Chemical dosimeters

Chemical dosimeters measure the absorbed dose by the quantitative determination of chemical change—that is, the G value of a suitable product—in a known chemical system. These are secondary dosimeters in the sense that the corresponding G values must be established with reference to a primary, absolute dosimeter. The primary dosimeters are usually physical in nature calorimeters, ionization chambers, or charge measuring devices with particles of known energy. However, the primary dosimeters are generally cumbersome, whereas the chemical dosimeters are convenient to handle. On the other hand, the chemical dosimeters are not suitable for low-dose measurements. 

In principle, a large variety of radiation chemical changes of a semipermanent nature induced in a solution can be utilized for dosimetric purpose. The practical suitability is determined by satisfying as many of the following requirements as possible  

The response of the dosimeter should be linear to the dose—that is, the G value of the product should be independent of the dose or the dose rate. The useful dose range may be stipulated as -1-106 Gy, and it may be difficult to design a chemical system that preserves linearity over the entire range. In any case a useful chemical dosimeter must have linearity over most of the intermediate dose range. 

The product G value should be independent of the incident particle LET otherwise, the LET dependence of the yield must have been well established. 

The product yield should be independent of temperature and insensitive to variation of experimental conditions during the course of radiolysis such as accumulation of radiolytic products, change in pH and so forth. 

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Industrial sterilization cycles tend to vary considerably, not only from manufacturer to manufacturer, but often from product type to product type, depending on the bioburden present on a given load. Chemical indicators have historically been used only to differentiate between sterilized and nonsterilized packages. More recent developments have resulted in the availability of chemical dosimeters of sufficient accuracy to permit their appHcation either as total monitors or as critical detectors of specific parameters. 

Chemical dosimeters based on ferrous sulfate, ferrous cupric sulfate, or ceric sulfate are generally used. Color-change process indicators are also used, but these cannot measure the radiation dose, only the extent of sterilization. 

Since 1925, The International Commission on Radiation Units and Measurements at Bethesda, Maryland has been publishing reports updating the definitions and units for measurements of various radiation-related quantities. Of these ICRU Reports, special mention may be made of reports no. 19 (1971) [radiation quantities and units], 33 (1980) [radiation quantities and units], 36 (1983) [microdosimetry], 47 (1992) [thermoluminiscent dosimetry], and 51 (1993) [radiation protection dosimetry]. A succinct description of various devices used in dosimetry, such as ionization chambers, chemical and solid-state dosimeters, and personnel (pocket) dosimeters, will be found in Spinks and Woods (1990). In this section, we will only consider some chemical dosimeters in a little detail. For a survey of the field the reader is referred to Kase et at, (1985, 1987), McLaughlin (1982), and to the International Atomic Energy Agency (1977). Of the earlier publications, many useful information can still be gleaned from Hine and Brownell (1956), Holm and Berry (1970), and Shapiro (1972). 

A large variety of aqueous and a few nonaqueous solutions have been used or proposed as chemical dosimeters with respective dose ranges for use (Spinks and Woods, 1990 Draganic and Draganic, 1971). Of these, a special mention may be made of the hydrated electron dosimeter for pulse radiolytic use (l(h2 to 10+2 Gy per pulse). It is composed of an aqueous solution of 10 mM ethanol (or 0.7 mM H2) with 0.1 to 10 mM NaOH. Concentration of hydrated electrons formed in the solution by the absorption of radiation is monitored by fast spectrophotometry, which is then used for dosimetry with the known G value of the hydrated electron. 

However, by far the most widely used chemical dosimeters are the Fricke (ferrous sulfate) and the ceric sulfate dosimeter, which will now be described. 

Scientists are often faced with problems directly related to the problem of equivalence of the radiation effects produced by different types of radiation. The most typical are the two following problems. The first has to do with prediction of the radiation effect produced by a given type of radiation based on the data of radiative transformations produced by another type of radiation. This problem is very closely related to the problem of determining the limits of applicability of dosimetric systems (especially, of liquid chemical dosimeters). The second problem concerns the choice of equivalent radiation that can be substituted for a difficult-to-study type of radiation we might be interested in. 

The problem of equivalence of the radiation effects produced by different types of radiation is very important in dosimetry of ionizing radiation (especially, for liquid chemical dosimeters). As we have shown in Section IX.A, in the general case, the radiation effect in a condensed... 

Armstrong WA, Facey RA, Grant DW, Flumphreys WG (1963) A tissue-equivalent chemical dosimeter sensitive to 1 rad. Can J Chem 41 1575-1577... 

Inasmuch as the irradiation dose can change from pulse to pulse, it is desirable and often necessary to monitor the dose delivered by each pulse. This can be done by means of various devices a toroidal coil placed around the electron beam [116], a secondary emission chamber placed just before the exit window of the accelerator [13], a charge collector placed in the proximity of the irradiation cell [117], or, electron beam energy permitting, behind the irradiation cell [118], With these devices, a parameter is obtained representing the relative dose of each pulse calibration against chemical dosimeters provides the knowledge of the absolute irradiation dose associated with each individual pulse. 

The chemical dosimeter that is used most frequently is the thiocyanate dosimeter [119]. Other chemical dosimeters for pulse radiolysis are ferrocyanide [119], modified Fricke (Super-Fricke) [119], hydrated electron [120], 02-saturated solutions of potassium iodide [112], and N20-saturated solutions of methylviologen and formate [118]. The C(N02)4 (tetranitromethane, TNM) dosimeter is used in pulse radiolysis experiments with simultaneous optical and conductometric detection [121-124]. The composition and characteristics of the various chemical dosimeters used for pulse radiolysis with optical detection are listed in Table 8. 

Table 8. Chemical dosimeters for pulse radiolysis with optical detection. 

In practical application scanning can be manipulated on-the-fly within a chromatographic separation to obtain maximum information. In metabolism studies or as a chemical dosimeter, the structural feature of the parent compound and its unique neutral loss occurring on collisional activation, marks the metabolite. The mass of the metabolite is then obtained from the TSP mass spectrum at Q1 and the product ion spectrum of the metabolite molecule ion is obtained by product ion scanning. Recent publications have discussed additional applications of both tandem mass spectrometry (26. 271 and thermospray tandem mass spectrometry (28) in metabolite structure elucidation. 

The hydrolysis of methyl acetate is only weakly stimulated by ultrasound [182—184] and so this reaction would seem to be a poor contender in the pursuit of a chemical dosimeter. Despite this, Fogler and Barnes [ 183] have used this hydrolysis to investigate sonochemical reaction conditions. With a cup-horn type reactor they observed a temperature dependent optimum power input for this system (56 W at 40 °C, 61 W at 35 °C, and 67 W at 30 °C). The reaction itself has not been used more generally as a dosimeter. 

The terephthalate chemical dosimeter can be prepared by dissolving terephthalic acid (TA) 1.5.1(T3 mol-1 and NaOH 5.10-3 mol-1 in a phosphate buffer at pH = 7.4. The fluorescence generated by the product (hydroxyterephthalate) is measured at 425 nm with an excitation wavelength of 315 nm. The TA dosimeter solution can... 

In conclusion it is important to note that the above chemical dosimeters do not measure the same effects. The TA probe is a specific dosimeter for HO radicals, while the others are more general—thus both I and Fe2+ can also be oxidized by H02 , H202, or indeed other species and such processes do not occur at the same rate (e.g. the rate of production of I2 from I oxidation and the formation of H202 in water can be monitored independently and are not the same [174]). Chemical dosimeters are strongly frequency-dependent, thus the production of iodine in air saturated KI solutions is 6 times faster at 514 kHz than at 20 kHz [174], They are also strongly dependent on experimental conditions, especially with respect to the gas content. 

Since this chapter appears in a volume devoted to sonochemistry, chemical probes would appear to be the most attractive since they could allow direct comparisons with other chemical reactions. Chemical dosimeters are generally used to test the effect of an ultrasonic device on the total volume of the reactor. Local measurements can however be made with very small cells containing the dosimeter which could be moved inside the reaction vessel as with a coated thermocouple. Most of these chemical probes are derived from reactions carried out in an homogeneous medium, e.g. Weissler s solution, the Fricke dosimeter, or the oxidation of terephthalate anions. Among these the latter shows promise in that despite the fact that to date it has been much less used than Weissler s reaction it seems to have higher sensitivity and better reproducibility. 

Ideally when a chemical dosimeter is used to test or assess an ultrasonic device, care should be taken to match the system under study with the dosimeter type. The optimum conditions determined for a reactor using a chemical probe may well not be the same optimum as that required for the chemical system under investigation. Similar observations apply to the use of sonoluminescence. 

In this connection it is interesting to note that Dvornik and co-workers (17) have recently proposed a low-level chemical dosimeter made of 10 vol. chlorobenzene, 10 vol. % ethanol in 2,2,4-trimethylpentane using a spectrophotometric method for C determination. This should be of considerable use in measuring the Intensity of ionizing radiation when of a low-level. 

A. Chemical Dosimeters. Chemical dosimeters are systems in which measurable chemical changes are produced by ionizing radiation. Radiation produces acids in the system, the amount of which can be determined from visible color changes, or, more accurately, by titration or pH readings. Most chemical systems of practical size are useful only for gamma doses of hundreds to millions of cGy. However, small volume detectors can be found which measure doses in the range of a few cGy to several thousand cGy. 

Chemical Dosimeter - A type of dosimeter that uses a chemical change to measure the radiation. Chemoprophylaxis - Administration of a chemical to prevent the development of an infection or the progression of an infection to active manifest disease. 

The extrapolation of any chemical dosimeter calibrated at much lower dose rates cannot be used directly because the relative concentration of radicals produced with such pulses is so high that radical-radical reactions may predominate so that the yields of products may be different. 

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