Conseil Scientifique et Technique du SPhN RESEARCH PROPOSAL

Transcription

1 Conseil Scientifique et Technique du SPhN RESEARCH PROPOSAL Title: S 3 (Super Separator Spectrometer): Beam dump and spectrometer design Experiment carried out at: SPIRAL2 - GANIL Spokes person(s): A. Drouart, J. A. Nolen (ANL Argonne), A.C.C. Villari (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, M. Zielinska List of DAPNIA divisions and number of people involved: SENAC, SACM, SIS (number of people to be determined) 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 costs covered by ESFRI Travel budget/year for SPhN: travel costs covered by ESFRI - If already evaluated by another Scientific Committee: Approved by SPIRAL2 Scientific Advisory Committee

2 Beam dump and ion-optics design for the S 3 project Summary The S 3 (Super Separator Spectrometer) project has already been described in a previous CSTS document (October 2006) as a Letter of Intent. In this proposal, we ask for specific actions in order for the project to go on in two aspects. - The beam dump of the spectrometer will received unprecedented dose of heavy ions. It is therefore required to investigate the potential hazards due to this local accumulation of ions: radioprotection safety measured required, optimum beam dump design to reduce its influence on environment (photon/neutron emission ). On this point we ask for the help of the SENAC in order to perform realistic calculations and to help on the beam dump design. - The optics of the spectrometer is a critical part of the project. Different Physics cases have been studied and requirements on the spectrometer characteristics have been deduced. Existing separators have been studied and some rough sketches of configuration proposed. Nevertheless, the design of the spectrometer requires careful and extended studies performed by specialists in that domain. Man power should be dedicated to this study. Brief introduction to S 3 From 2011 the LINAG accelerator in GANIL will be able to produce stable heavy ion beams of unprecedented currents. They will be a factor 10 to 100 more intense than present beams. These stable ion beams will enable to deepen our knowledge in many aspects of nuclear physics by various experiments: - synthesis and spectroscopy of super heavy elements - synthesis and spectroscopy of nuclei at or beyond the proton drip line - study of production mechanisms and reaction products distributions - production and study of isomers - study of ground state properties of rare nuclei - To use these high intensities above particles per second, which is beyond the limit of any detector, it is necessary to develop a new kind of device. It should separate the interesting nuclei which are only a tiny part of the transmitted ions after the target most of them are beam ions that do not interact with the target. The aim is to obtain after separation counting rates of the order of 1 khz or less, compatible with a standard detection. The S 3 spectrometer will be design to achieve this goal. S 3 is a device which includes: - A highly durable target, able to sustain high beam currents - Two separation/selection stages (magnetic and electro-magnetic) in order to select and if possible identify the nuclei of interest. The rejection of the beam is a major aspect of this selection. - A detection setting adapted to the running experiment.

3 The detection setting could be: - an implantation/decay detector, to measure delayed alpha, electron and gamma spectroscopy. - a gas catcher, followed by an ion trap for laser spectroscopy and high resolution mass measurements. - a Coulex measurement set-up, (e.g. comparable to the one used at Isolde). Beam dump Expected intensities and beams S 3 should operate with the highest intensities available for a wide range of heavy ion beams; from very light ions to heavy lead or uranium beams. According to the physics cases, the most useful beams in the first experiments of S 3 would be: - 48 Ca beam for the study of Superheavy elements in hot fusion reactions (Uranium targets and heavier) Fe beams for the spectroscopy of Hassium (Z=108) with a lead target 58 Ni beams for the production of 100 Sn with 46 Ti or 48 Cr targets. According to recent results on the most advanced ion sources, intensities from 10 to 20 pµa (1 pµa is equivalent to ion/sec) have been reached for such ions. Several consequences are expected from the beam material interaction: - Heating of the beam dump, which may be very localized - Production of radioactive nuclei with various decay times. - Production of volatile/gaseous material The beam dump design depends also on the beam spot size after rejection. For this reason a close connection with the Optics working group is foreseen. Preliminary calculations In the frame of the safety status of the accelerator building of SPIRAL2, the SENAC has already made some preliminary calculations on the potential radiological hazards linked to the heavy ion beam dump. These calculations were performed with the PHITS code based on the QMD Model (for more details see Bâtiment accélérateur et salles d expérience : étude préliminaire du statut réglementaire des installations, Internal Report DAPNIA/SENAC A. Van Lauwe and V. Blideanu, ref DAPNIA/SENAC/E/07-262/NT-A. ) for Ar and Ca beam. Argon being one of the most intense beams available, its case is interesting for an upper limit. The conclusions show that a large number of ions are produced in the beam dump, lots of them being unstable and taking part in the beam dump radioactivity especially after the beam stops. Different reaction mechanisms were taken into consideration: - Fusion of beam and beam dump material - Quasi fusion - Scattering with excitation of both nuclei Activities were estimated and some results are reported in the next table (extracted from [1]):

4 48 Calcium Activity(Bq) 40 Argon Activity(Bq) Time after beam stop none 2,37E+10 1,72E+11 1 minute 2,19E+10 1,64E+11 1 hour 1,75E+10 1,34E+11 1 day 1,00E+10 9,67E+10 1 week 6,84E+09 7,31E+10 1 year 5,49E+08 3,70E+09 These results were used to demonstrate that, as far as the safety level of the whole facility is concerned, the beam dump for heavy ions is less critical than the deuteron beam dump. Nevertheless, in the scope of the S3 project, it is important to know precisely what will be the activation of the beam dump. It has consequences on the design of the beam dump and its setting up in the beam line. As far a radiation hazards are concerned, other aspects can also be addresses: - Target material irradiation, and disposal of these materials after use - Accidental loss of the beam at different points of the spectrometer - Beam Dump Working Group A WG has been organised in the frame of the S 3 collaboration. It already includes: - A physicist for calculation of the nuclei production in the beam dump (GANIL) - A physicist specialized in material resistance (CIRIL) - A safety engineer (GANIL) - A member of the optics WG as contact A task leader for this WG and a thermodynamics specialist (for heating simulations) are still required. A status report on the beam dump is required for September Test experiment A test experiment should be performed at GANIL in December Its goal is to bring preliminary data about the production of nuclei with a 48 Ca beam at an energy of 10 MeV/n impinging on a copper target. Gamma emission will be measured with a near-by germanium detector. Information about gamma activities decay times could also be gained. Production rates could be compared with different codes. Optics of the spectrometer Characteristics of reaction products S 3 is designed to select and analyze nuclei produced in fusion-evaporation reaction. Several specific features of such kind of reactions have to be considered for the optical design: - The evaporation residues are produced with a 0 angle, along the beam axis. - the reaction channels are multiple. Specifically, various evaporation channels are often opened after the fusion reaction. They include multi-neutron evaporations, but also charged particle evaporations. The interesting nuclei may not be the dominant species. - they are produced with rather low recoil velocities, depending on the mass ratio of the projectile and the target. This low energy has consequences on the ions distributions: - The scattering if only in the target leads to large angular distributions. - The charge state distributions of the ions (including beam ions) are broad, which implies a large Bρ distribution.

5 Finally, due to the high intensities, the energy deposit in the target is very large. In order to dissipate this power over the largest target area, we think to use a large rectangular beam spot on a rotating target: ±1mm in one direction but ±1cm in the direction orthogonal to the target rotation. This has direct implications on the optical design. Therefore, we require from the spectrometer: - A very high rejection power - A large angular and momentum acceptance - A significant mass selection of the nuclei. Experiment simulations and deduced requirements In order to estimate these quantities, we have simulated key experiments in various different conditions: Direct kinematics: Synthesis of Z=116 Superheavy element. 48 Ca Cm n Symmetric kinematics: Production of 100 Sn for ion trap detection 56 Ni + 46 Ti 100 Sn + 4n Inverse kinematics: production of 68 Se for Coulex excitation 58 Ni + 12 C 68 Se + 2n [Note that these reactions were mainly chosen for their broad range of kinematics. They are not first step experiments for S 3 ] From these simulations, we derive the main characteristics required for S 3 : 1 st case: light projectile on heavy target fusion reaction Angular acceptance: +/- 80 mrad X and Y (solid angle of 20msr) Brho acceptance: +/- 10% Variable focal plane (from 50 x 50 mm to ) M/q selection 1/350 resolution Beam rejection: Electric rigidity: <3MV 2 nd case: symmetric fusion reaction Angular acceptance: +/- 50 mrad X and Y Brho acceptance: +/- 10% Variable focal plane (from 50 x 50 mm to ) M/q selection 1/350 resolution Beam rejection: Electric rigidity: <8MV 3 rd case: inverse kinematics fusion reaction Angular acceptance: +/- 15 mrad X and Y Brho acceptance: +/- 5% Variable focal plane (from 50 x 50 mm to ) M/q selection 1/350 resolution Beam rejection: (rejection is difficult to reach, but beam current are lower for heavy ions) Electric rigidity: <40MV 4 th case: deep-inelastic Apart from fusion-evaporation reaction, we also considered the case of deep inelastic reactions for the production of neutron rich radioactive nuclei. Their kinematics strongly differs, since the interesting products are no more at zero degree. This would imply a drastically different configuration for the spectrometer. Therefore, we have chosen to put this case aside, not making it a priority of the design. We will consider it only if an acceptable and compatible solution appears. Angular acceptance: +/- 150 mrad X and Y, for angles >0

6 Brho acceptance: +/- 10% Variable focal plane (from 50 x 50 mm to ) M/q selection 1/100 resolution Beam rejection: (not at 0 ) Existing Separators around the world We studied the separators presently in operation in different laboratories, and also have a look at different projects. Vassilissa Separator (Dubna) The Vassilissa separator is a vacuum separator used in Dubna for the study of superheavy elements produced in hot fusion reaction, mainly with a 48 Ca beam on actinides target. It has a very good rejection power and one of the highest angular acceptances (15msr). It has a limited mass resolution (2-3% for A~300) and is not effective in symmetric or inverse kinematics. Daresbury RMS (Oak Ridge) The RMS is a double step separator used for the production of medium-heavy nuclei using fusion-evaporation reactions. It has a very high rejection power even for symmetric or inverse kinematics reaction. It has also a good mass resolution (1/450 at the final focal point). Meanwhile, its acceptance is relatively low: 13msr in angle and ±5% in M/q. RITU (Jyväskylä)

7 Ritu is a gas-filled separator. The charge equilibration of the ions in the gas leads to trajectories which are independent of the charge state. Gas separators have a very good rejection power in direct kinematics but may require H 2 gas for very asymmetric reactions. They have also a wide momentum and angular acceptance. Due to the high straggling in the gas, they have almost no mass resolution, and poor Z resolution. They enable a good cooling of the primary target which is put inside the gas. There is still debate about the possibility to handle very high intensities (>10pµA) inside the gas. HIRA Project (New Delhi) The HIRA project combines two different spectrometers as two branches of the same device. One branch is a high resolution vacuum spectrometer (good mass resolution, small acceptance) while the other is a gas filled spectrometer (high acceptance, poor mass resolution). Since there is a required compromise between acceptance and resolution, this configuration enables to choose the best line according to the experimental needs. A possible configuration for one separation stage: Vassilissa-like Wien filters ensemble Vassilissa is a QQQEEEQQQD separator (Q: magnetic Quadrupole; E: Electric Dipole; D, magnetic Dipole). In our simulations, we replace the electric dipoles by Wien filters, with crossed E and B fields so that the all ions with velocities close to the beam velocity are not deviated. We calculate with the Zgoubi ray-tracing code the resulting trajectories up to the middle of the second electric dipole. Simulations are done for a direct kinematics reaction ( 48 Ca U n) and symmetric reaction ( 40 Ca+ 40 Ca 78 Zr+2n) We show here some second order calculations for the two configurations and two different reactions:

8 48 Ca U n Wien filter E=270kV/m, B= e-04 T Pure electric Dipole E=270kV/m Z=112, Q=27+, Bρ=0.584 Z=112, Q=27+, Bρ= Ca, 17+ Bρ= Ca, 17+ Bρ=0.891 With the direct kinematics, there is little practical difference between the two configurations. We could see that the focalisation of the Evaporation residue is slightly better with the WFs. In this case, the Magnetic correction is very small and the electric filed are at the small value for both configurations.

9 40 Ca+ 40 Ca 78 Zr+2n Wien filter B=4.35x10-2 T ; E=900kV/m Pure electric Dipole E=450kV/m 78 Zr, 19+, Bρ= Zr, 19+, Bρ= Ca, 16+, Bρ= Ca, 16+ Bρ=0.602 In symmetric kinematics, Vassilissa is unable to separate the evaporation residues from the beam. It is event worse with the higher charge states of the 40 Ca. The WF configuration can do it without ambiguity. Nevertheless, note that for the same deviation of 15cm, the electric dipole field is twice bigger (900kV/m, still well within realistic limits). The simulations show that if we replace a simple electric dipole by a Wien filter, it might greatly enhance the separation in the case of symmetric kinematics. The configuration studied here lacks of mass resolution, so does not fulfil all the requirements for S 3, but it may be a part of the line. There are other configurations where an electric dipole is used (like in the Oak Ridge RMS). It seems worthwhile to investigate wether its replacement by a Wien filter increases the rejection. Status and perspective of the optics simulations So far, we have quantified the basic characteristics of S 3 according to key experiment simulations. We have studied existing separators and have a look at some of the possible options. Some preliminary simulations have been done on a Wien filter configurations. Additional simulations are currently performed at the Argonne National Laboratory to study a wide acceptance momentum separator, similar to the first part of the Oak Ridge RMS. In parallel, an engineer from IPN Orsay works part time on the project, more specifically on the very specific (large, flat beam spot) focalization of the beam on the primary target. An ionoptics engineer from GANIL should also join the optical group. Nevertheless, a lot of work has still to be done to confirm the possible option, make a conceptual design and then work on a detailed study of the chosen design. Considering the

10 tight schedule of the project, this has to be completed as soon as possible. This will not be possible without the help of experts in ion-optics and engineers which could work in parallel on the different elements of the spectrometer. Conclusions Regarding the expertise of the SENAC in the calculations of induced radioactivity and other radioactivity hazards, we ask that they could actively take part in the project in the design of the beam dump of S 3, but also on other points where such problem arise. They will work in cooperation with a GANIL Team. On the point of the optics design, it is vital that we add to our group an expertise in ionoptical calculations and spectrometer design. It is clear that the Working Group is lacking of manpower during this critical phase. One person working at full-time on the subject is necessary. In the next step (detailed design of the various optical elements, setting up of the spectrometer in the LINAG experimental area), an engineering expertise will also be required. Note on SPIRAL2 Week In the frame the SPIRAL2 week (from 26 th to 30 th of November), most of the S 3 working groups will have technical meetings. A brief outlook of this week will be giving during the CSTS oral presentation.

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