This document discusses the structure of atoms, radioactivity, and uses evidence about radiation and the universe to support the Big Bang theory. It describes: 1) The basic structure of atoms including protons, neutrons, electrons, isotopes, and radioisotopes. 2) Different types of radiation including alpha, beta, and gamma rays and how they are produced and can be blocked. 3) How radioisotopes are used in medical tracers and carbon dating to measure material ages based on half-life decay. 4) Evidence from light spectroscopy showing redshift of spectra from distant galaxies, which suggests everything is moving away from a single point supporting the Big Bang theory of an expanding universe.

1. Radiation and the Universe
b.Radioactivity
Mr S Thompson

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

3. 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
An isotope is an atom with a different number of neutrons:
Each isotope has 8 protons – if it didn’t then it just
wouldn’t be oxygen any more.
Notice that the mass number is different. How many
neutrons does each isotope have?
A “radioisotope” is simply an isotope that is radioactive –
e.g. carbon 14, which is used in carbon dating.

5. 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”.

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

7. 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.

8. Blocking Radiation
Each type of radiation can be blocked by different materials:
Sheet of
paper
Few mm of
aluminium
Few cm of
lead

9. Deflection by Magnetic Fields
Alpha and beta particles
have a charge:
+
+
-
2 protons, 2 neutrons,
therefore charge = +2
1 electron, therefore
charge = -1
Because of this charge, they will be deflected by electric and
magnetic fields:
+
-

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

11. Uses of radioactivity
1) Medical uses – gamma rays can be used to destroy
cancerous cells or to sterilise medical instruments
2) Tracers – a tracer is a small amount of radioactive material
used to detect things, e.g. a leak in a pipe:
Gamma
source
Tracers can also be used to develop
better plant fertilisers and in
medicine to detect tumours:
The radiation from the radioactive source is picked up above
the ground, enabling the leak in the pipe to be detected.

12. Uses of radioactivity 2
Rollers
Beta
emitter
Beta
detector
Paper

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

14. 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…
At start
there are 16
radioisotopes
After 1 half
life half have
decayed
(that’s 8)
After 3 half
lives another
2 have
decayed (14
altogether)
After 2 half
lives another
half have
decayed (12
altogether)
= radioisotope = new atom formed

15. A radioactive decay graph
Time
Count
1 half life

16. Dating materials using half-lives
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.
8
8
Answer: The sample was originally completely uranium…
…of the
sample was
uranium
4
8
2
8
1
8
Now only 4/8 of
the uranium
remains – the
other 4/8 is lead
Now only 2/8 of
uranium remains
– the other 6/8
is lead
Now only 1/8 of
uranium remains
– the other 7/8
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.
1 half life
later…
1 half life
later…
1 half life
later…

17. 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

18. Evidence about the origins of
the universe…

19. Source of
light “Spectra”

20. If you pass the light through a gas something
different is seen…
helium
Some wavelengths of light
are absorbed by the gas –
an “absorption spectrum”.

21. After
helium
If the light source is moving away the absorption
spectra look a little different…
helium
Before

22. The absorption lines have all been “shifted”
towards the longer wavelength end (red end)…
After
Before
A similar effect happens with sound –
this is called “The Doppler Effect”
Hear Doppler Effect
This is called red
shift. The faster
the light source
moves the further
its light will be
“shifted”

23. Light from different stars and from the edge
of the universe also shows this “red-shift”.
This suggests that everything in the universe
is moving away from a single point.
This is the BIG
BANG theory

24. Red shift summary
Light from other galaxies has a longer wavelength than
expected. This shows that these galaxies are moving away
from us very quickly. This effect is seen to a greater
extent in galaxies that are further away from us. This
indicates that the further away the galaxy is, the faster it
is moving.
This evidence seems to suggest that everything in the
universe is moving away from a single point, and that this
process started around 15 billion years ago. This is the big
bang Theory.

25. Observing the Universe
Consider different types of telescope:
Ground-based telescopes Space-based telescopes
What are the advantages and disadvantages of each?