The ISOLDE facility

• Prof. Björn Jonson, Chalmers University of Technology, Göteborg, Sweden

• Karsten Riisager, Aarhus University, Denmark.

The Isotope Separator On-Line facility at CERN

Figure 1: The ISOLDE Facility. The 1.4 GeV proton beam enters from upper left into the shielded target area, covered by iron, concrete and soil. The experimental hall is situated at the upper right. Courtesy: Stefano Marzari.

The ISOLDE Facility is a world-leading laboratory for production and studies of radioactive nuclei. ISOLDE belongs to CERN’s accelerator complex situated on the border between Switzerland and France. The Facility has been in operation since its start in 1967.

The radioactive nuclei are produced in reactions of high-energy protons from the PS-Booster accelerator in thick targets (Fig.1). The typical proton energies are between 1 and 1.4 GeV. More than 25 different target materials are used. The target material is kept at an elevated temperature so that the produced radioactive atoms diffuse out of the target into different dedicated ion sources. Ionisation can take place in a hot plasma, on a hot surface or by laser excitation (Fig.2). By judicious combinations of target-ions sources a chemical selectivity may be obtained and has resulted in selective production of more than 70 of the chemical elements. The ions are swept out of the ion-source by an applied voltage, accelerated to 30-60 kV and directed into an electro-magnet where they are separated according to their mass (Fig. 3). In this way ISOLDE has been able to deliver more than 700 isotopically pure beams with intensities ranging from 1 to more than 10¹⁰ ions/s.

Figure 2: Overview of ISOLDE target area. Industrial robots (yellow) carry out the remote target handling. The photo to the right shows the front-end with its target container. Courtesy: Stefano Marzari.

Figure 3: The ISOLDE Facility is served by two heavily shielded isotope separators shown in this cut-away view. Left: GPS with one magnet. Right: HRS with two magnets and high resolving power. Courtesy: Stefano Marzari.

Figure 4: The backbone of the ISOLDE beam delivery. Experiments are attached to the ends of the beam lines. Courtesy: Stefano Marzari.

The versatility in isotope production is matched by the versatility in ion manipulation so that the physics studies take place in the energy range from 10^-6 eV to 3 MeV per nucleon. Science studies focuses mainly on modern nuclear structure physics but give opportunities to front-line studies in other fields. The main lines of research are:

• Nuclear Structure Physics
  • Precise determination of nuclear masses
  • Nuclear charge radii, spins and moments
  • Properties of excited nuclear states
  • Shell closures
  • Shape evolution and coexistence
  • Dripline phenomena, including halo nuclei
  • Exotic radioactive decay modes
• Nuclear Astrophysics
  • Nuclear masses, half-lives
  • Decay properties including beta-delayed particle emission
  • Low-energy reaction cross sections
• Atomic Physics
  • Atomic structure of radioactive elements
  • X-ray energy shifts
  • Intertwining of atomic and nuclear processes
• Solid State Physics
  • Surface and bulk studies
  • Diffusion dynamics
  • Semiconductors
  • Spintronics
• Life Sciences
  • Radioisotopes in medical diagnostics and therapy
  • Biochemistry
• Fundamental interactions
  • Neutrino properties
  • Scalar bosons
  • The CKM matrix
  • P- and T-violation

Figure 5: Science subjects studied at ISOLDE.

Achievements

Scientific results

• Extension of the Nuclear Chart

The first systematic studies of structural evolution in long chains of isotopes of noble gases, alkali elements and mercury were made at ISOLDE already during the first years of operation. Since then there has been a continuous technical development of target-ion-source combinations giving access to wide regions of the nuclear chart. The mapping of the outskirts of the nuclear chart was led by the ISOL facilities until the complementary in-flight technique entered the scene at the end of the seventies.

Figure 6: Sketch of the ISOLTRAP setup used for precision mass experiments. The photo shows the precision Penning Trap.

• Nuclear masses

High precision measurements of masses of radioactive ions were pioneered at ISOLDE in the ISOLTRAP[1] experiment (Fig. 6). A series of Penning traps are used for capturing, selectively cooling and measuring the mass of a small number of ions. The absolute mass measurements can be determined in conjunction with a reference mass measurement, such as ¹²C clusters, yielding a relative precision in mass below 10^-8. Furthermore isotopes with half-lives down to 60 ms have been measured successfully. The outcome since the start in the late 1980’s has been more than 400 new or improved mass values for radioactive isotopes. In recognition of this achievement Heinz-Jürgen Kluge was awarded the Lise Meitner Prize of the European Physical Society in 2006.

Figure 7: Schematic illustration of a collinear laser experiment.

Figure 8: Charge radii: from the famous odd-even staggering in Hg, observed at an early stage of ISOLDE, to the recent success with Pb isotopes.

• Shape staggering in light Hg isotopes

The long chains of isotopes of different elements available at ISOLDE allow systematic studies of nuclear properties as a function of neutron number. Atomic spectroscopy measurements (Fig. 7) for determination of charge radii, which give detailed information about the ground-state wave function, have been performed from the early days of ISOLDE. Among the most spectacular results is the very pronounced odd-even staggering of the nuclear charge radii discovered in the neutron-deficient mercury isotopes and interpreted as alternating oblate and prolate shapes: a large difference in the charge radius between the ground state and an isomeric state in ¹⁸⁵Hg was the first example of shape coexistence between a prolate ground state and an oblate isomer. Large odd-even staggering is also observed for light platinum isotopes, while the charge radii of neutron deficient lead isotopes around neutron mid-shell (N=104) show that the magic proton number (Z=82) results in predominantly spherical shapes (Fig. 8). This region is still being actively investigated at many laboratories, one example being Coulomb excitation reactions carried out at the REX-ISOLDE post accelerator.

• The Island of Inversion

Experiments at CERN, PS in the seventies gave the first indications for structural changes in the region (the so-called “Island of Inversion”) around 31Na, later identified as being due to prominent intruder configurations. Many laboratories participate in the ongoing elucidation of the extent of the structural changes, the strength of ISOLDE being the versatility of experimental tools that can be applied once an ion beam of sufficient intensity has been developed. For the isotopes ^30-33Mg, spanning from well outside to inside the island, experiments at ISOLDE have contributed via measurements of masses, spins, magnetic moments, E0 transition rates, level lifetimes, Coulomb excitation, transfer reactions etc. helping to determine exact ground state configurations and to identify the excited state 0⁺ configurations.

• Isomeric beams

Where the atomic hyperfine splitting is large the high selectivity of the Resonance Ionization Laser Ion Source is often sufficient to separate ions in the nuclear ground state from ions in an excited long-lived state (an isomer). The purified isomeric beams can be studied separately, allowing masses, radii, magnetic moments and other properties to be measured unambiguously. As one example Coulomb excitation of accelerated beams of the ground state and the isomeric state of ⁶⁸Cu allowed its level structure to be determined.

Figure 9: “The Guinness Book of Record” nucleus ¹¹Li: no known nucleus shows so many different beta-delayed decay modes.

• Beta-delayed multi-particle emission

Moving away from the line of stability the beta-decay Q-values increase while the separation energy of nucleons decrease which opens the possibility for prompt particle emission after beta decay. Initially beta-delayed proton (βp), alpha (βα) and neutron (βn) emission were used as tools for investigations of structure properties such as beta-strength functions and level densities. When even more exotic nuclei, approaching the drip-line, became available more exotic decay modes were discovered at ISOLDE: β2n, β3n, βt and βd. All these decay modes are present for the two-neutron halo nucleus ¹¹Li, where the β3n mode leads to a final state α + α + n + n + n + β + ν, a multi-fragmentation ‘en miniature’ (Fig. 9). Detailed studies of the multi-particle decays became possible from the late nineties with the advent of multi-segmented detector arrays. As an example the mechanism of the β2p decay from ³¹Ar was shown to be a sequential proton emission.

• Cluster emission

Decays where a nucleus emits a cluster with a mass in the range between an alpha particle and a typical fission fragment are referred to as heavy-particle radioactivity. This very rare type of radioactivity was studied at ISOLDE using polycarbonate sheets for detecting energetic clusters of mass greater than 5. The radioactive beam was collected on the sheet and the presence of cluster decay was found after etching. The damage caused by the heavy fragment resulted in an etched-track diameter and length that allowed a determination of the charge of the emitted cluster. In this way tiny branches of ¹⁴C emission were detected in the decays of ^222,224Ra.

• Dripline – at and beyond

The accelerated radioactive beams from REX ISOLDE allow studies in transfer reactions to nuclear resonance systems beyond the dripline. An example is the unbound nucleus ¹⁰Li, which is a binary subsystem of ¹¹Li, produced in a d (⁹Li,p)¹⁰Li reaction. This type of transfer reaction in the REX ISOLDE energy regime provides interesting complements to studies of unbound nuclei in reactions at relativistic energies. Jonson 2004.

Figure 10: Halo structures in light nuclei. The one-neutron halo ¹¹Be, the two-neutron halos ⁶He and ¹¹Li (illustrated schematically) and the two more challenging cases ⁸He and ¹⁴Be.

• Halo structure

Some nuclei very close to the dripline may develop a spatially extended structure, the halo structure (Fig. 10). This is basically a threshold phenomenon resulting from the presence of a bound state close to the continuum. Several key experiments proving the existence of nuclear halos have been carried out at ISOLDE. The magnetic moment and the electrical quadrupole moments of ⁹Li and ¹¹Li, measured by combining optical and beta-decay experiments, showed that the ⁹Li core was marginally affected by the dilute tail of neutron matter in ¹¹Li. The observation of the beta-delayed deuteron decay mode, which is closely linked to the two-neutron halo, was observed both for ⁶He and ¹¹Li. A measurement of the magnetic moment for the one-neutron halo nucleus ¹¹Be gave information on the d-wave component in its ground state. This, together with the charge radius, measured by frequency-comb based collinear laser spectroscopy, give input into ab inito descriptions of ¹¹Be.

• Waiting-point nuclei and element synthesis

Many of the exotic nuclides produced at ISOLDE give key information for nuclear processes involved in the element synthesis in astrophysical environments. New light on the understanding of the triple-alpha process, bridging the low-mass elements towards synthesis of heavier elements, has been gained from studies of the beta-delayed triple-alpha emission from ¹²B. These data, combined with data from the sister facility IGISOL, show that the rate of the triple-alpha process is dependent on the interference between the 0⁺ Hoyle state at 7.65 MeV in ¹²C and a broad 0⁺ resonance centred at an energy about 3 MeV higher. The abundance of heavier elements shows characteristic maxima at A≈ 80,130 and 195, which are due the neutron-magic nuclei in the r-process path that have longer half-lives and “wait” to decay. It is an experimental challenge to reach the heavier r-process nuclei. This was first achieved at ISOLDE for the A≈ 130 mass region where half-lives of the N=82 waiting point nuclides ¹³⁰Cd and ¹²⁹Ag were measured.

• Atomic spectroscopy of Francium

The element francium (Z=87) exists in nature but in very tiny amounts. Natural radioactivity gives rise to a small equilibrium amount of about 20 g ²²³Fr (T_1/2=21.8 m) present on earth at any moment. Francium is the heaviest alkali element and its electronic structure is of high theoretical interest. The production yields for Fr isotopes at ISOLDE (10⁹ atoms/s for ²²³Fr, corresponding to the equilibrium content in 10 t of uranium ore) allowed a measurement of the D₂ wavelength (λ = 717.97 (0.01) nm) and studies of Rydberg levels for several Fr isotopes.

• Beta-neutrino correlations

Several experimental methods have been and are used to probe the angular correlation in beta decay between the neutrino and the beta particle, thereby probing the detailed structure of the weak interaction. Rather than the neutrino one detects the recoil of the final nucleus, which is small and challenging to measure. One method exploits that the recoil effect is amplified by a subsequent particle emission. In this way experiments on ³²Ar in the nineties gave improved limits on a scalar component of the weak interaction.

• Octupole states

There is an extended, well-established region of nuclides showing octupole deformation (“pear-shaped”) in their ground states situated around A=225. Measurements of the β-decay of Ra and Fr isotopes, performed at ISOLDE, have contributed to the understanding of these reflection-asymmetric shaped nuclei. The level scheme of ²²⁵Ra established from studies of the β decay of ²²⁵Fr is one example. Here three sets of coupled bands having negative- and positive-parity components were identified from studies of conversion electrons and gamma rays.

Figure 11: Illustration of a mercury atom binding to three proteins. Courtesy: Lars Hemmingsen.

• Solid state physics, materials science and life sciences

The experimental programme at ISOLDE employing radioisotopes as probes for solid-state related studies has been very successful over the years. A rich variety of radioactive species ranging from ⁸Li to ²¹³Fr has been employed for studies with nuclear techniques, like Perturbed Angular Correlations (PAC) and Mössbauer Spectroscopy (MS), as well as with more traditional approaches like Deep Level Transient Spectroscopy (DLTS) and Photo Luminescence Spectroscopy (PL). Typical examples of applications are investigations of radiation damage, lattice sites of dopants, site selective doping of semiconductors, donor-acceptor interactions in semiconductors, diffusion studies, and studies of surfaces and interfaces. Materials relevant for use in spin transport electronics (spintronics) are studied with PAC and MS techniques. Recently biologic systems have been in focus of interest, where the binding of Ag, Pb and Hg to biomolecules has been investigated via PAC techniques. The results give information about the chemistry of heavy-metal-biomolecule interactions, which is important for the understanding of heavy-metal toxicity (Fig. 11). One has also performed in vivo studies of water transport in barley plants. Forkel-Wirth 1999

Technical developments.

ISOLDE has throughout its existence been leading developments in production of radioactive isotopes via the ISOL (Isotope Separation On-Line) method. Many results have also emerged on specialized techniques for manipulation of (radioactive) ions and measurements performed with them.

• The intensity of a radioactive beam depends on the intensity of the primary (driver) beam, the target thickness, the cross section for production of the specific radioactive isotope and of the efficiency of extracting the produced isotope from the target, purifying it, ionizing it and leading it into the experimental set-up. Many specialized targets and ion sources have been constructed in order to optimize the efficiency, but it is still challenging to predict the behaviour of a new target design in the fierce environment involving high-temperature chemistry and intense radiation. At an early stage of the exotic nuclei-physics programme at CERN a complementary installation ran using protons from the PS synchrotron on Ir or UC2 targets. These experiments convincingly demonstrated that the production cross sections for very exotic nuclei still increase when going from 600 MeV to 10-24 GeV. The present ISOLDE optimises production yields by a compromise between cross section and proton beam intensity.
• The Resonance Ionization Laser Ion Source (RILIS), Fig. 12, is very selective and operates through stepwise excitation of atoms to above the ionization threshold. It has been an important part of the Facility since 1992 and the technology has been transferred to many other laboratories.

Figure 12: The RILIS ion source: Combining three dedicated laser beams for selective ionization of Hg.

• Atomic-beam experiments with radioactive ions were pioneered at ISOLDE and still provide important and model independent information on nuclear ground state properties: charge radius, spin, magnetic dipole moment and electric quadrupole moment. Today most studies involve collinear laser spectroscopy where the ion (or atom) beam is overlaid with one or more laser beams.
• Penning and Paul traps allow sophisticated ion manipulation to be performed. Many of the procedures in use today were developed at the ISOLTRAP mass measurement experiment at ISOLDE, including use of ion beam coolers. See also the Scholarpedia article on The ion traps in nuclear physics.

Figure 13: 3D Sketch of the REX ISOLDE post-accelerator, without shielding. The platform in the upper right accommodates the REXTRAP and the REXEBIS. The insert is a photo of the accelerator. Courtesy: Stefano Marzari.

• The multiply charged ions needed for efficient post-acceleration are produced in a unique way in the REX-ISOLDE low energy stage. Singly charged ions from ISOLDE are stopped, bunched and cooled in a Penning trap (REXTRAP), transferred to an electron beam ion source (REX-EBIS) where they are bred to charge to mass ratios between 1/4.5 and 1/3 before magnetic separation and acceleration (Fig. 13). This essentially universal scheme allows post-acceleration of most of the beams produced by ISOLDE.

Figure 14: Ideas to the future superconducting linear accelerator HIE ISOLDE. Courtesy: Yacine Kadi.

• At the end of 2009 the CERN Research Board approved a new project, HIE ISOLDE, which will allow a great expansion of the physics opportunities at the ISOLDE Facility. There are three major elements of this upgrade of the post-accelerator complex: (i) An energy increase from the present 3 MeV/u at REX ISOLDE to 5.5 MeV in a first step and, eventually 10 MeV/u, an energy envisaged for 2015. The main component that allows entering into this interesting energy domain is a superconducting linear accelerator (Fig. 14). (ii) Higher beam intensity achieved through improvements of the PS Booster injector, the charge breeding efficiency and target-ion-sources. (iii) Improvements in the beam quality, in purity and emittance, which is a prerequisite for high-precision studies.

Impact on the research community

ISOLDE has from the outset been a purely international undertaking, starting from a growing realization in Europe in the sixties that progress in experimental nuclear physics needed installations far beyond what the individual countries could provide. It was then natural to approach CERN that could offer not only a suitable accelerator, the SC, but also an organisational structure that encourage international collaboration. The present ISOLDE is a brilliant example of an international environment, where collaborations are formed from physics interests regardless of nationality. The ensuing interaction between scientists fosters new ideas and even leads to cross fertilisation between different lines of physics. At the beginning ISOLDE, in the words of D.A. Bromley,"... got such a head start on the rest of the world activity in this field that people were very reluctant to attempt to mount a competitive operation." Today the situation is different since know-how accumulated at ISOLDE gradually has been transferred to and inspired the development of similar facilities world-wide. Many of today’s nuclear physicists, nuclear chemists and engineers have received training at ISOLDE. The user community is evolving dynamically and currently numbers between 400 and 500 scientists.

History

The feasibility of on-line production of short-lived radioactive isotopes was demonstrated already in 1951 by O. Kofoed-Hansen and K-O. Nielsen. They performed experiments on short-lived isotopes of noble-gas elements that were produced by connecting a target, irradiated by protons, directly to an isotope separator. Inspired by this, the European nuclear physics community proposed to build a general-purpose experiment for production of short-lived nuclei connected to the synchro-cyclotron (SC) at CERN. The project was approved in 1964 and the first experiment with the on-line isotope separator, named ISOLDE, was performed in 1967. The first four letters in the acronym has since then been the standard name for this type of radioactive isotope production, the ISOL technique. A technical development programme started to build target-ion-source systems that gave access to more and more elements. In 1972-74, a major upgrade of the SC beam intensity was performed. At the same time an improvement programme at ISOLDE resulted in a new layout called ISOLDE 2. The high intensity of produced isotopes and large scope of produced isotopes meant that ISOLDE had become a major international Facility to perform experiments on radioactive isotopes. The SC was shut down at the end of 1990 and CERN decided to move the ISOLDE activities to a beam from the PS Booster. A new experimental hall was built and the first experiments at the ISOLDE PSB Facility started in the middle of 1992 (Jonson & Richter, 2000). At the end of the nineties it was proposed to post accelerate the beams from ISOLDE. The REX ISOLDE accelerator was built and started its operation in 2001. A major upgrade program, HIE-ISOLDE, was approved in 2009.

Figure 15: Some of the major milestones in the development of ISOLDE from a single beam experiment at the CERN SC to its present position at the PS Booster as a major facility for handling, manipulating and studying of rare isotopes.

Organisational aspect

The ISOLDE facility is now run by CERN staff and forms an integral part of the CERN accelerator complex. In the first decades it was structured as a physics experiment run by the ISOLDE collaboration. The collaboration still maintains an important role in shaping the science program at ISOLDE and the technical developments around it and officially holds the Memorandum of Understanding with CERN that covers legal aspects. The member states of the collaboration are (February 2010) Belgium, CERN, Denmark, Finland, France, Germany, Italy, Norway, Romania, Spain, Sweden and the United Kingdom.

Proposals for doing experiments are submitted to CERN’s ISOLDE and Neutron Time-of-Flight Experiments Committee (INTC) and evaluated there. If accepted, experiments will be scheduled typically within 1-2 years. ISOLDE users benefit from the general user support services provided by CERN. Most experiments install their own experimental equipment, although some common equipment is in place, in particular for the solid state physics community. Among the larger (semi-) permanent experimental set-ups are the gamma detector array Miniball, the mass measurement ISOLTRAP, the collinear laser spectroscopy set-ups COLLAPS and CRIS, the ³He-⁴He dilution refrigerator NICOLE, the weak interaction experiment WITCH and the ultra high vacuum set-up ASPIC.

References

Jonson B. Phys. Rep. 389 (2004) 1 - 59, Light dripline nuclei.

Jonson B. and Richter A. Hyperfine Interactions 129 (2000) 1 - 22, More than three decades of ISOLDE physics.

Forkel-Wirth B. Rep. Prog. Phys. 62 (1999) 527 - 597, Exploring solid state physics properties with radioactive isotopes.

Internal references

• James Meiss (2007) Dynamical systems. Scholarpedia, 2(2):1629.

• Eugene M. Izhikevich (2007) Equilibrium. Scholarpedia, 2(10):2014.

• Philip Holmes and Eric T. Shea-Brown (2006) Stability. Scholarpedia, 1(10):1838.

Recommended reading

Hyperfine Interactions Vol. 129 (2000) 1 - 553, ISOLDE - a laboratory portrait.

P. Van Duppen, K. Riisager, J. Phys. G 38 (2011) 024005, Physics with REX-ISOLDE: from experiment to facility.

ISOLDE Web page.

See also

Mass measurements, Nuclear Astrophysics, Nuclear reactions and structure of unstable nuclei, The ion traps in nuclear physics

Sponsored by: Dr. Riccardo Guida, Institut de Physique Théorique, CEA & CNRS, Gif-sur-Yvette, France

Sponsored by: Dr. Nicolas Alamanos, CEA, IRFU; F-91191 Gif-sur-Yvette, France, France

Reviewed by: Anonymous

Reviewed by: Dr. Dominique Guillemaud-Mueller, Institut de Physique Nucléaire d’Orsay

Accepted on: 2010-07-08 07:23:48 GMT