Hot Fragments Research Articles

Contribution of hot spring cyanobacteria to the mysterious deaths of Lesser Flamingos at Lake Bogoria, Kenya

Cyanobacterial mats at hot springs on the shore of the alkaline Lake Bogoria, Kenya, were investigated regarding species community and cyanobacterial toxin content. The hepatotoxins microcystin-LR, -RR, -LF and -YR, and the neurotoxin anatoxin-a were present. The mats were dominated by Phormidium terebriformis, Oscillatoria willei, Spirulina subsalsa and Synechococcus bigranulatus. The concentration of microcystins in mat samples, ranged from 221 to 845 μg microcystin-LR equivalents g −1 DW of mat. Anatoxin-a concentrations ranged from 10 to 18 μg g −1 DW of mat. A contribution of the cyanobacterial toxins from the hot spring mats to the mass mortalities of Lesser Flamingos is suggested by: (a), the presence of hot spring cyanobacterial cells and cell fragments, and high concentrations of the cyanobacterial hepato- and neurotoxins in flamingo stomach contents and faecal pellets; (b), observations of neurological signs of bird poisoning at the lake. Cyanobacterial toxins in stomach contents, intestine and fecal pellets were 0.196 μg g −1 fresh weight (FW) for the microcystins and 4.34 μg g −1 FW for anatoxin-a. Intoxication with cyanobacterial toxins could occur by uptake of detached cyanobacterial cells from the mats, as the flamingos need to drink fresh or brackish water, and to wash their feathers daily, which they do in the vicinity of the hot springs, where salinity is lower than in the main body of water of the lake.

Production mechanism of hot nuclei in violent collisions in the Fermi energy domain

A production mechanism of highly excited nuclei formed in violent collisions in the Fermi energy domain is investigated. The collision of two nuclei is decomposed into several stages which are treated separately. Simplified exciton concept is used for the description of pre-equilibrium emission. A modified spectator–participant scenario is used where motion along classical Coulomb trajectories is assumed. The participant and one of the spectator zones undergo incomplete fusion. Excitation energies of both cold and hot fragment are determined. Results of the calculation are compared to recent experimental data in the Fermi energy domain. Data on hot projectile-like, mid-velocity and fusion-like sources are described consistently. Geometric aspects of pre-equilibrium emission are revealed. Explanations to previously unexplained experimental phenomena are given. Energy deposited into non-thermal degrees of freedom is estimated.

Mass and lifetime measurements at the storage ring ESR

We present results from two methods of direct mass measurements of relativistic exotic nuclei stored in the storage ring ESR. Schottky Mass Spectrometry (SMS) with electron-cooled ions is well suited for long-lived nuclei ( T 1/2⩾10 s), and the Isochronous mass measurements (IMS) for hot fragments ( T 1/2⩾1 μs). In addition, SMS provides information on nuclear lifetime for bare and highly ionized atoms.

Comparison of1AGeV197Au+Cdata with thermodynamics: The nature of the phase transition in nuclear multifragmentation

Multifragmentation MF results from 1A GeV Au on C have been compared with the Copenhagen statistical multifragmentation model ~SMM!. The complete charge, mass, and momentum reconstruction of the Au projectile was used to identify high momentum ejectiles leaving an excited remnant of mass A, charge Z, and excitation energy E* which subsequently multifragments. Measurement of the magnitude and multiplicity ~energy! dependence of the initial free volume and the breakup volume determines the variable volume parametrization of SMM. Very good agreement is obtained using SMM with the standard values of the SMM parameters. A large number of observables, including the fragment charge yield distributions, fragment multiplicity distributions, caloric curve, critical exponents, and the critical scaling function are explored in this comparison. The two stage structure of SMM is used to determine the effect of cooling of the primary hot fragments. Average fragment yields with Z>3 are essentially unaffected when the excitation energy is 170 the effective latent heat approaches zero. Thus for heavier systems this transition can be identified as a continuous thermal phase transition where a large nucleus breaks up into a number of smaller nuclei with only a minimal release of constituent nucleons. Z<2 particles are primarily emitted in the initial collision and after MF in the fragment deexcitation process.

I. The properties of hot Ca-like fragments from the 40Ca + 40Ca reaction at 35AMeV

The creation of hot Ca-like fragments was investigated in the 40Ca + 40Ca reaction at 35 AMeV. Using the AMPHORA 4π detector system, the primary projectile-like fragment was reconstructed and its properties were determined. Both primary and secondary distributions are compared with the predictions of a Monte Carlo code describing a heavy-ion collision as a two-step process. Some of the nucleons which are identified as participants in the first step are transferred in the second step to these final states, which correspond on the average to the maximum value of entropy (thermodynamic probability). The model allows for competition between mean-field effects and nucleon-nucleon interactions in the overlap zone of the interacting nuclei. The analysis presented here suggests a thermalized source picture of the decay of the projectile-like fragment. The validity of the reconstruction procedure for projectile-like fragments is discussed.

II. The intermediate velocity source in the 40Ca + 40Ca reaction at Elab = 35AMeV

The shape of the velocity distributions of charged particles projected on the beam direction can be explained if emissions from the hot projectile-like fragment and the target-like fragment are supplemented by an emission from an intermediate velocity source located between them. The creation of this source is predicted by a two-stage reaction model where, in the second stage, some of the nucleons identified in the first stage as participants form a group of clusters located in the region between the colliding nuclei. The cluster coalescence process is governed on the average by the maximum value of entropy, although its fluctuations are also significant. The properties of the intermediate velocity source are precisely described, including the isotopic composition of the emitted particles.

Hot fragmentation of nuclei

Today, we have a variety of reactions at hand that can be used to multi-fragment nuclei. In many of these reactions even several sources of fragments can be discerned and characterized. There is overwhelming evidence that these sources of fragments are hot. It is already less clear whether heat by itself is sufficient to initiate the fragment decay. What causes fragmentation, and when and how are the fragments (pre)formed? These questions have remained as much a challenge as the complementary class of questions to which they are related: What observations derive their significance from the liquid-gas phase behavior of extended nuclear matter? And, can we observe a phase transition in finite nuclei? Recent developments, largely coming from complex analyses of data sets measured in 4-pi-type experiments as well as from calculations based on advanced theoretical concepts, will be discussed.

Nuclear isotope thermometry

We discuss different aspects which might influence temperatures deduced from experimental isotopic yields in the multifragmentation process. It is shown that fluctuations due to the finite size of the system and distortions due to the decay of hot primary fragments conspire to blur the temperature determination in multifragmentation reactions. These facts suggest that caloric curves obtained through isotope thermometers, which were taken as evidence for a first-order phase transition in nuclear matter, should be investigated very carefully.

Isotopic composition of fragments in nuclear multifragmentation

In this Rapid Communication we present results from the analysis of the isotopic yields of fragments emitted in two selected reactions: the decay of the quasiprojectile in Au+Au peripheral collisions at 35 MeV/nucleon, and the disassembly of the unique source formed in Xe+Cu central reactions at 30 MeV/nucleon. We find that the relative yields of neutron-rich isotopes increase with the excitation energy of the emitting sources. In the framework of a statistical multifragmentation model which reproduces fairly well the experimental observables, such behavior can be explained with the increase of the $N/Z$ ratio of the hot primary fragments. This corresponds to the statistical evolution of the fragmentation mechanism as a function of the excitation energy, from the decay into few small fragments with a heavy residue to complete multifragmentation.

Vapor film collapse around hot liquid drops when an external pressure wave is supplied

An external pressure wave generated by a magnetic hammer is applied to a single or multiple hot LiNO 3 drops which have been in stable film boiling in subcooled ethanol. The process of vapor film collapse triggered by the pressure wave is studied by photographic observations and pressure history measurements. The vapor film begins to destabilize about 0.1 ms after pressure wave arrival at the drop. About 1.0 to 1.5 ms later, the vapor film begins to collapse from the lower part. Collapse of the vapor film develops from a portion around the drop preceded by a complex destabilization process. The ripple waves generated from a portion in the initial stage are concluded to be due to a Rayleigh-Taylor instability. Generation of interface disturbance and that of tiny bubbles as well as that of hot liquid fragments are observed, though no penetration of cold liquid jets can be observed in these experimental conditions

New Type of Shape Instability of Hot Nuclei and Nuclear Fragmentation

A novel mechanism of prompt nuclear fragmentation is proposed. Assuming micro-canonical or canonical equilibrium, it is shown that a strong enhancement of the accessible phase space volume, due to the diffuceness of the nuclear surface, leads to dynamical instabilities of hot nuclei and to prompt fragmentation. Equations are derived for the transition temperature T_T, for which the thermodynamical surface tension vanishes, as well as for the thermodynamical fissility parameter chi_td.

Constraints on cooling rates and permeabilities of pumice in an explosive eruption jet from colour and magnetic mineralogy

In explosive volcanic eruptions, vesicular magma droplets, produced by fragmentation, are propelled into the atmosphere where they are chilled to form pumices. The thermal history of droplets and the permeability of their internal bubble networks determine how much they are deformed in the eruption jet, and hence what information pumices record about the state of the magma at fragmentation. We study these aspects of the `Minoan' plinian eruption of Santorini Volcano by quantifying the rate of oxidation reactions that took place when air entered the hot magma fragments. In our experiments white Minoan pumices were heated for minutes to hours between 600 and 850°C, either in air, or in an atmosphere with an oxygen fugacity at the Ni–NiO buffer. Pumices were unchanged by heating at Ni–NiO. Those heated in air often became pink to dark pink, depending on heating time, and their Curie temperatures, as determined by magnetic susceptibility measurements, increased. We use oxidation rates deduced from these experiments, in conjunction with calculations of the rate of conductive cooling and of the rate at which air can enter a pumice, to constrain the conditions experienced by pumices during the eruption. Natural Minoan pumices less than about 5 cm in radius are white, whereas larger ones often have white rims and pink interiors with Curie temperatures higher than those of white material. We infer that small pumices were cooled before being oxidized, and that oxidation of the interiors of large clasts mostly took place during flight, at temperatures within a few tens of degrees of magmatic values. White rims of large pumices, despite being permeable, were cooled before oxidation could occur. Permeability developed in the liquid state, but did not develop early enough, with respect to cooling, or was not large enough to allow extreme oxidation. We give measurements of pumice permeabilities that should be close to magmatic values.

Experimental determination of fragment excitation energies in multifragmentation events

For 50 MeV/nucleon 129Xe+natSn multifragmentation events, we deduced, by means of correlation techniques, the multiplicities of the hydrogen and helium isotopes which were emitted by the hot primary excited fragments produced at the stage of the disassembly of an equilibrated hot source. We also derived the relative kinetic energy distributions between the primary clusters and the light charged particles that they evaporate. From the comparison between the secondary multiplicities observed experimentally and the multiplicities predicted by the GEMINI model, we concluded that the source breaks into primary fragments which are characterized by the same N/Z ratio as the combined system. Knowing the secondary light charged particle multiplicities and kinetic energies, we reconstructed the average charges of the hot fragments and we estimated their mean excitation energies. The fragment excitation energies are equal to 3.0 MeV/nucleon for the full range of intermediate mass fragment atomic number. This global constancy indicates that, on the average, thermodynamical equilibrium was achieved at the disassembly stage of the source.Received 13 March 1998DOI:https://doi.org/10.1103/PhysRevC.58.256©1998 American Physical Society

Isotopic and microcanonical temperatures in nuclear multifragmentation

A systematic comparison of different isotopic temperatures with the thermodynamical temperature of a multifragment system is made on the basis of the statistical multifragmentation model. It is demonstrated that isotopic temperatures are strongly affected by the secondary decays of hot primary fragments and the population of particle-stable excited states in final fragments. The He-Li temperatures, measured recently by the ALADIN group, are reproduced fairly well both as a function of excitation energy and bound charge. Our analysis confirms the anomaly in the nuclear caloric curve.

Process of Vapor Film Collapse around Hot Liquid Drops Induced by an External Pressure Wave.

An external pressure wave generated by a magnetic hammer is applied as an explosion trigger to a single or multiple hot LiNO3 drops which have been in stable film boiling in subcooled ethanol and the process of vapor film collapse is studied by photographic observation and pressure history measurement. The vapor film begins to destabilize within about 0.1ms after the pressure wave arrival to the drop position. At about 1.0-1.5ms later, the vapor film begins to collapse from the lower part. The vapor film initiates to collapse at a portion around the drop after a complex destabilization process. The ripple waves generated near the central part of the drop at the initial stage are supposed to be due to the Rayleigh-Taylor instability because of the wave length. Generation of interface disturbance and that of tiny bubbles or hot liquid fragments are observed, though no penatration of cold liquid jets can not be observed in this experimental condition.

The photodissociation dynamics of OClO between 306 and 370 nm: Fragment translational energy release and recoil anisotropy

The photodissociation OClO(à 2A2)→ClO(X̃ 2Π)+O(3P) was studied at wavelengths between 306 and 370 nm using photofragment translational energy spectroscopy. The flight time distributions and anisotropies of the recoiling fragments were measured with the photolysis wavelength tuned to 10 maxima of the structured absorption spectrum, corresponding to a vibronic excitation of the parent molecule with 9–18 quanta in the symmetric stretching coordinate on the à 2A2 surface. The translational energy distributions show that the ClO fragments are created in highly inverted vibrational state distributions which become extremely broad [v(Cl–O)∼1–15] with increasing excitation energy. The large fraction of vibrationally hot ClO fragments produced–particularly at λ&amp;lt;325 nm–could enhance various thermodynamically unfavorable atmospheric reactions in connection with ozone depletion. The main mechanistic features of the dissociation process, which account for the almost constant average translational energy and linearly increasing vibrational energy of ClO as a function of the excitation energy, can be interpreted, to a first approximation, as vibrational predissociation on the à 2A2 potential energy surface involving a relatively late exit barrier. From the measured translational energies the barrier height is estimated to be about 48 kJ/mol.

IMPROVEMENTS TO MODELING OF HOT FRAGMENT CONDUCTIVE IGNITION OF NITRAMINE-BASED PROPELLANTS

Hot fragments generated during the impact of shaped charge jets may penetrate the cartridge casing and induce propellant ignition. To simulate hot fragment conductive ignition, a comprehensive model has been formulated and validated by conducting detailed experiments. The model has been based on an assumed decomposition behavior of the various binder ingredients contained in the nitramine-composite propellant. The objective of this work is to describe the effects of incorporating an improved knowledge about the thermal decomposition behavior of these binder ingredients into the existing model. Rapid thermolysis experiments have revealed that essentially no decomposition occurs of the major ingredients CAB and ATEC over the range of temperatures from 450 to 600 K. The use of these findings has enabled an improved predictive capability of the propellant's rate of gasification, as well as the of the location of the Go/No-Go ignition boundary for both partially and fully confined enclosures. It should also be noted that the use of different reactions among CH2O and NO2 for describing the ignition event has shown only a limited impact on the location of Go/No-Go ignition boundary.

Structural Studies of Biodegradable Polymer with Fractal (Part 2). Fractal and Structure of Poly(L-Lactic Acid) in Fragmentation.

Degradation of biodegradable poly L-lactic acid was studied with alkaline hydrolysis and hot water. In the alkaline hydolysis, the sample shows a remarkable weight loss and at the same time the shape is changed with time. Koch curve was applied as a model of surface boundary for the degraded material. In the degradation with hot water, cracking and fragmentation occurs preferentially in the initial stage of weight loss. The change in shape of the degraded PLLA was analyzed by measuring box counting dimension and comparing the dimension with fractal image. The spider tree model of fractal figure is useful for cracking of PLLA. The overall cracks has a dimension of about 1.5. At the dimension of 1.7, the cracks form network and samples are pulverized. In conclusion, this treatment is useful for plastic garbage of PLLA.

Hot fragment conductive ignition of nitramine-based propellants

Hot fragments generated during the impact of shaped-charge jets may penetrate the cartridge casing and induce propellant ignition. The major objective of this study is to acquire a better understanding of hot fragment conductive ignition behavior of nitramine-bas ed XM39 and M43 propellants. The confinement effect of the cartridge is simulated by an enclosure within which the pyrolysis products can accumulate and significantly increase pressure. Both theory and experiments showed that the ignition threshold was a strong function of confinement. At low pressures, the XM39 propellant was more susceptible to ignition, because its binder decomposition is more exothermic than that of the M43 propellant. At highly confined conditions, which allow for chamber pressurization, the M43 ignition threshold became lower than that of XM39 propellant. The reduction of the ignition threshold is caused mainly by exothermic reactions between CH2O and NO2 species, which are enhanced under gas accumulation conditions.

Prevalence and Molecular Analysis of Two Hot Spots for Ectopic Recombination Leading to XX Maleness

Two hot spots of ectopic Xp-Yp recombination have previously been shown to be at the origin of XX maleness (Well et al., 1994, Nature Genet. 7:414-419). To get more insight into the molecular basis of the abnormal interchange, 25 Y(+) class 3 XX male patients were studied. The two hot spots were found to account for the aberrant exchange in more than 50% of the cases. In addition, there is a correlation between the prevalence of each hot spot and the degree of X-Y homology between the corresponding fragments. Sequencing of the X-Y junctions in six patiens, who carried a breakpoint mapping in either of these two hot spot fragments, showed that their precise lacations were different from one individual to the other. In particular, the results obtained here in four new patients exclude the possibility that the repeated elements, present in these X-Y homologous fragments, are responsible for the high incidence of X-Y interchanges observed. Moreover, the breakpoints of all 25 class 3 XX males were found to be arranged in the same order on the X and Y chromosomes. This suggests that most ectopic recombinations leading to class 3 XX maleness involve X-Y homologous sequences persisting from an ancestral larger block of homology on the short arms of both sex chromosomes.