Radioactivity

1. Radioactivity Atoms can be thought to be made up of 3 particles Click here for web work

2. Atomic Particles

3. Radioactivity When atoms are unstable they can eject alpha or beta particles or gamma rays to become more stable. These particles or rays are emitted from the nucleus of the atom. This is called radioactive decay.

4. Alpha Particles, a These are the nucleus of a helium atom. The nucleus consists of 2 protons and 2 neutrons. Alpha particles have a large mass and move slowly. Alpha particles are absorbed by a few cms of air or a thin piece of paper.

5. Beta Particles, b These are fast moving electrons. Beta particles are absorbed by thin aluminium( a few mms ). ( a neutron breaks down into a proton and electron inside the nucleus, the proton is left behind )

6. Gamma Rays, g These are electromagnetic rays( radiation ) that travel at 3.0 x 108 m s-1 Gamma rays can be partially absorbed by thick lead or concrete.

7. Ionisation Atoms contain the same number of protons and electrons. They are electrically neutral. Apha and beta particles or gamma rays can change the number of electrons in an atom. The neutral atom would then be charged. Definition : The gain or loss of electrons to form a charged particle.

8. Ionisation Density Alpha particles have the greatest ionisation density : they cause more ionisation than beta particles or gamma rays ( they are slow moving and have a big mass and so are completely absorbed )

9. Detection All ionising radiation blackens or fogs photographic plate. Dentists, radiographers , doctors and so on wear photographic film badges to monitor the quantity of ionising radiation they are exposed to. Click for radiation badge

10. Geiger Muller Tube • Ionising Radiation passes through the mica window. The gas inside is ionised producing negative and positively charged particles. • The negatively charged • particles are attracted • to the electrode and flow down it. • This is an electric current.

11. Effects of Radiation Radiation can kill or change the nature of living cells. This can have both good and bad effects.

12. Good points Can be used to kill cancerous cells : Radioactive isotope is implanted in the patient, Beta radiation emitted kills the cancerous cells. Gamma radiation or X Rays can be fired onto a brain tumour from outside to destroy it Can be used to kill bacteria on medical instruments / food

13. Bad points Can cause cancer Can kill healthy cells

14. Tracers • A gamma emitter can be injected into the blood stream • The gamma rays pass through the body and are detected • This allows blockages in veins to be identified

15. Activity The Activity , A, is the number of nuclei, N,that decay per second. Units are Becquerels, Bq

16. Activity Calculation

17. Activity Calculation 2 A patient is injected with 25 mg of radioactive technetium -99 which has an activity of 500 MBq. What activity would be measured in the storage case which contains 10 doses ? I dose = 500 MBq 10 doses = 500 x 10 = 5000 MBq.

18. Absorbed Dose, D This is the energy , E, absorbed per kilogram, m, of absorbing material. Units are Grays, Gy

19. Absorbed Dose, D, Calculation

20. Radiation weighting factor,wR This is a measure of the potential biological effect. Alpha has the highest,wR, it causes most ionisation.

21. Equivalent Dose,H H = D x wR Units are Sieverts, Sv Biological risk depends on : Absorbed Dose Type of radiation Body organs exposed Length of exposure

22. Calculation A worker is exposed to material producing an absorbed dose of 10 mGy with a weighting factor of 3. Calculate the equivalent dose. WR = 3 D = 10 mGy H = ? H = D x wr H = 10 x 3 = 30 mSv

23. Calculation 2

24. Half Life, t½ The time for the activity to drop to half its original value. Ranges from a few seconds to millions of years.

25. Half Life calculation 1 A radioactive isotope has a half life of 5 days. Its original activity is 800 kBq, calculate the activity after 20 days. No of half lives = 20 / 5 = 4 No of t½s 0 1 2 3 4 Activity, kBq 800 400 200 100 50

26. Half Life calculation 2 For the previous isotope calculate the fraction active after 4 half lives. No of t½s 0 1 2 3 4 Fraction active

27. Half Life Graph • t½ is the time for the activity to drop to half its original value : • From an activity of 200 to 100 Bq the time interval is 3 – 0 = 3 s. • From an activity of 100 to 50 Bq the time interval is 6 -3 = 3s.

28. Safety This sign needs to be displayed Always use forceps to handle source Cover cuts with plasters / gloves Never bring a source close to eyes Keep in suitable container Work with source for minimum time

29. Reducing Dose equivalent • Use shielding e.g. lead for gamma sources • Keep source as far away as possible from operator • Minimise the time working with source

30. Nuclear Reactors Advantages Reliable Small mass of fuel needed to generate lots of electrical energy No contribution to global warming Disadvantage Small quantity of waste produced which is highly active and has a long half life

31. Nuclear Fission Neutron is fired at Large Uranium Nucleus Nucleus splits into smaller fragments releasing more neutrons and energy

32. Chain Reaction A nucleus splits , releasing two neutrons. These neutrons cause 2 Uranium nuclei to split releasing 4 neutrons ……..

33. Control of Nuclear Reactions As more of the control rods are lowered into the reactor, more neutrons are absorbed and the rate of reaction decreases.

34. Fuel Rods The Uranium fuel is made into pellets. Lots of pellets are made into a rod and lots of rods are bundled together to form a fuel element.

35. Coolant The fuel elements are inserted into a reactor core. During the nuclear reactions large quantities of heat energy are released. Carbon Dioxide gas is pumped through the reactor core to absorb this heat energy. The hot gas is used to turn water into steam in a heat exchanger.

36. Containment The reactor core is contained within thick concrete to stop ionising radiation escaping. In case of gas leakage the air pressure inside the plant is lower than outside.

37. Waste The waste products from the reactions are highly active with a long half life. At present this waste is stored. Some people want to dispose of it underground but there are concerns about leakage.

38. Nuclear fusion Two small nuclei fuse or join together to form a more massive nuclei. There are no highly active waste products. The reactants are readily available. This reaction occurs in the sun.