Radiation and the Universeb.Radioactivity

Mr S Thompson

The structure of the atomELECTRON –

negative, mass nearly

nothing

PROTON – positive,

same mass as neutron

(“1”)

NEUTRON – neutral,

same mass as proton

(“1”)

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)

IsotopesAn 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.

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”.

Types of radiation1) 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

IonisationRadiation 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.

Blocking RadiationEach type of radiation can be blocked by different materials:

Sheet of paper

Few mm of aluminium

Few cm of lead

Deflection by Magnetic Fields

Alpha and beta particles have a charge:

++

-

2 protons, 2 neutrons, therefore charge =

+21 electron, therefore

charge = -1

Because of this charge, they will be deflected by electric and magnetic fields:

+

-

+

Background Radiation

Radon gas

Food

Cosmic rays

Gamma rays

Medical

Nuclear power

13% are man-made

Uses of radioactivity1) 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.

Uses of radioactivity 2

Rollers

Beta emitter

Beta detector

Paper

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

Half lifeThe 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 radioisotope

s

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

A radioactive decay graph

Time

Count

1 half life

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

8Now 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

leadSo 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…

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

Evidence about the origins of the universe…

Source of light “Spectra

”

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”.

After

helium

If the light source is moving away the absorption spectra look a little different…

helium

Before

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”

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

Red shift summaryLight 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.

Observing the UniverseConsider different types of telescope:

Ground-based telescopes

Space-based telescopes

What are the advantages and disadvantages of each?