The Fricke Dosimeter

The chemical change in the Fricke dosimeter is the oxidation of ferrous ions in acidic aerated solutions. It is prepared from a -1 mM solution of ferrous or fer-roammonium sulfate with 1 mM NaCl in air-saturated 0.4 M H2S04. Addition of the chloride inhibits the oxidation of ferrous ions by organic impurities, so that elaborate reagent purification is not necessary. Nevertheless, the use of redistilled water is recommended for each extensive use. Absorption due to the ferric ion is monitored at its peak -304-305 nm. The dose in the solution is calculated from the formula 

Chapter 11 Radiation Chemical Applications in Science and Industry 

Chemical synthesis is one important aspect of the application of radiation-chemical reactions in industry. Various kinds of radiation-induced syntheses are available, some of which will be described here. There are also nonsynthetic applications including, but not limited to, food irradiation, waste treatment, and sterilization by irradiation. Some of these will be taken up in the next section. 

It is clear from this discussion that the dose requirement and unit cost will be lower if the material has a higher molar mass M and the reaction has a high G value. Thus, the best candidates will be a polymeric material and a chain reaction. Quite often, a free-radical irradiation is used. The radiation source of choice is usually a 60Co - y facility, although electron beam irradiation is also used. Since most radiation-chemical reactions used in industry can also be brought about by other conventional means such as thermal, or photochemical processes, the processing cost must be below 10 t. kg-1 to be competitive, since it is unlikely that the cost of irradiation will come down in future. It should remembered that in figuring the irradiation cost one has to include the cost of operation, maintenance, and the like. (Danno, 1960). 

Various industrial pilot plants and full-scale operations, using radiation-chemical processing have been reported, with production rates -50 to -1000 tons per year (Spinks and Woods, 1990 Chutny and Kucera, 1974). Production rates less than -50 tons per year are not considered viable. These operations are or have been conducted in countries such as the United States, the former U.S.S.R., Japan, and France. However, some operations have also been reported in the former Czechoslovakia and Romania, especially in connection with petroleum industry. In the United States, chlorination of benzene to gammexane (hexachlorocyclohexane) was hotly pursued at one time by radiation or photoinitiation. Since the early seventies the activity has dwindled, presumably due to lack of demand and environmental considerations. 

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The quantitative aspects of track reactions are involved some details will be presented in Chapter 7. The LET effect is known for H2 and H202 yields in aqueous radiation chemistry. The yields of secondary reactions that depend on either the molecular or the radical yield are affected similarly. Thus, the yield of Fe3+ ion in the Fricke dosimeter system and the initiation yield of radiation-induced polymerization decrease with LET. Numerous examples of LET effects are known in radiation chemistry (Allen, 1961 Falconer and Burton, 1963 Burns and Barker, 1965) and in radiation biology (Lamerton, 1963). 

Several authors have made restricted comparisons between experiment and calculations of diffusion theory. Thus, Turner et al. (1983, 1988) considered G(Fe3+) in the Fricke dosimeter as a function of electron energy, and Zaider and Brenner (1984) dealt with the shape of the decay curve of eh (vide supra). These comparisons are not very rigorous, since many other determining experiments were left out. Subsequently, more critical examinations have been made by La Verne and Pimblott (1991), Pimblott and Green (1995), Pimblott et al. (1996), and Pimblott and LaVeme (1997). These authors have compared their... 

Figure 7 Effect of ion energy on the response of the Fricke dosimeter. The solid lines and filled...

Hydrated electron yields decrease with increasing MZ jE, but they do not seem to decrease to zero. Experiments have been performed on aerated and deaerated Fricke dosimeter solutions using Ni and ions [93]. One half of the difference in the ferric ion yields of these two systems is equal to the H atom yield. The Fricke dosimeter is highly acidic so the electrons are converted to H atoms and to a first approximation the initial H atom yield can be assumed to be zero (see below). There is considerable scatter in the data of the very heavy ions, but they seem to indicate that hydrated electron yields decrease to a lower limit of about 0.1 electron/100 eV. The hydrated electron distribution is wider than that of the other water products because of the delocalization due to solvation. This dispersion probably allows some hydrated electrons to escape the heavy ion track at even the highest value of MZ jE. 

Experimental approaches to measure the radial dose distribution are also in progress [119], and it was found that the distribution follows r law in the inner region of a critical distance and obeys r law outside of that region. LaVerne and Schuler reported the considerable decrease in the radiation chemical yield for ferric production in the Fricke dosimeter, suggesting a model of a deposited energy density in an ion track, which depends on the LET and the atomic number of an irradiation particle [120,121]. 

The coolant water at around 300 °C is irradiated mainly at the core of the reactor. At the initial stage to determine the G-values of water decomposition products at elevated temperatures, the Fricke dosimeter was chosen [10-14] because the mechanism of the reaction has been established. Since the reactions in neutral solution are of practical interest, intensive measurement of the G-values of water decomposition products at elevated temperatures in neutral solutions has been done [15 21]. 

Dosimetry was based either directly on the Fricke Dosimeter (I) (using GFes+ = 15.5) or on an n-on-p solar cell (18) that was frequently calibrated against the Fricke dosimeter. 

A.S.T.M., Standard Method of Test for Adsorbed Gamma Radiation Dose in the Fricke Dosimeter, D 1671-63. 

The time required to raise or lower the source is 1.5 minutes, comparable to an irradiation dose in the irradiation position of approximately 35,000 rads (transient dose). The dose rate in the irradiation position is approximately 60,000 rads per minute. Calibration of the ferrous-cupric dosimeter is determined by comparison with the Fricke dosimeter, when irradiated at a position in the cell having a lower dose rate. The dose rate in the calibration position is approximately 5 X 103 rads per minute with a transient dose of approximately 3 X 103 rads. Calibrations made at such a position when using a G(Fe3+) = 15.6 for the Fricke dosimeter gave a G(Fe3+) of 0.66 for the ferrous-copper dosimeter. 

It has been established (5, Jf) that by adding cupric sulfate to modify the Fricke dosimeter, it is possible to reduce the ( (Fe3+) drastically from 15.6 to 0.66. 

Effect of Ferrous Ion Concentration. The reactions involved in the ferrous-cupric dosimeter, as described by Hart (1) are independent of oxygen concentration, and one would not expect to observe a change in the ferric yield ( (Fe3+) in this system when increasing the dose, as occurs in the Fricke dosimeter. This would indicate that the dose limit of this dosimeter is a function of the initial ferrous ion concentration and is not influenced by the oxygen concentration. To explore this idea, we increased the ferrous ion concentration from 0.001M to 0.01M while keeping the... 

Fig. 3. Compilation of radiolytic yields of the Fricke dosimeter in aerated conditions for various ions and energy. The squares are unpublished results obtained from 34 MeV protons, 1 GeV carbon ions and 2 GeV argon ions.

One of the earliest devices for the measurement of radiation dosage, the Fricke dosimeter, is based on the oxidation of the ferrous ion by OH radicals produced in the radiolysis of a dilute aqueous solution of ferrous sulfate ... 

Several methods exist for the identification and quantification of the HO and H02 radicals generated by the sonolysis of water. These species can oxidize ionic moieties e.g. Fe2+ into Fe3+ (the Fricke dosimeter) and I- into iodine. In addition, either can dimerize to form hydrogen peroxide (Schemes 2 and 3), which can then be titrated using conventional techniques. The HO will also react with terephtha-late anion in aqueous solution to produce hydroxyterephthalate anion, a fluorescent material which can then be estimated using fluorimetry. 

Price and Lenz [188] compared the TA probe with the Fricke dosimeter (Fe2+ — Fe3+), using a Sonics and Materials VC 600 horn operating at 20 kHz. 

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. 

Dosimetry. The dose rate of 2.0 X 1018 e.v. grams"1 min."1 in water vapor was based on the yield of hydrogen from ethylene, using G(H2) = 1.31 (23). Using this dosimetry, the yield of nitrogen from 700 torr N20 irradiated in the same experimental set-up at room temperature was G(N2) = 10.0 0.3. The relative dose rate was checked periodically with the Fricke dosimeter. The energy absorbed by each component in a mixture was calculated, assuming it to be directly proportional to the electron density of that component. 

The irradiations were carried out at room temperature with dose-rates between 0.40 X 1018 and 4.66 X 1018 e.v./ml./hr. The absorbed doses were measured by the use of the Fricke dosimeter, taking G(Fe3+) = 15.6 (9). 

Irradiation Procedure. Irradiation was carried out with a Siemens x-ray set operated at 220 kv. and 20 ma., with 2 mm. Al filtration. The sample solutions in irradiation vessels of borosilicate glass (23) were flushed with appropriate gases before and during irradiation. The dose rate for each irradiation vessel was determined with the Fricke dosimeter (36), assuming 15.5 molecules of Fe3+ (36) formed per 100 e.v. absorbed energy. The dose rate was 1.94 krad min."1. 

For irradiation of the solutions a panorama 60Co-source of 2.05 X 104 Ci (dose rate 6.25 X 1018 e.v. ml.-1 min.-1) was used (22). A number of samples were irradiated with a second 60Co-source (Gamma Cell 220 dose rate 6.9 X 1017 e.v. ml."1 min."1). The dose rate was determined by means of the Fricke dosimeter using G(Fe8+) = 15.6. 

Chloride ions are used in the Fricke dosimeter as hydroxyl radical scavengers (2). Since adding chloride ions does not decrease the yield of ferric ion (except in the presence of organic impurities), it can be argued that chlorine atoms and hence Cl2" radical ions react to oxidize ferrous ion. Using the present technique we have measured the effect of ferrous ions on the rate of decay of the Cl2" transient. Ferrous ions increased this rate, and a rate constant for Reaction 3 was determined h = 3.8 0.3 X 107M 1 sec."1 at pH 2.1... 

All samples were irradiated with 60Co y-rays at a dose-rate of 1.2 X 1018 e.v./gram/min. as determined by the Fricke dosimeter [G(Fe3+) = 15.5, esoo = 2180 at 24°C.]. All yields are expressed as G values (molecules per 100 e.v. of absorbed energy). Energy deposition in the solids and concentrated solutions was taken to be proportional to electron density. 

Connally and Gevantmann (2) used the spectrophotometric method and found, within 5-10%, agreement with values extrapolated from measurements with the Fricke dosimeter. 

The system should be checked against the Fricke-dosimeter to prove the adequacy of the oxygenation. 

Figure 3 presents a typical y-ray calibration of LiF thermoluminescence using a dose rate of 0.90 rads/sec. The y-ray source was calibrated against the Fricke dosimeter. 

Comparison of the hydrated electron absorption with the Fricke dosimeter gave the same results as were obtained by Fielden and Hart (2) for the product of G(e m) and e, the molar extinction coefficient of the hydrated electron at 7000 A. 

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