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This is a list of radioactive nuclides (sometimes also called isotopes), ordered by half-life from shortest to longest, in seconds, minutes, hours, days and years. Current methods make it difficult to measure half-lives between approximately 10 −19 and 10 −10 seconds.
Instead, the half-life is defined in terms of probability: "Half-life is the time required for exactly half of the entities to decay on average". In other words, the probability of a radioactive atom decaying within its half-life is 50%. [2] For example, the accompanying image is a simulation of many identical atoms undergoing radioactive decay.
The half-life of tritium is 12.3 years. The electrons from beta emission from tritium are so low in energy (average decay energy 5.7 keV) that a Geiger counter cannot be used to detect them. An advantage of the low energy of the decay is that it is easy to shield, since the low energy electrons penetrate only to shallow depths, reducing the ...
As time passes after such material has formed, uranium-234 in the sample with a half-life of 245,000 years decays to thorium-230. [4] Thorium-230 is itself radioactive with a half-life of 75,000 years, [ 4 ] so instead of accumulating indefinitely (as for instance is the case for the uranium–lead system), thorium-230 instead approaches ...
11 C decays by positron emission with a half-life of ca. 20 min. 11 C is one of the isotopes often used in positron emission tomography. [3] 14 C decays by beta decay, with a half-life of 5730 years. It is continuously produced in the upper atmosphere of the earth, so it occurs at a trace level in the environment.
This is a list of some of the more commonly known problems that are NP-complete when expressed as decision problems. As there are thousands of such problems known, this list is in no way comprehensive. Many problems of this type can be found in Garey & Johnson (1979).
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In clinical practice, this means that it takes 4 to 5 times the half-life for a drug's serum concentration to reach steady state after regular dosing is started, stopped, or the dose changed. So, for example, digoxin has a half-life (or t 1 / 2 ) of 24–36 h; this means that a change in the dose will take the best part of a week to ...