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P6 Radioactive P6 Radioactive MaterialsMaterials

M Barker

Shirebrook Academy

OCR 21OCR 21stst Century Century

21/04/23P6.1 Why are some materials P6.1 Why are some materials radioactive?radioactive?

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The structure of the atomThe structure of the atomELECTRON –

negative, mass nearly

nothing

PROTON – positive,

same mass as neutron

(“1”)

NEUTRON – neutral,

same mass as proton

(“1”)

21/04/23Introduction to Introduction to RadioactivityRadioactivity

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.

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Background RadiationBackground Radiation

Radon gas

Food

Cosmic rays

Gamma rays

Medical

Nuclear power

13% are man-made

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Structure of the atomStructure of the atom

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…

I did an experiment (with my colleagues Geiger and Marsden)

that proved this idea was wrong. I called it the “Scattering

Experiment”

Ernest Rutherford, British scientist:

21/04/2321/04/23The Rutherford Scattering The Rutherford Scattering ExperimentExperiment

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”.

21/04/23What keeps a nucleus What keeps a nucleus together?together?

+

+

+++

-

The “strong force”

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Nuclear Fusion in starsNuclear Fusion in starsProton NeutronNuclear fusion happens in stars when hydrogen nuclei

are brought close enough together:

Einstein (1879-1955)

We can calculate how much energy this reaction releases using my famous E=mc2

equation.

21/04/23IsotopesIsotopesAn 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.

21/04/23Types of radiationTypes of radiation1) Alpha () – an atom decays into a new atom and emits an alpha particle (2 protons and 2 ______ – the nucleus of a ______ atom)

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.

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.

Unstable nucleus

Unstable nucleus

Unstable nucleus

New nucleus

New nucleus

New nucleus

Alpha particle

Beta particle

Gamma radiation

Words – frequency, proton, energy, neutrons, helium, beta

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Blocking RadiationBlocking RadiationEach 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

21/04/23Changes in Mass and Proton Changes in Mass and Proton NumberNumber

Alpha decay:

237

93

4

2Am

241

95Np α+

90

39

0

-1Sr

90

38Y β+

Beta decay:

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A radioactive decay graphA radioactive decay graph

Time

Activity (Bq) “1 Becquerel” means “1 radioactive count per

second”

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Half lifeHalf 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 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

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A radioactive decay graphA radioactive decay graph

Time

Count

1 half life

1 half life

1 half life

21/04/23P6.2 Using Radioactive P6.2 Using Radioactive MaterialsMaterials

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IonisationIonisationRadiation 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.

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Uses of Radioactivity 1Uses of Radioactivity 1Sterilising 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.

21/04/2321/04/23Uses of Radioactivity 2 - Uses of Radioactivity 2 - TracersTracers

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 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.

For medicinal tracers, you would probably use a beta source with a short half life – why?

21/04/2321/04/23Uses of Radioactivity 3 - Treating Uses of Radioactivity 3 - Treating CancerCancer

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?

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Exposure to RadiationExposure to RadiationPeople 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).

21/04/23Background Radiation by Background Radiation by LocationLocation

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

21/04/23How we are exposed to How we are exposed to RadiationRadiation

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

21/04/23Nuclear fissionNuclear fission

Uranium nucleus

Unstable

nucleus New nuclei (e.g. barium

and krypton)

More neutron

s

Neutron

21/04/23Chain reactionsChain reactions

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

21/04/23Fission in Nuclear power Fission in Nuclear power stationsstations

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.

21/04/23Disposing of radioactive Disposing of radioactive wastewaste

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 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:

The containers are then kept in stores, often _________.

Words – glass, steel, underground, radioactive