The LNL radioactive beam facility
|Gianfranco Prete (2010), Scholarpedia, 5(5):9751.||doi:10.4249/scholarpedia.9751||revision #143504 [link to/cite this article]|
Dr. Gianfranco Prete accepted the invitation on 9 July 2009 (self-imposed deadline: 9 September 2009).
The Istituto Nazionale di Fisica Nucleare (INFN) operates two nuclear physics laboratories for low and intermediate energies in Italy: the Laboratori Nazionali di Legnaro (LNL) near Padova and the Laboratori Nazionali del Sud (LNS) in Catania. LNS runs a 15 MV MP-Tandem and a K800 Superconductive Cyclotron (SC) mainly dedicated at studying nuclear physics at Fermi energy. At LNL it is in operation a 15 MV XTU-Tandem with a superconductive Linac (ALPI) as post accelerator. Recently a superconductive RFQ (PIAVE) was developed allowing the stand-alone operation of the linac system. Two small Van der Graaf accelerators are also in operation and dedicated to applied physics. An effort was made by INFN to stay at forefront of nuclear physics research addressing the production of Radioactive Ion Beams (RIB) to extend the study of nuclear physics far from the valley of stability. At LNS the project EXCYT (Exotics with CYclotron and Tandem) was completed and FRIBs (Fragmentation Ion Beams), taking advance of the 30 AMeV cyclotron beams, is under development. At LNL the project SPES (Selective Production of Exotic Species) is under construction.
Both EXCYT and SPES are ISOL facilities: EXCYT is based on the SC cyclotron beams as primary beams, a graphite direct target and the Tandem as reaccelerator. Proton-rich light ion beams like 8,9Li and 21Na are reaccelerated with intensities in the order of 104-105pps. SPES is a project to develop a RIB facility as an intermediate step toward the future European ISOL facility EURISOL. The SPES project is part of the INFN Road Map for the Nuclear Physics developed in Italy supported by both LNL (Legnaro) and LNS (Catania). The SPES project is based on the ISOL method with an UCx Direct Target and makes use of a proton driver of at least 40 MeV energy and 200 microA current. Neutron-rich radioactive beams will be produced by uranium fission at an expected fission rate in the target in the order of 1013 fissions per second. The key feature of SPES is to provide high intensity and high-quality beams of neutron-rich nuclei to perform forefront research in nuclear structure, reaction dynamics and interdisciplinary fields like medical, biological and material sciences. The exotic isotopes will be re-accelerated by the ALPI superconducting linac at energies of 10 AMeV and higher for masses in the region of A=130 amu with an expected rate on the secondary target of 109pps.
Starting from a nucleus on the stability line and adding successively neutrons one observes that the binding energy of the last neutron decreases steadily until it vanishes and the nucleus decays by neutron emission. The position in the nuclear chart where this happens defines the neutron drip line. It lies much farther away from the valley of stability than the corresponding drip line associated with protons, owing the absence of electrical repulsion between neutrons. The location of the neutron drip line is largely unknown as we have experimental data only for nuclei with mass up to around 30. The interest in the study of nuclei with large neutron excess is not only focused on the location of the drip line but also on the investigation of the density dependence of the effective interaction between the nucleons for exotic N/Z ratios. In fact, changes of the nuclear density and size in nuclei with increasing N/Z ratios are expected to lead to different nuclear symmetries and new excitation modes. While in the case of some very light nuclei a halo structure has been identified, for heavier nuclei the formation of a neutron skin has been predicted. The evolution of nuclear properties towards the neutron drip line depends on how the shell structure changes as a function of neutron excess. This evolution has consequences on the ground state properties (spin, parity, and electromagnetic moments) and on the single-particle and collective excitations. In particular, studies of neutron-rich nuclei beyond doubly magic 132Sn are of key importance to investigate the single-particle structure above the N=82 shell closure and find out how the effective interaction between valence nucleons behaves far from stability. New modes of collective motion are also expected in connection with the formation of a neutron skin, namely oscillations of the skin against the core, similar to the soft dipole mode already identified in the case of very light halo nuclei. Presently, neither the thickness nor the detailed properties of the neutron skin of exotic nuclei are known. This information is needed to enable a quantitative description of compact systems like neutron stars, where exotic nuclei forming a Coulomb lattice are immersed in a sea of free neutrons, a system which is expected to display the properties of both finite and infinite (nuclear matter) objects. Despite the large number of experimental studies, so far it is still not possible to predict reliably the limits of nuclear stability or the behavior of the Nuclear Equation of State (NEOS) at low and high baryon densities. In particular, the asymmetry term in the NEOS is largely unknown but in the region close to saturation. However, it is just this energy which plays an important role in setting the stability limits. For this reason, it is quite challenging to investigate the behaviour of nuclear matter far from stability. Although the SPES energy range is somewhat limited for studies of this kind, the neutron-rich ion beams of SPES will allow one to further extend the investigation of the NEOS along the isospin coordinate, in a region where it is largely unknown at low as well as high excitation energy.
SPES facility overviewIn order to better underline the framework in which the SPES project is going to be developed, a short description of the Legnaro Laboratory will be briefly outlined in the following. The Legnaro National Laboratory (figure 1) is involved in the nuclear physics framework since the '60th, when important gamma rays spectroscopy results have been achieved using . The heavy-ion accelerator complex started its operation in the '80th with the 16MV Tandem XTU, followed in the '90th by the Linac ALPI. The Tandem XTU was used stand alone or as an injector to ALPI. In the last years a new performant super-conducting RFQ injector (PIAVE) was designed and put in operation for ALPI. This injector is based on an ECR Ion Source (placed on a 350 kV platform) and on a super-conducting RFQ able to accelerate ions with A/q < 8.5 up to 1.2 AMeV.
The PIAVE-ALPI accelerator complex results in a superconducting heavy ion linac, composed by the RFQ section and three QWR sections for a total of 80 cavities installed as shown in figure 2.Its operate routinely at an equivalent voltage of 40MV and may exceed 70 MV in optimized conditions (all the resonators operating at the designed voltage, normalized transit time factor and synchronous phase taken into account). Due to external constrains the linac was constructed in a bended configuration: it is composed by two branches connected by an achromatic and isochronous U-bend. It uses three different kinds of cavities: Low Beta, Medium Beta and High Beta according to the different velocity along the acceleration path. In the last years the whole system came into routine operation and the medium energy QWR section was upgraded with a new Nb sputtered coating in substitution to the original Pb layer sputtered cavities. An upgrade program is on the way, to improve the final energy substituting poor performing QWR with new one and adding more cavities in the low beta section for a better acceleration of heavy ion beams. A further energy improvement has been tested recently installing a stripper station before the U-bend, in this condition the energy increased by 20% (from 6.8 to 8.1 MeV/A for 136Xe23) with the drawback of a reduced transmission (30%). In figure 3 the final energies for different masses are reported according to the actual and upgraded state of the linac.
The superconductive accelerator complex will be used as re-accelerator for the exotic beam. This is the reason why the area where the SPES facility will be constructed has been chosen to be very close to the existing PIAVE-ALPI. The linac will be a very important part of the new facility: as post-accelerator it allows to have RIBs of final energies in the order of 10 - 13 AMeV for A = 130. This open the possibility to perform multi-nucleon transfer reactions able to extend the range of neutron-rich isotopes that could be studied. Three experimental halls are already available for the scientific activities where quite a number of modern and performant apparatus are installed. They are intended for nuclear structure and nuclear dynamics studies. Among them we can cite GARFIELD and 8PLP multi-detector systems for the detection, with solid angles approaching 4π, light and heavy particles; GASP, a gamma detector array; RIPEN, a set-up for neutron detection and PRISMA, a high acceptance magnetic spectrometer. Other apparatus have been time to time installed and coupled to the existing installations (for example HECTOR-GARFIELD, CLARA-PRISMA etc.) for gamma-particle coincidence experiments. At the moment the AGATA demonstrator is installed at the PRISMA site to perform its first campaign of experiments. Interdisciplinary physics is also an historical brick of the LNL knowledge. On the other side of the laboratory area, with respect to SPES, two small Van der Graaf accelerator complexes, CN (7MV) and AN2000 (2MV), are in operation for low energy activities mainly devoted to material physics, dosimetry, radio-biology, detector development and others applications.
The SPES lay-out is shown in figures 4 and 5. For safety reasons the ISOL facility is constructed at level -5 m. In figure 4 the ISOL facility is located in the white area, housing the cyclotron proton driver, the two RIB targets, the High Resolution Mass Spectrometer (HRMS) and the transfer lines. The target development laboratory (not shown) will be constructed at level 0 over the ISOL facility. Two laboratories for applied physics and applications are planned: one at the same level of the ISOL facility which makes use of the Cyclotron proton beam and another at level 0 where the TRASCO high intensity linac will be used as driver for the Neutron Facility.
Figure 5 shows the transfer line for the exotic beam. The general configuration of the SPES layout follows the EXCYT facility, the ISOL facility for proton-rich nuclei in operation at LNS (Catania, Italy). The production target and the first mass selection element are housed in a high radiation bunker and mounted on a high voltage platform. Before the High Resolution Mass Spectrometer a cryopanel is installed to prevent the beam line to be contaminated by radioactive gasses. After the HRMS the selected isotope is stopped inside the Charge Breeder and extracted with increased charge. Before to reach the PIAVE-ALPI reaccelerator, a final mass selector (CB_MassSelector) cleans the beam from the contamination introduced by the Charge Breeder itself.The most critical element of the SPES ISOL facility is the Direct Target. The proposed target represents up to day an innovation in term of capability of sustaining the primary beam power. The design is carefully oriented to optimize the radiative cooling taking advantage of the target system high operating temperature, that is in the order of 2000°C. An extensive simulation of the target behavior for thermal and release properties is at the bases of the target-ion-source design. Experimental work to bench mark the simulations was carried out at HRIBF, the Oak Ridge National Laboratory ISOL facility (USA). The production target is designed following the ISOLDE (CERN) and EXCYT (LNS, Catania) projects devoting special care to the system safety and radiation protection. According to the estimated level of activation in the production target area (1013Bq) a special infrastructure needs to be designed. The use of up-to-date techniques of nuclear engineering will result in a high security level of the installation: the control system will integrate radiation management and survey as well as facility operation and safety infrastructures. Redundancies and fault tolerant PLC will be adopted in the low level layer of the control system while EPICS and LabView will be used in the general architecture and user front-end. The interaction of the proton beam with the UCx target will produce fission fragments of neutron-rich isotopes that will be extracted by thermal motion and ionized at 1+ by a source directly connected with the production target. Several kinds of sources are foreseen according to the beam of interest. A laser source for producing a beam as pure as possible will be implemented in collaboration with INFN-Pavia. The selection and the transport of the low intensity exotic beam at low energy is a challenging task. Techniques already applied to the EXCYT beam are of reference for SPES; they include the High Resolution Mass Spectrometer, the online identification station and several systems for low current beam diagnostics. Before the injection in the PIAVE-ALPI Linac, the Charge Breeder is an essential element for an effective reacceleration as it increases the charge state from 1+ to n+. The SPES Charge Breeder is based on ECR method and aims to produce ions with A/q less than 6 for A~130.
The SPES project will be one of the main nuclear physics developments in Italy for the next years. It is organized as a wide collaboration among the INFN Divisions, Italian Universities and international laboratories. A strong link and support was established with ISOLDE (CERN, CH) and HRIBF (ORNL, USA). SPIRAL2 (GANIL, F) and SPES collaborate in the frame of LEA (Laboratorio Europeo Associato) which aims to share the technical developments and the scientific goals in the field of nuclear physics with exotic beams. SPES is an up-to-date project in this field and it represents a very competitive step forward to the European project EURISOL. The relevance of the project is not only related to the nuclear physics research but also to astrophysics and applied physics: mainly for nuclear medicine, material research and nuclear power energy. The possibility to operate at the same time the ALPI Superconductive Linac, the high current RFQ and the 2 exit ports Cyclotron, gives a large improvement to the research capabilities at LNL. The first exotic beam at SPES is expected in 2014.