Penetration of protons

This obvious supposition occured to be false—the addition of stilbene causes no changes in the results of the reaction (Chang and Johnson 1965, 1966). It follows that an effective contact between PhCH=CHPh and (PhC=CPh) , 2Na is less than probable. As known, (PhC=CPh) , 2Na is a polymeric aggregate [(PhC=CPh) , 2Na+] . Bulk stilbene molecules are incapable of diffusing inside the aggregate. At the same time, the penetration of proton (or methanol) seems to be possible. 

At ordinary temperature and pressure quartz is the stable modification of silica. Quartz does not noticeably react with water but is readily attacked by HF. A less dense metastable modification of Si02 called silica W [after Weiss and Weiss (35)] reacts readily with water and forms a yellow silver silicate in contact with an aqueous solution of AgNOs. Another newly discovered but dense modification of Si02, called silica C [after Coes (6)], which is also metastable under ordinary conditions, is so unreactive that even the smallest crystals are not noticeably attacked by HF. The rates of the reactions of Si02 are determined by the polarizability of the O-2 ions. The O-2 ions are most polarizable in the low density form (silica W) and least polarizable in the high density form (silica C). The reactions of silica with water are initiated by the penetration of protons into the electron clouds of the O-2 ions and the rate of proton penetration increases with increasing polarizability of the anions. The phenomena described are strictly rate phenomena, because neither quartz nor the two metastable forms of silica are in equilibrium with HF. 

The drastic effect of the state of polarization upon the rates of chemical reactions is well known to the analytical chemist. Some metal sulfides—e.g., NiS— cannot be precipitated from an acidified solution but once formed will resist acid attack because the penetration of protons into S 2 ions which are polarized by cations such as Ni+2 ions is a reaction with a high energy barrier. 

When samples of goethite of differing crystallinities are compared, the titration curves are found to reflect these diflFerences by increased spread of the curves at pH values distant from the pzc [55]. These are described in this model by increases in the capacitance terms [55]. The observed behaviour probably reflects increased penetration of protons into pores between domains. In this case, we might think of the changes as reflecting changes in surface roughness and thus in the mean distance between planes. That this should be reflected in a capacitance term in not entirely inappropriate. 

Studies on a large number of aromatic compounds have revealed that for CTAB the largest shift occurs for the alkyl chain protons near the surfactant headgroup, whereas in SDS nearly all proton signals are shifted significantly " ". For SDS, the most pronounced shifts are observed for protons around the centre of the chain. This result has been interpreted in terms of deeper penetration of... 

Compared to EDS, which uses 10-100 keV electrons, PEXE provides orders-of-magnitude improvement in the detection limits for trace elements. This is a consequence of the much reduced background associated with the deceleration of ions (called bremsstrahlun compared to that generated by the stopping of the electrons, and of the similarity of the cross sections for ioiuzing atoms by ions and electrons. Detailed comparison of PIXE with XRF showed that PDCE should be preferred for the analysis of thin samples, surfrce layers, and samples with limited amounts of materials. XRF is better (or bulk analysis and thick specimens because the somewhat shallow penetration of the ions (e.g., tens of pm for protons) limits the analytical volume in PIXE. 

Si(Li) spectroscopy, with the capability of simultaneous quantitative analysis of 72 elements ranging from sodium through to uranium in solid, liquid, thin film and aerosol filter samples. The penetrating power of protons allows sampling of depths of several tens of microns, and the beam itself may be focussed, rastered or varied in energy. The use of a proton beam as an excitation source offers several advantages over other X-ray techniques, for example there is a higher rate of data accumulation across the entire spectrum which allows for faster analysis. 

Dalgarno and Griffing (1958) made a detailed theoretical analysis of the ionization produced by a beam of protons penetrating a gas of H atoms. They find that the W value remains constant at around 36 eV, to within 2.5 eV, for proton energies of 10 KeV and up. However, below about 100 KeV, the near constancy of the W value is also partially due to the fact that the beam is a near equilibrium composition of protons and H atoms because of charge exchange. Therefore, at... 

Nuclei which are radioactively unstable usually decay by the emission of one of three particles from the nucleus, traditionally labelled a, p and y particles. The largest, slowest and least penetrating of these are the a particles, which turn out to be the nucleus of the helium atom - i.e., two protons and two neutrons, with an overall charge of + 2. Decay by a emission is restricted to the heavier elements, and can be summarized in the following general equation ... 

The entrainment of superfluid protons by rotating superfluid neutrons leads to appearance of proton vortices and to generation of a homogeneous mean magnetic field with amplitude B and direction parallel to the axis of rotation of the star [20], This field will penetrate into the quark core through the normal cores of quark vortices. 

The complications for fhe fheoretical description of proton fransporf in the interfacial region befween polymer and water are caused by the flexibility of fhe side chains, fheir random distributions at polymeric aggregates, and their partial penetration into the bulk of water-filled pores. The importance of an appropriate flexibilify of hydrated side chains has been explored recently in extensive molecular modeling studies. Continuum dielectric approaches and molecular dynamics simulations have been utilized to explore the effects of sfafic inferfacial charge distributions on proton mobility in single-pore environments of Molecular level simulations were employed... 

Amantadine (C) specifically affects the replication of influenza A (RNA) viruses, the causative agent of true influenza. These viruses are endocytosed into the cell. Release of viral DNA requires protons from the acidic content of endosomes to penetrate the virus. Presumably, amantadine blocks a channel protein in the viral coat that permits influx of protons thus, uncoating is prevented. Moreover, amantadine inhibits viral maturation. The drug is also used prophylactically and, if possible, must be taken before the outoreak of symptoms. It also is an antiparkinsonian... 

The emission of a helium nucleus in the final stage regenerates the initial carbon-12. The latter thus plays the role of a catalyst. The overall result is the fusion of four protons into a helium nucleus. At high temperatures, this cycle dominates over the proton-proton chain. Indeed thermal agitation facilitates penetration of the relatively high electrical barrier between proton and carbon nucleus. Whatever hydrogen fusion mechanism is prevalent, the star s mass determines the rate at which it consumes its nuclear fuel, and hence also its lifetime. The higher its mass, the more quickly it bums. 

Improving the physical selectivity. The most straightforward method to improve the efficacy of radiation therapy is to improve the physical selectivity of the irradiation, i.e., by using beams with better physical characteristics (penetration, collimation, etc.). This is the rationale for the introduction of proton beams no radiobiological advantage is expected. 

Figure 20 Comparison of calculated extended Bragg peaks of protons and carbon ions ( C) at different penetration depths. (From Ref. 45.)...

Alpha particle (symbol a) A positively charged particle emitted by certain radioactive materials it is made up of two neutrons and two protons bound together, thus, it is identical to a helium atom. It is the least penetrating of the three most common types of radiation (alpha, beta, gamma) emitted by a radioactive material and can be stopped, for example, by a sheet of paper. It is not dangerous to... 

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