Background radiation
When unstable atoms give off particles that can be harmful to humans. materials occur naturally and, as a result, everyone is exposed to a low-level of radiation every day. This exposure comes from a mixture of natural and man-made sources.
The actual amount of Energy carried by particles from a radioactive substance, or spreading out from a source. that a person is exposed to depends on where they live, the job they do and many other factors.
Low level nuclear radiation that is always present from natural and man-made sources, eg cosmic rays from the Sun, radioactive rocks in the Earth's crust, radioactive radon gas in the air. is low level nuclear radiation that is always present from natural and man-made sources eg cosmic rays from the Sun. Scientists must always take into consideration the amount of background radiation when working or experimenting with radioactive sources and discount it from their results.
Background radiation affects everyone mainly by Process of exposing an object to a source of radiation. Eg fruit exposed to gamma rays in order to destroy bacteria is said to have been irradiated., but a small amount is from being contaminated by An isotope of an atom of an element that releases ionising radiation. Isotopes of an element have more or fewer neutrons than each other. Also called a radioactive isotope. in the food and drink that is consumed.
Measuring amounts of radiation
The activity of a radioactive source is the number of decays per second from the unstable The nucleus controls what happens inside the cell. Chromosomes are structures found in the nucleus of most cells. The plural of nucleus is nuclei. present in the source.
Radioactivity can be measured in counts per second or per minute.
Since The process in which unstable atomic nuclei break apart or change, releasing radiation as they do so. is a random process, it is always good practice to determine the average count rate rather than to measure the counts that occur in just one second or one minute.
Radioactivity can be detected using a Geiger-Muller tube connected to a counter.
When alpha (\(\alpha\)) particles, beta (\(\beta\)) particles or gamma (\(\gamma\)) rays enter the GM tube the counter clicks and the count is displayed on the screen.
The number of counts per second or per minute is called the count rate.
Measuring the background radiation
Remove all known sources of radioactivity from the room.
Set the counter to zero.
Switch on and start a stop clock.
After 20 minutes switch off. Record the count.
Divide the count by 20 to calculate the count rate per minute.
The background count rate is measured over a period of 20 minutes because of the random nature of radioactive decay.
Dividing by 20 enables the average count rate per minute to be determined.
Background count rate is typically 18 counts per minute which does not present a serious health risk to humans.
Extended syllabus content: Determining a correct count rate
If you are studying the Extended syllabus, you will also need to know about how to determine a corrected count rate. Click 'show more' for this content:
If a radioactive source is being measured, a more accurate value can be determined if the background radiation in the area is taken away from the reading. This is called a corrected count rate.
How radiation can be detected
The ionising effect of radiation is used in the Geiger-Muller (GM) tube as a means of detecting the radiation.
The GM tube is a hollow cylinder filled with a gas at low pressure. The tube has a thin window made of mica at one end. There is a central electrode inside the GM tube. A high voltage supply is connected across the casing of the tube and the central electrode as shown in the following diagram.
When alpha (\(\alpha\)), beta (\(\beta\)) or gamma (\(\gamma\)) radiation enters the tube it produces ions in the gas. The ions created in the gas enable the tube to conduct. A current is produced in the tube for a short time. The current produces a voltage pulse. Each voltage pulse corresponds to one ionising radiation entering the GM tube. The voltage pulse is amplified and counted.
The greater the level of radiation, the more ionisation in the tube so the greater the number of counts.
The GM tube counting the number of ionisations may not provide a completely accurate reading, as the number of counts will simply keep increasing.
The quantity activity gives an indication of how radioactive a substance is. Activity is the number of radioactive atoms which disintegrate and emit radioactivity per second.
Activity is measured in units called Units of activity. Usually shortened to Bq. (Bq) named after Henri Becquerel, the French scientist.
\(Activity = \frac{number~of~disintegrations}{time~taken}\)
\(A = \frac{N}{t}\)
Time is measured in seconds \((s)\). The number of disintegrations has no units.
The number of disintegrations cannot be determined easily in practical work, but the count of radioactive particles detected by a Geiger Muller counter is a useful approximation at this level, and can give an indication of the rate of change of activity.
Video: Measuring the activity of a radioactive sample
Watch this video to see the procedure for measuring the activity of a radioactive sample, taking into account the background radiation.
Extended syllabus content: Background radiation measurements
If you are studying the Extended syllabus, you will also need to know how to use measurements of background radiation to determine a corrected count rate. Click 'show more' for this content:
Before the source is used the background count rate is measured using a Geiger Muller tube connected to a counter. The count rate from the source is then measured at regular fixed intervals over a period of time.
The background count rate is subtracted from each measurement of the count rate and so the actual count rate from the source is calculated (known as the 'corrected count rate'). An example of this is shown in the table below.
| Time (hours) | Corrected count rate (counts per minute) |
|---|---|
| 0 | 200 |
| 1 | 110 |
| 2 | 57 |
| 3 | 37 |
| 4 | 20 |
| 5 | 13 |
The three types of nuclear emission
The The production and release of something. of radiation from a nucleus is Sudden and without prior thought. and random in direction. Three types of ionising radiation are:
Alpha particle
A type of ionising radiation consisting of 2 protons and 2 neutrons. particle \(\alpha\)- is a helium nucleus, two protons and two neutrons. It has a large mass, compared to other ionising radiations, and a strong positive charge.
Beta particle
A type of ionising radiation consisting of a single electron. particle \(\beta\)- is a fast-moving electron. It has a very small mass and a negative charge.
Gamma ray
A The shortest wavelength and highest energy part of the EM spectrum. Produced by radioactive materials. \(\gamma\)- is a high-energy electromagnetic wave. Gamma rays are caused by changes within the nucleus. They are part of the electromagnetic spectrum and so travel at the speed of light. They have no mass and no charge.
Penetrating power
Each type of radiation has a different ability to penetrate materials. The material is said to have absorbed the radiation.
The energy of the three radiations is absorbed by the material through which the radiation passes. The amount of energy which is absorbed depends on the type of radiation and the type of the absorbing material.
The range of the alpha radiation in an absorbing material is less than that of beta or gamma. The alpha radiation transfers more energy to an absorber than beta or gamma radiation. Alpha radiation is absorbed by the thickness of the skin or by a few centimetres of air.
Beta radiation is more penetrating than alpha radiation. It can pass through the skin, but it is absorbed by a few centimetres of body tissue or a few millimetres of aluminium.
Gamma radiation is the most penetrating of the three radiations. It can easily penetrate body tissue. It requires several centimetres of lead or about one metre of concrete to absorb it.
| Radiation | Range (cm) | Ionising power | Can pass through paper? | Can pass through 5mm of aluminium? | Can pass through 5cm of lead? |
|---|---|---|---|---|---|
| Alpha | 3-5 | Highly ionising | No | No | No |
| Beta | About 15 | Ionising | Yes | No | No |
| Gamma | Much longer | Weakly ionising | Yes | Yes | No - although some will still get through |
Extended syllabus content: Deflection of alpha, beta and gamma radiation
If you are studying the Extended syllabus, you will also need to describe the deflection of alpha, beta and gamma radiation. Click 'show more' for this content:
| Radiation | Electric and magnetic deflection |
|---|---|
| Alpha | Deflected towards negative plate in electric fields and upward in magnetic fields |
| Beta | Deflected towards positive plate in electric fields and downwards in magnetic fields |
| Gamma | Not deflected in electric nor magnetic fields |
Extended syllabus content: Ionising effects
If you are studying the Extended syllabus, you will also need to know about Ionising effects. Click 'show more' for this content:
The effect of ionisation that alpha, beta and gamma emissions can have depends upon their kinetic energy and electrical charge. Even though alpha particles are slow, they are the most ionising because they have the greatest mass and the most charge (+2). Beta particles are faster but have significantly less mass and have a -1 charge, so they are less ionising. Gamma emissions have no mass and no charge so are the least ionising.
Radioactive decay
Radioactive decay is a change in an unstable nucleus that can result in the emission of alpha particles or beta particles and/or gamma radiation. These changes are spontaneous and random.
A nucleus changes into a new element by emitting alpha or beta particles. These changes are described using nuclear equations.
Extended syllabus content: Decay equations
If you are studying the Extended syllabus, you will also need to know about decay equations. Click 'show more' for this content:
Alpha decay (two protons and two neutrons) changes the nucleon number of the element, decreasing by four and decreasing the proton number by two. An alpha particle is the same as a helium-4 nucleus.
Example
\(_{86}^{219}\textrm{Rn} \rightarrow _{84}^{215}\textrm{Po} + _{2}^{4}\textrm{He}\)
Beta decay changes the proton number, increasing by one (the nucleus gains a proton) but the nucleon number remains unchanged (it gains a proton but loses a neutron by ejecting an electron, so a beta particle is an electron).
Example
\(_{6}^{14}\textrm{C} \rightarrow _{7}^{14}\textrm{N} + _{-1}^{0}\textrm{e}\)
A type of ionising radiation that is also part of the EM spectrum. It has no mass. is pure energy and will not change the structure of the nucleus in any way.
Half-life
The process in which unstable atomic nuclei break apart or change, releasing radiation as they do so. is a random process. A block of When unstable atoms give off particles that can be harmful to humans. material will contain many trillions of The nucleus controls what happens inside the cell. Chromosomes are structures found in the nucleus of most cells. The plural of nucleus is nuclei. and not all nuclei are likely to decay at the same time, so it is impossible to tell when a particular nucleus will decay.
It is not possible to say which particular nucleus will decay next but given that there are so many of them, it is possible to say that a certain number will decay in a certain time. Scientists cannot tell when a particular nucleus will decay but they can use statistical methods to tell when half the unstable nuclei in a sample will have decayed. This is called the The time it takes for the number of nuclei of a radioactive isotope in a sample to halve. Also defined as the time it takes for the count rate from a sample containing a radioactive isotope to fall to half its starting level..
Half-life is the time it takes for half of the unstable nuclei in a sample to decay or for the activity of the sample to halve or for the count rate to halve.
Count rate is the number of decays recorded each second by a detector, such as the Geiger-Muller tube. This also known as the Activity of the source. One decay per second is known as one Becquerel (Bq).
The illustration below shows how a radioactive sample is decaying over time.
From the start of timing, it takes two days for the count to halve from 80 down to 40. It takes another two days for the count rate to halve again, this time from 40 to 20.
Note that this second two-day period does not see the count drop to zero, only that it halves again. A third, two-day period from four days to six days see the count rate halving again from 20 down to 10.
This process continues and although the count rate might get very small, it does not drop to zero completely.
The half-life of radioactive carbon-14 is 5,730 years. If a sample of a tree (for example) contains 64 grams (g) of radioactive carbon after 5,730 years it will contain 32 g, after another 5,730 years that will have halved again to 16 g.
Extended syllabus content: Calculations using half-life
If you are studying the Extended syllabus, you will also need to know about calculations using half-life. Click 'show more' for this content:
It should be possible to state how much of a sample remains or what the activity or count should become after a given length of time. This could be stated as a fraction, decimal or ratio.
For example, the amount of a sample remaining after four half-lives could be expressed as:
A fraction – a \(\frac{1}{2}\) of a \(\frac{1}{2}\) of a \(\frac{1}{2}\) of a \(\frac{1}{2}\) remains which is \(\frac{1}{2}\) × \(\frac{1}{2}\) × \(\frac{1}{2}\) × \(\frac{1}{2}\) = \(\frac{1}{16}\) of the original sample.
A decimal – \(\frac{1}{16}\) = 0.0625 of the original sample.
A ratio – given in the form 'activity after n half-lives : initial activity'. In this case 1:16.
This could then be incorporated into other data. So if the half-life is two days, four half-lives is 8 days. If a sample has a count rate of 3200 Becquerel (Bq) at the start, its count rate after 8 days would be \(\frac{1}{16}\)th of 3200 Bq = 200 Bq.
Example
The half-life of cobalt-60 is 5 years. If there are 100 g of cobalt-60 in a sample, how much will be left after 15 years?
15 years is three half-lives so the fraction remaining will be \(( \frac{1}{2})^{3}\) = \(\frac{1}{8}\) = 12.5 g.
As a ratio of what was present originally compared to what was left, this would be 100:12.5 or 1:0.125.
Question
What is the half-life of a sample where the activity drops from 1200 Bq down to 300 Bq in 10 days?
Answer
Half of 1200 is 600, half of 600 is 300. So it takes two half-lives to drop from 1200 Bq to 300 Bq, which is 10 days. Therefore, one half-life is five days.
Example
Technetium-99 is used in medicine and emits gamma rays. The original sample has a measured activity of 2000 Bq. Over a twelve-hour period this has dropped to 500 Bq. What is the half-life?
After one half-life 2000 Bq drops to 1000 Bq. After another half-life 1000 Bq drops to 500 Bq. So there have been two half lives in the twelve-hour period. This means one half-life is six hours.
Tip: It is important to subtract background radiation from half-life graphs or data if it has not already been removed. This gives a more accurate result.
Irradiation
Extended syllabus content: Types of radiation emitted
If you are studying the Extended syllabus, you will also need to know about the types of radiation emitted and how the half-life of an isotope determines which isotope is used for applications. Click 'show more' for this content:
Exposing objects to beams of radiation is called Process of exposing an object to a source of radiation. Eg fruit exposed to gamma rays in order to destroy bacteria is said to have been irradiated.. The term applies to all types of radiation including radiation from the The nucleus controls what happens inside the cell. Chromosomes are structures found in the nucleus of most cells. The plural of nucleus is nuclei. of atoms.
Irradiation from The process in which unstable atomic nuclei break apart or change, releasing radiation as they do so. can damage living cells. This can be put to good use as well as being a hazard.
Irradiation for sterilisation
Irradiation can be used to preserve fruit sold in supermarkets by exposing the fruit to a radioactive source - typically cobalt-60. The The shortest wavelength and highest energy part of the EM spectrum. Produced by radioactive materials. emitted by the cobalt will destroy any bacteria on the fruit but will not change the fruit in any significant way. The process of irradiation does not cause the irradiated object to become radioactive.
Medical irradiation
Doctors also use radioactive sources for a number of reasons, eg:
sterilisation of surgical instruments
beams of gamma rays, called a gamma knife, can be used to kill cancerous tumours deep inside the body
These beams are aimed at the tumour from many different directions to maximise the dose on the tumour but to minimise the dose on the surrounding soft tissue. This technique can damage healthy tissue, so careful calculations are done to establish the best dose - enough to kill the tumour, but not so much so that the healthy tissue is damaged.
In medical applications that involve using radioactive sources, efforts are made to ensure that irradiation does not cause any long-term effects. This is done by considering:
the nature of decay (alpha, beta or gamma)
the The time it takes for the number of nuclei of a radioactive isotope in a sample to halve. Also defined as the time it takes for the count rate from a sample containing a radioactive isotope to fall to half its starting level. (long enough for the isotope to produce useful measurements, but short enough for the radioactive sources to decay to safe levels soon after use)
Poison level.
If the half-life chosen is too long, the damaging effects of the radiation would last for too long and the dose received would continue to rise.
Advantages and disadvantages of irradiation
Advantages
- sterilisation can be done without high temperatures
- it can be used to kill bacteria on things that would melt
Disadvantages
- it may not kill all bacteria on an object
- it can be very harmful - standing in the environment where objects are being treated by irradiation could expose people’s cells to damage and A random and spontaneous change in the structure of a gene, chromosome or number of chromosomes.
Uses of radiation
Radioactive materials are moved, used and stored in a safe way.
Uses of radioactivity in the home: smoke detectors
One type of smoke detector uses Americium-241, an alpha particle source, to detect smoke.
The alpha particles pass between the two charged metal plates, causing air particles to ionise (split into positive and negative ions).
The Electrically charged particle, formed when an atom or molecule gains or loses electrons. are attracted to the oppositely charged metal plates causing a current to flow.
When smoke enters between the plates, some of the alpha particles are absorbed causing less ionisation to take place.
This means a smaller than normal current flows so the alarm sounds.
An alpha source is used because alpha radiation does not penetrate very far.
It is absorbed by a few centimetres of air.
This means that as long as the detector is high up on a wall, or the ceiling, it is safe for humans to be in the same room.
The source should have a long half-life so that the smoke detector does not have to be replaced too frequently, and so that the count rate remains almost constant each day.
Uses of radioactivity in agriculture: food preservation
Gamma radiation from a radioactive source such as cobalt-60 can be used to kill bacteria on fresh food and enable it to keep for longer.
Harmful bacteria, moulds and yeasts grow on foods, and can be killed by gamma rays.
This helps to prolong the shelf life of fresh fruit, vegetables, spices, fish and chicken, reduces food poisoning and food waste, and helps to keep prices down.
Using radiation in this way does not make the food radioactive as the food itself never comes into contact with the radioactive source.
It is the energy from the radioactive source in the form of gamma rays that kills the bacteria that can cause food poisoning in a similar way that heat energy kills bacteria when food is heated.
Once the radiation treatment has stopped, the food quickly loses this energy in the same way that cooked food quickly cools down.
Other uses of radiation in agriculture include:
Plant seeds can be exposed to radiation to produce new, stronger and better plants.
Radiation can be used to control the number of dangerous insects. This decreases the use of dangerous pesticides.
Radioactive sources are used in machines that measure the thickness of eggshells to screen out thin, breakable eggs before they are packaged in egg cartons.
Many of our foods are packaged in polyethylene film (also known as shrink wrap). This is exposed to radiation so that it can be heated above its usual melting point and wrapped around foods to provide an airtight protective covering.
Uses of radiation: in industry
To control the thickness of metal or paper
Radioactive sources are used in industry to control the thickness of metal or paper as it is rolled into thin sheet.
An Material that sends out energy. is placed on one side of a sheet and a detector on the other.
If the thickness of the sheet remains constant the activity will not change.
If there is a change in thickness, the activity increases or decreases.
This can trigger the rollers to squeeze harder or less hard to maintain the correct thickness.
A beta source is used because beta radiation can penetrate paper or thin aluminium, but the amount of penetrating will vary sufficiently as thickness changes.
The source should have a long The time it takes for the number of nuclei of a radioactive isotope in a sample to halve. Also defined as the time it takes for the count rate from a sample containing a radioactive isotope to fall to half its starting level. so that the count rate remains almost constant each day and so that it does not need to be replaced too frequently.
The effects of radiation on the human body
When unstable atoms give off particles that can be harmful to humans. materials are hazardous. Nuclear radiation can To ionise is to convert an uncharged atom or molecule into a charged particle by adding or removing electrons. chemicals within a body, which changes the way the cells behave. It can also deposit large amounts of energy into the body, which can damage or destroy cells completely.
Some of the effects that radiation has on a human body are shown below.
| Body part | Effect |
|---|---|
| Eyes | High doses can cause cataracts. |
| Thyroid | Radioactive iodine can build up and cause cancer, particularly during growth. |
| Lungs | Breathing in radioisotopes can damage DNA. |
| Stomach | Radioactive isotopes can sit in the stomach and irradiate for a long time. |
| Reproductive organs | High doses can cause sterility or mutations. |
| Skin | Radiation can burn skin or cause cancer. |
| Bone marrow | Radiation can cause leukaemia and other diseases of the blood. |
Managing the risks
The risk associated with radioactive materials depends on the amount of exposure. Being exposed to highly radioactive materials or being exposed to radioactive materials for long periods of time or on a regular basis increases the dose received which, in turn, increases the risk.
Safety precautions
Given that radioactive materials are hazardous, certain precautions can be taken to reduce the risk of using radioactive sources. These include:
Keep radioactive sources like technetium-99 shielded (preferably in a lead-lined box) when not in use.
Only move them when absolutely necessary and following strict protocols.
Wear protective clothing to prevent the body becoming contaminated should radioactive isotopes leak out.
Avoid contact with bare skin and do not attempt to taste the sources.
Wear face masks to avoid breathing in materials.
Handle radioactive materials with tongs in order to keep a safer distance from sources.
Monitor exposure using detector badges, etc.
Extended syllabus content: Safety precautions extended
If you are studying the Extended syllabus, you will also need to be able to explain safety precautions for all ionising radiation. Click 'show more' for this content:
Working safety with radiation also means reducing the exposure time, increasing the distance between the source and the person using it, and using shielding to absorb radiation.
Quiz
Test your knowledge with this quiz on radioactive decay.
Teaching resources
Are you a physics teacher looking for more resources? Share this short video with your students, in which science presenter Jon Chase explains some of the uses of radioactivity and how unstable nuclei decay release ionising radiation:
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