1 Origin and nature of nuclear radiation

1. 1 Origin and nature of nuclear radiation

2. Properties of ,  and  radiations (2) Mass (nucleon unit) 4 1/1850 0 Effect of Fields Very small deflection Large deflection No deflection Ionizing power Strong Weak (10% of ) Very weak (0.01% of )

3. Properties of , and  radiations (3) Range in air ~5 cm ~5 m ~500 m Penetrating power Stopped by a sheet of paper Stopped by 5 mm of aluminium Never fully absorbed : reduced to half by 25 mm of lead

4. Properties of ,  and  radiations (4) Detectors • Photographic film • Ionization chamber • Cloud chamber • Spark counter • Thin window GM tube • Photographic film • Cloud chamber • GM tube • Photographic film • Cloud chamber • GM tube Radioactive transmutation No transmutation

5. 5 Deflection in electric & magnetic field a In electric field  electric field  (+) radioactive source  +  (–) mass of  >> mass of   deflection:  << 

6. 5 Deflection in electric & magnetic field F b In magnetic field B  I  radioactive source B-field (into paper)  Flow of a particles = Flow of positive charges = Direction of Current Flow of b particles = Flow of negative charges = Opposite direction of current

7. b In magnetic field  radiation tracks in a very strong B-field (Photo credit: Lord Blackett’s Estate)

8. Radiation Detectors • Photographic Film • To detect ,  and  radiations • Spark counter • To detect -particles • Ionization Chamber • To detect -particles • Cloud Chamber • To detect  and  particles • Geiger-Müller Tube • To detect ,  and  radiations

9. Photographic Film • The photographic film has been blackened by radioactivity except in the shadow of the key.

10. Earthed grid To the positive terminal of the EHT supply Spark Counter • The spark counter consists of positively charged wire mounted under an earthed metal grid. • It produces sparks in the presence of ionized particles. • It can only be used to detect α-radiation.

11. a source Metal cylinder Brass rod (+) 2 kV d.c Radioactive source electrometer • When radiation enters the metal can, the gas inside is ionized. • Under the influence o the electric field, electrons move towards the anode while the positive ions move towards the cathode. • As a result, a small ionization current is produced and is recorded by an electrometer.

12. Ionizing current (I) a source b source V Note • 1. When the applied voltage (V) is increased, the ionization current is larger since more ions and electrons can reach the electrodes. Until a certain voltage, all ion-pairs produced reach the electrodes and a saturated ionization current is obtained. • 2. The saturated ionization current is increased with the rate of producing ion-pairs. Therefore, ionization chamber is suitable for detecting a-particles and b-particles since their ionizing powers are relatively strong. But the ionizing power of g-rays is very weak; it cannot be measured by the ionization chamber.

13. Cloud Chamber (1) • The diagrams below show a diffusion cloud chamber and its structure.

14. Cloud Chamber (2) • The felt ring round the top of the chamber is soaked with alcohol. • The cooled chamber is full of alcohol vapour. • A weak radioactive source inside the chamber emits radiation that produces ions along its path. • The alcohol vapour which diffuses downwards from the top condenses around the ions. • The resulting tiny alcohol drops show up as a track in the bright light

15. Cloud Chamber Tracks (1) a radiation Having a strong ionizing power, the heavy  particles give straight and thicktracks of about the same length.

16. Tracks of  rays can hardly be seen. b radiation They are twisted because the particles are small in mass and bounce off from air molecules on collision.

17. Cloud Chamber Tracks (3) • Under diffusion cloud chamber, • Alpha source gives thick , straight tracks ; • Beta source produces thin, twisted tracks. They are small in mass and so bounce off from air molecules on collision. • Gamma source gives scattered, thin tracks. Gamma rays remove electrons from air molecules. These electrons behave like beta particles.

18. GM Counter • When ionizing radiation enters the GM tube, ions and free electrons are formed. • A flow of charge takes place and causes a pulse of current. • The pulse of current is amplified and counted electronically.

19. Central wire as anode (+) Aluminium tube radiation 400 V d.c Mica end-window Argon gas at low pressure counter GM tube • A GM tube is filled with argon gas and a high voltage (~ 400 V) is applied to the central wire.

20. Central wire as anode (+) Aluminium tube radiation 400 V d.c Mica end-window Argon gas at low pressure counter GM tube • When radiation enters the tube, it pulls an electron from an argon atom and produces an ion-pair. • The resulting electrons rapidly accelerated towards the anode and cause more ions formed as they collide with argon gas atoms. • In this way, one electron can lead to the release of 108 electrons. An avalanche of electrons is produced. • When the electrons reach the anode, a pulse is created and can be counted by the GM counter.

21. 2p 2n Beta decay e- p n Gamma emission energy X Y + He  X X* Y X + + e A 4 A4 A A A A 0 Z 2 Z 2 1 Z Z Z+ 1 Z 1 Three types of decay Alphadecay

22. Example 1

24. The points plotted do not fall exactly on the curve. The fluctuations are due to the random nature of dice throwing. • Radioactive decay is also random in nature because, like the dice ‘decay’ the chance of certain nuclei decaying at a particular time is random.

25. no. of undecayed nuclei • Always decreasing. • Decrease rapidly in the beginning. • Decreases gently finally. • Becomes zero after a long time. Time / s

26. Decay rate (Activity) = Activity of a radioactive isotope (1) • Let N(t) be the number of radioactive nuclei in a sample at time t. The `-’ sign indicates that N(t) decreases with time • The decay rate is directly proportional to N(t). The SI unit of activity is the becquerel (Bq). The constant k is called the decay constant. A large value of k corresponds to rapid decay.

27. Activity of a radioactive isotope (2) • k can be interpreted as the probability per unit time that any individual nucleus will decay. From ,

28. no. of undecayed nuclei • 0 – 4 s: 2000  1000 4 – 8 s: 1000  500 8 – 12 s: 500  250 12 – 16 s: 250  125 • Half life = 4 s It takes 4 s for half to decay Time / s

29. Activity A A0 ½A0 ¼A0 ⅛A0 Half life t1/2 Half-life t½

30. Half-life (1) • The graph shows the number of remaining nuclei N(t) as a function of time.

31. Half-life (2) • The half-life t1/2 is the time required for the number of radioactive nuclei to decrease to one-half the original number No. • At t = t1/2, N(t) = No/2, obtaining • Taking logarithms to base e, gives

32. Cloud Chamber Tracks (2)

33. dN(t) = -kN(t) dt Rate of decay undecayed nucleus decayed nucleus

34. Radiation hazard • Ionizing effect can destroy or damage living cells. • Radioactive gas and dust cannot be removed once taken in. • Gamma rays are dangerous due to strong penetrating power

35. Background radiation

36. Hazards due to sealed and unsealed sources (1) • Hazards due to sealed sources • α-particles usually do not present any external radiation hazard because they are unable to penetrate to dead layer of skin. But, extremely precautions must be taken to prevent α-emitters from getting into the body. • β-particles never constitute a whole-body external radiation hazard due to their short range in tissue. • γ-rays have very high penetrating power and require greater care to avoid receiving excess dosage.

37. Hazards due to sealed and unsealed sources (2) • Hazards due to unsealed sources • Unsealed sources usually constitute some kind of internal hazard. This is the absorption and retention of radionuclides into specific organs of the body through intake of the materials present in air and in water. • The radionuclides may be rapidly absorbed by the organs causing damage to these organs.

38. Radioactive doses • The radiation emitted transfers energy to the organs and causes damage. • The level of damage depends on 1. energy absorbed by the body 2. type of radiation 3. the parts of human body

39. Effective dose = Absorbed dose x Radiation weighting factor x tissue weighting factor

40. Handling precautions • The weak sources used at school should always by lifted with forceps. • The sources should never by held near the eyes. • The source should be kept in their boxes (lead container) when not in use. • Take great care not to drop the sources when handling them. • Carefully plan the experiments to minimize the time the source is used.

41. Uses of radioisotopes • Medical uses • Treatment of body cancer • Investigation of Thyroid Gland (甲狀腺) • Radon-222 (a emitted) • Iodine-131 (g emitted)

42. Industrial uses

43. Archaeological Use (Carbon-14 dating) Carbon-14 exists due to formation by bombardment of nitrogen-14 in atmosphere by neutrons ejected from nuclei by cosmic rays ( ) and this forms radioactive carbon dioxide. Living plants or trees absorb and give out carbon dioxide, so the percentage of C-14 in their tissue remains unchanged. After death, no fresh CO2 taken in. C-14 starts to decay with a half-life of 5.7  103 years. By measuring the activity of C-14 ( ), the age of carbon containing material (e.g. wood, linen, charcoal) can be estimated.

44. End of this chapter

45. Side view Zinc sulphide screen Gold foil -source microscope Evacuated metal box To vacuum pump Alpha-Scattering Experiment(1) • A beam of -particles was directed at a thin sheet of gold-foil and the scattered -particles were detected using a small zinc sulphide screen viewed through a microscope in a vacuum chamber.

46. Alpha-scattering Experiment (2) • From the experiment it was found that • most of the -particles passed through the foil unaffected, • a few were deflected at very large angles, • some were nearly reflected back in the direction from which they had come.

47. 10-15 m 10-10m Rutherford’s atomic model • Rutherford’s assumptions: • All the atom’s positive charge is concentrated in a relatively small volume, called the nucleus of the atom • The electrons surround the nucleus at relatively large distance. • Most of the atom’s mass is concentrated in its nucleus.

48. Difficulties of Rutherford’s model • The Rutherford model was unable to explain why atoms emit line spectra. The main difficulties are: • It predicts that light of a continuous range of frequencies will be emitted; • It predicts atoms are unstable—electrons should quickly spiral into the nucleus.

49. Mass and Energy • The mass-energy relationship • Einstein showed that mass and energy are equivalent. • E = mc2 • Mass defect • The difference between the mass of an atom and the mass of its particles taken separately is called the mass defect (Δm). • Δm = Zmp +Nmn- Mnucleus • The mass defect is small compared with the total mass of the atom.

50. Unified Atomic Mass Unit • The unified atomic mass unit (u) is defined as one twelfth of the mass of the carbon atom which contains six protons, six neutrons and six electrons. • 1 u = 1.660566 × 10-27 kg • Energy equivalence of mass • 1 u = 931.5 MeV • It is a useful quantity to calculate the energy change in nuclear transformations.