Radioactive isotopes mean life

FIGURE 61 The decay of radiocarbon. Radiocarbon is a radioactive isotope whose half-life is 5730 + 40 years. This means that half of the original amount of radiocarbon in any carbon-containing sample will have disintegrated after 5730 years. Half of the remaining radiocarbon will have disintegrated after 11,400 years, and so forth. After about 50,000 years the amount of radiocarbon remaining in any sample is so small that older remains cannot be dated reliably. 

Another isotopic anomaly, discovered in Allende inclusions, concerns magnesium, for which an intrinsically low abundance in these samples makes its isotope ratios sensitive to small effects. Certain of the inclusions show a correlation between 26Mg and 27 Al, indicating an origin of excess 26Mg from radioactive decay of 26 A1 (mean life 1 Myr), the existence of which had previously been postulated as a heat source for meteorite parent bodies (Fig. 3.32). Other short-lived activites that seem to have been alive in the early Solar System are 10Be (mean life 2.2 Myr) from a correlation of 10B with 9Be, and 41Ca (mean life 0.15 Myr) from a correlation of... 

ISOTOPES There are a total of 59 radioactive isotopes for bismuth, ranging in half-lives from a few milliseconds to thousands of years. At one time it was thought that there was just one stable isotope (Bi-209), but it was later found that Bi-209 is radioactive with a half-life of 19,000,000,000,000,000,000 years. Such a long half-life means that BI-209 has not completely disintegrated and Is still found In nature, and Is thus considered stable. In this case, BI-209 makes up 100% of Bismuth s natural abundance. 

The abundances of radioactive isotopes over time in the galaxy can be modeled based on the above considerations. With an approximately constant production rate, the abundance of a stable nuclide will grow and will be proportional to the time over which it has been produced. In contrast, the abundance of a radionuclide will reach a steady state between production and decay in about eight mean lifetimes. (We will use mean life (t) instead of... 

Half-lives are remarkably constant and not affected by external conditions. Some radioactive isotopes have half-lives that are less than a millionth of a second, while others have half-lives of more than a billion years. For example, uranium-238 has a half-life of 4.5 billion years, which means that in 4.5 billion years, half the uranium in Earth today will be lead. 

Drugs that have a radioactive isotope incorporated into their structures are invariably used in studies to establish safe withdrawal periods. The presence of the radioactive isotope is harmless to the animal, but offers a highly sensitive means of determining the biological half-life of the drug and the total residue concentrations in different tissues in different animals. 

Exponentials play a useful role in understanding nuclear disintegrations and half-lives. For example, 14C, a radioactive isotope of carbon used for carbon dating, has a half-life / /2 = 5730 years before it converts into stable 147V. This means that the number... 

Like all radioactive isotopes, C-14 decays at a predictable rate. Its half-life of 5,730 years means that one-half the amount of C-14 normally present in a living organism is present in an organism that has been dead for 5,730 years. By suitable manipulation of the mathematics involved in half-life calculations, the approximate age of the remains of plants and animals can be determined. 

A radioactive isotope of phosphorus has a half-life of 14 days (that means it takes 14 days for the amount of to fall to half its original value, and another 14 days to fall to a quarter, and so on). We can plot a graph of amount of ip against time in days from these figures ... 

Tantalum-180m is radioactive. The suffix m means that the isotope is metastable, that is, it changes to a more stable state over time. A radioactive isotope is one that breaks apart and gives off some form of radiation. The half life of a radioactive element is the time it takes for half of a sample of the element to break down. Tantalum-180m has a half life... 

There are 280 naturally occurring nuclides that make up the 83 stable and long-lived elements. These are all the elements up to Bi with Z = 83, except for unstable Tc (Z = 43) and Pm (Z = 61) that only have short-lived isotopes, but the long-lived Th and U bring the total back to 83. Here long-lived or short-lived is with respect to the half-life of an isotope against radioactive decay and the age of the solar system. Long-lived means then an element is still present in measurable quantities since the solar system formed 4.6 Gyr ago, and radioactive isotopes with half-lives above 0.6 Gyr usually qualify... 

Like all radioactive isotopes, it decays. U-238 decays very slowly, its half-life being the same as the age of the earth. This means that it is barely radioactive, less so than many other isotopes in rocks and sand. U-238 has a specific radioactivity of 12.4 kBq/g, and U-235 80 kBq/g, so natural uranium is 13 kBq/g. In decay it generates 0.1 watts/t, and this is enough to warm the earth s mantle. 

A patient is given 0.050 mg of technetium-99m (where m means metastable— an unstable but long-lived state), a radioactive isotope with a half-life of about 6.0 hours. How long until the radioactive... 

Al-26 comes in two forms -- one with a 6.4 second half-life and one with a 7 million year half-life. Some radioisotopes have three different half-lives. So, for radioactive isotopes, we sometimes have to add to our symbol to tell which decay mode we mean. In the case of Al-26, the 6.4 second activity is a "higher energy state" and is called Al-26m. In class we will discuss decay schemes of radioisotopes we encounter at UWNR are produced by absorbing a neutron in a stable element and are "neutron rich" — which... 

The speed of the radioactive decay (disintegration) is governed by only the nature of the nucleus, and often is characterized by the time that it takes for one half of a sample of a radioactive isotope to disintegrate. This time is called half-life. Some isotopes have very short half-life e.g., 2.4 s in the case of They disintegrate very quickly. Others have very long half-life e.g. 4.5 x 10 years in the case of This means that it takes 4.5 billion years (approximately the life span of the Earth) for 1 g of radioactive to become half a gram of radioactive in this process, the end product is lead through a multistep process. 

It is assumed that the average heat generation due to the radioactive processes described above was around 8x 10 6 J/g of rock (Birch, 1954). If this value is extrapolated to the Earth s crust (to a depth of 35 km), the result is 53 J/cm2/year. It seems clear that the crust of the primeval Earth contained about four times as much 40K as today. The higher half-life of 238U means that the amount of this isotope present about four billion years ago was around twice today s value. The corresponding factor for 235U, with a half-life of 7x 108 years, is 64. Thus, the value of... 

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