Detecting radioactivity
Definition
Radiation detector
A device or material that gives an observable response when ionising radiation reaches it.
Definition
Count rate
The number of counts a detector records each second.
Definition
Corrected count rate
The count rate due to the source alone, found by subtracting the background count rate from the measured count rate.
- Ionising radiation cannot be seen, heard, smelt, tasted or felt, so a detector is always needed to show that it is present.
- Every detector works by responding to the ionisation that the radiation causes as it passes through a material.
- The two methods to be able to describe are photographic film and the Geiger-Muller tube.
- Photographic film records the total exposure built up over hours, days or weeks.
- A Geiger-Muller tube gives an immediate reading, count by count, as the radiation arrives.
- A count rate is worked out from a count and a time using count rate=counttime\text{count rate}=\dfrac{\text{count}}{\text{time}}count rate=timecount, giving an answer in counts per second.
Photographic film
- Photographic film is coated with an emulsion that contains grains of light-sensitive silver salts.
- Ionising radiation passing through the emulsion ionises the silver salts and changes the grains it hits, in the same way that light does.
- The film is sealed inside a light-proof wrapper, so any change to the emulsion must have been caused by radiation rather than by light.
- When the film is later developed, every changed grain turns into dark metallic silver and that part of the film appears blackened.
- The greater the exposure to radiation, the more grains are changed, so the darker the developed film becomes.
- Comparing the darkening with an unexposed control film shows both that radiation was present and roughly how much of it was absorbed.
- A film badge worn by a radiographer or a nuclear worker holds the film behind several windows made of different materials, typically an open window, plastic, aluminium and lead.
- Radiation that darkens the film behind the lead window must be highly penetrating, so comparing the darkening behind each window shows which types of radiation the worker met.
- The badge is collected and developed at set intervals, so it gives a permanent record of the dose received but no warning at the time.
Key Idea
Photographic film detects radiation because ionising radiation darkens the film when it is developed, and greater exposure produces greater darkening.
The Geiger-Muller tube
Definition
Geiger-Muller tube
A gas-filled tube that produces a short pulse of current every time ionising radiation enters it, which a counter registers as one count.
- The tube is a sealed metal cylinder holding argon gas at low pressure, with a thin mica window at one end that is thin enough to let alpha and beta radiation through.
- A thin wire anode runs along the centre of the tube and the metal wall acts as the cathode, with a potential difference of several hundred volts between them.
- When ionising radiation enters the tube it ionises argon atoms, producing positive ions and free electrons.
- The free electrons are accelerated towards the central wire by the electric field and gain enough energy to ionise further argon atoms on the way.
- This chain of ionisation, called an avalanche, turns a single entering particle into a large number of charge carriers.
- The charge arriving at the wire causes a brief pulse of current in the circuit connected to the tube.
- The pulse is passed to a counter or ratemeter, which registers one count and often produces an audible click.
- Dividing the count by the counting time gives the count rate, and a larger count rate means more radiation is reaching the detector.
- The tube records only the radiation that actually enters its window, so the count rate is always smaller than the number of decays happening in the source.
Practical
Measuring count rate with a Geiger-Muller tube
- Aim: to measure the background count rate and use it to find the corrected count rate of a sealed radioactive source.
- Apparatus: Geiger-Muller tube with a mica window, digital counter or ratemeter, stand, clamp and boss, metre rule, stopwatch, sealed source, source handling tongs and a lead-lined source store.
- Variables: the presence and position of the source is the independent variable, the count is the dependent variable, and the tube, the counting time, the distance and the room are all kept the same.
- Method, measuring the background:
- Clamp the tube horizontally at bench height with the mica window pointing along the bench, and check that every source is locked in the lead-lined store on the far side of the room.
- Reset the counter to zero and start the stopwatch at the same moment.
- Record the count after exactly 300 s300\ \text{s}300 s, because a long counting time reduces the effect of the random nature of decay on the result.
- Repeat twice more, calculate the mean background count and divide it by 300 s300\ \text{s}300 s to obtain the background count rate.
- Method, measuring the source:
- Use tongs to lift the source from its store, hold it at arm's length and keep it pointing away from yourself and from everyone else.
- Clamp the source so that its front face is exactly 5.0 cm5.0\ \text{cm}5.0 cm from the mica window, measured with a metre rule laid on the bench.
- Reset the counter, start the stopwatch and record the count after exactly 60 s60\ \text{s}60 s.
- Repeat the timed count three times without moving the source or the tube, then calculate the mean count.
- Return the source to the lead-lined store as soon as the last reading is taken and record how long it was out.
- Results: with no source the counter creeps up unevenly to a small total, and with the source in place the total after the same time is many times larger.
- Maths: divide each mean count by its counting time to get a count rate in counts per second, then use corrected count rate=measured count rate−background count rate\text{corrected count rate}=\text{measured count rate}-\text{background count rate}corrected count rate=measured count rate−background count rate.
- Watch out: the repeats never match exactly because decay is random, so always take a mean; nudging the tube or the source between readings changes the count rate on its own and ruins the comparison.
- Safety: handle sealed sources with tongs only, keep them at arm's length and pointing away from the body, keep the time out of store as short as possible, and wash your hands at the end.
Example
Finding a corrected count rate
- With no source present the counter records 138138138 counts in 300 s300\ \text{s}300 s.
- The background count rate is 138300=0.46\dfrac{138}{300}=0.46300138=0.46 counts per second.
- With the source clamped in place the counter records 126612661266 counts in 60 s60\ \text{s}60 s.
- The measured count rate is 126660=21.1\dfrac{1266}{60}=21.1601266=21.1 counts per second.
- The corrected count rate is 21.1−0.46=20.621.1-0.46=20.621.1−0.46=20.6 counts per second to three significant figures.
Comparing the two methods
- Both methods respond to ionisation, but they give different kinds of information.
- Photographic film shows nothing until it has been developed, so it cannot warn anyone while the exposure is happening.
- A Geiger-Muller tube responds instantly, so it can be used to search for a source or to watch a count rate change from moment to moment.
- Film records the total dose collected over a long period, which a single reading from a tube cannot show.
- Film is small, cheap and needs no power supply, so it is convenient to wear as a badge all day.
- A tube needs a high-voltage supply and a counter, so it is used as laboratory or survey equipment rather than as personal monitoring.
- Film behind different absorbers shows which types of radiation were present, while a tube shows this only when absorbers are deliberately placed in front of the window.
Exam technique
Describing a detection method
- Give the response together with its cause, so state that radiation darkens the developed film, or that radiation ionises the gas inside the tube.
- Use the word ionises when describing the tube, because the ionisation of the gas is the point that earns the mark.
- Say that the counter records the pulses, rather than saying that the clicks measure the radiation.
- Set out the background subtraction as a separate line of working whenever a question supplies both a background reading and a source reading.
- Convert both readings to counts per second before comparing counts that were taken over different times.
Common Mistake
- Do not say that a Geiger-Muller tube measures the number of decays in the source, because it counts only the radiation that enters its window.
- Do not say that photographic film gives an immediate reading, because it has to be developed first.
- Do not forget to subtract the background count rate when a question gives you a background reading.
- Do not compare a count taken over 30 s30\ \text{s}30 s with a count taken over 60 s60\ \text{s}60 s without turning both into counts per second.
- Do not write that the film is darkened by light, because the light-proof wrapper rules light out.
Self review
- Explain why a detector is needed to show that ionising radiation is present.
- Describe how photographic film in a film badge shows that a worker has been exposed to radiation.
- Describe what happens inside a Geiger-Muller tube from the moment radiation enters the window to the moment a count is registered.
- A counter records 900900900 counts in 120 s120\ \text{s}120 s and the background count rate is 0.50.50.5 counts per second, so calculate the corrected count rate.
- Give one advantage of a film badge and one advantage of a Geiger-Muller tube.
