Ferrous dosimeter

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. 

Irradiation Conditions. The gamma (cobalt-60) radiation facility and the source calibration are described by Holm and Jarrett (4). Irradiation doses were 3-4 Mrad and 6-7.5 Mrad at 9 X 102 rads per second for the screening study. Irradiation temperatures were 5, —30, and — 90°C. The gamma source was calibrated with the ferrous sulfate-cupric sulfate dosimeter. 

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

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... 

Product analysis of y-R was carried out in a Pyrex tube with an inner diameter of 1.0 cm at room temperature (r.t.). After y-R, the reaction mixtures were directly analyzed by GC and GC-MS. The G values were calculated from the product yields, and the absorbed dose measured by the ferrous sulfate dosimeter (Fricke dosimeter). 

A dosimeter suitable for measuring the radiation intensity at doses in the range of 10 to 8 X 1CP rads consists of a solution of 0.001 M ferrous sulfate-0.01M cupric sulfate in 0.0ION sulfuric add. If the recommended concentrations are used, the dosimeter is reproducible to 0.3% and stable after irradiation to approximately 2% per week. The dose received by the recommended dosimeter can be calculated, if read at 25°C., by converting the change in absorbance (AA) using the equation dose (rads) =... 

HPhe Fricke dosimeter (ferrous sulfate solutions) has been used to measure A the radiation intensity of various types of ionizing radiation sources since its development by Fricke and Morse in 1927 (2). It is widely accepted because it yields accurate and reproducible results with a minimum of care. This system meets many of the requirements specified for an ideal dosimeter (5, 9) however, it has a limited dose range, and for our applications it has been necessary to develop a dosimeter covering larger doses. Of the systems reviewed (6, 7), two (ferrous sulfate-cupric sulfate and ceric sulfate) showed the most promise for use with the radiation sources at the U. S. Army Natick Laboratories (8). Of these, the ferrous-cupric system has received the most use, and this paper describes our experience in using this system and suggests procedures by which it may be used by others with equal success. 

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. 

Since oxygen is not consumed in the over-all reaction, it does not affect G Fe3+). This dosimeter shows no dose rate effect under the usual conditions of 7-ray irradiation since all of the above reactions except Reaction 6 are fast, and this reaction has a half-life of 14 seconds in 0.001M ferrous sulfate (1). 

Figure 1. Effect of initial sulfuric acid concentration on ferric ion yield (AA), measured after irradiating of standard ferrous-cupric dosimeter to approximately 400,000 rads...

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... 

Effect of Cupric Concentration. The cupric ion concentration was varied between 0.0001M and 0.1 M to determine its effect on the standard ferrous copper dosimeter. Figure 4 shows that as the cupric ion concentration is increased, the G(Fe3+) decreases. A change of 0.005M from an initial cupric ion concentration of 0.01M results in a 10% variation in the ferric yield. 

Reproducibility. To determine the maximum variation in results that one might expect when using the ferrous sulfate-cupric sulfate system as a routine dosimeter, we prepared a number of dosimeters containing the standard solution and irradiated them in a No. 10 can similar to that previously described. These dosimeters gave a dose rate for the position of... 

There is no question that the ferrous-cupric dosimeter can be used for routine dose measurements if the unirradiated solution is read at the start of the irradiation and not used as a blank against which the irradiated solutions are compared. The irradiated solutions can be held up to 1 month before reading without affecting the results by more than 10%. If it is inconvenient to prepare the solution fresh before each day of irradiation, the... 

Figure 6. Storage stability of standard ferrous-cupric dosimeter, unirradiated and irradiated, to various initial absorbances Ferric ion concentration...

Figure 7. Relation of irradiation time to absorbance curves for fresh preirradiated and stored solutions of standard ferrous-cupric dosimeter...

For routine use, the dose the standard ferrous-cupric dosimeter (0.001 M FeSC>4, 0.01M CuS(>4, in 0.01 H2SO4) has received can be calculated, if read at 25°C., from the following equation ... 

To calibrate the cobalt source, three systems are most often used ferrous sulfate, ferrous sulfate-cupric sulfate, and ceric sulfate. Dosimeters of these solutions are prepared by filling 5-ml. chemical-resistant glass ampoules with approximately 5 ml. of solution and flame-sealing the ampoules. The ampoules are then arranged in phantoms of Masonite or similar materials (Figure 13) to simulate the food items. These phantoms are placed in containers similar to those used for food products, and arranged in the conveyor carrier in which they are transported into the irradiation cell. Because of the upper dose limit of the ferrous sulfate and ferrous sulfate-cupric sulfate dosimeters (40,000 and 800,000 rads, respectively), these systems can be used only to establish the dose rate in the facility and not to monitor the total dose during food irradiation. The ceric dosimeter which... 

Exposures, for both the gamma-irradiated samples at the Argonne facility and the x-irradiated samples, were based on the standard ferrous sulfate dosimeter solution (using a G value of 15.6) (12, 15). In the latter case, the mica window sample holder used for sample irradiations was filled with the dosimetric solution for dosage estimations. 

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 ... 

For quantitative studies in radiation chemistry, it is essential that the energy input into the irradiated volume should be accurately determined. For this purpose, the most versatile and reliable method is the ferrous sulfate dosimeter, proposed by Fricke and Morse. The method involves the use of an air-saturated solution of 10 M ferrous sulfate and 10 M sodium chloride in 0.8 N sulfuric acid. On exposure of the solution to ionizing radiations, the ferrous ion is oxidized to ferric ion, which may conveniently be determined accurately by spectrophotometry. The amount of chemical change is proportional to the total energy-input, independent of dose rate, and (within wide limits) independent of the concentration of ferrous ion, ferric ion, and oxygen. The main reactions involved are as follows. 

Dosimetry. The intensity of the flux at the sample site was measured periodically with a ferrous sulfate dosimeter, using GFe(iii) = 15.5. Energy absorbed in liquid samples was based on this dosimetry and was corrected for the electron density of the samples. To determine energy absorbed in the vapor samples, nitrous oxide was irradiated, at comparable electron densities, in the vessel used for the hydrocarbons the G value for nitrogen production was taken to be 11.0 (7). 

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. 

Dosimetry is the measurement of absorbed dose. The unit of absorbed dose is the gray (Gy). Because dose is a measure of absorbed energy, calorime-try is the fundamental method of measurement. However, calorimetry suffers from being insensitive, complex, slow and highly demanding in technical skills and experience. Primary dose measurement is usually done with substances that are chemically changed quantitatively in response to the amount of radiation absorbed. For most purposes the standard primary system is the Fricke or ferrous sulfate dosimeter. In this system, which consists of a solution of ferrous sulfate in dilute sulfuric acid, ferrous ions Fe are oxidized by absorbtion of radiation to ferric ions Fricke dosimeters are usually presented in glass... 

Irradiation Procedures. G(Fem) of 15.6 for the ferrous sulfate dosimeter was determined in our laboratory by Hochanadel and Ghorm-ley (II). G-values reported here are based on total energy absorbed by the solutions. The energy absorbed in concentrated sodium nitrate solutions relative to the ferrous sulfate dosimeter was taken to be in the ratio of electron densities since energy absorption in 60Co irradiations is caused essentially only by Compton recoil electrons. 

Irradiations were within a 60Co source (— 2000 curies) with a dose rate of about 1018 e.v. ml.-1 min. 1 in the ferrous sulfate dosimeter. Changes in cerium (IV) concentration were followed with a Cary recording spectrophotometer at 340 mfx. No detectable effect on measurements at 340 m[a could be attributed to coloration of the S 18-260 silica windows during irradiation. Therefore, irradiations and spectrophotometric measurements were made in the same cell. 

All chemicals used were reagent grade. Acid solutions were prepared with HC104 unless otherwise noted. The Fricke ferrous sulfate dosimeter was employed in the usual manner. 

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... 

Radiation doses are measured with a ferrous sulfate dosimeter based on a value of G(Fe3+) = 15.5 (28). For radiation doses approaching... 

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