The GSI plans for an international accelerator facility for beams of ions and antiprotons

ELSEVIER

Nuclear Physics A719 (2003)

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www.elsevier.com/locate/npe

The GSI plans for an international and antiprotons

accelerator

facility

for beams of ions

K. Siimmerer” “Ges. f. Schwerionenforschung

mbH, Planckstr.

1, D-64291

Darmstadt,

Germany

GSI proposes to build a next-generation facility for research with relativistic beams of ions and antiprotons. This facility allows a broad range of topics in nuclear and astrophysics, plasma and atomic physics to be addressed. The topic most interesting in the context of this conference is physics with high-intensity beams of exotic nuclei. In addition, a short overview of the opportunities in the other fields of nuclear physics is given. 1. INTRODUCTION For more than a decade, GSI has operated a high-energy accelerator facility that can provide beams of all ions up to uranium with energies up to around 1 A GeV. This facility, based on the SIS18 synchrotron, is equipped with unique instrumentation: a high-resolution fragment separator, FRS, and an experimental storage ring, ESR. Together with experimental setups for fixed-target experiments (FOPI, ALADIN/LAND, and KaoS), this facility allowed to perform interesting studies in a wide variety of nuclear physics fields. In the field of nuclear structure physics, highlights from the last decade include investigations of halo nuclei, decay studies of doubly-magic nuclei (e.g. iooSn), the proof that high-energy projectile fission is a rich source of neutron-rich nuclei, the first observation of the 2-phonon giant dipole resonance in nuclei and (very recently) of the 2-proton decay of a nuclear ground state in 45Fe. Unique mass measurements over wide ranges of the nuclear chart were done by analyzing revolution times of fragment beams inside the ESR. Highlights from other fields of nuclear physics include e.g. the resolution of deeply-bound pionic states in the FRS, the first observation of bound-state P-decay in the ESR, sensitive tests of quantum electrodynamics (QED) by studying X-rays from H-like uranium in the ESR, and the observation that kaon masses in compressed hadronic matter differ from those in free space. This list may be incomplete and biased by the perspective of the author, but it certainly proves that high-energy heavy ion beams are a rich source of interesting physics information, provided that sophisticated instrumentation is available. Based on the successful experience with the present GSI facility, and after extensive discussions with many colleagues from various fields of physics all over the world, GSI has submitted a proposal to expand its present complex towards a next-generation facility for research with relativistic beams of ions and antiprotons [I]. Its fields of research can be specified as follows: 0375.9474/03/$ - see front matter doi: lO.l016/SO375-9474(03)00927-S

0 2003 Elsevier

Science

B.V

All rights

reserved.

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e Investigations with intense beams of exotic nuclei, that cover hitherto unexplored regions in the field of nuclear structure or nuclear astrophysics (in particular along the r-process path); e Hadronic matter at the sub-nuclear level studied with beams of cooled antiprotons; 0 Compressed hadronic matter investigated through nucleus-nucleus collisions at energies in the 20-30 A GeV range; o Basic research of high-density plasmas, relevant for astrophysics, physics of condensed matter and inertial-confinement fusion; l

QED studies in extremely strong electromagnetic fields.

UNILAi

Figure 1. Layout of the proposed International Accelerator Facility at GSI. Experiments with radioactive ion beams involve the new Super-FRS and the associated experimental setups, the collector ring CR and a new experimental storage ring NESR. Cooled antiproton beams are used in the high-energy cooler/storage ring HESR. Nuclear-collision experiments are fed directly from SISSOO.

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The proposed facility for this kind of research is shown schematically in Fig.]. The centerpiece of this complex of accelators, storage rings, and experiments is a double-ring synchrotron, SIS100/2007 that consists of two rings stacked on top of each other. The 100 Tm synchrotron SISlOO can accelerate low charge states of heavy ions like e.g. 23*U28+ to 2.7 A GeV or protons to 29 GeV. Its stronger brother, SIS200, will accelerate (after stripping to q = 73+) uranium up to 23 A GeV and protons up to 60 GeV. The primary ion beams from SISlOO with maximum intensities of z 1 x 1012 ions/s will either be directed to a production target to create exotic fragments or will be used as short bunches in plasma physics experiments. 29 GeV protons will be converted to 3 GeV antiprotons which will be collected in the collector ring CR and stored and cooled in the new experimental storage ring NESR. After reinjection into SISlOO they will be transferred to the high-energy storage ring HESR for pp collisions in the PANDA experiment. High-energy heavy-ion beams from SISZOO will be mainly used for studies of compressed baryonic matter in a dedicated Nuclear Collisions cave (CBM experiment, Fig.1). In the following, I will describe in some detail the first of the topics listed above, the research with exotic beams. The other topics will be treated in a more cursory fashion. 2. RESEARCH

WITH

EXOTIC

BEAMS

2.1. The proposed Super-FRS The proposed GSI facility will probably be the only one worldwide that can provide ion beams up to uranium with such high energies that even high-2 ions are almost fully stripped. This allows heavy fragments of uranium to be investigated; both with respect to their nuclear structure (relevant for the upper end of the r-process) and with respect to their low-energy fission properties. Studies of neutron-rich nuclei produced by uranium fission are also of prime interest and define the acceptance for which the Super-FRS has to be designed. In momentum the new Super-FRS will accept &/p = &2.5%, and a@, = +2Omrad. At the same time, the momentum in angle aQz = &4Omrad resolution of the present FRS of 1500 will be kept. This requires large magnet apertures obtainable only with superconducting magnets. At the same time, the required purity of the fragment beams imposes a two-stage layout consisting of a pre-separator and a main separator, each equipped with an achromatic energy degrader [a]. The large apertures require careful compensations of higher-order ion-optical aberrations using hexapole and octupole magnets. Fig.2 shows the ion optics of the Super-FRS in the bending plane. Both sections are designed as achromatic separators. The two degraders operate at different fragment energies and thus impose different selection cuts in the N-Z-plane. The preseparator allows detectors to be placed in the focal planes of the main separator without overloading them with too high countrates. At the same time, contaminants produced in reactions with the first, degrader are effectively removed in the main separator. 2.2. Reactions with relativistic radioactive beams -At the present GSI facility, the ALADIX/LAND setup allows to study nuclear reactions in complete kinematics. For the new facility, a much more versatile setup for Reactions wit,h Relativistic Radioactive Beams (R3B) is foreseen (Fig.3). Coulomb and nuclear breakup reactions plus giant resonances will be studied in complete kinematics similar to

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Pre-Separator

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Main-Separator

Figure 2. Ion-optical layout of the proposed high-acceptance fragment separator SuperFRS [2]. The separation properties for the example of 132Sn produced in 238U projectile fission at 1.5 A GeV at the different focal planes are indicated. Due to the large acceptance of the Super-FRS in both angle and momentum, a two-stage separation scheme has to be applied.

AALADI?J/LAJD, while the range of nuclei available for secondary reactions will be greatly enhanced due to the much higher secondary-beam intensities. The larger bending power of the new superconducting dipole magnet allows a better mass resolution of the fragments; at the same time, the study of breakup reactions where light charged particles are emitted in addition to neutrons becomes feasible. A completely new feature is the detection of recoil protons from the liquid-H2 reaction target with an array of strip detectors around 90 degrees. 2.3. Experiments with stored and cooled fragment beams Already the present GSI facility allows to inject radioactive beams into the ESR cooler/storage ring. The efficiency for this injection procedure, however, is rather poor. The new facility (Fig.4) has been designed to overcome this limitation. To this end, the st,orage ring has been split into two parts with different tasks: the collector ring CR has been designed to capture most of the hot fragments within a momentum band of 12.5%. Bunch rotation and stochastic cooling allows fast compression of this momentum spread down to N *0.05% (but imposes a fixed injection energy of 740 A MeV). If the fragments

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Figure 3. Experimental setup for reactions with relativistic radioactive beams in complete kinematics. After the reaction target, the secondary fragments are momentum-analyzed in a large superconducting dipole magnet. Emitted neutrons are detected in an upgraded LAND neutron detector, whereas light charged particles are tracked in a set of new wire chambers. Around the target, detectors for y-rays, recoil protons, and a Cherenkov detector for fragment velocity measurements are foreseen.

are sufficiently long-lived, they can be transferred to the cooler/storage ring NESR, where they can be electron-cooled to still lower momentum spread (< 1 x 10P4). At present, a passive energy degrader is foreseen to slow down the fragments if the experiment requires an energy lower than 740 A MeV. Experiments involving short-lived species have to be performed in the CR (e.g. mass measurements with the time-of-flight method using the isochronous operation mode of the CR). Mass measurements with the Schottky method can be performed in the NESR. This ring will be equipped with an internal gas target for in-ring reaction experiments. A particular challenge is the plan to perform elastic and inelastic electron scattering on radioactive ions by colliding them with a counterpropagating electron beam from a small electron storage ring. The electron spectrometer foreseen for this purpose has to reach both high efficiency and high resolution, which is not easy to obtain simultaneously and therefore will require intense R&D in the future. 2.4. Experiments with low-energy exotic nuclei The low-energy branch of the Super-FRS is meant to combine the advantages of a high-energy projectile-fragment separator, i.e. clean, universal, and fast separation, with that of an ISOL-type facility, namely to provide “thin” sources for decay spectroscopy plus low-emittance beam properties. The key installation to achieve this is a passive monochromator (Fig.5), that involves the dispersion of the residual momentum spread of the separated high-energy fragment beam onto a monoenergetic degrader, where the faster ions impinge on thicker layers of matter and the slower ones on thinner layers, such that the resulting focussed beam has a minimum energy spread (which allows e.g.

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Exotic nuclei fmm

Figure 4. Layout of the collector ring CR and the cooler/storage ring NESR. The CR is designed to accept large-emittance fragment beams with maximum efficiency and stochastically precool them. Subsequent electron cooling and in-ring experiments with either a gas jet target or electron-ion collisions take place inside the NESR.

stopping in about 1 m of He gas at 2 bar). i’l’ith the resulting energy-bunched slow beams, a large variety of experiments can be adressed, in a similar way as they are performed or foreseen at present or future ISOLtype facilities. Examples include decay spectroscopy after stopping, Coulomb excitation or collinear laser spectroscopy of slow beams, transfer to precision traps for mass measurements etc. 3. NUCLEUS-NUCLEUS

AND

PROTON-ANTIPROTON

COLLISIONS

=It GSI, nucleus-nucleus collisions have up to now been studied in fixed-target experiments up to about 2 A GeV incident energy. At other laboratories, much higher energies have been explored in fixed-target experiments at AGS (11 A GeV) and CERY (158 A GeV)! and with collider experiments at RHIC (‘200 A GeV). Construction of the 3.4 TeV Large Hadron Collider (LHC) at CERN has been started. GSI’s motivation to do nucleusnucleus collisions at a comparatively low energy of 20-30 A GeV is twofold: (i) The RHIC and LHC experiments are aimed at studying regions of the nuclear-matter phase diagram at low density but at high temperature, a region which is assumed to

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: 8 I-0 Figure 5. Ion-optical layout of the energy bunches with its 90 degree dipole magnet. The magnet-plus-degrader arrangement allows to focus a quasi monoenergetic beam into e.g. a short-length gas cell. Note the large apertures of the magnets.

be close to the big-bang situation. Complementary to that, the proposed Compressed Baryonic Matter (CBM) experiment at GSI will explore a region that is closer to that expected for neutron stars: very- high density, but only moderate temperature. (ii) Experimental indications suggest that around 35 A GeV the relative amount of strangeness production is maximum. As strange quarks are a better testing ground for QCD than light quarks, this energy range is thought best suited for probing the role of strangeness in the ultra-dense nuclear medium. Experimental tools foreseen for the CBM experiment are, among others, a superconducting dipole magnet equipped with a multi-pixel Si tracker for momentum determination and displaced-vertex reconstruction, a ring-imaging Cerenkov detector and a transition-radiation detector plus a large time-of-flight wall for particle identification. This universal detector will allow to track the enormous multiplicities of both leptons and hadrons and at the same time identify rare channels. While GSI has a long tradition in exploring central nucleus-nucleus collisions, reactions with GeV antiprotons have previously performed mainly at CERN or Fermilab. The new accelerator complex now opens new and exciting possibilities to perform precision hadronspectroscopy experiments with cooled antiprotons. This requires fast-extracted beams of 29 GeV protons (50 ns width) to be converted into 3 GeV p which will be captured in the CR and then stacked in the NESR. After reinjection into and acceleration or deceleration in SISlOO, they mill be transferred to the HESR where they interact with a Hz gas jet or cluster target. A multipurpose detector (“PANDA”)for both calorimetric and tracking capabilities is foreseen in the HESR.

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4. OUTLOOK The proposal sketched in this contribution has been reviewed by the German Science Council, an advisory body to the Federal Government of Germany. Very recently, a final positive statement of the Science Council has been released. If international support plus adequate funding can be obtained, the first stage of the facility (SISlOO, Super-FRS and fixed-target experiments with uncooled antiprotons) could become operational near the end of 2009, followed by the various storage rings by the end of 2010. The full-scale facility including SIS200 and e--cooling in HESR could go into operation in 2012.

REFERENCES 1. 2.

“An International Accelerator Facility for Beams of Ions and Antiprotons”, Conceptual Design Report, GSI Darmstadt (November 2001). H. Geissel et al.; Proc. EMIS-14, Victoria, British Columbia, Canada, May 5-11, 2002; to be published in Nucl. Instr. Meth. in Phys. Res. B.