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 and they are currently selling for as little as £5, and just recently was the London fire brigade holding a campaign where they were handing out free smoke alarms.
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. 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. 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 (alpha source).
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. 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.
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.
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. 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. 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 (Alpha particle). As a consequence of alpha decay, losing protons and neutrons means that the element has changed into a completely different element.
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.
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 ...
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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.
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 it passes through.
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). 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. 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. 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.
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.
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.
Name of radioactive source
Type of radiation
Half-life
V
? and ß radiation
270 days
W
? - radiation
432 years
X
? - radiation
38 days
Y
ß - radiation
351 years
Z
? - radiation
0 years
From this graph we can clearly see that this graph follows the usual pattern of a half life graph. The graph evidently shows that at each data point the time for the radioactivity level in Becquerel's to drop by half is equal every time, which represents a half life. The half life of a source is the time taken for the radioactivity quantity to fall by half. To elaborate, from 100 Becquerel's to 50 Becquerel's the time taken is 432 years; the time taken for 50 Becquerel's to drop half to 25 Becquerel's is also 432 years and this pattern carries on. This shows us that the half life of this source is 432 years. The alpha radioactive source with a half life of 432 years is Americium 241, a by-product of plutonium production.
This type of curve can also be called an exponential decay as the line never seems to meet the x axis and zero. From the graph, to the right of the origin, we can see that the level of radioactivity is declining and the line seems to flatten at the x sis and reach zero. However, a closer examination will show that the line is in fact slightly above the x axis. The decay series table below may be able to provide a better understanding of how americium 241 isotopes decay.
Half Life Cycle
(Years)
(432)
2
(864)
3
(1296)
4
(1728)
5
(2160)
6
(2592)
7
(3024)
8
(3456)
9
(3888)
Radioactivity
Level in Becquerel's
50
25
2.5
6.25
3.125
.5625
0.78125
0.390625
0.1953525
From this table we can clearly see how the half life of an isotope affects its radioactivity level. The table shows that as the decay series of the isotope gets longer, the number of half lives increase and the level of radioactivity is decreased and is halved every 432 years. Because the radioactivity level is constantly decreasing and halving after every half life, it means that the radioactivity level of the source (americium 241) will never reach zero. The radioactivity will get smaller but never will it meet the x axis on the half lie graph and reach zero.
Why Source W (Americium 241)
There are a number of reasons to explain why my choice, source (W) is the best radioactive source for a smoke detector. The main reason was because of its very low penetrating power. Alpha particles are absorbed by everything. Therefore, alpha particles are absorbed within the detector, while most of the gamma rays escape harmlessly; meaning a 0% dose of radiation is exposed to occupants of a house. Despite its radioactivity, americium's isotopes do not break down in the digestive system if swallowed, however if americium is in a soluble form it could be potentially dangerous as it could affect the skeleton as it decays alpha and gamma emissions.
One other main advantage of this particular alpha source is that it is not expensive. 1 gram of americium could cost you up to £1000, however that one gram is enough to power over 3 million smoke alarms. On top of this is the alpha source sensibility. Some people say this is a negative although equally some think it is positive. Alpha particles are so sensitive that they do not even need smoke to set a detector off, alpha particles can sense the particles of combustion. This means that a potential fire could be stopped at its earliest before it causes more harm.
The radioactive source I chose was always going to be an alpha source. This is because Beta and Gamma radioactive particles are not that easily absorbed like alpha are. Therefore radioactive emissions will be able to escape the smoke detector and cause harm. In addition, beta and gamma radiation have such high penetrating power, which means they can easily penetrate your skin and affect cells inside.
As we are not going to use beta or gamma radioactive sources, this leaves us with two alpha radioactive sources. Source X and Source W. To decide which alpha source is better to use we have to look at the half life of each source. In this case, the longer the half life of the radioactive source, the better. If a source with a short half life was to be used in a smoke detector, then the smoke detector would have to be replaced every now and again, which is costly. This means a source with a long half life would be ideal for a smoke detector as the detector would not have to be replaced often, hence I chose source W with a very long half life of 432 years over source X with a half life of 138 days.
As I was given the facts and figures, there was no proof that they were reliable or valid.
The reliability is referred to how consistent and similar the measurements are (concordant data). A measurement is said to be reliable if future measurements are seen to be similar in the same situation. In our case we know that the figures given to us are reliable. We know this because the graph shows that the alpha source is decaying at the same rate plus the half life figures (time in years) are correct. The validity refers to whether the facts and figures are correct and whether they are what they claim to be. In our case, we know that the half life figures are valid and we know that they are valid as they have been published in the very latest edition of a physics textbook and evidence shows that the measurements have been proven to be accurate.
Advantages of ionisation smoke alarms/alpha sources
* 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 ionisation smoke alarms/alpha sources
* 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
Glossary
Word
Definition
Smoke Alarm
A device that can detect smoke, a household detector that was created to detect fires
Ionisation
This is the process where an atom or molecule is converted into an ion due to the subtraction or addition of an electron
Penetration
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
Radiation
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
Reliability
Whether the information being used is reliable and trustworthy
Validity
Whether the information being used is recent and proven to be accurate
Bibliography
http://www.darvill.clara.net/nucrad/sources.htm
http://www.physics.isu.edu/radinf/gamma.htm
http://home.howstuffworks.com/home-improvement/household-safety/fire/smoke.htm
http://www.google.com/images/alphadecay
http://www.google.com/images/betadecay
http://www.google.com/images/gammadecay
