Viable Nucleus Research Articles

Search for a viable nucleus–nucleus potential in heavy-ion nuclear reactions

We have constructed an empirical formulae for the fusion and interaction barriers using experimental values available till date. The fusion barriers so obtained have been compared with different model predictions based on the proximity, Woods-Saxon and double folding potentials along with several empirical formulas, time dependent Hartree-Fock theories, and the experimental results. The comparison allows us to find the best model, which is nothing but the present empirical formula only. Most remarkably, the fusion barrier and radius show excellent consonance with the experimental findings for the reactions meant for synthesis of the superheavy elements also. Furthermore, it is seen that substitution of the predicted fusion barrier and radius in classic Wong formula [C. Wong, Phys. Rev. Lett. {31}, 766 (1973)] for the total fusion cross sections satisfies very well with the experiments. Similarly, current interaction barrier predictions have also been compared well with a few experimental results available and Bass potential model meant for the interaction barrier predictions. Importantly, the present formulae for the fusion as well as interaction barrier will have practical implications in carrying out the physics research near the Coulomb barrier energies. Furthermore, present fusion barrier and radius provide us a good nucleus-nucleus potential useful for numerous theoretical applications.

Population size, abundance, habitat relationships and the result of a translocation programme in the Gran Canaria Blue ChaffinchFringilla polatzeki

SummaryThe Gran Canaria Blue ChaffinchFringilla polatzekiis a threatened, endemic, forest‐dwelling bird species of the Canary Islands, whose core population at the end of the 20th century was restricted to the pine forests of Inagua Nature Reserve (38 km2). A translocation programme released birds from a breeding centre into the nearby (<3 km) pine forests of La Cumbre in the years following 2010. From 2015 to 2019 the La Cumbre population was reinforced by translocation of wild juveniles from the source population of Inagua. We estimate the population size, the spatial variation of abundance, and recent temporal changes in density of the species in Inagua and La Cumbre by means of line transects, distance sampling, and habitat suitability modelling using random forests. The average density of the Blue Chaffinch in Inagua Nature Reserve was 10.2 birds/km2in spring 2019, with a population estimated at 362 birds (95% CI: 257–489). The most important variables affecting the distribution of the Blue Chaffinch in Inagua were the amount of precipitation during the summer (July–September), the solar radiation in June, and the northern position in the reserve, highlighting the importance of abiotic factors related to thermal and hydric stress during the breeding season. The density was considerably lower in the translocated population inhabiting 21 km2of pine forests in La Cumbre (3.3 birds/km2), with an estimate of 68 Blue Chaffinches (35–141) breeding freely in the wild. The translocation programme successfully contributed to the establishment of a second viable nucleus, accounting for 16% of the total population within a time span of 10 years. This result reinforces the role of translocations in preventing extinctions of endangered species with very low population sizes restricted to only one isolated area.

Fragmentation and low density as major conservation challenges for the southernmost populations of the European wildcat.

Knowledge of population dynamics of threatened species in the wild is key to effective conservation actions. However, at present, there are many examples of endangered animals for which their current situation is unknown, and not just in remote areas and less developed countries. We have explored this topic by studying the paradigmatic case of the European wildcat (Felis silvestris silvestris), an endangered small carnivore whose status has been subjectively established on the basis of non-systematic approaches and opportunistic records. Little is known about its demographic situation, prompting the need for information to improve conservation measures. However, the secretive behaviour of felines along with its low density in natural conditions have prevented the gathering of sufficient data. We developed a field sampling strategy for one of the largest populations (Andalusia, South Spain, 87,268 km2), based on a logistically viable systematic non-intrusive survey by camera-trapping. This study offers the first large-scale estimation for any European wildcat population, based on analytical approaches applied on Species Distribution Models. A hierarchical approach based on a Maxent model for distribution estimation was used, along with Generalised Linear Models for density estimation from explicit spatial capture-recapture data. Our results show that the distribution range is smaller and more highly fragmented than previously assumed. The overall estimated density was very low (0.069 ±0.0019 wildcats/km2) and the protected areas network seems to be insufficient to cover a significant part of the population or a viable nucleus in demographic terms. Indeed, the most important areas remain unprotected. Our main recommendations are to improve the protected area network and/or vigilance programs in hunting estates, in addition to studying and improving connectivity between the main population patches.

Cross-Nucleation between Concomitantly Crystallizing α- and γ-Phases in Polypivalolactone: Secondary Nucleation of One Polymorph on Another

Cross-nucleation between polymorphs occurs when a new crystal structure nucleates on the surface of a pre-existing crystal of a different modification. The understanding of the phenomenon is still mostly phenomenological and qualitative. Here, we report quantitative measurements of cross-nucleation rate of the α phase of polypivalolactone on its γ phase, during melt crystallization over a large temperature range. The cross-nucleation kinetics is well modeled as heterogeneous secondary nucleation of one polymorph on another, in which the formation of a viable α-phase nucleus on the surface of γ-crystals is the rate-limiting step. According to this analysis, cross-nucleation can occur because the interfacial energy between the two structures is small, and the nucleation free energy barrier is similar to that for secondary nucleation within the same polymorph in crystal growth. Some peculiar aspects of cross-nucleation with respect to conventional heterogeneous nucleation, arising from the continuous growth ...

Gellan gum-based hydrogels for intervertebral disc tissue-engineering applications

Intervertebral disc (IVD) degeneration is a challenging clinical problem that urgently demands viable nucleus pulposus (NP) implant materials. The best suited biomaterial for NP regeneration has yet to be identified, but it is believed that biodegradable hydrogel-based materials are promising candidates. In this work, we have developed ionic- and photo-crosslinked methacrylated gellan gum (GG-MA) hydrogels to be used in acellular and cellular tissue-engineering strategies for the regeneration of IVDs. The physicochemical properties of the developed hydrogels were investigated by Fourier-transform infrared spectroscopy, (1) H nuclear magnetic resonance and differential scanning calorimetry. The swelling ability and degradation rate of hydrogels were also analysed in phosphate-buffered saline solution at physiological pH for a period of 30 days. Additionally, the morphology and mechanical properties of the hydrogels were assessed under a scanning electron microscope and dynamic compression, respectively. An in vitro study was carried out to screen possible cytotoxicity of the gellan gum-based hydrogels by culturing rat lung fibroblasts (L929 cells) with hydrogel leachables up to 7 days. The results demonstrated that gellan gum was successfully methacrylated. We observed that the produced GG-MA hydrogels possess improved mechanical properties and lower water uptake ability and degradation rate as compared to gellan gum. This work also revealed that GG-MA hydrogels are non-cytotoxic in vitro, thus being promising biomaterials to be used in IVD tissue-engineering strategies.

Multidimensional Kinetics of Nucleation in Liquid-Vapor Systems

A metastable system is described as an ensemble of locally isolated statistically independent centers, with a possibility of one and only one nucleus of a close-to- critical radius emerging on each one of these centers. It is assumed that this event occurs as a result of fluctuations in a heterophase subsystem and leads to the formation of a viable nucleus at a given point in space. The process of the emergence of this nucleus is treated as the first crossing of the potential barrier by a Brownian particle. Proceeding from the principles of nonequilibrium thermodynamics, dynamic equations of bubble (droplet) growth are derived, which correspond to the Onsager relations. These formulas are used as Langevin equations in multidimensional phase space and are related to the respective Fokker-Planck equation whose solution enables one to determine the local rate of emergence of a viable nucleus and, as a consequence, the rate of its emergence in the entire system. An alternative expression is given for the rate of homogeneous steady-state nucleation, which differs from the classical expression by the pre-exponential factor and, in the case where one parameter (radius) may be sufficient, gives close limits of attainable superheat (supersaturation). Given the expression for the nonequilibrium work of bubble (droplet) and the distribution of heterogeneous centers, the obtained result may be readily generalized to the case of heterogeneous nucleation.

In vivo macrophage recruitment by murine intervertebral disc cells.

An in vivo murine experiment was conducted to measure the capacities of viable intervertebral disc cells to recruit inflammatory cells. The objective was to determine whether compounds secreted from viable cells induce inflammation or whether inflammation in disc herniation simply requires exposure to structural cell or matrix components. Three tissue preparations were inserted into the right lower peritoneal cavity of male mice: tissue with viable annulus fibrosus and nucleus pulposus cells, tissue with viable annulus fibrosus cells, or devitalized annulus fibrosus and nucleus pulposus tissue. Controls included sham-operated and nonoperated groups. Mice were killed 1, 2, or 7 days after surgery. Macrophage recruitment occurred after exposure to viable disc tissue but not after exposure to devitalized disc components; recruitment increased over time. Viable disc cells play a role in the etiology of inflammation in disc herniation.

Boiling of Superheated Liquids and Frenkel “Islands”

Versions of the exponential model of a Frenkel “island” are suggested. The nucleation of the new phase is treated as a fluctuation “surge” in the system. For the case when a bubble on the surface may be in several equilibrium states, a formula is given which is based on the concept that the formation of a viable nucleus is a homogeneous Markovian process. Calculations are performed for several organic liquids and water. The parameters are obtained at which the model shows the best correlation with experimental data.

Survival and Mutation in Neurospora Exposed at Nuclear Detonations

In the experiments to be described here, the conidia of a fungus, Neurospora crassa, were exposed to radiation from nuclear devices. Biological information of three types was obtained: first, the survival of conidia under conditions of forced heterokaryosis, where the viability of two or more genetically different nuclei per cell is a prerequisite of survival; second, the survival on supplemented medium, where survivors need contain but a single viable nucleus per cell; and third, the frequency of nuclei carrying recessive lethal mutations. These effects were measured after exposure at several stations, both inside and outside the seven-inch-thick lead neutron hemispheres in detonations A and B. It must be emphasized at the beginning that those conclusions which depend on exact knowledge of the radiation dose must not be taken too seriously, because the material is sufficiently resistant to radiation to necessitate its being placed where no physical measurement of total rep could be made by available methods, not a surprising eventuality when it is realized that dose rates of the order of billions of rep per second were involved. The magnitude of 'the error involved in estimates by extrapolation is unknown. However, the findings justify several conclusions which are independent of absolute dose measurements; namely, that the radiation from these devices kills the conidia by damaging the cell nuclei, and not through damage to other cell constituents; that a slow neutron component of the radiation was detected biologically, and that exposure under anoxic conditions (nitrogen atmosphere) had no protective effect. On the whole, the results give no reason to believe there is anything unique about the biological effects of radiation from the devices; anything which could not bSe duplicated by mixed radiation from laboratory sources. In order to understand the results we must first discuss the special properties of the material which facilitate these conclusions. HETEROKARYOSIS AND THE ANALYSIS OF RADIATION EFFECTS The stock used in the present experiments contains two genetically different types of nuclei. This condition is known as heterokaryosis. One of the nuclear components carries a mutation leading to a requirement for arginine, whereas the other component carries mutations leading to a requirement for methionine and to a drastic morphological modification known 'Work performed for the Atomic Energy Commission under Contract No. W-7405

Autogamy in Paramecium polycaryum

Mass cultures of a stock of Paramecium polycaryum maintained over a period of several years showed abundant and frequent nuclear reorganization stages resembling those of ex‐conjugant and ex‐autogamous animals of other species of Paramecium. Conjugation has never been reported for P. polycaryum, nor has it been found in these studies. Cytological examination of stained preparations revealed a process of autogamy in P. polycaryum, closely similar to that described previously for P. aurelia. As a rule, all four of the micronuclei, the typical vegetative number in P. polycaryum, engage in the first prezygotic division which is characterized by the formation of prophase crescents. Variable numbers of the eight nuclei continue with the second division. A maximum of sixteen nuclei may result. Apparently, only one of these normally completes the third prezygotic division to form the gametic nuclei, although more than one may initiate it. A fusion nucleus (synkaryon) arises in, or near, a paroral cone, thus paralleling autogamy in P. aurelia. A series of postzygotic divisions produces eight definitive nuclei, four of which become macronuclear anlagen and four remain micronuclei. The first division of the synkaryon results, possibly, in the formation of a viable nucleus and a non‐viable one, as in ex‐conjugants of P. caudatum. After the last micronuclear division, a skein evolves from the old macronucleus which has become flattened and leaf‐like. The skein rapidly segments into “sausages” which transform into spherical fragments, about thirty in number. Two cell divisions restore the normal vegetative nuclear complex.