Escape from incident

Not many readers will handle radioaetive materials, but this incident and the previous one do show how easy it is to overlook some of the routes by which hazardous materials or effects can escape from containment. 

Backscattered electrons, however, do give some elemental information about the sample because they are more energetic than secondary electrons and escape from farther within the sample [45,46], On the molecular level, the electron beam can interact with the nucleus of an atom and be scattered with minimal loss of energy. These incident electrons may be scattered more than once and then ejected from the sample as backscattered electrons. The back-scattered electrons originate from a greater depth within the sample and are... 

The primary safety feature for any installation should be its evacuation mechanisms for its personnel. If personnel cannot escape from an incident they may be affected from it. Personnel must first be aware that an incident has occurred, and then have an available means to escape or evacuate from it. An adequate means of escape should be provided from all buildings, process areas, elevated structures, and offshore installations. Provision of an adequate means of escape is listed in most national safety regulations for the industry as a whole as well as local building code ordinances. 

XRF spectrometry is based on the principle that primary X-rays (from an X-ray tube or radioactive source) are incident upon a sample and create inner shell (K, L, M) vacancies in the atoms of the surface layers. These vacancies de-excite by the production of a secondary (fluorescent) X-ray whose energy is characteristic of the elements present in the sample. Some of these characteristic X-rays escape from the sample and are counted and their energies measured. Comparison of these energies with known values for each element (e.g., Van Grieken and Markowicz 1993, Parsons 1997) allow the elements present in the sample to be identified and quantified. 

Other examples cited by Goodman include the poisoning of 200 French soldiers by Chinese reformers in Hanoi on 27 June 1908, all of whom recovered. One of the intoxicated soldiers saw ants on his bed, a second fled to a tree to escape from a hallucinated tiger and a third took aim at birds in the sky. Another incident was the abortive attempt by Soviet agents in 1959 to poison the staff of Radio Free Europe in Munich by putting atropine in saltshakers in the cafeteria. A double agent foiled this effort. 

We should note that the photoelectric effect often leaves an inner shell vacancy in the atom that previously contained the ejected electron. This vacancy will be filled by an atomic transition, called fluorescence, and generally produces an X-ray photon. In an interesting twist of fate, the X-ray photon will have an energy that is just below the sharp rise in the attenuation coefficient due to conservation of momentum and can often escape from the absorber. Recall that the direction of the fluorescence photon will be uncorrelated with the direction of the incident photon and a fraction will be emitted backwards from the absorber. The absorber will thus emit its own characteristic X-rays when it is irradiated with high-energy photons. 

The Mossbauer-effect experiment can also be applied to the study of surfaces in the variation known as conversion electron Mossbauer spectroscopy (CEMS). Here, what is monitored as a function of incident y-ray energy is not absorption, but the emission of electrons through a process of internal conversion (i.e., as a byproduct of the absorption of Mossbauer y rays). Since the conversion electrons can only escape from the surface layers of the solid, data are selectively acquired for the surface region, arising from the Mossbauer effect in the (most commonly iron) atoms of the surface layers. The monitoring of emitted electrons results in a mirror image of the usual absorption spectrum. Transmission and CEM spectra of vivianite [Ee3(P04)2-8H20] are illustrated in Fig. 2.49 (after Tricker et al., 1979]. 

Excited electronic states thus give rise to photon emission with a yield smaller than unity on the other hand, absorption of these photons produces, in turn, excited electronic states, also with a yield smaller than unity. Consequently, if one neglects the possibility for the photons to escape from the solid, a quasi-equilibrium is established between these two forms of energy between which the near totality of the incident energy is recovered. However, every conversion from one form to the other is accompanied by a release of thermal energy. If the irradiated system does not use the energy in either of these forms for certain definite purposes, such as chemical reaction for instance, the totality of this energy will be finally converted into heat. 

Figure 7.4 Schematic comparison of Auger peak intensity with other electrons escaped from a solid surface. E0 indicates energy of incident electrons. The kinetic energy of electrons can be divided into three regions I, II and III from low to high. (Reproduced with permission from M. Prutton and M.M. El Gomati, Scanning Auger Electron Microscopy, John Wiley Sons Ltd, Chichester. 2006 John Wiley Sons.)...

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