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

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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.

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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.

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Smoke Alarm

A device that can detect smoke, a household detector that was created to detect fires


This is the process where an atom or molecule is converted into an ion due to the subtraction or addition of an electron


The ability to get through something, in this case the ability to get through the skin

Half life

The time required for the radioactivity level to drop by half its original value

Alpha Decay

A type of radioactive decay where the ‘unstable’ nucleus emits an alpha particle

Beta Decay

A type of radioactive decay where the ‘unstable’ nucleus emits a beta particle.

Gamma Decay

This is a type of decay reaction where gamma emissions or given off


Energy that is transmitted in the form of rays, waves or particles

Ionisation Chamber

An ionization chamber is a device used to detect particles in the air and is used in a smoke detector


Whether the information being used is reliable and trustworthy


Whether the information being used is recent and proven to be accurate





















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This student written piece of work is one of many that can be found in our GCSE Radioactivity section.

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Here's what a star student thought of this essay

3 star(s)

Response to the 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, ...

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Response to the 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.

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.

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.

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Reviewed by alkyone 18/02/2012

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