Research Activity

     Physics

The high performances of CHIMERA allow to investigate different aspects of nuclear collisions between heavy ions at intermediate energies. The experiments involve both stable and unstable beams produced by in-flight fragmentation (FRIBS@LNS) and the new FRAISE@LNS facility. Here is a (non-exhaustive) list of the main investigated topic:

• Study of the isospin influence and mass dependence of the reaction mechanisms both at Fermi [8,9,10,11] and low energies (≈10 AMeV) [12].
• Study of the isospin dependence of the asymmetry term of the nuclear equation of state both at low (LNS) [13] and at high nuclear density (GSI-Darmstadt) [14].
• Study of Isospin equilibration phenomenon at Fermi energies [15].
• Study of population and nuclei decay, looking for cluster structures in exotic nuclei [16] and stable nuclei [17,18].
• Study of gamma decay of the Pygmy Dipole Resonance in neutron rich isotope as ⁶⁸Ni [19].
• Study of the population and decay of states of astrophysical interest, eg. Hoyle state of ¹²C, both in gamma decay channels and in many particle decay channels [20].
• Study of the excitation function of the fusion barrier with indirect methods (distribution of the height of the fusion barrier) [21].
• Development of methods for the nuclear density determination in neck fragmentation processes [22].

Figure 1: Neutron enrichment of IMF fragment emitted in neck fragmentation processes, compared to fragments emitted in sequential processes (left). Comparison of the dynamic component with an SMF model for different parameterizations of the symmetry energy (right) [13].

References

[1] A. Pagano et al., Nucl. Phys. A 681, 331 (2001).

[2] A. Pagano et al., Nucl. Phys. A 734, 504c (2004).

[3] E. Geraci et al., Nucl. Phys. A 732, 173 (2004).

[4] E. De Filippo and Pagano A. Eur. Phys. Jour. A 50 (2014) 32.

[5] A. Pagano, Nuclear Physics News International, 22:1 (2012)25.

[6] E.V. Pagano et al., Nucl. Instr. and Methods A 889, 83 (2018).

[7] E.V. Pagano et al, Nucl. Instr. and Methods A 905, 47 (2018).

[8] E. De Filippo et al., Phys. Rev. C 71, 044602 (2005).

[9] E. De Filippo et al., Phys. Rev. C 71, 064604 (2005).

[10] F. Amorini et al., Phys. Rev. Lett. 102, 112701 (2009).

[11] G. Cardella et al., Phys. Rev. C 85, 012093 (2012).

[12] S. Pirrone et al., Eur. Phys. Jour. A 55, 22 (2019).

[13] E. De Filippo et al., Phys. Rev. C 86, 014610 (2012).

[14] P. Russotto et al. Phys. Rev. C 94, 034608 (2016).

[15] M. Papa et al. Phys. Rev. C 91, 041601(R) (2015).

[16] D. Dell'aquila et al, Phys. Rev. C 93, 024611c(2016).

[17] Ad.R. Raduta et al.,  Physics Letters B 705 (2011); B.Borderie et al., Phys. Lett. B 755 (2016).

[18] J. Bishop et al., Phys Rev. C 100, 034320 (2019).

[19] N.S. Martorana et al., Phys. Lett. B782 (2018) 112.

[20] G. Cardella et al., EPJ Web of Conf. 165, 01009 (2017).

[21] A. Trzcinska et al., Acta Phys. Pol. B 49, 393 (2018).

[22] A. Pagano et al., Eur. Phys. J. A 56, 102 (2020).

     The experiments

ASY-EOS

The ASY-EOS experiment was carried out at the GSI laboratory in Darmstadt (Germany) in May 2011, using a complex detection system consisting of:

• the GSI LAND detector for neutrons and charged particles;
• 8 rings of the CHIMERA multi-detector;
• the plastic scintillator wall of the GSI Aladin detection apparatus (AToF-Wall);
• 4 rings of the MicroBall detector from Washington University (USA);
• 35 modules of the KraTTA hodoscope (Kraków, Poland).

As shown schematically in the following image:

In order to study the behavior of the symmetry energy at densities higher than saturation density (ρ₀), the elliptic flow of neutrons and charged particles was measured in Au+Au, Ru+Ru, and Zr+Zr collisions at 400 MeV/A. By comparing the experimental results with QMD-type transport models, a constraint on the symmetry energy was obtained, summarized in a value of L - the slope of the symmetry energy around saturation density - equal to 72 ± 13 MeV, which represents one of the few laboratory-derived constraints on high-density symmetry energy. This result allows for an estimation of the Mass-Radius relationship of a neutron star, a topic that has become of great interest in recent years in the era of "Multi-Messenger Astronomy," ushered in by the recent discovery of gravitational waves and data produced by new X-ray telescopes aboard space stations.

The CHI-NEXT group plans to perform new measurements on this topic at GSI within the R3B collaboration, aiming to produce new and more stringent constraints on the symmetry energy at higher densities than previously tested.

More details at

1. Russotto et al., Phys. Rev. C 034608 (2016)
2. Russotto et al., arXiv:2105.09233 [nucl-ex]

CHIFAR

The CHIFAR experiment takes its name from the first use in coupling of the 4π CHIMERA detector and the FARCOS correlator (in its final version), which are the two detection devices for charged particles of the CHI-NEXT Collaboration. CHIFAR, whose spoke-persons are: Emanuele V. Pagano, Enrico De Filippo and Paolo Russotto, was fully realized in November 2019 at the Laboratori Nazionali del Sud di Catania using the superconducting cyclotron as a particle accelerator. With CHIFAR we study the dynamic evolution of nuclear reactions involving heavy ions. The collisions involved were: ¹²⁴Sn+⁶⁴Ni, ¹²⁴Sn+⁶⁴Zn, ¹¹²Sn+⁵⁸Ni,  at the energy of 20 AMeV. Thanks to the comparison between neutron rich systems and neutron poor ones, it is possible to highlight the role of Isospin in the Nucleus-Nucleus interaction, which is an important degree of freedom in the study of the equation of state of nuclear matter. The study also plays a role in nuclear astrophysics, particularly in the study of exotic astrophysical objects such as neutron stars. The CHIFAR experiment takes place in the groove traced by the REVERSE / ISOSPIN experiments, performed with CHIMERA in the 2000 and 2004 campaigns, and INKIISSY experiment performed in the 2013 campaign in which the same colliding systems were studied at the higher energy of 35 AMeV.

CluB

The aim of the experiment is the investigation of exotic cluster configurations in the Boron isotope ¹³B. The formation of such structures is favoured in light neutron rich systems and in particular, approaching the drip line, as consequence of exotic clustering configurations, the nuclei are more unstable, strongly deformed and easy to break-up. According to the calculation of the Antisymmetrized Molecular dynamics model, the ground state of this particular isotope of the Boron, which has a magic number of neutrons N=8, is a 3/2^- state with a normalp-shell closure configuration while the excited states of ¹³B exhibit cluster configurations, ¹²Be+p, ⁹Li+α, ¹⁰Be+t, with large deformation. Other exotic configurations, not theoretically predicted, are possible, as for example ⁷Li+⁶He, which was already observed, but with very low statistic, in a previous experiment performed by the CHIMERA group at Laboratori Nazionali del Sud. In order to study the exotic clustering configurations and their relative branching ratio, break-up reactions of 55 MeV/nucleon ¹³B beam, impinging on CH₂ target (proton), will be used.The isotope of ¹³B will be produced in a cocktail beam, with a suitable production rate, by the LNS - FRAISE facility, with ¹⁸O primary beam delivered by K-800 Superconductive Cyclotron. Each isotope produced by projectile fragmentation will be identified by a tagging system, consisting in a Micro Channel Plate (MCP) and a Double Sided Silicon Strip Detector (DSSSD, 140 μm thick), installed along the beam line. The unambiguous identification of fragmentation products will be obtained by correlating the energy loss (∆E) in the DSSSD and the time of flight (TOF) from the MCP to the DSSSD (≈13 m). In order to investigate the different cluster configurations of ¹³B, the reconstruction of the complete event, an accurate isotopic identification of the reaction products and a very good angular resolution, especially at forward angles is necessary. For these reasons, the experiment will be performed by using the powerful array FARCOS. In particular, 8 FARCOS telescopes will be allocated at around zero degrees, coupled to the full 4π CHIMERA array, in order to be able to select with very good accuracy the true break-up channels by detecting and identifying all the produced fragments and the recoiling proton. The experiment was approved by the LNS-PAC, but because of the pandemic emergency and of the upgrading of different LNS devices and facility, it was postponed.

CLIR

The CLIR (Cluster in Light Ion Reactions) experiment was performed with the aim to investigate the cluster structure in light neutron-rich radioactive nuclei. It is in fact well known in literature that self-conjugated nuclei, such as ⁸Be, ¹²C, ¹⁶O and ²⁰N, can present a cluster structure of α particles, as a consequence of the reorganization of nucleons in much more stable sub-units. Furthermore, cluster structures have been observed even in neutron-richer isotopes, in which the “extra” nucleons can act as valence particles between such α particles sub-units, exchanged among them, in a similar way to the exchange of valence electrons in molecules. Moreover, the reorganization of nucleons can also cause the formation of exotic structures, like in carbon neutron-rich isotopes, in which 3α particles are arranged linearly or triangularly, while the valence neutrons reside among them.The CLIR experiment was performed at INFN-Laboratori Nazionali del Sud by producing a Radioactive Ion Beam by means of the FRIBs facility; a cocktail beam was produced by fragmenting an ¹⁸O (55 MeV/u) primary beam on a ⁹Be 1000 μm-thick fragmentation target, containing several isotopes of interest, such as ⁶He, ^7,8,9Li, ^10,11,12Be, ^13,14,15B and ^16,17C. Several study cases are in fact present in this cocktail beam, such as ¹⁰Be, for which states have been already studied previously at LNS, suggesting the existence of a 13.5 MeV state, Carbon isotopes, for which previous results have not been able to give exhaustive confirmation on the existence of some suggested states due to low detection yield at small angles, Boron isotopes, like ¹³B, for which several exotic cluster states have been theorized (see CLuB experiment). Tagging of the cocktail beam ions was done using the ΔE-TOF technique, performed by means of a tagging system formed by a MCP detector and a DSSSD 156 μm-thick, allowing to perform identification in change and mass of the several isotope species. Experimental data was gathered by four FARCOS arrays, coupled with the CHIMERA 4π multidetector. In particular, two reaction targets were used: a (CH₂)_n polyethylene 50 μm-thick target and a ¹²C 75 μm-thick target. Due to the reaction kinematic, forward focused with a rather narrow cone, the FARCOS arrays were placed at small angles, between 1° and 8.5° polar angles, allowing to gather most of the reaction products. Calibrations of the FARCOS stages are currently nearing completion, while also a preliminary work on ion selection has been performed, showing evidence of break-up products for ¹⁰Be clustering state (⁶He + ⁴He).

HOYLE

The aim of this experiment was the branching ratio measurement of the γ-decay of the Hoyle state and of the 9.64 3−α excited level of 12C. The γ-decay of 12C levels above the particle emission threshold plays a crucial role in the production of 12C in astrophysical environments. The Hoyle state is important in the helium burning phase, while the 9.64 MeV level can be involved in higher temperature explosive environments. The experiment was carried out at the INFN-LNS using an α beam at 64 MeV on a carbon target. The CHIMERA multidetector was used to detect both γ and charged products. The results about this experiment are reported in: G.Cardella et al., "The γ decay of the Hoyle and higher excitation energy
states of 12C", J. Phys.: Conf. Ser. 1643 (2020) 012145; G. Cardella etal., Journ. of Phys.: Conf. Ser. 1668, 012004 (2020); F. Favela et al.,“First results of the Hoyle-Gamma experiment: study of the excited levels in carbon-12 gamma decay”, Journal of Physics: Conf. Series 1078 (2018) 012010; G. Cardella et al., in print in PRC.

Pygmy

The goal of the Pygmy experiment was the investigation of the γ-decay of the Pygmy Dipole Resonance (PDR) in the unstable nucleus 68Ni using an isoscalar probe, i.e. a carbon target. The excitation has been
produced in reactions where a 68Ni beam, obtained by the fragmentation of a 70Zn primary beam at INFN-LNS, impinged on a carbon target. The γ decay was detected using the CsI(Tl) detectors of the CHIMERA
 multidetector sphere. The 68Ni isotope as well as other heavy ion fragments were detected using the FARCOS array. The study of the PDR in neutron rich nuclei is important for several aspects of nuclear physics.
This mode has a strong relation with the symmetry energy and it has been used as a further tool to constrain it. Furthermore, the PDR might have a remarkable influence on the astrophysical r-process nucleo-synthesis,
since the presence of an even small dipole strength around the neutron separation energy strongly enhances the neutron capture cross sections.
The results about this experiment are reported in: On the nature of the Pygmy Dipole Resonance in 68Ni”, Il Nuovo Cimento 41 C (2018) 199; N.S.
Martorana et al., "Experimental study of the pygmy dipole resonance in the 68Ni nucleus", Acta Physica Polonica B 49 (2018) 475; N.S. Martorana et al., "First measurement of the isoscalar excitation above the neutron emission threshold of the Pygmy   Dipole Resonance in 68Ni". Physics
Letters B 782 (2018) 112–116.

SIKO

This experiment was performed with the aim of experimentally investigating alpha condensation in light nuclei. For alpha conjugate nuclei, when the density is below a specific value relative to the nuclear saturation density, it was theoretically postulated a phase change in the structure of the nucleus, from nuclear liquid to a diffuse gas of alpha particles. Below this value of density, the dominant degrees of freedom are those of alpha particles and for their bosonic nature, it is possible to describe the ground state of the nucleus as an exotic system analogous to the atomic Bose –Einstein condensate.The more dilute structure modifies the penetrability factor of the Coulomb barrier, with the consequence of enhancing the emission of alpha gas states. Moreover, in the decay of these states, as the result of the reduction of the Coulomb barrier, the products are emitted with a kinetic energy lower than those coming from a geometric cluster configuration. In order to search for α-gas states in ¹²C, ¹⁶O, ²⁰Ne, ²⁴Mg, and ²⁸Si, the SIKO experiment was performed in Catania, at Laboratori Nazionali del Sud, by using FARCOS detector coupled to the 4π CHIMERA apparatus and a beam of ¹⁶O delivered by the Superconductive Cyclotron at different energies, 160, 280 and 400 MeV, impinging on a Carbon target. The analysis did not provide any evidence of alpha condensation, in particular, for the heavier isotopes, probably the underlying fermionic structure of the α particles is sufficient to prevent this α-gas nature. 

BARRIERS

This experiment was realized at INFN-Labotatori Nazionali del Sud, by using the CHIMERA 4π multidetector, in order to measure the barrier distribution for the systems ²⁴Mg+⁹⁰Zr and ²⁴Mg+⁹²Zr, by backscattering method.According to this method the barrier is obtained by the dependence on the beam energy, of the number of the projectiles quasielastically backward emitted, normalized to the Rutherford scattered ions.The measurements were performed at different TANDEM beam energies, in the step of 0.5 MeV, spanning the range, in the laboratory reference frame, between 68 and 88.5 MeV.The experimental results were compared to the theoretical predictions of the coupled channels calculations and a strong discrepancy was found. The standard couple channels method predicts very similar shape of the barrier distributions for both targets, because according to this method, significant structures are due to the strong deformation of the projectile ²⁴Mg, which consists of six alpha particles.Experimentally it was found that the structures are visible in the case of the ⁹⁰Zr target which has a lower level density compared to the target of ⁹²Zr, for which the structures are smoothed out by the excitations of the single particle levels. A possible explanation of this discrepancy is that the dissipation of the kinetic energy of the relative motion between the projectile and the target into internal degrees of freedom of the nuclei, strongly influences the barrier tunnelling.A good agreement of the experimental data with the theoretical model is achieved by merging the coupled channels model to the random matrix theory, for which the dissipation of the energy is taken into account.

EQUILIBRATION and EQUILIBRATION-2

These experiments concern the study of the isospin equilibration, coming during the nuclear reaction, comparing symmetric and asymmetric systems respect to the target and projectile N/Z ration, as for example in the reactions  40Ca+27Al and 40Ca+40Ca at 40 MeV/A.

The isospin equilibration processi is to be associated to the nuclear systems spontaneus trend to equalize the ratio N/Z in the phases space. Besides, it depends in a peculiar way from the isovectorial part of the interaction, that is from the symmetry energy at the involved densities. For this reason this phenomenon can be used to investigate on the interaction forces in extreme condition of density and on varying  the exitation energy.

In particular in the EQUILIBRATION experiment the  aim was the measurement of the global variable <Dz>, defined as the medium value of a class of selected events of the derivate of the total dipole momentum evaluated in the asinptotic phase in the beam direction. In previous studies it has been shown, as this variable is associable to the dynamic of the equilibratio process and as it is sensible to the symmetry term of the nuclear matter EOS.

The EQUILIBRATION data were compared with the results of the CoMD-III model (molecular dynamics). Analysis and results for binary process are reported in [1]. Good fit results were obtained to consider  for the symmetry energy term Esym, and stifness Y, respectively the values of 32 MeV and 0.9+-0.1.

[1] M.Papa et al; PRC 91 041601(R) (2015) and references therein

ISODEC

The neutron richness of a compound nucleus is expected to play a crucial role in its formation and in the competition between the various de-excitation channels, whose study can provide information about fundamental nuclear quantities such as level density, fission barrier and viscosity.

Novel information on these nuclear properties can be gained from the variation of the fusion cross section across long isotopic chains of compound nuclei, extending from the neutron-rich to neutron-poor side, all produced with comparable excitation energies and angular momenta.

ISODEC experiment aims at studying these effects, by looking at the reactions 86Kr + 48Ca and 78Kr + 40Ca, at the laboratory energy of 10MeV/nucleon, performed with CHIMERA. Velocity and energy spectra, mass and charge distributions, as well as dynamic features of reaction products were studied. The analysis highlighted clear differences for the two systems in the contributions arising from different reaction mechanisms, showing in the composite system produced by central collisions a slightly higher fusion-evaporation cross section and a strongly pronounced probability of fission-like processes for the neutron poor system with respect to the neutron rich one. Similar results were found exploring the break-up process of the quasi-projectile system formed in more peripheral collisions. Presently, the influence of the isospin parameter on the thermometric characteristics of the systems is under observation. The thermal evaporation from compound nucleus and from the Quasi-Projectile have been studied with different thermometers, based on the slope of the alpha particle’s energy spectra and on the ratio of isotope yields. In fusion-reactions higher temperature has been found for the system with higher neutron enrichment, independently of the nature of the method used for the determination of the temperature, while in contrast, for the Quasi-Projectile, the observed temperature is lower for the neutron rich system.

Future radioactive ion beam facilities, as for example SPES@LNL, will offer the opportunity to produce excited nuclei in unexplored regions of the nuclear landscape, offering access to nuclear matter at extreme isospin asymmetries approaching the limits of nuclear stability, besides providing additional constraints on the sophisticated models attempting to describe statistical and/or dynamical properties in the formation and decay of nuclei.

CHIMERA detector upgrade

The Chimera detector upgrade consists in :

- check, treatment, restore of the Silicon and CsI(Tl) detectors of the CHIMERA Rings;

- HV-LV  power modules renewal, and development of adapting interface;

- construction of 50 news mother boards, to change the old and broken ones. For this point we needed to recreate the beggining electronic project schemes, this was done in a succeful way thanks to the INFN sezione di Catania, Electronic Service;

- complete renewal of detector cabling system to solve the problem of the flanges, unable to keep the connectors for their degradation. Besides, we take advantage of this operation to improve the transport quality of the signals and the immunity by electromagnetic interferences. Besides interfaces card will be realized for connecting the old cables, coming from the Pre-amplifiers of the detectors. Finally is foreseen the realization of interface card to transform the signal from single ended to differential type.

  schema del nuovo sistema di cablaggio segnali CHIMERA

NEUTRON DETECTION

Since 2017 the CHI-NEXT group has undertaken a study aimed at developing a new detector that can simultaneously measure neutrons and charged particles, with high energy and angular resolution and with a reasonable detection efficiency for neutrons, NArCoS (Neutron ARray for COrrelation Studies). Such a device will be needed to adequately measure the abundant neutrons that will be produced in collisions between exotic ions currently under development in numerous laboratories around the world. Its important characteristics will allow to measure correlations in relative linear momentum between neutrons and charged particles, allowing the study of the nuclear interaction by "turning off" the Coulomb one. Initially the study mainly concerned simulations. We then moved on at testing new plastic materials, which, when suitably assembled in a stack configuration, can satisfy the required characteristics. The project has received new lymph and strength thanks to the funding of the recent PRIN (2020-21) project, ANCHISE which is focused precisely on the development of a detection prototype.

STUDY OF REACTIONS INDUCED BY RADIOACTIVE BEAMS AND IDENTIFICATION SYSTEM

The CHI-NEXT research group is also involved in the study of reactions induced by radioactive beams, produced by the in-flight technique at the INFN-LNS. Many experiments in this research field have been carried out through the FRIBS@LNS apparatus. The FRIBs@LNS (in Flight Radioactive Ion Beams at LNS) facility produces RIBs (Radioactive Ion Beams) at intermediate energy (20-50 MeV/A) using the In-Flight fragmentation technique. Exotic beams, from ⁶He to ⁶⁸Ni, have been produced from 2001 to 2019, following the pioneering, using the fragmentation of various stable beams accelerated by the LNS Superconducting Cyclotron (CS) on a Beryllium target, placed at the exit of the CS. The fragmentation beam is a cocktail beam whose elements have to be identified event-by-event in order to off-line select the ions of interest. An advanced tagging system is employed in the CHIMERA beam line. The system consists of a large surface Micro Channel Plate (MCP) detector producing the start of Time Of Flight (TOF) measurements and of a DSSSD (Doubled Sided Silicon Strip Detector), with thickness of 140-150 µm, which provides the energy loss information, the position and the stop of TOF measurements. A PPAC (Parallel Plate Avalanche Counter) position sensitive detector can be used to complete the trajectory measurement.

Scheme of the tagging system used in the CHIMERA beam line.

The upgrade project of the superconducting cyclotron opens new perspectives and opportunities for fragmentation beam studies at LNS. Indeed, the expected availability of a primary beam with high intensity can be used to produce very intense RIBs beam and to get good intensities even for more exotic isotopes. One of the benefits of this upgrade will be the possibility to extend the Isospin physics studies, in the range of Fermi energies, using reactions with high Isospin asymmetric projectiles. Another relevant goal will be the study of cluster physics (nuclear molecule) expected to be very prominent, in many exotic nuclei, when moving away from stability valley. Moreover, it will be also possible to extend the studies about the nuclear structures of unstable ions and to carry out reactions of astrophysics interest using low energy/degraded beams. To profit of this intensity upgrade, the construction of a new fragment separator named FRAISE (FRAgment In-flight Separator) is going on. In many cases, the high beam intensity will not allow the use of the tagging system currently employed in the FRIBS facility. Therefore, an important upgrade of the tagging system for the CHIMERA multidetector is ongoing to meet requirements of this intensity upgrade. Moreover, a new rotating fragmentation target is going to be studied and installed. In this respect, an array of detectors based on the SiC technology is in development. In detail, the array will be made of single detection pads with a surface of 5×5 mm² and thickness of 100 µm, arranged to cover an area of ≈ 60×30 mm² . Two SiC arrays, with same features, will be located at a distance of few cm, in order to reduce the dead area and to obtain a reconstruction of the trajectory. Such a segmentation will allow to have a maximum value of ≈ 10⁷ pps. The event-by-event identification of the beam will be obtained using the  ∆E-ToF method.

Scheme of the SiC array in development for the diagnostics and tagging systems of the FRAISE facility.