Emitted particles

With the exception of the scanning probe microscopies, most surface analysis teclmiques involve scattering of one type or another, as illustrated in figure A1.7.11. A particle is incident onto a surface, and its interaction with the surface either causes a change to the particles energy and/or trajectory, or the interaction induces the emission of a secondary particle(s). The particles that interact with the surface can be electrons, ions, photons or even heat. An analysis of the mass, energy and/or trajectory of the emitted particles, or the dependence of the emitted particle yield on a property of the incident particles, is used to infer infomiation about the surface. Although these probes are indirect, they do provide reliable infomiation about the surface composition and structure. 

Conservation of Energy. Because the naturally occurring radioactive materials continued to emit particles, and thus the associated energy, without any decrease in intensity, the question of the source of this energy arose. Whereas the conservation of energy was a firmly estabUshed law of physics, the origin of the energy in the radioactivity was unknown. 

Because the various types of particle can appear in both primary excitation and secondary emission, most authors and reviewers have found it convenient to group the techniques in a matrix, in which the columns refer to the nature of the exciting particle and the rows to the nature of the emitted particle [1.1-1.9]. Such a matrix of techniques is given in Tab. 1.1., which uses the acronyms now accepted. The meanings of the acronyms, together with some of the alternatives that have appeared in the literature, are given in Listing 1. 

The average number of emitted particles X per incident primary ion is given by the secondary yield Y (X ) ... 

Eor analysis of emitted particles, solid state surface barrier detectors (SBD) are used inside the scattering chamber to measure the number and energy of the reaction products. Stopper foils are used to prevent scattered projectiles from reaching the detector. Depth profiles can be obtained from the energy spectra, because reaction products emitted in deeper layers have less energy than reaction products emitted from the surface. The concentration in the corresponding layer can be determined from the intensity of reaction products with a certain energy. 

The cross-section curve a(E) gives the dependence of the nuclear cross-section on the projectile energy, E. The measured energy spectra of emitted particles or the excitation curve N(Eq) wiU depend on the depth profile N(x) of the analyzed isotope and on the cross-section curve (t(E(x)), where E(x) gives the energy of the projectiles at a depth x. Evaluation of the depth profile N (x) from measured energy spectra or excitation curves often requires a tedious evaluation procedure if the cross-section curve has a complex structure. It is simplified for two special types of behavior of the cross-section curve ... 

When, in NRA, energy spectra of emitted particles are analyzed, a sufficiently thick foil in front of the detector is usually used to absorb the scattered projectiles. This reduces the depth resolution of NRA, because of energy loss straggling of the reaction products in the foil. 

Atoms with the same value of Zbut different values of A are isotopes (Table 11.1). Many isotopes are stable but others are naturally or artificially radioactive, i.e. their atomic nuclei disintegrate, emitting particles or radiation. This changes the nuclear structure of the atom and often results in the production of a different element. 

Dose is related to the amount of radiation energy absorbed by people or equipment. If the radiation comes from a small volume compared with the exposure distance, it is idealized as a point source (Figure 8.3-4). Radiation source, S, emits particles at a constant rate equally in all directions (isotropic). The number of particles that impact the area is S t Tr where Tr is a geometric effect that corrects for the spreading of the radiation according to ratio of the area exposed to the area of a sphere at this distance i.e. the solid angle - subtended by the receptor (equation 8.3-4). 

Booths are often used for work prtKedures with momentum-driven emissions. In such cases the capture devices must be placed to rake advantage of this momentum. For example, a spray paint booth would have the exhaust location downstream of the painting location, most likely at the back of the booth. The capture devices in the back wall should be suitable to reduce the momenriim of the emitted particles in such a way that they are not reflected back into the work area. Floor exhaust should be able to keep the heavier particles dowm so that they cannot be a source for secondary emissions. 

The development of particle accelerators grew out of the discovery of radioactivity in uranium by Henri Becquerel in Paris in 1896. Some years later, due to the work of Ernest Rutherford and others, it was found that the radioactivity discovered by Becquerel was the emission o particles with kinetic energies o several MeV from uranium nuclei. Research using the emitted particles began shortly thereafter. It was soon realized that if scientists were to learn more about the properties of subatomic particles, they had to be accelerated to energies greater than those attained in natural radioactivity. 

The same method of analysis of emitted particles was applied to elucidate the origin of non-stability of piezoquartz resonators with silver electrodes [35]. 

Using several physical and chemical techniques typical for the method of semiconductor sensors one can identify emitted particles. 

Nuclear reaction analysis (NRA) also identifies emitted particles which are different from the incident ones. In order to avoid permanent radioactivity, the energy of the projectile is maintained below 6 MeV, so that it is used primarily to determine the concentration and depth of light elements (Z < 9) in the near surface of solids. 

National Academy of Sciences-National Research Council, Health Effects of Alpha-Emitting Particles in the Respiratory Tract, Report of AD Hoc Committee on Hot Particles, U. S. Environmental Protection Agency, EPA 520/4-76-013, October 1976. 

The sample collected from the middle of a copper smelter stack representing emitted particles was highly complex. 

An alternative for the low detection efficiencies of the emitted particles is to ionize them with a UV laser beam, either in a resonant or non-resonant way [37]. In this way the ionization efficiency increases about a thousandfold and the attractive prospect of doing SNMS under static conditions at sensitivities comparable to those of... 

Isotope Particle emitted Particle energy (MeV) Half-life... 

Radioactive unstable atomic nuclei spontaneously emitting particles and energy Radioactive Tracer a radioactive substance used to monitor the movement and behavior of a chemical in biological processes and chemical reactions... 

Because certain sources emit particles with characteristic elemental signatures, in principle one ought to be able to measure the composition of particles in the atmosphere and then work backward to calculate how much each source contributed to obtain the final, observed particle composition. This approach involves the use of receptor models, defined as models that assess contributions from various sources based on observations at sampling sites (the receptors) (Gordon, 1988). 

HSCTs also emit particles and S02, with the latter being oxidized to H2S04 and sulfate particles. Measurements of particle concentrations in the plume of the Concorde SST showed much larger particle concentrations than anticipated (Fahey et al., 1995a). Furthermore, a much larger portion of the S02 in the exhaust was oxidized to H2S04 particles than expected based on the OH levels measured in the exhaust plume (Hanisco et al., 1997), suggesting that there are some as yet unknown mechanisms of S02 oxidation in the plume. 

Some degree of fractionation as function of distance from the power station smoke stack is to be expected coarse particles will fall out in the immediate vicinity of the power station, whereas fine fly ash will be transported further, and gaseous emissions might be expected to be transported the furthest. Thus, from the point of view of environmental health, not only the chemical composition of emitted particles and aerosols, but also their size, is relevant (Teinemaa et al. 2002). As particulate matter is dominated by basic oxides (e.g., CaO) and gaseous emissions by acidic gases (e.g., CO2, SO2), this fractionation will influence the pH of... 

As the fluorescence is dependent on the particle sizes (58), a shift of the excitation threshold with the emission wavelength indicates the presence of particles of different sizes in the solution which shows size distribution of the aggregates (41,43). In the case where this excitation spectrum is analogous to the absorption spectrum, the threshold is a measure of the average size of the emitting particles. 

Walter Kanfmarm also reported the determination of the charge-to-mass ratio of cathode rays (abont 10 emu g- ) in a paper he submitted in April 1897 (7). Kanfmarm also based his result on magnetic deflection measurements however, he concluded that the hypothesis of cathode rays as emitted particles could not explain his data. (One of the outstanding questions in the study of cathode rays in the late 1890s was whether they were particles or electromagnetic waves. Thomson and Kanfmarm were typical of their coimtrymen most British researchers leaned toward the particulate hypothesis and most Germans toward waves.) Today Kanfmarm is better known for his careful measurements of the velocity-dependent mass of the electron published over several years beginning in 1901 these results were later explained by special relativity. 

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