Breakup Temperature Research Articles

Isobar correlations bearing information on the properties of hot disassembling nuclear sources

Two-particle correlations based on the multiplicity of selected isobars are found to be sensitive to the parameterization of the fragments' binding energies and the breakup volume assumed in the model calculations. The properties of these correlations have been examined in the framework of the Statistical Multifragmentation Model as a function of the breakup temperature. The model calculations suggest that the maxima of these correlation functions occur at well separated temperatures as the breakup volumes used in the model vary from 3 to 6 times that at normal density. These volumes are within the range assumed in most statistical calculations and supported by experiments. Besides their position, the height and width of the maxima are also found to be sensitive to the parameterization of the fragments' binding energy. The magnitude of all these effects also depends on the isobars considered in the correlations. We found that, due to an interplay between the symmetry energy and the volume dependent terms of the Helmholtz free energy, in the case of nearly symmetric sources, correlations involving light mirror nuclei seem to enhance these effects. We suggest that the proposed correlation functions could be used to extract information on the fragments' energies and on the breakup volume of nuclear sources.

Riesgo de heladas por inversión térmica en la huerta de Murcia: incidencia en la actividad agraria

En el Sureste peninsular son frecuentes las situaciones anticiclónicas alimentadas en invierno por advecciones de aire polares/árticas continentales. Bajo estas condiciones, las cuencas y valles fluviales intrabéticos son bastante proclives al desarrollo de inversiones térmicas. Un claro ejemplo lo constituye la Vega Media del Segura en los meses de invierno, con génesis de heladas relativamente frecuentes en sectores localizados. Como complemento al estudio de los factores físicos, se realiza un profundo estudio del parcelario agrícola de la huerta de Murcia y los posibles efectos que puedan generar las bajas temperaturas sobre los principales cultivos del área de estudio, centrados especialmente en los mayoritarios cítricos. Finalmente se proponen una serie medidas anti-heladas activas, pasivas en relación a las características del área de estudio y un sistema empírico de predicción de temperatura mínima, a fin de que los agricultores posean herramientas para mitigar los efectos de las heladas en la Vega Media del Segura.

Internal and kinetic temperatures of fragments in the framework of a nuclear statistical multifragmentation model

The agreement between the fragments' internal and kinetic temperatures with the breakup temperature is investigated using a Statistical Multifragmentation Model which makes no a priori as- sumption on the relationship between them. We thus examine the conditions for obtaining such agreement and find that, in the framework of our model, this holds only in a relatively narrow range of excitation energy. The role played by the qualitative shape of the fragments' state densities is also examined. Our results suggest that the internal temperature of the light fragments may be affected by this quantity, whose behavior may lead to constant internal temperatures over a wide excitation energy range. It thus suggests that the nuclear thermometry may provide valuable information on the nuclear state density.

Thermal instabilities and Rayleigh breakup of ultrathin silver nanowires grown in helium nanodroplets.

Ag nanowires with diameters below 6 nm are grown within vortex containing superfluid helium nanodroplets and deposited onto a heatable substrate at cryogenic temperatures. The experimental setup allows an unbiased investigation of the inherent stability of pristine silver nanowires, which is virtually impossible with other methods due to chemical processes or templates involved in standard production routes. We demonstrate by experiment and by adaption of a theoretical model that initially continuous wires disintegrate into chains of spheres. This phenomenon is well described by a Rayleigh-like breakup mechanism when the substrate is heated to room temperature. Our findings clarify the recent discussions on the cause of the observed segmented patterns, where a breakup during deposition [Gomez et al., Phys. Rev. Lett., 2012, 108, 155302] or mechanisms intrinsic to the helium droplet mediated growth process [Spence et al., Phys. Chem. Chem. Phys., 2014, 16, 6903] have been proposed. The experimental setup confirms the validity of previous suggestions derived from bulk superfluid helium experiments [Gordon et al., Phys. Chem. Chem. Phys., 2014, 16, 25229] for the helium droplet system, and further allows a much more accurate determination of the breakup temperature.

Peligrosidad de heladas por inversión térmica en la huerta de Murcia

<p>En el Sureste peninsular son frecuentes las situaciones anticiclónicas alimentadas en<br />invierno por advecciones de aire polares/árticas continentales. Bajo estas condiciones, las<br />cuencas y valles fluviales intrabéticos son bastante proclives al desarrollo de inversiones<br />térmicas. Un claro ejemplo lo constituye la Vega Media del Segura en los meses de invierno.<br />En el presente artículo se analizan tales factores y las situaciones de inversión térmica más<br />importantes registradas en esta Vega durante el período 2000-2013. Como complemento al<br />estudio, se realiza una propuesta de cartografía de peligrosidad de heladas en la Huerta de<br />Murcia para los diferentes meses invernales.<br />Palabras claves: Inversión térmica, anticiclón,</p>

Baryon annihilation and regeneration in heavy ion collisions

The role of baryon-antibaryon annihilation during the hadronic stage of a relativistic heavy ion collision is explored by simulating the chemical evolution of a hadron gas. Beginning with a chemically equilibrated gas at an initial temperature of 170 MeV, the chemical composition of a representative hydrodynamic cell is followed throughout the hadronic stage. The cell's volume changes with time according to a parametrization that mimics a three-dimensional hydrodynamic expansion. The chemical evolution includes both annihilation and regeneration of baryons, consistent with detailed balance. During the hadronic stage, the number of baryons drops by approximately 40% for the case in which there is no net baryonic charge. When the calculations are performed without the baryon regenerating processes, e.g., $5\ensuremath{\pi}\ensuremath{\rightarrow}p\overline{p}$, the loss of baryons was found to be closer to 50%. After accounting for annihilation, yields are consistent with measurements from the ALICE Collaboration at the Large Hadron Collider (LHC). Baryon annihilation is shown to alter the extracted chemical breakup temperature by significantly changing the $p/\ensuremath{\pi}$ ratio. Assuming that annihilation cross sections are independent of the strangeness and isospin of the annihilating baryon and antibaryon, the loss of strange baryons from annihilation is found to be similar.

Effects of geometric constraints on the nuclear multifragmentation process

We include in statistical model calculations the facts that in the nuclear multifragmentation process the fragments are produced within a given volume and have a finite size. The corrections associated with these constraints affect the partition modes and, as a consequence, other observables in the process. In particular, we find that the favored fragmenting modes strongly suppress the collective flow energy, leading to much lower values than those obtained from unconstrained calculations. For a given total excitation energy, this leads to a nontrivial correlation between the breakup temperature and the collective expansion velocity. In particular, we find that under some conditions, the temperature of the fragmenting system may increase as a function of this expansion velocity, contrary to what might be expected.

Isotopic effects in multifragmentation and the nuclear equation of state

Isotopic effects in spectator fragmentations following heavy-ion collisions at relativistic energies are investigated using data from recent exclusive experiments with SIS beams at GSI. Reactions of 12C on 112,124Sn at incident energies 300 and 600 MeV per nucleon were studied with the INDRA multidetector while the fragmentation of stable 124Sn and radioactive 107Sn and 124La projectiles was studied with the ALADIN spectrometer. The global characteristics of the reactions are very similar. This includes the rise and fall of fragment production and deduced observables as, e.g., the breakup temperature obtained from double ratios of isotope yields. The mass distributions depend strongly on the neutron-to-proton ratio of the decaying system, as expected for a simultaneous statistical breakup. The ratios of light-isotope yields from neutron-rich and neutron-poor systems follow the law of isoscaling. The deduced scaling parameters decrease strongly with increasing centrality to values smaller than 50% of those obtained for the peripheral event groups. This is not compensated by an equivalent rise of the breakup temperatures which suggests a reduction of the symmetry term required in a liquid-drop description of the fragments at freeze-out.

Multifragmentation and the phase transition: A systematic study of the multifragmentation of1AGeVAu, La, and Kr

A systematic analysis of the multifragmentation (MF) in fully reconstructed events from 1A GeV Au, La and Kr collisions with C has been performed. This data is used to provide a definitive test of the variable volume version of the statistical multifragmentation model (SMM). A single set of SMM parameters directly determined by the data and the semi-empiricalmass formula are used after the adjustable inverse level density parameter, $\epsilon_{o}$ is determined by the fragment distributions. The results from SMM for second stage multiplicity, size of the biggest fragment and the intermediate mass fragments are in excellent agreement with the data. Multifragmentation thresholds have been obtained for all three systems using SMM prior to secondary decay. The data indicate that both thermal excitation energy $E_{th}^{*}$ and the isotope ratio temperature $T_{He-DT}$ decrease with increase in system size at the critical point. The breakup temperature obtained from SMM also shows the same trend as seen in the data. The SMM model is used to study the nature of the MF phase transition. The caloric curve for Kr exhibits back-bending (finite latent heat) while the caloric curves for Au and La are consistent with a continuous phase transition (nearly zero latent heat) and the values of the critical exponents $\tau$, $\beta$ and $\gamma$, both from data and SMM, are close to those for a 'liquid-gas' system for Au and La. We conclude that the larger Coulomb expansion energy in Au and La reduces the latent heat required for MF and changes the nature of the phase transition. Thus the Coulomb energy plays a major role in nuclear MF.

Multifragmentation and the phase transition: A systematic study of the multifragmentation of 1A GeV Au, La and Kr

A systematic analysis of the multifragmentation (MF) in fully reconstructed events from 1A GeV Au, La and Kr collisions with C has been performed. Detailed comparisons of the various fragment properties are presented as a function of excitation energy, E*th. The charged particle multiplicity from MF stage shows a saturation beyond E*th ∼ 8 MeV/nucleon for Kr. The universal behavior of intermediate mass fragment yields and of the size of the largest fragment is observed only for Au and La when scaled with size of the system. The Kr data are found to lack this property. Moments of the fragment size distribution show that the Kr MF is different than the MF of Au and La. A power law behavior is observed for Au and La with exponent τ>2, while for Kr τ<2. The results are compared with the statistical multifragmentation model (SMM). A single value of all the parameters of the model fits the data for all the three systems. The breakup of Au and La is consistent with a continuous phase transition. The data indicate that both E*th and the isotope ratio temperature THc-DT decrease with increase in system size at the critical point. The breakup temperature obtained from SMM also shows the same trend as seen in data. This trend is attributed primarily to the increasing Coulomb energy with finite size effects playing a smaller role. The percolation and Ising model studies for finite size neutral matter show behavior which is opposite to the one seen in the present work.

Temperature measurements in central collisions

At incident energies in excess of about E/A=50 MeV, a rapid collective expansion of the combined system may occur during the later stages of a central collision between heavy nuclei. The emission of the observed particle species occurs after initial overlap and before the density of the system passes below about 0.1Po, corresponding to an expansion rate dependent on the total emission time which is less than 100 fm/c. The short emission times motivate the description of such collisions via dynamical models or via statistical models which assume equilibrium at a single breakup density and temperature. This latter class of models includes bulk multifragmentation models such as the Statistical Multifragmentation Model (SMM) which displays a phase transition in sub-saturation density [1]. Calculations predict enhanced heat capacities for finite nuclear systems at temperatures of the order of 4-6 MeV due to the transformation from the Fermi liquid which can be found in the interior of large nuclei in their ground and low excited states to a gas phase consisting of free nucleons and light clusters.

Electron-irradiation-induced deep levels in n-type 6H–SiC

The fluence-dependent properties and the annealing behavior of electron-irradiation-induced deep levels in n-type 6H–SiC have been studied using deep-level transient spectroscopy (DLTS). Sample annealing reveals that the dominant DLTS signal at EC−0.36 eV (labeled as E1 by others) consists of two overlapping deep levels (labeled as ED3L and ED3H). The breakup temperature of the defect ED3L is about 700 °C. The ED3H center together with another deep level located at EC−0.44 eV (so-called E2) can withstand high-temperature annealing up to 1600 °C. It is argued that the involvement of the defect ED3L is the reason that various concentration ratios of E1/E2 were observed in the previous work. The revised value of the capture cross section of the deep-level ED3H has been measured after removing ED3L by annealing. A deep level found at EC−0.50 eV is identified as a vacancy–impurity complex since it was found to have a lower saturated concentration and weak thermal stability. Two other deep levels, EC−0.27 eV and EC−0.32 eV, which were not observed by others because of the carrier freeze-out effect, are also reported.

Temperature and energy partition in fragmentation

We study fragmentation of small atomistic clusters via molecular dynamics. We calculate the time scales related to fragment formation and emission. We also show that some degree of thermalization is achieved during the expansion process, which allows the determination of a local temperature. In this way we can calculate the break-up temperature as a function of excitation energy, i.e. the fragmentation caloric curve. Fragmentation appears as a rather constant temperature region of the caloric curve. Furthermore, we show that different definitions of temperature, related to different degrees of freedom, yield very similar values.

Development of Mg–V alloys by physical vapour deposition Part 1 – Bulk and surface characterisation

A series of Mg–V alloys were produced by physical vapour deposition (PVD). Both c and a lattice parameters, as well as the c/a ratio, decreased with increasing vanadium content. The deposits were characterised by a strong basal texture. The solid solubility of vanadium in magnesium was extended to ∼ 17 wt-% V: Grain refinement occurred with increasing solute content. The solid solution breakup temperature decreased as the vanadium content in the alloys increased. Pure vanadium precipitated when solid solubility was not retained. A thin oxide layer of between 10 and 20 nm existed at the surfaces of the alloys and consisted predominantly of hydromagnesite at the outermost surface with Mg(OH)2 in an excess of MgO underneath. No evidence of vanadium oxide in the surface film was found. Magnesium oxide was also found to exist between the grains of the deposits, creating a graded metal/oxide interface.

Breakup temperature of target spectators in 197Au +197Au collisions at E/A = 1000 MeV

Breakup temperatures were deduced from double ratios of isotope yields for target spectators produced in the reaction 197Au + 197Au at 1000 MeV per nucleon. Pairs of 3,4He and 6,7Li isotopes and pairs of 3,4He and H isotopes (p, d and d, t) yield consistent temperatures after feeding corrections, based on the quantum statistical model, are applied. The temperatures rise with decreasing impact parameter from 4 MeV for peripheral to about 10 MeV for the most central collisions. The good agreement with the breakup temperatures measured previously for projectile spectators at an incident energy of 600 MeV per nucleon confirms the universality established for the spectator decayat relativistic bombarding energies. The measured temperatures also agree with the breakup temperatures predicted by the statistical multifragmentation model. For these calculations a relation between the initial excitation energy and mass was derived which gives good simultaneous agreement for the fragment charge correlations. The energy spectra of light charged particles, measured at τlab = 150°, exhibit Maxwellian shapes with inverse slope parameters much higher than the breakup temperatures. The statistical multifragmentation model, because Coulomb repulsion and sequential decay processes are included, yields light-particle spectra with inverse slope parameters higher than the breakup temperatures but considerably below the measured values. The systematic behavior of the differences suggests that they are caused by light-charged-particle emission prior to the final breakup stage.

Breakup temperature of target spectators in 197Au + 197Au collisions at E/A = 1000 MeV

Breakup temperatures were deduced from double ratios of isotope yields for target spectators produced in the reaction Au + Au at 1000 MeV per nucleon. Pairs of $^{3,4}$He and $^{6,7}$Li isotopes and pairs of $^{3,4}$He and H isotopes (p, d and d, t) yield consistent temperatures after feeding corrections, based on the quantum statistical model, are applied. The temperatures rise with decreasing impact parameter from 4 MeV for peripheral to about 10 MeV for the most central collisions. The good agreement with the breakup temperatures measured previously for projectile spectators at an incident energy of 600 MeV per nucleon confirms the observed universality of the spectator decay at relativistic bombarding energies. The measured temperatures also agree with the breakup temperatures predicted by the statistical multifragmentation model. For these calculations a relation between the initial excitation energy and mass was derived which gives good simultaneous agreement for the fragment charge correlations. The energy spectra of light charged particles, measured at $\theta_{lab}$ = 150$^{\circ}$, exhibit Maxwellian shapes with inverse slope parameters much higher than the breakup temperatures. The statistical multifragmentation model, because Coulomb repulsion and sequential decay processes are included, yields light-particle spectra with inverse slope parameters higher than the breakup temperatures but considerably below the measured values. The systematic behavior of the differences suggests that they are caused by light-charged-particle emission prior to the final breakup stage. PACS numbers: 25.70.Mn, 25.70.Pq, 25.75.-q

The effect of helium breakup on two-temperature accretion disks including cooling processes

It is shown that for two-temperature steady state accretion disks with viscosity generated be neutron collisions there is a lower (upper) bound on the viscosity (accretion rate). Assuming 4 He breakup as the source for the neutrons in accretion disks with the usual α viscosity prescription, taking into account cooling due to breakup ignition, thermalization of the reactant particles and radiation losses, we have found absence of steady-state solutions for accretion rates below M˙ c 17 =3.14×10 2 α 2 M 34 . It is also shown that, close to the breakup temperature, unsaturated inverse Comptonization cooling dominates over other processes and breakup is unlikely to occur. For bremsstrahlung, radiation losses are small compared to other cooling processes

Flow effects on transverse momentum spectra in ultrarelativistic nuclear collisions

We discuss fits of the Landau-Milekhin model to the transverse momentum spectra measured in 200 GeV/{ital c} nucleus-nucleus collisions. It is observed that the data are fit by a range of anticorrelated values of the breakup temperature and the average transverse hydrodynamic velocity. These fits indicate that a better understanding of transverse flow in ultrarelativistic nuclear collisions is required to uniquely determine the breakup temperature of the system.

Entropy from fragment yields in nucleus-nucleus collisions around 100 MeV/nucleon.

Recent experimental data for light particle and complex fragment emission are compared to the results of a microcanonical statistical fragmentation model. The entropy produced in the reactions is extracted by varying the breakup temperature and density. The best fit to the data is nearly independent of the temperature and density. The extracted entropy is somewhat higher for light particles than for complex fragments.

Fragmentation of hot quantum drops.

Statistical and quantum dynamical expansion calculations for finite and hot quantum drops are performed. Quantum dynamical evolution trajectories are calculated and compared with static equation of state calculations. Density distributions are studied and evidence for volume dependent multifragmentation processes is obtained for most breakup temperatures. The breakup trajectories in the T,rho plane fluctuate below the critical values of the equation of state results. Small as well as large fragments are found in U-shape spectra. At temperatures above the breakup temperature the quantum drops vaporize and at temperatures below the breakup temperature the hot drops evaporate some nucleons. For averaged initial densities of the order of equilibrium density, matter does not fragment near the critical point. Thus, it appears unlikely that the multifragmentation phenomenon can be used to study the isothermal critical region of liquid-vapor phase transitions.