Radıoactıvıty
12 Temmuz 2007
RADIOACTIVITY
Radioactivity basically is the property of some atoms to spontaneously give off energy as particles or rays. The atoms that make up the radioactive materials are the source of radiation.
Radioactivity of atoms depends on some basis. The balance of the forces in the nucleus of an atom determines whether a nucleus is stable or unstable. An atom is unstable (radioactive) if these forces are unbalanced; if the nucleus has an excess of internal energy. Instability of an atom’s nucleus may result from an excess of either neutrons or protons. A radioactive atom will attempt to reach stability by giving off nucleons (protons or neutrons), as well as other particles, or by releasing energy in other forms.
These unstable atoms become stable as they go through radioactive decays. These decay types will be further discussed under the sub-title Decay Types.
Except for Hydrogen, all atoms nuclei have more than one proton. Since like charges repel, all nuclei are unstable. The electrostatic force is not the only force present in a nucleus. Protons do repel each other, but also there is the strong force in the nucleus, which acts to overcome the electrostatic force of repulsion within the nucleus, and it binds nucleons into a package. This force has a characteristic. It decreases far more rapidly with distance than an electrostatic force. Force exerted by one nucleon on another nucleon falls to zero within the nucleus. The strong force between two adjacent nucleons, doesnt contribute anything to the binding of the nucleons on the other side of the nucleus. Electrostatic force in the nucleus never falls off to zero. Proton in one nuclear region repel proton in all other regions. Such repulsions are toned down by the intervening nucleon because they help separate the protons.
When nuclei carry large numbers of proton without enough intermingled neutrons to dilute the electrostatic repulsions, the result is an unstable nucleus. Fission is a possible consequence of this instability only in Uranium-235 and other fissile material. Other mechanisms used by all isotopes, including Uranium-235, include ejection of small nuclear fragments and high-energy electromagnetic radiation in order to achieve stability.
Required definitions:
Radionuclides isotopes whose nuclei emit particles or energy
Radioactivity the emission itself
Radioactive materials that have the ability to be radio nuclides
Types of Radioactive Decays
As stated before, there are some types of decays that substances go through in order to become stabilized. These ways are listed below.
Alpha Decay:
Alpha particles (symbol a ) are a type of ionizing radiation ejected by the nuclei of some unstable atoms. They are large subatomic fragments consisting of 2 protons and 2 neutrons. Ernest Rutherford, an English scientist, discovered alpha particles in 1899 while working with uranium. Rutherford’s studies contributed to our understanding of the atom and its nucleus through the Rutherford-Bohr planetary model of the atom. An alpha particle is identical to a helium nucleus having two protons and two neutrons. It is a relatively heavy, high-energy particle, with a positive charge of +2 from its two protons. Alpha particles have a velocity in air of approximately one-twentieth the speed of light, depending upon the individual particle’s energy. When the ratio of neutrons to protons in the nucleus is too low, certain atoms restore the balance by emitting alpha particles. Alpha emitting atoms tend to be very large atoms (that is, they have high atomic numbers). With some exceptions, naturally occurring alpha emitters have atomic numbers of at least 82 (the element lead). The nucleus is initially in an unstable energy state. An internal change takes place in the unstable nucleus and an alpha particle is ejected leaving a decay product. The atom has then lost two protons along with two neutrons.
Since the number of protons in the nucleus of an atom determines the element, the loss of an alpha particle actually changes the atom to a different element. For example, polonium-210 is an alpha emitter. During radioactive decay, it loses two protons, and becomes a lead-206 atom, which is stable (i.e., non radioactive).
Beta Decay:
Beta particles are subatomic particles ejected from the nucleus of some radioactive atoms. They are equivalent to electrons. The difference is that beta particles originate in the nucleus and electrons originate outside the nucleus. Henri Becquerel is credited with the discovery of beta particles. In 1900, he showed that beta particles were identical to electrons, which had recently been discovered by Joseph John Thompson. Beta particles have an electrical charge of -1. Beta particles have a mass of 549 millionths of one atomic mass unit, or AMU, which is about 1/2000 of the mass of a proton or neutron. The speed of individual beta particles depends on how much energy they have, and varies over a wide range.
While atoms that are radioactive emit beta particles, beta particles themselves are not radioactive. It is their energy, in the form of speed that causes harm to living cells. When transferred, this energy can break chemical bonds and form ions. Beta particle emission occurs when the ratio of neutrons to protons in the nucleus is too high. Scientists think that an excess neutron transforms into a proton and an electron. The proton stays in the nucleus and the electron is ejected energetically.
This process decreases the number of neutrons by one and increases the number of protons by one. Since the number of protons in the nucleus of an atom determines the element, the conversion of a neutron to a proton actually changes the radio nuclide to a different element.
Often, gamma ray emission accompanies the emission of a beta particle. When the beta particle ejection doesn’t rid the nucleus of the extra energy, the nucleus releases the remaining excess energy in the form of a gamma photon.
Beta particles travel several feet in open air and are easily stopped by solid materials. When a beta particle has lost its energy, it is like any other loose electron. Whether in the outdoor environment or in the body, these electrons are then picked up by a positive ion.
Gamma Decay:
A gamma ray is a packet of electromagnetic energy–a photon. Gamma photons are the most energetic photons in the electromagnetic spectrum. Gamma rays (gamma photons) are emitted from the nucleus of some unstable (radioactive) atoms. Physicists credit French physicist Henri Becquerel with discovering gamma radiation. In 1896, he discovered that uranium minerals could expose a photographic plate through a heavy opaque paper. Roentgen had recently discovered x-rays, and Becquerel reasoned that uranium emitted some invisible light similar to x-rays. He called it “metallic phosphorescence.”
In reality, Becquerel had found gamma radiation being emitted by radium-226. Radium-226 is part of the uranium decay chain and commonly occurs with uranium.
Gamma radiation is very high-energy ionizing radiation. Gamma photons have about 10,000 times as much energy as the photons in the visible range of the electromagnetic spectrum.
Gamma photons have no mass and no electrical charge - they are pure electromagnetic energy.
Because of their high energy, gamma photons travel at the speed of light and can cover hundreds to thousands of meters in air before spending their energy. They can pass through many kinds of materials, including human tissue. Very dense materials, such as lead, are commonly used as shielding to slow or stop gamma photons.
Their wavelengths are so short that they must be measured in nanometers, billionths of a meter. They range from 3/100ths to 3/1,000ths of a nanometer.
Gamma rays and x-rays, like visible, infrared, and ultraviolet light, are part of the electromagnetic spectrum. While gamma rays and x-rays pose the same hazard, they differ in their origin. Gamma rays originate in the nucleus. X-rays originate in the electron fields surrounding the nucleus.
Gamma radiation emission occurs when the nucleus of a radioactive atom has too much energy. It often follows the emission of a beta particle. Cesium-137 provides an example of radioactive decay by gamma radiation. Scientists think that a neutron transforms to a proton and a beta particle. The additional proton changes the atom to barium-137. The nucleus ejects the beta particle. However, the nucleus still has too much energy and ejects a gamma photon (gamma radiation) to become more stable. Gamma rays exist only as long as they have energy. Once their energy is spent, whether in air or in solid materials, they cease to exist. The same is true for x-rays.
Most people’s primary source of gamma exposure is naturally occurring radio nuclides, particularly potassium-40, which is found in soil and water, as well as meats and high-potassium foods such as bananas. Radium is also a source of gamma exposure. However, the increasing use of nuclear medicine (e.g., imaging procedures such as CAT scans) contributes an increasing proportion of the total for many people. Also, some man-made radionuclides that have been released to the environment emit gamma rays.
Most exposure to gamma and x-rays is direct external exposure. Most gamma and x-rays can easily travel several meters through air and penetrate several centimeters in tissue. Some have enough energy to pass through the body, exposing all organs. X-ray exposure of the public is almost always in the controlled environment of dental and medical procedures.
Although they are generally classified as an external hazard, gamma-emitting radionuclides do not have to enter the body to be a hazard. Gamma emitters can also be inhaled, or ingested with water or food, and cause exposures to organs inside the body. Depending on the radionuclide, they may be retained in tissue, or cleared via the urine or feces.
Both direct (external) and internal exposure to gamma rays or X-rays is of concern. Gamma rays can travel much farther than alpha or beta particles and have enough energy to pass entirely through the body, potentially exposing all organs. A large portion gamma radiation largely passes through the body without interacting with tissue–the body is mostly empty space at the atomic level and gamma rays are small in size. By contrast, alpha and beta particles inside the body lose all their energy by colliding with tissue and causing damage. X-rays behave in a similar way, but have slightly lower energy.
Gamma rays do not directly ionize atoms in tissue. Instead, they transfer energy to atomic particles such as electrons (which are essentially the same as beta particles). These energized particles then interact with tissue to form ions, in the same way radio nuclide-emitted alpha and beta particles would. However, because gamma rays have more penetrating energy than alpha and beta particles, the indirect ionizations they cause generally occur farther into tissue (that is, farther from the source of radiation).
Because of the gamma ray’s penetrating power and ability to travel great distances, it is considered the primary hazard to the general population during most radiological emergencies. In fact, when the term “radiation sickness” is used to describe the effects of large exposures in short time periods, the most severe damage almost certainly results from gamma radiation.
Neutron Radiation
Neutron radiation is energy released from an atom in the form of neutral particles called neutrons. Neutrons are part of the basic building blocks of atoms. They have no charge and are about the same mass as a proton. Due to ion-producing collisions with matter and absorption/decay processes, neutrons are a type of ionizing radiation. They were discovered by James Chadwick, who received the 1935 Nobel Prize in physics for his work.
During the fission process, as well as certain decay processes, neutrons are emitted from the nucleus of an atom. This is neutron radiation. Combining alpha-emitting isotopes with beryllium produces a neutron source. Accelerators are another means of producing neutron radiation. In the upper atmosphere, the interaction of cosmic radiation with air also produces neutron radiation.
Neutron radiation is used by researchers to investigate the sub-atomic structure of matter. It has been used in the security arena as part of a technology that can investigate the existence of explosives and other dangerous items. The medical community relies on neutron radiation for production of medical isotopes and certain direct therapies.
Significant neutron radiation in the human environment is rare. Possibilities include an improperly handled neutron source and an unexpected criticality accident. Neither of these occurrences are likely in an area of human population due to the secure locations where these materials are handled.
Positron Decay:
A positron is a particle that has the same mass as an electron but has a positive charge. Positron decay will occur when there are too many protons in the nucleus, but there isn’t enough energy to emit an alpha particle.
In this case, a proton is converted into a neutron, and nucleus emits a positron and a neutrino. This increases the number of neutrons by one, decreases the number of protons by one, and leaves the atomic mass unchanged. By changing the numbers of protons, however, positron decay transforms the nuclide into a different element.
Electron Capture:
Electron capture will occur when there are too many protons in the nucleus, and there isn’t enough energy to emit a positron.
In this case, a proton in the nucleus, forming a neutron and a neutrino, captures one of the orbital electrons. Since the proton is essentially changed to a neutron, the number of neutrons increases by 1, the number of protons decreases by 1, and the atomic mass remains unchanged. By changing the number or protons, electron capture transforms the nuclide into a new element.
Isometric Transition:
If, after previous attempts at stabilization, the nucleus still has excess energy, it can emit energy without changing the number of protons or neutrons. Processes that accomplish this are called “isomeric transition.”
Isomeric transition includes gamma ray emission, as well as internal conversion. Internal conversion is an alternate mechanism of shedding excess energy. In internal conversion, the excess energy of the nucleus is transmitted to one of the orbital electrons, and the electron may be ejected from the atom. This process usually competes with gamma radiation. Internal conversion can occur only if the amount of energy given to the orbital electron exceeds its binding energy.
Penetrating Abilities of Decays:
Decays differ each other in many ways. As well as their differences on the structure of the atom, their penetrating ability differs too. Heres a diagram to show you the difference:
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