Radioactive decay radioisotopic dating

The Kinetics of Radioactive Decay Rate of Radioactive Decay Radioisotopic Dating... 

The fluorescence decay time is one of the most important characteristics of a fluorescent molecule because it defines the time window of observation of dynamic phenomena. As illustrated in Figure 3.2, no accurate information on the rate of phenomena occurring at time-scales shorter than about t/100 ( private life of the molecule) or longer than about 10t ( death of the molecule) can be obtained, whereas at intermediate times ( public life of the molecule) the time evolution of phenomena can be followed. It is interesting to note that a similar situation is found in the use of radioisotopes for dating the period (i.e. the time constant of the exponential radioactive decay) must be of the same order of magnitude as the age of the object to be dated (Figure 3.2). 

The alkali metals are not found free in nature, because they are so easily oxidized. They are most economically produced by electrolysis of their molten salts. Sodium (2.6% abundance by mass) and potassium (2.4% abundance) are very common in the earth s crust. The other lA metals are quite rare. Francium consists only of short-lived radioactive isotopes formed by alpha-particle emission from actinium (Section 26-4). Both potassium and cesium also have natural radioisotopes. Potassium-40 is important in the potassium-argon radioactive decay method of dating ancient objects (Section 26-12). The properties of the alkali metals vary regularly as the group is descended (Table 23-1). 

With its predictable and unchanging rates, radioactive decay has provided scientists with a technique for determining the age of fossils, geological formations, and human artifacts. Using a knowledge of the half-life of a given radioisotope, one can estimate the age of an object in which the iso- tope is found. Four different isotopes are commonly used for dating objects carbon-14, uranium-238, rubidium-87, and potassium-40. Now look at one of these techniques in more detail. 

Discovery of Radioactivity Nuclear Notation Radioactive Decay Detecting Radioactivity Half Life and Radioisotope Dating ChemLab The Radioactive Decay of Pennium ... 

Expressing the decay rate (activity) for radioactive of an object in radioisotopic dating) (773) ... 

Fe beta particles are counted with a proportional detector or its gamma rays are analyzed with a Ge detector and spectrometer. The sample is then measured for Fe content with a thin Ge detector and spectrometer or xenon-filled X-ray proportional detector with a thin (e.g., 140 mg cm ) beryllium absorber. The Fe count rate is adjusted for background, the Fe contribution, self-absorption in the plated sample, and the chemical yield, and converted to the disintegration rate. The activity of both radioisotopes is corrected for radioactive decay from the sampling date. 

RADIOACTIVE DECAY RATES AND DETECTION OF RADIOACTIVITY (SECTIONS 21.4 AND 21.5) The SI unit forthe activity of a radioactive source is the becquerel (Bq), defined as one nuclear disintegration per second. A related imit, the curie (Ci), corresponds to 3.7 X lO disintegrations per second. Nuclear decay is a first-order process. The decay rate (activity) is therefore directly proportional to the number of radioactive nuclei. The half-life of a radionuclide, which is a constant independent of temperatiu e, is the time needed for one-half of the nuclei to decay. Some radioisotopes can be used to date objects C, for example, is used to date organic objects. Geiger counters and scintillation counters coimt the emissions from radioactive samples. The ease of detection of radioisotopes also permits their use as radiotracers to foUow elements through reactions. 

Radical or free radical (10.4) Radioactive dating (10.7) Radioactive decay (10.1) Radioactive nuclei (10.1) Radioisotope (10.2)... 

Radiocarbon dating (43) has probably gained the widest general recognition (see Radioisotopes). Developed in the late 1940s, it depends on the formation of the radioactive isotope and its decay, with a half-life of 5730 yr. After forms in the upper stratosphere through nuclear reactions of... 

The decay of °Th leads to radioisotopes of other elements, ultimately concluding with the stable isotope lead-206. Happily, some of the oldest rocks on Earth, called zircons, contain no lead when they are formed. This means that the amount of lead they accumulate over time from uranium decay reflects their age. Until the rocks crystallized, uranium atoms could move freely through the molten magma from which they formed, and decayed uranium could be replenished. Solidification of a zircon does for uranium what an organism s death does for radiocarbon it stops the influx of fresh radioactive material, and the decay clock starts ticking. Because of U s long half-life, zircons can be dated back to the Earth s earliest days. 

Because radioactive isotopes seem to decay at very constant rates, they can be used as clocks. One of the first radioactive dating techniques involved the use of the radioisotope carbon-14. Carbon-14 is produced in the upper atmosphere when neutrons (produced by cosmic rays from space) collide with nitrogen-14 molecules in the reaction shown below ... 

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