The Nuclear Atom

To understand radioactivity we have to understand nuclear physics. To understand nuclear physics we have to understand the nucleus.

• It’s part of an atom.
• Atoms are made up a nucleus and electrons.
• The nucleus is made up of protons and neutrons.

But how do we know this?

Task:

Research and answer the following question: Ernest Rutherford hypothesised that atoms are mostly empty space but have a dense central nucleus. He used a gold foil scattering experiment to prove it. Describe how his experiment was set up, and what his evidence was to verify his hypothesis.

We know about what is in the nucleus of an atom from its symbol representation...

We can use the periodic table to help us work out the nucleon number (atomic number) and proton number (mass number) for different atoms.

Isotopes

Nuclei of the same element must have the same number of protons, but they can have different numbers of neutrons.

The three isotopes of hydrogen...

Now you try it... Worksheet, Solutions

Physics for You pp352-353

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Radioactive Particles

There is a new force that overcomes all this...

The strong nuclear force (SNF). The SNF is very strong, but very short range.

How do we know that the SNF exists? What does CERN stand for? What does it do?

Imagine a large nucleus for an element such as Uranium .

How many protons are there?

And how many neutrons?

The protons that are close together are held in place by the SNF between themselves and the neutrons. BUT…

The protons that are at opposite ends of the nucleus are too far apart for the SNF to reach between them. In this case the electric force takes over. Now the protons at opposite ends of the nucleus repel each other and the nucleus becomes unstable.

Question:

How can the nucleus become more stable?

Emit some protons and become smaller, thus more stable –This is called Alpha decay.

Alpha Decay

In alpha decay a helium nucleus is emitted, so the nucleon number changes by…

And the proton number changes by…

Complete the following nuclear equations for alpha decay…

Beta Decay

Sometimes a nucleus can have too many neutrons and need to convert a neutron into a proton. If it was neutral to start with, then turns into a proton, what else must it produce, so that the charge balances? This is called beta decay.

If beta decay happens, how much does the nucleon number change by?

What about the proton number?

Complete the following nuclear equations for beta decay…

Gamma Decay

 Some nuclei just have too much energy, and need to get rid of some to become more stable. They do this by emitting a gamma wave.

Question: If a typical gamma ray has a frequency of f≈10²²Hz. What is the wavelength of gamma radiation?

Emitting gamma radiation doesn’t change the nuclear structure of an atom.

Now find out more about the different types of radioactive particles… worksheet.

Physics for You pp 355-357

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Half Life

Activity is all about how many nuclei in a radioactive sample decay in a second. This is called the Bequerel (Bq).

Radioactive decay is a totally random event. You can never predict whether or when exactly an unstable nucleus will decay. It's just like throwing a 6 when rolling dice (we'll use this analogy in a minute).

Because we cannot predict when a nucleus will decay, the best we can do is use probability to suggest how likely it is to happen.

We can say how long it will take for half of a sample of radioactive nuclei to decay. This is called it's half life. This should be the same for any quantity of a given radioactive element, but will differ from one element to another.

When we look at the decay of a radioactive sample, we get a graph that is exponential. It tends to but never actually reaches zero. This is why we must use half life as a measurement of decay rather than "full life".

 

Task:

Now do this experiment with dice to recreate a radioactive decay curve, and thus work out the half life of your sample of dice.

A nice applet of the decay of a radioactive sample, illustrating half life.

Physics for You pp354, 256-7, 362, questions q1 and 2 pp363.

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Detection and Safety of Radioactive Particles

Radioactive particles are difficult to detect without specialist equipment. We use the fact that radioactive particles ionise the other particles around them when we detect them.

Ionization is when a neutrally charged particles gains or loses electrons so as to become charged, and thus form an ion.

Both the Spark Counter, and the GM Tube rely on this principle to count the number of radiations per second.

Background Radiation

Notice that the GM tube gives a count even when there is no radiation near it? This is because there is radiation all around us. Most things are radioactive to a very small degree, and this causes part of what we call the backround radiation.

The main contributor to the background radiation though was the Big Bang! Ref: IB Physics

Penetrating Potential

This is how far each type of radiation will penetrate different types of materials (ie: what can they get through)

 

Now compile a table to summarise the penetrating potential and ionisation of each of the different radioactive particles...

Particle	Penetrating Potential	Ionisation
Alpha (a)	Stopped by paper, skin of a few cm of air.	Strongly ionises air and other media
Beta (b)	Stopped by thin non-dense metal (eg: Aluminium)	Weakly ionises air and other media
Gamma (g)	Very difficult to stop. Dense materials like lead are used to reduce it's penetration but cannot stop it completely	Very weakly ionises air and other media

Safety from Radiation

You have found out a little about what stops radioactive particles (eg:Lead). We must use this information to protect ourselves from radioaction that we may expose ourselves to everyday (eg: Hospital radiographers retreat behind lead screens when taking photographs, photographic film badges, etc)

Tasks:

Write a paper explaining either the dangers of exposure to ionizing radiation (this should link to what you know about cellular biology and DNA), or the uses of ionizing radiation in archeological dating and medical radiotherapy.

Challenging: How does the IAEA protect us from radiation?

Physics for You pp348-365

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Nuclear Power

Possibly the most important use for radioactive elements is in harnessing the energy their reactions give out to generate electricity. At the moment we do this using nuclear fission of Uranium.

We can split up a big (unstable) nucleus by bombarding it with small particles (like electrons).

The masses of the products (Krypton, Barium and 3 neutrons) is less that the mass of the reactants (Uranium and a neutron). Where does this mass go?

E=mc² - The mass gets converted into energy!

Question:

How much energy would we get if we converted 1g of mass into energy?

Hint: The speed of light is 300000000m/s.

 

This is what happens in a nuclear power station, only there is a chain reation...

Task:

Draw a schematic to show how a standard fossil fuel power station works.

Nuclear power stations work in a similar way to those powered by fossil fuels in that they are both ‘thermal power stations’. They both use their fuels to produce thermal energy, which evaporates and pressurizes steam, which in turn drives a turbine.

 

The difference between the fossil and nuclear power stations is how they produce the thermal energy to evaporate and pressurize the steam. Nuclear power stations do this in the reactor. There are different designs of nuclear reactor, but they have some shared ‘critical components’. These are:

• Fuel rods
• Moderator
• Coolant
• Control rods
• Containment building

Task:

1. Watch the video on nuclear power

2. Explain, using the diagram to see how it all fits together, the role of each of these ‘critical’ components.

3. What sort of things can go wrong? Investigate the Chernobly disaster in 1986. What are the long term consequences of the disaster?

Physics for You pp 348-349, questions 1-7 pp151.

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