Sensitive volume

Fig. 2.4.3 (a) Image of a flat oil-glass interface centered along the sensitive volume. The length of the pulses was set to 5 ps to excite a slice thicker than the PSF width. The dashed line displays the step sample interface, (b) PSF obtained as the derivative of the step image shown in (a). It is symmetrical and has a linewidth of about 2.3 pm. 

Ability to absorb Ability to seek light Small light-sensitive volume. 

Gamma interference can also be minimized by reducing the pressure of the gas inside the chamber. The reduction in pressure reduces the number of atoms within the sensitive volume and has the same effect as reducing the volume. 

Ionization chamber sensitivity to gamma rays can also be reduced by increasing chamber sensitivity to neutrons. This is accomplished by increasing the boron-coated area, as shown in Figure 18. Both ionization chambers shown in Figure 18 have the same sensitive volume. 

The simple one-hit-one-effect hypothesis of Dessauer (D7) still serves as the basis for most modern expositions of this type. According to this so-called target or treffer theory, the lethal effect of radiation is observed whenever an ionization is produced inside one or more sensitive volume. The lines of evidence which support this point of view have been analyzed by Lea (L7, L8). They are... 

Studying the electron tracks with the Monte Carlo method, the authors of Refs. 302 and 303 have used the so-called stochastic approach, within which one fixes a simultaneous picture of the spatial distribution of excitation and ionization events. The tracks found this way are sets of spatial points where the inelastic scattering events took place. With this at hand it proves to be possible to calculate the energy absorption spectrum in sensitive volumes of the irradiated medium303 and to calculate the shape of the line and the slope of electronic spin echo signals.302 Such a... 

Besides the electrical devices described above, mechanical systems for temperature measurement are not uncommon in chemical reactors. As an example, systems consisting in a bulb connected to a temperature-sensitive volume or pressure ele-... 

Single-particle optical counters cannot be easily used for measuring high aerosol concentrations. Whitby and Liu (1967) have shown that if many small particles are in the illuminated sensitive volume of this type of instrument at the same time, significant errors are introduced because the particles are considered by the detector to be fewer in number and greater in size. Thus there is a practical upper limit on the aerosol concentration that can be measured by such an instrument, being on the order of 1000 to 10,000 particles per cubic centimeter. 

Example 17.9 An optical particle counter samples an aerosol at a flow rate of 175 cm3/min into a sensitive volume of 0.01 cm. What is the implied maximum particle concentration (in particles per liter) that can be sampled ... 

To have a maximum of one particle in the sensitive volume at any time, on average there should be no more than 100 particles per cubic centimeter, or 100,000 particles per liter. 

Element (units) Physiological concentrations eta-aas sensitivity Volume required for analysis11... 

The receiving optics separated the light scattered from the sensitive volume at the angles 1 , 3°, 6°, 12°, and 24° for independent measurements. The system designed to accomplish this task is also shown in... 

As previously indicated, a final determination of the particle size distribution requires knowledge of the sensitive volume which is actually a function of particle diameter and composition. The first step in quantifying the sensitive volume was to investigate the light intensity distribu-... 

Figure 9. Sensitive volume and optical field of view near laser focus...

As previously indicated, determination of an actual particle size distribution requires an expression for the volumetric sampling rate V (or frontal area Ag X the flow velocity) as a function of particle diameter. This can be achieved if the instrument parameter f I (/-number times the length of the sensitive volume) can be determined. Since f and I are the most uncertain because of spherical aberration of the lens and resolution deficiencies of the collecting optics, / I was calibrated out by determining the actual frontal area at the 1/e intensity points as described above (Figure 9). For the counter under consideration the following relation holds ... 

From Equations 2 and 3 it can be seen that wider particle size distributions need to be corrected as shown. In the present study, an additional sensitive volume check was made using an electron microscope count of the particles on a millipore filter placed in the delivery tube for a fixed time. Results indicated that the analysis and observations were consistent. This also applies to calibration measurements with silicon oil mists. 

A GM tube is usually of the general construction shown in Fig. 5.20. If the detector is to be sensitive to P radiation, it is fitted with a thin end-window (see Fig. 5.25) that allows the P particles to penetrate into the detector s sensitive volume. GM tubes are widely used because they are quite sensitive to radiation, but simple and rugged in construction. 

There are various types and configurations of semiconductor detectors. One type, the surface barrier detector, is especially useful for alpha spectrometry. This type of detector uses a semiconductor wafer, and the sensitive volume of the detector is the surface layer of the wafer. It detects a radiation very effectively, because the a particles are not absorbed before they reach the sensitive volume (the usual problem with other detectors). 

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