Physics · 5 Nuclear physics

5.2.1 Detection of radioactivity

Before you start

An atom has a small nucleus of protons and neutrons, with electrons outside it (5.1).
A rate is an amount divided by the time it took, in the same way that speed is distance divided by time.
There are 60 seconds in a minute, so a 'per minute' value is 60 times the 'per second' value.

Syllabus objectives covered (5)

Know what is meant by background radiation
Know the sources that make a significant contribution to background radiation including: (a) radon gas (in the air) (b) rocks and buildings (c) food and drink (d) cosmic rays
Know that ionising nuclear radiation can be measured using a detector connected to a counter
Use count rate measured in counts / s or counts / minute
Use measurements of background radiation to determine a corrected count rate

Part 1 of 5

Background radiation is always there

Some nuclei are unstable. They emit radiation from the nucleus, and this radiation is ionising: it has enough energy to knock electrons out of the atoms it passes, which turns those atoms into ions.
Ionising radiation is not only found near a radioactive source in a laboratory. A small amount of it is passing through you, and through every room, all the time.
Background radiation is the ionising radiation that is present in the environment all the time, from natural and artificial sources. It is there even when there is no radioactive source anywhere near you.
This has a practical result that the rest of this lesson depends on. A radiation detector never reads zero. Switch it on in an empty room and it still records counts, and every one of those counts is background radiation.
The level of background radiation is not the same everywhere. It changes from place to place, because the sources of it, described next, are not spread evenly.

Part 2 of 5

Where background radiation comes from

The syllabus names four sources that make a significant contribution. Learn all four by name, and learn one reason for each, because a question can ask you to explain a difference between two places.
Radon gas (in the air). Radon is a radioactive gas. It is produced when uranium in rocks and soil decays, and it seeps out of the ground into the air. Indoors it can build up, especially in rooms with poor ventilation such as cellars.
Rocks and buildings. Some rocks, such as granite, contain radioactive isotopes. Stone, brick and concrete are made from rock material, so the walls of a building can emit radiation too.
Food and drink. Plants take in radioactive isotopes, such as potassium-40 from the soil and carbon-14 from the air, so food and drink contain small amounts of radioactive material. Because you eat and drink it, your own body contains radioactive isotopes.
Cosmic rays. These are high-energy particles that reach the Earth from space, including from the Sun. The atmosphere absorbs much of this radiation, so there is more of it at high altitude, on mountains and in aircraft, where there is less air above you.
There are also artificial sources, such as medical X-rays and waste from the nuclear industry. They are part of background radiation, but the four sources above are the ones the syllabus requires you to name.
The trap here is naming the place instead of the source. 'The ground' is vague. 'Radon gas released from rocks and soil' or 'radioactive isotopes in granite' names the actual source.

Part 3 of 5

Detecting radiation: a detector connected to a counter

You cannot see or feel ionising radiation, so you measure it with two pieces of equipment that work together: a detector and a counter.
The detector responds to the radiation. One detector is the Geiger-Müller tube (G-M tube). It contains a gas at low pressure. When ionising radiation enters the tube, it ionises some of the gas, and this causes a short pulse of electric current.
The counter is connected to the detector by a cable. It counts the pulses. Each pulse is one count, so the counter shows the number of times radiation was detected.
Keep the two jobs separate in your answers. The detector detects the radiation; the counter counts the pulses that the detector sends it. 'A G-M tube measures radiation' leaves out the counter.
apparatus diagram — not drawn yet
Two side-by-side panels on a bench. Left panel, labelled 'background': a G-M tube clamped horizontally, connected by a cable to a box labelled 'counter' with a digital display, and no source nearby. Right panel, labelled 'with source': the same G-M tube, cable and counter, with a small radioactive source held in tongs a few centimetres in front of the window of the tube.
The same detector and counter are used for both readings. The left reading is background only. The right reading is the source plus background.
Some counters show the total number of counts since they were started. Others, called ratemeters, show the number of counts per second or per minute directly.
The counter only records radiation that actually enters the detector. Radiation that leaves the source in other directions is never counted, so the number of counts depends on where the detector is placed.

Part 4 of 5

Count rate

A total number of counts means little on its own. 500 counts in 1 minute and 500 counts in 1 hour are very different amounts of radiation. To compare readings you need the count rate: the number of counts per unit time.
No formula is given on Paper 4, so you must know this one.
$count rate = \frac{number of counts}{time taken}$
The unit comes from what you divided by. Divide by a time in seconds and the unit is counts/s. Divide by a time in minutes and the unit is counts/minute.
To change between the two units, think about which number must be bigger. In one minute there are 60 seconds, so there are 60 times as many counts in a minute as in a second.
So multiply by 60 to go from counts/s to counts/minute, and divide by 60 to go from counts/minute to counts/s. For example, $0.50$ counts/s is $0.50 \times 60 = 30$ counts/minute.
Radioactive emission is random, so the number of counts in equal time intervals is not the same each time. Counting for a longer time, or repeating the count and taking the mean, gives a more reliable count rate.
Always write the unit. A count rate of '450' is not an answer until you say whether it is 450 counts/s or 450 counts/minute, which differ by a factor of 60.

Part 5 of 5

Corrected count rate

When a source is in front of the detector, the counter records radiation from the source and background radiation. The detector cannot tell them apart.
To find the count rate from the source alone, you first measure the background count rate. Remove all sources (or put them far away), switch on the detector and counter, and record the counts over a measured time.
Then put the source in place and measure the count rate again with the same detector in the same position. Subtract the background from it to get the corrected count rate.
$corrected count rate = measured count rate - background count rate$
graph diagram — not drawn yet
A single vertical bar labelled 'measured count rate' with the vertical axis labelled 'count rate / counts per minute'; the bar is split into a short lower section shaded grey and labelled 'background' and a taller upper section labelled 'from the source = corrected count rate'.
The measured reading is background plus source. Removing the grey part leaves the part due to the source.
Subtract rates, not totals. If the background was counted for 4 minutes and the source for 2 minutes, the two totals cover different times. Divide each total by its own time first, then subtract.
Use the same unit for both. You cannot subtract a background in counts/s from a reading in counts/minute. Convert one of them first, using the factor of 60.
The correction matters most when the reading is small. If the measured count rate is $765$ counts/minute and the background is $24$ counts/minute, the background is only about $3 %$ of the reading. If the measured count rate is $30$ counts/minute, the same background is most of it: the source gives only $30 - 24 = 6$ counts/minute.
You will use corrected count rates again when you work with half-life, so build the habit now: correct for background before you do anything else with a count rate.

Worked examples

Example 1. A G-M tube connected to a counter records $1350$ counts in $3.0$ minutes. Calculate the count rate in (a) counts/minute and (b) counts/s.

Show the working

Count rate is the number of counts divided by the time. The time is given in minutes, so dividing by it gives counts/minute directly.
$count rate = \frac{1350}{3.0} = 450 counts/minute$
For counts/s, check which number must be smaller: there are fewer counts in one second than in one minute, so divide by 60.
$\frac{450}{60} = 7.5 counts/s$
Check another way: $3.0$ minutes is $180$ s, and $1350 \div 180 = 7.5$ counts/s. The two methods agree.

Answer: (a) $450$ counts/minute (b) $7.5$ counts/s

Example 2. With no source present, a counter records $96$ counts in $4.0$ minutes. A source is then placed in front of the detector, and the counter records $1530$ counts in $2.0$ minutes. Determine the corrected count rate in counts/minute.

Show the working

The two totals were counted over different times, so they cannot be subtracted as they are. Find each count rate first.
Background count rate:
$\frac{96}{4.0} = 24 counts/minute$
Measured count rate with the source in place:
$\frac{1530}{2.0} = 765 counts/minute$
The measured reading includes the background, so subtract it to leave the count rate due to the source.
$765 - 24 = 741 counts/minute$
The wrong route is $1530 - 96 = 1434$ . That subtracts 4 minutes of background from 2 minutes of reading, and it is not even a rate.

Answer: $741$ counts/minute

Example 3. The background count rate in a laboratory is $0.40$ counts/s. With a source in place, the count rate is $210$ counts/minute. Calculate the corrected count rate in counts/s.

Show the working

The two rates are in different units. The answer is wanted in counts/s, so convert the reading to counts/s first. Divide by 60, because there are fewer counts in a second than in a minute.
$\frac{210}{60} = 3.5 counts/s$
Both rates are now in counts/s, so subtract the background.
$3.5 - 0.40 = 3.1 counts/s$
Check in the other unit: the background is $0.40 \times 60 = 24$ counts/minute, $210 - 24 = 186$ counts/minute, and $186 \div 60 = 3.1$ counts/s. Same answer.

Answer: $3.1$ counts/s

Example 4. A student measures the background count rate in two places: the cellar of an old granite house, which has no windows, and a wooden hut. The count rate in the cellar is higher. Suggest two reasons for this.

Show the working

Start from the four named sources and ask which of them would be different in the two places. Food and drink and cosmic rays are much the same in both, so look at rocks, buildings and radon.
Rocks and buildings: granite contains radioactive isotopes, so the stone walls of the cellar emit radiation. Wood is not made from rock.
Radon gas: radon seeps out of rocks and soil into the air. A cellar is below ground level and has no windows, so the gas is not ventilated away and builds up in the air.
Each reason names a source and says why it is greater in the cellar. A reason that only says 'it is underground' does not name a source.

Answer: The granite walls contain radioactive isotopes and emit radiation. Radon gas from the rocks and soil builds up in the cellar because it is poorly ventilated.

Where people go wrong

Mistake: Subtracting the background total from the measured total when they were counted over different times.

Instead: Divide each total by its own time to get two count rates, then subtract the rates.

A total depends on how long you counted. Only rates over the same unit of time can be compared or subtracted.

Mistake: Subtracting a background in counts/s from a reading in counts/minute.

Instead: Convert one value first: multiply counts/s by 60 to get counts/minute, or divide counts/minute by 60 to get counts/s.

The two numbers differ by a factor of 60, so the subtraction gives a meaningless result.

Mistake: Expecting the counter to read zero when no source is present, or treating counts from an empty room as a fault.

Instead: Those counts are background radiation, which is present everywhere all the time.

This is exactly why a corrected count rate is needed: the background is in every reading you take.

Mistake: Thinking background radiation comes only from nuclear power stations or nuclear waste.

Instead: All four sources the syllabus names are natural: radon gas, rocks and buildings, food and drink, and cosmic rays.

Background radiation would still be present if there were no nuclear industry at all.

Mistake: Giving a count rate as a bare number, such as '450'.

Instead: Write the unit every time: 450 counts/minute or 450 counts/s.

The two possible units differ by a factor of 60, and a missing unit normally loses the final calculation mark.

Check yourself

State what is meant by background radiation.
Answer
The ionising radiation that is present in the environment all the time, from natural and artificial sources, even when no radioactive source is nearby.
Name the four sources that the syllabus lists as making a significant contribution to background radiation.
Answer
Radon gas in the air; rocks and buildings; food and drink; cosmic rays.
Why does a pilot on a long flight receive more radiation from cosmic rays than a person at sea level?
Answer
Cosmic rays come from space, and the atmosphere absorbs much of them. At high altitude there is less air above the aircraft to absorb them.
A G-M tube is connected to a counter. State what each one does.
Answer
The G-M tube detects the ionising radiation: each time radiation enters it, it produces a pulse. The counter counts these pulses.
Convert a count rate of $3.6$ counts/s into counts/minute.
Answer
$3.6 \times 60 = 216$ counts/minute
A counter records $600$ counts in $2.5$ minutes. Calculate the count rate in counts/minute and in counts/s.
Answer
$600 \div 2.5 = 240$ counts/minute, and $240 \div 60 = 4.0$ counts/s
The measured count rate near a source is $85$ counts/minute. The background count rate is $20$ counts/minute. Calculate the corrected count rate.
Answer
$85 - 20 = 65$ counts/minute
The background count rate is $0.30$ counts/s. The measured count rate near a source is $47$ counts/minute. Calculate the corrected count rate in counts/minute.
Answer
Background $= 0.30 \times 60 = 18$ counts/minute, so the corrected count rate is $47 - 18 = 29$ counts/minute.

Exam questions

Try each part first. Open the mark scheme after you have written your answer.

Question 1. A student investigates a radioactive source using a G-M tube connected to a counter. Before bringing the source into the laboratory, she records $150$ counts in $5.0$ minutes. She then places the source in front of the G-M tube and records $2280$ counts in $2.0$ minutes.

(a) State what is meant by background radiation. 1 mark No calculator

Mark scheme

B1 (ionising) radiation present in the environment / surroundings all the time; insufficient: radiation from the source

(b) Calculate the background count rate in counts/s. 2 marks

Mark scheme

C1 150 / 5.0 = 30 counts/minute or 150 / 300 seen
A1 0.50 counts/s; allow 0.5 counts/s

(c) Determine the corrected count rate due to the source, in counts/minute. 3 marks

Mark scheme

C1 measured count rate = 2280 / 2.0 = 1140 counts/minute
C1 background count rate taken away in the same unit, 1140 - 30 (or 19 - 0.50 = 18.5 counts/s); ecf from (b)
A1 1110 counts/minute; allow 18.5 counts/s correctly converted to 1110 counts/minute

(d) Explain why the student measures the background radiation before she uses the source. 2 marks No calculator

Mark scheme

B1 the counter also records background radiation when the source is present
B1 subtracting the background leaves the count rate due to the source alone, owtte

Question 2. Background radiation is measured in several different places.

(a) State two sources that make a significant contribution to background radiation. 2 marks No calculator

Mark scheme

B2 any two from: radon gas (in the air); rocks / soil / buildings / building materials; food / drink; cosmic rays (from space / the Sun); allow medical sources (e.g. X-rays); one mark each, and rocks with buildings, or food with drink, scores only one mark

(b) The background count rate in the basement of a stone house is higher than in a wooden garden shed. Suggest two reasons for this. 2 marks No calculator

Mark scheme

B1 the stone walls contain radioactive isotopes / rock emits radiation
B1 radon gas (from rocks and soil) builds up in the basement because it is poorly ventilated / below ground

(c) Explain why the count rate due to cosmic rays is higher on top of a high mountain than at sea level. 2 marks No calculator

Mark scheme

B1 cosmic rays come from space / from the Sun
B1 there is less atmosphere above the mountain top to absorb them, ORA

Question 3. A student uses a detector connected to a counter to compare the background radiation at two places, A and B.

(a) Describe how the student measures the background count rate at one place. 3 marks No calculator

Mark scheme

B1 all radioactive sources removed / placed far away
B1 counts recorded on the counter over a measured time (a long time, or repeated and averaged)
B1 count rate = number of counts / time

(b) The background count rate at A is $1.2$ counts/s. At B it is $66$ counts/minute. Deduce which place has the higher background count rate. Show how you decide. 2 marks No calculator

Mark scheme

C1 converts to the same unit: 1.2 counts/s = 72 counts/minute, or 66 counts/minute = 1.1 counts/s
A1 A has the higher background count rate

How this is examined

Extended: finding the count rate from the source alone by taking away the background: a Calculate or Determine of two or three marks in a practical context, with the background given as a count over a time or as a rate, possibly in a different unit from the reading Each total turned into a rate by dividing by its own time; both rates in the same unit (use the factor of 60); then measured count rate minus background count rate. The final answer needs its unit, counts/s or counts/minute.
Working out a count rate from a number of counts and a time, in the unit asked for: a Calculate of one or two marks: a number of counts recorded in a stated time, with the answer wanted in counts/s or counts/minute Number of counts divided by time, then the factor of 60 applied in the right direction: multiply to go from counts/s to counts/minute, divide to go the other way. The answer must carry the unit.
Naming the sources of background radiation and explaining why it is higher in one place: a State of one or two marks asking for sources, or a Suggest or Explain of two marks comparing two places, such as a cellar and a hut or a mountain top and sea level Sources named as the syllabus names them: radon gas, rocks and buildings, food and drink, cosmic rays. For a comparison, a source plus the reason it is greater there, for example 'radon gas from the rocks builds up because the cellar is poorly ventilated' or 'less atmosphere above to absorb cosmic rays'.
Describing how to measure background radiation with a detector and a counter: a Describe of two or three marks set in a practical context, which can lead into a corrected count rate calculation Three points: remove all radioactive sources (or put them far away); record the counts on a counter connected to a detector such as a G-M tube over a measured time, a long one or repeated; divide the counts by the time to get the background count rate.
What background radiation is, and why a detector never reads zero: a one-mark State asking what is meant by background radiation, or an Explain of why a reading is taken before the source is brought in 'Ionising radiation that is present in the environment all the time'. For the Explain: the detector also counts background radiation, so it must be subtracted to leave the count rate from the source alone.

Words to know

Background radiation: The ionising radiation that is present in the environment all the time, from natural and artificial sources.
Radon: A radioactive gas produced when uranium in rocks and soil decays. It seeps into the air and can build up in poorly ventilated rooms.
Cosmic rays: High-energy particles that reach the Earth from space, including from the Sun. Much of this radiation is absorbed by the atmosphere.
Detector: A device that responds to ionising radiation, such as a Geiger-Müller tube, which produces a pulse each time radiation enters it.
Counter: The device connected to a detector that counts the pulses the detector produces.
Count rate: The number of counts per unit time, in counts/s or counts/minute.
Corrected count rate: The measured count rate minus the background count rate, giving the count rate due to the source alone.

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