P. Zambon

Results of the ASY-EOS experiment at GSI : the symmetry energy at suprasaturation density

Directed and elliptic flows of neutrons and light charged particles were measured for the reaction 197Au+197Au at 400 MeV/nucleon incident energy within the ASY-EOS experimental campaign at the GSI laboratory. The detection system consisted of the Large Area Neutron Detector LAND, combined with parts of the CHIMERA multidetector, of the ALADIN Time-of-flight Wall, and of the Washington-University Microball detector. The latter three arrays were used for the event characterization and reaction-plane reconstruction. In addition, an array of triple telescopes, KRATTA, was used for complementary measurements of the isotopic composition and flows of light charged particles. From the comparison of the elliptic flow ratio of neutrons with respect to charged particles with UrQMD predictions, a value \gamma = 0.72 \pm 0.19 is obtained for the power-law coefficient describing the density dependence of the potential part in the parametrization of the symmetry energy. It represents a new and more stringent constraint for the regime of supra-saturation density and confirms, with a considerably smaller uncertainty, the moderately soft to linear density dependence deduced from the earlier FOPI-LAND data. The densities probed are shown to reach beyond twice saturation.

Investigation of the Dependence of CsI(Tl) Scintillation Time Constants and Intensities on Particle's Energy, Charge and Mass Through Direct Fitting of Digitized Waveforms

The dependence of the CsI(Tl) scintillation time constants and intensities on the particle ionization profile is at the basis of the Charge and Mass identification technique for Light Charged Products in Intermediate Energy Nuclear Physics multi-detector arrays. The possibility of storing the digitized waveforms in true beam experiments allows performing further offline analyses and makes available a data set related to the interaction of different particle types in a wide energy range.

Experimental investigation of the impact of inter-strip incidence on the signal shape in Double Sided Silicon Strip Detectors for particle identification

We are currently developing a novel detection system, named FARCOS (Femtoscope Array for Correlation and Spectroscopy) and featuring high angular and energy resolution and able to reconstruct particles momentum at high precision for different physical cases in multi-fragmentation nuclear physics experiments. Performing a Pulse Shape Analysis on signals coming from the ΔE stages -Double Sided Silicon Strip Detectors (DSSSDs) - we aim at identifying in charge and mass even particles stopping in the first detection layer. In the framework of the full characterization of the DSSSD response in terms of both the linearity of the amplitude response and of the efficiency and collection properties as a function of the point of incidence, we got evidences of significant alterations of the signals shape in case of particle injection in the inter-strip region. Such an effect, which depends also on the type and energy of the impinging particle, cannot be ascribed only to a mere charge division, especially in case of signals with “wrong” polarity, but to more complex phenomena - for instance charge induction on neighbor strips. In this work we present the results of the qualification campaign carried out exploiting the monochromatic pulsed proton beam facility available at LaBeC of INFN - Sezione di Firenze, focusing the attention on the aforementioned “inter-strip effect”. We interpreted the measurements also with the aid of dedicated 3D simulations performed with a custom semi-analytical code, which gives a better insight on the physics occurring in the device. The impact of the “inter-strip effect” on particle identification scatter plots is also discussed.

A 3-D simulation code of electron-hole transport and signal formation with coulomb repulsion and thermal diffusion in 2-D semiconductor detectors

We developed a simulation code for electron-hole transport and signal formation in semiconductor detectors having 2D geometry, either in linear or cylindrical coordinates. Thermal diffusion and Coulomb interaction between the carriers - essential for the correct simulation of high-density ionization tracks - have been included preserving their full 3D nature. In this way we can solve transport problems in the frequent case of 2D detector geometry with a precise treatment of the Coulomb interaction up to high charge levels. The induced current and induced charge signals at arbitrary electrodes are obtained with a microscopic approach, i.e. by computing the trajectories of the individual carriers. Extensive use of parallel programming has been employed in order to optimize the code for multi-core CPUs. As a realistic case study we present the space-time evolution of the electron-hole cloud resulting from a 7.5 MeV 7Li ion impinging on a double-sided silicon strip detector meant for the identification of charged particles in nuclear physics experiments.

Simulation of the 3-D Coulomb explosion of the electron-hole distribution at high injection levels in 2-D semiconductor detectors

We developed a novel simulation code for 3-D electron-hole transport and signal formation in semiconductor detectors. The code includes thermal diffusion and Coulomb interaction between the carriers - essential for the correct simulation of high-density ionization tracks. The present work focuses on dedicated numerical techniques to correctly simulate the explosion of high-density charge carrier clouds down to the first instants of the time evolution, when extremely high field intensities are reached due to Coulomb interaction between carriers. The case study of a p+nn+ diode irradiated by 1 keV photons on under different bias and charge injection conditions up to 107 e.h pairs is presented and discussed. This case well represents the operating conditions foreseen in the imaging detectors for the novel FEL X-ray facilities and the potential of the developed simulation code.

FARCOS: A versatile and modular Femtoscopy Array for Correlations and Spectroscopy

In the framework of multi-fragmentation experiments the evolution towards two(or more) particle correlations with stable and radioactive beams calls for the development of a novel detection system featuring high angular and energy resolution and able to reconstruct the particles momentum at high precision. The proposed detection system, named FARCOS (Femtoscopy ARray for COrrelations and Spectroscopy) will be beneficial for different physical cases. To this aim we are building a prototype detection system featuring four telescopes. Each telescope features an active area of 6.4 cm × 6.4 cm and is composed of three detection stages. The first ΔE stage is a Double Sided Silicon Strip Detector (DSSSD), 300 μm thick, featuring 32 × 32 strips. The second ΔE stage is again a DSSSD, 1500 μm thick, featuring 32 × 32 strips. The third stage, acting as calorimeter, is composed by four CsI(TI) crystals with an active area of 3.2 cm × 3.2 cm and an absorption length of 6 cm forming a 2 × 2 matrix. The scintillators are readout by a Silicon photodiode 300 μm thick. The paper presents the relevant features of FARCOS and the expected performance of the silicon detection layers, with a special focus on the requirements of the frontend electronics for the DSSSD and the qualification of the DSSSD. In addition we report the first results of the preliminary on-beam tests.

Laser mapping of the inter-strip response in double sided silicon strip detectors for particle identification

Silicon microstrips are widely used in nuclear physics experiments when high granularity and high resolution is required (e.g. particle-particle correlation). In addition microstrip detectors are used in the field of X-ray detection for position resolved X-ray spectroscopy. In the framework of the construction of a novel Femtoscope ARray for Correlation and Spectroscopy, named FARCOS, we are performing a thorough characterization of the silicon detector layers in terms of the efficiency and charge collection properties as a function of the point of incidence and type of interaction. We have experimental evidence—as previously observed also in the literature for other strip detectors—that inter-strip incidence alters the signal shape not only for the trivial charge division but also affecting the shape of the induced signal on neighbor strips. The phenomenon can be more or less pronounced depending on the energy and ion type but it is always present. In order to surgically probe the response of the inter-strip region we tested the detector both in back and in front injection with an infrared pulsed laser at two different wavelengths (705 nm and 904 nm). Depending on the side of interaction, on the position of incidence and on the absorption length, the shapes of the induced signals are significantly altered and signals of opposite polarity or bipolar signals arise. In order to get a complete picture of the physical phenomena at the basis of the aforementioned behavior we performed detailed 3D device simulations with a custom semi-analytical code and compared the results with the experimental data. The paper focuses on the description of the experimental measurements and their exhaustive description and interpretation.

Probing the Merits of Different Event Parameters for the Identification of Light Charged Particles in CHIMERA CsI(Tl) Detectors With Digital Pulse Shape Analysis

We investigated the merits of different event parameters in the identification of Light Charged Particles (LCPs) with CsI(Tl) scintillators read out by photodiodes at high incident energy (400 MeV/u). This investigation is made possible by digital signal processing the output signals. As in the conventional analogue case, the digitized signals allow the discrimination of light charged particles by computing the fast and slow components. In addition other identification parameters as the rise time of the output pulses of the CsI(Tl) come out nearly for free. Aim of this paper is the investigation of novel identification plots and the probe of their merits, in particular at relativistic energies.

Mapping the amplitude and position response of double sided silicon strip detectors with monochromatic single protons

We are currently developing a novel detection system featuring high angular and energy resolution and able to reconstruct the particles momentum at high precision for different physical cases in multi-fragmentation nuclear physics experiments, based on Double Sided Silicon Strip Detectors as ΔE stages aimed at performing also pulse shape analysis for fragments stopping therein. We carried out a detailed qualification of the performance of the strip detectors. In order to provide the detector response matrix we used a pulsed monoenergetic proton beam. The paper reviews the pulsed proton beam facility of the LaBeC of INFN - Sezione di Firenze and present the results of the characterization of the DSSSD prototypes with the pulsed proton beam. In particular we will discuss the amplitude and time response mapping and we will present the analysis of the dependence of the signal shape as a function of the position of interaction.

Probing the Symmetry Term of the Nuclear Equation of State at High Baryonic Densities

In the ASY-EOS experiment flows of neutrons and light charged particles were measured for 197Au+197Au collisions at 400 MeV/nucleon, in order to investigate the strength of the symmetry term of the nuclear equation of state at supra-saturation densities. By comparing the experimental data with the UrQMD transport model predictions, we have extracted a new constraint in agreement with the moderately soft to linear density dependence obtained in the former analysis on FOPI-LAND data, but reducing the associated uncertainty by a factor ~ 2.