Dosimeter system

The quantitative aspects of track reactions are involved some details will be presented in Chapter 7. The LET effect is known for H2 and H202 yields in aqueous radiation chemistry. The yields of secondary reactions that depend on either the molecular or the radical yield are affected similarly. Thus, the yield of Fe3+ ion in the Fricke dosimeter system and the initiation yield of radiation-induced polymerization decrease with LET. Numerous examples of LET effects are known in radiation chemistry (Allen, 1961 Falconer and Burton, 1963 Burns and Barker, 1965) and in radiation biology (Lamerton, 1963). 

There are several dosimeter systems being used for determining fhe absorbed dose. The simplest ones are thin-film dosimeters, which are suitable for use in routine practice. 

The requirement for sensitivity of the dosimeter system is to detect a concentration of 18 parts per trillion in 8 hours. At this safe limit concentration, the bubbler will collect 1.5 X 10 g of contaminant when sampling air at 30 cc/min for an 8-hour period. A sufficient amount of contaminant in solution must be retained for analysis. 

A shutter and cutoff filters are usually placed between the light source and the irradiation cell to minimize photolysis effects. For example, when the MV +/ formate dosimeter is used (see Table 8), it is essential to exclude UV light to prevent photolytic generation of MV+, which is also the product of this dosimeter system. Similar considerations apply to the ferrocyanide and Fricke dosimeters where photo-oxidation of Fe to Fe can occur. Different filters are sometimes mounted on a wheel that can be remotely controlled, possibly by a computer, as it is done for the shutter. 

The production of the (SCN)2 radical has been carefully studied, and the G value accurately determined. With its broad absorption band and generous molar absorptivity (8472 = 7580 dm mol" cm" ), this radiolysis dosimeter system provides an excellent means of calibrating the radiation radical yield (Figure 3). The oxidation of SCN" by HO (reaction 30) is virtually diffusion controlled, as is the subsequent complexation of the radical with SCN . Since... 

In the following text, a few dosimeter systems will be mentioned that are commonly used in radiation chemistry. For a more detailed treatment of chemical and physical dosimetry, see O Chap. 49 in Vol. 5 on Dosimetry Methods. However, more attention will be paid to pulse dosimetry that is used in pulse radiolysis investigations. 

Besides the well-established aqueous dosimeter solutions, new systems are also under development and test and the introduction of the tetrazolium solutions for dose control in radiation processing is one example of these new dosimeter systems. 

The liquid dosimeter systems used most frequently in radiation processing are shown in O Table 49.1. 

Dosimeter system Method of analysis Useful dose range (Gy) Typical precision limits [k = 2) Reference... 

Various calorimetric dosimeters have been designed for use in radiation dosimetry in medical applications and in radiation processing. Calorimetry is often used to calibrate chemical and solid-state dosimeter systems, but, until recently, it was rarely applied for routine... 

The same conclusions can be drawn regarding electrochemical sensors (Korotcenkov et al. 2011, etc). Electrochemical sensors are suitable only for low-concentration, parts-per-million ranges. In addition, their life expectancy is only 2-5 years. Moreover, depending on the application, life expectancy may be much shorter. However, the electrochemical gas sensors have very low power consumption, respond quickly to gas, and are not affected by humidity. These sensors can also be exposed to gas periodically, which maximizes sensor life. Therefore, one can conclude that electrochemical sensors are a good choice for portable instruments and alarm/dosimeter systems, including Ughtweight, personal monitor/alarm devices, rather than for continuous monitors. 

In the field of radiation methods of control, development work was performed in order to create the X-ray detectors with a low content of silver. X-ray TV systems with improved performance for automatic interpretation of the X-ray TV images, portable radiometers and dosimeters, creation of portable equipment for radioscopy of the welded joints of pipelines, etc. 

Chemical dosimeters measure the absorbed dose by the quantitative determination of chemical change—that is, the G value of a suitable product—in a known chemical system. These are secondary dosimeters in the sense that the corresponding G values must be established with reference to a primary, absolute dosimeter. The primary dosimeters are usually physical in nature calorimeters, ionization chambers, or charge measuring devices with particles of known energy. However, the primary dosimeters are generally cumbersome, whereas the chemical dosimeters are convenient to handle. On the other hand, the chemical dosimeters are not suitable for low-dose measurements. 

The response of the dosimeter should be linear to the dose—that is, the G value of the product should be independent of the dose or the dose rate. The useful dose range may be stipulated as -1-106 Gy, and it may be difficult to design a chemical system that preserves linearity over the entire range. In any case a useful chemical dosimeter must have linearity over most of the intermediate dose range. 

Figure 8.5. Schematic view of continuous flow reaction setup for IL/scCC>2 systems (adapted from reference [81]). C compressor, CT cold trap, D dosimeter, DP depressuriser, F flowmeter, M mixer, MF metal filter, P HPLC pump, PT pressure transducer and thermocouple, R reactor, S styrene...

Hydrated electron yields decrease with increasing MZ jE, but they do not seem to decrease to zero. Experiments have been performed on aerated and deaerated Fricke dosimeter solutions using Ni and ions [93]. One half of the difference in the ferric ion yields of these two systems is equal to the H atom yield. The Fricke dosimeter is highly acidic so the electrons are converted to H atoms and to a first approximation the initial H atom yield can be assumed to be zero (see below). There is considerable scatter in the data of the very heavy ions, but they seem to indicate that hydrated electron yields decrease to a lower limit of about 0.1 electron/100 eV. The hydrated electron distribution is wider than that of the other water products because of the delocalization due to solvation. This dispersion probably allows some hydrated electrons to escape the heavy ion track at even the highest value of MZ jE. 

As an aside, we note that the FDEMS sensor input information can also be used to detect the onset of phase separation in toughened thermoset systems and to monitor cure in thin film coatings and adhesive bond lines. It is particularly important that the FDEMS sensor is also very sensitive to changes in the mechanical properties of the resin due to degradation. As such, it can be used for accelerated aging studies and as a dosimeter to monitoring the composite part during use to determine the knockdown in the required performance properties with time. 

Alanine dosimeters are based on the ability of 1-a alanine (a crystalline amino acid) to form a very stable free radical when subjected to ionizing radiation. The alanine free radical yields an electron paramagnetic resonance (EPR) signal that is dose dependent, yet independent of the dose rate, energy type, and relatively insensitive to temperature and humidity. Alanine dosimeters are available in the form of pellets or films and can be used for doses ranging from 10 Gy to 200 kGy. A reference calibration service using the alanine EPR system was developed and the scans were sent to the service center by mail. Currently the available system allows transferring the EPR scan to a NIST server for a calibration certificate. This way the procedure has been shortened from days to hours. ... 

Noise—with the assessment of permissible noise levels lor communication and warning signals and the development of technology for noise abatement and control. Developments have included an audio dosimeter to replace conventional sound-level meters, discriminating earmuffs, and a noise control muffler system to reduce pneumatic drill noise. 

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. 

When a system is being considered as a routine dosimeter, it is desirable to investigate the effect of altering various parameters—e.g., acid concentrations, reagent concentrations, and storage time—on the results obtained. To obtain this information, a series of experiments was conducted. 

---