Dosimetry reactions

Polystyrene fflm doped with traws-stilbene has been found useful as a chemical dosimeter for ionizing radiation [68]. The dosimeter was effective for the photon energy range from 4.7 MeV to 1.25 keV (mean) and for rates above 300 rads/s. The mechanism for the dosimetry reaction, isomerization of trans- to cis-stilbene, was shown to be analogous to the mechanism proposed for radiolytic isomerization. 

In practice, the (F) function in a given point inside the sample is determined by the unfolding method using a set of activation threshold detector foils. The most commonly used dosimetry reactions are also summarized in Sect. 36.3 of the Appendix of this Volume. With the knowledge of the 0(F) and [Pg.1677]

In order to estimate the induced activity, the neutron dose value and the effect of interfering reactions, the distributions of thermal, epithermal, and fast neutron flux densities should be determined in a bulk media or a phantom. The most commonly used dosimetry reactions are given in O Sect. 36.3 of the Appendix of this Volume. In the case of a phantom, the combination of the activation detectors and the physical integration methods is recommended for the determination of the volume averaged dose equivalent rates and through it the total dose absorbed by a given organ (Csikai et al. 1988). 

The types of dosimetry reaction can be divided into fission and non-fission, threshold and non-threshold. The choice of the set of reactions to be used for a particular plication depends on the intensity of the flux, the duration of the irradiation, the type of reactor spectrum (thermal reactor, fast reactor, core, shielding, vessel) and the neutron energy range of importance in the effect being monitored. 

Eor virtually all radiopharmaceuticals, the primary safety consideration is that of radiation dosimetry. Chemical toxicity, although it must be considered, generally is a function of the nonradio active components of the injectate. These are often unreacted precursors of the intended radioactive product, present in excess to faciUtate the final labeling reaction, or intended product labeled with the daughter of the original radioactive label. 

Dosimetry. Ion current measurements required for absolute dosimetry were performed with a Cary 31 ionization chamber and vibrating reed electrometer. Dry nitrogen was used as filling gas for the chamber, and a W value of 34.9 e.v./ion pair was assumed for H-3 beta rays in N2 (27). Deuterium pressures in each of the reaction mixtures were great enough to ensure that less than 1% of the H-3 beta rays reached the walls of the reaction vessel (7). 

Interaction with a chemical indicator Can be highly specific, if suitable indicator. Can measure total exposure over time (dosimetry), if a non-reversible reaction is used. Can allow operation at a convenient wavelength, when gas has no convenient absorption in that spectral range. Poisoning can occur, and is easily fouled. Sensitive to groups of chemicals, e.g. acid gases, rather than to a specific gas. May exhibit non-reversible behaviour, which, in many cases, may be undesirable. May need water vapour present, to act as a catalyst, if dry reaction is too slow. 

Lifetime for Excited Water. tHio s.h8o, where fe.Hto, is the rate constant reaction of solute with excited water (reaction of solute with H30 in one model and with H20 in the other model), is a constant which can be derived from results summarized by Equations 1-14 as discussed below and which is independent of any constant errors in absolute dosimetry. Let Gh,o denote the yield of H20 which disappears intraspur by a first-order process with resultant H2, H202, and H20 formation. Let a denote the number of H2 molecules formed for each H20 which disappears intraspur. 

Wright HA, Magee JL, Hamm RN, Chatterjee A, Turner JE, Klots CE (1985) Calculations of physical and chemical reactions produced in irradiated water containing DNA. Radiat Protect Dosimetry 13 133-136... 

For dosimetry, the reaction of the reaction of OH with DMSO which yields methanesulfinic acid (92% Veltwisch et al. 1980 Chap. 3.2) is usually used. This allows one to put the conductance signals on a quantitative basis (calculation of G values), and the rates of reactions that are kinetically of first order can be determined for the time dependence of the signal evolution. DMSO dosimetry yields only a relative dose. For the determination of second-order rate constants, however, the exact dose must be known, and this can be determined by the zero conductivity change dosimetry or neutralization kinetics dosimetry (Schuchmann et al. 1991). 

Animal studies designed to quantitatively identify organ and system toxicity need to be carried out. Dosimetry studies to quantitatively identify levels of toxicant in blood or urine either as reaction products or elemental phosphorus need to be considered (see Section 2.5.1). 

Another application of track detectors is dosimetry of a particles and neutrons. For neutron dosimetry the track detectors may be covered with uranium foils in which the neutrons induce fission. Alternatively, the detectors may be covered with a foil containing B or Li, and the a particles produced by (n, a) reactions are recorded. 

I, and I. Radioimmunoassay combines the specificity of an immunochemical reaction with the sensitivity of isotope analysis (F4, S19) and is currently developing rapidly for the analysis of steroids, peptide hormones, and specific proteins (G9). Enzymes can be determined with labeled substrates (01). The requirements of standardization and dosimetry make it probable that these methods will continue to be based on relatively large automatic instruments. However, the greatest problems are unlikely to be in the counting equipment but in sample handling and processing and in standardization of the system. 

The essential apparatus for pressure measurement and analysis, and other important aspects such as furnaces and temperature control, are reviewed for thermal, photochemical and radiochemical systems. The latter two also involve sources of radiation, filters and actinometry or dosimetry. There are three main analytical techniques chemical, gas chromatographic and spectroscopic. Apart from the almost obsolete method of analysis by derivative formation, the first technique is also concerned with the use of traps to indicate the presence of free radicals and provide an effective measure of their concentration. Isotopes may be used for labelling and producing an isotope effect. Easily the most important analytical technique which has a wide application is gas chromatography (both GLC and Gsc). Intrinsic problems are those concerned with types of carrier gases, detectors, columns and temperature programming, whereas sampling methods have a direct role in gas-phase kinetic studies. Identification of reactants and products have to be confirmed usually by spectroscopic methods, mainly IR and mass spectroscopy. The latter two are also used for direct analysis as may trv, visible and ESR spectroscopy, nmr spectroscopy is confined to the study of solution reactions... 

Biotransformation and generation of reactive intermediate metabolites are associated with a variety of toxicities and idiosyncratic reactions.37 Toxicologists should always consider how drug disposition and fate contribute to toxicity, as target organ dosimetry, biotransformation, and detoxification reactions can be important determinants of toxicity. In all cases, understanding how biotransformation may differ across species, with emphasis on human metabolism, is an important component in determining whether preclinical effects are predictive of and relevant for human safety evaluation. 

A very important point occurs in the transmission of acoustic power into a liquid which is termed the cavitation threshold. When very low power ultrasound is passed through a liquid and the power is gradually increased, a point is reached at which the intensity of sonication is sufficient to cause cavitation in the fluid. It is only at powers above the cavitation threshold that the majority of sonochemical effects occur because only then can the great energies associated with cavitational collapse be released into the fluid. In the medical profession, where the use of ultrasonic scanning techniques is widespread, keeping scanning intensities below the cavitation threshold is of vital importance. As soon as the irradiation power used in the medical scan rises above this critical value, cavitation is induced and, as a consequence, unwanted even possibly hazardous chemical reactions may occur in the body. Thus, for both chemical and medical reasons there is a considerable drive towards the determination of the exact point at which cavitation occurs in liquid media, particularly in aqueous systems. Historically, therefore, the determination of the cavitation threshold was one of the major drives in dosimetry. 

Although in principle almost any chemical reaction could be used as a standard for sonochemical dosimetry, we will focus attention on those few examples of classical reactions which have been studied as potential dosimeters. They are listed in Table 6. 

A dosimetry method based on the detection of the H radical involves its reaction with DPPH to generate DPPH2 which can easily be monitored by UV spectrometry. With this method, Verrall et al. reported a linear correlation between percent degradation of DPPH and free energy of cavitation [185]. 

The reaction can easily be monitored by HPLC, NMR, or by simple weighing of the addition product [197]. The rate increase with ultrasound not only depends on the mechanical effects (mass transfer improvement) but also on some electronic effects as it has recently been shown that the reaction mechanism involves a single electron transfer step which can be stimulated by ultrasound [198]. Hence the development of this chemical probe could provide a very good dosimetry system since it involves both the mechanical and sonochemical effect of ultrasound. 

There are two reasons why a sonochemist should be interested in dosimetry. The first is that it might be possible to calculate the power required to perform an operation in order that the economics of scale-up can be assessed. The second reason relates to the need to normalize results obtained in different laboratories or with different equipment. If a suitable dosimeter is chosen then it should be possible to perform sonochemical reactions anywhere under precisely comparable conditions. 

The results at the lower Febetron dose rates of 1024-1025 e.v./gram sec. show a decrease in the ratio of cyclohexene to bicyclohexyl. This may be caused by a reduction in the reaction of thermal hydrogen atoms with radicals and indicate a return to the low dose rate (i.e., 1016 e.v./gram sec.) mechanism. However, dosimetry at these dose rates at the present time is not sufficiently accurate to warrant further discussion. 

In radiolysis, the excitation energy is transferred to the bulk, and reactant properties are primarily determined by the solvent and solutes. This allows the initiation chemistry (and dosimetry) to remain invariant and reproducible as the reagent system of interest is varied. One potential drawback of this situation is that the limiting rate of intermediate formation is usually controlled by the concentration limit of the precursor species. In photolysis, the energy is transferred directly to the chromophore, which often directly produces the reactive intermediate. Limitations imposed by second-order formation reactions occur less frequently. Accurate actinometry depends on many factors, however. 

Ionic recombination rate constants were determined as described for XeF, by analyzing the later, linear, portion of the transformed kinetic curve and using ozone dosimetry and integrated kinetic traces. The overall exciplex fluorescence was isolated by subtraction of the X-ray and dimer xenon fluorescence intensities from the integrated fluorescence as before. However, the experimental resolution of the exciplex fluorescence yield into neutral reaction and ionic recombination components was not experimentally possible for this system. Therefore the fractional yields for each of the XeBr formation pathways, excited state reaction and ionic recombination, were calculated using experimental measurements of relative XeBr emission yields. 

---