Ferrous sulfate dosimeter

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). 

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. 

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). 

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. 

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

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. 

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. 

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). 

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... 

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... 

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 ... 

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. 

Let us assume that we want to measure the G-value, number of molecular changes per 100 e.v., of Ce4+ — Ce3+ in a ceric-sulfate solution as a function of the concentration. We measure the dose in a Fricke dosimeter vial or ferrous-cupric dosimeter vial at the same place as the vial containing the ceric solution, and then, as is usual, we correct for the difference in the energy transfer coefficient at 1.25 Mev. and for the difference in density of the solutions. However, as shown in Equation 14 and in Table V and Figures 3 and 4, these corrections are entirely inadequate because of the large difference in buildup factors. For 0.4M ceric sulfate solution, the correction caused by the buildup factor is 72% at fit r = 1 122% at /At r = 2 and 155% at fit r = 4. 

The most well-known liquid chemical dosimeter, the Fricke solution is based on the radiation-induced oxidation of ferrous ions, Fe(II), to ferric ions, Fe(III), in acidic media (E1026 Fricke Reference Standard Dosimeter, ASTM standard). The standard Fricke solution consists of 0.001 mol dm ferrous ammonium sulfate (Fe(NH4)2(S04)2(6H20)), or ferrous sulfate (FeS04(7H20)), and 0.4 mol dm H2SO4 in aerated aqueous solution. It is applicable for measuring doses in the 40 to 400 Gy range. 

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... 

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