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J. Helgesson

Publications and source records attributed to J. Helgesson.

12 recordsLinked to original sources
arXiv2006

Correlations and Characterization of Emitting Sources

Dynamical and thermal characterizations of excited nuclear systems produced during the collisions between two heavy ions at intermediate incident energies are presented by means of a review of experimental and theoretical work performed in the last two decades. Intensity interferometry, applied to both charged particles (light particles and intermediate mass fragments) and to uncharged radiation (gamma rays and neutrons) has provided relevant information about the space-time properties of nuclear reactions. The volume, lifetime, density and relative chronology of particle emission from decaying nuclear sources has been extensively explored and has provided valuable information about the dynamics of heavy-ion collisions. Similar correlation techniques applied to coincidences between light particles and complex fragments are also presented as a tool to determine the internal excitation energy of excited primary fragments as it appears in secondary-decay phenomena.

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arXiv2005

Light cluster production in E/A = 61 MeV 36Ar + 112,124Sn reactions

Experimental kinetic energy distributions and small-angle two-particle correlation functions involving deuterons and tritons are compared for 36Ar+ 112,124Sn collisions at E/A = 61 MeV (i.e. for systems similar in size, but with different isospin content). A larger triton yield is observed from the more neutron-rich system, as predicted by IBUU simulations, while the emission times of the light clusters are found to be the same for the two Sn-target systems. For both systems, the time sequence tau_{d} < tau_{p} < tau_{t}, is deduced for charged particles emitted from the intermediate velocity source.

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arXiv2005

The complement: a solution to liquid drop finite size effects in phase transitions

The effects of the finite size of a liquid drop undergoing a phase transition are described in terms of the complement, the largest (but still mesoscopic) drop representing the liquid in equilibrium with the vapor. Vapor cluster concentrations, pressure and density from fixed mean density lattice gas (Ising) model calculations are explained in terms of the complement. Accounting for this finite size effect is key to determining the infinite nuclear matter phase diagram from experimental data.

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arXiv2004

Correlation functions and emission time sequence of light charged particles from projectile-like fragment source in E/A = 44 and 77 MeV 40Ar + 27Al collisions

Two-particle correlation functions, involving protons, deuterons, tritons, and alpha-particles, have been measured at very forward angles (0.7 deg < theta_lab < 7 deg), in order to study projectile-like fragment (PLF) emission in E/A = 44 and 77 MeV 40Ar + 27Al collisions. Peaks, originating from resonance decays, are larger at E/A = 44 than at 77 MeV. This reflects the larger relative importance of independently emitted light particles, as compared to two-particle decay from unstable fragments, at the higher beam energy. The time sequence of the light charged particles, emitted from the PLF, has been deduced from particle-velocity-gated correlation functions (discarding the contribution from resonance decays). Alpha-particles are found to have an average emission time shorter than protons but longer than tritons and deuterons.

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arXiv2004

Analysis of charged particle emission sources and coalescence in E/A = 61 MeV $^{36}$Ar + $^{27}$Al, $^{112}$Sn and $^{124}$Sn collisions

Single-particle kinetic energy spectra and two-particle small angle correlations of protons ($p$), deuterons ($d$) and tritons ($t$) have been measured simultaneously in 61A MeV $^{36}$Ar + $^{27}$Al, $^{112}$Sn and $^{124}$Sn collisions. Characteristics of the emission sources have been derived from a ``source identification plot'' ($β_{source}$--$E_{CM}$ plot), constructed from the single-particle invariant spectra, and compared to the complementary results from two-particle correlation functions. Furthermore, the source identification plot has been used to determine the conditions when the coalescence mechanism can be applied for composite particles. In our data, this is the case only for the Ar + Al reaction, where $p$, $d$ and $t$ are found to originate from a common source of emission (from the overlap region between target and projectile). In this case, the coalescence model parameter, $\tilde{p}_0$ -- the radius of the complex particle emission source in momentum space, has been analyzed.

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arXiv2003

Isospin effects on two-particle correlation functions in E/A = 61 MeV 36Ar + 112,124Sn reactions

Small-angle, two-particle correlation functions have been measured for 36Ar+ 112,124Sn collisions at E/A = 61 MeV. Total momentum gated neutron-proton (np) and proton-proton (pp) correlations are stronger for the 124Sn-target. Some of the correlation functions for particle pairs involving deuterons or tritons (nd, pt, and nt) also show a dependence on the isospin of the emitting source.

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arXiv1998

Transport Simulations with Pi and Delta In-Medium Properties

Transport simulations including in-medium properties derived in a microscopic pi + nucleon-hole + delta-hole model in infinite nuclear matter are presented. In-medium pion dispersion relations, partial delta decay widths, pion absorption cross sections and delta cross sections are incorporated into the transport description by means of a local-density approximation. Strong modifications of pi and delta production and absorption rates are found, but only small effects on pion observables.

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arXiv1997

Effects of Spin-Isospin Modes in Transport Simulations

In-medium properties derived for nuclear matter in a microscopic pi + nucleon-hole + delta-hole model are incorporated into transport simulations of nuclear collisions by means of a local-density approximation and by utilizing a local medium frame. Certain features of the transport results differ from those based on the corresponding vacuum properties. Comparisons of the pi and delta production rates, as well as pion energy spectra, are discussed in particular.

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arXiv1997

Quantum-Statistical Correlations and Single Particle Distributions for Slowly Expanding Systems with Temperature Profile

Competition among particle evaporation, temperature gradient and flow is investigated in a phenomenological manner, based on a simultaneous analysis of quantum statistical correlations and momentum distributions for a non-relativistic, spherically symmetric, three-dimensionally expanding, finite source. The parameters of the model emission function are constrained by fits to neutron and proton momentum distributions and correlation functions in intermediate energy heavy-ion collisions. The temperature gradient is related to the momentum dependence of the radius parameters of the two-particle correlation function, as well as to the momentum-dependent temperature parameter of the single particle spectrum, while a long duration of particle evaporation is found to be responsible for the low relative momentum behavior of the two-particle correlations.

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arXiv1995

Treatment of Pionic Modes at the Nuclear Surface for Transport Descriptions

Dispersion relations and amplitudes of collective pionic modes are derived in a pi + nucleon-hole + delta-hole model for use in transport descriptions by means of a local density approximation. It is discussed how pionic modes can be converted to real particles when penetrating the nuclear surface and how earlier treatments can be improved. When the surface is stationary only free pions emerge. The time-dependent situation is also addressed, as is the conversion of non-physical (i.e. unperturbed delta-hole) modes to real particles when the nuclear density vanishes. A simplified one-dimensional scenario is used to investigate the reflection and transmission of pionic modes at the nuclear surface. It is found that reflection of pionic modes is rather unlikely, but the process can be incorporated into transport descriptions by the use of approximate local transmission coefficients.

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arXiv1994

Spin-Isospin Modes in Heavy-Ion Collisions I: Nuclear Matter at Finite Temperatures

With a view towards implementation in microscopic transport simulations of heavy-ion collisions, the properties of spin-isospin modes are studied in nuclear matter consisting of nucleons and Delta isobars that interact by the exchange of pi and rho mesons. For a standard p-wave interaction and an effective g' short-range interaction, the dispersion relations for the spin- isospin modes, and the associated amplitudes, are calculated at various nuclear densities and temperatures, within the random-phase approximation. Quantities of physical interest are then extracted, including the total and partial Delta decay widths and the Delta cross sections in the nuclear medium. The self-consistent inclusion of the Delta width has a strong effect on the Delta cross sections at twice normal nuclear density, as compared with the result of ignoring the width. Generally, the obtained quantities exhibit a strong density dependence, but are fairly insensitive to the temperature, at least up to T=25 MeV. Finally, it is described how these in-medium effects may be consistently included into microscopic transport simulations of nuclear collisions, and the improvements over previous approaches are discussed.

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