Ceric 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. 

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... 

Tphe oxalic-acid dosimeter has a history comparable to that of the ceric-sulfate dosimeter. Much work has been carried out, promising results have been obtained, and an abundance of publications have appeared but only few people really use these systems. Still both the ceric-sulfate dosimeter and the oxalic-acid dosimeter may be applied successfully, once they are debugged by the people who are to use them in the daily routine. Both systems deserve much attention as they are among the few promising candidates of aqueous chemical dosimeters for use in the megarad range. 

The oxalic-acid dosimeter has some substantial advantages over the ceric-sulfate dosimeter, which made us investigate it more closely (1) it is quite insensitive to impurities, (2) it has very good energy-absorption characteristics, and (3) the system is very stable to normal storage before and after irradiation. The system also has some drawbacks (1) the decomposition of oxalic-acid does not proceed linearly with the absorbed dose, and (2) the chemical yield is not fully independent of the radiation conditions. Other difficulties have been reported and have hampered the practical use. 

The use of the ceric sulfate dosimeter solution is based on the radiolytic reduction of the ceric ions to cerous ions in an aqueous addic solution (Weiss 1952). 

The ceric sulfate dosimeter is sensitive to impurities, but this effect can be decreased by the addition of scavengers, e.g., cerous ions, or by preirradiation of the solution to a dose of approximately 1 kGy. Other unfavorable characteristics of the solution are its light sensitivity, energy dependence below 0.1 MeV, dose rate dependence above 10 Gy s and the need to dilute the irradiated solutions for the spectrophotometric evaluation. The temperature coefficient of the solution during irradiation is not significant in the range of 10-62°C (Matthews 1982). 

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. 

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. 

Ceric Sulfate (or Ceric-Cerous Sulfate) Dosimeter... 

The response of the dosimeter is based on the difference in ceric ion concentration before and after irradiation. The initial concentration of ceric sulfate (or ceric ammonium sulfate) can be varied between 2 x 10 and 5 x 10 mol dm in an aqueous solution... 

The ceric-cerous sulfate dosimeter is a chemical dosimeter acting either as a routine dosimeter or as a reference standard dosimeter for the measurement of high dose levels (Matthews, 1982). A routine dosimeter is used in radiation processing facilities for dose mapping. A reference standard dosimeter is used to calibrate radiation fields and routine dosimeters. The dosimeter is based on the reduction of cerium(IV) to cerium(in) in an aqueous solution by radiation (Matthews, 1971). Doses in the range 0.5 to 50 kGy can be determined by conventional spectrophotometric analysis in the ultraviolet region, or by measuring the difference in the electrochemical potential between the irradiated and non-irradiated solutions in an electrochemical potentiometer (Matthews, 1972 Church et al., 1976). As most dosimeters, the... 

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