Thorium longest lived isotope

The element exists as an intermediate in uranium and thorium minerals through their decay. There is no stable isotope. The longest-living isotope has a half-life of 8.3 hours. In the crust of the Earth, the total steady-state mass is estimated at a few tens of grams. Thus astatine is the rarest element (record ). A few atoms of this relative of iodine can be found in all uranium ore. It exhibits certain metallic properties. 

Radium is element number 88, in which all of its isotopes are radioactive hence, what little radium is found on Earth is mostly as a trace element in uranium ores. The most common isotope has a mass number of 226 with a half-life of 1,604 years. The second longest-lived isotope is radium 228, with a half-life of 5.77 years. The other isotopes have much shorter half-lives ranging from microseconds to days. Radium is constantly being formed as part of the radioactive decay series of uranium and thorium. Because it decays so quickly, however, only minute quantities of radium ever exist at any one time. 

Astatine is the heaviest member of group 17 and is known only in the form of radioactive isotopes, all of which have short half-lives. The longest lived isotope is At (fi = 8.1 h). Several isotopes are present naturally as transient products of the decay of uranium and thorium minerals At is formed from the 3-decay of Po, but the path competes with decay to Pb (the dominant decay, see Figure 2.3). Other isotopes are artificially prepared, e.g. "At (an a-... 

This reaction is the lission of uranium nucleus by protons accelerated to very high energies. When such a fast proton hits uranium nucleus it produces something like an explosion with ejection of a multitude of particles, namely, six protons and 21 neutrons. Of course, the reaction is not due to a blind chance but is based on careful theoretical predictions. Uranium may be replaced with thorium. The reaction product, francium-212, for some time was considered to be the longest-lived isotope (a half-life of 23 min) but later the half-life was found to be only 19 min. 

In today s parlance, we call the radium emanation radon-222. (Like radium, the word radon comes from the Latin radius, for ray or beam. ) The alpha decay of radium-226 produces radon-222 and helium-4. The thorium emanation is radon-220, but the decay scheme from thorium-232 is more involved (see Problem 19.11). Rn-222 and Rn-220 are the two longest-lived isotopes of radon, the heaviest and rarest of the noble gases. (There are now 36 known isotopes of radon with mass numbers ranging from 193 to 228.) For many years, Dorn was generally credited as the sole discoverer of radon. However, as noted above, Ernest Rutherford and his co-workers, particularly Frederick Soddy, should be given at least equal billing. 

It was known [38] before 1910 that the longest-lived (still radioactive) descendant of Rn ( radium D ), °Pb(ti/2 = 21 years), cannot be chemically separated from nonradioactive lead, but it was first shown [44,45] in 1914 that lead isolated from uranium minerals can be mainly Pb and lead from thorium minerals can be predominantly ° Pb, compared to the usual isotope mixture with 207.2 amu. Less than ten years later, it was established that isotopy is not restricted to + values above 205 but that the majority of elements show two or more stable isotopes in mass-spectra. 

Th. The isotope Th, with a half-life of 7340 years, is the longest-lived radioactive daughter of U (Table 5.4). It is present at very low concentration in aged irradiated thorium. 

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