Collimation narrow

Ans. Radiotherapy and chemotherapy use radiation and chemicals, respectively, to cause fatal mutations in the DNA of cancer cells. Cancer cells are damaged more by radiotherapy and chemotherapy than are normal cells because cancer cells grow more rapidly. Improved radiotherapy causes minimal damage to healthy tissues by delivering the minimum effective radiation dose. The radiation is confined as much as possible to the cancerous tissues by using collimated (narrowed and focused) radiation beams. Chemotherapy is similarly improved by choosing chemicals that attack cancer cells more selectively than healthy cells. 

Distance is important Where radiation spreads in all directions from a source, its intensity drops rapidly with distance from the source. At two metres, the intensity is 1/4 that at one metre. At three metres, it is l/9th that at one metre. (This is the inverse square law.) So remote handling of materials, even just using long tongs, can greatly reduce dose. Note that this will not apply to a collimated (narrow and straight) or focused beam of radiatioa Also, in some cases, so-called scattering and secondary radiation can affect the principle. 

An effusive beam of atoms or molecules (see Ramsey, 1956 in fhe bibliography) is produced by pumping fhem fhrough a narrow slif, fypically 20 pm wide and 1 cm long, wifh a pressure of a few forr on fhe source side of fhe slif. The beam may be further collimated by suifable apertures along if. 

Figure 18.5 Schematic view of a diffraction experiment, (a) A narrow beam of x-rays (red) is taken out from the x-ray source through a collimating device. When the primary beam hits the crystal, most of it passes straight through, but some is diffracted by the crystal. These diffracted beams, which leave the crystal in many different directions, are recorded on a detector, either a piece of x-ray film or an area detector, (b) A diffraction pattern from a crystal of the enzyme RuBisCo using monochromatic radiation (compare with Figure 18.2b, the pattern using polychromatic radiation). The crystal was rotated one degree while this pattern was recorded.

Detection limits for various elements by TXRF on Si wafers are shown in Fig. 4.13. Synchrotron radiation (SR) enables bright and horizontally polarized X-ray excitation of narrow collimation that reduces the Compton scatter of silicon. Recent developments in the field of SR-TXRF and extreme ultra violet (EUV) lithography nurture our hope for improved sensitivity down to the range of less than 10 atoms cm ... 

Fig. 1-4. Simple absorption experiment. Note three important elements of optical system. Two others, the collimator (to give narrow, parallel beam) and the monochromator, have been omitted.

Excitation by Ao is the kind of excitation to which Equation 6-4 and its sequels apply this excitation leads to the absorption effects of the previous section. It can be treated successfully (as indicated above) by narrow, parallel beam geometry, which is certainly applicable to the well-collimated beam of a good spectrograph with a detector of small... 

Which directs them toweurds the analyzer slits. Alternatively, they may be extracted by the field penetration of the high voltage on the focusing electrodes. In both instances the ion beam is usually focused, collimated and accelerated to provide a beam of narrow energy dispersion that is capable of traversing the analyzer section of the mass spectrometer. In modern mass spectrometers the ionization source and analyzer sections are usually differentially pumped, allowing the source to operate at a distinctly higher... 

Figure 2.21. A, High-intensity point source lamp B, parabolic mirror C, light baffle D, narrow slit E, collimating lens F, Coming filters G, reaction cell or series of cells H, focusing lens I, photomultiplier.

The cross-section of the primary X-ray beam is extended and not an ideal point. This fact results in a blurring of the recorded scattering pattern. By keeping the cross-section tiny, modern equipment is close to the point-focus collimation approximation - because, in general, the features of the scattering patterns are relatively broad. Care must be taken, if narrow peaks like equatorial streaks (cf. p. 166) are observed and discussed. The solution is either to desmear the scattering pattern or to correct the determined structure parameters for the integral breadth of the beam profile (Sect. 9.7). 

Information on particle size may be obtained from the sedimentation of particles in dilute suspensions. The use of pipette techniques can be rather tedious and care is required to ensure that measurements are sufficiently precise. Instruments such as X-ray or photo-sedimentometers serve to automate this method in a non-intrusive manner. The attenuation of a narrow collimated beam of radiation passing horizontally through a sample of suspension is related to the mass of solid material in the path of the beam. This attenuation can be monitored at a fixed height in the suspension, or can be monitored as the beam is raised at a known rate. This latter procedure serves to reduce the time required to obtain sufficient data from which the particle size distribution may be calculated. This technique is limited to the analysis of particles whose settling behaviour follows Stokes law, as discussed in Section 3.3.4, and to conditions where any diffusive motion of particles is negligible. 

The third step in the structure determination is collection of the X-ray diffraction data. This may be done with a diffractometer in which a narrow collimated pencil source of X-rays is aimed at the crystal and the intensities and positions of the diffracted beams are measured automatically. The computer-controlled diffractometer is able to measure the angles to within less than one-hundredth of a degree. If sufficient time is allowed, very weak spots can be counted. Today, diffractometers are more likely to be used for preliminary measurements, while the major data collection is done with an area detector, an... 

Whatever the source of X rays, the beam is directed through a collimator, a narrow metal tube that selects and reflects the X rays into parallel paths, producing a narrow beam. After collimation, beam diameter can be further... 

Time-resolved spectroscopy is performed using a pump-probe method in which a short-pulsed laser is used to initiate a T-jump and a mid-IR probe laser is used to monitor the transient IR absorbance in the sample. A schematic of the entire instrument is shown in Fig. 17.4. For clarity, only key components are shown. In the description that follows, only those components will be described. A continuous-wave (CW) lead-salt (PbSe) diode laser (output power <1 mW) tuned to a specific vibrational mode of the RNA molecule probes the transient absorbance of the sample. The linewidth of the probe laser is quite narrow (<0.5 cm-1) and sets the spectral resolution of the time-resolved experiments. The divergent output of the diode laser is collected and collimated by a gold coated off-axis... 

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