Radioactivity.

1. 04/01/12
EdExcel Additional Science
Putting Radiation to Use
W Richards
The Weald School

2. 04/01/12
The structure of the atom
ELECTRON –
negative, mass
nearly nothing
PROTON –
positive, same
NEUTRON – mass as
neutral, same neutron (“1”)
mass as proton
(“1”)

3. 04/01/12
The structure of the atom
Particle Relative Mass Relative Charge
Proton 1 +1
Neutron 1 0
Electron 0 -1
MASS NUMBER = number of
protons + number of neutrons
SYMBOL
PROTON NUMBER = number of
protons (obviously)

4. Isotopes
04/01/12
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?
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.

5. 04/01/12
Introduction to Radioactivity
Some substances are classed as “radioactive” – this means
that they are unstable and continuously give out radiation:
Radiation
The nucleus is more stable after emitting some radiation – this
is called “radioactice decay” and the activity is measured in
“Becquerels (Bq)”.

6. Types of radiation
04/01/12
1) Alpha (α) – an atom decays into a new
atom and emits an alpha particle (2
protons and 2 ______ – the nucleus of a
Unstable New Alpha ______ atom)
nucleus nucleus particle
2) Beta (β) – an atom decays into a new
atom by changing a neutron into a
_______ and electron. The fast moving,
Beta high energy electron is called a _____
particle particle.
Unstable New
nucleus nucleus 3) Gamma – after α or β decay surplus
______ is sometimes emitted. This is
called gamma radiation and has a very
high ______ with short wavelength.
The atom is not changed.
Unstable New Gamma Words – frequency, proton,
nucleus nucleus radiation energy, neutrons, helium, beta

7. 04/01/12
Half life
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 1 half After 2 half After 3 half
At start life half have lives another lives another
there are 16 decayed half have 2 have
radioisotopes (that’s 8) decayed (12 decayed (14
altogether) altogether)

8. 04/01/12
A radioactive decay graph
Count
Time
1 half life

9. Dating materials using half-lives
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Question: Uranium decays into lead. The half life of uranium is
4,000,000,000 years. A sample of radioactive rock contains 7 times as
much lead as it does uranium. Calculate the age of the sample.
Answer: The sample was originally completely uranium…
1 half life 1 half life 1 half life
later… later… later…
8 4 2 1
8 8 8 8
…of the Now only 4/8 of Now only 2/8 of Now only 1/8 of
sample was the uranium uranium remains uranium remains
uranium remains – the – the other 6/8 – the other 7/8
other 4/8 is lead is lead is lead
So it must have taken 3 half lives for the sample to decay until only 1/8
remained (which means that there is 7 times as much lead). Each half
life is 4,000,000,000 years so the sample is 12,000,000,000 years old.

10. 04/01/12
An exam question…
Potassium decays into argon. The half life of potassium is
1.3 billion years. A sample of rock from Mars is found to
contain three argon atoms for every atom of potassium.
How old is the rock?
(3 marks)
The rock must be 2 half lives old – 2.6 billion years

11. 04/01/12
Ionisation
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.

12. Blocking Radiation
04/01/12
Each type of radiation can be blocked by different materials:
α
β
γ
Sheet of Few mm of Few cm of
paper aluminium lead

13. 04/01/12
Uses of radioactivity 1
Sterilising medical instruments
Gamma rays can be used to kill and sterilise
germs without the need for heating.

14. 04/01/12
Uses of radioactivity 2
Beta
detector
Rollers Paper
Beta
emitter

15. 04/01/12
Uses of radioactivity 3
Smoke detectors
Alpha
emitter
+ve electrode
-ve electrode
Alarm
If smoke enters here a
Ionised air particles current no longer flows

16. 04/01/12
Uses of Radioactivity 4 - Treating Cancer
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.

17. 04/01/12
Dangers of radioactivity
Radiation will ionise atoms in living
Alpha cells – this can damage them and
cause cancer or leukaemia.
Beta
Gamma
OUTSIDE the body β and γ are
more dangerous as α radiation
is blocked by the skin.
INSIDE the body an α source
causes the most damage
because it is the most ionising.

18. 04/01/12
Background Radiation
13% are
man-made
Radon gas
Food
Cosmic rays
Gamma rays
Medical
Nuclear power

19. 04/01/12
Protecting the Earth
1) The Atmosphere 2) The Magnetic Field
The Earth has a number of The Earth’s magnetic field
layers that reflect radiation reflects charged particles
back into space. from solar wind away from us.