Chapter 19

1. Chapter 19 Radioactivity Presented by April Senger

2. Radioactivity • Radioactivity is a process by which an unstable nucleus emits one or more particles or energy in the form of electromagnetic radiation • Nuclear radiation is charged particles or energy emitted by an unstable nucleus • When unstable nuclei break apart to become stable Presented by April Senger

3. Radioactive Facts • Radon is an example of a radioactive element • Radioactive elements are unstable (above atomic #83) • They become stable by releasing energy or particles • The new particle can become another isotope or a whole new element called nuclear decay Presented by April Senger

4. Alpha Particles • Alpha particles are a positively charged particle that consists of 2 protons and 2 neutrons (He nucleus) • U is a radioactive element that decays by alpha release which are the weakest and can be stopped by a sheet of paper Presented by April Senger

5. Example of Alpha • Radium-226 decays into Radon-222 by emitting an alpha particle • During alpha decay, the nucleus gives up two protons and two neutrons • The mass number and atomic numbers will always add up with the reactants and products • 226 222 4 Ra = Rn + He • 88 86 2 Presented by April Senger

6. Beta Particles • Beta particles are an electron emitted during decay of a neutron in the nucleus (super fast) • Beta particles come from a neutron that changes itself into a proton and electron • The smaller faster particle can be stopped by 3 mm of Al or 10 mm of wood Presented by April Senger

7. Beta Example • During Beta Decay, the nucleus gains a proton and loses a neutron • The mass number and atomic numbers also add up in beta decay • 14 14 0 • C N + e • 6 7 -1 Presented by April Senger

8. Gamma Radiation • Gamma rays are high-energy EM radiation emitted by a nucleus during radioactive decay • Gamma rays were found by Marie Curie (radium) and do not have an electric charge or mass • They can only be stopped through 60 cm of Al or 7 cm of Pb • Decay w/ gamma rays doesn’t change the atomic # because they have no charge or mass. They only change the total energy Presented by April Senger

9. Concept Check • Pretend you are given three radioactive rocks-one an alpha emitter, one a beta emitter, and one a gamma emitter. You can throw one away, but you must hold one of the remaining two in your hand and place the other in your pocket. What rocks would you pick to minimize your exposure to radiation? • Hold the alpha, beta in your pocket and throw the gamma Presented by April Senger

10. Nothing New To The Earth • Nuclear radiation can damage living cells, causing radiation sickness and birth defects • Nuclear radiation is used in medicine to diagnose and treat disease • Nuclear fission is an alternative to fossil fuels as a source of energy • Background energy is nuclear radiation that arises naturally from cosmic rays and from radioactive isotopes in the soil and air Presented by April Senger

11. Background Radiation • The atmosphere acts as a blanket for most radiation from space • If you were in Denver you would be exposed to twice as much radiation…Why? • Flying back and forth between New York and San Francisco a few times is about the same as a chest X-ray thus we limit air time for employees Presented by April Senger

12. Radon is often found in basements and cellars and has to be monitored and tested for Smoke alarms have an alpha emitting isotope in it that detects smoke in the air Radiation can cause genetic mutations Radioactive tracers are a material added to a substance so that the substance’s location can be detected later Nuclear wastes are difficult and dangerous to store (outerspace) Laser Eye surgery and Chemotherapy More From Around Us Presented by April Senger

13. Radioactivity Continued • The human diet includes about 200 g of potassium and 20 mg of the K is radioactive gamma emitters • Coal plants produce 13,000 tons of radioactive thorium and uranium each year • Most of this is not released into the atmosphere but does have to be buried as waste product Presented by April Senger

14. Strong Nuclear Force • The interaction that binds protons and neutrons together in a substance • Otto Hahn and Fritz Strassman did experiments with U in hopes of finding heavy nuclei but actually found fission • They did not understand what it was but they knew that they had separated the protons and neutrons • Protons are positive and should repel each other but the neutrons help to stablize them Presented by April Senger

15. SNF Continued • The strong nuclear force is powerful enough to attract p and n and override the p repulsion • The strong nuclear force is only good over very short distances • If there are too many protons or neutrons in the nuclei, it becomes unstable or radioactive • Some of the energy released is in the form of alpha, beta, and gamma rays during radioactive decay Presented by April Senger

16. Concept Check • How are protons in the nucleus both attracted and repelled? • They are electrically repelled by the positive and positive push away from each other • The attraction in is due to SNF • If SNF is greater than electrical repulsion they will be stable • If not, they will decay and release particles to become stable Presented by April Senger

17. More About Neutrons • Neutrons change into a proton and electron if they do not have protons present • When there is more neutrons than protons they want to make this conversion • Strong nuclear forces are only good over short distances • When an atom gets over 83 protons it becomes radioactive • Man made elements exist for fractions of seconds for this reason Presented by April Senger

18. Concept Check • Which is more sensitive to distance? SNF or Electric Forces? • SNF decreases attraction rapidly • Electrical forces remain more powerful if the distance between them increases Presented by April Senger

19. How Can Small Elements Be Radioactive? • They have more neutrons than protons • The protons can not keep the neutrons from turning themselves into protons and electrons • C-14 is an example that emits beta particles to remain stable Presented by April Senger

20. Transmutation • When one element turns into another element to become more stable it is called a transmutation • Uranium-238 undergoes alpha decay and turns into Thorium-234 • Thorium-234 is also radioactive and undergoes beta decay to be come Protactinium-234 • Note the mass in both alpha and beta decays Presented by April Senger

21. Concept Check • What finally becomes of all the uranium that undergoes radioactive decay? • All uranium eventually becomes lead • Look at page 331 figure 19.11 for a visual of how Uranium changes over time Presented by April Senger

22. Half-Life • The time required for half a sample of radioactive nuclei to decay is called half-life • The shorter the half-life the more radioactive • Radioactive decay is useful in dating ancient findings • Carbon-14 is an unstable isotope that is often used in a process called carbon dating. The beta decay changes the isotope to C-12. We study their ratios to determine the age. • After one half-life you will have ½ the sample Presented by April Senger

23. Half Done With Half-Life • After two half-lives you will have ¼ the sample • After 3 half-lives you will have 1/8 the sample • All radioactive elements have different half-lives lasting from nanoseconds to billions of years. • The ratios of how much K-40 and Ar-40 that is remaining can be used to approximate the date of the rock sample Presented by April Senger

24. Concept Check • If you have a sample of a radioactive isotope that has a half-life of one day, how much of the original sample remains at the end of the second day? Third day? • ¼ and 1/8 • What becomes of the decayed atoms of the sample? • They trasmutate into different elements • With equal quantities, which results in higher counting rate on a radiation detector, short or long half-life? • Shorter half-life is more radioactive Presented by April Senger

25. Carbon Dating • Radiation enters the Earth’s atm every day • A neutron + Nitrogen-14 combine to form Carbon-14 and a proton • There is about fourteen C-14 : 100 billion C-12 atoms • C-14 decays but is replaced by new C-14 from the atm • C-14 has a half-life of 5730 years • If an object is made of carbon, its ratio of C-14 to C-12 can be used to date its age • The ratio of carbon has not be constant over time so our ages are approximations Presented by April Senger

26. Concept Check • You extract 1.0 g of carbon from an ancient ax handle and find that it is about ¼ of the carbon in a freshly cut tree branch. How old is the handle? • ¼ is two half-lives • 5730 + 5730 is about 11,000ish years Presented by April Senger

27. How Old is Old? • Carbon dating is only good for carbon containing objects and are less than 50,000 years • We can use Uranium and other radioactive isotopes that have longer half-lives to date much further in the past • Rocks on Earth have been dated as old as 3.7 million years • The Moon has rocks as old as 4.2 billion years • The estimated age of our solar system is 4.6 billion years Presented by April Senger