Reproducibility, dosimeter

The performance of the dosimeter should be reproducible with a precision of a few percent. Dosimeter response should be insensitive to minor changes in its composition and it should be stable under normal conditions of exposure to air or light and the like. 

Other transient radicals such as (SCN)2 [78], carbonate radical (COj ) [79], Ag and Ag " [80], and benzophenone ketyl and anion radicals [81] have been observed from room temperature to 400°C in supercritical water. The (SCN)2 radical formation in aqueous solution has been widely taken as a standard and useful dosimeter in pulse radiolysis study [82,83], The lifetime of the (SCN)2 radical is longer than 10 psec at room temperature and becomes shorter with increasing temperature. This dosimeter is not useful anymore at elevated temperatures. The absorption spectrum of the (SCN)2 radical again shows a red shift with increasing temperature, but the degree of the shift is not significant as compared with the case of the hydrated electron. It is known that the (SCN) radical is equilibrated with SCN , and precise dynamic equilibration as a function of temperature has been analyzed to reproduce the observation [78],... 

Each type of dosimeter requires a specific procedure to ensure accurate and reproducible results, such as postirradiation heat treatment. Others need to be stabilized for a certain time (up to 24 h with some) before readings of absorbance are taken. The absorbance reading can be done by conventional spectrophotometry or other, more involved methods. 

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. 

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... 

Pugin [37] monitored the formation of an organolithium compound (butyl lithium in THF) and compared it to thermal and erosion measurements. He found a linear correlation between the rate of reaction for this process and the temperature rise of a coated thermocouple. This is an interesting result, but the slope of the line will almost certainly depend on the surface condition of the lithium pieces, on their size, and on the location of the solid relative to the ultrasonic source. These parameters should be carefully controlled in order to get reproducible results using this dosimeter. 

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. 

Fig. 9.4 Dependence of the peak-to-peak amplitudes of the ESR spectra of an irradiated lithium formate ESR dosimeter on microwave power and modulation amplitude. The settings for the experiments are marked in the figure. The diagram is reproduced from [21c] with permission from Dr. T.A. Vestad...

Fig. 9.6 ESR spectrum of irradiated self-calibrated alanine dosimeter. The diagram is reproduced from [4] with permission from Springer. An editing error in [4] was corrected...

The monitoring of dose can be performed with different techniques chemical systems (Fricke and ceric solutions [31]), radiochromic compounds (dyes [32, 33]), polymeric tapes (polyethylene [34], poly(vinyl chloride) [35, 36], poly(methyl methacrylate) [37-40], epoxy resin [41]), radiation thermoluminescence phosphors [42, 43]. Several requirements are imposed for a proper dosimeter similarity with processed material in respect with linear energy transfer, reproducibility, sensitivity, lack of the influence of humidity, stability after irradiation, easy to calibrate, appropriate dose range and dose rate, linearity and independency on the type of radiation, the response being constant in time, lack of post-irradiation modification. 

The dichromate dosimeter solution is of importance mainly for radiation sterilization and food irradiation applications both for gamma and electron dosimetry. Due to its very good reproducibility, the system is classified as a reference standard system (ASTM E 2628-2009) in the 5-50 kGy dose range and used widely also as a transfer standard dosimeter. 

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