Nuclear-Physics(Radioactivity)_pearson international igcse curriculum.pptx

Physics

1. Atoms, Ions and Isotopes What is the difference between an ion and an isotope of an atom? Explain why ions are often formed in chemical reaction. Why are isotopes often unstable? What is radiation and radioactivity?

2. Atomic, mass and isotopes

3. Atomic, mass and isotopes neutrons electrons protons and neutrons nucleus electrons protons and neutrons

4.   Mass Charge Location Proton       Neutron       Electron       The nucleus is tiny compared to the atom as a whole: the radius of an atom is about 0.1 nm (1 × 10 -10   m). the radius of a nucleus (1 × 10 -14  m) is less than   of the radius of an atom.  

5.   Mass Charge Location Proton  1  +1  Nucleus Neutron  1  0 Nucleus  Electron    -1  Orbiting the nucleus   Mass Charge Location Proton  1  +1  Nucleus Neutron  1  0 Nucleus  Electron  -1  Orbiting the nucleus

6. Charged Atoms When atoms lose or gain electrons, they become ions. Naturally, elements are neutrally charged. Why? This is because they contain the same number of protons (+) as electrons (-).

7. Isotopes When atoms lose or gain neutrons, they become isotopes of that element. Isotopes have different mass numbers, but are still atoms of the same element because their numbers of protons remain the same. The chemical properties of isotopes are the same.

8. History of the Atom Your task: Create a brief timeline summarising the major developments in our ideas about atoms. Include the key thoughts and experiments undertaken by Democritus, John Dalton, JJ Thomson, Ernest Rutherford, Niels Bohr and James Chadwick. You will present your work in 20mins to the class.

9. The Rutherford Scattering Experiment Also known as the alpha scattering or gold leaf experiment. It was known at the time that alpha particles have a 2+ charge. These particles were fired at a very thin piece of gold leaf. It was expected that most particles would pass straight through, a small % being deflected through small angles. …what happened next was very surprising.

10. The Results How can each of the data sets below be explained by the nuclear model? Most particles pass through, only some with minor deflections. A small number are deflected through a large angle. An even smaller number are deflected 180° (they bounced back)! Extension: why can the plum pudding model not explain all data sets above?

11. Most particles pass through, only some with minor deflections. Like charges repel, opposites attract. As both the alpha particles and nucleus were positively charged, they would repel one another. As the alpha particle and electrons were oppositely charged, there would be a very small attraction between these. However, most of an atom is empty space so there is very little/no attraction/repulsion experienced by most alpha particles. The plum pudding model can also explain these minor deflections.

12. A small number are deflected through a large angle. The bigger the force of repulsion (more densely packed charges), the greater the angle of deflection. Any alpha particles that got very close to the nucleus would be deflected by large angles. The plum pudding model cannot explain these large deflections.

13. An even smaller number are deflected 180° (they bounced back)! Alpha particles that come close enough to the nucleus, will strike it and be rebounded. This can only happen if the nucleus is very dense (lots and lots of charges in one small place). The plum pudding model definitely can’t explain this one.

14. Some Specifics Gold was used as the target metal because it can be rolled into very thin foils. Also, it has a nucleus with lots of positive charge (Z = 79) so causes a large repulsive force to deflect the alphas. The chamber of the apparatus was evacuated so that alphas weren’t slowed down by collisions with air molecules. The narrow beam of alpha particles was obtained from a sample of radium mounted in a lead box. After passing through the foil, the alphas were detected by a microscope fitted with a fluorescent screen. When an alpha hit the screen, it caused a little flash of light called a scintillation. These scintillations were counted by the observer carrying out the experiment. The microscope could be rotated to various angles to detect alphas deflected at those angles. What does Z refer to? What does evacuated mean in science? Why was the radium mounted in a lead box? Why couldn’t the alpha particles be directly detected?

15. What’s this?

16. What do you know about radiation?

17. Henri Becquerel He left uranium salts on top of a photographic plate When he came back, he noticed the plate had changed He didn’t know what it was, but asked Marie Curie to help him find out

18. Marie Curie The first, and only, person to win a Nobel prize in both Chemistry and Physics Discovered radioactivity – knew it was radiation, but didn’t understand where it came from Discovered many new elements, including Polonium (after her native Poland) Marie Curie - TED Ed

19. Background Radiation Your task: Copy and complete the following sentences. Use the words in black at the bottom, each word may be used more than once. An isotope of an element is an atom that contains the same number of _________ as that element, but a different number of _________. An ion is an atom that has lost or gained _________. An ion that has lost _________ will have a _________ charge, an ion that has gained _________ will have a _________ charge. Protons Neutrons Electrons Positive Negative

20. Background radiation It’s fair to estimate 500 000 cosmic rays pass through you per hour. From the air, 30 000 radon atoms decay each hour in your lungs. From our food, 15 000 000 atoms of potassium-40 decay inside you each hour. From rocks and building materials, 200 000 000 gamma rays pass through you each hour Most of us live our lives without being seriously harmed by all of this background radiation.

21. Background radiation either comes from natural or artificial sources. Natural – for example cosmic rays (including CMBR) and organic matter. Artificial – for example medical uses, nuclear waste, nuclear weapons. 1. What is the biggest source of background radiation (on average), according to the chart? 2. What is the biggest source of artificial radiation?

22. Ionising Radiation What does ‘ionising’ mean? Explain how being exposed to ionising radiation could cause cancers. Damage can be caused via irradiation and contamination . What are the differences between these underlined terms? nucleus cloud of electrons

23. Radon Gas Is produced naturally in bedrock when radium atoms decay into radon atoms.

24. Radon is a hazardous gas. It is produced naturally in rocks. Granite rock emits radon gas which is a particular hazard if it builds up in enclosed spaces and is breathed in. Depending on the bedrock beneath the ground radon hazards are different around the country. Radon gas emits alpha radiation (more on this in future lessons). Exposure to a source outside your body is called irradiation. If radiation gets inside your body it is called contamination. Radon is not an irradiation risk, however if you become contaminated the risk could be severe; you may die or become very ill! Where does radon gas come from? What do the levels of radon gas depend on? What source of radiation is emitted by radon gas? What is meant by irradiating? What areas of Britain have high levels of radon gas?

25. Types of Radiation When a nucleus is unstable it can easily change (known as decay). You cannot predict which nucleus in a sample will decay next, or when a specific nucleus will decay. It is a completely random process. Define the following terms. Isotope Decay Ionising radiation Background radiation GM Tube

26. Summarising the Types of Radiation Type of Radiation Composition Charge Penetrating Power Ionising Power Alpha Beta Gamma

27. Alpha Radiation Alpha, α : A particle made of two protons and two neutrons Overall +2 charge (because of the two protons) Mass number of 4 (2 protons + 2 neutrons) Very heavy Highly ionising Can be blocked by a sheet of paper

28. Beta Negative Radiation Beta, β⁻ : A particle made of one electron, ejected from the nucleus Formed when a neutron splits into a proton and an electron, the electron is pushed out and the proton stays in the nucleus. So the nucleus loses a neutron, but gains a proton. - 1 charge Negligible mass Very light and fast A little ionising (not as much as alpha) Can be blocked by a sheet of aluminium Note, beta-positive (positron) emission also exists, but is not covered in this course.

29. Gamma Radiation Gamma, γ : Electromagnetic radiation (it’s a type of light, found on the EM spectrum) No overall charge (neutral) No mass Very, very fast (travels at the speed of light) Not particularly ionising Blocked by a block of lead

30. Which radiation could this be? Paper Aluminum Thick lead

31. Which radiation could this be? Paper Aluminum Thick lead

32. Which radiation could this be? Paper Aluminum Thick lead Counts: 100 Counts: Counts: Counts: 100

33. Which radiation could this be? Paper Aluminum Thick lead Counts: 100 Counts: 100 Counts: 3 Counts: 100

34. Which radiation could this be? Paper Aluminum Thick lead Counts: 50 Counts: 20 Counts: Counts: 100

35. Magnetic and Electric Field Deflection – Extended Only Explain the paths of the three radiations below.

36. Alpha particles will experience a force which follows Fleming’s left hand rule. Beta particles will experience a force in opposition to Fleming’s left hand rule.

37. Your Task Explain how laboratory technicians can check which type of radiation their sources are emitting and check the activity levels of the sources.

38. Nuclear Equations Just as with chemical equations, nuclear equations representing decay must be balanced. What numbers are missing from the equations below?

39. Alpha Decay Can be represented with He or α . Two protons and two neutrons emitted. Z decreases by 2. A decreases by 4. N decreases by 2. 208 81

40. Beta⁻ Can be represented with e or β . A neutron changes into a proton, an electron and an antineutrino (the proton remains in the nucleus ). You do not need to show or discuss the antineutrino at iGCSE level. Z increases by 1 A remains the same N decreases by 1

41. Nuclear Equations You should use your periodic table to write the nuclear equations for the decays below. The first two have been done for you as an example. Americium- 241 decays by alpha emission. Carbon – 14 decays by beta ⁻ emission. Bismuth – 211 decays by alpha emission Polonium – 204 decays by alpha emission Radon – 224 decays by alpha emission Uranium – 235 decays by alpha emission Neptunium – 237 decays by alpha emission   6. Strontium – 90 decays by beta emission 7. Phosphorus – 32 decays by beta emission 8. Nickel – 63 decays by beta emission 9. Lead – 209 decays by beta emission 10. Hydrogen – 3 decays by beta emission

42. Radioactive Decay We’ve already discussed decay and the changes undergone by a nucleus to become stable in a lot of detail. Some key terms to remember: Radioactive: Emitting ionising particles of radiation (e.g. alpha or gamma). Decay: The process by which a nucleus emits ionising radiation to become more stable. Activity: The number of unstable nuclei that decay each second (measured in Bq ). Counts per minute/second, etc: Rate of emissions.

43. Radioactive Decay Decay is both spontaneous and random . We can’t predict which nucleus will decay next in a sample, nor when one nucleus will decay. Half-life ( T 1/2 ) is the time taken for half of a sample of radioactive nuclei to decay. This enables to predict how many and how long , but not which one and when .

44. The -------- of the particles in a ----------- substance are unstable. This means that at some point they will -------- by emitting either an -------- or beta particle or energy in the form of gamma radiation. It is impossible to predict when any particular nucleus will decay but we can --------- the time for ------ of the nuclei of the isotope to decay this is called the half-life of the isotope .

45. Half life calculations We can calculate half life from decay curves. We simply look at the decay or count rate at a specific time, and look at how long it takes for that number to half.

46. Half-life calculations In general, you can work out the count rate or the number of unstable nuclei left after n half-lives by dividing the initial value by 2 to the power n (i.e. 2 multiplied by itself n times, n being the number of half-lives). You can write this as an equation: N.B: count rate refers to the amount of radiation given off in a fixed amount of time, usually measured in counts per second (cps) or counts per minute ( cpm ).  

47. Worked example A particular radioactive isotope has a half-life of 6.0 hours. A sample of this isotope contains 60 000 radioactive nuclei. Calculate the number of radioactive nuclei of this isotope remaining after 24 hours. n = 4 because 24 hours equals 4 half-lives for this isotope. So, the number of radioactive nuclei of the isotope remaining after 24 hours = 60000 ÷ 2 4 = 60000 ÷ 16 = 3750 Units?

48. Half-life questions answers True or false? Half-life means half the time taken for a radioactive isotope to decay The activity of a radioactive source is the number of nuclei decaying per second. The activity of a radioactive source doesn’t change. True or false? The activity of a source reduces to one-sixteenth of its original value after four half-lives. The radioactive isotope sodium-25 has a half-life of 1 minute. What fraction of it remains after 3 minutes? A 1/3 B 1/4 C 1/6 D 1/8 False True False True

49. The medical tracer, technetium-99m, has a half-life of 6 hours. A sample gives a count rate of 2400 counts per second at 11:00 am on Monday How many half-lives will it take for the count to drop to 300 counts per second? How long will it take for the count to drop to 300 counts per second? What day and time will it be when the count is 300 counts per second? Xenon-133 is a radioactive gas used for diagnosing lung problems. In 15 days its activity falls to 1/8 of its original value. What is its half-life? The half-life of the radioactive isotope sodium-24 is 15 hours. A sample has a count rate of 240 counts per minute ( cpm ). Its count rate 60 hours later will be: A 15 cpm B 30 cpm C 40 cpm D 60 cpm 3 half-lives 18 hours 5:00 am on Tuesday 5 days

50. A radioactive isotope of silver has a half-life of 20 minutes. A sample gives a count rate of 6400 counts per second at 9 am. At what time will the count rate be about 200 counts per second? A sample of bone from a living animal contains carbon-14. Its activity is 80 counts per minute. The half-life of carbon-14 is 5730 years. How old is an antler with activity of 20 counts per minute? Cobalt-60 sources are used for sterilising medical instruments. It has a half-life of 5.27 years. What percentage of a source remains after 10.54 years? After the activity drops too low to be used, the cobalt-60 source must be disposed of safely. If the source must be stored until its activity has dropped to less than one-thousandth of its activity today, it would have to be stored for at least : 10:40 am 11460 years 25 % 52.7 years

51. Half-life questions answers 1 a false b true c false 2 true 3 D 1/8 4 a 3 half-lives b 18 hours c 5 am Tuesday 5 5 days 6 A 15 cpm 7 10:40 am 8 11460 years old 9 a 25% b 52.7 years

52. Tracers Radiopharmaceuticals are introduced to the body (intravenously, orally or via inhalation). The radiation emitted as the tracer decays can be tracked by specific imaging equipment. In order to be detected, the radiation must high a high enough penetrating power to be transmitted through the body. What would be a suitable half-life for a medical tracer?

53. X-ray Radiotherapy – Breast Cancer

54. Gamma Knife – Used to Treat Brain Tumour

55. Proton Beam Therapy – Treatment of Cancer at the Base of the Skull

56. PET - Positron Emission Tomography Positron emission tomography (PET) is a gamma imaging technique that uses radiotracers that emit positrons, the antimatter counterparts of electrons. The gamma rays used for imaging are produced when a positron meets an electron inside the patient’s body, an encounter that annihilates both electron and positron and produces two gamma rays travelling in opposite directions.  By mapping gamma rays that arrive at the same time the PET system is able to produce an image with good spatial resolution.  The computer works out the position of the source by “drawing lines” between gamma rays that arrive at the same time at the detector (within nanoseconds of each other ). The imaging is undertaken using a gamma camera (more on this later).

57. What are Positrons? Positrons are antielectrons, they have the same mass and penetrative power as an electron but the opposite charge. When a positron meets an electron, they will annihilate one another. Their mass is converted into energy (as gamma photons). The photons will travel in equal and opposite trajectories. They’re emitted from the nucleus of a radioisotope during decay but can’t exist inside the nucleus as a stable particle.

58. The Physics of PET The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide or ‘tracer’, which is introduced into the body on a biologically active molecule. The biologically active molecule (tracer) chosen for PET is usually fludeoxyglucose  (FDG ). After emission inside the body, the positrons travel about 1mm and then interact with electrons in the patient’s body: gamma rays are produced by the process of annihilation. These are detected by an imaging system outside the body. Three-dimensional images of tracer concentration within the body are then constructed by computer analysis. In modern PET scans, three-dimensional imaging is often accomplished with the aid of a CT X-ray scan performed on the patient during the same session, in the same machine. One obstacle to the wider use of PET is the difficulty and cost of producing and transporting the radiopharmaceuticals which are usually extremely short-lived. For example, the half-life of Fluoene-18 used in FDG is two hours only. To make the radioisotope requires a very expensive cyclotron, as well as a production line to make the radiopharmaceuticals.