Radioactivity

1. Radioactivity • Particles emitted from nuclei as a result of nuclear instability. • Common types of radiation: • Alpha • Beta • Gamma Nucleons = anything in the nucleus

2. Half-lives • Radioactive decay rates • Half-life – time it takes for half of the particles are emitted.

3. Alpha radioactivity • Alpha particle is a nucleus of the element helium = two protons and two neutrons • Range is less than a tenth of a millimeter inside the body. • Main hazard is when it is ingested. • Can have destructive power in its short range.

4. Einstein E = mc2 The difference in mass between the reactant and products isn’t missing it was turned into energy. E = energy M = mass C = speed of light, 3x 10^8 m/s

5. The energy is found (moslty) in the kinetic energy of the alpha particle and daughter nucleus moving away from each other.

7. Example 1: The iridium-168 isotope is known to go through alpha decays. Write out a decay equation that shows this process. • Start by looking up iridium on your periodic table so that you can find out its atomic number. • Then write down the most basic decay reaction; show what you started with (the iridium is your • parent nucleus), and how it has decayed by emitting an alpha particle and some other nuclei.

8. Masses Proton = 1.007316 amu or 1.6727 x 10-24 g Neutron = 1.008701 amu or 1.6750 x 10-24 g Electron = 0.000549 amu or 9.110 x 10-28 g 1 amu = 1.66 x 10-27 kg

9. Beta particles • A neutron turns into a proton and electron. • The proton stays in the nucleus but the electron is released. • High energy electrons have greater range of penetration than alpha particles, less than gamma rays. • Greatest hazard if ingested.

10. Beta

11. Example 3: Write out the beta negative decay reaction for calcium-46. As with the alpha decay in Example 1, first find your parent nucleus on the periodic table and write out a basic decay reaction...

12. Beta negative decay • A neutron is converted to a proton and an electron. • They found that the electron of a beta particle is accompanied by a neutral particle called an antineutrino • Called antimatter (opposite spin as nuetrons)

13. Beta positive decay • A proton turns into a neutron and emits a positron. • Exact same mass of electron but have a +1e charge • (antimatter version of electrons)

14. Half-life • The nuclei of radioactive atoms are unstable. They break down and change into a completely different type of atom. • Radioactive decay • Ex. Carbon-14 decays to nitrogen-14 when it emits beta radiation.

15. Its not possible to predict when an individual atom might decay . • It is possible to measure how long it takes for half the nuclei of a piece of radioactive material to decay.

16. Half-life = the time it takes for the number of nuclei of the isotope in a sample to halve.

17. Ex. The half-life of carbon-14 is 5,715 years, but the half-life of francium-223 is just 20 minutes.

18. Radioactive particles • Alpha particles = (+) charged • Beta particles = (-) charged • Gamma rays = (o) nuetral, electromagnetic waves (light waves) Rutherford figured out the charged by placing an electric and magnetic field around the radioactive element to see where the particles curved.

20. Gamma Decay • Passes through anything but the densest of matter. • Happens because the nucleus has just been through a lot, spitting out other subatomic particles so on… • Releases energy, not particles

21. Example 5: The argon-40 that was produced in Example 4 is in an excited state, so it releases a burst of gamma radiation. Write the equation for this.

22. The daughter nucleus may still be unstable and will have to go through several decays in order to be stable, this is called decay series.

24. Radiation risks • Geiger counters calculate radiation. • There's radiation everywhere that has been here forever so it isn’t harmful. • Large doses of radiation is harmful. Rad: a rad is the older unit used to describe each kilogram of tissue exposed absorbing 0.01 J of energy.

25. Gray (Gy): one Gray means each kilogram of material absorbs 1 Joule of energy. • So, 1 Gy = 100 rad • Sievert (Sv): is a modified version of Grays, because it takes into account the relative Biological Effectiveness (RBE) of a particular kind of radiation. The more dangerous a particular kind of radiation is to a person, the more the original Grays are multiplied to give Sieverts.

26. In normal situations, a person can expect to be exposed to about 0.5 mSv in a year. Any exposure of about 6 Sv or higher will be fatal.

27. Radiation sickness • Radiation can ionize cells. • Radiation is knocking electrons off the cells, interfering with cell division. • Many people that survived the initial blast of the bombs used at Hiroshima and Nagasaki died from radiation sickness a few days later.

28. Genetic damage • Damage to the actual DNA of cells. • Can result in cancer, show up after several years.

29. Alpha has a high ionization rate, but can not easily penetrate matter. A layer of clothes or even the top layer of skin (which is dead anyways) can stop it. The alpha particles can only move through the air about 5 cm before being stopped. Alpha radiation is really only a danger if you either breathe in or swallow the source of the alpha radiation.

30. • Beta does not ionize as easily, but it can penetrate matter more easily, traveling about 0.50 m through the air and about 1 cm into a body. This means that although beta radiation can be a bit more of a risk, it is still most dangerous if the source is ingested.

31. • Gamma can easily penetrate your body, since it is EMR with a high frequency. Although it doesn't ionize much, it causes the most damage to a person. Even being near an unshielded source of gamma radiation for a short period of time is very dangerous!

32. https://www.youtube.com/watch?v=FU6y1XIADdg

33. Half life • Time it takes for half of the parent atoms to transmutate into something lese (through alpha or beta decays, or another process)

34. Example 1: The half life of Carbon-14 is 5730 years. Explain what you would expect to happen over a long period of time. Imagine a sample of carbon that originally had 100 of these carbon-14 atoms. In reality we would need the sample to have many more atoms, since statistics are really only reliable for large numbers.

35. During the first few hundred years or so we would notice that some of the carbon-14 atoms have transmutated into some other element. In fact, a lot of them have changed. Since we started with a lot of the carbon-14 atoms, there is the greatest chance of seeing quite a few change. It would be like throwing 100 quarters into the air; since there are so many, you've got a really good chance of seeing a 50-50 split between heads and tails when they hit the ground.

36. By the time 5730 years have passed, we would expect to only have 50 carbon-14 atoms remaining. Remember, the half life is the time it takes for half of them to change. There are still the same total number of atoms, just not as many carbon-14 as we started with.

37. Some people think that if we wait another 5730 years, all of the carbon-14 will be gone... nope! Remember, half life is the time it takes for half the atoms to decay. So, after the next 5730 years we would expect 25 carbon-14 to be left; that's half of the 50 that we had after the first half life.

38. And so on, and so on... Eventually, after about six or seven half lives have passed, the number of carbon-14 atoms becomes so small that probabilities fall apart and you basically have the last few atoms decay whenever.

40. Half-life • T1/2 the time required for half the amount of a radioactive nuclide to decay. • Less stable nuclides decay very quickly and have shorter half-lives Ex.

41. Decay series • Series of radioactive nuclides produced by successive radioactive decay until a stable nuclide is reached. • Parent nuclide – the heaviest nuclide of each decay series. • Daughter nuclide – nuclides produced by the decay of the parent nuclides.

42. Half life equation

43. Example 2: Marie Curie had a 765g sample of polonium-210 (half life = 138d) in a box. After 3.8 years of refining radium, she goes to the box to get her polonium. Determine how much polonium-210 is in the box. First, we need to get our units for time the same, so figure out how many days there are in 3.8 years. 365 days = 1 year 1387 days

44. Now we can figure out how many half lives have passed (the “n” value in the formula). n = total length of time/half life n = 1387 days/138 d half-life = 10.0507 half lives

45. Now we figure out how much polonium-210 is remaining.

46. By the time Madame Curie gets back to her box, she’ll only find that 0.72g of polonium-210 is remaining. There is still a total of 765g of stuff in the box, but only 0.72g of it is polonium-210. The other 764.28g of stuff would be other elements that the polonium-210 decayed into.

47. Example 3: You have 75g of lead-212. If it has a half life of 10.6h, determine how long it will take until only 9.3g remains. This question is tougher than the previous example. If you write out the formula, here’s what you get…

48. Now, if you’re good with logarithms in math, you can go ahead and solve it. But in chemcommyou are not required use logs… we can do it an easier way! Try the following... 1. Type 75 into your calculator and divide by 2. You should get 37.5. So, after one half life you’ve got 37.5g left.

49. 2. Divide 37.5 by 2 to get 18.75… so after two half lives you’ve got 18.75g. 3. Divide 18.75 by 2 to get 9.375. After three half lives have passed you’ve got 9.375g left. That’s pretty close to the 9.3g in the question, so after just a little more than three half lives you should have 9.3g left over.

50. Fission • The process of causing a large nucleus (A > 120) to split into multiple smaller nuclei, releasing energy in the process. • It can start when the large nuclei absorbs a neutron, causing it to become unstable to the point that it falls apart. • This is the reaction that we use in nuclear power plants and early nuclear weapons. • Fission is relatively easy to do, but also leaves us with lots of nuclear waste that must be stored for thousands of years before it is safe.