Chain fission

Using this data the spectral dependence of the absolute quantum yield of the main chain fission of PMMA was calculated follow-... 

As enzymatic oxidative transformation of the PVA polymer can act as a multiple simultaneous event on the polymer with concurrent chain fission by the appropriate enzymes, the polymer can be broken down into small oligomers that can be channelled into the primary metabolism. This picture is not complete because PVA is usually more or less acetylated. The DH is a pivotal factor in almost every aspect of PVA application. Surprisingly there are very few data dealing with the enzymes involved in the deacetylation of not fuUy hydrolysed PVA polymer. In technical processes, esterase enzymes are widely applied to deal with PVAc structures. A good example is from the pulp and paper industry [85], where PVAc, a component of stickies , is hydrolysed to the less sticky PVA. Esterases from natural sources are known to accept the acetyl residues on the polymer as substrate but little detailed knowledge exists about the identity of acetyl esterases in the PVA degradative environment [86]. 

The discovery in 1938-1939 of nuclear fission of uranium, which led ultimately to the discovery of nuclear power, heralded a new, extraordinarily fruitful stage in Ya.B. s scientific activity. His interests were concentrated on the study of the mechanism of fission of heavy nuclei and, what proved particularly important, on the development of a theory of the chain fission reaction of uranium. During 1939-1943 Ya.B. wrote several papers which laid the foundation for this subject and were of fundamental value. We note that four of these papers, written in collaboration with Yu. B. Khariton, were done practically in two years before the war. The papers of this series form the foundation of modern physics of reactors and nuclear power they are widely known and do not require special commentary—a short review of the basic physical results is eloquent enough. 

The paper of 1939 [1 ], On the Chain Decay of the Main Uranium Isotope, studies the effects of elastic and non-elastic neutron moderation and concludes that chain fission reactions by fast neutrons in pure metallic natural uranium are impossible. The 1940 paper, On the Chain Decay of Uranium under the Influence of Slow Neutrons [2 ], is classic in the best sense of this word its value is difficult to overestimate. The theoretical study performed showed clearly that the effect of resonance absorption of neutrons by nuclei of 238U is a governing factor in the calculation of the coefficient of neutron breeding in an unbounded medium it was concluded that a self-sustained chain reaction in a homogeneous natural uranium-light water system is impossible. 

The second paper of 1940 [3 ], entitled Kinetics of Uranium Chain Decay, is no less significant than the first. This pioneering work yielded a whole series of brilliant results for the first time, the need to take into account the role of delayed neutrons in the kinetics of chain nuclear reactions was shown (it is precisely the delayed neutrons which ensure easy control of nuclear reactors), the influence of heating on the kinetics of a chain process was considered in detail, and a number of conclusions were reached which are of much importance for the theory of reactor control. This same paper predicted the formation in the process of chain fission of new, previously unknown, nuclei which strongly absorb neutrons, a prediction which was later fully confirmed. 

In the 1941 paper with Yu. B. Khariton [40], the problem of the critical size of a sample of 235 U in the fission of nuclei by fast neutrons was considered. The calculations showed that, in order to sustain a chain fission reaction by fast neutrons in a sample of 235 U surrounded by a heavy neutron reflector, it is sufficient to have only ten kilograms of pure 235U isotope. Here also a theory is given which allows calculation of the critical mass of... 

Ya.B. s work on the problem of the chain fission of nuclei, together with his work on detonation and shock waves, were the scientific foundation of Ya.B. s practical activity in a collective which was carrying out a very important state assignment. This activity was acknowledged by awarding Ya.B. the highest decorations of the USSR. 

On prolonged reaction, the molecular weight of the polymer decreases apparently because of chain fission. It has been suggested that the fission results from attack by the acid anion on the chain ether groups ... 

Fermi was an Italian physicist who emigrated to the USA in 1939 and continued his studies there. Fermi produced a nuclear chain fission reaction by bombarding the isotope 23SU with neutrons. For his this achievement he won the 1938 Nobel. Prize in Physics. 

Any alkyl group present in the paraffin can be replaced by a nitro group, i.e. chain fission takes place. Thus, isopentane yields nine nitroparaffins. The fission reaction increases as the temperature rises. 

When chain fission of the alkoxy radical occurs on the other side of the free radical group, the reaction will not yield volatile aldehydes but will instead form nonvolatile aldehydo-glycerides. Volatile oxidation products can be removed in the refining process... 

Although earlier reports suggested that there was no side chain fission during metabolization of phenothiazine drugs,... 

Nuclear reaction(s) producing noble gas isotope(s) from stable or long-lived isotope during an irradiation in a nuclear reactor, or parent isotopes with half-lives less than 10 years, typical or suspected abundance at the time of formation of the solar system. For longer-lived parent isotopes, current abundance (%) of element. For actinides, " He yields are the number of atoms produced per decay chain, fission Xe yields are branching ratio for Xe. Yields for other isotopes are given in Table 2. 

A breeder reactor is a type of nuclear reactor that is capable of generating fissile material (material that can sustain a chain fission reaction) faster than it uses it up. This is accomplished by using the neutrons given off in the fission reaction to generate additional isotopes capable of fusion. Typically this involves the use of either thorium to generate fissile uranium or uranium to generate fissile plutonium. 

While the theory of the nuclear chain fission mechanism in uranium set forth herein is based on the best presently known experimental evidence, we do not wish to be bound thereby, as additional experimental data later discovered may modify the theory disclosed. Any such modification of theory, however, will in no way effect the results to be obtained in the practice of the invention herein described and claimed. 

The present invention relates to nuclear physics and more particularly to an improved means and method of cooling a reactive composition used to sustain a chain fission reaction. 

However, in the case of long-chain alkylated cyclopentanes, the chain fission dominates the spectra. In fossil fuels the monoalkylated cyclohexanes are characterized by the m/z 83 fragment One should be aware of the problem that fragmentation of alkylated cycloalkanes could be followed by rearrangement of the ions formed hence m/z 83 could also be formed from the methylcyclopentyl ion. Of course we must bear in mind the fact that the molecular weight of monocyclic alkanes is the same as for mono-unsaturated alkenes and, when investigating mixtures, this point is of importance (see also the discussion on fossil fuels. Section VILA). 

Chain fission may begin at a weak point of the polymer and then run along a part or the whole of the molecule. The radical depolymerization of poly(methyl methacrylate) and dehydrochlorination of PVC are noteworthy examples of this unzipping reaction. 

The most important secondary products formed during oxidation are those of lower molecular weight resulting from chain fission. Many aldehydes, ketones, alcohols, acids, esters, lactones, ethers and hydrocarbons have been identified. These... 

The movement of double bonds in long-chain unsaturated acids has been known since Varrentrapp converted oleic acid to palmitic by fusion with alkali, an observation which led to the invalid conclusion that oleic acid was the A2 or A3 acid. Double bond migration unaccompanied by chain fission occurs under milder conditions. The reaction also occurs more easily with methylene-interrupted polyene acids to give products with conjugated unsaturation which are easily recognized by ultraviolet spectroscopy (Section 9.3). 

Figure 21.15 Chain fission reaction in which each fission produces two neutrons. The process leads to an accelerating rate of fission, with the number of fissions potentially doubling at each stage. 

The first controlled and sustained fission reaction was operated successfully in 1942 at the University of Chicago with comparatively impure and chain breakers. The first branching chain fission reaction was successfully carried out in a test explosion with pure Pu in 1945 near Alamogordo, New Mexico,... 

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