Radiation source, electron-capture

For many of the analytical techniques discussed below, it is necessary to have a source of X-rays. There are three ways in which X-rays can be produced in an X-ray tube, by using a radioactive source, or by the use of synchrotron radiation (see Section 12.6). Radioactive sources consist of a radioactive element or compound which spontaneously produces X-rays of fixed energy, depending on the decay process characteristic of the radioactive material (see Section 10.3). Nuclear processes such as electron capture can result in X-ray (or y ray) emission. Thus many radioactive isotopes produce electromagnetic radiation in the X-ray region of the spectrum, for example 3He, 241Am, and 57Co. These sources tend to produce pure X-ray spectra (without the continuous radiation), but are of low intensity. They can be used as a source in portable X-ray devices, but can be hazardous to handle because they cannot be switched off. In contrast, synchrotron radiation provides an... 

With all electron capture detectors which use tritium as the source of beta radiation, alcohols and water should be rigorously avoided as solvents for the sample and oxygen or air as a matrix for a gas sample should similarly be avoided if possible. These precautions are mentioned because each of these materials can exchange or react with the tritium in the cell resulting in decreased sensitivity or decreased detector life. 

Co(bipy)3(C104)3-3H20 has been used as a source of 14.4 keV gamma radiation and it has been shown that the daughter iron nucleus is stabilized as Fe(III)(bipy)3(C104)3 within 10 seconds after electron capture 587). 

Besides the generators described above there are X-ray sources based on radioactive materials to provide the excitation of the sample. The advantage of using these materials is that an isotope can be selected to provide a mono-energetic beam of radiation that is optimized for the specific application. One method consists to select a radionuclide that is transformed by internal electron capture (lEC). This mode of decomposition corresponds to the transition of one level-K electron into the nucleus of the atom. For a nuclide X, the phenomenon is summarized as follows ... 

The electron capture detector has a layer of Ni63 as a (3-radiation source. The radioactivity of the radiation source amounts from 10 up to 15 mC (370 to 550 MBq). 

An ionization instrument for the analysis of gas has been developed in which the gas passes through a small chamber where it is irradiated by a small radioactive source. For a constant source of radiation, the ions produced in the gas d nd on the flow velocity of the gas and on its temperature, pressure and atomic composition. The dependence of the ionization on the atomic composition is a consequence of the different ionization potentials of the differrat types of atoms of the gas and the different probabilities for electron capture and collision. The ion current is collected on an electrode and measured. This current is a function of the gas pressure and velocity since the higher the pressure, the more ions form, while at higher velocity, the fewer ions are collected as more ions are removed by the gas prior to collection. Such ionization instruments are used in gas chromatographs and other instruments as well as in smoke detection systems (the normal radiation source is Am, usually 40 kBq), where secondary electrons condense on smoke particles, leading to lower mobility for the electrons and a decreased ion current. 

Radioactive heavy elements such as uranium, thorium, or plutonium are used as nuclear fuel radium is used in the radiography of metals and radon is used as a surface label to study surface reactions, as well, in the determination of radium or thorium. Among the lighter isotopes, Ni is used in electron capture detectors for GC analysis, in radiocarbon dating and as a tracer, and tritium in nuclear fusion and as a tracer in the studies of reactions. Many radioactive elements are used as a source of radiation, in medicine to diagnose disease, and for treatment. 

X-radiation is a product of radioactive decay of certain isotopes. The term gamma ray is often used for an X-ray resulting from such a decay process. Alpha and beta decay and electron capture processes can result in the release of gamma rays. Table 8.4 lists some common radioisotopes used as XRF sources. 

In Tab. 11.4, some characteristics of radio sources emitting X-ray or y-ray lines are listed. The X-ray emitting sources usually contain nuclides that decay by means of the electron-capture mechanism. During the decay, an inner shell electron is captured by the neutron-defident nucleus, transforming a proton into a neutron. This results in a daughter nudide that has a vacancy in one of its inner shells, which results in the emission of corresponding characteristic radiation. For example, when a Fe-nudeus (26 protons and 29 neutrons) captures a K-electron and becomes a Mn nudeus, a Mn K-L3 2 (Mn-K,) or K-M3 2 (Mn-Kp) photon will be emitted. Other sources (such as Am or Co) emit y-rays of suitable energy as a result of different nudear transformations. 

Spectra from Radioactive Sources X-radiation is often a product of radioactive decay processes. Gamma rays, which are indistinguishable from X-rays, are produced in intranuclear reactions. Many a and /3 emission processes (see Section 32A-2) leave a nucleus in an excited state the nucleus then releases one or more quanta of gamma rays as it returns to its ground state. Electron capture or K capture... 

It is important to place in perspective the relative ionizing radiation dose acquired in common laboratory settings. The most commonly encountered source is a Ni source used in gas chromatographic electron capture detectors and in ion mobility spectrometers (Ref. 1). In both instruments, the source is sealed and has a radioactivity of 15 mCi. The exposures cited refer only to normal operation it does not consider exposures if the device is dismantled or allowed to overheat. 

Analysis of spectra such as Fraunhofer lines is called absorption spectroscopy because it deals with atoms capturing a photon that bumps an electron into a higher energy state. Emission spectroscopy uses an external source of energy—heat, radiation, or an... 

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