Radiation dose reduction

Radiation dose reduction by graded structures in the heat source of a °Sr radioisotope battery... 

A radioisotope battery is one of the choice for energy source of meteorological obseiwation and development of undersea and space[l]. We have considered a strontium-90 (half-life 28.8y) heat-source model of a radioisotope battery and improved it in two aspects—radiation dose reduction and improvement of thermal conductivity—adding graded structure to the model[2]. The present study reports the dose reduction of bremsstrahlung photons from -ray of - "Sr and its daughter nuclide yttrium-90. The calculation was carried out by a continuous energy Monte Carlo code, MCNP 4A[3]. 

In the mixture, average atomic number decreases with the ratio of BN. Since radiative stopping power of -ray is proportional to the square of average atomic number, the probabihty of production of bremsstrahlung photons can be reduced in the new layer, and also the spectrum of the photons becomes softer than that of the "SrTiOa layer. These are factors for radiation dose reduction. On the other hand, there is another factor for radiation dose increase since generated position of bremsstrahlung photons approaches the detection position. Consequently, radiation dose on the surface of the heat source can be minimized by arranging those two factors. 

Using 4-slices multidetector spiral CT, the scan time can be reduced dramatically compared to conventional spiral CT, with consequent radiation dose reduction, or the volume to be investigated can be extended. 

The resistance of an organism to a sterilizing agent can be described by means of the D-value. For heat and radiation treatments, respectively, this is defined as the time taken at a fixed temperature or the radiation dose required to achieve a 90% reduction in viable cells (i.e. a 1 log cycle reduction in survivors Fig. 20.2k). The calculation of the D-value assumes a linear type A survivor curve (Fig. 20.1), and must be corrected to allow for any deviation from linearity with type B or C curves. Some typical D-values for resistant bacterial spores are given in Table 23.2 (Chapter 23). 

It is possible to remove radon decay products from indoor air by filtration. The effects of air cleaning on dose levels are described by Jonassen (1987). However, there are major uncertainties in the effectiveness of air cleaning to remove the decay products because the particles are also removed. When the particles are removed, the "unattached fraction increases and although there are fewer decay products, they are more effective in depositing their dose of radiation to the lung tissue. Thus, there will. be much lower dose reduction than there is radioactivity reduction. It, therefore, may be more protective of health to control the radon rather than its decay products. 

Denaturation of proteins by irradiation can occur in small amounts at moderate or high doses. Some reduction in the availability of certain amino acids (lysine, methionine, etc.) has been reported in the proteins of irradiated foods. However, these effects are rather small, even at high radiation doses. 

Nickel oligomers prepared in the presence of PA (Amax = 540 nm) (Section 20.4.2) may also act as catalysts for the reduction of Ni by hypophosphite ions. This requires, as shown by pulse radiolysis, a critical nuclearity, while free Ni cannot be reduced directly by H2PO2. Very low radiation dose conditions, just initiating the formation of a few supercritical nuclei, will lead to large particles of nickel [96]. 

The possible formation of an alloyed or a core-shell cluster depends on the kinetic competition between, on one hand, the irreversible release of the metal ions displaced by the excess ions of the more noble metal after electron transfer and, on the other hand, the radiation-induced reduction of both metal ions, which depends on the dose rate (Table 5). The pulse radiolysis study of a mixed system [66] (Fig. 7) suggested that a very fast and total reduction by the means of a powerful and sudden irradiation delivered for instance by an electron beam (EB) should prevent the intermetal electron transfer and produce alloyed clusters. Indeed, such a decisive effect of the dose rate has been demonstrated [102]. However, the competition imposed by the metal displacement is more or less serious, because, depending on the couple of metals, the process may not occur [53], or, on the contrary, may last only hours, minutes, or even seconds [102]. 

The increased incidence of food-borne diseases from shellfish contaminated with Vibrio species have brought forward renewed interest in irradiation of shellfish and other seafood. Results are very promising for clams and oysters without killing the molluscs [65]. The radiation decimal reduction dose (Dio) determined for Vibrio cholerae 01 biotype El Tor inoculated into various molluscs was 0.14 kGy [66]. Similar radiation sensitivity of this organism was found in inoculated fish fillets and shrimp tails [67]. The radiation dose to eliminate as high as 10 CFU/g Vibrio spp. in oysters was 1.2 kGy [68]. The radiation Dio value for V. cholerae 01 biotype El Tor in marine snails was 0.11 kGy [69]. 

Lakritz et al. (32) reported that radiation doses of less than 10 kGy (at 0 to 4 C) produced minimal changes in the micro structure of bovine longissimus dorsi muscle. At doses of 30 kGy or higher, myofibril fragmentation and decreased tensile strength were noted. Lakritz and Maerker (33) reported reductions of 8% and 42% in the activities of lysosomal enzymes and acid phosphatase of irradiated (10 kGy) bovine longissimus dorsi muscle tissue. 

Regardless of the type of lethality induced by a sterilization process—whether it be heat, chemical, or radiation—micro-organisms, upon exposure to adequate levels of such treatments, will die according to a logarithmic relationship between the concentration or population of living cells and the time exposure or radiation dose to the treatment. This relationship between the microbial population and time may be linear or nonlinear, as seen in Figure 1. The D value, or the time or dose required for a one-log reduction in the microbial population, may be calculated from these plots. 

Mrad, a radiation dose of 1.2 Mrads would produce a 6-log reduction in the concentration of B. pumilus spores. Greater probability allowances (e.g., 10 3) would permit lower radiation doses. ... 

Also, validated reduced bioburden (e.g., 0-1 colony-forming units (CFU) per unit surface area or mL) would allow for a reduction in the radiation dose required to achieve a sterility assurance level of 10A... 

If it is assumed that the radiation sterilizer equipment and facilities have been qualified and microbiological studies have been conducted as previously outlined, the next step in the validation process is the complete evaluation of the radiation sterilization cycle. Tests are conducted to determine the effect of minimum and maximum product density on the ability of the minimum or nominal radiation dose—determined during the microbiological studies to produce a given log reduction in the biological indicator population—to sterilize the load. For example, it was found that a 0.2-Mrad dose of cobalt-60 will produce a 1-log reduction in the population of B. pumilus. The microbial load of a one-package polyvinyl chloride (PVC) device (intravenous administration site) was estimated to be approximately 1000. A probability of a nonsterility level of 10 6 is desired, therefore theoretically, the minimum dose necessary to produce a 9-log reduction in the microbial population is 1.8 Mrad. 

Technetium is a very suitable radionuclide for medical applications. Owing to its short half-life, radiation doses to the human organism are very low. The problems of protein labeling with technetium have been reviewed by several authors " . The following methods of technetium reduction are used in protein labeling by ascorbate aloneby ascorbate and Fe + by Fe"+ by Sn"+ 84.85,93-9S). trolytic reduction 84. ss). jjy gjjZ+.tartarate by Sn -citrate (Tab. 3). 

Figure 6 These CT (computerized tomography) scans of the head show a large metastatic cancer of the brain (A) in a patient enrolled in a Pharmacyclics, Inc. sponsored gadolinium(III) texaphyrin (PCI-0120) Phase Ib/II clinical trial. The tumor mass and surrounding edema cause the brain tissue to shift across the midline in (A). After intravenous injection of PCI-0120 (ten treatments) and radiation therapy (ten treatments, total radiation dose of 30 Gy), the tumor is eliminated, leaving some residual edema with the reduction of the shift, as illustrated in (B)...

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