Lifetime, fission

It gives reasonable fission barriers, fission isomer energies, and fission isomer lifetimes [KUM86]. Agreement with the fission lifetimes is also quite reasonable (see Fig. 7). This did require the introduction of two additional parameters (the strength and the A A -dependence) for the nuclear part of the fragment-fragment interaction. 

Hopefully, our results (some of which have been presented previously [KUM83]) for the superheavy nuclei will also stimulate other theorists to recalculate fission lifetimes, and, especially, fusion cross-sections for the suggested target-projectile combinations. These cross-sections are expected to be small and the corresponding experiments are expected to be quite difficult. 

FIG. 14.13. SponUneous fission lifetimes. (From Slnitinsky and Bjomholm.)... 

At least 21 tellurium isotopes are known, with mass numbers from 114 to 134. Of these, eight are stable, ie, 120, 122—126, 128, 130. The others are radioactive and have lifetimes from 2 min to 154 d the heaviest six, 131m, 131,132, 133m, 133, and 134, are fission products (see Radioisotopes). 

For polychlorinated biphenyls (PCBs), rate constants were highly dependent on the number of chlorine atoms, and calculated atmospheric lifetimes varied from 2 d for 3-chlorobiphenyl to 34 d for 236-25 pentachlorobiphenyl (Anderson and Hites 1996). It was estimated that loss by hydroxy-lation in the atmosphere was a primary process for the removal of PCBs from the environment. It was later shown that the products were chlorinated benzoic acids produced by initial reaction with a hydroxyl radical at the 1-position followed by transannular dioxygenation at the 2- and 5-positions followed by ring fission (Brubaker and Hites 1998). Reactions of hydroxyl radicals with polychlorinated dibenzo[l,4]dioxins and dibenzofurans also play an important role for their removal from the atmosphere (Brubaker and Hites 1997). The gas phase and the particulate phase are in equilibrium, and the results show that gas-phase reactions with hydroxyl radicals are important for the... 

The mode of fission of some azo compounds into alkyl radicals and nitrogen has been studied by Pryor and Smith<8) using the following postulates (1) A molecule that decomposes by a concerted scission of both C—N bonds will not undergo cage return and will have a rate constant independent of viscosity (2) a molecule that decomposes by a stepwise scission of the C—N bonds can undergo cage recombination and the rate constant for decomposition will decrease with solvent viscosity increase provided that the lifetime of the radicals produced by the initial homolysis is of the same order... 

There are two distinct advantages of the self-powered neutron detector (a) very little instrumentation is required—only a millivoltmeter or an electrometer, and (b) the emitter material has a much greater lifetime than boron or U235 lining (used in wide range fission chambers). 

One should remember in this connection that many cases of spontaneous fission of excited (isomer) nuclear states with lifetimes about 10 2 sec (quite close to the time of addition of a new link to the growing polyformaldehyde chain near absolute zero of temperature) were observed and successfully studied after the pioneering works of Dubna scientists. 

Unfortunately, precise knowledge of the distribution of direct yield among several competing isobars is generally not available furthermore, the radioactive half-lives involved are frequently completely unknown since the fission process gives rise directly to between 300 and 400 radioactive species, and the separation of such a complex mixture usually involves a time which is quite long compared with the lifetimes of interest. We do know that each isobaric chain is formed directly as a number of different isobars and that the width of the isobaric yield distribution is such that to account for 90% or more of a chain one must consider at least three or perhaps four chemical elements. 

If the time scale of neutron capture reactions is very much less than 3 -decay lifetimes, then rapid neutron capture or the r process occurs. For r-process nucleosynthesis, one needs large neutron densities, 1028/m3, which lead to capture times of the order of fractions of a second. The astrophysical environment where such processes can occur is now thought to be in supernovas. In the r process, a large number of sequential captures will occur until the process is terminated by neutron emission or, in the case of the heavy elements, fission or (3-delayed fission. The lighter seed nuclei capture neutrons until they reach the point where (3 -decay lifetimes have... 

Recently more sophisticated models have been developed for describing the steady state vaporization behavior of U Zr C,./-8 and U Nb C, y9 in high temperature, high pressure H2. The principal purpose of these studies was to assess the performance of these fuel systems in nuclear thermal propulsion (NTP) applications using H2 propellant. Vaporization rates and melting temperatures may be the two primary processes that limit the operating temperature and lifetime of the structural and fissionable materials in NTP systems. When the atom fraction of U is <0.1, the... 

The model, with major extensions to nuclear fission, leads to surprising predictions for the lifetimes of superheavy nuclei. Suggestions for heavy-ion synthesis are presented. 

A major thrust of recent work on FTMS of large biomolecules has dealt with questions concerning the lifetime of molecular ions formed by high-energy particle bombardment. Chait and Field have reported that a large fraction of the molecular ions of chlorophyll A formed by 252Cf fission fragment ionization decomposes with lifetimes of less than a few... 

The lifetimes of molecular fluorescence emissions are determined by the competition between radiative and nonradiative processes. If the radiative channel is dominant, as in the anthracene molecule, the fluorescence quantum yield is about unity-and the lifetime lies in the nanosecond range. In molecular assemblies, however, due to the cooperative emission of interacting molecules, much shorter lifetimes—in the picosecond or even in the femtosecond range—can theoretically be expected an upper limit has been calculated for 2D excitons [see (3.15) and Fig. 3.7] and for /V-multilayer systems with 100 > N > 2.78 The nonradiative molecular process is local, so unless fluorescence is in resonance by fission (Section II.C.2), its contribution to the lifetime of the molecular-assembly emission remains constant it is usually overwhelmed by the radiative process.118121 The phenomenon of collective spontaneous emission is often related to Dicke s model of superradiance,144 with the difference that only a very small density of excitation is involved. Direct measurement of such short radiative lifetimes of collective emissions, in the picosecond range, have recently been reported for two very different 2D systems ... 

On-line gas chemical studies of dubnium have been mostly performed with Db. This nuclide can be produced in the reaction Bk( 0,5n) at a beam energy of about 100 MeV. It has a half-life of 34 5 s and decays with 67 % by emission of two sequential a particles via 258Lr (T1/2=4.4 s) to the long-lived 254Md (Ti/2=28m). In addition, 262Db has a spontaneous fission decay branch of 33%. Hence, identification of each separated labeled molecule is based on either detection of two characteristic a-particles and their lifetimes or on the detection of a spontaneous fission decay. 

If a hydrogen atom is abstracted from an alkane by an alkyl radical, both the initial and final state of the reaction involve neutral species and it is only the transition state where some limited charge separation can be assumed. In the case of a homolytic O—H bond fission, however, the initial state possesses a certain polarity and possible changes in polarity during the reaction depend on both the lifetime of the transition state and the nature of the attacking radical. If the unpaired electron is localized mainly on oxygen in the reactant radical, the polarity of the final state will be close to that of the initial state and any solvent effect will primarily depend on the solvation of the transition state. Solvent effects can then be expected since the electron and proton transfers are not synchronous. 

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