THE FRAGMENT SEPARATOR

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 [1]. Exotic beams, from ⁶He to ⁶⁸Ni, have been produced since 2001 following the pioneering work of G. Raciti [2], using fragmentation of various stable beams accelerated by the LNS Superconducting Cyclotron (SC) on a Beryllium target, placed at the exit of the SC. One of the most important characteristics of the fragment separator is that it makes use of the magnets of the transport beam line (figure 1). Therefore, the beam can be delivered, in principle, to any of the LNS experimental halls.

TAGGING SYSTEM

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 [1]. An advanced tagging system is employed in the CHIMERA beam line [3]. 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. A few examples of ΔE-TOF correlation plots are shown here.

DIAGNOSTIC SYSTEM

The diagnostic system allows to obtain an optimal beam transport from the Beryllium target to the final user point [6,7]. The diagnostic system currently in operation is made of plastic scintillators and DSSSD (16*16 strips of 3*48 mm²), recently installed and tested along the beam line. Thanks to the DSSSD it is possible to obtain a measurement of beam profile. Moreover, using the ΔE-ToF identification method, measuring the ToF with respect to the RadioFrequency of the CS, it is possible to get an event-by-event identification of the transported ions. This allows checking the isotopic composition of fragmentation beam along all the beam line. In some specific cases, one can use an Aluminum degrader, placed between the two dipoles of the fragment separator. The use of the degrader is fundamental in the case in which high-purity beams are needed [1,8].

RESULTS

The study of the diproton decay of the 6.18 MeV excited level of ¹⁸Ne, carried out in the 20° beam line with the HODO detector, was the first important result obtained with FRIBS facility [9]. After, many tests and experiments were performed in the CHIMERA beam line. As an example, measurements of the angular distributions of neutron transfer reactions were obtained with the kinematical coincidence method [10]. Another interesting result was the study of cluster structure and levels of ¹⁰Be, with the first observation of a level at 13.1 MeV. This level has been proposed as the 6⁺ member of the rotational band based on the second 0⁺ state at 6.137 MeV [4]. Moreover, the isoscalar excitation of the Pygmy Dipole Resonance in ⁶⁸Ni was investigated for the first time, detecting the gamma decay by using the CHIMERA detector [5,11]. Recently, a research group from the University of Warsaw and Dubna JINR has realized an experiment with the aim to study the beta-delayed α decay in ¹¹Be, using an optic Time Projection Chamber (TPC) installed in the 0° experimental hall. In this case, the exotic beam was produced with high purity of ¹¹Be ( 95 %) using an Aluminum degrader, placed between the two dipoles of Fragment Separator [8].

LIST OF AVAILABLE BEAMS

PERSPECTIVES

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 [1]. 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 and diagnostic systems is ongoing to meet requirements of this intensity upgrade. Moreover, a new rotating fragmentation target is going to be studied and installed. A detailed description of the physics cases achievable by using the FRAISE facility is reported in ref. [1].

References

[1] P. Russotto et al., Jour. of Phys.: Conf. Series, 1014 (2018) 012016 and ref. therein
[2] G. Raciti et al., NIM B, 266 (2008) 4632
[3] I. Lombardo et al., Nucl. Phys. B - Proceedings Supplements 215.1 (2011), pp. 272-274
[4] D. DellAquila et al., Phys. Rev. C 93, (2016) 024611
[5] N.S. Martorana et al., Phys. Lett. B, 782 (2018), pp. 112-116
[6] A. Amato et al., LNS Activity Report, 2009
[7] E.V. Pagano et al., LNS Activity Report, 2015-2016
[8] N.S. Martorana et al., LNS Activity Report, 2018-2019
[9] G. Raciti et al., Phys. Rev. Lett. 100, (2008) 192503
[10] L. Acosta et al., NIM A, 715 (2013) 56
[11] N.S. Martorana et al., Acta Phys. Pol. B 49 (2018).

Work-in-progress of the FRAISE facility

Extra pictures and plots