INTRODUCTION
Experiments with radioactive (exotic) nuclei allow to explore the properties of isotopes that have a proton-to-neutron ratio very different from the stable ones, measure cross sections of important reactions for the stellar nucleosynthesis occurring in explosive astrophysical environments, constrain the isospin-dependent nucleon-nucleon interaction in neutron-rich nuclei and in neutron stars, synthesize superheavy elements and test physics beyond the standard model.
While several large-scale and small-scale Radioactive Ion Beam (RIB) facilities are actually operating worldwide, future infrastructures like SPES (LNL-INFN, Italy), SPIRAL2 (France), HIE-ISOLDE (CERN), FRIB (USA), FAIR (Germany), EURISOL (Europe) are aimed at delivering RIBs with the highest intensity and purity and with good ion optical quality for investigating unreachable parts of the nuclear chart.
Along with the construction of new RIB infrastructures, a continuous development of detection arrays is under way. Depending on the radioactive ion incident energy and on the class of reactions to be studied, different experimental set-ups were built for the detection of charged particles. In this paper we describe the EXOTIC project, consisting of the EXOTIC RIB facility and the associated experimental apparatus,developed at LNL-INFN, Italy.
MATERIALS AND METHODS
EXOTIC facility andexperimental program
The EXOTIC facility [1-4] is dedicated to the in-flight production of low-energy light RIBs, by inverse kinematics nuclear reactions using the intense heavy-ion beams from the LNL Tandem XTU accelerator hitting a light gas target (H2, D2, 3He, 4He). The main characteristics of the facility, displayed in the left-hand side of Fig. 1, are a large RIB acceptance of the optics elementsand a large capability to suppress all the unwanted scattered beams. It consists of: i) a production gas target that is a 5 cm-long cylindrical cell with entrance (
So far, RIBs of 7Be,8B,17F,15O,8Li, 10C and11C, in the energy range 3-5 MeV/nucleon have been delivered with intensities about 106, 103, 105, 4*104, 105, 5*103 and 2*105 pps, respectively, and with a high purity of 98-99% (apart from the 8B that has a lower purity). This renders the EXOTIC facility competitive compared with other first generation small-scale in-flight facilities.
The envisioned experimental program at EXOTIC aims at:
1) studying reaction mechanisms induced by light exotic nuclei impinging on medium- and heavy-mass targets at incident energies near the Coulomb barrier of the colliding system. In this energy range, the peculiar features of exotic nuclei, such as excess of neutrons or protons, low binding energy, halo structure, neutron or proton dominated surface, influence the elastic scattering and the fusion process giving a picture that is rather different from that of well bound species (for a review see for instance [5]). Despite the efforts carried out so far, the understanding of nuclear reaction mechanisms in collisions involving exotic and weakly-bound nuclei is still a very challenging task. In the considered measurements the charged products emitted in direct nuclear reactions (elastic and inelastic scattering, nucleon transfer, breakup of the weakly bound projectile) and the light charged particles emitted in fusion-evaporation reactions should be charge and mass identified. A Full-Width-at-Half-Maximum (FWHM) energy resolution of ~250-400 keV is needed in the most demanding cases for discriminating the elastic from the inelastic scattering of the projectile from the target, depending on the considered colliding nuclei: ~250 (400) keV for a 11Be (17F) projectile impinging on a 58Ni or 208Pb target. A large detection solid angle is requested to compensate the low RIB intensity, in the most favorable cases limited to a few orders of magnitude less than typical stable beams, and to allow detection of coincident breakup particles emitted at large relative angles while a high granularity would allow detection of coincident breakup particles emitted at small relative angles. A FWHM time resolution of ~1-1,5 ns is sufficient for discriminating protons,
2) studying
3) performing measurements of astrophysical interest with RIBs impinging on solid or gas light targets in inverse kinematics: among the different processes of stellar nucleosynthesis forming elements heavier than 9Be, the rapid proton-capture and
4) performing measurements of fusion-evaporation cross sections at near- and sub-barrier energies. In this kind of experiments, the EXOTIC facility designed for the in-flight production of low-energy light RIBs, is employed as a separator of evaporation residues from the incident beam (stable or RIB). The evaporation residues are transported and detected at the focal plane of the facility.
Experimental set-up
The design of a high-performance detection system suitable for the above mentioned experiments must meet several requirements:
event-by-event beam tracking capabilities to account for the typical poor emittance of in-flight produced RIBs in conjuction with a good time resolution for Time of Flight (TOF) measurements and a fast signal for handling counting rates up to 106 Hz;
charge and mass identification of the reaction products with the highest achievable energy resolution;
a solid angle coverage as large as possible;
high segmentation to achieve good angular resolution and for reducing pile up events and low-energy events coming from the radioactive decay of the elastically scattered projectiles;
flexibility in order to be suitable for different experimental needs.
The experimental set-up [10] installed at the focal plane of the facility and displayed in the right-hand side of Fig. 2 consists of: (a) the RIB tracking system and (b) EXPADES, a new charged-particle telescope array. It satisfies the previously mentioned requisites for studies with low-energy light RIBs, moreover, it has the additional advantages of compactness andportability. The components of the EXPADES array can be easily reconfigured to suit many experiments while it can be used as an ancillary detection system with
The two Parallel Plate Avalanche Counters (PPACs) of the tracking system, are position-sensitive, fast, high-transparency detectors, radiation hard which can sustain counting rates up to ~106 Hz. They are placed 909 mm (PPAC A) and 365 mm (PPAC B) upstream the reaction target (see Fig. 2, right-hand side). PPAC B is positioned at the entrance of the reaction chamber (element 10 in the schematic layout of the facility shown in Fig. 1, left-hand side) to provide an event-by-event reconstruction of the trajectory of the RIB particles and a time reference for TOF measurements. The PPACs are filled with isobutane (C4H10) at a working pressure of 10-20 mbar and have entrance and exit windows that are made of 1,5
EXPADES is an array of eight telescopes arranged in a cylindrical configuration around the reaction target (see Fig. 2, right-hand side). The telescope structure is flexible and is composed of two Double Side Silicon Strip Detectors (DSSSDs) and/or an Ionization Chamber (IC), depending on the experimental requests. We use 40/60
The choice of the electronic front end of the DSSSDs was based on a compromise between the requirement for high granularity, good energy and good time resolution and that to maintain low the overall cost. Application Specific Integrated Circuit (ASIC)-based electronics was employed for the treatment of the Eres signals. ASIC electronics allows us to handle 32 energy signals of each side of the 300
In some experiments, the unambiguous identification by means of the
The low-noise charge-sensitive preamplifiers for the
to have a compact set-up (detectors + electronics);
to minimize the internal and external connections and
to overcome the environmental noise at the EXOTIC beamline.
In this way, we manage to keep as low as possible the DSSSDs electronic thresholds, typically 300-500 keV.
The distance of the EXPADES telescopes from the target can be varied continuously from a minimum value of 105 mm to a maximum of 225 mm, which corresponds to an angular resolution for a pixel from
The whole EXPADES array and the PPACB are housed in the reaction chamber, placed at the final focal plane of the EXOTIC facility.To allow the realization of experiments with RIBs impinging on both solid and gas reaction targets, a small chamber housing the PPAC Bwas built. When requested, this small chamber isolates, through a 2
RESULTS AND DISCUSSION
Studies on nuclear reactions and astrophysics
The RIBs of the EXOTIC facility and the above described experimental set-up have been used so far, in the framework of international collaborations, for the study of nuclear reaction dynamicsat Coulomb barrier energies [11-16] and
CONCLUSIONS
Summary and perspectives
Finally, first encouraging results have been obtained for the use of the EXOTIC facility for sub-barrier fusion-evaporation cross section measurements [19]. The fusion reactions can be induced by the stable beams of the LNL Tandem XTU accelerator and also by the neutron-rich RIBs of the SPES (Selective Production of Exotic Species) ISOL-type facility, in construction at INFN-LNL.