A radioactivity

This is a radioactive element. It occurs in minute traces in barium and thorium minerals, but it can be produced by irradiation of bismuth in a nuclear reactor. (The study of its chemistry presents great difficulty because of its intense a radiation). 

Radon, sealed in small capsules called seeds , has been used as a radioactive substance in medicine, but is being superseded by more convenient artificially-produced radioisotopes. 

Deuterium is used as a moderator to slow down neutrons. Tritium atoms are also present but in much smaller proportions. Tritium is readily produced in nuclear reactors and is used in the production of the hydrogen (fusion) bomb. It is also used as a radioactive agent in making luminous paints, and as a tracer. 

Seventeen isotopes of potassium are known. Ordinary potassium is composed of three isotopes, one of which is 40oK (0.0118%), a radioactive isotope with a half-life of 1.28 x IO9 years. 

Also present in the first test tube is a synthetic analog of ATP in which both the 2 and 3 hydroxyl groups have been replaced by hydrogens This compound is called 2 3 dideoxyadenosme triphosphate (ddATP) Similarly ddTTP is added to the second tube ddGTP to the third and ddCTP to the fourth Each tube also contains a primer The primer is a short section of the complementary DNA strand which has been labeled with a radioactive isotope of phosphorus ( P) When the electrophoresis gel is examined at the end of the experiment the positions of the DNAs formed by chain extension of the primer are located by a technique called autoradiography which detects the particles emitted by the P isotope... 

There are many potential advantages to kinetic methods of analysis, perhaps the most important of which is the ability to use chemical reactions that are slow to reach equilibrium. In this chapter we examine three techniques that rely on measurements made while the analytical system is under kinetic rather than thermodynamic control chemical kinetic techniques, in which the rate of a chemical reaction is measured radiochemical techniques, in which a radioactive element s rate of nuclear decay is measured and flow injection analysis, in which the analyte is injected into a continuously flowing carrier stream, where its mixing and reaction with reagents in the stream are controlled by the kinetic processes of convection and diffusion. 

The rate of decay, or activity, for a radioactive isotope follows first-order kinetics... 

An important characteristic property of a radioactive isotope is its half-life, fj/2, which is the amount of time required for half of the radioactive atoms to disintegrate. For first-order kinetics the half-life is independent of concentration and is given as... 

The time required for half of the initial number of a radioactive isotope s atoms to disintegrate (ti/2). 

A form of internal standardization in which a radioactive form of the analyte serves as the internal standard. 

Isotope Dilution Another important quantitative radiochemical method is isotope dilution. In this method of analysis a sample of analyte, called a tracer, is prepared in a radioactive form with a known activity. Ax, for its radioactive decay. A measured mass of the tracer, Wf, is added to a sample containing an unknown mass, w, of a nonradioactive analyte, and the material is homogenized. The sample is then processed to isolate wa grams of purified analyte, containing both radioactive and nonradioactive materials. The activity of the isolated sample, A, is measured. If all the analyte, both radioactive and nonradioactive, is recovered, then A and Ax will be equal. Normally, some of the analyte is lost during isolation and purification. In this case A is less than Ax, and... 

One example of a characterization application is the determination of a sample s age based on the kinetics for the decay of a radioactive isotope present in the sample. The most common example is carbon-14 dating, which is used to determine the age of natural organic materials. 

Radiochemical methods of analysis take advantage of the decay of radioactive isotopes. A direct measurement of the rate at which a radioactive isotope decays may be used to determine its concentration in a sample. For analytes that are not naturally radioactive, neutron activation often can be used to induce radioactivity. Isotope dilution, in which a radioactively labeled form of an analyte is spiked into the sample, can be used as an internal standard for quantitative work. 

These chemical effects become important in medicine because living systems operate mostly through the reactions of enzymes, which catalyze all sorts of metabolic reactions but are very sensitive to small changes in their environment. Such sensitivity can lead to preferential absorption of some deleterious isotopes in place of the more normal, beneficial ones. One example in metabolic systems can be found in the incorporation of a radioactive strontium isotope in place of calcium. 

Greater sensitivity is attained if an additional ionization mode is included. This can be in the form of a radioactive source, a heated filament, or a plasma or glow discharge. 

Special techniques for experimentation with the actinide elements other than Th and U have been devised because of the potential health ha2ard to the experimenter and the small amounts available (15). In addition, iavestigations are frequently carried out with the substance present ia very low coaceatratioa as a radioactive tracer. Such procedures coatiaue to be used to some exteat with the heaviest actinide elements, where only a few score atoms may be available they were used ia the earHest work for all the transuranium elements. Tracer studies offer a method for obtaining knowledge of oxidation states, formation of complex ions, and the solubiHty of various compounds. These techniques are not appHcable to crystallography, metallurgy, and spectroscopic studies. 

External stimulus flow meters are generally electrical in nature. These devices derive their signal from the interaction of the fluid motion with some external stimulus such as a magnetic field, laser energy, an ultrasonic beam, or a radioactive tracer. 

There should be specific, saturable binding to the receptor, accompanied by pharmacological characteristics appropriate to the functional effects, demonstrable using a radioactive, eg, tritium or iodine-125, ligand to label the receptor. Radioligand binding assays (1,6) have become a significant means by which to identify and characterize receptors and enzymes (see Immunoassays Radioactive tracers). Isolation of the receptor or expression of the receptor in another cell, eg, an oocyte can be used to confirm the existence of a discrete entity. 

Nuclear wastes are classified according to the level of radioactivity. Low level wastes (LLW) from reactors arise primarily from the cooling water, either because of leakage from fuel or activation of impurities by neutron absorption. Most LLW will be disposed of in near-surface faciHties at various locations around the United States. Mixed wastes are those having both a ha2ardous and a radioactive component. Transuranic (TRU) waste containing plutonium comes from chemical processes related to nuclear weapons production. These are to be placed in underground salt deposits in New Mexico (see... 

Radiochemical tracers, compounds labeled with radioisotopes (qv), have become one of the most powerful tools for detection and analysis in research, and to a limited extent in clinical diagnosis (see Medical IMAGING TECHNOLOGY). A molecule or chemical is labeled using a radioisotope either by substituting a radioactive atom for a corresponding stable atom in the compound, such as substituting for H, for or for P, and for for... 

Any radioactive nucUde or isotope of an element can be used as a radioactive tracer, eg, chromium-51 [14392-02-0] cobalt-60 [10198-40-0] tin-110 [15700-33-1] and mercury-203 [13982-78-0],hut the preponderance ofuse has been for carbon-14 [14762-75-5],hydj ogen-3 [10028-17-8] (tritium), sulfur-35 [15117-53-0], phosphoms-32, and iodine-125 [14158-31 -7]. More recendy phosphoms-33 has become available and is used to replace sulfur-35 and phosphoms-32 in many appUcations. By far the greater number of radioactive tracers produced are based on carbon-14 and hydrogen-3 because carbon and hydrogen exist in a large majority of the known natural and synthetic chemical compounds. 

In this decay process, only one particle is emitted and, because energy is conserved, for each level in the daughter nucleus there is a unique a-particle energy. This means that a measurement of the differences in the energies of the a-particles emitted in a radioactive decay gives expHcidy the differences in the energies of the levels in the daughter nucleus. 

Background Radiation. If the radiation from a radioactive source is measured, the spectmm also includes contributions from the radiations from the surrounding environment. This includes radiations from the radioactivity in the materials in and around the detector, including the stmcture of the building or nearby earth. There is also cosmic radiation that comes from space and interacts with the earth and atmosphere to produce radiations that may enter the detector, and thus is observed. 

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