Nuclear Physics Objectives:

1. Nuclear Physics Objectives: -Identify four types of nuclear radiation and their properties -Balance equations for nuclear decay -Calculate the half life of a radioactive isotope -Distinguish between fission and fusion, and provide examples of each. -Recognize the equivalence of mass and energy, and why small losses in mass release large amounts of energy

2. Radioactivity is the process by which an unstable nucleus emits one or more particles or energy in the form of electromagnetic radiation. Nuclear radiation are the particles that are released from the nucleus during radioactive decay There are 4 different types of nuclear radiation.

3. The alpha particle is a positively charged radiation that is essentially a helium nucleus This means the alpha particle has 2 protons and 2 neutrons and a net charge of +2 The beta particle is a negatively charged radiation that is essentially a nuclear electron This means that the beta particle is formed when a neutron breaks into a proton and an electron. The electron is ejected from the nucleus. The proton is not ejected from the nucleus.

4. Gamma rays are high energy electromagnetic radiation, which is essentially high frequency light. Gamma rays have no mass and no charge. The last form of radiation is the neutron. Neutrons are electrically neutral meaning they carry no charge. Of the 4 types of radiation each has different levels of penetration through materials.

5. Alpha Particles are the least penetrating and are barely able to pass through a sheet of paper. Beta Particles are slightly more penetrating and easily pass through paper but are stopped by 3 mm of aluminum foil Gamma rays are very high energy and can penetrate 7 cm into lead. Neutrons are the most penetrating and it takes a block of lead 15 cm thick to stop them.

6. Nuclear Decay A nucleus gives up 2 protons and 2 neutrons during alpha decay. The neutron in the nucleus breaks into a proton and an electron, and the electron is ejected during beta decay. Many times during alpha and beta decay gamma rays are also released.

7. Radioactive Decay Rates Half-life is the time required for half of a sample of a radioactive substance to disintegrate by radioactive decay. For example Iodine –131 undergoes beta decay to produce Xenon - 131 The Half-life of Iodine –131 is 8 days. This means that every 8 days half of the Iodine –131 has transformed into Xenon -131 So if you start with 100 g of Iodine –131, 8 days later you have 50 g of Iodine -131

8. Every 8 days you will lose half of the Iodine sample. How much would you have left on day 16 if you started with 200 g of Iodine -131? What fraction of the Iodine –131 is left on day 16? How much would you have left on day 24 if you started with 200 g of Iodine –131? What fraction of the Iodine –131 is left on day 24?

9. Predict the products of the following Radioactive Alpha decays. Thorium -219 Hafnium -156 Plutonium -239 Uranium - 235 Uranium - 238

10. Predict the products of the following Radioactive Beta decays. Carbon -14 Potassium -40 Nickel -63 Hydrogen -3 Boron -12

11. Consider the following Alpha Decays if each isotope started at 400 grams. • How much Thorium –219 is left after 3 half-lives? • How many half-lives have gone by if there is only 1/8 of the Hafnium –156 left? • How many half-lives have gone by if there is only 25 grams of Plutonium -239 • What fraction of Uranium – 235 is left if there are only 12.5 grams remaining • How much Uranium – 238 remains after 6 half lives

12. Nuclear Fission and Fusion We already know that like charges repel and unlike charges attract. In the nucleus there is a strong electrical repulsive force that pushes the protons apart. Why doesn’t the nucleus fly apart? The nucleus doesn’t fly apart because there is a more powerful force at work known as the strong nuclear force. This force hold neutrons and protons together.

13. Nuclear Instability While neutrons and protons are attracted to each other these attractions only act over short distances, and if the nucleus becomes to large it becomes unstable all nuclei with over 83 protons are unstable If a small nucleus contains to many neutrons the nucleus will also become unstable Because the strong nuclear force only acts over a very short distance as the nucleus gets bigger the strong nuclear force is diminished leading to instability.

14. Nuclear Fission Fission is the process by which a large unstable nucleus splits into two pieces and releases both neutrons and energy During a fission reaction the overall mass before the reaction is greater than the overall mass after the reaction. The mass that is lost is converted into a large amount of energy. Small amounts of mass are converted into large amounts of energy.

15. Chain Reactions Fission reactions can be started by adding neutrons to the nucleus of an unstable atom. The neutron causes the already unstable atom to become so unstable that it breaks in half forming two new atoms and releasing both energy and neutrons. The neutrons released in the reaction then fly out cause other atoms to break apart, and these atoms release more neutrons causing more atoms to break apart starting a nuclear chain reaction.

16. Controlling Chain Reactions. Chain Reactions can be controlled however by several different methods. For a chain reaction to continue the fissionable material must be present in a large enough amount called the critical mass. If the amount of material is to small then most of the neutrons will not hit more fissionable atoms but will escape harmlessly. In nuclear power plant control rods are used which absorb the neutrons that keep a nuclear chain reaction going. For this reason power plants don’t produce the explosive power of nuclear bombs.

17. Nuclear Fusion A nuclear fusion reaction occurs when two small nuclei are forced together under extreme heat and pressure and fuse into one nucleus. Nuclear fusion reactions need large amounts of energy to begin, but once they start tremendous amounts of energy are released. Nuclear fusion reactions power our sun and millions of tons of hydrogen are fused together to form helium every second, releasing the light and heat energy that we see and feel from the sun.

18. Understanding Check Questions • A 2 kg blob of putty moving at 3 m/s slams into a 2 kg blob of putty at rest. • Calculate the speed of of the two stuck together blobs of putty after the collision • b) Calculate the speed of the two blobs if the one at rest was 4 kg instead of 2 kg. • 1.5 m/s • 1 m/s

19. Understanding Check Questions In terms of Impulse and Momentum, why are airbags in automobiles a good idea? Airbags increase your stopping time in a head on collision. Greater time of impact means less force of impact.

20. Understanding Check Questions You can’t throw a raw egg against a wall without breaking it, but you can throw it at the same speed into a sagging sheet without breaking it. Explain. The time it takes to stop is extended. More time means less force, and a less-likely broken egg

21. Understanding Check Questions If you topple from your treehouse, you’ll continuously gain momentum as you fall to the ground below. Doesn’t this violate the law of conservation of momentum? If the system is you and the Earth, then your momentum toward the earth is equal and opposite to the Earth’s momentum toward you. There’s no net change because while your falling down, Earth is falling (much less noticeably) up.

22. Understanding Check Questions • A bug and the windshield of a fast moving car collide. Indicate whether each statement is true or false • The forces of impact on the bug and car are the same. • The impulses on the bug and on the car are the same size. • The changes in speed of the bug and the car are the same. • The changes in momentum of the bug and the car are the same. TRUE TRUE FALSE TRUE

23. Understanding Check Questions Comic strip hero SuperJones meets an asteroid in outerspace and hurls it at 100 m/s. The asteroid is 1000 times more massive than SuperJones is. In the strip SuperJones is seen at rest after the throw. Taking physics into account, what would be his recoil speed? What is this in Miles / hour? His recoil speed would be 100,000 m/s and converting this to miles/ hour we find out it is a whopping 224,000 mi/h