DOI /epja/i Toward the drip lines and the superheavy island of stability with the Super Separator Spectrometer S 3

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1 EPJ A Hadrons and Nuclei EPJ.org your physics journal Eur. Phys. J. A (2015) 51: 66 DOI /epja/i Toward the drip lines and the superheavy island of stability with the Super Separator Spectrometer S 3 F. Déchery, A. Drouart, H. Savajols, J. Nolen, M. Authier, A.M. Amthor, D. Boutin, O. Delferriére, B. Gall, A. Hue, B. Laune, F. Le Blanc, S. Manikonda, J. Payet, M.-H. Stodel, E. Traykov and D. Uriot

2 Eur. Phys. J. A (2015) 51: 66 DOI /epja/i Special Article Tools for Experiment and Theory THE EUROPEAN PHYSICAL JOURNAL A Toward the drip lines and the superheavy island of stability with the Super Separator Spectrometer S 3 On behalf of the S 3 Collaboration F. Déchery 1,2,a, A. Drouart 3, H. Savajols 4,J.Nolen 5, M. Authier 3, A.M. Amthor 6, D. Boutin 1,2,O.Delferriére 3, B. Gall 1,2,A.Hue 7, B. Laune 7,F.LeBlanc 1,2, S. Manikonda 8,J.Payet 3, M.-H. Stodel 4,E.Traykov 4, and D. Uriot 3 1 Université de Strasbourg, IPHC, 67037, Strasbourg, France 2 CNRS, UMR7178, Strasbourg, France 3 Irfu, CEA-Saclay, Gif-sur-Yvette, France 4 GANIL, Caen, France 5 Argonne National Laboratory, Argonne, IL 60439, USA 6 Bucknell University, Lewisburg, PA, USA 7 IPNO, Université Paris-Sud 11, CNRS/IN2P3, 91406, Orsay, France 8 AML Superconductivity and Magnetics, Palm Bay, Florida 32905, USA Received: 15 January 2015 / Revised: 24 March 2015 Published online: 1 June 2015 c Società Italiana di Fisica / Springer-Verlag 2015 Communicated by N. Alamanos Abstract. The Super Separator Spectrometer S 3 is a major experimental system developed for SPIRAL2. It has been designed for physics experiments with very low cross sections by taking full advantage of the very high intensity stable beams to be produced by LINAG, the superconducting linear accelerator at GANIL. These intensities will open new opportunities in several physics domains using fusion evaporation reactions, principally: super-heavy and very heavy element properties, spectroscopy at and beyond the dripline, and isomer and ground-state properties. The common feature of these experiments is the requirement to separate very rare events from intense backgrounds. S 3 accomplishes this with a large acceptance, a high background rejection efficiency, and a physical mass separation. This article will present the technical specifications and optical constraints needed to achieve these physical goals. The optical layout of the spectrometer will be presented, focusing on technical elements of the target system, the superconducting multipole magnets used to correct high-order optical aberrations, the electric and magnetic dipoles, and the open multipole triplet used for primary beam rejection. The expected system performance will be presented for three experimental cases using 3 specific optical modes of the spectrometer. 1 The SPIRAL2 project SPIRAL2, the second generation System for On-Line Production of Radioactive Ions [1] is a project for fundamental nuclear physics and interdisciplinary research based on a high-intensity linear particle accelerator. The project involves over 600 scientists from 34 countries. It will solidify for the Grand Accélérateur National d Ions Lourds (GANIL) a leading place in global nuclear physics research, especially for the production and study of exotic nuclei in the low-energy range from kev/u to MeV/u.The baseline project was presented in a white book [2] and is described in detail in the final technical design report [3] including all planned options and future extensions. The core element of SPIRAL2 is the latestgeneration superconducting LINear Accelerator at GANIL a fabien.dechery@iphc.cnrs.fr (LINAG) [4 6]. The SPIRAL2 accelerator may be divided into sub-sections according to specific function. First are the electron cyclotron resonance (ECR) ion sources (producing deuterons or heavy ions), followed by a preaccelerator and beam shaping system. An exhaustive description of the SPIRAL2 accelerator is presented in the technical refs. [7 10] with the full set of parameters and the procedure for fine tuning. LINAG will provide deuterons and stable heavy-ion beams ranging from helium to uranium [11]. The beam energies for heavy ions will be up to 14.5MeV/u with intensities up to ions per second, several orders of magnitude higher than available at existing facilities. These very high intensities are made possible by improvements in electron cyclotron resonance ion sources. Two ECR ion sources have been developed: the first is developed by the Service des Accélérateurs, de Cryogénie et de Magnétisme, of the Institut de Recherche sur les lois Fondamentales de

3 Page 2 of 16 Eur. Phys. J. A (2015) 51: 66 Fig. 1. Extension of the SPIRAL complex in two phases. Phase 1: The very high stable beam intensities of the LINAG (1) will be carried to the NFS or S 3 experimental rooms (2), and through S 3, to the low-energy DESIR facility (4) (phase 1+). Phase 2: The production building (3) will provide radioactive ion beams (RIB) by means of fissions to be sent to DESIR or to the existing GANIL facility. l Univers (IRFU/SACM) for deuterons and protons [12], designed for exotic beam production by induced uranium fission; and the second is a next-generation A-PHENIX V3 type developed by the Laboratoire de Physique Subatomique et de Cosmologie (LPSC) of Grenoble for highly charged heavy ions [13]. Phase 1 of SPIRAL2 will include three experimental areas (cf. fig. 1): the Super Separator Spectrometer (S 3 ) [14 17] built to synthetise rare isotopes, Neutrons For Science (NFS) built to produce a high-intensity neutron beam from deuterons [18], and the Decay, Excitation and Storage of Radioactive Ions facility (DESIR) [19] that will use radioactive low-energy beams coming from SPI- RAL1, S 3 or, in a next phase, produced by the targetsource ensemble of the production building to perform experiments in decay spectroscopy, laser spectroscopy, mass spectrometry, ion trapping, etc. 2S 3 research programs S 3 will open new opportunities for the study of rare isotopes characterized by very low production cross sections like superheavy and exotic nuclei far from stability as well as for the study of reaction mechanisms. These physics cases have in common the necessity to discriminate rare events of interest from dominating background events. The principal areas of research fit into four broad categories [2,20]: Super-Heavy Element (SHE) synthesis, Z > 104, and very heavy element (VHE) production by fusionevaporation reactions for spectroscopy or measurement of ground state properties and isomeric state studies. Detailed decay spectroscopy studies and high-precision mass measurements could be possible depending on the production rate. Production and spectroscopy of neutron-deficient nuclei close to the proton drip-line. Neutron-deficient and N = Z nuclei will be produced by fusion-evaporation to study, for example, single particle structure, collectivity effects, shape coexistence, or ground-state properties. Production and spectroscopy of neutron-rich nuclei, produced by multi-nucleon transfer to study, for example, single particle structure and the evolution of magicity. Ion-ion atomic interactions to study electronic exchange cross sections for plasma physics. This research axis is developed by the Fast-Ion Slow-Ion Collisions (FISIC) Collaboration [21]. Specific research programs have been proposed for S 3, represented by 16 Letters of Intent (LoIs) [22,23], which were signed by 170 physicists. After commissioning, S 3 is expected to run experiments between 16 and 31 weeks per year to perform these and other physics programs. In the area of super-heavy element research, the high intensities will allow detailed studies of previously observed elements, information about their isospindependent properties, cross section systematics (mechanisms of fusion and fission), but also measurement of ground-state properties masses, charge radii or transactinide chemistry depending on the available detection. Spectroscopic information should be accessible for elements up to Darmstadtium (Z = 110) and studies of collectivity will be possible around the islands of deformation ( 254 No, 270 Hs). The hunt for new K-isomers will bring information about the single particle states of very heavy elements. This will help to deduce the single particle structure of the super-heavy elements around the socalled super-heavy island of stability. S 3 aims to produce new isotopes to bridge the gap between neutrondeficient heavy elements, typically produced by cold fusion, and neutron-rich heavy elements, typically produced by hot fusion ( 48 Ca on actinide targets). One of the major open questions to be addressed by S 3 is the connection of the decay patterns observed in 48 Ca-induced hot fusion reactions on actinide targets (observed at the gas-filled separator GFRS of the FLNR [24,25]) to the decay chains obtained for cold fusion reactions with 208 Pb and 209 Bi targets (observed at the velocity filter SHIP of GSI [26] and later at the gas-filled separator GARIS of RIKEN [27]). The decay chains of the isotopes produced by hot fusion reactions terminate with the fission of unknown isotopes and are thus not connected to isotopes with known alpha decays to provide mass identification. The mass resolving power of S 3 should be able to independently validate the mass number of the produced nuclei. 3 Technical specifications and optical constraints The S 3 design has been optimized for fusion-evaporation production, since that is the production reaction central to the physics program. Representative experiments have

4 Eur. Phys. J. A (2015) 51: 66 Page 3 of 16 Table 1. Properties of input distributions used to simulate the spectrometer performance for the three key reactions studied: the target thickness (backing and stripper thickness are about 2 μm of titanium and 50 μg/cm 2 of carbon, respectively), the mean recoil energy, the momentum and angular distributions (RMS), the mean charge state and charge state distribution width (RMS) [29], central magnetic and electric rigidities, and geometric transverse emittances (RMS). The beam spot on target is given by: X(RMS) =0.5mm and ΔY =10mm(Y (RMS) =2.89 mm), so ǫ XX (RMS) =X(RMS) σ θ (RMS) and ǫ YY (RMS)=ΔY σ θ (RMS). N Reactions τ E σ dp σ θ Q (dq) Bρ Eρ ǫ XX ǫ YY p μ g/cm 2 MeV % mrad Mean (RMS) T m M V mm mrad mm mrad Ti( 58 Ni, 4n) 100 Sn (1.8) Pb( 48 Ca,2n) 254 No (2.1) U( 22 Ne, 5n) 255 No (1.5) Fig. 2. Schematic representation of the S 3 design: the primary beam strikes the target, a first separator stage or momentum achromat (MA) rejects a large majority of the unreacted primary beam, and the second spectrometer stage an m/q or mass separator (MS) brings additional selectivity. Alternatively, a secondary reaction target can be installed between the two stages for reaction studies with secondary beams. been defined by the collaboration to guide its detailed development: a symmetric reaction leading to the synthesis of a neutron-deficient exotic nucleus 46 Ti( 58 Ni,4n) 100 Sn, a very heavy element synthesis from two doubly magic nuclei 208 Pb( 48 Ca,2n) 254 No, and another isotope of nobelium produced by the very asymmetric reaction 238 U( 22 Ne,5n) 255 No. The three experiments have been chosen with increasing emittances in order to cover a wide range of kinematics. Each production case has been simulated with the S3Fusion code [28] and the produced particle distributions where used in Monte Carlo simulations to estimate the performance and to guide the design of the optical structure. See table 1 for a summary of the reaction properties. Particle distributions are generated from beam-target interactions including the fusion-evaporation reaction kinematics and multi-scattering processes occurring in the target material. The momentum spread, angular straggling, charge state and rigidity distributions quantified in these simulations help to define the global technical specifications for the S 3 layout. The optical constraints necessary to satisfy the proposed research programs have mostly been derived from the representative experiments given above. The optical design, presented schematically in fig. 2, must provide the following characteristics: very strong suppression of the primary beam at 0 (up to a factor of ); magnetic rigidity acceptance of ±7% for a given charge state; charge state acceptance of ±10%, corresponding to a range of five charge states centered at 20+ for the reference momentum; angular acceptance of ±50 mrad in both transverse planes; a maximum magnetic rigidity of Bρ max =1.8Tm in the momentum achromat (MA) stage and a maximum electric rigidity of Eρ max = 12MV in the mass separator (MS) stage; a mass resolving power of 300 at FWHM. All acceptance values angular and rigidity have been specified to achieve a recoil transmission better than 50% for the reference cases ( 100 Sn and 254 No) if the most populated charge states are selected. Rare isotopes are produced together with many contaminants through alternate reaction channels. Contaminant rejection is achieved in S 3 in a two-step process. Firstly, in the momentum achromat stage, selection by magnetic rigidity reduces transmission of contaminants, and unreacted primary beam charge states by at least a factor of Secondly, an electromagnetic mass separator stage provides a physical separation of products by m/q at an achromatic final focal plane. For S 3, the goal is to distinguish neighboring masses up to A 300, which is the basis of the resolving power specification above. At the same time, S 3 retains the flexibility to maximize either the transmission or the mass resolution, at some expense to the other property, in dedicated optical modes depending on the priority of a particular experiment. 4 Detection systems Two complementary detection systems will be located at the final focal plane for delayed studies: the implantation and decay station SIRIUS, for Spectroscopy and Identification of Rare Isotopes Using S 3 [30] (see fig. 3), designed to perform proton, alpha, electron and gamma decay spectroscopy, and the Rare Elements in-gas Laser Ion Source and Spectroscopy at S 3, REGLIS3 [31], an additional selection, ionization, and low-energy beam creation and manipulation system. This second system will include an online laser spectroscopy system as well as the capability to send the isotope beam through the low-energy branch either to the DESIR facility or to a multipurpose experimental room next to the S 3 vault, where various set-ups can be installed to perform ground state property measurements (e.g. a tape station, etc.).

5 Page 4 of 16 Eur. Phys. J. A (2015) 51: 66 Fig. 3. SIRIUS detection system. Left: full detection system including trackers and the implantation and decay station shown following the final multipole of S 3 and including the focal plane vacuum chamber. Middle: Expanded view of the silicon box as it will be inserted and the surrounding germanium detectors. Right: Larger view of the silicon box mounted on its frame, including the implantation and the four tunnel detectors, with a view of one pixelised tunnel pad. The SIRIUS system includes tracking detectors that provide the recoil time of flight and the trajectories for mass identification. A silicon box is composed of a Double Sided Silicon Strip Detector (DSSD), where the recoils are implanted, and four tunnel detectors. surrounded by germanium detectors. They measure the recoil energy and γ-rays,α-particles electrons and protons from decays. The ion optical system is tuned to maximize the recoil transmission in the 10cm 10cm DSSD, while preserving the mass resolving power needed at the m/q dispersive focal plane. The REGLIS3 system includes a gas catcher to slow down the selected ions, up to 10 9 particles per second, with an overall efficiency between 4% and 24% depending on the physics case (including transmission through S 3, thermalization, diffusion, gas cell extraction, neutralization, laser ionization and transport efficiency). The thermalized isotopes are first ionized with a laser providing chemical selection and then formed into a low-energy beam. The ions are then pre-selected by a Radio Frequency Quadrupole system (RFQ) and/or a Multi-Reflection Time-Of-Flight Mass Separator (MR-TOF-MS), accelerated up to 40keV, and transported to the DESIR facility or to the multipurpose experimental room. In this system, the ion optics will be tuned to maximize the recoil transmission onto the 5 cm diameter gas cell window, with no need for physical mass resolution. In some cases some mass resolution might still be desirable, because a high particle rate into the gas cell reduces extraction efficiency of the desired ions. This may be achieved (at some cost to transmission) to perform an additional selection of the nuclei with a slit system to reduce the total implantation rate. The very high LINAG intensities will induce a high gamma flux at the primary beam target that precludes any prompt spectroscopy. For this reason, the S 3 project will be mainly dedicated to delayed spectroscopy. Prompt studies will still be possible, however, at the secondary target point located at the intermediate focus at the middle of S 3. Here exotic beams from the first target may be used to perform secondary reactions like Coulomb excitation or deep inelastic transfer, or to study reaction mechanisms. Different detectors could be installed at this secondary target point [32], for example, PARIS [33], EX- OGAM2 [34], or part of AGATA [35,36]. The coupling of the separator with these systems has been considered in the design. The FISIC project will also take place at the intermediate focal point, to study the interaction of a selected beam charge state from the first half of S 3 with a low-energy beam from a specialized perpendicular beam line. A specialized detection system has also been developed for this purpose [21]. 5 Optical design The S 3 system is a two-stage optical structure, combining a large-acceptance momentum achromat with a highresolution mass separator [37] (cf. fig. 4). The MA stage acts like a powerful filter to ensure a 99.9% rejection of the LINAG primary beam in a dedicated beam dump area, and the MS stage provides selectivity in mass-to-charge ratio. Symmetry was a criterion in the initial design of the optical structure as it leads to the natural cancellation of many geometric and chromatic aberrations [38]. Due to mechanical constraints, the symmetry of the two stages

6 Eur. Phys. J. A (2015) 51: 66 Page 5 of 16 Fig. 4. The S 3 experimental hall with the two-stage optical structure including the main elements dipoles, Superconducting Multipole Triplet (SMT), detection systems and the different focal planes (F i with i = 0 4): target point F 0, magnetic dispersive plane F 1, achromatic focal plane F 2, electric dispersive plane F 3, final focal plane F 4. has been partially broken but is still largely preserved in the four half-stages. Each stage includes two dipoles (D) and four multipole triplets (MMM); each multipole (M) includes independent, superimposed quadrupole, sextupole and octupole components (except for the second triplet, which has no octupoles, see sect. 6). The full system is divided into eight MMM[D/2] cells. The combination of two cells MMM[D/2] + [D/2]MMM = MMMDMMM corresponds to a half-stage with a specific function. Each full stage is achromatic. The momentum achromat (MA) stage uses two identical magnetic dipoles (D M ) to produce a p/q dispersive intermediate image, where the primary beam rejection is performed. The mass separator (MS) stage uses an electrostatic dipole (D E ) coupled to a magnetic dipole to provide an achromatic image at the final focal plane that is dispersive in m/q. Identical superconducting multipole triplets are used throughout the system except for the triplet following the first dipole, which is an open-sided, room-temperature multipole triplet. The open-sided triplet allows high intensity charge states to be stopped outside the acceptance of the system avoiding excessive heat load, radiation damage, or in-system scattering that would occur in a closed superconducting triplet if used in this location. Movable beam dumps, including a multi-finger system, are located at the p/q dispersive plane in order to stop a significant part (> 99,9%) of the primary beam power. The full S 3 optical structure is given by: (F 0 )-MMMD M MMM-(F 1 )-MMMD M MMM-(F 2 ) -MMMD E MMM-(F 3 )-MMMD M MMM-(F 4 ), where F i are the different focal planes. The first focus, F 0, is at the location of the production target. F 1 is a momentum dispersive focal plane for initial purification and primary beam rejection, where the multi-finger system and the high-power beam dump are located. F 2 is the achromatic focal plane at the end of the momentum achromat. A collimator may be placed at F 2 to reduce transmission of scattered particles. F 3 is the energy dispersive focal Fig. 5. Technical drawing of the high-velocity stable and radioactive rotating target chamber [39]. plane (dispersive in pv/q due to the electrostatic dipole). F 4 is the final focal plane of the spectrometer, which is achromatic but dispersive in mass-to-charge ratio. 6 Technical elements 6.1 Target station A rotating target has been designed [39] (cf. fig. 5) to withstand the high LINAG beam intensities by effectively spreading the deposited power over a large surface. The target is confined in a specialized target chamber. Two chambers have been designed for stable and radioactive targets: a large chamber dedicated to stable targets, which are mounted on large 37cm radius wheels and rotating at up to 3000RPM, and a smaller chamber dedicated to radioactive targets, which are mounted on small 8cm radius wheels and rotating at up to 5000RPM. Initially, the

7 Page 6 of 16 Eur. Phys. J. A (2015) 51: 66 beam spot at the target will be circular or elliptical. After a future upgrade, it could also be possible to sweep the beam spot vertically (perpendicular to the local rotational motion of the wheel) with a high frequency, using two vertically bending electric dipoles as a beam raster. Different beam spot distributions are being considered: a Gaussian distribution in both x and y with standard deviations of σ X =0.5mm and σ Y =2.5mm, as might be achieved by detuning the beam spot in the vertical direction or a gaussian distribution in x with standard deviation of σ X =0.5mm and a uniform distribution in y with a fullwidth Y = 10mm, as might be produced by a beam raster. The second beam spot geometry has been considered in our simulations. A first target wheel prototype has already been successfully tested at GANIL [39]. Table 2. Dipole properties: W is the dipole weight, Gap corresponds to the full distance between the poles (vertical for magnetic dipoles) or electrodes (horizontal for the electric dipole), θ is the deviation angle of the dipole, ρ is the bending radius, Bρ/Eρ max is the maximal magnetic or electric rigidity. Dipoles Magnetic 1 & 2 Magnetic 3 Electric W (tons) Gap (mm) θ ( ) ρ (m) Bρ/Eρ max 1.8 T m 1.8 T m 12M V 6.2 Magnetic and electrostatic dipoles Two magnetic dipole geometries are used: two 22 degree bend dipoles for the momentum achromat and one 36 degree bend dipole for the mass separator. Within the good field region of ±15 cm, the transverse field homogeneity obtained in OPERA3D [40] was better than with low saturation, which is around 4% at I max 800A, corresponding to about 8 kgauss magnetic field. The three magnetic dipoles designed by GANIL are complete and the elements have been constructed and delivered by the SIGMAPHI company [41] after magnetic field studies (cf. fig. 6). A 22 degree bend angle electrostatic dipole is also used in the mass separator. The electrostatic dipole design is being optimized with OPERA3D simulations by IPN Orsay. The basic requirements determined by the desired separator resolution, acceptance, and rigidity are: dipole horizontal gap is 20cm, radius of curvature ρ =4m, electric field E = 3 MV/m(±300 kv), vertical acceptance 25 cm. Extensive electrostatic simulations have been done to support a mechanical design with peak surface fields limited to 8MV/m on the electrodes and much smaller surface fields along the support insulators. The second half of the anode includes a gap along its center in the horizontal plane to pass rigid beam particles that are not stopped in the momentum achromat. These ions are stopped in a specific beam dump [42]. This should reduce the contamination at the final focal plane due to primary beam particles that would otherwise scatter on the anode and is considered necessary to achieve the design goal for primary beam suppression. The electrodes are profiled to achieve the required vertical extent of the good field region with minimum height of the electrodes. The parameters of the anode gap and electrode profiles were optimized to meet the overall beam optical requirements. The positive and negative high voltage power supplies are being designed starting from the concept high voltage multipliers integrated with the electric dipole s vacuum chamber as currently being used at the ATLAS FMA separator and the ISAC EMMA separator. These supplies are being designed for peak voltages of ±480kV a) to make sure they operate reliably at required voltages by conditioning to at least 20% higher, and b) to possibly operate at higher than Fig. 6. Picture of one magnetic dipole (left) and technical drawing of the electrostatic dipole in its vacuum chamber (right). the base-line requirement of ±300 kv if the design peak fields can be exceeded in practice. The dipole properties are summarized in table 2. All the dipoles are presently treated analytically in the tracking code TraceWin but the implementation of 3D field maps of these elements is in progress. The latest simulations include a 3D field map of the electrostatic dipole. They show that the aberrations coming from a realistic field can be corrected. With an adapted tuning of the magnets, the performances are not significantly deteriorated, notably the mass resolution at the final focal plane. We are confident that we can similarly deal with realistic field maps for the magnetic dipoles. 6.3 Multipoles The multipole magnets were developed to achieve high m/q resolving power at the final focal plane of S 3 together with a large acceptance. Large acceptance and optical flexibility of S 3 require multipoles with large apertures and high field uniformity. Drift spaces should be minimized while still accommodating all necessary equipment. This helps to preserve a large acceptance while reducing growth in image aberrations that degrades resolution. Each multipole or singlet is composed of superimposed quadrupole, sextupole and octupole fields, plus a dipole steerer. The required field strength for quadrupoles, sextupoles and octupoles at the reference radii of 150mm are 1.2T, 0.4T and 0.2T, respectively. The effective length is 350mm.

8 Eur. Phys. J. A (2015) 51: 66 Page 7 of 16 Fig. 7. Transverse cross section (intersection with YZ-plane) of the singlet showing two double layers for octupole, five double layers for quadrupole, three double for the sextupole and one double layer for dipole coil. The red and blue colours represent the direction of currents. A field uniformity of B/B 10 4 is required in the volume defined by the reference radii and the effective length. The singlets are arranged in triplets in their cryostats, for a total of 12 sets of coils and 11 power supplies per triplet, since two of the dipole steerers are coupled together. The seven superconducting triplets are currently under construction and will be delivered to GANIL during A single superconducting magnet for S 3 has already been built and tested. A new winding configuration for transverse magnetic fields of a given multipole order has been developed for the S 3 multipoles, which closely approximates a pure cos(nθ) current density distribution over the entire length of the coil, including the coil ends [43]. Such cos(nθ) winding configuration makes it possible to achieve the necessary field uniformity in the interior of the large aspect ratio (aperture/length) superconducting multipoles to be used. Each singlet is implemented as four concentric cos(nθ) type superconducting magnets wound on a mandrel. The superconducting multiplets are 450 mm long with roughly 86mm drift space between neighboring elements in the triplet, and the warm-bore diameter is 336 mm. The dipole correction coils are the outermost of the nested coils, with the sextupole, qadrupole and octupole coils at successively smaller radii (cf. fig. 7). A warm cylindrical iron yoke outside the coils will contain most of the external field and provide a small enhancement to the interior field. The designs are based on a NbTi superconductor from Supercon with a non-copper critical current density of about 2825A/mm 2. The analysis shows that the required field strengths for the quadrupole can be achieved with a current of 415A, and the octupole and sextupole coils reach their required fields at currents of 260A and 365A, respectively. Fig. 8. Amplitudes of the six most important multipole harmonics at 15 cm normalized to the main multipole component for quadrupole (top), sextupole (middle) and octupole (bottom) fields versus the longitudinal coordinate. C k coefficients give the components of the field associated to the N-th order with N =2k [46]. Three-dimensional field maps based on these simulations were exported and used in the TraceWin raytracing code [44, 45] for simulations of the performance of the ion optical system. The field gradient integral and higher multipole harmonics have been studied in detail (cf. fig. 8) [47]. The field decomposition shows that the harmonic amplitudes are mostly less than a factor of 10 4 of the main field, except for one component (twice the order of the main field) which is less than 10 3 of the main field. The presence of these non-natural harmonics is due to a field perturbation made by the power supply connector. The first partial singlet was cold tested in January and February of 2014, and integral harmonics were measured at room temperature by the manufacturer, Advanced Magnet Labs (AML), in March Measurement of the relative error in the field gradient integral of the quadrupole shows that the sum of the gradient integral including multipoles up to the 14 th harmonic at a radius

9 Page 8 of 16 Eur. Phys. J. A (2015) 51: 66 Fig. 9. Left: OPERA3D Monte Carlo simulation of several charge states of unreacted primary beam going through the open multipole triplet. Simulations of different experimental cases have been performed in order to specify the necessary size of the beam dump fingers and shutters, the vertical size of the shutters opening, and the effects on beam trajectories of the magnetic field throughout the beam dump chamber. Right: Design drawing of the open triplet including the vacuum chamber, upstream and downstream beam dump chambers, and upstream and downstream beamlines and magnetic elements. of 15cm is < 0.15%, i.e. much less than the < 3% requirement. The measurements show that the performance of the coils far exceeds the specified requirements. Additional measurements of the octupole and other coils, as well as more in depth analysis of other contributions will be performed and reported in the future. In order to use measured field maps in simulations following commissioning, a new 3D fast mapper with reduced errors is being designed in a collaboration including RISP/Korea, ANL, and GANIL. 6.4 Open triplet and beam rejection system Rejection of 99.9% of the very high intensity primary beam is performed in the momentum achromat stage. The multipole triplet following the first dipole has been jointly designed by GANIL [48] and CEA/IRFU/SACM to be open on the high-rigidity side (with a vertical acceptance of ±50mm) in order to safely transport the primary beam charge states out of the acceptance (cf. fig. 9). The outer charge states are transported through a delta vacuum chamber with large horizontal extension and out to a specialized beam dump system. This system is located at the momentum dispersive plane; it includes two shutters and one fixed plate at the exit of the first dipole and, farther downstream, two shutters and a large beam dump at the dispersive focal plane. Also five 15mm wide movable beam dumps, or fingers, that can be used to stop primary beam charge states when they are within the acceptance without blocking all of the interspersed reaction products. All of these elements are independently cooled with pressurized water. The quadrupoles in the first half of the momentum achromat are tuned to provide a good focus of the beam charge states on the fingers. The elements of the beam dump have been designed to sustain the very high power deposition expected from LINAG beams, up to 50kW over the full dump system, with individual fingers designed to withstand a peak density of 5kW/cm 2 each. The beam spot size on the fingers varies from 2.6 to 1.3mm (RMS). Unlike the closed triplets used elsewhere throughout S 3, the open triplet consists of resistive magnets working at room temperature, and only sextupole correction coils could be accommodated within the quadrupole pole tips. The sextupole fields in the open multipoles are produced as the residual field of two overlapping, opposite dipole fields of different transverse extent. Despite the field quality, which is poorer than a standard sextupole, these corrections are mandatory for the successful operation of the spectrometer. A superconducting option was considered in detail [49] and produced good field quality, but simulations showed significant losses of the extracted primary beam on the magnet vacuum chamber, incompatible with the use of superconducting magnets. 7S 3 optical modes The design of S 3 is highly versatile and allows the development of various optical modes dedicated to different experimental programs and detection systems. Four standard optical modes have been developed to meet the needs of experiments under particular conditions that are already foreseen. Variations or other specialized modes will likely be developed in future, given the flexibility of S 3, as new experimental needs arise.

10 Eur. Phys. J. A (2015) 51: 66 Page 9 of 16 Fig. 10. Horizontal beam envelopes (3 RMS) for the central charge state and central momentum for the 100 Sn experimental case (cf. table 1) for different optical modes: HR (top), HT (middle), CM (bottom). The contribution that would be expected due to dispersion is indicated by the dispersion coefficient T[1][6] = δ/x (cm/%) (in red) versus the z (m) longitudinal coordinate [44]. 7.1 High Resolution (HR) The high mass resolution optical mode has been developed to perform experiments when the highest possible mass selectivity is needed, for example, for the spectroscopy of nuclei in the N = Z region, where contaminants dominate the desired products, and for the synthesis and identification of superheavy elements from neighbouring isotopes with very small δm M. This mode can be used to reduce the counting rate on the detectors by means of a slit system at the m/q dispersive image, or to measure the mass-over-charge ratio using tracking detectors. Horizontal beam envelopes in the dipoles are optimized for a beam with 50mrad divergence (cf. fig. 10). Due to remaining aberrations, the total angular horizontal acceptance is not symmetric and around 90mrad ( 50/ + 40) (cf. fig. 11). Momentum acceptance is roughly ±8.5% and the first order mass resolving power is R M = 536. The sextupole and octupole elements throughout the system are optimized entirely toward achieving a high mass resolving power at F 4. The obtained mass resolving power is R M = 382. The focal plane width at F 4 for Q 0 ± 2 charge states (Q 0 26+) is ±50mm (cf. fig. 13). 7.2 High Transmission (HT) The high transmission optical mode has been developed to increase the transmission of the recoils of interest while keeping a reasonable mass resolving power. This optical mode is used for large emittance reactions; the target is placed 285mm closer to the entrance of the first triplet compared to the HR mode, and the horizontal beam envelopes in the dipoles are optimized so that a 70mrad divergence beam will fill the horizontal acceptance (cf. fig. 10). The adjustment leads to an increase of the fullwidth horizontal angular acceptance in Monte Carlo simulations from 90mrad to 120mrad ( 70/+50) (cf. fig. 11). This modification reduces the m/q dispersion at F 4 so that up to Q 0 ± 3 charge states (for Q 0 26+) will fall

11 Page 10 of 16 Eur. Phys. J. A (2015) 51: 66 Fig. 11. Angular X Y (top) and angular-momentum X dp/p (bottom) acceptance plots of the three main optical modes: HR (left), HT (middle), and CM (right). The ions transmitted are indicated in red [44]. within the DSSD acceptance windows for the 100 Sn reference case (cf. fig. 13), while decreasing the mass resolving power to about R M = 300 in first order. The reduced focal plane size and the larger angular acceptance both increase the effects of optical aberrations, which will reduce the achievable mass resolving power. After the tuning all the sextupole and octupole fields, the mass resolving power is found to be R M = Converging Mode (CM) The converging mode has been developed to maximize the transmission of the nuclei of interest through the REGLIS3 gas cell entrance window (cf. fig. 10). This mode could also be used to increase implantation efficiency onto the SIRIUS DSSD when mass separation is not needed. In this mode there is no usable mass resolution. In first order, the setting is identical to the high transmission mode, except that the last two triplets of the mass separator stage are tuned to reduce the size for Q 0 ±4 charge states (for Q 0 26+) to match the 5cm diameter acceptance of the gas cell (cf. fig. 13). The reduced size of the final beam distribution comes at the expense of the loss of the achromatic image at the end of the MS stage. The total angular horizontal acceptance becomes around ±80mrad (cf. fig. 11). All sextupole and octupole elements can be turned off without reducing the transmission, except for the two first triplets that will be tuned for the primary beam rejection at F High Beam Rejection (HBR) A high beam rejection mode could be used to run at the highest possible beam intensities and will be used for the lowest cross section experiments, for example the production of superheavy elements. This optical mode is presently identical to the high transmission mode in first order, but could make use of an alternate first order tune depending on the necessary mass resolution. The sextupoles and octupoles in the first and second triplets are used to minimize the primary beam spot size at the fingers, located at F 1, in order to maximize the rejection of unreacted beam particles Rutherford scattered within the target. The sextupoles and octupoles in the third and fourth triplets are used to minimize the achromatic beam spot size at F 2. The small spot at F 2 allows a collimator at this position to block the largest possible fraction of unwanted ions scattered from upstream surfaces. The sextupole and octupole elements of the mass separator stage are then used to maximise the mass resolving power at F 4 as in the HR mode. As this mode has fewer correction elements dedicated to the higher-order corrections at the final focal plane, the final mass resolving power for the high beam rejection mode will necessarily be lower than for the high mass resolution mode. This optical tuning is an example of an alternative mode. Thanks to its great flexibility, S 3 can be adapted to the specific needs of a given experiment. 8 Correction of aberrations Higher-order aberrations are divergences from the firstorder simulations due to the finite extent of the beam envelope and by the detailed features and imperfections of the primary electromagnetic elements, quadrupoles and dipoles. Aberrations degrade the basic properties of the spectrometer, like angular or momentum acceptance and mass or momentum resolution. In large acceptance,

12 Eur. Phys. J. A (2015) 51: 66 Page 11 of 16 Fig. 12. Beam spot (X-Y ) and emittance plots (X-X and Y -Y )of 100 Sn 26+ charge state (see details in table 1) at F 4, using the high-resolution optical mode with all correctors OFF (3 top pads), and with all correctors on (3 bottom pads). The higher-order mass resolving power for the uncorrected system would be only about R M = 26, far below what is necessary to accomplish the physics goals of S 3 [44]. high-resolution optical systems it is particularly crucial to manage these higher-order effects to produce the desired final properties. The S 3 concept relies on the active correction of optical aberrations by sextupole and octupole coils embedded in the quadrupole elements. Depending on the optical mode, the aberrations can be minimized at different specific points along the beam line: e.g. at F 1 to minimize the beam spot size on beam blockers and thus enhance the primary beam rejection, at F 2 to match the collimator aperture or to focus on an optional secondary target, or at F 4 to maximize the separation between different masses in the transmitted nuclei. In each case, the adjustable sextupoles and octupoles are tuned to provide the desired optical performance. The most challenging singular task in the tuning of S 3 is achieving the mass resolution for the HR mode; in this case, all the octupoles and sextupoles of the full system are adjusted to maximize the resolution of neighboring masses in the final focal plane. The six frames in fig. 12, show the final focal plane distributions without any corrections (top three frames) and with sextupoles and octupoles tuned for maximum mass resolution (bottom three frames). We see that the second- and third-order aberrations (those most coupled to the sextupole and octupole correctors) are strongly suppressed; the final mass resolution is close to the value calculated in 1st order (cf. sect. 7). If random (uniform distribution) fluctuations are imposed on the sextupolar and octupolar field values, the resolution is not significantly affected for amplitudes of 0.1% (peak-topeak) or less. Larger amplitudes of 1% have been observed to lead to a 20% degradation of the average mass resolution across the focal plane. For this reason, the power supply stability is required to be better than 0.1%. Some corrector fields (for elements 12 through 16, depending on the mode) have low values, i.e. less than 5% of the nominal current given at Bρ max. Studies have shown that these can be set to zero without any impact on the mass resolution, provided the remaining active correctors are retuned slightly to compensate. This will help avoid above average instabilities sometimes observed in magnet power supplies when operated at very low current. Simulation studies have also shown that the transmission of the nuclides of interest is fairly insensitive (within 2% for all charge states) to the correction fields used, which is the reason why the correction elements are not used in the converging mode. 9 Optical performances and discussion Different optical modes have been developed and coupled to the different detection systems, i.e. SIRIUS and REGLIS3, to meet the needs of the proposed experimental cases, each represented by a key experiment defined by the S 3 Collaboration. All of the optical modes discussed above are compatible with the two detection systems, with performance estimates for the three experimental cases presented in table 3. The optical and detection configuration should ultimately be optimized based on the needs of each experiment. The extreme regions of the nuclear chart expected to be the focus of experimental work at S 3 using fusionevaporation reactions are mainly exotic neutron-deficient

13 Page 12 of 16 Eur. Phys. J. A (2015) 51: 66 Fig. 13. Distribution of selected species at the final focal plane, showing the five transmitted charge states (Q 0 = 26+) of the nuclide of interest and its two neighbouring masses (A=99, 100, and 101), for the 100 Sn experiment in high mass resolution mode (HR, top), high transmission mode (HT, middle), and converging mode (CM, bottom). Detector apertures are drawn in red (DSSD for HR and HT modes, gas cell for CM). Under true experimental conditions, a roughly continuous distribution of contaminants from the many open reaction channels would be observed in the focal plane superimposed on these distributions [44]. nuclei (e.g. along the N = Z line) and superheavy elements. The success of the experiments will be determined by the transmission and, if needed, by the m/q selectivity offered by the mass separator stage. The mass resolving power required is determined either by the need to resolve the desired species from the nearest neighbouring masses or by the need to reduce the rate of contaminants in the detection systems. For the SIRIUS detection, the allowable contaminant rate is simply the counting rate limit per pixel in the implantation DSSD. For the REGLIS3 gas cell, the relevant criterion is the total particle rate in the gas cell. Because of the multiple evaporation channels, ions with similar m/q ratios will reach the final focal plane. In some cases this contamination can be many orders of magnitude greater than the rate of the nucleus of interest (e.g. N = Z nuclei, with up to a factor 10 8 for the representative 100 Sn case). This problem is less critical in the superheavy element region, where one evaporation channel is generally dominant. Experiments may also be contaminated by target-like and fission products with kinematic properties close to the nuclide of interest. Direct transmission of beam-like ions is not possible due to the large differences in electric rigidity between beam-like products and the nuclides of interest. Random ions could reach the final focal plane because of multiple scatterings along the spectrometer. Due to the many factors affecting these very rare events, it is extremely difficult to estimate the resulting contaminant rate. This will be studied and optimised during the first operation of S 3. The optical performances shown in table 3 include the full system up to the final focal plane, taking into account detector and system apertures. Some additional transmission and resolution losses may be expected in practice due to the finite position resolution of the tracking system and scattering or stopping in parts of the full detection system (tracking detectors, silicon box, etc.). Still, the current system design has achieved the desired mass resolving power and recoil transmission rate. The results shown for the worst case experiment (production of 255 No, with the largest residue emittance) indicate a very promising mass resolution performance of S 3 over a wide range of reactions. The variation of the mass resolving power in HR and HT modes (cf. table 3) versus the recoil kinetic energy E ER (in MeV) (cf. table 1) shows a roughly linear behaviour that can be simply parametrised by: { M/δM HR =2.04 E ER M/δM HT =1.95 E ER The transmission of the system is presented in table 3. T FP (%) gives the transmission to the final focal plane taking into account the relative population of charge states but neglecting losses due to finite detector acceptance and slits or fingers for primary beam rejection at F 1. The transmission T Rej (%), on the other hand, does take into account losses due to the primary beam blockers. Transmission values are given for an optical setting tuned on the most populated charge state of the nuclide of interest, although other charge states could be preferable depending on the number and population of primary beam charge states within the momentum acceptance for a given setting. T DSSD (%) is the transmission within the 10 10cm 2 acceptance window of the SIRIUS detection system, and T Cell (%) the transmission within the 5cm aperture diameter acceptance of the gas cell entrance of REGLIS3. The transmission presented in table 3 shows high performance for experiment 1 with T DSSD (%) HR =51.6 and T Cell (%) CM =56.8 that both reach the 50% transmission specification defined as the design goal by the collaboration. The coupling efficiency to the SIRIUS DSSD, given by T DSSD (%)/T Rej (%), is close to 100% for all experiments, both for the high transmission mode and for the converging mode. Also, the coupling efficiency to the gas cell, given by T Cell (%)/T Rej (%) ratio, is good (up to 88% for experiment 1). The efficient coupling to the focal plane detection systems is ensured by the available HT and MC optical modes. If the highest mass resolution is needed, T DSSD (%)/T Rej (%) for HR mode in experiment 2

14 Eur. Phys. J. A (2015) 51: 66 Page 13 of 16 Table 3. Performances of the High Resolution, High Transmission and Converging Mode at the final focal plane for the three experiments presented in table 1 and the selected charge states: δm 1 is the separation in number of FWHM between two m neighbouring masses; is the associated mass resolving power (1 FWHM) relative to the recoil mass and weighted by charge δm states population and their optical transmission including fingers; T FP(%) the transmission up to the focal plane; T Rej(%) the transmission taking into account the primary beam charge states rejection using movable fingers; T DSSD(%) is the transmission in the cm 2 DSSD acceptance window; and T Cell (%) the transmission in the 5cm aperture diameter widow of the gas cell. Statistical errors on transmission values are less that 0.2%. Experiments Optical modes δm 1 m/δm T FP(%) T Rej(%) T DSSD(%) T Cell (%) HR Ti( 58 Ni, 4n) 100 Sn 26+ (1) HT CM N/A N/A HR Pb( 48 Ca,2n) 254 No 18+ (2) HT CM N/A N/A HR U( 22 Ne, 5n) 255 No 8.5/9+ (3) HT CM N/A N/A seems to indicate that the charge state acceptance on the DSSD would benefit from the development of an intermediate optical mode between HR and HT, to maximize the transmission while keeping the mass resolving power as high as possible. Finally, experiments with very broad magnetic rigidity distributions due to lower recoil charge states (like experiment 3) strongly reduce the number of charge states transmitted within the detector acceptance and will make it more efficient in some cases to centre the system on non-integer values of Q 0 (i.e for 255 No). For this reason, each transmission result presented for experiment 3 has been individually optimised by keeping the best transmission obtained using either 8.5+ or 9+ tuning as the central charge state. The variation of transmission T DSSD (%) in the DSSD acceptance window versus the recoil kinetic energy (in MeV) shows a power law behaviour that can be roughly parameterised by: T DSSD (%) HR = EER T DSSD (%) HT = EER T DSSD (%) CM = E The same functional dependence is observed for the transmission T Cell (%) into the gas cell aperture which is given roughly by: T Cell (%) HR = EER T Cell (%) HT = EER T Cell (%) CM = E Note that both T DSSD (%) and T Cell (%) are calculated assuming that the detectors are located at the exact focal plane position. This assumption is valid for the gas cell but a correction factor, or coupling efficiency factor, should be applied to the particles reaching the focal plane depending on the SIRIUS configuration. Further studies of the optical coupling to the SIRIUS detection systems have been performed [50], and the possibility to move the focal plane ER ER downstream to improve the transmission without significant losses of resolving power has been demonstrated separately based on a re-tuning of the last quadrupole triplet and a re-optimization of the higher order corrections. This focal plane shift is only possible when the final focal plane is not constrained to be located at the slit system position, i.e. when using tracking detectors. If a physical selection using a slit system is needed at the final focal plane, greater efficiency is ensured by the removal of the unnecessary tracking system. This coupling efficiency factor strongly depends on the detection system configuration needed in a given experiment. 10 Conclusion The S 3 spectrometer has been optimized for the study of nuclear structure by delayed spectroscopy of nuclei produced by fusion-evaporation reactions. These reactions lead to low-energy recoils, with large angular, momentum and charge state distributions. Further, the nuclei of interest are emitted around zero degrees together with a large amount of contaminant nuclei, mainly beam-like ions, target-like ions, and other evaporation channels. S 3 combines a momentum achromat and a mass separator to provide a very strong selectivity for the nuclei of interest. Its innovative quadrupoles are large aperture superconducting magnets with superimposed sextupole and octupole correctors, in order to achieve both a high transmission and a high mass resolving power at the final focal plane. Using advanced target technologies, the very high heavy-ion beam intensities delivered by the LINAG will allow the synthesis of very rare isotopes on large areas of the nuclear chart, particularly in the N = Z and super-heavy element regions. The principle of the two stage structure the momentum achromat and the mass separator has been presented with an emphasis on the main optical elements (dipoles, superconducting multipole triplets, and