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Radioactive alpha decay in smoke detectors

Overview

        This paper provides the information of the radioactivity, the process of radioactive decay, particularly in the emission of alpha particle (alpha decay) and its applications in the recent technology of smoke detectors. The purpose and objectives of the researchers in doing the research of this topic is that the researchers are mostly interested in the development of radioactivity within our daily lives and thereby a further research, deep understanding and thorough analysis to increase the knowledge about radioactive decay is absolutely necessary. In fact, a fully detailed research on the alpha decay for Americium-241, who discovered it, as an alpha emitter and is commonly produced from an artificial transmutation and used in smoke detectors, is the main objective of this research.

Furthermore, in detail, many objectives and common but necessary inquiries are inquired by the researchers for radioactive alpha decay understanding, such as what cause the smoke detectors to work and what cause the alpha particles to be inflicted in an alpha decay or what chemical substance or any external factors that trigger the smoke detectors to be working, what happens within the process of Americium-241 alpha decay, what is the result, effect and functionality in smoke detectors and how the smoke detectors work in the cause of Americium-241 decay existence. The effect of Americium-241 towards environment, human's body and technology will also be explained briefly in this paper.

  1. ATOMS AND NUCLEAR FORCE

Atoms are fundamental units that compose matter. In a single atom, massive nucleons (composed of protons (positively charged) and neutrons (neutral)) are surrounded by electrons (negatively charged) in their orbits based on their energy levels, ranging from the lowest energy level (nearest to the nucleons) to the highest energy level (furthest to the nucleons). Generally, an atom's size (the radius from the most outside electron to the nucleons) is measured by using the unit of Ångström, whereas 1 Ångström is the same as 10-10 meters or 0.1 nanometers (Russell, 2008).

The numbers of protons inside the nucleons of an atom reflects the element of the atom itself. Thus, different elements have different number of protons inside their nucleons (or also known as atomic number). For example, Americium is placed on number 95 in periodic table. This means, Americium element has an atomic number of 95 and it has 95 protons in its nucleons. Most elements have different atomic mass or the different number of neutrons, but with the same amount of protons, which is called "isotopes". For example, the isotope Americium-243 (243Am) has 95 protons and 148 neutrons, so the atomic mass is 243. Another isotope of this element is Americium-241 (241Am), which has 95 protons and 146 neutrons, so the atomic mass is 241.

Quarks are the elementary particles that form protons and neutrons in an atom. Within this subatomic particle level, there is a strong binding force between protons and neutrons to form nuclei. This force, which is the strong residual force or mostly known as nuclear force. Moreover, protons and neutrons tend to repel themselves due to their like charges by a repulsive electromagnetic charge. However, they are bound to form a nucleus by a stronger attractive nuclear force, so that the weaker repulsive force does not affect the form of bound nucleus.

Nuclei have their own stable amount of nucleons. All elements that have the amount of nucleons less than 208 are stable elements. As there are more nucleons in a nucleus, the radius of nucleus increases and the attractive nuclear force weakens. Hence, the outer nucleons have the tendency to repel with other nucleons. This behavior causes the element to be unstable, and it is found that elements with amount of nucleons greater than 208 possess this instability. As the radius of nucleons is increased, the radioactive behavior is increased as well and it tends to decay to a completely stable element (stable to alpha, beta and gamma decay).

Within the level of hadron component, protons and neutrons component are bound by a binding force, or known as the term of quantum chromodynamics (QCD). Generally, there are interactions between quarks and gluons within protons and neutrons. This interaction is mostly known as color charge and three quarks in a proton/neutron tend to exchange their color one another. These three quarks in a proton/neutron is assumed as Red, Green and Blue, because the combination of these three colors is white or color-neutral.

  1. HISTORY OF RADIOACTIVITY

In 1896, Antoine Henri Becquerel conducted an experiment where he could not use the sun radiation as the source of the energy he needed. When he put photographic plates together with a crystal contained an amount of uranium, some queer and surprising phenomenon occurred. The uranium emitted an invisible radiation itself without the presence of any energy source. Although Becquerel was aware of this phenomenon, he did not look further into this case as he did not study about radioactivity. (Slowiczek, 2008)

On 1898, Marie Curie and Pierre Curie did an experiment to discover the substance that could turn air into a electricity conductor with pitchblende and uranium. Both of them found that the pitchblende (UO2) had the ability to produce current several hundred times stronger than pure uranium could. This unknown phenomenon was also confused them, and they thought an active substance must had existed within the pitchblende to encounter this phenomenon, and so, they introduced the term of "Radioactive" (Slowiczek, 2008)

In 1911, Ernest Rutherford performed an experiment where he ejected an alpha particle beam emitted from the decay of a radioactive material onto a thin gold foil. He observed that most of the particles passed through the gold foil perfectly without any collisions, supposing the gold foil was composed of mostly empty space, instead of a solid matter. Nevertheless, some particles collided with gold particles and bounced back, showing that the gold foil was a solid matter. In this experiment, Rutherford showed that atoms consisted of empty space with nucleus in the centre. Rutherford also showed that unstable elements possess a radioactive behavior and they decay over time. He was also the first to conduct an artificial transmutation between elements (Slowiczek, 2008).

  1. RADIOACTIVITY

The term of radioactivity means an atom emission from an unstable atomic nucleus. It happens because the reaction between repulsive electromagnetic force and nuclear force affects particular nucleus and the size of nucleons affect the radioactive behavior as well. This unstable nuclear isotope produces radiation, such as alpha (α), beta () and gamma (γ) radiation (Nave, n.d.)

Most unstable elements decay through any decay process, whether it is alpha decay, beta decay or gamma radiation to produce a completely stable nuclide. Some elements need to undergo through several decays before a stable nuclide is formed. Decay chain is a process of several steps of radioactive decays that proceeds from an unstable parent nuclide to a daughter nuclide with unstable nuclear behavior (or rather called as radioactivity behavior). This daughter nuclide decays and produces radiation within a sequence until a stable nuclide is produced. Some isotopes may take 3-4 steps until a stable daughter nuclide is produces, while some may take more than 10 decays to produce one stable nuclide.

Alpha Decay

Alpha decay is one of radioactivity's types where an alpha particle is emitted by the nucleus of atom and then decays into an atom with (X-4) atomic mass and (Z-2) atomic number, whereas X means the original number of atomic mass and Z is the original atomic number. For example, alpha decay happens when Berkelium-245 decays into Americium-241 with the addition of alpha particle emission. Usually alpha decays happen in a solid nucleus that has a large amount of ratio between protons to neutrons.

When anunstable atom decays through an alpha decay an emits an alpha particle, its daughter nuclide's atomic mass decreases by four because of the release of four nucleons and its atomic number decreases by two since the parent nuclide release two protons in the decay process. Thus, the new atom becomes a new element with different amount of protons and nucleons with the parent nuclide. For instance, an isotope berkelium-245 transforms into isotope of Americium-241 through an alpha decay process.

AMERICIUM-241

Americium-241 (Am-241) was first produced by Dr. Glenn Seaborg and his co-workers in 1944. This element is named after America, since it was discovered in the University of Chicago, USA (Visual Elements - Americium, n.d.).

This element can be seen as a silvery white metal which is able to be be tarnished slowly in dry air at room temperature. It is humanly produced artificially by alpha decay from brekelium isotopes (EVS, 2005). The atomic mass of Americium-241 is 245.056823 u and it has a half-life of 432 years. The possible parent of this element is beta decay from Pu-241 (Plutonium-241), electron capture from Cm-241 (Curium-241) and alpha decay from Bk-245 (Berkelium-245). The excess mass that is emitted by this element is 52930 KeV (Americium-241, 2009). If Americium-241 goes through another alpha decay, it will transform into Neptunium-237 as the daughter nuclide.

For the application, this element is usually used in smoke detectors. Smoke detectors usually depend on the combination of the decay of this element and the alpha particle that was emitted in the decay process. Other technology and applications that usually use Americium-241 are gamma radiograhy, target material in nuclear reactores and crystallography (EVS, 2005).

Enviromental americium was generated by the atmospheric testing of nuclear weapons which stopped worldwide in 1980s. The accidents happened from weapon production factories which caused localized contaminations and thus, americium oxide is very common in the environment. Approximately, there are 0.01 picocurioes of this element in every surface soil (EVS, 2005). This element is not easily dissolved yet it may be dissolved through chemical and biological process.

        It is uncommon for people to come in contact with Americium-241. However, when this element get into the human body, this element will most likely to stay in the bone, muscle and liver. This element has a big possibility of staying in the human body for years and will increase the risk of developing cancer if it is exposed by radiation. A way for this element to get into the human body is through inhalation. When it does get into the human lungs, it will most likely to remain there. The particle size and the chemical form effect its outcome. If the easily dissolved chemical form is inhaled by the human lung, it will pass into the bloodstream. However, if the less easily dissolved chemical forms is inhaled by the human lungs, the element will go to the lung's natural defense system, get swallowed naturally by the human itself and passes from the body through the faeces (EVS, 2005).

SMOKE DETECTOR

Smoke detectors, which we can see everywhere nowadays, are basically consist an alarm. The detectors themselves can run with a simple 9V battery to 120V house electric current. As explained before, smoke detectors differ on how they work, some use light, some use the radiation emission from a radioactive material and many more.

There are smoke detectors that use light and they are usually called photoelectric smoke detectors. These detectors use a light source to produce light and then use the smoke particles that enter inside the detector to bounce the light to the sensor. They usually use photodiode for the sensor, because of its sensitiveness against light. These detectors are quite big and ineffective because the light will bounce only if the smoke is thick enough to bounce the light photons. Otherwise, the light will only just go straight and will never hit the sensor or it will not bounce accurately to the light sensor to set off the alarm. Therefore, the alarm will not trigger.

On the other hand, smoke detectors that use radiation emitted from a radioactive material are smaller and more effective against small amount of smoke. They call it ionization smoke detectors because the detectors use ionization radiation and it consists an ionization chamber in the mechanism of alarm triggering.

Ionization radiation is radiation which uses energy to knock off electron out of some atoms, causing the atoms to be charged, leaving a free electron. While the ionization chamber itself is actually a normal chamber that the ionization radiation takes place. The chamber consists of two plates charged by either batteries or other current. One of the plates is charged by a positive charge, while the other plate is charged by a negative charge. The other part in these detectors is the source of the radiation; they use a very small amount of americium-241 as the source of the radiation which they put it near the plates in the chamber.

Apart from using sensor like photoelectric detectors, ionization smoke detectors make use of the current level. When the americium-241 is released, they ionized the atoms in the air, creating an ionization radiation. As explained before, the radiation will cause the atoms to lose electron and to be charged. The electrons that are knocked off will attracted to the positive charged plate, while the charged atoms will attracted to the negative charged plate. This activity will make a small amount of current, but when the smoke enters the chamber, they will disrupt the activity because the particles of the smoke attach themselves to the both atoms and electrons, preventing them to reach the plate. This will eventually drop the current level that was produced by the charged atoms and the electrons before. When the current drops into a certain level, the alarm will trigger automatically.

        In a smoke detector, the alpha radiation emitted from the Americium-241 material is very small and it is harmful towards human body. Since Americium-241 can be distributed in the air, a person can inhale the element isotope of Americium-241. Because of its harmfulness towards human body, it is suggested that people do not play with any application or technology devices that use a substance of Americium-241.

There are many advantages of having detectors in either private or public places. these advantages are the reduction of losses due to fire, the minimization of expenditure spent due to the fire and the decrease of pollution which can harm the human body as well as the Earth itself by this smoke detector device. Furthermore, smokers will have their smoking problems cured by the presence of smoke detectors as they will avoid smoking in buildings that have smoke detectors installed. Hence, the probability of contracting smoking diseases, such as cancer, heart diseases, etc. can be reduced. Millions of lives can also be saved by smoke detectors and saving the expense of millions of dollars because of the fire occurred. (Chaudhari, n.d.).

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