Nuclear decay characteristics

The isotope 242Cm was first isolated among the products of a-bombard-ment of 239Pu, and its discovery actually preceded that of americium. Isotopes of the other elements were first identified in products from the first hydrogen bomb explosion (1952) or in cyclotron bombardments. Although Cm, Bk and Cf have been obtained in macro amounts (Table 28-2), much of the chemical information has been obtained on the tracer scale. The remaining elements have been characterized only by their chemical behavior on the tracer scale in conjunction with their specific nuclear decay characteristics. 

Methods of incorporating inorganic radiochemicals into liquid and solgel scintillators that exhibit high scintillation yield are described. Liquid-scintillation measurements have been made at the National Bureau of Standards on 31 different radionuclides for a variety of application in radionuclide metrology. Sample preparation techniques are described for a number of radionuclides that differ markedly in their chemical behavior as well as in their nuclear-decay characteristics. Particular emphasis is given to radionuclides such as Fe and... 

Much of the basic chemistry of thorium and uranium tvas known in 1942, but the nuclear decay characteristics of most of the (FPs) were not. Furthermore, the chemistry of many of the FPs and transuranic (TRU) elements was not known in sufficient detail. Promethium, technetium, and all the TRU elements were new to science and much had to be inferred from an element s position in the periodic table. The chemical and physical effects of radiation imposed additional difficulties and imcerlainties in the proposed processes, as they do even today. 

Figure 1.2 Schematic diagram to show X-ray emission to fill vacancy caused by nuclear decay. An L-shell electron (A) is shown filling a K-shell vacancy (B). In doing so, it emits a characteristic K X-ray.

NAA is gamma ray spectroscopy that uses the slow thermal neutrons from a nuclear reactor to excite the nucleus of an atom. When an atom absorbs a thermal neutron, its atomic mass increases by one and the nucleus becomes unstable. One or more nuclear reactions then take place that release gamma-rays with energies characteristic of the particular nuclear decay reactions, along with other radiation (Fig. 4.14). While... 

Name two nuclear decay processes in which characteristic X-rays are possibly emitted. 

The half-life (fi/z) is the time required for one-half of a given quantity of a substance to undergo change. Not all radioactive isotopes decay at the same rate. The rate of nuclear decay is generally represented in terms of the half-life of the isotope. Each isotope has its own characteristic half-life that may be as short as a few millionths of a second or as long as a billion years. Half-lives of some naturally occurring and s)mthetic isotopes are given in Table 10.2. 

Most countries have one or more suppliers of radiochemicals. To locate suppliers, the simplest way is often to contact the nearest nuclear c ter, as it may be a producer of radionuclides, the national radiation safety organization, or the annual Byers Guide of common nuclear journals. Product catalogs list the type of radioactive sources and compounds available, purity of the products, maximum and specific activities, radiation decay characteristics, accuracy of standards, labeling position, etc. 

The left-hand side of Equation 19.1, dNjdt, is the number of disintegrations taking place per unit time. Each radionuclide has its own characteristic nuclear-decay constant. The minus sign indicates that the decay results in a decrease in N. Equation 19.1 indicates that the rate of decay of any radionuclide is directly proportional... 

Radioactive decay is a first-order kinetic process. Recall that a first-order process has a characteristic half-life, which is the time required for half of any given quantity of a substance to react. (Section 14.4) Nuclear decay rates are commonly expressed in terms of half-lives. Each isotope has its own characteristic half-life. For example, the half-life of strontium-90 is 28.8 yr ( FIGURE 21.6). If we start with lO.O g of strontium-90, only 5.0 g of that isotope remains alter 28.8 yr, 2.5 g remains after another 28.8 yr, and so on. Strontium-90 decays to yttrium-90 ... 

Half-lives as short as millionths of a second and as long as billions of years are known. The half-lives of some radioisotopes are listed in TABLE 21.5. One important feature of half-Uves for nuclear decay is that they are unaffected by external conditions such as temperature, pressure, or state of chemical combination. Unlike toxic chemicals, therefore, radioactive atoms cannot be rendered harmless by chemical reaction or by any other practical treatment. At this point, we can do nothing but allow these nuclei to lose radioactivity at their characteristic rates. In the meantime, we must take precautions to prevent radioisotopes, such as those produced in nuclear power plants (Section 21.7), from entering the environment because of the damage radiation can cause. 

The formation of a radionuclide in an elemental constituent of a sample being bombarded by a source of nuclear particles ceases when the bombardment period is over. At that time, it becomes necessary to measure the radioactivity of the radionuclide of the element of interest. Since each radionuclide has specific decay characteristics, then an experimenter must detect and measure its radiations by some type of a counting system. 

Using two detectors simultaneously, one has to detect the 122 keV y quanta coming from the Mossbauer source as a start signal. This actually marks the birth of the 14.4 keV Mossbauer excited level (see the decay scheme inO Fig. 25.7). Then, with the other detector, the 14.4 keV y quantum is detected, and the elapsed time measured (stop signal). With this method, Mossbauer spectra can be recorded in any time interval after the nuclear decay of Co. Recording characteristic X-rays following the electron capture, part of the aftereffect events can be filtered off. Due to the coincidence technique, very low activities can only be used, and therefore these measurements are rather time consuming (several weeks or months). 

A NUREG document (Reilly et al (1991)) discusses all of the various methods of passive assay of nuclear materials - neutron and gamma. Of particular relevance to us is a detailed assessment of the decay characteristics of the plutonium isotopes and the various regions of their spectra and their relevance to plutonium measurement (Sampson (1991)). The region around 100 keV is... 

It was emphasized in [1] that the nuclear decay properties of the isotope to be used in these studies must be well known and have unique decay characteristics suitable for detection and positive identification on an atom-at-a-time basis in order to verify that it is from the element whose chemistry is to be studied It must have a half-life comparable to the proposed chemical separation procedure as well as a reasonable production and detection rate to permit statistically significant results to be obtained, and must give the same results for a few atoms as for macro amounts. For the transactinide elements, production rates range from a few atoms per minute for rutherfordium (Rf, Z = 104) to only about one atom per day in the case of elements 108 (hassium, Hs), 112, and 114, the heaviest elements studied to date with chemical techniques. Details of these chemical investigations are outlined in Liquid-Phase Chemistry of Superheavy Elements and Gas-Phase Chemistry of SuperheavyElements . 

Detection of the nuclide via its characteristic nuclear decay properties. 

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