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Radioactivity

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Radioactivity entered history at the onset of major transitions in science and itself propelled these changes. The field's brief trajectory, spanning three decades, contrasts starkly with the interest it generated and the far reaching results it produced. The story of radioactivity provides a window into the era's science and its cultural matrix. It illuminates the scientific process and the ongoing human quest for understanding. Radioactivity has the dual attractions of a fascinating history and dramatic consequences for humanity. The atomic bomb, nuclear power and changing relations of science to government and the military are obvious results. Though important in their own right, to concentrate on these outcomes would risk distorting the history of radioactivity by viewing the past through the eyes of the present. The radioactivity researchers worked in a very different environment from scientists several decades later. To them radioactivity was an enigma, a discovery with many possibilities for investigation, rather than a prelude to unknowable future developments.

Definition

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Radioactivity is the process of the spontaneous decay and transformation of unstable atomic nuclei accompanied with the emission of nuclear particles or electromagnetic radiation.

Radioactivity

Radioactivity is an important tool for studying nuclear physics without this the present knowledge of nuclear physics would have been very much incomplete natural radioactivity was the chance discovery of Henri Becquerel. Radioactivity is a spontaneous and self disruptive activity exhibited by several heavy elements, like uranium.

Radioactive Elements

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A radioactive element is one that has no stable isotopes. There are eleven radioactive elements that are found on Earth. All versions of the element's atoms are unstable and break apart. Many other elements such as lead and carbon, have a few radioactive isotopes. However, these are very rare compared to their stable version. Several artificial radioactive elements have been made in laboratories. These elements are so unstable that many only exist for a few days or for an even shorter period of time. Most radioactive elements are metals. These metals are all very dense and a small sample weights a lot. The most common radioactive elements on Earth are the metals thorium and uranium. Other radioactive metals, such as radium and actinium are much rarer. Three of the radioactive elements found on earth are not metals. Polonium and astatine are metalloids. Radon is the only radioactive element that is a nonmetal. This element is a gas. It is the only radioactive member of the group of elements called the noble gases.

Uses 

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Nuclear radiation and sources of radioactivity, that is, radionuclide, have become a necessary part of our daily lives. The quantity and quality of our food, our health, general well-being and consequently our extended life span are due in large part to radioactive sources and their numerous applications in medicine, biology, agriculture, industry and electric power generation. The significance of the role that radioactivity plays to improve our lives was commemorated with the postage stamp issued by France in 1965. The stamp illustrates an artistic depiction of an atom together with drawings representing four fields where radioactivity and nuclear energy play a significant role in development, namely, medicine, agriculture, industry and nuclear power for electricity.

Types 

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Radioactivity is a nuclear phenomenon, was first discovered by Henri Becquerel in 1896. Hence, it is the process by which a nuclei undergoes disintegration and emits either alpha or beta and gamma radiations. During the radioactive process, the atom changes its atomic number and chemical identity. An atom, with unstable nuclei and performs radioactivity is called radioisotopes.
Radioactivity may be classified as natural and artificial. The phenomenon of spontaneous emission of rays such as $\alpha$, $\beta$ and $\gamma$ by heavy elements having atomic number greater than 82 is called natural radioactivity. Radium-226 and potassium-40 are examples for natural radioactivity. Artificial or induced radioactivity was discovered by Curie and Joliet in 1934, when they were studying the disintegration of light elements by $\alpha$ particles. They found that when light elements such as boron and aluminium were bombarded with $\alpha$ particles, an unstable nucleus was formed and this nucleus disintegrates spontaneously. The artificial radioactive substance emit electrons, neutrons, positrons or $\gamma$ rays. They follow the same laws of decay as natural radioactivity. Cobalt-60 and phosphorus-32 are examples for artificial radioactivity.

Units

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The term radioactivity refers the disintegration rate of a radioisotope and it is measured by the unit Curie(Ci). One curie is the number of disintegration per second (dps) from 1 gram of radium and it is found to be 3.7$\times10^{10}$ dps. Smaller units such as millicurie (mCi) and microcurie($\mu$Ci) are also used to measure activity.

1 mCi = 3.7$\times10^{7}$dps
and
1 $\mu$Ci = 3.7$\times10^{4}$ dps

Becquerel (Bq) is the SI unit of activity and it is equal to 1 dps.1mCi = 37MBq

Discovery 

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Radioactivity was first described in 1896 by Antoine Henri Becquerel (1852-1908) shortly after the discovery of X-rays by Konrad Wilhelm Rontgen (1845-1923) in 1895. Radioactivity was accidentally discovered by the exposure producing effect on a photographic plate by a mineral containing uranium-pitchblende, when wrapped in a black paper and kept in the dark. Soon after the fundamental discoveries, Pierre Curie (1859-1906) together with his wife Maria Curie-Sklodowska (1867-1934) extracted two new sources of radioactivity from pitchblende- radium and polonium. In 1898, Ernest Rutherford (1871-1937) found that there are at least two components of the radiation emitted by these elements. In 1899, the same physicist distinguished alpha and beta particles. The following year, Paul Villard (1860-1934) discovered and described the gamma rays emitted by radium.At the beginning of 1930s, other important discoveries contributed considerably to our knowledge of radioactivity. A further significant step in the study of radioactivity consisted in the discovery of artificial radionuclides by Frederic Joliot (1900-1958) and Irene Curie(1897-1956) in 1934. At present, we have identified something like 2600 nuclides: 260 stable nuclides, 25 very long lived naturally occurring radionuclides and more than 2300 man made radionuclides. The recent advances in particle accelerators, nuclear instrumentation and experimental techniques have led to an increased ability to prepare new nuclides.

Alpha Beta Gamma Radiations

Detector

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The methods used to detect the different radiations depends on the various types of emitter and the detection purposes. Ionization chamber is one of the examples of radiation detectors. A particle with high energy can separate electrons from the surface of atoms when it strikes. This high energy particle subjected between two parallel plates with opposite charges. If the ion moves towards the opposite charged plates, ionization takes place, this process producing current. Geiger Muller counter is working with this principle. The given figure shows the schematic diagram of Geiger Muller counter.

Radioactivity Detector

Causes 

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As its name implies, radioactivity is the act of emitting radiation spontaneously. This is done by an atomic nucleus that, for some reason, is unstable; it wants to give up some energy in order to shift to a more stable configuration. During the first half of the twentieth century, much of modern physics was devoted to exploring why this happens, with the result that nuclear decay was fairly well understood by 1960.

Effects 

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Some of the adverse effects of radiation is listed below:

  • The exposure of 200 rems or higher dose causes the hair lose.
  • If 5000rems dose is incident on the human body, it will destruct the brain cell directly. And it kills the blood vessels and nerve cells.
  • Exposure of different radiation will badly affect the thyroid gland
  • If radiation dose is exceed 100 rems, the number of lymphocyte in the blood will be reduced and causes infection.
  • The high dose exposure of radiation like 1000-5000 rems immediately lead to death due to heart failure.
  • Radiation will affect the intestinal tract also. This leads to nausea, diarrhea and vomiting.

Measuring Radioactivity

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In the measurement process, the object to be observed is always affected by an undetermined interaction between the observer and the observed. As a result, the measured magnitudes are always reproduced with a certain inherent uncertainty caused by the instrument. This uncertainty in the measurements makes the use of error theory essential. When we measure radioactive substances, the situation becomes even more complicated, because the radioactivity decay is a random process. In radioactivity, counting two types of fluctuations are basically generated, one related to the activity of the sample, when the half-life of the radionuclide is short and another caused by the random nature of radioactivity decay, which modifies the disintegration rates with time. Since the measurement of radioactivity involves values with different degrees of reliability and validity, the principles of counting statistics must be applied. In many types of measurement, such as mass, volume, time, length etc., the measured quantity has a given value and only the measurement conditions introduce statistical variations. The situation is different, however, in a radioactivity measurements. The radioactive decay process follows Poison statistics, so a sample's activity value is not a specific value but a mean value that varies with time.

Examples 

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The examples of radioactivity are;

$\alpha$ decay: Emission of a doubly ionized 4He nucleus with two units of positive charge.

$\beta$ decay:
  • $\beta^{-}$ decay: Emission of an electron and antineutrino
  • $\beta^{+}$ decay: Emission of a positron and neutrino.
  • Electron Capture (EC): The nucleus captures one electron which is in an atomic shell outside the nucleus.
  • Double $\beta$ decay: Emission of two electrons (or positrons) and two antineutrinos (neutrinos) at the same time. There may also occur double \beta decay by the emission of two electrons without the emission of two neutrinos.
$\beta^{-}$, $\beta^{+}$ and EC are collectively called $\beta$ decay

$\gamma$ decay: Emission of electromagnetic radiation with energies in the range of a few keV up to a few MeV.
  • Internal Conversion(IC): The nucleus gives its excitation energy to an atomic electron and the electron is ejected from the atom.
  • Internal pair creation:The nucleus gives its excitation energy to create an $e^{-}$-$e^{+}$ pair which carries away the remaining energy.
Spontaneous Fission (SF): The nucleus spontaneously breaks into two nuclei with about the same mass with 0-10 neutrons emitted.
  • Heavy Cluster Radioactivities: Light nuclei like 14C and 24Ne are emitted in a highly asymmetric spontaneous fission but without neutron emission.

How do Radioactive Isotopes Decay?

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During radioactive decay, an atom is transformed into an isotope of another element. Sometimes that new isotope is stable and sometimes the new isotope is radioactive. If it is radioactive, it will decay. When one radioactive isotope decays or is transformed into another radioactive isotope which in turn decays, a decay chain is formed. A decay chain can contain two or more radioactive isotopes and always ends with a stable isotope.The rate of radioactive decay is called activity and is expressed as the number of disintegration per second. Some natural radioisotopes have long decay chain.