Nuclear Physics

The Atom An atom consists of negatively charged electrons, positively charged protons, and neutrons (particles consisting of no electrical charge). The protons and neutrons are found in the nucleus. The electrons occupy the electron cloud-the space surrounding the positively charged nucleus.

What holds the nucleus together? The negatively charged electrons that surround the positively charged nucleus are held in place by an attractive electromagnetic force. There is an electromagnetic repulsive force between the protons (like charges repel). They do not fly apart however, due to the strong nuclear force (also called the strong force) which acts between the protons and neutrons in the nucleus holding them together. This strong force is 100 times stronger than the repulsive force between protons. The range of the strong force is short-only about the radius of a proton. It is attractive and is of the same strength between protons and neutrons.

The strong force In order for a particle to be pulled from the nucleus, work must be done to overcome the attractive force. Doing work adds energy to the system. Therefore, the assembled nucleus has less energy than the separate protons and neutrons that make it up. The difference is the binding energy of the nucleus. Because the nucleus has less energy, all binding energies are negative.

E = mc2 Einstein showed that mass and energy are equivalent, so the binding energy can be expressed in the form of an equivalent amount of mass. E=mc2 (c is the speed of light) c=3.00 x108 m/s Example: the mass of a proton is 1.007276 u and the mass of a neutron is 1.008665 u. What do you expect the mass of a helium nucleus to be? (note: 1 u = 1.6605 x 10-27 kg) If the mass of the helium nucleus were equal to the mass of 2 protons and 2 neutrons, the mass would be 4.031882 u. Careful measurement shows that the mass of the nucleus is only 4.002603 u. The actual mass of the nucleus is less than the individual components by 0.029279 u. This difference in mass is called the mass defect. Binding energy can be determined through the use of the equation E=mc2.

Calculating mass defect and binding energy 1 u = 931.49 MeV of energy. E=mc2=(1.6605x10-27)(2.9979 x 108)2 E=(1.4294x10-10 J) (1 eV/1.60217 x 10-19J) =9.3149x108eV E=931.49 MeV Find the mass defect and the binding energy of hydrogen-3. The mass of the hydrogen-3 isotope is 3.016049 u. The mass of a hydrogen atom is 1.007825 u, and the mass of a neutron is 1.008665 u.

Which weighs more? What weighs more, an electron and a proton, or a hydrogen atom? On a two-pan balance, you put 2 neutrons and 1 proton on one side and you put a hydrogen-3 nucleus on the other side. Which side weighs more?

Radioactivity Radioisotopes are unstable isotopes whose nuclei gain stability by spontaneously undergoing changes. Because the range of the strong force is so short that in large nuclei the repulsive force is greater resulting in an unstable nuclei. When a radioactive isotope decays, some of the energy holding the nucleus together is released in the form of a radioactive particle with mass and kinetic energy. These changes are accompanied by the emission of large amounts of energy. Radioactive decay is the process by which materials give off this energy. The penetrating rays and particles that are emitted during these changes are called radiation. Eventually unstable radioactive isotopes are transformed into stable isotopes of a different element.

Radioactive Isotopes All elements consist of at least one radioactive isotope. Isotopes that have too many or too few neutrons (atomic mass larger or smaller than the average) tend to be radioactive. All isotopes with an atomic number greater than 83 are radioactive.

Identify the radioactive isotope 10 11H 614C 816O 714N 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Identify the radioactive isotope 10 Chlorine-35 Carbon-12 Lead-207 Potassium-40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Types of Radiation Alpha radiation Beta radiation Gamma radiation

Alpha Radiation Alpha radiation consists of helium nuclei that are emitted from a radioactive isotope. Alpha particles consist of two protons and two neutrons. Alpha particles have a 2+ charge. The symbol for an alpha particle is 42He or α. Alpha particles are the most massive of the radioactive particles (4 amu), the most damaging, and are the least penetrating (easily stopped by a piece of paper).

What is the product when plutonium-238 undergoes alpha decay? 10 Uranium-234 Thallium-206 Lead-206 Radium-226 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Explanation of Answer 23894Pu  42He + 23492U

What is the product when bismuth-210 undergoes alpha decay? Radium-226 Lead-206 Thallium-206 Uranium-232 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Explanation of Answer 21083Bi  42He + 20681Tl

What is the product when polonium-210 undergoes alpha decay? Bismuth-206 Lead-206 Radium-206 Thorium-206 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Beta Particles Beta particles consist of fast moving electrons formed by the decomposition of a neutron in an atom. The neutron decomposes into a proton and an electron-the proton remains in the nucleus and the electron is emitted. Beta particles have a 1- charge. The symbol for a beta particle is o-1e or β. Beta particles are 8000 x lighter than an alpha particle, are less damaging, but are more penetrating (stopped by aluminum foil or thin pieces of wood).

What is the product when carbon-14 undergoes beta decay? 10 Carbon-13 Nitrogen-14 Oxygen-14 Boron-10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Explanation of Answer 146C  0-1e + 147N

What is the product when strontium-90 undergoes beta decay? 10 Rubidium-90 Krypton-91 Strontium-91 Yttrium-90 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

What is the product when potassium-40 undergoes beta decay? 10 Calcium-40 Scandium-40 Argon-40 Chlorine 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Gamma Radiation Gamma radiation is high energy electromagnetic radiation. Gamma rays are emitted along with alpha or beta particles. Gamma rays have no mass or charge. The symbol for gamma rays is ooγ Gamma rays are extremely penetrating and potentially dangerous (stopped only by several meters of concrete or several centimeters of lead).

Other Nuclear Reactions Fission-when a large nucleus is bombarded with neutrons, a division of the nucleus into 2 smaller nuclei occurs resulting in a large release of energy. This energy is used in nuclear power plants and in atomic bombs. Fusion-nuclei with small masses combine to form a nucleus with a larger mass. This type of reaction occurs in the sun and in hydrogen bombs. The high temperature needed to start a fusion reaction is produced by a fission reaction.

Half-Life A half-life is the time it takes for one -half of the nuclei of a sample of radioactive isotopes to undergo radioactive decay. Half-lives may be as short as a fraction of a second or as long as billions of years. For examples, see the chart on page 810.

Determining Half-Lives In order to solve problems involving half-lives, the following equation may be used: # of half-lives = total time/time of one half-life To determine the amount of sample left, the following equation may be used: amount left = starting amount/ 2# of half-lives

The half-life of carbon-14 is 5700 years The half-life of carbon-14 is 5700 years. If a 10 gram sample undergoes decay for 17,100 years, how many half-lives has the sample undergone? 10 5 3 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Cobalt-60 is a radioactive element used as a source of radiation in the treatment of cancer. Cobalt-60 has a half-life of five years. If a hospital starts with a 1000-mg supply, how much will remain after 10 years? 1000 mg 750 mg 500 mg 250 mg 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Polonium-210 has a half-life of 138 days. How much of a 2 Polonium-210 has a half-life of 138 days. How much of a 2.34 kg sample will remain after four years? 10 0.644 mg 1.50 mg 1.51 g 10.6 g 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24