Ferrous-cupric 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. 

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. 

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... 

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 ... 

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. 

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. 

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) =... 

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... 

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... 

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