Radioactive elements, half-life

The nature of the radioactive decay is characteristic of the element it can be used to fingerprint die substance. Decay continues until bodi die original element and its daughter isotopes are non-radioactive. The half-life, i.e. die time taken for half of an element s atoms to become non-radioactive, varies from millions of years for some elements to fractions of a second for odiers. 

The degree of activation of the sample is measured by post-irradiation spectroscopy, usually performed with high-purity semiconductors. The time-resolved intensity measurements of one of the several spectral lines enables to get the half-life of the radioactive element and the total number of nuclear reactions occurred. In fact, the intensity of a given spectral line associated with the decay of the radioactive elements decreases with time as Aft) = Aoexp[—t/r], where Aq indicates the initial number of nuclei (at t = 0) and r is the decay time constant related to the element half-life (r = In2/ /2), which can be measured. Integrating this relation from t = 0 to the total acquisition time, and weighting it with the detector efficiency and natural abundance lines, the total number of reactions N can be derived. Then, if one compares this number with the value obtained from the convolution of... 

The half-life of a radioactive decay is the period of time required for half of the initial amount of the substance to disintegrate. The shorter the half-life of a radioactive decay, the higher the rate of radioactive decay and the more radioactivity. The half-life is the characteristic property of each element. 

Lanthanum-138 is very rare and is radioactive. Its half life is about 100 billion years. The half life of a radioactive element is the time it takes for half of a sample of the element to break down. Only 5 grams of a 10-gram sample of lanthanum-138 will remain after 100 billion years. The other 5 grams would have broken down to form a new isotope. 

Radioactive substances decay into stable end products with rates widely varying with the half-life periods of the elements. Half-life is the time period over which an element decays into half of its initial concentration. The rate of disintegration follows first-order kinetics. It is interesting to note that the half-life period of isotopes may vary enormously, from billions of years to milliseconds. 

Equations 13.31 and 13.32 are only valid if the radioactive element in the tracer has a half-life that is considerably longer than the time needed to conduct the analysis. If this is not the case, then the decrease in activity is due both to the effect of dilution and the natural decrease in the isotope s activity. Some common radioactive isotopes for use in isotope dilution are listed in Table 13.1. 

Each of the elements has a number of isotopes (2,4), all radioactive and some of which can be obtained in isotopicaHy pure form. More than 200 in number and mosdy synthetic in origin, they are produced by neutron or charged-particle induced transmutations (2,4). The known radioactive isotopes are distributed among the 15 elements approximately as follows actinium and thorium, 25 each protactinium, 20 uranium, neptunium, plutonium, americium, curium, californium, einsteinium, and fermium, 15 each herkelium, mendelevium, nobehum, and lawrencium, 10 each. There is frequently a need for values to be assigned for the atomic weights of the actinide elements. Any precise experimental work would require a value for the isotope or isotopic mixture being used, but where there is a purely formal demand for atomic weights, mass numbers that are chosen on the basis of half-life and availabiUty have customarily been used. A Hst of these is provided in Table 1. 

Radon is the heaviest of the hehum-group elements and the heaviest of the normal gaseous elements. It is strongly radioactive. The most common isotope, Rn, has a half-life of 3.825 days (49). Radon s scarcity and radioactivity have severely limited the examination of its physical properties, and the values given ki Table 3 are much more uncertain than are the values Hsted for the other elements. 

The same chemical separation research was done on thorium ores, leading to the discovery of a completely different set of radioactivities. Although the chemists made fundamental distinctions among the radioactivities based on chemical properties, it was often simpler to distinguish the radiation by the rate at which the radioactivity decayed. For uranium and thorium the level of radioactivity was independent of time. For most of the radioactivities separated from these elements, however, the activity showed an observable decrease with time and it was found that the rate of decrease was characteristic of each radioactive species. Each species had a unique half-life, ie, the time during which the activity was reduced to half of its initial value. 

The alkali metals form a homogeneous group of extremely reactive elements which illustrate well the similarities and trends to be expected from the periodic classification, as discussed in Chapter 2. Their physical and chemical properties are readily interpreted in terms of their simple electronic configuration, ns, and for this reason they have been extensively studied by the full range of experimental and theoretical techniques. Compounds of sodium and potassium have been known from ancient times and both elements are essential for animal life. They are also major items of trade, commerce and chemical industry. Lithium was first recognized as a separate element at the beginning of the nineteenth eentury but did not assume major industrial importance until about 40 y ago. Rubidium and caesium are of considerable academic interest but so far have few industrial applications. Francium, the elusive element 87, has only fleeting existence in nature due to its very short radioactive half-life, and this delayed its discovery until 1939. 

Table 21.1 summarizes a number of properties of these elements. The difficulties in attaining high purity has led to frequent revision of the estimates of several of these properties. Each element has a number of naturally occurring isotopes and, in the case of zirconium and hafnium, the least abundant of these is radioactive, though with a very long half-life ( Zr, 2.76%, 3.6 x 10 y Hf, 0.162%, 2.0 X 10 5 y). 

Although the nucleus of the uranium atom is relatively stable, it is radioactive, and will remain that way for many years. The half-life of U-238 is over 4.5 billion years the half-life of U-235 is over 700 million years. (Half-life refers to the amount of time it takes for one half of the radioactive material to undergo radioactive decay, turning into a more stable atom.) Because of uranium radiation, and to a lesser extent other radioactive elements such as radium and radon, uranium mineral deposits emit a finite quantity of radiation that require precautions to protect workers at the mining site. Gamma radiation is the... 

Parenthetical names refer to radioactive elements the mass number (not the atomic weight) of the Isotope with largest half-life is usually given. 

Values in parentheses refer to the isotope of longest known half-life for radioactive elements. 

This can result in a radioactive product from the A(n, t)A reaction where A is the stable element, n is a thermal neutron, A is the radioactive product of one atomic mass unit greater than A, and y is the prompt gamma ray resulting from the reaction. A is usually a beta and/or gamma emitter of reasonably long half-life. Where access to a nuclear reactor has been convenient, thermal neutron activation analysis has proven to be an extremely valuable nondestructive analytical tool and in many cases, the only method for performing specific analyses at high sensitivities... 

Half-lives span a very wide range (Table 17.5). Consider strontium-90, for which the half-life is 28 a. This nuclide is present in nuclear fallout, the fine dust that settles from clouds of airborne particles after the explosion of a nuclear bomb, and may also be present in the accidental release of radioactive materials into the air. Because it is chemically very similar to calcium, strontium may accompany that element through the environment and become incorporated into bones once there, it continues to emit radiation for many years. About 10 half-lives (for strontium-90, 280 a) must pass before the activity of a sample has fallen to 1/1000 of its initial value. Iodine-131, which was released in the accidental fire at the Chernobyl nuclear power plant, has a half-life of only 8.05 d, but it accumulates in the thyroid gland. Several cases of thyroid cancer have been linked to iodine-131 exposure from the accident. Plutonium-239 has a half-life of 24 ka (24000 years). Consequently, very long term storage facilities are required for plutonium waste, and land contaminated with plutonium cannot be inhabited again for thousands of years without expensive remediation efforts. 

The actinoid elements (or actinides An) constitute a series of 14 elements which are formed by the progressive filling of the 5/ electron shell and follow actinium in the periodic table (atomic numbers 90-103). All of the isotopes of the actinide elements are radioactive and only four of the primordial isotopes, Th, and " " Pu, have a sufficient long half-life for there to be any of these left in nature. 

The discovery of two new elements started a frenetic race to find more. Actinium was soon unearthed (Debierne 1900) and many other substances were isolated from U and Th which also seemed to be new elements. One of these was discovered somewhat fortuitously. Several workers had noticed that the radioactivity of Th salts seemed to vary randomly with time and they noticed that the variation correlated with drafts in the lab, appearing to reflect a radioactive emanation which could be blown away from the surface of the Th. This Th-emanation was not attracted by charge and appeared to be a gas, °Rn, as it turns out, although Rutherford at first speculated that it was Th vapor. Rutherford swept some of the Th-emanation into a jar and repeatedly measured its ability to ionize air in order to assess its radioactivity. He was therefore the first to report an exponential decrease in radioactivity with time, and his 1900 paper on the subject introduced the familiar equation dN/dt = - iN, as well as the concept of half-lives (Rutherford 1900a). His measured half-life for the Th emanation of 60 seconds was remarkably close to our present assessment of 55.6 seconds for °Rn. 

Increasingly, new attempts to use basic chemistry to separate substances from radioactive material were meeting with failure. In many cases, two substances which were known to have different radioactive properties and molecular masses simply could not be separated from one another and appeared chemically identical. By 1910, this problem led Soddy to speculate that there were different forms of the same element (Soddy 1910). By 1913 he was confident of this interpretation and coined the term isotope to describe the various types of each element, recognizing that each isotope had a distinct mass and half-life (Soddy 1913b). In the same year he wrote that radiothorium, ionium, thorium, U-X, and radioactinium are a group of isotopic elements, the calculated atomic masses of which vary from 228-234 (a completely accurate statement- we now call these isotopes Th, °Th, Th, Th respectively). Soddy received the... 

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