Radioactive atoms, direct

A modem variant is to count the number of atoms directly in a mass spectrometer.) The practical limit is about 50000 y since by this time the activity has fallen to about 0.2% of its original valuable and becomes submerged in the background counts. is also extremely valuable as a radioactive tracer for mechanistic studies using labelled compounds, and many such compounds, particularly organic ones, are commercially available (p. 310). 

Artificial radioactive atoms are produced either as a by-product of fission of uranium or plutonium atoms in a nuclear reactor or by bombarding stable atoms with particles, such as neutrons or protons, directed at... 

The SI unit of activity is the becquerel (Bq) 1 Bq = that quantity of radioactive material in which there is 1 transformation/second. Since activity is proportional to the number of atoms of the radioactive material, the quantity of any radioactive material is usually expressed in curies, regardless of its purity or concentration. The transformation of radioactive nuclei is a random process, and the number of transformations is directly proportional to the number of radioactive atoms present. For any pure radioactive substance, the rate of decay is usually described by its radiological half-life, TR, i.e., the time it... 

Chemical forms with at least one radioactive atomic nucleus are radioactive substances. The capability of atomic nuclei to undergo spontaneous nuclear transformation is called radioactivity. Nuclear transformations are accompanied by emission of nuclear radiation (Severa and Bar 1991). The average number of nuclei that disintegrate per unit time (= activity) is directly proportional to the total number of radioactive nuclei. The time for 50% of the original nuclei to disintegrate (= half-life or Tb 1/2) is equal to In 2/decay constant for that element (Kiefer 1990). Radiations... 

Man-made radioactive atoms are produced either as a by-product of fission of uranium atoms in a nuclear reactor or by bombarding stable atoms with particles, such as neutrons, directed at the stable atoms with high velocity. These artificially produced radioactive elements usually decay by emission of particles, such as positive or negative beta particles and one or more high energy photons (gamma rays). Unstable (radioactive) atoms of any element can be produced. 

N may be expressed in any convenient units because whatever is used cancels out in the ratio NJN2. A common unit is "counts per minute" (cpm) because, through Equation 26-1, the observed cpm are a direct measure of the number of radioactive atoms present. 

The rate of radioactive decay of an element is the number of atoms emitting a radioactive ray per a unit time. The rate of decay is directly proportional to the initial amount of substance and the structure of the nuclei. On the other hand, the rate of decay is independent of the physical and chemical properties of a radioactive atom. Temperature does not affect the rate of decay. The rate of... 

The features of the molecular excitation spectra are independent of the size and structure of the molecule and are mainly determined by the radioactive atom and its nearest neighbors directly bound to it. 

From the chemical point of view, the most direct and dramatic consequence of the )3-decay is undoubtedly the sudden change of chemical identity undergone by the radioactive atom, which drastically affects all its properties, including the ability to form, or maintain, chemical bonds. If the radioactive atom is chemically combined, the change of its atomic number is often sufficient to cause the disruption of the molecule, particularly when the nuclide formed from the decay is a chemically inert, noble gas atom. Other important chemical consequences follow directly from the intrinsic physical characteristics of the nuclear transformation. 

The activity of the sample is also directly proportional to the number of radioactive atoms. 

There are three methods commonly used to ascertain the quantity of radioactive atoms in a given sample film exposure, Geiger-Miiller counting, and scintillation spectrometry. None of these techniques measures the number of /3 particles directly, but rather monitors the results of collisions between the j3 particles emitted from the radioactive atoms and some component of the assay system. 

The rapid expansion of lectin-based applications for the detection and quantification of glycoconjugates has been led by the development of commercially available, purified and chemically derivatized lectins, and in some cases, anti-lectin antibodies. Over 50 purified plant lectins are sold commercially by a number of producers and vendors, with this number growing annually. Equally important is the ease by which investigators can obtain lectins labeled with various fluorescent dyes, haptenic moieties, biotin, and radioactive atoms, as well as conjugated to enzymes and solid-phase supports. These derivatized lectins are useful for either direct or indirect detection and quantification techniques, or for the physical separation of particulate-bound or soluble glycoconjugates. Table 4 lists many of the commercially available lectin reagents and sources. 

In PET, radioactive substances that emit positrons are introduced into a patient s bloodstream. As the radioactive atoms decay, the positrons they emit collide with electrons, producing gamma rays that escape from the body and are detected by an array of instruments surrounding the patient. Computer analysis of the amount and direction of gamma ray production, and comparison of the data collected for people with and without certain brain disorders provides doctors with valuable information. For example, PET scans of the brain have been used to study the movement of the medication L-dopa in the brains of people suffering from Parkinson s disease. In these procedures, fluorine-18 atoms are attached to L-dopa molecules, which are then injected into a patient. Each flourine-18 decays and emits a positron that generates gamma rays when it meets an electron. 

To get a PET scan of a patient, a solution that contains a positron emitting substance is introduced into the body. The positrons that the radioactive atoms emit collide with electrons, and they annihilate each other, creating two gamma photons that move out in opposite directions. 

Once an animal or plant dies, it ceases to accumulate C02 and the level of 14C slowly decays as it disintegrates by 6-emission. It takes approximately 5600 years for the number of 14C atoms to decay by one half this is called the half-life of the isotope. This rate of disintegration is directly proportional to the number of radioactive atoms present. Since the decay constant for 14C is known, by measuring the decay rate of the object at the present time (and knowing the original decay rate) it is possible to calculate the age of the object. 

In well-designed experiments the mean hot atom lifetime is much shorter than the mean thermal reactive lifetime. The MNR technique thus oflFers good utility for precise equilibrium kinetics studies wdth the radioactive atoms and unstable radicals produced using nuclear recoil methods. Small residual nonthermal reaction yields are invariably observed in recoil experiments, but these have no direct bearing on the validity of the MNR equilibrium hypothesis. 

An experimental procedure for the direct measurement of self diffusion is the radiotracer technique. Here, molecules are marked with radioactive atoms, e.g. with C, and deposited onto the surface of the crystal at a time t = 0. Thereafter, layers are sUced off the crystal surface sequentially at times t > 0 using a microtome. A typical layer thickness is 5 /rm. The crystal is thus dissected layer by layer and the quantity of tracer molecules in each layer is analysed with a spatial resolution given by the layer thicknesses [31, 32]. 

The mass-velocity relation introduced in this way, for the convenience of retaining the conservation of momentum in the relativity system of mechanics, has been directly verified by experiments on the m/e ratios of electrons with differing velocities. Some of the /3-particles emitted by radioactive atoms have velocities which amount to a considerable fraction of c, and m/e (measured in deflexion experiments) is much higher than for slow electrons. The variation is well expressed by Einstein s mass equation. It is much more convenient to attribute the variability to m than to e, since an electron is a... 

Each radioisotope formed in an element subject to a nuclear particle bombardment is uniquely characterized by its half-life (rate of decay) and the types and energies of the radiations it emits as it decays. Therefore, a positive identification of the radioelement is possible. The amount of element in the bombarded or activated sample can be determined directly from a measurement of the radionuclide s radioactivity because the induced radioactivity is directly proportional to the number of atoms of the stable isotope in the sample and to the intensity (flux) of the nuclear particles interacting with the stable nuclei. 

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