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Homogeneous nucleation of quark gluon plasma, finite size effects and longlived metastable objects
(1998)
The general formalism of homogeneous nucleation theory is applied to study the hadronization pattern of the ultra-relativistic quark-gluon plasma (QGP) undergoing a first order phase transition. A coalescence model is proposed to describe the evolution dynamics of hadronic clusters produced in the nucle- ation process. The size distribution of the nucleated clusters is important for the description of the plasma conversion. The model is most sensitive to the initial conditions of the QGP thermalization, time evolution of the energy den- sity, and the interfacial energy of the plasma hadronic matter interface. The rapidly expanding QGP is first supercooled by about T = T Tc = 4 6%. Then it reheats again up to the critical temperature Tc. Finally it breaks up into hadronic clusters and small droplets of plasma. This fast dynamics occurs within the first 5 10 fm/c. The finite size e ects and fluctuations near the critical temperature are studied. It is shown that a drop of longitudinally expanding QGP of the transverse radius below 4.5 fm can display a long-lived metastability. However, both in the rapid and in the delayed hadronization scenario, the bulk pion yield is emitted by sources as large as 3 4.5 fm. This may be detected experimentally both by a HBT interferometry signal and by the analysis of the rapidity distributions of particles in narrow pT -intervals at small |pT | on an event-by-event basis. PACS numbers: 12.38.Mh, 24.10.Pa, 25.75.-q, 64.60.Qb
In dieser Arbeit wurden thermodynamische Eigenschaften eines chiralen Quark Meson Modelles untersucht. Das chirale Quark Meson Model beschreibt die starke Wechselwirkung über den Austausch von Mesonen und zudem die thermische und dichteabhängige Entwicklung der Quarkmassen im Medium über die chirale Symmetrie.Im SU(2) Model wurde zunächst in mean field approximation gearbeitet, um im Anschluss den divergenten Vakuumterm mit einzubeziehen. Nach eingehender Untersuchung der Ergebnisse, wurden dann die thermischen Mesonenfluktuationen studiert. In beiden Ansätzen verschiebt die Nullpunktsenergie den chiralen Phasenübergang zu höheren Temperaturen, wodurch die Massen bei höheren Temperaturen entarten. Beide Ansätze wurden dann zu einem gemeinsamen Modell kombiniert, um den Einfluss der Mesonenfluktuationen auf Ordnungsparameter, Massen und thermodynamische Grössen zu untersuchen. Als Fazit der Studie kann behauptet werden, dass sich der Einfluss der Mesonenfluktuationen in grösserem Maÿ auf die Thermodynamik, als auf den Ordnungsparameter und die Massen auswirkt. Im SU(3) Modell wurden ebenfalls regularisiert und zudem Vektormesonen mitberücksichtigt, welche die Repulsion zwischen den einzelnen Freiheitsgraden modelliert. Die Zustandsgleichung wird durch den Vakuum Term etwas softer und zeigt ein ähnliches Verhalten im niederen Energiebereich. Untersucht wurde neben der Temperatur T, die Elektron Baryon Rate Ye, die Sigma Meson Masse noch der Einfluss der Vektorkopplung. Aus der Zustandsgleichung konntendann Isentropen im T-mu Phasendiagramm errechnet werden, welche in naher Zukunft Aufschluss über eine dritte Familie von kompakten Sternen in Zusammenhang mit der entsprechenden Supernova Explosion geben könnte. Um die Existenz von kompakten Sternen genauer zu analysieren, wurde das chiraleSU(3) Quark Meson Modell bei T = 0 benutzt, um über die aus dem Formalismusgewonnenen Grössen Druck und Energiedichte die Tolmann-Oppenheimer-Volkoff zu lösen. Diese stellen die Masse-Radius Beziehungen kompakter Objekte dar. Auf der Suche nach Twin Stern Lösungen aus dem chiralen SU(3) Quark Meson Model wurde zunächst ein Modell für Hybridsterne entwickelt. Im untersuchten Parameterbereich fanden wir Hybrid Stern Lösungen, bei welchen der Einfluss der Quarkmaterie auf die Stabilität des Sternes untersucht wurde, denn das Einsetzen des Phasenüberganges übt einen zusätzlichen gravitativen Zug auf die hadronische Kruste aus. Der Stern ist stabil, wenn der Druck der Quarkmaterie diesem zusätzlichen Zug standzuhalten vermag. Für einen zu grossen Sprung in der Energiedichte werden die Lösungen jedoch instabil. Zwillingssterne waren nicht unter den Lösungen, da der Übergangsdruck relativklein sein muss, während der Energiedichtesprung eher gross sein sollte. Das Auftreten zweier stabiler Äste in der Masse Radius Relation kann allerdingsmit dem SU(3) Modell und entsprechendem chiralen Phasenübergang modelliert werden. Für einen gewissen Parameterbereich einhergehend mit kleinem Wert des Vakuum Druckes B konnten Nicht-Linearitäten in der Zustangsgleichungzur Lösung der TOV Gleichung beitragen. Im Weitern ist das Zusammenspiel der Vektorkopplung und der Sigma Mesonen Masse einflussreich auf die Lösungen, welche auf Kausalität, Stabilität und neben der 2 Sonnenmassen Bedingung noch auf Restriktionen vom millisecond pulsar PSR J1748-2446ad untersucht wurden.Mit Weltraummissionen wie etwa NICER (Neutron star Interior CompositionExploreR) sollte die Radiusbestimmung kompakter Objekte in Zukunft bis auf einen Kilometer genau bestimmt werden können. Die Entdeckung von zweiSternen mit der gleichen Masse und unterschiedlichen Radien wäre in der Tat ein Beweis für die Existenz von Zwillingssternen, welche dann die Theorie des Phasenüberganges in dichter Materie untermauern würde. Das Kollaps-Szenario eines Zwillingssternes würde weiteren Aufschluss über Neutrino-Emmissivität, Gamma-ray burster und Gravitationswellen Signale geben können. Dynamische Simulationen in allgemein relativistischem Kontext für compact star merger mit den hier diskutierten Zustandsgleichungen sind bereits in Planung, um Eigenschaftenwie beispielsweise das Temperatur- und Dichteprofil solcher Objekte genauer zu analysieren.
Problems of production and study of new neutron-enriched heavy nuclei are discussed. Low-energy multinucleon transfer reactions are shown to be quite appropriate for this purpose. Reactions with actinide beams and targets are of special interest for synthesis of new neutron-enriched transfermium nuclei and not-yet-known nuclei with closed neutron shell N = 126 having the largest impact on the astrophysical r-process. The estimated cross sections for the production of these nuclei look very promising for planning such experiments at currently available accelerators. These experiments, however, are rather expensive and difficult to perform because of low intensities of the massive projectile beams and problems of separating and detecting the heavy reaction products. Thus, realistic predictions of the corresponding cross sections for different projectile-target combinations are definitely required. Some uncertainty still remains in the values of several parameters used for describing the low-energy nuclear dynamics. This uncertainty does not allow one to perform very accurate predictions for the productions of new heavier-than-target (trans-target) nuclei in multinucle on transfer reactions. Nevertheless these predictions are rather promising (large cross sections) to start such experiments at available accelerators if the problem of separation of heavy transfer reaction products would be solved.
The present limits of the upper part of the nuclear map are rather close to the beta stability line while the unexplored area of heavy neutron rich nuclides (also those located along the neutron closed shell N = 126 to the right hand side of the stability line) is extremely important for nuclear astrophysics investigations and, in particular, for the understanding of the r-process of astrophysical nucleogenesis. For elements with Z > 100 only neutron deficient isotopes (located to the left of the stability line) have been synthesized so far. The “north-east” area of the nuclear map can be reached neither in fusion–fission reactions nor in fragmentation processes widely used nowadays for the production of new nuclei. Multi-nucleon transfer processes in near barrier collisions of heavy ions seem to be the only reaction mechanism allowing us to produce and explore neutron rich heavy nuclei including those located at the superheavy island of stability. Neutron capture process can be also considered as an alternative method for the production of long-lived neutron rich superheavy nuclei. Strong neutron fluxes might be provided by nuclear reactors and nuclear explosions in laboratory frame and by supernova explosions in nature.
There are only 3 methods for the production of heavy and superheavy (SH) nuclei, namely, fusion reactions, a sequence of neutron capture and beta(-) decay and multinucleon transfer reactions. Low values of the fusion cross sections and very short half-lives of nuclei with Z<120 put obstacles in synthesis of new elements. At the same time, an important area of SH isotopes located between those produced in the cold and hot fusion reactions remains unstudied yet. This gap could be filled in fusion reactions of 48Ca with available lighter isotopes of Pu, Am, and Cm. New neutron-enriched isotopes of SH elements may be produced with the use of a 48Ca beam if a 250Cm target would be prepared. In this case we get a real chance to reach the island of stability owing to a possible beta(+) decay of 291114 and 287112 nuclei formed in this reaction with a cross section of about 0.8 pb. A macroscopic amount of the long-living SH nuclei located at the island of stability may be produced by using the pulsed nuclear reactors of the next generation only if the neutron fluence per pulse will be increased by about three orders of magnitude. Multinucleon transfer processes look quite promising for the production and study of neutron-rich heavy nuclei located in upper part of the nuclear map not reachable by other reaction mechanisms. Reactions with actinide beams and targets are of special interest for synthesis of new neutron-enriched transfermium nuclei and not-yet-known nuclei with closed neutron shell N=126 having the largest impact on the astrophysical r-process. The estimated cross sections for the production of these nuclei allows one to plan such experiments at currently available accelerators.
This Ph. D. thesis with the title "Characterisation of laser-driven radiation beams: Gamma-ray dosimetry and Monte Carlo simulations of optimised target geometry for record-breaking efficiency of MeV gamma-sources" is dedicated to the study of the acceleration of electrons by intense sub-picosecond laser pulses propagating in a sub-millimeter plasma with near-critical electron density (NCD) and resulting generation of the gamma bremsstrahlung and positrons in the targets of different materials and thickness.
Laser-driven particle acceleration is an area of increasing scientific interest since the recent development of short pulse, high-intensity laser systems. The interaction of intense high-energy, short-pulse lasers with solid targets leads to the production of high-energy electrons in the relativistic laser intensity regime of more than 1018 W /cm2. These electrons play the leading role in the first stage of the interaction of laser with matter, which leads to the creation of laser sources of particles and radiation. Therefore, the optimisation of the electron beam parameters in the direction of increasing the effective temperature and beam charge, together with a slight divergence, plays a decisive role, especially for further detection and characterisation of laser-driven photon and positron beams.
In the context of this work, experiments were carried out at the PHELIX laser system (Petawatt High-Energy Laser for Heavy Ion eXperiments) at GSI Helmholtz Center for Heavy-Ion Research GmbH in Darmstadt, Germany. This thesis presents a thermoluminescence dosimetry (TLD) based method for the measurement of bremsstrahlung spectra in the energy range from 30 keV to 100 MeV. The results of the TLD measurements reinforced the observed tendency towards the strong increase of the mean electron energy and number of super-ponderomotive electrons. In the case of laser interaction with long-scale NCD-plasmas, the dose caused by the gamma-radiation measured in the direction of the laser pulse propagation showed a 1000-fold increase compared to the high contrast shots onto plane foils and doses measured perpendicular to the laser propagation direction for all used combinations of targets and laser parameters.
In this thesis I present novel characterisation method using a combination of TLD measurements and Monte Carlo FLUKA simulations applicable to laser-driven beams. The thermoluminescence detector-based spectrometry method for simultaneous detection of electrons and photons from relativistic laser-induced plasmas initially developed by Behrens et al. (Behrens et al., 2003) and further applied in experiments at PHELIX laser (Horst et al., 2015) delivered good spectral information from keV energies up to some MeV, but as it was presented in (Horst et al., 2015) this method was not really suitable to resolve the content of photon spectra above 10 MeV because of the dominant presence of electrons. Therefore, I created new evaluation method of the incident electron spectra from the readings of TLDs. For this purpose, by means of MatLab programming language an unfolding algorithm was written. It was based on a sequential enumeration of matching data series of the dose values measured by the dosimeters and calculated with of FLUKA-simulations. The significant advantage of this method is the ability to obtain the spectrum of incident electrons in the low energy range from 1 keV, which is very difficult to measure reliably using traditional electron spectrometers.
The results of the evaluation of the effective temperature of super-ponderomotive electrons retrieved from the measured TLD-doses by means of the Monte-Carlo simulations demonstrated, that application of low density polymer foam layers irradiated by the relativistic sub-ps laser pulse provided a strong increase of the electron effective temperature from 1.5 - 2 MeV in the case of the relativistic laser interaction with a metallic foil up to 13 MeV for the laser shots onto the pre-ionized foam and more than 10 times higher charge carried by relativistic electrons.
The progressive simulation method of whole electron spectra described with two -temperatures Maxwellian distribution function has been developed and the results of dose simulations were compared with the acquired experimental data. The advanced feature of this method, which distinguishes it from the results of the simulation of the photon spectrum using the interaction with the target of mono-energetic electron beams (Nilgün Demir, 2013; Nilgün Demir, 2019) or the initial electron spectrum expressed as a function of one electron temperature (Fiorini, 2012), is the ability to simulate the initial electron spectrum described by the Maxwellian distribution function with two temperatures.
The important objective of this thesis was dedicated to the study and characterisation of laser-driven photon beams. In addition to this, the positron beams were evaluated. The investigation of bremsstrahlung photons and positrons spectra from high Z targets by varying the target thickness from 10 µm to 4 mm in simulated models of the interactions of electron spectra with Maxwellian distribution functions allowed to define an optimal thickness when the fluences of photons and positrons are maximal. Furthermore based on the results of FLUKA simulations the gold material was found to be the most suitable for the future experiments as e − γ target because of its highest bremsstrahlung yield.
Additionally Monte Carlo simulations were performed applying the obtained electron beam parameters from the electron acceleration process in laser-plasma interactions simulated with particle-in-cell (PIC) code for two laser energies of 20 J and 200 J. The corresponding electron spectra were imported into a Monte Carlo code FLUKA to simulate the production process of bremsstrahlung photons and positrons in Au converter. FLUKA simulations showed the record conversion of efficiency in MeV gammas can reach 10%, which reinforces the generation of positrons. The obtained results demonstrate the advantages of long-scale plasmas of near critical density (NCD) to increase the parameters of MeV particles and photon beams generated in relativistic laser-plasma interaction. The efficiency of the laser-driven generation of MeV electrons and photons by application of low-density polymer foams is essentially enhanced.
Hot hypernuclear matter is investigated in an explicit SU(3) quark model based on a mean field description of nonoverlapping baryon bags bound by the self-consistent exchange of scalar sigma, zeta and vector omega,phi mesons. The sigma, omega mean fields are assumed to couple to the u, d-quarks while the zeta ,phi mean fields are coupled to the s-quark. The coupling constants of the mean fields with the quarks are assumed to satisfy SU(6) symmetry. The calculations take into account the medium dependence of the bag parameter on the scalar fields sigma, zeta. We consider only the octet baryons N,Lambda,Sigma, Xi in hypernuclear matter. An ideal gas of the strange mesons K and K is introduced to keep zero net strangeness density. Our results for symmetric hypernuclear matter show that a phase transition takes place at a critical temperature around 180 MeV in which the scalar mean fields sigma, zeta take nonzero values at zero baryon density. Furthermore, the bag contants of the baryons decrease significantly at and above this critical temperature indicating the onset of quark deconfinement. The present results imply that the onset of quark deconfinement in SU(3) hypernuclear matter is much stronger than in SU(2) nuclear matter. PACS:21.65.+f, 24.85.+p, 12.39Ba
The properties of strange hadronic matter are studied in the context of the modified quark-meson coupling model using two substantially di erent sets of hyperon-hyperon (Y Y ) interactions. The first set is based on the Nijmegen hard core potential model D with slightly attractive Y Y interactions. The second potential set is based on the recent SU(3) extension of the Nijmegen soft-core potential NSC97 with strongly attractive Y Y interactions which may allow for deeply bound hypernuclear matter. The results show that, for the first potential set, the hyperon does not appear at all in the bulk at any baryon density and for all strangeness fractions. The binding energy curves of the resulting N system vary smoothly with density and the system is stable (or metastable if we include the weak force). However, the situation is drastically changed when using the second set where the hyperons appear in the system at large baryon densities above a critical strangeness fraction. We find strange hadronic matter undergoes a first order phase transition from a N system to a N for strangeness fractions fS > 1.2 and baryonic densities exceeding twice ordinary nuclear matter density. Furthermore, it is found that the system built of N is deeply bound. This phase transition a ects significantly the equation of state which becomes much softer and a substantial drop in energy density and pressure are detected as the phase transition takes place. PACS:21.65.+f, 24.85.+p, 12.39Ba
A new technique developed for measuring nuclear reactions at low momentum transfer with stored beams in inverse kinematics was successfully used to study isoscalar giant resonances. The experiment was carried out at the experimental heavy-ion storage ring (ESR) at the GSI facility using a stored 58Ni beam at 100 MeV/u and an internal helium gas-jet target. In these measurements, inelastically scattered α-recoils at very forward center-of-mass angles (θcm ≤ 1.5°) were detected with a dedicated setup, including ultra-high vacuum compatible detectors. Experimental results indicate a dominant contribution of the isoscalar giant monopole resonance at this very forward angular range. It was found that the monopole contribution exhausts 79+12−11% of the energy-weighted sum rule (EWSR), which agrees with measurements performed in normal kinematics. This opens up the opportunity to investigate the giant resonances in a large domain of unstable and exotic nuclei in the near future. It is a fundamental milestone towards new nuclear reaction studies with stored ion beams.
Fossil dental remains are an archive of unique information for paleobiological studies. Computed microtomography based on X-ray microfocus sources (X-μCT) and Synchrotron Radiation (SR-μCT) allow subtle quantification at the micron and sub-micron scale of the meso- and microstructural signature imprinted in the mineralized tissues, such as enamel and dentine, through high-resolution “virtual histology”. Nonetheless, depending on the degree of alterations undergone during fossilization, X-ray analyses of tooth tissues do not always provide distinct imaging contrasts, thus preventing the extraction of essential morphological and anatomical details. We illustrate here by three examples the successful application of neutron microtomography (n-μCT) in cases where X-rays have previously failed to deliver contrasts between dental tissues of fossilized specimen.