 2(l.0087 u = 4.0320 u].

• Note, however, that there is a difference of 0.0305 u between the measured mass (4.0015 u) and the calculated mass, 4.0320 u, of a He nucleus.

Mass Defect and Nuclear Binding Energy

• This difference in mass is called the nuclear mass defect. • The nuclear mass defect is the difference between the mass

of a nucleus and the sum of the masses of its constituent particles.

• The mass defect is converted into energy units by using Einstein's equation, E = mc2.

• This energy is generally referred to as the nuclear binding energy, which is the energy released when a nucleus is formed from its constituent particles.

• This energy must be supplied to a nucleus to separate it into its constituent particles.

Relationship Between Nuclear Stability and the Neutron/Proton Ratio

• The stability of atomic nuclei is affected by the ratio of the neutrons to protons that compose them.

• The stability of a nucleus partially depends on the even-odd relationship of the number of protons and neutrons.

• Most stable nuclei (157 of them) have even numbers of both protons and neutrons.

• There are 55 stable nuclei having an even number of protons and an odd number of neutrons.

• Fifty stable nuclei have an odd number of protons and an even number of neutrons.

• Only four stable nuclei having odd numbers of both protons and neutrons are known.

Types of Nuclear Reactions• Because of the difference in stability of different nuclei, there are four

types of nuclear reactions. In each type, a small amount of the mass of the reactants is converted into energy, forming products of greater stability.

• 1. Radioactive decay refers to the emission of an alpha particle, a beta particle, or gamma radiation and the formation of a slightly lighter and more stable nucleus.

• 2. A nucleus is bombarded with alpha particles, protons, deuterons (deuterium nuclei), neutrons, or other particles. The unstable nucleus emits a proton or a neutron and becomes more stable in a process called nuclear disintegration.

• 3. Fission refers to the process in which a very heavy nucleus splits to form medium-weight nuclei.

• 4. Fusion refers to the process in which lightweight nuclei combine to form heavier, more stable nuclei

The Phenomenon of Radioactivity

• In 1896 Henri Becquerel (bek-rel) (1852-1908) was studying the properties of uranium compounds.

• He was particularly interested in the ability of these compounds to fluoresce or give off visible light after being exposed to sunlight.

• By accident Becquerel found that, whether they flouresce or not, all uranium compounds give off invisible rays. He discovered that these rays penetrate the lightproof covering of a photographic plate and affect the film as if it had been exposed to light rays directly.

• Substances that give off such invisible rays are radioactive.

• Radioactivity is the process whereby an unstable nucleus forms a more stable nucleus by the release of high energy particles and radiation.

The Curies

• Becquerel was very interested in the source of radioactivity. At his suggestion, Marie Curie (1867-1934) and Pierre Curie (1859-1906) began to investigate the properties of uranium and its ores.

• They soon learned that uranium and uranium compounds are only mildly radioactive.

• They also discovered that one uranium ore, pitchblende, has four times the amount of radioactivity expected on the basis of its uranium content.

• The Curies discovered two new radioactive metallic elements in pitchblende in 1898.

• These elements, polonium and radium, account for the high radioactivity of pitchblende.

• Radium is more than a million times as radioactive as the same mass of uranium.

• Polonium is five thousand times as radioactive as the same mass of radium.

Naturally Occurring Radioactive Nuclides

• Because of their radioactivity, radioactive nuclides have several unusual properties. Some of these properties are:

• 1. They affect the light-sensitive emulsion on a photographic film.

• 2. They produce an electric charge in the surrounding air.

• 3. They produce fluorescence with certain other compounds.

• 4. Their radiations have special physiological effects.

• 5. They undergo radioactive decay.

• Half-life is the length of time during which half of a given number of atoms of a radioactive nuclide decays.

Nature of radiation• The radiation given off by radioactive nuclides can

be separated into three different kinds of particles and rays.

• 1. The (alpha) particles are helium nuclei. Their mass is nearly four times that of a protium atom. They have a charge of +2 and move at speeds that are approximately one-tenth the speed of light. Because of their relatively low speed, they have low penetrating ability. A thin sheet of aluminum foil or a sheet of paper stops them. They burn flesh and ionize air easily, however.

• Nuclear Symbol -2

4He0

0

Nature of radiation

• 2. The (beta) particles are electrons. They travel at speeds close to the speed of light, with penetrating ability about 100 times greater than that of alpha particles.

• Nuclear Symbol

1

1H

-1

0e

Nature of radiation• 3. The - (gamma) rays are high-energy

electromagnetic waves. They are the same kind of radiation as visible light, but are of much shorter wavelength and higher frequency. Gamma rays are produced when nuclear particles undergo transitions in nuclear-energy levels. They are the most penetrating of the radiations given off by radioactive nuclides. Alpha and beta particles are seldom, if ever, given off simultaneously from the same nucleus. Gamma rays, however, are often produced along with either alpha or beta particles.

• Nuclear Symbol-0

0

Nuclear Equations• nuclear equation - an equation in which only

nuclei are represented

• Nuclear symbols must be used in nuclear equations. You will need to know the symbol of each of the following particles.– alpha electron– beta proton– gamma neutron– positron deuteron

Sample Equations

• Write the nuclear equation for each of the following reactions.– alpha decay of U-238

– beta decay of Th-234

– alpha decay of Rn-222

– beta decay of Pa-234

Artificial Radiation

• transuranium elements - elements with more than 92 protons

• nuclear bombardment - striking the nucleus with another particle at a very high speed

• When you see the word bombardment, this means a particle must be added to the reactant side of a nuclear equaiton.

Nuclear Bombardment Equations

• Write the equation for the bombardment of U-238 by a neutron resulting in the formation of Np-239 and the release of another particle.

• Write the equation for the deuteron bombardment of Pu-239 resulting in beta emission and formation of another element.

• Assignment: Page 801, 1-4; Page 805, #5,

• Page 808, 15-19; Page 809, 20-23; Page 810, 24.