Uranium isotopes decay rate

The Oklo Phenomenon. Naturally occurring uranium consists mainly of and fissionable The isotopic ratio can be calculated from the relative decay rates of the two isotopes. Because decays faster than the isotopic ratio decreases with time. In 1997, the isotopic abundance of 235u... 

The sequences of radioactive decays that lead to lead are well-known and the rates of decay have been carefully measured. We shall consider the sequence based upon the relatively slow decomposition of the most abundant uranium isotope, mass 238 (natural abundance, 99%) ... 

Th and 231Pa are ubiquitous components of recently deposited deep-sea sediments because they are produced uniformly throughout the ocean from the decay of dissolved uranium isotopes and they are actively collected onto sinking particles. The distribution with depth of these nuclides in deep-sea sediments may be modeled to estimate rates of sedimentation extending over the past 200 to 300 thousand years. These techniques complement 14C dating methods that only extend to about 40 thousand years before the present. 

Pb. The total amount and age of uranium combined with the differences in decay rate of the two uranium isotopes leads to the production of distinct ° Pb/ ° Pb lead isotope ratios uniquely related to mineralization (e.g., Gulson 1986 Holkefa/. 2003). 

Ernest Rutherford, however, used the alpha decay of uranium, which produces helium, to estimate the ages of several uranium ores in 1906. By measuring the ratio of helium to uranium and the current rate of helium production (that is, the current decay rate of uranium), he deduced that the minerals were at least 440 million years old. All this was, of course, before anyone knew anything about isotopes. 

The one-body theory of a decay applies strictiy to even-even a emitters only. The odd-nucleon a emitters, especially in ground-state transitions, decay at a slower rate than that suggested by the simple one-body formulation as applied to even-even nuclei. Consider the data in Figure 7.9 that shows the a-decay half-lives of the even-even and odd A uranium isotopes. The odd A nuclei have substantially longer half-lives than their even-even neighbors do. 

The uranium isotopes often encountered in the radioanalytical chemistry laboratory are listed in Table 6.2. In natural uranium, the relative decay rates at equilibrium are 1.0 Bq 1.0 Bq and 0.045 Bq Emiched (containing relatively more and and depleted (relatively more U) combinations are also encountered, as are in neutron-irradiated mixtures and from some processes. These uranium isotopes emit alpha particles, characteristic 13-keV L X rays, and generally several weak gamma rays. Several isotopes have numerous minor alpha-particle or gamma-ray transitions that are not listed. 

The half-life of the uranium isotope is about 1 X 10 times larger than the half-life of the polonium isotope. Unlike the rate constants for chemical reactions, moreover, the rate constants for nuclear decay are unaffected by changes in environmental conditions, such as temperature and pressure (see Table 17.1). 

The term half-life (VJ is defined as the required time for the activity of a radioactive substance to decay to half of its initial value. For some nuclides like this would take 4.468 10 years while in other uranium isotopes like this would occur after 68.9 years (see Table 1.2). Thus, the rate of emission of alpha particles from a given number of nuclei would be about 64.5 million times higher than from the same number of nuclei. 

The Voyager space vehicle launched in 1977 contained a gold-plated copper disk electroplated with a patch of pure uranium-238 isotope. If this disk were recovered by some advanced alien civilization, the disk s age could be determined from the radioactive decay rate of the isotope. 

Neutron-rich lanthanide isotopes occur in the fission of uranium or plutonium and ate separated during the reprocessing of nuclear fuel wastes (see Nuclearreactors). Lanthanide isotopes can be produced by neutron bombardment, by radioactive decay of neighboring atoms, and by nuclear reactions in accelerators where the rate earths ate bombarded with charged particles. The rare-earth content of solid samples can be determined by neutron... 

Half-lives can be interpreted in terms of the level of radiation of the corresponding isotopes. Uranium has a very long half-life (4.5 X 109 yr), so it gives off radiation very slowly. At the opposite extreme is fermium-258, which decays with a half-life of 3.8 X 10-4 s. You would expect the rate of decay to be quite high. Within a second virtually all the radiation from fermium-258 is gone. Species such as this produce very high radiation during their brief existence. 

Many scientists thought that Earth must have formed as long as 3.3 billion years ago, but their evidence was confusing and inconsistent. They knew that some of the lead on Earth was primordial, i.e., it dated from the time the planet formed. But they also understood that some lead had formed later from the radioactive decay of uranium and thorium. Different isotopes of uranium decay at different rates into two distinctive forms or isotopes of lead lead-206 and lead-207. In addition, radioactive thorium decays into lead-208. Thus, far from being static, the isotopic composition of lead on Earth was dynamic and constantly changing, and the various proportions of lead isotopes over hundreds of millions of years in different regions of the planet were keys to dating Earth s past. A comparison of the ratio of various lead isotopes in Earth s crust today with the ratio of lead isotopes in meteorites formed at the same time as the solar system would establish Earth s age. Early twentieth century physicists had worked out the equation for the planet s age, but they could not solve it because they did not know the isotopic composition of Earth s primordial lead. Once that number was measured, it could be inserted into the equation and blip, as Patterson put it, out would come the age of the Earth. ... 

The best sealed-in minerals are zircons, zirconium silicate minerals which are formed when melted lava on the flanks of volcanoes solidifies. When the zircons crystallize out, they incorporate radioactive uranium (in particular 238U), which decays in several steps, leading Anally to the lead isotope 208Pb. The rate of decay is very low, as the half-life of uranium-238 is 4.5 x 109 years. Thus, the U-Pb-zircon method for age determination of Precambrian rock is very important. The fossils studied by Schopf were sandwiched between two lava layers (Schopf, 1999). The volcanic layers were dated to 3.458 0.0019 x 109 years and 3.471 0.005 x 109 years the age of the fossil layer (Apex chert) was thus determined to be about 3.465xlO9 years. 

Due to the extremely slow rate of decay, the total amount of natural thorium in the earth remains almost the same, but it can be moved from place to place by nature and people. For example, when rocks are broken up by wind and water, thorium or its compounds becomes a part of the soil. When it rains, the thorium-containing soil can be washed into rivers and lakes. Also, activities such as burning coal that contains small amounts of thorium, mining or milling thorium, or making products that contain thorium also release thorium into the environment. Smaller amounts of other isotopes of thorium are produced usually as decay products of uranium-238, uranium-235, and thorium-232, and as unwanted products of nuclear reactions. 

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