1. P6 Radioactive Materials
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P6 Radioactive Materials
OCR 21st Century
M Barker
Shirebrook Academy

2. P6.1 Why are some materials radioactive?
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P6.1 Why are some materials radioactive?

3. The structure of the atom
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ELECTRON – negative, mass nearly nothing
PROTON – positive, same mass as neutron (“1”)
NEUTRON – neutral, same mass as proton (“1”)

4. Introduction to Radioactivity
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Some substances are classed as “radioactive” – this means that they are unstable and continuously give out radiation at random intervals:
Radiation
The nucleus is more stable after emitting some radiation – this is called “radioactive decay”. This process is NOT affected by temperature or other physical conditions.

5. Background Radiation 13% are man-made Radon gas Food Cosmic rays
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13% are man-made
Radon gas
Food
Cosmic rays
Gamma rays
Medical
Nuclear power

6. Structure of the atom
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A hundred years ago people thought that the atom looked like a “plum pudding” – a sphere of positive charge with negatively charged electrons spread through it…
Ernest Rutherford, British scientist:
I did an experiment (with my colleagues Geiger and Marsden) that proved this idea was wrong. I called it the “Scattering Experiment”

7. The Rutherford Scattering Experiment
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Alpha particles (positive charge, part of helium atom)
Thin gold foil
Most particles passed through, 1/8000 were deflected by more than 900
Conclusion – atom is made up of a small, positively charged nucleus surrounded by electrons orbiting in a “cloud”.

8. What keeps a nucleus together?
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The “strong force”

9. Nuclear Fusion in stars
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Nuclear fusion happens in stars when hydrogen nuclei are brought close enough together:
Proton
Neutron
Einstein ( )
We can calculate how much energy this reaction releases using my famous E=mc2 equation.

10. Isotopes
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An isotope is an atom with a different number of neutrons:
Notice that the mass number is different. How many neutrons does each isotope have? O 8 16 O 8 17 O 8 18
Each isotope has 8 protons – if it didn’t then it just wouldn’t be oxygen any more.
A “radioisotope” is simply an isotope that is radioactive – e.g. carbon 14, which is used in carbon dating.

11. Types of radiation
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1) Alpha () – an atom decays into a new atom and emits an alpha particle (2 protons and 2 ______ – the nucleus of a ______ atom)
Unstable nucleus
New nucleus
Alpha particle
2) Beta () – an atom decays into a new atom by changing a neutron into a _______ and electron. The fast moving, high energy electron is called a _____ particle.
Beta particle
New nucleus
Unstable nucleus
3) Gamma – after  or  decay surplus ______ is sometimes emitted. This is called gamma radiation and has a very high ______ with short wavelength. The atom is not changed.
Words – frequency, proton, energy, neutrons, helium, beta
Unstable nucleus
New nucleus
Gamma radiation

12. Sheet of paper (or 6cm of air will do)
Blocking Radiation
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Each type of radiation can be blocked by different materials:   
Sheet of paper (or 6cm of air will do)
Few mm of aluminium
Few cm of lead

13. Changes in Mass and Proton Number
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Alpha decay: Am
241 95 Np α +
237 4 93 2
Beta decay: Sr 90 38 Y β + 90 39 -1

14. A radioactive decay graph
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Activity (Bq)
“1 Becquerel” means “1 radioactive count per second”
Time

15. Half life
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The decay of radioisotopes can be used to measure the material’s age. The HALF-LIFE of an atom is the time taken for HALF of the radioisotopes in a sample to decay…
= radioisotope
= new atom formed
After 2 half lives another half have decayed (12 altogether)
After 3 half lives another 2 have decayed (14 altogether)
After 1 half life half have decayed (that’s 8)
At start there are 16 radioisotopes

16. A radioactive decay graph
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Count
1 half life
1 half life
1 half life
Time

17. P6.2 Using Radioactive Materials
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P6.2 Using Radioactive Materials

18. Ionisation
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Radiation is dangerous because it “ionises” atoms – in other words, it turns them into ions by “knocking off” electrons:
Alpha radiation is the most ionising (basically, because it’s the biggest). Ionisation causes cells in living tissue to mutate, usually causing cancer, and causes molecules to break into bits called ions that then take part in other reactions.

19. Uses of Radioactivity 1 Sterilising medical instruments
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Sterilising medical instruments
Gamma rays can be used to kill and sterilise germs without the need for heating. The same technique can be used to kill microbes in food so that it lasts longer.

20. Uses of Radioactivity 2 - Tracers
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A tracer is a small amount of radioactive material used to detect things, e.g. a leak in a pipe:
Gamma source
The radiation from the radioactive source is picked up above the ground, enabling the leak in the pipe to be detected.
Tracers can also be used in medicine to detect tumours:
For medicinal tracers, you would probably use a beta source with a short half life – why?

21. Uses of Radioactivity 3 - Treating Cancer
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High energy gamma radiation can be used to kill cancerous cells. However, care must be taken in order to enure that the gamma radiation does not affect normal tissue as well.
Radioactive iodine can be used to treat thyroid cancer. Iodine is needed by the thyroid so it naturally collects there. Radioactive iodine will then give out beta radiation and kill cancerous cells.
What sort of half life would you want the radioactive iodine to have?

22. Radiation dose is measured in units called “sieverts” (Sv).
Exposure to Radiation
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People like me work with radiation a lot so we need to wear a “dosimeter” to record our exposure to radiation:
Radiation dose is measured in units called “sieverts” (Sv).

23. Background Radiation by Location
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In 1986 an explosion occurred at the Chernobyl nuclear power plant. Here is a “radiation map” showing the background radiation immediately after the event:
Other “risky” areas could be mining underground, being in a plane, working in an x-ray department etc

24. How we are exposed to Radiation
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We can be exposed to radiation by “irradiation” or by “contamination”:
Irradiation
“Irradiation” is when radiation “hits” us from the outside, like background radiation.
Contamination
“Contamination” is when we take radioactive sources in, like the case of Alexander Litvinenko
Gamma

25. New nuclei (e.g. barium and krypton)
Nuclear fission
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More neutrons
Neutron
Uranium nucleus
Unstable nucleus
New nuclei (e.g. barium and krypton)

26. Chain reactions
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Each fission reaction releases far more energy than burning the same mass of coal would do!

27. Fission in Nuclear power stations
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These fission reactions occur in the fuel rods and they become very hot. Water (a coolant) cools the rods (which then turns to steam) and the control rods are moved in and out to control the amount of fission reactions taking place.

28. Disposing of radioactive waste
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The key to dealing with radioactive waste is to IMMOBILISE it. There are a number of ways of doing this depending on how __________ the waste is:
High level waste is immobilised by mixing with ____ making ingredients, melting and pouring the glass into steel containers.
Intermediate waste is set in cement in _____ drums.
The containers are then kept in stores, often _________.
Words – glass, steel, underground, radioactive