Coincidences source volume

Figure 4.5 Principal trajectories of electrons with different kinetic energies in an electrostatic deflection analyser. The centre of the detector plane coincides with the focal point B of the principal ray at a U°p corresponding to the E°in. Principal rays, which also start in the source volume Q, but differ in their kinetic energy by A kin, reach the detector plane at different positions, thus illustrating the energy dispersion of the analyser. In the case shown the detector plane is aligned perpendicular to the principal rays it should be noted that the actual focal plane of a CMA is inclined in a clockwise direction with respect to this plane.

Note that the sensitivity of a PET scanner increases as the square of the detector efficiency, which depends on the scintillation decay time and stopping power of the detector. This is why LSO, LYSO and GSO detectors are preferred to Nal(Tl) or BGO detectors (see Table 2.1). In 2D acquisitions, system sensitivity is compromised because of the use of septa between detector rings, whereas these septa are retracted or absent in 3D acquisition, and hence the sensitivity is increased by a factor of 4-8. However, in 3D mode, random and scatter coincidences increase significantly, the scatter fraction being 30—40% compared to 15-20% in 2D mode. The overall sensitivities of PET scanners for a small-volume source of activity are about 0.2-0.5% for 2D acquisition and about 2-10% for 3D acquisition, compared to 0.01-0.03% for SPECT studies (Cherry et al, 2003). The greater sensitivity of the PET scanner results from the absence of collimators in data acquisition. 

The application of the quasi-cw, ultraviolet laser source to kinetic studies was demonstrated in the laser photolysis/laser-induced fluorescence experiments shown schematically in Figure 2. Chemical reactions were initiated by 193-nm photolysis of N2O in N20/H20/hydrocarbon/helium gas mixtures. The 0( D) atoms formed by photodissociation rapidly converted to OH through reaction with H2O, and time-resolved OH concentrations were measured as functions of hydrocarbon number density by laser-induced fluorescence. Hydroxyl radical fluorescence was excited by pumping the nearly coincident Pjl, Qi3, and Qi3 (0,0) band transitions at 308.16 nm, (15) and radiation emanating from the reaction volume in a downward direction was skimmed by black-anodized collimators, focused by quartz lenses, selected by a bandpass filter (308.3 nm peak, 8 nm FWHM), and detected by an RCA 8850 photomultiplier operating in the photon-counting mode. 

The positron annihilation lifetime spectroscopy (PALS) iq>paratus used to measure free volume in this work consisted of an automated EG G Ortec fast-fast coincidence system. The 1.3 MBq NaCl source was a 2 mm t source sandwiched between two Ti foils (2.54 pm foils). The source gave a two-component best fit to 99.99% pure, aimealed, chemically polished aluminum (x = 169 2 ps, Ii = 99.2 0.4 %, X2 = 850 25 ps, I2 = 0.8 0.4%). No source correction was used in the analysis of the data with the PFPOSFIT program (9). Measurements were made in air at 50% relative humidity with temperature control of 0.7 C. 

Fig. 6.21 shows the located AE events that occurred from the initial loading up to that time the cracking load was reached. First hair line cracks were visible on the bottom side where the bending causes tension of the concrete. Only those results are displayed whieh satisfy the ehosen erite-rion for good localizations. Located AE coincide with the topography of two evolving cracks. Results from a later loading step, where sudden erack growing occurred, are shown in Fig. 6.22. Numerous AE events were detected and located around one opening crack. Further active cracks, however, do not express in the location results at all. Waves will not propagate directly when there are open cracks in the volume and therefore the source localization method will fail.

Sedriks et al. [20] and Bakulin et al. [19] found that Ecggn of B/Al MMCs was active to that of their monolithic matrix aUojrs in aerated NaCl solutions. That behavior does not appear to comply with the mixed-potential theory. Bakulin et al. [79], however, found that hot-pressed stacks of aluminum foil processed in the same way as the MMC (but without the BFs) have Ecorr values that are active to the MMCs. The only difference between the monolithic aluminum and the hot-pressed stacks of aluminum foil was crevices in the diffusion bonds between adjacent foils. The crevices, which are sources of additional anodic sites, can polarize the stacks to active potentials. Thus, the B/Al MMCs are actually noble to the matrix material processed in the same way, emd the Ecorr values actually coincide with the mixed-potential theory. Sedriks et al. [20] found that increasing the volume fi ac-tion of BF caused anodic current densities (w.r.t. matrix area) to increase. This implies that BF-matrix interfaces, which increase with BF content, were also sources of anodic sites. Evans and Braddick [89] also reported that BF-matrix interface regions were severely attacked in an oxygenated NaCl solution. These reports indicate that the BF-matrix and foil-foil interfaces are major causes of corrosion. 

---