Calibration radiation sources

For the calibration of most infrared ear thermometers the sensitivities S0 and R0 and the temperature coefficients Sj and a for both sensors have to be determined. Typically a two-step calibration is performed. In the first step the ambient sensor is calibrated by immersing it into two different temperature controlled baths. In the second step the thermopile sensor is calibrated by measuring the output signal while placing it before two different blackbody radiation sources. 

There are not many uses for polonium. Probably the most important is as a source of alpha particles (nuclei of helium atoms) and high-energy neutrons for research and radiation studies. It is also used to calibrate radiation-detection devices. 

Blackbody radiation sources are accurate radiant energy standards of known flux and spectral distribulion. They are used for calibrating other infrared sources, detectors, and optical systems. The radiating properties of a blackbody source are described by Planck s law. Energy distribution... 

In principle, there are two ways to achieve the radiometric calibration of an instrument measuring solar radiation. The first is by comparison to a standard radiation source of known output and the second by comparison to a prototype standard instrument that is capable in measuring the same radiometric quantity. The fist can be applied to broadband detectors only if their spectral response over the whole range of the radiation source is known with sufficient accuracy. The second method requires that the standard instrument has exactly the same spectral response, which is rather unlikely to occur. 

Radiation-Density Gauges Gamma radiation may be used to measure the density of material inside a pipe or process vessel. The equipment is basically the same as for level measurement, except that here the pipe or vessel must be filled over the effective, irradiated sample volume. The source is mounted on one side of the pipe or vessel and the detector on the other side with appropriate safety radiation shielding surrounding the installation. Cesium 137 is used as the radiation source for path lengths under 610 mm (24 in) and cobalt 60 above 610 mm. The detector is usually an ionization gauge. The absorption of the gamma radiation is a function of density. Since the absorption path includes the pipe or vessel walls, an empirical calibration is used. Appropriate corrections must be made for the source intensity decay with time. 

A major detraction for LS AAS has always been the relatively short linear region of the calibration curves, typically not more than two orders of magnitude in concentration. The limits of the linear working range arise from stray radiation and the finite width of the emission lines of the radiation source, which is not monochromatic and just three to five times narrower than the absorption profile. With HR-CS AAS, there is no theoretical limit to the calibration range, only the practical limits imposed by the size of the array detector, the increasing possibility of spectral interferences, and the ability to clean the atomizer after extremely high analyte concentrations have been introduced. 

The typical routine determination of a number of elements in a set of similar sample solutions will therefore no longer be the determination of element A in all the samples, followed by a change of radiation source, wavelength, flame conditions, burner height and so on, and determination of element B in the same samples, and so on, as it is common practice in LS FAAS. In HR-CS FAAS it will rather consist of a calibration of the instrument for all the elements of interest, followed by a determination of all elements in sample 1, all elements in sample 2, all elements in sample 3, and so on. It might be worth mentioning that, at least for a limited number of elements, the total analysis time required for HR-CS FAAS will be even shorter than that for a simultaneous ICP AES measurement, because of the much shorter equilibration time required for a typical AAS burner after changing the sample solution, compared to the spray chambers used in ICP AES. 

EXAFS data (Rh K-edge ((23220 eV) or Ir Lm-edge (13419 eV)) were collected in transmission mode on station 9.2 of the Daresbury Synchrotron Radiation Source, operating at 2 GeV with an average current of 150 mA. A water-cooled Si(220) double crystal monochromator was used, with its angle calibrated by running an edge scan of a 5 pm Rh or Ir foil. For each sample 2-10 scans were recorded at room temperature in the... 

Accurate measurement of photon flux is essential in order to calculate quantum efficiency (whether based on absorbed photons flux or the flux of photons impinging on the surface). The response of commercial sensors is wavelength dependence, hence, the readout, which is in energy flux units (W cm ), is usually calibrated according to either the 365 nm line of mercury or the 254 nm line, depending on the sensor. For more details on radiation sources see de Lasa et al. (2005). A different method for measuring photon flux is actinometry. This method was very popular in the past, however, very few researchers still use it due to its complexity and the time it consumes. 

The calibration of excitation and emission monochromator wavelengths should be checked regularly by the use of sharp lines from the instrument s own radiation source (e.g. xenon lines at 450.1, 462.4,... 

Determining the accuracy of the analytical methods for environmental samples and for calibrating radiation instrumentation requires that standard, certified radioactive sources with known concentrations of uranium. 

To study the structural sensitivity of poly silanes to ionizing radiation, a number of samples were irradiated with a calibrated Co source, and the degraded materials were analyzed by GPC in a manner similar to that described for the determination of photochemical quantum yields (59). In radiation processes, the slopes of the plots of molecular weight versus absorbed dose yield the G values for scissioning, G(s), and cross-linking, G(x), rather than the respective quantum yields. These values, which represent the number of chain breaks or cross-links per 100 eV of absorbed dose, are indicative of the relative radiation sensitivity of the material. The data for a number of polysilanes are given in Table IV. Also included in Table IV for comparison is the value for a commercial sample of poly(methyl methacrylate) run under the same conditions. The G(s) value of this sample compares favorably with that reported in the literature (83). 

Absorption or scattering of radioactive radiation is applied in industry for measurement of thickness or for material testing. For example, the production of paper, plastic or metal foils or sheets can be controlled continuously by passing these materials between an encapsulated radionuclide as the radiation source and a detector combined with a ratemeter, as shown in Fig. 20.2. After appropriate calibration, the ratemeter directly indicates the thickness. The radionuclide is chosen in such a way that the radiation emitted is eflFectively absorbed in the materials to be checked. Thus, the thickness of plastic foils is measured by use of f emitters, whereas Cs or other y emitters are used for measuring the thickness of metal sheets. 

In addition, the LMRI elaborates and distributes radioactivity standards and references, and provides calibrations, measurements and testings in radioactivity and dosimetry, for measuring instruments and ionising radiation sources. These services are intended for research, industry and medicine. 

Intentional uses of radium today are primarily in the treatment of cancer using a radiation source and as a neutron source in research and instrument calibration. Earlier uses of radium in paints and as a treatment for other illnesses and health-rejuvenating tonics were halted after its toxicity was recognized (see below). 

Accordingly, from the photomultiplier signal the number of analyte atoms brought from a given amount of sample into the radiation source can be calculated directly. As all constants in the calculation shown, however, are not known apriori, AES in practice is a relative method and a calibration has to be performed. The determination of the calibration function is an important part of the working procedure. The calibration function relates the intensity of a spectral line (I) to the concentration c of an element in the sample. From the work of Scheibe [336] and Lomakin [337] the following relationship between absolute intensities and elemental concentrations was proposed ... 

The ratio (Ix/Iu) and the calibration constants cu for a given element and line depend on the radiation source and the working conditions selected but also on the spectral apparatus [338, 339] ... 

The devices used for calibration of radiation sources and test chambers are discussed in Chapter 5 and Chapter 6. Neither the UV filter radiometer nor the luxmeter provides information on the spectral power distribution (SPD, the plot of radiation intensity vs. wavelength) of sources. Upon delivery, the device should have been calibrated by the manufacturer against a standard lamp and provided with a response curve. If the meters are used as received, they are well suited for measuring evenness of irradiance across the sample area and changes in total output with time. 

Neither filter radiometers nor luxmeters can be used to obtain an absolute measurement of irradiance or to compare irradiance between sources unless they are calibrated specifically for each source (Tpnnesen and Karlsen, 1997). A spectro-radiometer is needed for a detailed estimate of the SPD but at present such equipment is not widely used on a regular basis because of cost and convenience. The total irradiance (i.e., actual number of photons) can be determined by chemical actinom-etry using a reaction of known photochemical efficiency (Chapter 3 and Chapter 6). The chemical actinometer listed in the ICH guideline (quinine hydrochloride) has its limitations and its suitability as actinometer has been questioned (Baertschi, 1997 Bovina et al., 1998 Drew, 1998). This actinometer is not suitable for calibration of option 1 radiation sources (Thatcher et al., 2001a, b). An alternative actinometer (2-nitrobenzaldehyde) has recently been discussed for calibration of option 1 (and other sources) for UV irradiance (Allen et al., 2000). 

Whatever the source of radiation used, the dose delivered to the biological samples is determined by the time of exposure to radiations. Thus the dose delivered by the radiation source must be measured with precision. Dosimetry can be performed with a ferrous sulfate solution (Fricke and Morse, 1927), thermoluminescent dosimeters, bleaching of films (Hart and Fricke, 1967), Perspex dosimetry (Berry and Marshall, 1969), or calibration with standard enzymes (Beauregard et al., 1980 Beauregard and Potier, 1982 Lo et al., 1982). In many laboratories, control enzymes with known D37 are added to protein preparations as internal standards so that any variation between experiments could be corrected for. Because of the better precision of dose rate in Gammacell irradiators, this precaution is not necessary. 

Many researchers have used this technique, originally suggested by Fery [83], to measure flame temperatures. The flame is commonly seeded with sodium salt. The salt vaporizes in the flame and dissociates into sodium atoms and other products. These excited atoms emit at a specific wavelength. This radiation is then compared to a calibrated reference source, such as a tungsten filament lamp. The temperature of this source is adjusted, until it matches the seed radiation. To the naked eye, the background source appears to disappear when it is at... 

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