21. 5 Measuring Radioactivity

1. 21. 5 Measuring Radioactivity • One can use a device like this Geiger counter to measure the amount of activity present in a radioactive sample. • The ionizing radiation creates ions, which conduct a current that is detected by the instrument.

2. 21.6 Energy Changes in Nuclear Reactions • There is a tremendous amount of energy stored in nuclei. • Einstein’s famous equation, E = mc2, relates directly to the calculation of this energy. • In the types of chemical reactions we have encountered previously, the amount of mass converted to energy has been minimal. • However, these energies are many thousands of times greater in nuclear reactions.

3. He 238 92 234 90 4 2 U Th +  • For example, the mass change for the decay of 1 mol of uranium-238 is −0.0046 g.

4. The change in energy, E, is then E = (m) c2 E= (−4.6  10−6 kg)(3.00  108 m/s)2 E= −4.1  1011 J

5. Nuclear Binding Energies • The masses of nuclei are always less than the masses of the individual nucleons of which they are composed. • Ex: He nucleus

6. The mass difference between a nucleus and its constituent nucleons is called the mass defect. • The mass defect represents the energy that was lost when the nucleus was formed. • Nuclear binding energy is the energy required to separate a nucleus into its individual nucleons. • The binding energy is due to the attractive forces that hold the nucleons together. • Ex: He nucleus • Heavy nuclei gain stability and give off energy when fragmented into two mid-sized nuclei. (see fig 21.13)

7. 21.7 Nuclear Fission • The splitting (fission) of heavy nuclei is an exothermic process. • Nuclear fission is the type of reaction carried out in nuclear reactors.

8. Bombardment of the radioactive nuclide with a neutron starts the process. • Neutrons released in the transmutation strike other nuclei, causing their decay and the production of more neutrons. • This process continues in what we call a nuclear chain reaction.

9. If there are not enough radioactive nuclides in the path of the ejected neutrons, the chain reaction will die out. • Therefore, there must be a certain minimum amount of fissionable material present for the chain reaction to be sustained, called the critical mass.

10. Nuclear Reactors In nuclear reactors the heat generated by the reaction is used to produce steam that turns a turbine connected to a generator.

11. The reaction is kept in check by the use of control rods. • These block the paths of some neutrons, keeping the system from reaching a dangerous supercritical mass.

12. 21.8 Nuclear Fusion • Energy is produced when light nuclei are fused into heavier ones. • Example: 4 H  He • Net reaction for fusion in many stars. • Mass of four hydrogen atoms is greater than the mass of the He atom produced. • Mass difference accounts for energy released according to E = mc2 • There are many different kinds of fusion reactions (pg 924 lists some). • Deuterium-tritium fusion most efficient for fusion power plants.

13. 21.8 Nuclear Fusion • Fusion would be a superior method of generating power. • The good news is that the products of the reaction are not radioactive. • The bad news is that in order to achieve fusion, the material must be in the plasma state at several million kelvins.

14. Tokamak apparati like the one shown show promise for carrying out these reactions. • They use magnetic fields to heat the material.