Ionisation smoke alarms use an ionisation chamber and a source of ionising radiation to detect smoke.

Smoke Alarms

What Is It?

A smoke alarm is a device that can detect smoke; alternatively it can also be called fire alarm systems or household detectors. The smoke alarm was created to detect fires and would consequently give a warning in the form of a signal or an alarm. Smoke alarms are fitted to the ceiling and are usually a very small disk shape size.

There are two main types of smoke alarms. The first one and probably the more commercial and cheaper one is the physical process alarm (Ionisation). These are usually powered by a single disposable battery. The other main type of smoke alarm is photoelectric, which basically works by optical detection.[1] This report is on Ionisation smoke alarms.

How Do They Work?

Ionisation smoke alarms use an ionisation chamber and a source of ionising radiation to detect smoke. These are the more inexpensive ones however; one major disadvantage of ionisation detectors is that they are sensitive to very small particles of smoke. An ion is an atom with a positive or negative charge.[2] To ionise means to remove an electron from an atom and purposely create positive and negative ions. Inside the ionisation chamber is a tiny amount of americium 241.[3]

The reason why americium is often used instead of other radioactive elements is that americium 241 has a very long half life of 432 years. It is a very good source of alpha particles. The americium gives off alpha particle radiation consequently stealing electrons from nearby oxygen and nitrogen atoms. Also in the ionisation smoke detector is 2 metal plates approximately 1cm apart from each other.[4] These plates are attached to the battery or house electricity, giving one plate a positive charge and the other a negative charge. Therefore, when ionisation occurs you are left with one free electron with a negative charge and one atom missing the electron with a positive charge. [5]

The charged ions then migrate to their oppositely charged plate. So, the negative ion will be attracted to the positive plate and the positive ion would be attracted to the negative metal plate. The ions would then ‘gravitate’ in a regular stream; this creates a small electric current.[6]

If a fire breaks out, the smoke particles enter the smoke alarm; they start to clog up the ionisation chamber. The particles interrupt the stream of ions or electric current by combining with the ionised oxygen and nitrogen. Once the particles have attached themselves onto the ions they have effectively shut off the electric current.[7] The circuit in the detector spots this straight away and triggers the alarm. Ionisation smoke alarms are strong at detecting low levels of smoke.

Alpha Radiation

Alpha particles are absorbed by everything. A couple centimetres of air or a very thin sheet of paper can easily absorb alpha particles. An alpha particle loses its energy as when it smashes into air; it breaks the air atoms into pieces and eventually loses all of its energy and stops, causing no harm.[8] Alpha decay is a process that an unstable atom can use to become more stable. During alpha decay, an unstable atom nucleus releases two neutrons and two protons. As a consequence of alpha decay, losing protons and neutrons means that the element has changed into a completely different element.[9]

Many people believe that just because alpha particles can be absorbed by air they assume that alpha is the least dangerous out of the three types of radiation; when in fact alpha particles can be up to 20 times more dangerous than other types of radiation. Whilst alpha cannot penetrate your skin, people can eat or drink something contaminated with an alpha source.[10]

This would leave you with alpha particles in your body, which is very dangerous as they can ionise atoms in your cells. If a cell has been ionised in your body, then the cell is most likely to do what it is not supposed to do as its instructions are scrambled. Scientists have also said that an ionised cell may turn cancerous and start multiplying uncontrollably. Alpha particles do have a low penetrating power although they can still be dangerous as they can ionise cells in your body.

Beta Radiation

Beta particles have a longer range in air. Therefore, unlike alpha particles, beta particles are not absorbed by everything. Beta can still pass through a few centre metres of air and will get through paper although a thin sheet of lead will stop it. As beta has a longer range in air, you need to keep your distance from a beta source to be safe. Beta particles do not break up atoms as much when it passes; therefore they do no ionize as strongly. This means that beta radiation will cause less harm to a cell as ...

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Beta Radiation

Similar to alpha decay, beta decay is a process that an unstable cell can use to become more stable. However beta decay is different. During beta decay, a neutron in an atoms nucleus turns into a proton and an electron (beta particle). [12]This leaves the nucleus with one more proton than it had originally started with. Since the atom has gained an extra proton, it changes from one element to another. The electron that was also created during the process, fly’s away from the nucleus.

Although the number of protons and neutrons (mass number) in an atom change, the mass number of the element always stays the same as you have lost a neutron but gained a proton. Again beta radiation can be harmful if something with a beta substance in it is swallowed. However, as they can ionize and cause disruption to your cells, they have much less ionizing strength; therefore you could say that alpha particles are more dangerous. [13]However, beta particles have more penetrating power, which means that they can get through your skin and affect cells inside you quicker and easier than alpha particles.

Gamma Radiation

Gamma radiation is a type of electromagnetic radiation. Gamma radiation is very high in energy and is only emitted in the aftermath of alpha or beta decay. There is no pure gamma emitter. After a decay reaction, the nucleus is still left with unnecessary energy which it needs to get rid of [14]. Thus, instead of emitting an alpha or beta particle, the nucleus gets rid of its excess energy in the form of a gamma ray. The gamma ray is very high in energy and frequency and that’s why it is a danger.[15]

Alpha and beta particles ionize the atoms as they pass them but gamma rays do not ionize as much. This means that they do not lose much energy as they travel, as they do not interact as much when they pass. Therefore gamma rays have high penetrating power and a very long range.

Gammas rays past through most materials and are quite difficult to stop. You would need a large amount of lead or concrete in order to block gamma rays, although even then some gamma rays still pass through. Gamma rays have no mass and no charge.[16]

Gamma rays are the most dangerous form of radiation and can cause serious damage to the body. Whilst they do not ionize as much as alpha and beta particles do, they are high in energy and frequency and have high penetration power, thus they can easily penetrate your skin. This means that gamma rays can affect your DNA, which is not very good. It is important people keep their distance from a gamma source in order not to receive much radiation.

There are some benefits of gamma radiation. Gamma rays can be used for sterilizing medical equipment. They can be used to see inside patients and they can be used as a tracer. In some cases gamma can also be used to kill cancerous cells.[16]

Why Is It Safe?

The risk of harm from exposure of radiation from a smoke alarm is considered to be small. Alpha radiation, which is emitted by the radioactive source used to make the detector operate, won’t even penetrate through air for more than a few centimetres and so it stays within the chamber of the alarm.[17] Gamma radiation emitted by the source is of low intensity and so it is shielded by the alarm. Gamma radiation intensity decreases really fast while its distance is increasing from the source.

The gamma radiation at 1 metre from an unshielded americium-241 source from activity found in domestic smoke alarms would expose a person to about 3,000 times less than the radiation dose they would receive from natural background radiation. Everyone is exposed to natural background radiation; this includes radiation from rocks and soil and cosmic rays from the sun.

Also, the actual amount received from the detector would be much less than this since the americium-241 source is shielded by the metal chamber and a person would not normally be at a distance of 1 metre from the smoke detector for any length of time. The amount radiation that people receive from smoke alarms installed in their house and is very small.

The probability of the source being removed from the chamber and then swallowed is extremely rare and wouldn’t happen. So therefore the internal radiation hazard is again very small. A case has happened, where a worker in a factory making smoke alarms swallowed two of the radioactive sources. Even in this case, the radiation exposure was small as the sources passed through his body with very little leakage.[18]

How You Dispose Of It?

The Radiation Health Committee has recommended that the preferred method of disposal for small numbers of smoke alarms is to include them in the domestic rubbish. The Committee considered this action acceptable because, the amount of radioactive material in each smoke alarm is extremely small and, from environmental and public health perspectives, the disposal of individual smoke alarms with domestic rubbish does not represent any hazard as the radioactive material is securely bound in a metal foil within the smoke alarm and, the amount of naturally occurring alpha emitting radioactivity in normal soils is equivalent to a dozen or more smoke alarms in every cubic metre.[19] The dispersal of smoke alarms, even in large numbers, through refuse land-fill sites is therefore not significant in comparison.

Method

I collected the information I needed to find out about people’s thoughts using a questionnaire. I asked 15 people from all different age groups.

The way that I made the experiment valid is by making it fair. A way I made it fair is that in my questionnaire I had to ask everyone the same questions. It was important that I asked everyone the same questions because I was able to compare several different answers to the same question and choose the average response to it. Another way I made it fair is by only changing one factor in the experiment- and the factor that I changed was the people that I questioned. It was important to question more than one person so I can get more than one answer, so that I know what most people would think about smoke alarms

The way that I made the experiment reliable is by averaging the results from the questionnaire. It was best to average the results because I could then find out what the average response to the questionnaire is and then analyse the results to find out why the response had turned out like that. I could also find an anomalous result, in case someone who is questioned has a different idea then everyone else, and then I can find out why that person thinks like that.

These are the questions I asked:

1. Do you own a smoke alarm?
2. How many do you own?
3. Where are they in your home?
4. How often do you replace your smoke alarm?
5. Do you believe smoke alarms are safe?
6. Did you know that there is radioactive material in you smoke alarm?
7. Do you know how a smoke alarm works?
8. Do you have any concerns on smoke alarms?

Results

Question 1:

Question 2: How many do you own?

Question 3:

Question 4:

Question 5:

Question 6: Did you know that there is a radioactive material in your smoke alarm?

Question 7: Do you know how a smoke alarm works?

Question 8: Do you have any concerns on smoke alarms? All of them answered no, but said knowing there is radioactive material was worrying

Evaluation

This is a hard experiment to judge by the reliability and validity of it because of the type of method used. There isn’t really an incorrect way of making a questionnaire, as long as it reflected the subject. I couldn’t keep the test reliable because there isn’t a way to repeat the test because I can’t ask the same people as I would get the same results- unless they changed their minds about what they thought.

I could have improved my experiment by making it more valid. A way I could improve it is by questioning people from different age groups, for example if I had only asked 20 people under the age of 30. This would make it more valid as the ages would all be similar as in my questionnaire I asked different ages of people and not just one set of age group. I would also ask people separately as although the subject is smoke alarms they might change their view just because of the people around them this would make their answers valid therefore making my results valid.

I can extend my investigation further by asking more people on their view, like asking 100 people that way I would get more results on what people think on smoke alarms. I could also extent it by asking 50 people from one city and 50 people from another. For example if I asked 50 people from Bristol and 50 people from London, that way I can compare what two cities think and the different perspectives they may have. I could also find out whether their views differ from one city to another. I would make this reliable by looking for anomalies and researching further why this person’s view was different, and by averaging results. I would make this valid by asking them all the same questions and only changing one thing which is the people I ask.

Conclusion

Most of the people I asked said they had a smoke alarm 2/15 people I asked said they didn’t . This shows that not everyone owns a smoke alarm.

The second question which was ‘How many do you own?’ 2/15 people I asked said that they owned 1 smoke alarm, 6/15 people I asked said they had 2 smoke alarms, 2/15 said they had 3 smoke alarms, 1 person said they had 4, 1 person said they had 5 and 1 person said they had 6 smoke alarms. These results show that most people had 2 smoke alarms.

The third question I asked was ‘Where are they in your home?’ these answers were based on the amount of smoke alarm the person owned. 15 smoke alarms were in the hallway. People put their smoke alarms midway of their house so the smoke alarm can detect the smoke from all around. The most people also had one upstairs and one downstairs.

The fourth question was ‘How often you replace your smoke alarm?’ most people said they replace their smoke alarm every ten years, this is because they have now made smoke alarms which they last for a long time so most people go for this to avoid the hassle of changing in so often.

The fifth question was ‘do you believe smoke alarms are safe?’ most people said that they did believe they were safe some said they did not know and only one person said they did not think they were safe. I think most people said they were safe as they save lives.

The sixth question was ‘Did you know that there is radioactive material in you smoke alarm?’ 13/15 people said they did know this shows they have an understanding of what is in their smoke alarm.

The seventh question was ‘do you know how a smoke alarm works?’ 8/15 people said they did not know how it works which shows that they don’t know how it works.

The eighth question was ‘do you have any concerns about smoke alarms?’ all of them said no but radioactive material inside is worrying. However seeing as they were not worried about this shows that they know the radioactive material is not very dangerous.

Argument

Advantages of buying a smoke alarm

Smoke alarms are devices that save lives
The risk of death from a fire is three times higher in a house without a smoke alarm
Smoke alarm can detect smoke from the early stages of a fire before the fire can do damage and cause harm
Cheaper than other types of smoke alarms
The Alpha source used in the alarm has a very long half-life of 432 years, meaning the alarm will last a very long time, no need to replace regularly
Alpha source has low penetrating power
Alpha source is absorbed by air
Reliable

Disadvantages of smoke alarms

Contains radioactive material
Battery needs replacing regularly
Not effective in sensing smouldering , smoking fires
Tends to go off when cooking
If the alpha source is swallowed or something is contaminated with an alpha source and is swallowed, then it would be very dangerous as they ionise cells in the body.
Needs regular cleaning and testing
Very sensitive and vulnerable to the smallest particles
Less sensitive to the larger particles in dense smoke found in slow smouldering fires
More prone to false alarms as it can detect particles too small to be visible

People may be concerned that smoke alarms can be a health risk as they contain radioactive material however the plastic casing on the smoke alarm is enough to stop the alpha particles. Our skin is also enough protection to stop the radioactivity. This means that the radioactive source in the smoke alarm will only be a risk if consumed.

So I think having a smoke alarm is very important although there are health risks having a smoke alarm can help save lives and I think that over rules all the risks of having a smoke alarm. And although smoke alarms are vulnerable to small air particle this shows it is reliable as while your asleep you could have forgotten about something and having a smoke alarm their helps you and improves your safety as it saves lives.

Glossary

Bibliography

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[18]

[19]

Peer Reviews

Here's what a star student thought of this essay

alkyone

18th of Feb 2012

Quality of writing

The structure of the essay is good: the information is in the right order and easy to follow. However, there are some grammatical mistakes throughout the essay making the author's point being made difficult to understand in some cases. A mistake in the horizontal axis of the first graph can be seen, one bar should be labelled as yes and the other as no instead of hallway and living room. The use of glossary in the end of the essay shows that the students has a good understanding of the precise meaning of scientific terms being used in the essay. The writer meets the expectations required for GCSE standard, even though a better work could be done in the second part of the essay compared to an excellent standard reached in the first part.

Level of analysis

The writer has a good understanding of background physics knowledge required to understand how a smoke alarm works and explains it in depth in the first part. The use of a wide range of sources of information which are clearly stated in the bibliography section indicates that the author researched the topic very well to develop a good understanding shown in the clear description of a smoke alarm and the scientific concepts behind its function. In the second part, a survey was conducted, however the aim of the survey is not clearly stated. Even though the author states that factors were controlled, there is hardly any reference to specific factors the student kept the same. It is stated that reliability cannot increase as a repeat using the same subjects would not provide any meaningful data; this is a wrong idea: reliability can increase by surveying a larger sample of people. The graphical representation of results obtained makes comparison between possible answers easily observable. In the conclusion section, the writer makes a good use of the results obtained to conclude about several aspects of the perception of people for smoke alarms. A conclusion can be more easily reached if an aim is provided, to show how the results account for what the student was testing for.

Response to question

The author demonstates a good understanding of how a smoke alarm with an ionisation chamber functions, although several improvements could be made to the second part of the essay consisting of a questionnaire. In the first part of the essay, a clear description of how a smoke alarm functions is provided. The author gives a detailed explanation of how each component of a smoke alarm works to trigger the alarm when smoke is present in terms of concepts of physics. In the second part of the essay, there is no specific aim or hypothesis stated. In the method section there is an extensive focus on validity, while more information of how the survey was conducted and explanation of the choice of questions could be provided.

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