Conseil Scientifique et Technique du SPhN RESEARCH PROPOSAL

Transcription

1 Conseil Scientifique et Technique du SPhN RESEARCH PROPOSAL Title: Design of the Super Separator Spectrometer S 3 Experiment carried out at: SPIRAL2 - GANIL Spokes person(s): A. Drouart, J. A. Nolen (ANL Argonne), H. Savajols (GANIL) Contact person at SPhN: A. Drouart Experimental team at SPhN: R. Dayras, A. Gillibert, A. Görgen, W. Korten, J. Ljungvall, A. Obertelli, B. Sulignano, Ch. Theisen, List of DAPNIA divisions and number of people involved: SENAC(2), SACM (1), SIS List of the laboratories and/or universities in the collaboration and number of people involved: Permanent staff involved in working groups: ANL Argonne (3), CSNSM Orsay (2), FL Dubna (1), GANIL (6), GSI (2), IPHC (2), IPNO (3), Jyväskylä University (1), KU Leuven (1), Mainz University (1), MSU (1), TANU (1), Vinca Institute (1) SCHEDULE Possible starting date of the project and preparation time [months]: Total beam time requested: non relevant Expected data analysis duration [months]: non relevant REQUESTED BUDGET Total investment costs for the collaboration: ~10M Share of the total investment costs for SPhN: to be determined Investment/year for SPhN: to be determined Total travel budget for SPhN: travel cost covered by ESFRI Travel budget/year for SPhN: travel cost covered by ESFRI If already evaluated by another Scientific Committee: Letter of Intent Approved by SPIRAL2 Scientific Advisory Committee Technical Proposal to be submitted to the SPIRAL2 SAC in June 2008

2 Design of the Super Separator Spectrometer S 3 Introduction The high intensity stable beams of heavy ions provided by the SPIRAL2 LINAG will allow nuclear structure studies to be performed over a large range of the Segré chart. The main topic of physics that has been emphasized by the S 3 collaboration is the delayed study of nuclei produced by fusionevaporation reaction. Two regions of interests have been specifically chosen as areas of interests: super heavy elements and intermediate mass neutron deficient nuclei, specifically in the 100 Sn region. Nevertheless, other aspects could also be addressed and they will be briefly discussed in this document. The S 3 device will be coupled to state-of-the-art detection systems and employ advanced target technologies to handle the intense primary beam. In this document, we will present the different physics cases that have been chosen as priority for the design of S 3. Simulations of such reactions have lead to the definition of the required characteristics for S 3 in terms of beam, target, separator/spectrometer, and detection. We have estimated different difficulty levels for these different aspects. Finally, we detail technical proposals for the design of the separator/spectrometer. We proposed two different layouts: - A sequential design, which aims at fulfilling all the requirements with a unique, innovative set-up. - A parallel design, where S 3 is composed of two branches, one optimized for experiments requiring high transmission, the other focusing on mass resolution. Note The 10 th of June, the S 3 collaboration will present to the SPIRAL2 Scientific Advisory Committee a technical proposal for the S 3 project, which includes the topics covered here and additional details on nuclear safety, low energy branch set-up and connection to the DESIR area, which are only briefly mentioned here (reports are under progress). Physics Cases & Requirements Superheavy Elements Present Status of Research In the physics case of the SPIRAL2 white book, the field of superheavy element research was identified as one of the major subjects which can be exploited by the stable beam part of the facility with the high heavy ion beam intensities offered by its linear driver accelerator LINAG. There it was illustrated that the fundamental question "Is there a limit, in terms of number of protons and neutrons, to the existence of nuclei?" can be attacked by employing a broad range of techniques. The main goal, the synthesis of ever heavier new elements until the "island of stability" is eventually reached, will be driven by applying the most modern, existing and yet to be developed, techniques of nuclear spectroscopy, reaction mechanism studies and even chemistry of heavy and superheavy nuclei. The highly intense stable beams available at SPIRAL 2 together with a highly efficient separator and/or spectrometer like the S 3 set-up are an ideal combination for low cross section experiments. The present status of the field is summarized in Fig. 1. One of the major open questions is the connection of the decay patterns observed in 48 Ca induced reactions on actinide targets at the gasfilled separator of the FLNR, Dubna [1] (brown framed region in Fig. 1). In contrast to the decay chains leading to the heaviest nuclei in reactions with 208 Pb and 209 Bi targets first observed at the velocity filter SHIP of GSI, Darmstadt [2] (green framed region in Fig. 1) and later at the gas-filled separator GARIS of RIKEN, Tokyo [3], those are not connected to isotopes with known α decays what would settle the unambiguous identification, but end all in fission assigned to unknown isotopes. First attempts to reproduce the data for 48 Ca U were already successfully performed at GSI [4]. Also for the Z assignment a group of chemists from the PSI in Switzerland has made impressive progress [5]. The set-up proposed here has the capability to contribute essentially to the solution of this puzzle. Nuclear structure studies in terms of in-beam and decay spectroscopy have almost reached the heaviest known nuclei. They have provided the first information on the structure of actinides and transactinides [6]. The nuclides studied e.g. at the velocity filter SHIP at GSI in terms of decay

3 spectroscopy are marked in blue in Fig. 1. The development of single particle levels towards high Z and A is a major ingredient for the localisation of the next shell gap in Z and N. To pursue this task the trends of the first excited states have to be followed into the region, which is framed red in Fig. 1. As a start-up a series of reactions has been defined as first day projects for the combination of LINAG and a separator/spectrometer like the here proposed S 3. shell correction MeV decay mode Fig. 1: Excerpt of the chart nuclides in the region of superheavy elements. Indicated regions: Green: nuclides where the decay chains of the heaviest at SHIP synthesised elements (Z= ). Brown: at Dubna observed decay patterns for 48Ca-induced reactions on actinide targets. Blue: isotopes for which nuclear structure data has been collected at SHIP (bold line: new or improved data; hatched area: reproduced or confirmed data). Red: region of interest for near future investigations. The background pattern: shell correction energies according to R. Smolanczuk et al. [7]. First Day Experiments One of the major open questions is the connection of the decay patterns observed in the 48 Ca induced reactions on actinide targets at the gas-filled separator of the FLNR, Dubna [1] (see Fig. 1). As the Q/A of the heavy ion linac will be limited in the first construction phase to 1/3, reactions with relatively light projectiles (A<~40) on actinide targets are the natural choice. For the first day experiments, reactions of various silicon, sulphur and calcium isotopes will offer already exciting opportunities to perform forefront experiments in the context of SHE synthesis, reaction mechanism, nuclear spectroscopy and even chemistry studies. Fig. 2 illustrates the nuclei which become accessible with the reaction series described in the following Ca U * Theoretically possible xn evaporation residues (ER) from this series of experiment are ranging from (3n ER channel with 48 Ca beam) which was the first and up to now only isotope in this hot fusion region for which the same decay patterns have been observed outside the FLNR, at the velocity filter SHIP at GSI [4], to (3n ER channel with 42 Ca) which was observed in cold fusion. In this way, the link can be established between the island, to which the decay patterns obtained in 48 Ca-induced reactions have been assigned, and the edge of the "main land" reached in cold fusion studies. In addition, the highly intense stable beams provided by LINAG could possibly make nuclear structure studies feasible also for the very heavy isotopes produced in these reactions S U Hs*: The first evidence of the element hassium was produced in the reaction 58 Fe Pb as the isotope 265 Hs [8]. Nowadays various additional isotopes are known as decay products from heavier

4 nuclides produced in cold and hot fusion reactions as well as from the reactions 25,26 Mg Cm [9]. The latter reactions are leading to isotopes with reasonable high cross sections and long life times that chemical studies and eventually the assignment of hassium to the Fe-Ru-Os group in the periodic table became possible [10]. The series of reactions possible with sulfur isotopes span the whole region and allow for detailed chemical studies. In addition and even more important for the understanding of nuclear matter under the extreme conditions of high Z and A, they serve also for reaction mechanism and nuclear structure investigations entering the region in between the two groups of heaviest known nuclei Si U Sg*: The understanding of the reaction mechanism, which is governing nuclear reactions around the Coulomb barrier as a function of isospin, is essential to successfully design future experiments to reach ever-higher Z and eventually localise the region of spherical superheavy nuclei. Present day capabilities are strongly limited by the maximum available beam intensities and only marginally significant data could be collected e.g. for the reaction 30 Si U [11,12]. To really understand the reaction dynamics the high beam currents from the SPIRAL 2 LINAG are necessary. Beyond this scope, they will also allow for nuclear structure studies protruding into the neutron rich region. The trend of single particle levels as a function of proton and neutron number, especially in the region around the deformed subshells of Z=108 and N=162, will yield most important information also on the localisation of the spherical closed shells at Z = 114, 120 or 126, and N = 184. The seaborgium isotopes and their daughter products produces in this reaction series together with hassium isotopes mentioned above are well in the centre of this region of special interest. One interesting feature expected in this region is K-isomerism which has been observed up to 270 Ds (red circles in Fig. 2). Those isomeric states might be characterised by special properties like different spontaneous fission-α branching ratios, interesting population schemes and even longer lifetimes than the ground state. On the basis of this package of first day experiments, its results, and the lessons learned by those studies, the main quest in SHE research, the search for the "island of stability" could be attacked. The highly intense stable beams of the SPIRAL 2 LINAG together with a state of the art separator and spectrometer as envisioned with S 3 will make this facility a unique place on global scale for this type of research. To settle the question of connecting the decay patterns observed for 48 Ca induced reactions to the known nuclei the systematic described above will help, as the mass identification capability of S 3 will do. The synthesis of new elements and the desire to investigate the structure of those neutron rich nuclei call for the use of actinide targets Ca U * S U Hs* Si U Sg* (higher Z) actinide targets? link hot to cold fusion x-section systematics transactinide chemistry hunt for K-isomers

5 Fig. 2: Series of first day experiments proposed with 238 U targets and various silicon, sulphur and calcium projectiles. The possible compound nuclei and xn ER channels are indicated as well as the regions where K- isomers have been identified (red circles) and looked for (pink circle). Requirements Optics As a reference for kinematics conditions, we took the fusion reaction of 48 Ca beam on a 248 Cm target. More asymmetric reaction could also be foreseen, in which cases we have slower residues and larger angular distributions, but larger differences between the beam and the evaporation residues. The following table sums up the different results Beam 48 Ca Recoil E [MeV/n] <Bρ> [Tm] <Eρ> [MV] <Q> <V > [cm/ns] θ (±2σ) [mrad] dq dp/p [%] ± 8 ±0.2* ± 55 ± 2 [23-27]=>67% ±2.3 - Evaporation residues are kinematically clearly separated from the beam. The rejection is then easier, but it is also the case where it must reach a very high value of A large angular and charge state acceptance is required - Secondary reaction is not possible due to the low energy recoils. - Beam rejection: Even very improbable charge states or extreme tails of the beam geometry have to be carefully checked to reach this value. - Mass resolution of 1/350 would be welcome but is not mandatory. SHE identification is done through decay measurements. - Final focal plane size should be less than 20cm for gamma spectroscopy efficiency purposes. For chemistry experiments it should be of a few square centimeters. Targets Primary target should be able to sustain the highest intensities. Target thicknesses range from 100 to 500 µg/cm 2 with a possible backing (carbon, titanium). The use of actinides targets is mandatory for the synthesis of heavy elements and of the utmost importance for the spectroscopy of new isotopes. Detection The basic detection for SHE research involves an alpha/electron/x/gamma decay array at the focal plane. The highest efficiency and resolution are required. Ground state properties and chemistry studies could also be performed. The evaporation residues are then implanted in a gas catcher and brought to a specific apparatus. Neutron deficient nuclei Physics Challenges in the 100 Sn Region For the 100 Sn region, the key issue is a set of precision tests of the shell model, and more generally, of the concept of a simple mean field with single particle motion. 100 Sn is a doubly magic nucleus with N=Z and situated right at the proton drip-line. This makes it quite different from 16 O, 40 Ca or 208 Pb. The experimental challenges are to precisely locate the position of states near the Fermi surface, and eventually to investigate the degree to which their wave functions reflect pure single particle motion. With this framework established, the residual interactions between particles can be explored, both in isospin (protons and neutrons) and in particle and hole character. This information forms the solid basis for description of all proton-drip-line nuclei. With intense beams of stable nuclei, this region is uniquely accessible. 100 Sn (core states), 100 In (particle-hole coupling), 99 In, 96,97 Cd, and 94 Ag (hole states) are all within reach. Indeed experiments

6 are being discussed now with 100pna beams. With 10pµa beams, studies of nuclei across the N=Z line should be possible, reaching the dripline, even for the even-even nuclei. For some of these very neutron-poor nuclei, two-proton emission becomes favoured, possibly enhanced by a proton-skin effect. Above 100 Sn the physics interest is to understand how deformation helps restore relative stability and to determine the onset of deformation. It is conceivable that an island of N=Z nuclei lies outside the proton drip-line, so the topology of binding becomes interesting. The immediate N=Z nuclei above 100 Sn become very fast alpha emitters in their ground states, so would not survive the transit in the separator. However, overshooting to the barium and heavier nuclei, like 112 Ba, the ground states are re-stabilized by deformation and become relatively long-lived once more. Search for Proton-Emitting Nuclei Beyond 185 Bi Proton emitting ground states have been observed in the majority of odd-z elements between tin (Z=50) and bismuth (Z=83). The proton emission process is determined by an interplay between the nuclear attraction and Coulomb and centrifugal barrier effects. The proton decay rates yield information on the structure of the parent and daughter states through the spectroscopic factor. To date no proton emitting nuclei beyond 185 Bi have been found. Another interesting case is proton emission from odd-odd nuclei, where the effect of coupling to an odd neutron on the proton emission can be investigated. Detailed Decay Spectroscopy along the Proton Drip-Line beyond Pb The increased beam intensities available with LINAG will make the production of larger quantities of exotic nuclei at the proton drip-line possible. With the addition of a state-of-the-art focal plane spectrometer, detailed spectroscopy of nuclei produced at the nb level and below will be achievable through charged-particle and γ/conversion electron coincidence techniques. The study of isomeric states will also be possible with the same experimental set-up. In studies of odd-mass nuclei, these experiments will allow the evolution of single-particle states to be mapped out far from the valley of beta stability. Detailed measurements may also reveal whether information has been missed in studies performed using only α decay, which can have an effect on the deduced masses. Experimental Approaches Coulomb Excitation The core physics opportunity here is to identify the development of nuclear collective modes going away from 100 Sn. The secondary beams from S 3 are really too low in energy, but with these fluxes many not-too-exotic nuclei could be studied. The evolution of collectivity is certainly an interesting issue. β-decay Decay studies in this region are physics-rich, especially the investigation of GT-decay strength and analog states. These studies will be hampered by the problem of channel selection. Implantation on a tape or into flowing gas can help suppress the very long-lived activities, but this gives one or two orders of magnitude improvement, not the six orders needed. Only the super-allowed cases may be amenable as they decay in 50ms as opposed to many seconds for the bulk of the nuclei. Search for isomers This is a physics-rich opportunity, as the isomers are very sensitive to both the location of single particle states and to the correlations between nucleons. It is perhaps the most physics-rich opportunity as it can answer many of the physics challenges, but has a good experimental trigger which can be used in this high rate environment. Observation of gamma decaying isomers and the spectroscopy of the decay paths and rates can determine these quantities, with some theoretical input from the shell model. Both of these nuclear properties need determining at the drip line in order to ascertain if low binding energies / coupling to the continuum / proton skins cause significant modification of structure. To quantify them, working near a doubly magic nucleus is the ideal place. Search for charged particle decays

7 (beta-delayed protons, proton radioactivity, superallowed alpha decays, multiple nucleon and cluster decays). This again is a very rich physics topic, as the decay rates, especially for proton emitters, are sensitive to structure. Unfortunately, at the moment no candidates are known, though they should exist. Requirements Optics As a reference, we used the symmetric fusion reaction 58 Ni+ 46 Ti 100 Sn +4n. The characteristics are summed up in this table: E [MeV/n] <Bρ> [Tm] <Eρ> [MV] <Q> <V > [cm/ns] θ (±2σ) [mrad] dp/p (±2σ) [%] Beam Ni ±8.6 ±0.2* Recoil 100 Sn Target Parameters 46 Ti ±28.6 ± ±5 ±0.2* - Those reactions will be performed using the so called low energy branch. A minimal 10% total efficiency can be considered if the number of particles stopped in the buffer gas cell does not exceed 10 8 pps. It is therefore important that the primary beam will be suppressed by S 3 prior to the gas cell. To increase the selectivity, laser ionization can be coupled to the gas cell (isotope selection) as well as a mass separator (a dipole with mass resolving power of 1500) behind the gas cell (see LISOL facility) (isotope selection). Furthermore, for later coupling to DESIR, all the low energy branch has to be set on a high voltage platform. - Separation of beam and ER is less than with direct kinematics. There is an overlap in the magnetic rigidity distributions but the velocity difference is still important. We can accept beam counting rates that are 1% of isobaric counting rates (i.e pps). - Angle and charge state distributions are large. - Mass resolution is critical in this case to reduce the very high counting rate of evaporation residues. - Isobaric selection must be reached. Due to the low energies, it is impossible to do it in-flight with a degrader. A gas catcher with a laser ionization selection or with a very high resolution dipole is considered at the end of S3. This will be the perfect device to study the 100 Sn with high resolution ion trap, for mass, charge, radius or spectroscopy measurements. Targets The targets of stable medium mass elements should sustain the highest intensities. Charge equilibration with carbon foils is required. Detection As pointed out in the previous section, several detection settings have to be used at the focal plane of S 3 : beta/particle/gamma spectroscopy. A gas catcher connected to a low energy branch is necessary for specific ground states studies: measurements of mass or charge radius by laser spectroscopy Other physics topics The previous topics have in common the fact that they all deal with the production of unstable nuclei by a fusion-evaporation reaction and their study in a detection set-up or gas catcher. In this section we address to other possibilities offered by the high intensity beams of LINAG. Secondary reactions Nuclei produced by fusion in inverse kinematics may be produced in sufficient quantity and at sufficient energy in order to perform an additional, high cross section reaction on a secondary target. Such experiments have specific requirements that we detail here.

8 Optics As an example of inverse kinematics, we choose the 58 Ni+ 12 C 68 Se+2n fusion-evaporation. <E> [MeV/n] <Bρ> [Tm] <Eρ> [MV] <Q> <V> [cm/ns] θ [mrad] dq Beam ± 5 (34%) Recoil ± 18.8 ± 2 (27%) [56-60]=90% (Bρ) [%] ±9 - The beam and ER are very similar in magnetic rigidity and in velocity. This makes the separation of both extremely difficult. - An initial primary beam suppression of 100 to 1000 can be done via a small beam stopper at zero degrees since the yield of recoils is suppressed at the center of the distribution. An annular acceptance chosen to enhance the 2n channel over the 1n1p and 2p channels can also be employed. - The forward focusing of the reaction reduces the dispersion of the evaporation residue. Nevertheless, tracking of the ions after separation is required. Emissive foil detectors are adapted to this purpose. - A mass selection is necessary (1%). The total counting rate at the detection plane must be less than 10 6 pps. - An isobaric identification is also required. It must be done in-flight to allow the secondary reaction. This could be done with a degrader in the spectrometer (in such case, the energy loss must not be too great) and with the identification through a detection system (e.g. Bragg chamber). In this latter case, a counting rate of less than 10 6 pps is mandatory. - The ER must be focused on a small spot on the secondary target ( a few square cm) Targets Light stable targets of less than 1mg/cm 2 are required in such experiments. Detection The detection setting for secondary reactions must deal with very low energy (<3 MeV/n) heavy ions. It should include: - Tracking devices in order to reconstruct the ion trajectories before the secondary target - Identification in Z in order to get rid of the isobaric contamination - Gamma detection around the secondary target. The Compton background due to the dominant contaminants could be much stronger than the interesting nucleus photopeak. Note A detailed study of the 58 Ni+ 12 C 68 Se+2n reaction has been made by Andreas Georgen (IRFU/SPhN). It concludes to the high level of difficulty for such an experiment. It will be easier to measure the B(E2) with a plunger, the goal can hence only be to measure the quadrupole moment. One should also keep in mind that a (pure) Se beam could be made at Isolde. Even though the intensity for 68 Se there will be very small, they will have higher energies at HIE-Isolde. The comparison between both facilities should be carefully studied. Transfer of a few nucleons Transfer of nucleons using light nuclei could be used with S 3 in order to produce high intensity of secondary radioactive beams that could impinge on a secondary target, and then perform a secondary reaction like another transfer, inelastic scattering

9 12 C( 13 C,2p) 11 Be Reaction I( 13 C) = pps (5kW) Theoretical σ : 2mb Production rate ~ evts/sec E beam = 10MeV/u Target 20mg/cm 2 of 12 C dp/p = 25% Beam Recoil <E> <Bρ> <Eρ> <Q> <V> θ [MeV/n] [Tm] [MV] [cm/ns] [mrad] ± 5 dq (Bρ) [%] ± >±10% - To get rid of the beam and optimize the cross section, the beam can impinge on the target with an angle with respect to the spectrometer or a degrader can be used since recoils will be fully stripped for these light nuclei. - The angular distribution of the interesting nuclei is very large. - Magnetic separation is needed in that case. Velocity separation is not efficient - Secondary reactions are possible - Bρmax = 1.8 Tm - The electric rigidity is beyond what is possible for the mass separator section. Pure magnetic separation can be used if the electric section can be bypassed. A non-limitative list of radioactive beams available by this technique is shown below. Beams Intensity (pps) Primary beam Beam power comment 18F F@10MeV/u 5kWatt 50*spiral1;*10 Rexisolde 14O O@10MeV/u 5kWatt 2*spiral1 13O O@10MeV/u 5kWatt 2*spiral1 20O Ne@10MeV/u 5kWatt 10*Rexisolde 11Be C@10MeV/u 5kWatt 12Be N@10MeV/u 5kWatt 105@50MeVLise 10C C@10MeV/u 5kWatt 500*Rexisolde 16C O@10MeV/u 5kWatt 100*Rexisolde 22Mg Mg@10MeV/u 5kWatt 1000*Rexisolde 21F Ne@10MeV/u 5kWatt Lise E>50 MeV/A I=107 22F Ne@10MeV/u 5kWatt 34Si S@10MeV/u 5kWatt 32Al S@10MeV/u 5kWatt 30Mg S@10MeV/u 5kWatt * Rexisolde 38S Ar@10MeV/u 5kWatt 45cl Ca@10MeV/u 5kWatt 44cl Ca@10MeV/u 5kWatt 52Fe Fe@10MeV/u 5kWatt 62Fe Ni@10MeV/u 5kWatt 48Cr Cr@10MeV/u 5kWatt 56Ni Ni@10MeV/u 5kWatt 1000*Rexisolde 56Ni Ni@10MeV/u 5kWatt 100*Rexisolde 52Ti Cr@10MeV/u 5kWatt Deep inelastics scattering Deep inelastic scattering enables the transfer of a large number of nucleons. In this way, neutron rich nuclei can be reached outside the fission bumps. Reaction Target I( 86 Kr) = pps (5kW) 1mg/cm 2 of Bi 86 Kr MeV/u

10 Beam Recoil <E> <Bρ> <Eρ> <Q> <V> θ [MeV/n] [Tm] [MV] [cm/ns] [mrad] ± 5 dq (Bρ) [%] ± 200 ± 2 >±10% - Such nuclei are typically studied by prompt spectroscopy around target. In this case, the beam intensity is limited by the counting rate of the gamma array around the target (e.g. ~ pps with a high counting rate detector like AGATA). The very high intensity of LINAG is of no use here and other devices (VAMOS, PRISMA) are more adapted for these experiments. - DIC is at the grazing angle (non zero degree) - Very high angular acceptance is needed, only a small part of the azimuthal angle will be covered - Secondary reaction is possible with the restrictions seen in the inverse kinematics cases. - Considering that only delayed measurements (not around the primary target) are possible, the interesting physics cases must be clearly identified. Conclusions Physics Priority The S 3 collaboration has fixed three different levels of priority to be taken into account on the design of the spectrometer: - High priority: level 3 - Medium priority: level 2 - Low priority: level 1 Scientific option SHE / VHE Synthesis and delayed spectroscopy 3 Chemistry 3 Ground state properties 3 Secondary Coulex with inverse kinematics 1 N = Z Ground state properties 3 Secondary Coulex with inverse kinematics 2 NRN Multi-nucleon transfers / prompt spectroscopy 1 LN Direct reaction in inverse kinematics 1 Global requirements To conclude this part: To reach very high efficiency, acceptances found in both angle and momentum are : Angular acceptance > +/- 60 mrad X and Y Bρ acceptance: +/- 10% For some reactions, the required angular and momentum acceptances exceed those values, but a technical solution that offers higher acceptance will not meet the other constraints (Beam rejection, mass resolution, etc) The beam rejection : Bρmax (reaction products) = 2Tm Eρmax = 30 MV (15 MeV/q) (Electric dipole limit) A good mass resolution is mostly required (1/350). This can be obtained by different techniques following the reaction to be considered (Mass separator, Energy-TOF, gas catcher laser ionization, ) We present in the after-the-next section two different options to fulfill these constrains.

11 Technical proposal Difficulty levels Complexity levels have been assigned for the different parts of the project. These risks are assessed from several foreseen aspects: - Technical complexity of the equipment - Availability or technical feasibility of the equipment - Cost of the equipment Primary beam The difficulties are linked to the availability of the highest intensities. - Physics with A/Q = 3 (deuteron injector + common ion source): Low difficulty coefficient 3 - Physics with A/Q = 3 (deuteron injector + high intensity ion source): Medium difficulty coefficient 2 - Physics with A/Q = 6 (source and injector optimized for heavy ions) : High difficulty coefficient 1 Targets Baseline The basic target for S 3 will consist of a high speed (>2000 t/min) rotating wheel, with a variable diameter depending on the material (10 to 60 cm). Cooling systems (He jet) are possible. Various diagnostics (electron beam) are considered for the on-line target monitoring. After the Fulis construction (high speed rotating wheel for GANIL), the IRFU/SIS has already an expertise in the design of such devices. For stable elements, high fusion temperature compound (PbS) should be used when available. They have already proven their durability. Actinide targets For actinides targets (Plutonium to Curium), specific safety measures have to be taken. Such targets have already been used in various laboratories in Europe, and the S 3 collaboration has established contacts in order to estimate the appropriate safety levels: - with GSI and Munich laboratories, which have already produced and used such targets - with the IRFU/SENAC for the nuclear safety calculations and estimation for technical solutions - with the IRSN for the administrative procedures and expertise in nuclear safety. The following procedure has been foreseen: - Targets are rented from a relevant laboratory (Dubna, Munich ). They are returned at the end of the experiment - Targets are mounted on the rotating wheel in a specific laboratory with adapted nuclear safety. - The wheel is put in a sealed target chamber, which is designed to limit the contamination both in inbeam conditions (limitation of sputtering by small solid angle). - The target chamber is transported up to the target point in the S 3 area. The zone around the target point is restricted and submitted to nuclear safety rules. - The target chamber is connected to the beam line and only then can be unsealed. - Specific procedures ensure that contamination is minimal in case of accident (fast valves up and down streams). Note that these studies are done in coordination with the Neutron for Science collaboration, which has also the need for radioactive targets. The target difficulty is linked to the necessity of using actinides targets for the experimental program. - No actinide required : Low difficulty coefficient 3 - Partly Actinides required : Medium difficulty coefficient 2 - Mainly Actinide required : High difficulty coefficient 1

12 Detection The difficulty level for the detection is linked to the complexity of the different detection settings required for the experiments. - Instrumentation is simple (Recoil decay tagging spectroscopy): low difficulty coefficient 3 - Instrumentation is complex (low energy branch): medium difficulty coefficient 2 - Instrumentation is highly complex (detection for low energy secondary reactions): High difficulty coefficient 1 Optics The difficulty level for the optics is linked with the overall complexity of the device and its cost: Energy Velocity (mass) filter: Low difficulty coefficient 3 Magnetic spectrometer: Medium difficulty coefficient 2 Magnetic and velocity (mass) filter: High difficulty coefficient 1 Another aspect is the possibility to use the spectrometer for the detection of reaction products at large angles. One technical possibility would be to use a beam swinger. It will enable to give the beam an incident angle from 5 to 10. It will require specific equipment before the target, as well as a specific target chamber and beam dump. In the next section, we propose two options for the design of the spectrometer. Summary of the difficulty levels PHYSICS PROGRAM (PRIORITY) Primary Target Optics Detection. beam SHE / Synthesis VHE delayed spectroscopy Chemistry Ground state properties Secondary Coulex with inverse kinematics Neutron Ground state properties deficient Secondary Coulex with inverse kinematics Deep Multi-nucleon transfers / prompt inelastic spectroscopy Direct reactions Direct reaction in inverse kinematics Considering these aspects, the collaboration has established that the minimal baseline for S 3 should be: - A source/injector optimized for heavy ions up to at least mass 60 - A high power target for stable elements - A 0 separator/spectrometer with high acceptance, high rejection power, and mass resolution. - A basic detection chamber, compatible with a gas catcher for low energy measurement. Even if they must be taken into account at the beginning for further upgrades, other aspects do not need to be fully implemented at the very beginning: - A/q=6 injector - Actinide targets - Rotation of the beam around the spectrometer axis - Detection set-ups for secondary reactions. - Detection set-ups for chemistry Their presence will be dictated by the interests of the physics community and the effective feasibility. Nuclear Safety As pointed out in the last CSTS, all these aspects are strongly linked with the nuclear safety that is required both by the high intensity beams (beam dump) and the possibility to use radioactive materials (target). This topic is currently being studied by the Irfu/SENAC and the SPIRAL2 safety group, with a strong collaboration with the optics and target working groups. A report is due for the 16 th of May.

13 Technical proposal for the optics For the optics design, we propose two different options. One is the sequential design, where a unique, versatile beam line in two stages is used for all the experiments. The other is a parallel design with two different, independent beam lines, the first being optimized for separation and the second for its mass selection. Sequential design In the sequential design, a momentum achromat is followed by a mass achromat. A simple sketch of the layout is given is the nearby figure. This configuration ensures that the beam can be separated from the reaction products first according to its magnetic rigidity, and second according to its mass. In this case the rejection of the beam should be optimum. On top of that, the first stage reduces the beam intensity by a factor of at least 1000, ensuring that the intensity is at no risk for the second part that requires strong electric fields. So far, a preliminary study of the momentum achromat has emphasized several aspects of the design: - The strong symmetries of the device allows for a natural cancellation of second order aberration. Nevertheless, it is still necessary to add up to 12 sextupoles for this first part. Thus the resulting momentum achromat is 21 meters long (at the limit for the available space in the S 3 room). This could be reduced and improved with the help of superconducting quadrupoles, which enable to couple quadrupolar and sextupolar fields, thus significantly reducing their encumbrance. - It has been simulated for three different reactions: direct, symmetric and inverse kinematics fusion reactions. The simulation shows that the beam will have different trajectories for each reaction. It is thus necessary to foresee three different zones for beam dumping: - in the second quarter of the line, in the Qpoles area. The vacuum chamber has to be carefully designed to stop up to 100% of the beam in direct kinematics. - in a free drift zone, which is made to accommodate for a beam dump stopping up to 70% of the beam. - in the medium dispersive plane, where adjustable fingers have to be set in order to stop specific charge states of the beam, mixed with the interesting reaction products. The next 3 figures show the beam trajectories for the direct, symmetric and inverse kinematics cases: Figure 1: direct kinematics 48 Ca+ 248 Cm (116)*

14 Figure 2: symmetric kinematics 58 Ni+ 46 Ti 100 Sn+4n Figure 3 : Inverse kinematics 58 Ni+ 12 C 68 Se+2n This design provides a nice object point at the exit of the momentum achromat, at the image point of the mass achromat. The counterpart of such rejection is the partial transmission of the interesting nuclei. In the case of the direct kinematics, it is expected that ~60% of the evaporation residue are transmitted, provided that the magnetic elements are large, with a 40cm diameter. Due to the presence of the fingers in the focal plane, ~40% of the 100 Sn are transmitted in the symmetric kinematics. The scattering on the finger should be eliminated by an anti-scattering aperture at the object point as well as by the mass selection of the second stage. This second stage has not been designed yet. Its mass resolution and its transmission have to be calculated according to the first stage outputs. The main advantage of this design is its versatility. It should also have a very good rejection power and a reasonable transmission. Its mass resolution is still to be determined. Its main drawback is its length and complexity, both in design and in operation (tuning of the fingers in the focal plane). Work in progress We are currently exploring the possibilities of simplifying the first magnetic stage. The second mass spectrometer stage is under study at the Argonne National laboratory, with parallel work at Saclay.

15 Parallel design In this design we propose to use two different devices in two parallel beam lines. The first one will be optimized for high transmission and the second for high mass resolution experiments. Each beam line has its own target point. Practically, the high transmission device could be a gas filled spectrometer, and the other a recoil mass spectrometer. The figure on the right shows a basic sketch of the design, with arbitrary optical elements and sizes. Gas filled separator (GFS) Basics of GFS Gas filled separators are already widely in use for the studies of fusion-evaporation products [13]. They are based on the principle of the constant charge exchange between the ions and a gas (typically H 2, He) at low (~1 mbar) pressure. When put in a magnetic field, the ions follow an average trajectory that does not depend on their charge state, but solely on their mass, velocity and atomic number. As a consequence, the acceptance is not limited by the large charge state distribution (hence of magnetic rigidity) neither of the beam or the evaporation residues. If the effective magnetic rigidities of the beam and the reaction products are different enough, both are cleanly separated in the dipole. Meanwhile, the scattering on the gas molecule induce a strong straggling of the nuclei. Thus, the mass resolution of such spectrometers are poor, of the order of a few %. Today, GFS are used in Riken (GARIS), GSI (TASCA), Jyväskylä (RITU), Dubna (DGFRS) and Berkeley (BGS) for superheavy elements and for neutron deficient nucleus synthesis and spectroscopy in direct or symmetric kinematics. The following table (courtesy of D. Ackermann) shows the characteristics of different GFS: Separator DGFRS GARIS BGS TASCA TASCA RITU LTM* SIM* Configuration DQhQv DQhQvD QvDhD DQhQv DQvQh QvDQhQv Length / m Bend. angle / deg Bρmax / Tm Dispersion / mm/% Solid angle / msr Transmission / % * TASCA can operates in two different mode: a large transmission mode (LTM) and a small image mode (SIM) for the purpose of chemistry experiments Transmission is given for the evaporation residues of a 48 Ca+ 208 Pb like reaction. The rejection powers of GFS are fairly high. RITU reaches for 48 Ca+ 208 Pb fusion. GARIS has a rejection power of for 64 Ni+ 208 Pb. The BGS, with its large bending angle, is stated as having rejection >10 15 for beam like particles and >10 4 for target like particles. Nevertheless, its high dispersion leads to a lower transmission (relatively to its high angular acceptances), because nuclei are dispersed out of the focal plane detection. Considerations for S 3 The following criteria are important in our application: - Gas interaction: the high intensity beam will loose a large amount of energy in the gas. While charge space effect is unforeseen (experiments have already been done with 100mA proton beams), gas heating will probably occur. This effect may lower the local pressure around the beam trajectory and perturb the charge equilibration. Nevertheless, experiments with 2pµA Argon beams have been done with TASCA and haven t shown any major consequences. - High rejection power: a rather large bending angle is required. A compromise has to be made with the transmission and the dispersion at the focal plane, but the BGS is a proof of feasibility. A bending

16 angle of can be considered. Like GARIS, a last small dipole before the focal plane helps to remove low rigidity background. - High transmission. Simulation made on the TASCA GFS show that a DQQ configuration is optimal when the target can be put very close to the dipole entrance. Due to the few magnetic elements, large apertures are technically (and financially) feasible. - Target: an advantage of gas filling is the natural cooling of the target by convection. This is critical in the case of the high power beams of S 3. - Beam dump: the beam dump is well localized without being too strongly focussed. It is also cooled by the gas. The separation of the beam dump from the focal place is critical, as it has been shown with RITU. - General design: Gas filled spectrometer have the advantage of a simple design, involving only classical magnetic elements. Work in progress A discussion is under progress with the Jyväskylä (RITU) and GSI (TASCA) laboratories on the optimization of a GFS. A DQQD configuration, with large aperture magnetic elements for high transmission is considered as a promising design. It is currently studied in GANIL. Recoil mass spectrometers (RMS) Basics of RMS Spectrometers combine magnetic and electric fields in order to separate the ions according to their magnetic rigidity (p/q) and electric rigidity (mv 2 /q). It is possible to disperse the ions according to their velocity or their M/q ratio (RMS). Velocity filters (LISE in GANIL, SHIP in GSI) are known to have good rejection and fair transmission, but they have no mass resolution. On the other hand RMS (FMA at Argonne, Oakridge RMS) can have very good mass resolution (M/ M ~ 400), but they lack the high transmission. A larger acceptance (in angle or energy) means larger optical aberrations that impaired the resolution. Moreover, their acceptance is mainly limited by the size of the electric dipoles: in order to reach a good dispersion, they must be long (1 meter or more) and the electric field must be strong, thus the electrodes must be close to each other (~10cm). On top of that, electric dipoles are sensitive to the impact of charged particles. It is absolutely necessary that the primary beam do not touch the electrodes. The new EMMA project [15] proposes a RMS with the largest acceptance in the world. It has a full angular acceptance of 16msr, a mass acceptance of ±4% and an energy acceptance from -17% to +25%. Its 1 st order mass resolution is 481 and is reduced to 368 for an acceptance of ±3 (H and V) and de=±10%. It is well suited to fusion evaporation reactions, but also to few nucleon transfer reaction. Nevertheless, classic RMSs are not optimized for the separation of the beam, and their rejection power is estimated around Considerations for S 3 - Rejection. If the mass selection is assumed, the rejection power need only to reduce the beam counting rate below the isobar counting rate. E.g. in the case of medium ions produced by fusion reaction, the isobaric counting rate is of the order of 10 8 pps for 10pµA beam ( pps). Thus, the necessary rejection power is 10 8, in order to reach a beam particle counting rate negligible in comparison to the isobar counting rate. For superheavy elements, the fusion hindrance strongly decreases the production cross sections and so dramatically increases the need for beam rejection up to Electric dipole. As pointed out earlier, the beam shall not touch the electric dipole. It is then required that either the electrodes are short or widely spaced. This apparently excludes the use of long, bended electric dipoles that are classically used for mass spectrometers. Hence, the beam has to be suppressed in an earlier stage. The use of a magnetic spectrometer as first stage corresponds to the first sequential design. One alternative solution could be a wide velocity filter that suppresses the beam before a RMS. This is still a major limitation for the use of a RMS. Work in progress Designs of a recoil mass spectrometer are studied in Saclay and in Argonne (also as the second part of the sequential design). The goal is to optimize the beam rejection without sacrificing to the mass resolution. This RMS could be a simplified (lower acceptance) version of the sequential design.

17 Manpower for the optics design As far as the manpower is concerned, the following institutes are involved in the design: - ANL: One optics engineers and a post-doctorate student - Irfu/SACM: two optics engineers. One year post-doctoral position is financed through the ESFRI SPIRAL2 preparatory phase. - GANIL: one optics engineer and a magnet designer. A 2 years post-doctorate position is financed by the CNRS for optics design and physics simulations - 3men.years have been requested to the Agence National pour la Recherche for the detailed optics design. - Collaboration with the University of Jyväskylä and GSI for expertise on separators. Conclusion The S 3 collaboration has focused its interest on the delayed study of rare nuclei produced by fusion evaporation, with two region of interest: - Superheavy elements, for synthesis, decay spectroscopy and ground state properties. - Neutron deficient nuclei for decay spectroscopy and ground state properties. Others aspects could also be considered if the physics community manifests a stronger interest in them: - Secondary reactions (e.g. COULEX) with radioactive nuclei produced in a first reaction. These very complex experiments require a very good mass selection of the nuclei as well as a full tracking and identification of the low energy ions. They may be interesting in some specific cases, but other facilities (e.g.: ISOLDE) could be more suitable. - Transfer reactions on light nuclei could be used to produce high intensity radioactive light secondary beams. Again, some peculiar physics cases could benefit from S 3, but they will require a very good selection of reactions products that are not emitted at 0. - Deep inelastic reactions could be used to populate neutron rich nuclei outside the fission bumps. The structure of these nuclei is basically studied through prompt spectroscopy around the target. Thus, very high intensity beams are of no use in this case. They could be studied with secondary reactions with the previous reservations, and the additional characteristics that the reaction products are emitted at the grazing angle. Considering these physics cases, the collaboration has established that the minimal baseline for S 3 should be: - A source/injector optimized for heavy ions up to at least mass 60 - A high power target for stable elements - A 0 separator/spectrometer with high acceptance, high rejection power, and mass resolution. - A basic detection chamber, compatible with an implantation/decay detector and a gas catcher for low energy measurements and a beam line to the DESIR facility. Even if they must be taken into account at the beginning for further upgrades, other aspects do not need to be fully implemented at the very beginning: - A/q=6 injector (foreseen in the SPIRAL2 building layout) - Actinide targets (currently under study) - Non 0 incidence on target (possibility for a beam swinger, specific physics cases must be developed) - Detection set-ups for secondary reactions (seems very complex due to the tracking/identification requirements), specific physics cases must be developed) - Detection set-ups for chemistry Their presence will be dictated by the interests of the physics community and the effective feasibility. Simulations show that the envelope requirements are the following for the separator-spectrometer: - Angular acceptance > +/- 60 mrad X and Y - Bρ acceptance: +/- 10% - The beam rejection : Bρmax (reaction products) = 2Tm - Eρmax = 30 MV (15 MeV/q) (Electric dipole limit) - Mass resolution = 1/350. To fulfill these constrains, we propose two different options: