National 5 Physics

Nuclear Radiation

Nuclear Particles

There are three types of ionising radiation alpha, α, beta, β and gamma, γ. The first two are both particles, whereas gamma is electromagnetic radiation.

Ionising Radiation

Alpha, beta and gamma radiation are types of ionising radiation. What does the term ionising mean? They are called ionising because they cause anything they come in contact with to be ionised. They are able to knock electrons out of atoms (which then become ions) that they come into contact with. This is because of their very high energy. This ability to ionise other materials is why they are called ionising radiation.

Penetrating Ability

The different types of ionising radiation are able to penetrate different materials to different depths.

Activity

The activity of radioactive materials is measured in Becquerels (Bq) named after Henri Becquerel who is credited with discovering radioactivity in 1896. One becquerel is equivalent to one nuclear disintegration in one second. The activity of a radioactive material can be determined using the following equation:

Example

A sample of radioactive material with a half life of 65 years is measured to have 12600 disintegrations in a time of 7 minutes. What is the activity of the sample?
N = 12600
t = 7 × 60 = 420 s

Interactive activity question with solution

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Background Radiation

In our daily lives we are exposed to ionising radiation from a variety of natural sources. Naturally occuring background radiation is by far much greater than the amount of radiation produced by human processes. the pie chart below shows that 88% of background radiation is from naturally occuring sources. Of the 12 % from human processes almost all of this is from medical sources such as x-rays, cancer treatment and sterilisation of equipment. The chart below shows the values by percentage.

The average amount of background radiation in the UK is about 2.2 milli sieverts per year, but this will depend on where people live. For example in parts of Cornwall where there is a lot of igneous rock the annual average background radiation is around 7.8 milli sieverts. For the National 5 examination it is important to remember 2.2 mSv as the average background radiation level.

Effect on living tissue

What effects do the different types of ionising radiation have on cells?
There are three possibilities for the effects of ionising radiation:

1. Ionises material in the cell and the cell repairs the damage
2. Ionises material in the cell causing mutation which may then cause cancer or other problems
3. Kills the cell

Where cells are damaged, any ionisation of DNA can result in mutation which then can be passed on to cells reproduced from the damaged cell. This is very important if the cells affected are sperm or egg cells, because the mutation would then affect every cell of any baby formed with the damaged cell.

To ensure the safety of people it is important to be able to measure levels of radiation. In many areas where ionising radiation is used people will wear radiation badges.

Measuring Radiation

The techniques for measuring radiation began with photographic film. Each time an ionising particle hit the photgraphic film a silver deposit was formed. The number of silver deposits could then be counted when the film was developed.

Film badges are worn by people who work in jobs with radioactive materials. These badges operate in the same way as film.

Geiger Muller Tube is a device which uses the ionising capability of the radiation to complete an electric circuit. Each ionising particle can therefore be counted and this is recorded by a counting circuit.

An animation of how a Geiger-Muller tube operates is shown below: (move the cursor over the animation to restart it)

A spark counter can be used with alpha radiation because of its very high ionising capability. Each time an alpha particle is detected a spark is generated at the metal grill. Since alpha particles do not travel far this is only useful for detection right next to the source.

A scintillation counter is a device which generates a flash of light when struck by ionising radiation.

A cloud chamber uses a saturated vapour (often chilled ethanol) which will allow vapour trails to form as ionising radiation passes through. The vapour is chilled using electronic refrigeration or dry ice. As an ionising particle or ray passes through the equipment a vapor trail shows their passage through the equipment. Below is an animation of the vapour trails formed by an ionising source on the left hand side of the cloud chamber.(move the cursor over the animation to restart it)

Measuring the effects on tissue

Absorbed Dose (D)

The amount of energy that ionising radiation deposits in organic material is measured in Joules per kilogram. The absorbed dose of 1 Gray (Gy) is defined as one Joule of energy deposited in one kilogram of tissue.

Example

What is the absorbed dose when a 70 kg adult absorbs 0.2 J of ionising radiation?

This measure of energy absorbed does not indicate the how damaging the radiation absorbed is because it does not take consideration of the ionising capability of the radiation.

Interactive absorbed dose question with solution

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Interactive energy question from absorbed dose with solution

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Equivalent Dose (H)

In order to take into consideration the amount of damage the different types of ionising radiation can have a weighting factor (w_r) is introduced to the equation.

The weighting factor gives an indication of just how damaging the specific type of radition is. In the National 5 examination this data will be found in the data sheet. The equivalent dose is measured in Sieverts

Example

Calculate the equivalent dose for a radiation worker who has been exposed to 0.5 µGy of slow neutron radiation.

D = 0.5 µGy, w_r = 3

Interactive equivalent dose question with solution

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Equivalent Dose Rate ()

The rate at which the radiation is absorbed is also an important factor to consider. Absorbing 50 mSv over a year is quite different from absorbing this dose in 1 second. Clearly it is much more dangerous when it is absobed over a shorter time period. In order to take this into consideration we use the equivalent dose rate (). The time unit is not specified in the equation because there are so many different time measurements made including second, minute, hour, day, year etc.

The equation connecting equivalent dose to equivalent dose rate is:

A table of typical dose rates and their effects can be seen below:

Example

A hospital technician absorbs on average 10 µSv of x-ray radiation per week. There are 45 working weeks in the year. Will the technician be below the radiation dose rate of 20 mSv y^-1?

H = 10 µSv per week
H = 10 × 45 = 450 µSv

This is less than the 20 mSv y^-1 and so the technician is below the annual limit for radiation workers.

Annual Dose Limits

For the National 5 examination the following information is required to be learnt:

• Annual UK Background Radiation 2.2 mSv.
• Annual effective dose limit for member of the public: 1 mSv.
• Annual effective dose limit for radiation worker: 20 mSv.

Appications of Nuclear Radiation

Electricity generation by nuclear power stations has been operational in the UK since 1956 when Calder Hall started generating. In Scotland there are currently two nuclear power stations, Torness Power Station near Dunbar and Hunterston B Power Station in West Kilbride.
There are 4 key components in a nuclear power station:

Cancer treatment is another major use of radioactive materials. Ionising radiation can both cause cancer and cure it. Doctors routinely use gamma radiation in a machine called a gamma knife to target tumors in places such as the brain.

Other medical uses include sterilising medical equipment with gamma rays. Radioactive tracers such as iodine-131 is another use of ionising radiation. This is specificaly used to determine issues with the thyroid gland.

Industrial uses include measuring the thickness of materials such as aluminium foil, detection of gas pipe leaks and smoke detectors. In agriculture many supermarket strawberries have been irradited by gamma radiation to increase their shelf-life.

Half-Life

All radioactive processes are completely random processes. This means that we cannot tell when a specific unstable nucleus will decay. When investigating nuclear decay it has been found that the time it takes to half the activity of a sample of radioactive material is independent of how much material there is to begin with. This leads to the concept of half-life which is defined as the time it takes for the activity of a sample to reduce to half the initial value. For the National 5 examination it is important to use the concept of activity, because any answers not using this concept will not get any marks.

It is possible to carry out an experiment to determine the half-life of a radioactive isotope. Using the isotope sodium-24 (²⁴Na), an experiment was carried out to determine it's half-life. The first part of the experiment was to determine the average background radiation in the laboratory. This was done by taking several readings over a suitable timeframe and averaging them. In this experiment the average background count was found to be 12 counts per minute. All of the other readings are in counts per minute (cpm). The following table of results was obtained:

As you can see there is the recorded experimental value for the number of counts per minute in the middle column. This has then been corrected for the background count of 12 cpm by subtracting this value to give the corrected count rate. The values of time and corrected count rate are then plotted on a graph. The time taken to go from 100 cpm to 50 cpm and then from 50 cpm to 25 cpm are then determined by drawing lines on the graph. A second set of lines showing the time taken to go from 60 cpm to 30 cpm and then from 30 cpm to 15 cpm are also given as shown below

The time taken to go from 100 cpm to 50 cpm is 15 hours
The time taken to go from 50 cpm to 25 cpm is 15 hours (30 - 15)
The time taken to go from 60 cpm to 30 cpm is 15 hours (26 - 11)
The time taken to go from 30 cpm to 15 cpm is 15 hours (41 - 26)

As you can see from the above results the time taken to half the activity does not depend on where you start from. The half-life is a constant value of 15 hours for this isotope of sodium.

Example

A radioactive isotope has a a half life of 2 hours. How long would it take to reduce the activity to one sixteenth of its original value?

Example

A radioactive source has an initial activity of 200 kBq. After 12 days the activity of the source is 25 kBq. What is the half-life of this radioactive source?

From the table we can see it takes 3 half lives to go from 200 kBq to 25 kBq. This took 12 days. Therefore we need to divide the time by the number of half-lives to find out the length on an individual half-life.
12 ÷ 3 = 4 days for the half-life.

Nuclear Fission

When one large nucleus spits into two smaller nuclei we call this nuclear fission. This can be a spontaneus process due to the instability of the nucleus. This type of fission is not desirable for generating electricity because the random nature of spontaneous fission is not controllable. The other type of fission process is induced nuclear fission. This is where a nuetron collides with a nucleus and causes it to split apart into two smaller nuclei whilst also releasing more neutrons. This process can be controlled and is used to generate electricity in nuclear power stations. Each nuclear fission of a nuceus produces a sgnificant amount of energy, which is collected as heat energy. The heat energy is then transferred through heat exchanger to produce steam which drives a turbine, generating electricity.

A process where one neutron is used to generate multiple neutrons from the fragmentation can lead to a chain reaction. The uncontrolled production of excess neutrons leads to a nuclear bomb, or the type of accident that occured at Chernobyl in the Ukraine. In a nuclear power station the control rods, made of boron, remove excess neutrons and therfore keep the fission process controlled. The neutrons released from the fission of uranium-235 are moving too rapidly to cause effective fission and so in a nuclear power station a moderator made from graphite is used to slow these nuetrons down so that they cause further fission reactions.

Remember control rods made of boron, control the number of neutrons present.
The graphite moderator slows donw neutrons so they can cause further fission reactions.

Advantages of Nuclear Fission for Electricity Generation

1. Unlike fossil fuels, nuclear fuels do not produce carbon dioxide or sulphur dioxide
2. In terms of energy produced: 1kg of nuclear fuel = 2,900,000 kg of coal
3. Since comparatively small volumes of fuel are used, this can be easily transported by road or rail
4. Waste product produced in small volumes

Disadvantages of Nuclear Fission for Electricity Generation

1. Like fossil fuels, nuclear fuels are non-renewable energy resources
2. If there is an accident, large amounts of radioactive material could be released into the environment (highly unlikely with modern design)
3. Nuclear waste remains radioactive and is hazardous to health for thousands of years and it must be stored safely

Nuclear Fusion

Where two small nuclei join together to form one larger nucleus the process is called nuclear fusion. This is because tw nuclei are being fused together to form one new nucleus. This process is the nucler process going on in our Sun. Each time a pair of nuclei fuse together a small amount of mass is converted into energy according to Einstein's famous equation:

Experiments are being carried out here on Earth to try and reproduce the process in a workable reactor which would then be used to generate electricity. The reaction takes place at over 1 million ° celcius, and so the reaction has to be kept from touching any surface. The reactor shape is of a hollow doughnut. The gas plasma is heated to over 1 million Kelvin for fusion to take place.

Above is the fusion of tritium (H-3) with deuterium (H-2) to form helium (He) and a neutron.

Producing ectricity by this method would mean that there was an abundant source of energy with no waste product. The difficulty is containing the extremely high temperature gas contained in a suitable reaction vessel, especialy with the incredibly high temperatures required. To date any electricity produced has been less than the elecricity required to get the process running.