The ISOLDE RIB-facility at CERN has today been producing a vast range of radioactive beams since more than 30 years. The low-energy beams of ISOLDE will be complemented by a post-accelerator, REX-ISOLDE, currently being assembled. In order to convert the pseudo-DC, singly-charged beam from the ISOLDE mass separators into a cooled and bunched beam at higher charge states a novel scheme of trapping, cooling and charge-state breeding has been devised, using a linear Penning trap and an Electron Beam Ion Source (EBIS). This allows for subsequent acceleration by a short, cost-effective LINAC consisting of an RFQ, an IH-structure and three seven-gap resonators, reaching 0.8 - 2.2 MeV/u. The installation of REX-ISOLDE is well underway and the first post-accelerated radioactive beams are expected to be obtained during late 2000

Ames, F

Bollen, G

Bongers, H

Cederkäll, J

Davinson, T

Emhofer, S

Forstner, O

Gund, C

Habs, D

Huber, G

Huck, A

Huyse, M

Jonson, B

Kester, O

Kruglov, K

Kühnel, K U

Liljeby, L

Moukha, I

Nilsson, T

Nyman, G H

Oinonen, M

Ostrowski, A N

Podlech, H

Ratzinger, U

Ravn, H L

Reiter, P

Rensfelt, K G

Repnow, R

Richter, A

Schempp, A

Schmidt, P

Schrieder, G

Schwalm, D

Shotter, A C

Sieber, T

Simon, H

Skeppstedt, Ö

Thirolf, P G

Van den Bergh, P

Van Duppen, P

Von Hahn, R

Walter, G

Weissman, L

Welsch, C P

Wenander, F

Woods, P J

Äystö, J

CERN Document Server

REX-ISOLDE - POST-ACCELERATED RADIOACTIVEBEAMS AT CERN-ISOLDET. Nilsson1, J. Äystö1, O. Forstner1, H.L. Ravn1, M. Oinonen1, H. Simon1,12,J. Cederkäll1, L. Weissman1, D. Habs2, F. Ames2, O. Kester2, T. Sieber2,H. Bongers2, S. Emhofer2, P. Reiter2, P.G. Thirolf2, G. Bollen2, P. Schmidt3,G. Huber3, L. Liljeby4, Ö. Skeppstedt4, K.G. Rensfelt4, F. Wenander5, B. Jonson5,G. Nyman5, R. von Hahn6, H. Podlech6, R. Repnow6, C. Gund6, D. Schwalm6,A. Schempp7, K.-U. Kühnel7, C. Welsch7, U. Ratzinger8, G. Walter9, A. Huck9,K. Kruglov10, M. Huyse10, P. van den Bergh10, P. van Duppen10, A.C. Shotter11,A.N. Ostrowski11, T. Davinson11, P. J. Woods11, I. Moukha12, A. Richter12,G. Schrieder12, and the REX-ISOLDE collaboration1 CERN, CH-1211 Geneva 23, Switzerland2 Sektion Physik, LMU München,D-85748 Garching, Germany3 Johannes-Gutenberg Universität, D-55099 Mainz, Germany4 Manne Siegbahn Laboratory, S-10405 Stockholm, Sweden5 Chalmers University of Technology, S-41296 Göteborg, Sweden6 Max-Planck-Institut für Kernphysik, D-69117 Heidelberg, Germany7 Institut für Angewandte Physik, J.W. Goethe Universität, D-60325 Frankfurt, Germany8 GSI, D-64220 Darmstadt, Germany9 Universite Louis Pasteur, Strasbourg, France10 Instituut voor Kern- en Stralingsfysica, K.U. Leuven, B-3001 Leuven, Belgium11 University of Edinburgh, GB-Edinburgh EH9 3JZ, UK12 TU-Darmstadt, D-64289 Darmstadt, GermanyAbstract.The ISOLDE RIB-facility at CERN has today been producing a vast range of radioactive beams since more than 30years. The low-energy beams of ISOLDE will be complemented by a post-accelerator, REX-ISOLDE, currently beingassembled. In order to convert the pseudo-DC, singly-charged beam from the ISOLDE mass separators into a cooled andbunched beam at higher charge states, a novel scheme of trapping, cooling and charge-state breeding has been devised,using a linear Penning trap and an Electron Beam Ion Source (EBIS). This allows for subsequent acceleration by a short,cost-effective LINAC consisting of an RFQ, an IH-structure and three seven-gap resonators, reaching 0.8 - 2.2 MeV/u.The installation of REX-ISOLDE is well underway and the first post-accelerated radioactive beams are expected to beobtained during late 2000.IntroductionRadioactive Ion Beam (RIB) Facilities is presently atopic attracting interest from a large part of the nuclearphysics community. A number of new or upgraded facil-ities are currently being planned, built or commissionedworld-wide, and in a longer perspective, NUPECC (Eu-rope) and DOE (USA) independently have recommendedthe construction of a "second-generation" RIB facilitywhere radioactive beam intensities 100 times higher thanat the current facilities can be produced.The production of RIB is done according to two dif-ferent methods; one is to let an energetic heavy-ion beamimpinge on a thin production target and by subsequentin-flight (Bρ ∆E Bρ) separation of the fragments se-lect the wanted isotopes (1). The other method, whichis the one used at ISOLDE (2), is to let a driver beam(protons, neutrons or heavy ions) impinge on a thick pro-duction target where a huge amount of unstable nuclidesare created, and from this target volume extract, ioniseand separate the wanted isotopes.The driver beam of the ISOLDE facility is a pulsedCERN-OPEN-2001-07010/10/2000MINIBALL + aux. detectors7-gap resonatorsIH-structure4-rod RFQMass separatorREXEBISISOLDE beamFIGURE 1. Schematic view of the REX-ISOLDE installation in the ISOLDE experimental hall.proton beam from the PS-Booster synchrotron of 1.0 or1.4 GeV energy with a repetition rate of typical 1/2.4 s 1.The average current exceeds 2 µA. The high-energeticprotons induce spallation, fragmentation and fission, giv-ing rise to a large number of radioactive species. ISOLDEregularly uses a large variety of target materials (metalfoils, oxides and carbides) and 1+-ion sources (surface,plasma and laser ionisation) to produce more than 600isotopes of >60 elements. A constant development oftargets with respect to new materials with release of theoften short-lived species of interest (down to a few ms)is intensely pursued. The Resonant Ionisation Laser IonSource (RILIS) (3, 4) has been intensely and successfullyused at ISOLDE in recent years to improve the ionisa-tion efficiency, enhance selectivity and to ionise elementsnot otherwise achievable with the traditional ISOLDE ionsources.With this as a starting point, it is clear that the pre-requisites for building a post-accelerator at ISOLDE areoptimal, apart from the proliferation and intensities of theavailable radioactive beams, it also benefits from the largeknow-how in targetry and RIB handling and the existenceof CERN infrastructures.The REX-ISOLDE post-acceleratorUntil now, ISOLDE has been limited to study ra-dioactive nuclei at very low energies only, 60keV. TheREX-ISOLDE (5, 6) experiment will change this situa-tion dramatically; almost any nuclide currently producedat ISOLDE will be possible to accelerate to an energyin the range 0.8 - 2.2 MeV/u. This opens up a com-pletely new field of experiments with RIB with A<50,with an energy range tailored for Coulomb excitation andfew-nucleon transfer reactions, being well-proven spec-troscopic tools for stable beams. An overview picture ofREX-ISOLDE in its final state is shown in 1.REXTRAP - REXEBIS Cooling, bunching andcharge breedingThe RIB from the ISOLDE separator is a continuousbeam, with the intensity varying in time after the pro-ton impact as a function of the release characteristics andthe half-life of the nuclide in question. To transform thissingly-charged, "pseudo-DC" beam from the ISOLDEseparator to a multiple-charged (q/A > 1/4.5) pulsed0 20 40 60 80 100 12002040608010026NaArOHH2OTOF [ms]ionsFIGURE 2. TOF distribution of captured, cooled and bunched26Na (T1=2 = 1.07 s) ejected from REXTRAP.beam suitable for acceleration with acceptable losses re-quires novel techniques. Bunching of the radioactivebeam is required in the chosen acceleration scheme be-cause the charge-state breeding in the REXEBIS requiresa typical time of about 20 ms and the LINAC operateswith a duty factor of 10%. The basic technique applied toaccumulate and bunch the beam is to continuously injectthe radioactive ions into a large Penning trap, REXTRAP,where they are stopped by collisions with the atoms of abuffer gas, accumulated and finally extracted as pulses.REXTRAP consists of a 1 m long cylindrical trap struc-ture filled with buffer gas in a 3 T magnetic field. It ismounted on a high voltage platform close to 60 kV in or-der to retard the ions from 60 keV to some eV suitablefor injection into the trap. After the ions have passed thepotential barrier at the entrance of the trap, final deceler-ation is done by friction in the Ar buffer gas. The ionscan be fully captured if their energy loss during a sin-gle oscillation in the trap is larger than the energy spreadof the ISOLDE beam after deceleration, after which theycan not escape the entrance barrier. Typical cooling timesdown to room temperature are a few ms. By this coolingthe emittance of the extracted beam is considerably im-proved. The trapping and cooling are expected to reachalmost 100% efficiency. Recent tests with stable and ra-dioactive beams have shown capture efficiencies up to27% for ion beams from ISOLDE and 45% from an aux-iliary ion source. In fig 2 a TOF distribution of captured,cooled and bunched 26Na (T1=2 = 1.07 s) ejected fromREXTRAP is shown.Mass separator and LINACThe bunched beam is then transported to REXEBIS(Electron Beam Ion Source). An EBIS makes use of adense electron beam that is focused to a high current den-sity by a strong magnetic field of a solenoid. The elec-tron beam forms a radial potential well for the ions whilethe longitudinal confinement is performed by electric po-tentials applied to cylindrical electrodes surrounding theelectron beam. Trapped low-charged ions undergo step-wise ionisation via electron impact collisions until theions are extracted by changing the longitudinal potentialdistribution. The centroid of the charge distribution is de-termined by the product of the confinement time and theelectron beam current density. In the REXEBIS a super-conducting solenoid creates a magnetic field of 2 T, witha homogeneity of about 0.25% along the confinementlength of 0.8 m which compresses the 0.5 A, 5 keV elec-tron beam into a current density larger than 200 A/cm2.The isotopes used at REX-ISOLDE will require breedingtimes between 5 and 20 ms to reach a charge-to-mass ra-tio larger than 1/4.5. In order to decrease the injectionenergy into the RFQ to 5 keV/u the platform voltage hasto be switched from 60 kV (injection potential) down to20 kV, and rapidly back again to accept the next bunch ofions.The yield of the radioactive isotopes from ISOLDEcan be several orders of magnitude lower than the amountof residual gas ions from C, N, O and Ar coming out ofthe EBIS. Therefore, a mass separator similar to a Nier-spectrometer will be employed with a q/A-resolutionof about 1/150 which is sufficient to select the highlycharged rare radioactive ions from rest-gas contaminants.In the first stage of REX-ISOLDE LINAC the ions areaccelerated from 5 keV/u to 300 keV/u by a 4-rod RFQ.In order to match the beam from the RFQ into the ac-ceptances of the IH-structure a section consisting of twomagnetic quadrupole triplet lenses and a rebuncher is re-quired.The Interdigital-H-type (IH)-structure is an efficientdrift-tube structure, similar to structures like the GSIHLI-IH-structure or ’tank 1’ of the lead LINAC at CERN.A new feature of the REX-ISOLDE-IH resonator is thepossibility to vary the final energy between 1.1 and 1.2MeV/u by adjusting the gap voltage distribution via twocapacitive plungers and by adjusting the rf-power fed intothe resonator. The lower final energy of the IH-structureis important for deceleration of the ions down to 0.8MeV/u, since the deceleration from 1.2 MeV/u down to0.8 MeV/u through the 7-gap resonators would perform anon-acceptable phase spread at the target.The high-energy section (0.8 - 2.2 MeV/u) of theREX-ISOLDE LINAC consists of three 7-gap resonatorssimilar to those built for the high-current injector at MPIHeidelberg. These spiral resonators are designed and op-timised for synchronous velocities of β=5.4%, 6.0% and6.6%. Each resonator has a single resonance structure,which consists of a copper half shell and three arms at-tached to both sides of the shell. Each arm consists of twohollow profiles, surrounding the drift tubes and carryingthe cooling water. All LINAC structures are operated at101.28 MHz.Planned experiments at REX-ISOLDEThe REX-ISOLDE pilot experiments (7, 8)aims toprobe whether the magic numbers N=20 and N=28 areconserved when going to very neutron-rich nuclei byusing Coulomb excitation and transfer reaction experi-ments. Other experiments have hinted a weakening ofthe nuclear shell structure in the region of 32Mg (9) andbelow 48Ca (10, 11), with deduced sizable deformations.The main detector system to be used here is the state-of-the-art gamma-detector array MINIBALL (12) currentlybeing assembled by a large European collaboration. Tofacilitate Doppler corrections, charged particles will bedetected in a high-granularity, segmented silicon detectorwith “compact-disc” geometry, the CD. The residual nu-clei will be detected in a PPAC (Parallel Plate AvalancheCounter) downstreams from the reaction target.A large number of other experiments will also be us-ing REX-ISOLDE. Two further experiments (13, 14) arealready approved; both concerned with investigating theunbound sub-systems of halo nuclei by one-neutron pick-up reactions and elastic resonance scattering. E.g. theunbound sub-system 10Li of the halo nucleus 11Li will bestudied by the 9Li(d,p)10Li reaction and through the IASin 10Be by the 9Li(p,p’)9Li reaction. Other experimentsare foreseen in the short-term future concerning the struc-ture of nuclei along the N=Z line, proton radioactivity andnuclear astrophysics.The bunched and cooled beams from REXTRAP arealso well suited for other experiments without subsequentacceleration. In one approved experiment (15), the cooledbeam will be extracted and stored in an electromagnetictrap, combined with a retardation spectrometer. The lat-ter is used to measure the recoil momentum of the daugh-ter nuclei after beta decay, allowing studies of β-ν cor-relations. This could give information on possible scalarcurrent components in the nuclear beta decay, meaning“new physics” beyond the Standard Model. Furthermore,the higher available implantation energy gives rise to newpossibilities to use radioactive methods to probe the bulkproperties of condensed matters.OutlookThe REX-ISOLDE post-accelerator will keepISOLDE in a unique position in the world, with a un-precedented choice of radioactive beams for low-energyreactions. Together with the continuous beam develop-ment, this assures many years to come with ISOLDEresearch at the forefront of physics. The techniques usedin REX-ISOLDE are also highly interesting for future,second-generation ISOL-facilities, such studied in therecently started EURISOL (16) project. It is foreseen thatafter the commissioning phase and the first generation ofpilot experiments, REX-ISOLDE will be integrated intothe CERN accelerator structure.REFERENCES1. Geissel, H., Münzenberg, G., and Riisager, K.: Annu. Rev.Nucl. Part. Sci., 45, (1995) 163.2. Kugler, E., et al.: Nucl. Instr. Meth., B70, (1992) 41.3. Lettry, J., et al.: Rev. Sci. Instrum., 69 (2), (1998) 761–763.4. Köster, U., et al.: Nucl. Instr. and Meth., A160 (4), (2000)528–535.5. Habs, D., et al.: Nucl. Instr. and Meth., B139, (1998) 128–135.6. Habs, D., et al.: Hyperfine Interactions, ISOLDE Labora-tory portrait, acc. for publ.7. REX-ISOLDE collaboration: Radioactive beam experi-ments at isolde: Coulomb excitation and neutron transferreactions of exotic nuclei, CERN/ISC 94-25 P64, Novem-ber 1994.8. Scheit, H., et al.: Investigation of the single particle struc-ture of the neutron-rich sodium isotopes 27 31Na, CERN-ISTC-99-20 ISC-P-114, 1999.9. Motobayashi, T., et al.: Phys. Lett., B346, (1995) 9–14.10. Glasmacher, T., et al.: Phys. Lett., B395 (3-4), (1996) 163–168.11. Scheit, H., et al.: Phys. Rev. Lett., 77 (19), (1996) 3967–3970.12. Habs, D., et al.: Z.Phys., A358, (1997) 161–162.13. Axelsson, L., et al.: Study of the unbound nuclei 10Li and7He at REX ISOLDE, CERN/ISC 98-11 ISC-P-100, 1998.14. Axelsson, L., et al.: Investigations of neutron-rich nucleiat the dripline through their analogue states: The cases of10Li-10Be (T=2) and 17C-17N (T=5/2), CERN/ISC 98-23ISC-P-105, 1998.15. Beck, D., et al.: Search for new physics in beta-neutrinocorrelations using trapped ions and a retardation spectrom-eter, CERN/ISC 99-13 ISC-P-111, 1999.16. EU-network EURISOL: proposal to the EU, No. HPRI-1999-50016, 2000.

REX-ISOLDE: post-accelerated radioactive BEAMS at CERN-ISOLDE

REX-ISOLDE: post-accelerated radioactive BEAMS at CERN-ISOLDE

Abstract

Similar works

Full text

Available Versions

CERN Document Server