The Radioactive Ion Beam Facility Project for the Legnaro Laboratories

In the frame work of the Italian participation to the project of a high intensity proton facility for the energy amplifier and nuclear waste transmutations, the LNL has been proposed a project for the construction of a second generation facility for the production of radioactive ion beams (RIBs) by using the ISOL method. The final goal consists in the production of neutron rich RIBs with masses ranging from 80 to 160 by using primary beams of protons/ deuterons with energy of 100 MeV/u and 100 kW power. This project is proposed to be developed in about 10 years from now and intermediate milestones and experiments are foreseen and under consideration for the next INFN five year plan (1999-2003). In such period of time is proposed the construction of a deuteron accelerator of 20 MeV energy and 5 mA current, consisting of a RFQ (5 MeV) and a linac (20 MeV), and of a neutron area dedicated to the RIBs production and other applications. Besides the RIBs production, neutron beams for the BNCT applications and neutron physics are also planned. 1 METHODS OF PRODUCTION Two methods can be used to produce RIBs and are the Projectile Fragmentation (PF) and the post-acceleration of isotopes produced by spallation, fragmentation, or fission reactions with high-intensity proton/light-ion beams in thick targets (ISOL method). The ISOL method takes a complementary approach to the production of RIBs. In the ISOL scheme a primary accelerator (or a nuclear reactor) yields a production beam of charged particles (or neutrons), which is sent on a thick, hot target. The radioactive species thereby produced are transported by a transfer tube to an ion source, then mass separated and breeding to higher charge state; the resulting ions are separated by an isotope/isobar separator, post-accelerated and sent into the experimental area. The comparison and choice between the PF and ISOL methods is strongly dependent on the physics to investigate. One of the relevant parameters for a comparison of the ISOL and the PF production methods is the RIBs production luminosity, which is independent of cross-section. The maximum production luminosity expected for an ISOL facility is three or four order of magnitude higher than in the PF figure. This because the luminosity can be achieved through the combination of a thick production target and a very intense primary beam. The proposed researches which are done at near-Coulomb barrier energies and below, involves studies of nuclear structure, low-energy nuclear reactions, astrophysics, and materials sciences applications and are best served by intense, high-quality, low-energy RIBs provided by the ISOL approach. This because the post-accelerator, optimized for high quality beams, provide the easy energy variability, high-energy precision and small emittances as demanded by the experiments. The main drawback to the ISOL method is that the diffusion/desorption from the production target and ionization of radioactive fragments are strongly element-dependent and slower than in the PF method. However, this feature can be also an advantage of the ISOL method because it give Z selectivity and, consequently, enhance beam purity. 2 THE FACILITY CONCEPT We propose a two-accelerator ISOL-type facility to provide intense neutron-rich radioactive ion beams of highest quality, in the range of masses between 80 and 160. The proposed production mechanism is the fission induced by fast neutrons in fissile material targets. The fission mechanism has the advantage that very neutron rich radioactive beams can be produced. The conceptual design is based on a high intensity proton/deuteron linac as driver and on the availability of the heavy-ion accelerator ALPI as post accelerator [1]. The main idea is to use the primary beams to produce, by a converting target, an intense flux (~ 3 x 10 cm s) of neutrons. The use of a converting target partially solves the problems concerning the beam power dissipation and radiation damage in the radioisotope production target/ion source system. The purely nuclear mechanism of energy loss and the long collision length of neutrons allow rather large target thickness to be used for more effective isotope production. The primary accelerator is a sequence of RFQ-DTL linac which can deliver proton/deuteron beams in the energy range of 10 – 100 MeV/u at a beam power of 100 kW. The RFQ accelerates the beam up to 5 MeV while further acceleration up to 100 MeV/u is accomplished by the DTL linac. The facility is planned to be located below ground level to assist in the prompt neutron radiation shielding. The beams delivered by the driver will produce radionuclides by irradiating targets in a wellshielded dedicated area. The radionuclides will be extracted at 20 kV from the target and ionized to the 1+ charge state, charge breeded to obtain a mass over charge ratio of about 10, mass separated, and then sent either directly to an experimental area for research with ion traps, or to the secondary-beam accelerator which will be housed in the existing ALPI building. To fill the ALPI linac with these very low energy beam requires a new accelerating stage (pre-accelerator) up to about 1 MeV/u. The pre-accelerator consists of three lowfrequency RFQs similar to the ones of the PIAVE injector [2]. These low-energy secondary beams can be delivered to the experimental area, located in the ALPI building, for low-energy experiments (astrophysics). The further acceleration up to about 5 MeV/u (Sn) is accomplished by the ALPI linac and the radioactive beams will be delivered to the existing experimental areas. The capability of the ALPI linac to accelerate stable beams will remain independent of this program, both during construction and operation with radioactive beams. 3 SECONDARY BEAM INTENSITIES Extensive calculations of production rates of radioisotopes have been performed using Montecarlo codes LAHET [3] and FLUKA [4]. The goal of such a calculations is to provide realistic and accurate estimates of the production yields for secondary ion beams to guide the detailed accelerator design and providing input for the physics program. Since LAHET code does not transport neutrons with energy below 20 MeV, simulations in voluminous thick targets have been performed with the FLUKA code (version ’98). In particular, the fission process is computed using ENDF/B-6, JEF2.2 and JENDL3 nuclear data libraries. The most efficient production of neutron-rich isotopes is achieved via neutron induced fission of fissile materials (i.e. uranium and thorium). Neutrons are produced by stopping the primary proton/deuteron beam in a suitable target (stopping target). The stopping target must be chosen according to the primary beam in order to emit secondary neutrons in a forward narrow cone. It is understood that such a two-step device produces its best results when the secondary neutrons production is high. This could be accomplished, for instance, taking advantage of the deuteron stripping in reactions with light elements. The predicted fission fragments spectrum for high energy neutrons has been calculated with the FLUKA code and shown in Fig.1. Figure 1: Predicted fission fragments spectrum. 4 THE NEUTRON FACILITY SPES As an intermediate milestone of the full project the Legnaro Laboratories have been proposed a neutron facility (SPES) to be constructed in the next five years. It will result in a medium intensity neutron facility addressed to the national community and dedicated to the RIBs production as well as to the BNCT applications and to the neutron physics. Because its modularity the primary accelerator is planned to be constructed, at the beginning, in a minimal version and further implemented to accelerate beams up to 100 MeV/u. The primary accelerator consist in the sequence of RFQ-DTL linac limited in energy (10 MeV/u). The beam power to be considered is ranging between 10 and 100 kW. For the remaining part the facility is conceptually planned as described above, thus the secondary beams can be delivered for experiments at low-energy (traps, astrophysics,...) or pre-accelerated and injected in the ALPI linac for a further acceleration to higher energies. The limited energy of the primary accelerator will imply a lower neutron production rate; in any case the estimated neutron yield at the target level is 2 x 10 n/s (100 kW), high enough to satisfy the demand for an advanced RIBs facility. 5 NEUTRON PRODUCTION RATES AND BEAM INTENSITIES High energy neutron sources based on high current continuous wave (CW) deuteron or proton accelerators and thick targets of light nuclei provide the most suitable intensities for the aims of the project. The experimental data on deuteron and proton induced neutron source reactions were reviewed by Lone [5] and Barschall [6]. At deuteron energies above a few MeV, the d + Be and d + Li reactions have the highest cross sections for production of neutrons in the forward direction. The high energy neutrons measured from thick beryllium and lithium targets exhibit spectral shapes and angular distributions which are characteristic of the deuteron stripping process. At a comparable projectile energy the intensity and the average energy of neutrons at 0 from a thick beryllium target are highest for the deuterons. The dose rates from the d + Be reaction on thick targets are higher than those from the p + Be reaction at the same projectile energy. Also, the angular distributions of neutrons from the deuteron reactions are narrowed than those from the proton reactions at the same projectile energy. The intensity of the neutrons emitted in the forward direction increases rapidly with the deuteron bombarding energy. Independently from the stopping target, the deuterons seems the most appropriate projectiles for the production of neutrons. Stopping targets as beryllium and lithium both present characteristics which are suitable for the task. For some Red > 10 11 atoms/ μC Jellow 10 10 10 11 Green 10 9 10 10 Blue 10 8 10 9 200 MeV d + Be n + 238U application uranium stopping target is suitable too, even if it shows different angular and energetic distributions. In this contest, for the intermediate milestone of the project, the most suitable driver may consists of a deuteron accelerator of 20 MeV energy and power up to 100 kW. With minimal changes to the RFQ designed for the high intensity proton driver the same device allows to accelerate also deuterons. The RFQ is followed by a short section of the DTL linac. The expected yield of neutrons with energy above 1 MeV emitted in an angle between 0 and 10 is 2 x 10 (n/s), for 100 kW power in the stopping beryllium target. The expected beam intensities for a production deuteron beam of 20 MeV and 100 kW power are shown in Table 1. The stopping target is beryllium and the production target is uranium carbide with a thickness of 100 gcm. Table 1:. Expected beam intensities. The overall efficiency values are taken from Ref. [7]. * Value taken from Ref. [8].