Y-rays from

The radioactive source need not aiways be introduced into the body. Inoperable brain tumors can be treated with y rays from an extemai source, usuaiiy a sampie of cobait-60. The patient is placed in a position where the y-ray beam passes through the tumor. The patient is moved so that the y rays irradiate the tumor from several angles. In this manner the tumor receives a much higher dose of radiation than the dose received by any surrounding tissues. 

Resonant y-ray absorption is directly connected with nuclear resonance fluorescence. This is the re-emission of a (second) y-ray from the excited state of the absorber nucleus after resonance absorption. The transition back to the ground state occurs with the same mean lifetime t by the emission of a y-ray in an arbitrary direction, or by energy transfer from the nucleus to the K-shell via internal conversion and the ejection of conversion electrons (see footnote 1). Nuclear resonance fluorescence was the basis for the experiments that finally led to R. L. Mossbauer s discovery of nuclear y-resonance in ir ([1-3] in Chap. 1) and is the basis of Mossbauer experiments with synchrotron radiation which can be used instead of y-radiation from classical sources (see Chap. 9). 

Fig. 7.35 Ru Mossbauer spectra of Ruo.023Feo.977 at 4.2 K (source Rh in Ru metal), (a) unpolarized absorber (b) absorber magnetized parallel and (c) perpendicular to incident y-rays (from [110])...

Discovery of y-rays from Galaxy and diffuse background. 

The Mossbauer effect as a spectroscopic method probes transitions within an atom s nucleus and therefore requires a nucleus with low-lying excited states. The effect has been observed for 43 elements. For applications in bioinorganic chemistry, the 57Fe nucleus has the greatest relevance and the focus will be exclusively on this nucleus here. Mossbauer spectroscopy requires (a) the emission of y rays from... 

The electronic environment about the sample s nucleus influences the energy of the y ray necessary to cause the nuclear transition from the ground to the excited state. The energies of the y rays from the source can be varied by moving the source relative to the sample. In order to obtain the Mossbauer spectrum, the source is moved relative to the fixed sample, and the source... 

Gamma-ray spectrometers use scintillator detectors. These spectrometers sense y-rays from all directions, and hence have large "footprints" (commonly hundreds of kilometers diameter) with sizes determined by orbital elevation above the surface. The y-rays come from depths of less than a meter in the target material. 

Natural radiation, a-, (3- and y-rays from radioactive elements in the environment or intrinsic to materials, ionizes the materia] and produces paramagnetic defects or radicals. They are often quite stable and accumulate with time. The ESR signal intensity is proportional to the total dose of natural radiation, i.e., to the product between the annual radiation dose rate and the time elapsed after their formation or an event which zeroed the spin concentration. 

The y-rays from the source (which must be specially prepared to yield a high fraction of recoilless emission) are absorbed by the sample and re-emitted approximately isotropically. Compared with ordinary electronic absorption of y-rays by an equivalent material, detector (A) in the figure measures a decrease in transmitted y-ray intensity when... 

Figure 9.7 Schematic examples of two techniques to prepare a nuclear state with unequal populations of the internal magnetic suhstates (a) correlating the sequential emission of two y rays and (b) correlating the emission of a [3 particle with a subsequent y ray. (From de Shalit and Feshhach, 1974, p. 693.) Copyright 1974 John Wiley Sons, Inc. Reprinted hy permission of John Wiley Sons, Inc.

Therapeutic Procedures Therapeutic procedures—those in which radiation is used to kill diseased tissue—can involve either external or internal sources of radiation. External radiation therapy for the treatment of cancer is often carried out with y rays from a cobalt-60 source. The highly radioactive source is shielded by a thick lead container and has a small opening directed toward the site of the tumor. By focusing the radiation beam on the tumor and rotating the patient s body, the tumor receives the full exposure while the exposure of surrounding parts of the body is minimized. Nevertheless, sufficient exposure occurs so that most patients suffer some effects of radiation sickness. 

Cancerous tumors can be treated by irradiation with y rays from this cobalt-60 source. 

Polymer was irradiated at room temperature with y-rays from a 60Co source and the spectra measured at room temperature, after heating for various periods at 110°. Even after about 80% of the... 

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