Radioactive emissions excitation

The detection system employed for monitoring antibody interactions with antigens will depend on the type of IA performed. The majority are batch systems and are performed off-line. Quantification is achieved by either measuring the absorbance with spectrophotometer, excitation or emission with flourometer, radioactivity decay with a scintillation counter or visualization of direct precipitation. Typical formats used include microtiter plates or membranes or dipsticks or gels. The assay time can range from less than a minute for precipitation/agglutination assays to minutes for the dipsticks onfield assays and hours for microtiter plate-based laboratory assays (48). 

Edmond Becquerel (1820-1891) was the nineteenth-century scientist who studied the phosphorescence phenomenon most intensely. Continuing Stokes s research, he determined the excitation and emission spectra of diverse phosphors, determined the influence of temperature and other parameters, and measured the time between excitation and emission of phosphorescence and the duration time of this same phenomenon. For this purpose he constructed in 1858 the first phosphoroscope, with which he was capable of measuring lifetimes as short as 10-4 s. It was known that lifetimes considerably varied from one compound to the other, and he demonstrated in this sense that the phosphorescence of Iceland spar stayed visible for some seconds after irradiation, while that of the potassium platinum cyanide ended after 3.10 4 s. In 1861 Becquerel established an exponential law for the decay of phosphorescence, and postulated two different types of decay kinetics, i.e., exponential and hyperbolic, attributing them to monomolecular or bimolecular decay mechanisms. Becquerel criticized the use of the term fluorescence, a term introduced by Stokes, instead of employing the term phosphorescence, already assigned for this use [17, 19, 20], His son, Henri Becquerel (1852-1908), is assigned a special position in history because of his accidental discovery of radioactivity in 1896, when studying the luminescence of some uranium salts [17]. 

Various methods can be used to measure the intensity of radioactive emissions. These exploit the ability of radiation from radioactive isotopes to cause ionisation (Geiger-Miiller counting), to excite fluorophores (scintillation counting), or to cause exposure of light-sensitive photographic emulsion (autoradiography) (Slater 1990 Rickwood et al. 1993). 

FIGURE 32-6 Overview of the neutron activation process. The incident neutron is captured by the target nucleus to produce an excited compound nucleus, which de-excites with emission of a prompt gamma ray. The radioactive nucleus formed decays by emitting a beta particle. If an excited product nucleus is formed, a delayed gamma ray can be emitted. If decay is directly to the ground state of the product nucleus, no gamma ray is emitted. 

This contrasts with the production of radionuclides by neutron irradiation, e.g. in a nuclear reactor, where the neutron absorption and subsequ t de-excitation by emission of y-rays produces neutron rich isotopes, frequently radioactive, which are isotopic with the target. A typical reaction is... 

Compare with other analytical techniques, it must suffice here to add that fluorescence measurements are rapid, accurate and require only very small quantities of sample (nanomole or less) with high selectivity. The principal advantage of fluorescence over radioactivity and absorption sp>ectroscopy is the ability to separate compounds on the basis of either their excitation or emission sp>ectra, as opposed to a single sp>ectra. This advantage is further enhanced by commercial fluorescent dyes that have narrow and distinctly separated excitation and emission spiectra... 

In the 15 keV - 10 MeV region, line-forming processes such as nuclear excitation, radioactivity, positron annihilation, cyclotron emission and absorption become important, and when used as astrophysical tools, are almost certain to lead to fundamental new discoveries. Unique astrophysical information is contained in the spectral shift, line width, and line profiles. Detailed studies of these processes require the resolving power (E/AE = 500) of a Germanium spectrometer such as that employed on INTEGRAL. Lower resolution spectrometers (e.g. SIGMA, OSSE, COMPTEL) do not have sufficient energy resolution to study the parameters of these lines. The... 

The fluorescence detector (ED) is one of the most sensitive detectors in use, and can record both excitation and emission spectra. The excitation spectra are identical to the UV-visible absorption spectra however, emission spectra can provide additional information. Detectors that are used to detect isotope-labeled molecules measure the radioactivity present. Other analytical instruments, such as IR spectrometers, mass spectrometers, NMR spectrometers, electron spin resonance spectrometers, plasma emission and plasma absorption spectrometers can be connected to HPLCs for use as detectors, to provide further information on molecular structure. 

A second type of device commonly used to detect radiation instantly is called a scintillation counter. In a scintillation coimter, the radioactive emissions pass through a material (such as Nal or Csl) that emits ultraviolet or visible light in response to excitation by eneigetic particles. This light is detected and turned into an electrical signal that can be read on a meter. 

Classic examples are the spontaneous emission of light or spontaneous radioactive decay. In chemistry, an important class of monomolecular reactions is the predissociation of metastable (excited) species. An example is the fonnation of oxygen atoms in the upper atmosphere by predissociation of electronically excited O2 molecules [12, 13 and 14] ... 

Fluorescent materials are very important in medical research. Dyes such as fluorescein (21) can be attached to protein molecules, and the protein can be traced in a biological system by exciting the fluorescein and looking for its emissions. The use of a fluorescent material allows the detection of much smaller concentrations than would otherwise be possible. Because fluorescent materials can be activated by radioactivity, they are also used in scintillation counters to measure radiation (see Box 17.2). 

Scintillation The flash of light emitted when an electron in an excited state drops to a lower energy level. Scintillation counters are designed to measure the intensity of emissions from radioactive materials. 

Fig. 2.1 Nuclear resonance absorption of y-rays (Mossbauer effect) for nuclei with Z protons and N neutrons. The top left part shows the population of the excited state of the emitter by the radioactive decay of a mother isotope (Z, N ) via a- or P-emission, or K-capture (depending on the isotope). The right part shows the de-excitation of the absorber by re-emission of a y-photon or by radiationless emission of a conversion electron (thin arrows labeled y and e , respectively)...

For nuclear y-resonance absorption to occur, the y-radiation must be emitted by source nuclei of the same isotope as those to be explored in the absorber. This is usually a stable isotope. To obtain such nuclei in the desired excited meta-stable state for y-emission in the source, a long-living radioactive parent isotope is used, the decay of which passes through the Mossbauer level. Figure 3.6a shows such a transition cascade for Co, the y-source for Fe spectroscopy. The isotope has a half-life time //2 of 270 days and decays by K-capmre, yielding Fe in the 136 keV excited state ( Co nuclei capmre an electron from the K-shell which reduces the... 

In terms of atomic spectrometry, NAA is a method combining excitation by nuclear reaction with delayed de-excitation of the radioactive atoms produced by emission of ionising radiation (fi, y, X-ray). Measurement of delayed particles or radiations from the decay of a radioactive product of a neutron-induced nuclear reaction is known as simple or delayed-gamma NAA, and may be purely instrumental (INAA). The y-ray energies are characteristic of specific indicator radionuclides, and their intensities are proportional to the amounts of the various target nuclides in the sample. NAA can thus... 

Radioactive processes commonly leave atomic nuclei in an excited state from which they may relax to their ground state by the emission of 7-radiation. A typical example is the decay of cobalt through /3-capture ... 

When a slow neutron is captured by the nucleus of element X, another isotope of the same element is instantaneously formed, in an excited state because of the impact (labelled compound nucleus in Figure 2.13), which then de-excites by the emission of a gamma particle (and possibly other particles) from the nucleus to produce a radioactive nucleus. For example, when 23Na captures a neutron (signified by on, since neutrons have a mass of one unit, but no electrical charge), it becomes the radioactive nucleus 24Na, as follows ... 

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