Isolde

The ISOLDE (Isotope Separator On Line DEvice) Radioactive Ion Beam Facility, is an on-line isotope separator facility located at the centre of the CERN accelerator complex on the Franco-Swiss border. Created in 1964, the ISOLDE facility started delivering radioactive ion beams (RIBs) to users in 1967. Originally located at the synchro-cyclotron (SC) accelerator (CERN's first ever particle accelerator), the facility has been upgraded several times most notably in 1992 when the whole facility was moved to be connected to CERN's proton synchroton booster (PSB). ISOLDE is currently the longest-running facility in operation at CERN, with continuous developments of the facility and its experiments keeping ISOLDE at the forefront of science with RIBs. ISOLDE benefits a wide range of physics communities with applications covering nuclear, atomic, molecular and solid-state physics, but also biophysics and astrophysics, as well as high-precision experiments looking for physics beyond the Standard Model. The facility is operated by the ISOLDE Collaboration, comprising CERN and sixteen (mostly) European countries. As of 2019, close to 1,000 experimentalists around the world (including all continents) are coming to ISOLDE to perform typically 50 different experiments per year.…

The ISOLDE (Isotope Separator On Line DEvice) Radioactive Ion Beam Facility, is an on-line isotope separator facility located at the centre of the CERN accelerator complex on the Franco-Swiss border. Created in 1964, the ISOLDE facility started delivering radioactive ion beams (RIBs) to users in 1967. Originally located at the synchro-cyclotron (SC) accelerator (CERN's first ever particle accelerator), the facility has been upgraded several times most notably in 1992 when the whole facility was moved to be connected to CERN's proton synchroton booster (PSB). ISOLDE is currently the longest-running facility in operation at CERN, with continuous developments of the facility and its experiments keeping ISOLDE at the forefront of science with RIBs. ISOLDE benefits a wide range of physics communities with applications covering nuclear, atomic, molecular and solid-state physics, but also biophysics and astrophysics, as well as high-precision experiments looking for physics beyond the Standard Model. The facility is operated by the ISOLDE Collaboration, comprising CERN and sixteen (mostly) European countries. As of 2019, close to 1,000 experimentalists around the world (including all continents) are coming to ISOLDE to perform typically 50 different experiments per year. Radioactive nuclei are produced at ISOLDE by shooting a high-energy (1.4GeV) beam of protons delivered by CERN's PSB accelerator on a 20 cm thick target. Several target materials are used depending on the desired final isotopes that are requested by the experimentalists. The interaction of the proton beam with the target material produces radioactive species through spallation, fragmentation and fission reactions. They are subsequently extracted from the bulk of the target material through thermal diffusion processes by heating the target to about 2,000 °C. The cocktail of produced isotopes is ultimately filtered using one of ISOLDE's two magnetic dipole mass separators to yield the desired isobar of interest. The time required for the extraction process to occur is dictated by the nature of the desired isotope and/or that of the target material and places a lower limit on the half-life of isotopes which can be produced by this method, and is typically of the order of a few milliseconds. For an additional separation, the Resonance Ionisation Laser Ion Source (RILIS) uses lasers to ionise a particular element, which separates the radioisotopes by their atomic number. Once extracted, the isotopes are directed either to one of several low-energy nuclear physics experiments or an isotope-harvesting area. A major upgrade of the REX post-accelerator to the HIE-ISOLDE (High Intensity and Energy Upgrade) superconducting linac completed construction in 2018, allowing for the re-acceleration of radioisotopes to higher energies than previously achievable.

Background

Most atomic nuclei contain protons and neutrons. The number of protons determines the chemical element the nucleus belongs to. Different isotopes of the same element have different numbers of neutrons in their nuclei, but contain the same number of protons. For example, isotopes of carbon include carbon-12, carbon-13, carbon-14, which contain 6, 7, 8 neutrons respectively, but all contain 6 protons. Each isotope of an element has a different nuclear energy state, and may have different stability. A nuclide is a more general term than isotope, and refers to atoms that have any particular number of protons and neutrons. Stable nuclides are not radioactive and do not spontaneously undergo radioactive decay, so are more usually found in nature. Whereas unstable (i.e. radioactive) nuclides are not found in nature, unless there is a recent source of them, because they are shorter lived, and will spontaneously decay, in one or more steps, to more stable nuclides. For example, carbon-14 is unstable but is found in nature. Scientists use accelerators and nuclear reactors to produce radioactive nuclides. As a general trend, and among other factors, the neutron–proton ratio of a nuclide determines its stability. The value of this ratio for stable nuclides generally increases for larger nuclei with more protons and neutrons. Many unstable nuclides have neutron-proton ratios beyond the zone of stability. The time required to lose half of a quantity of a given nuclide through radioactive decays, the half-life, is a measure of how stable an isotope is. Nuclides can be visually represented on a table (Segré chart or table of nuclides) where the proton number is plotted against the neutron number.

History

In 1950, two Danish physicists Otto Kofoed-Hansen and Karl-Ove Nielsen discovered a new technique for producing radioisotopes which enabled production of isotopes with shorter half-lives than earlier methods. The Copenhagen experiment they carried out included a simplified version of the same elements used in modern on-line experiments. Ten years later, in Vienna, at a symposium about separating radioisotopes, plans for an 'on-line' isotope separator were published. Using these plans, CERN's Nuclear Chemistry Group (NCG) built a prototype on-line mass separator coupled to target and ion source, which was bombarded by a 600 MeV proton beam delivered by CERN's the Synchro-Cyclotron. The test was a success and showed that the SC was an ideal machine for on-line rare isotope production. The plan for an electromagnetic isotope separator was developed during 1963–4 by European nuclear physicists and, in late 1964, their proposal was accepted by the CERN Director-General and the ISOLDE project began. The "Finance Committee" for the project set up originally with five members, then extended to twelve to include two members per 'country' (including CERN). As the term "Finance Committee" had other connotations, it was decided 'until a better name was found' to call the project ISOLDE and the committee the ISOLDE Committee. In 1965, as the underground hall at CERN was being excavated, the isotope separator for ISOLDE was being constructed in Aarhus. In May 1966, the SC shut down for some major modifications. One of these modifications was the construction of a new tunnel to send proton beams to a future underground hall that would be dedicated to ISOLDE. Separator construction made good progress in 1966, along with the appointing of Arve Kjelberg as the first ISOLDE coordinator, and the underground hall was finished in 1967. On 16 October 1967, the first proton beams interacted with the target and the first experiments were successful in proving that the technique worked as expected. In 1969, the first paper was published with studies of various short-lived isotopes. Shortly after the ISOLDE experimental program started, some major improvements for SC were planned. In 1972 the SC shut down to upgrade its beam intensity by changing its radiofrequency system. The SC Improvement Program (SCIP) increased the primary proton beam intensity by about a factor of about 100. To be able to handle this high-intensity ISOLDE facility also needed some modifications to successfully extract the improved beam to ISOLDE. After necessary modifications, the new ISOLDE facility also known as ISOLDE 2 was launched in 1974. Its new target design combined with the increased beam intensity from the SC led to significant enhancements in the number of nuclides produced. However, after some time the external beam current from the SC started to be a limiting factor. The collaboration discussed the possibility of moving the facility to an accelerator that could reach higher current values but decided on building another separator with ultra-modern design, for the facility. The new high-resolution separator, ISOLDE 3, was in full use by the end of the 80s. In 1990 a new ion source RILIS was installed at the facility to selectively and efficiently produce radioactive beams. The SC was decommissioned in 1990, after having been in operation for more than three decades. As a consequence, the collaboration decided to relocate the ISOLDE facility to the Proton Synchrotron, and place the targets in an external beam from its 1 GeV booster. The construction of the new ISOLDE experimental hall started about three months prior to the decommissioning of the SC. With the relocation also came several upgrades. The most notable being the installation of two new magnetic dipole mass separators. One general-purpose separator with one bending magnet and the other one is a high-resolution separator with two bending magnets. The latter one is a reconstructed version of the ISOLDE 3. The first experiment at the new facility, known as ISOLDE PSB, was performed on 26 June 1992. In May 1995, two industrial robots were installed in the facility to handle the targets and ion sources units without human intervention. To diversify the scientific activities of the facility, a post-accelerator system called REX-ISOLDE (Radioactive beam EXperiments at ISOLDE) was approved in 1995 and inaugurated at the facility in 2001. With this new addition, nuclear reaction experiments which require a high-energy RIB could now be performed at ISOLDE. Additionally, REXTRAP operates as a Penning Trap for the REX-ISOLDE then transfers bunches of ions to REXEBIS, an Electron Beam Ion Source (EBIS), which traps the isotopes produced and further ionises them. The facility building was extended in 2005 to allow more experiments to be set up. ISCOOL, an ion cooler and buncher, increasing the beam quality for experiments was installed at the facility in 2007. In 2006, the International Advisory Board decided that upgrading ISOLDE hall with a linear post-accelerator design based on superconducting quarter-wave resonators would allow for a full-energy availability, crucially without the reduction of beam quality. The HIE-ISOLDE project was approved in December 2009, and involves an upgrade of the energy range from 3 MeV per nucleon, to 5 MeV, and lastly to 10 MeV per nucleon. The design also incorporated an intensity upgrade to make best use of the delivered proton beams. The upgrade project was split into three different phases, to be completed over a number of years. In late 2013 the construction of a new facility for medical research called CERN MEDICIS (MEDical Isotopes Collected from ISOLDE) started. Of the incident proton beams used at ISOLDE, only 10% are actually stopped in the targets and achieve their objective, while the remaining 90% are not used. The MEDICIS facility is designed to work with the remaining proton beams that have already passed a first target. The second target produces specific radioisotopes that are delivered to hospitals and research facilities and can be made injectable. In 2013, during the Long Shutdown 1, three ISOLDE buildings were demolished. They've been built again as a new single building with a new control room, a data storage room, three laser laboratories, a biology and materials laboratory, and a room for visitors. Another building extension for the MEDICIS project and several others equipped with electrical, cooling and ventilation systems to be used for the HIE-ISOLDE project in the future were also built. In addition, the robots which were installed for the handling of radioactive targets have been replaced with more modern robots. In 2015, for the first time, a radioactive isotope beam could be accelerated to an energy level of 4.3 MeV per nucleon in the ISOLDE facility thanks to the HIE-ISOLDE upgrades. In late 2017, the CERN-MEDICIS facility produced its first radioisotopes and by the end of 2020 had provided external nine hospitals and research facilities with 41 batches of radioisotopes. Phase 2 of the facility's HIE-ISOLDE upgrade was completed in 2018, which allows ISOLDE to accelerate radioactive beams up to 10 MeV per nucleon.

Facility and concept

The ISOLDE facility contains the Class A laboratories, buildings for the HIE-ISOLDE and MEDICIS projects, and the control rooms located in building 508. Before ISOLDE, the radioactive nuclides were transported from the production are to the laboratory for examination. At ISOLDE, all processes from the production to the measurements are connected and the radioactive material requires no extra transport. Due to this, ISOLDE is referred to as an on-line facility. At the ISOLDE facility, the main proton beam for reactions comes from the PSB. The incoming proton beam has an energy of 1.4 GeV and its average intensity varies up to 2 μA. The beam enters the facility and is directed towards one of two mass separators: the General Purpose Separator (GPS) and the High Resolution Separator (HRS). The separators have independently run target-ion source systems, delivering 60 keV RIBs. The targets used at ISOLDE allow for the quick production and extraction of radioactive nuclei. Targets consist sometimes of molten metal kept at high temperature (700 °C to 1400 °C), which result in long isotope release times. Heating the target to higher temperatures, typically above 2000 °C, makes for a faster release time. Using a target heavier than the desired isotope, results in production via spallation or fragmentation. The ion sources, used in combination with the targets at ISOLDE, produce an ion beam of (preferably) one chemical element. There are three types used: surface ion sources, plasma ion sources and laser ion sources. The surface ion sources consist of a metal tube with a high work function heated up to 2400 °C, so that the atom can be ionised. If an atom cannot be surface ionised, the plasma ion source is used. The plasma is produced by an ionised gas mixture and optimised using an additional magnetic field. The laser ion source used at ISOLDE is RILIS. The GPS is made with a double focusing magnet with a bending radius of 1.5 m and a bending angle of 70°. The resolution of the GPS is approximately 800. The GPS sends beams to an electronic switchyard, allowing three mass separated beams to be simultaneously extracted. The second separator, the HRS, consists of two dipole magnets, with bending radii of 1 m and bending angles of 90° and 60°, and an elaborate ion-optical system. The overall resolution of the HRS has been measured as 7000, which enables it to be used for experiments requiring higher mass resolution values. The GPS switchyard and HRS are connected to a common central beam-line used to provide beam to the various experimental setups located in the ISOLDE facility.

Solid-state physics laboratory

Attached to ISOLDE in building 508, is CERN's solid-state physics laboratory. Solid state physics research (SSP) accounts for 10–15% of the yearly allocation of beam time and uses about 20–25% of the overall number of experiments running at ISOLDE. The laboratory uses the technique of Time Differential Perturbed Angular Correlation (TDPAC) to probe the large quantity of available radioactive elements provided by ISOLDE. This technique has also been used to measure ferromagnetic and ferroelectric properties of materials, as well as providing ion beams for other facilities within ISOLDE. Additional methods used for SSP are tracer diffusion, online-Mössbauer spectroscopy ( 57 Mn) and photoluminescence with radioactive nuclei.

Results and discoveries

Below is the list of some physics activities done at ISOLDE facility. Extension of the table of nuclides by discovering new isotopes The ISOLDE facility continuously develops the nuclear chart, and was the first to study structural evolution in long chains of noble gas, alkali elements and mercury isotopes. High precision measurements of nuclear masses The ISOLTRAP experimental setup Is able to make high precision measurements of nuclear masses by using a series of Penning traps. The experiment has been able to measure isotopes with very short half-lives ( −8 . For his work on "key contributions to the masses..." of isotopes at ISOLTRAP, among other work, Heinz-Jürgen Kluge was a recipient of the Lise Meitner Prize in 2006. Discovery of shape staggering in light Hg isotopes Atomic nuclei are usually spherical, however gradual changes in nuclear shape can occur when the number of neutrons of a given element changes. Research published in 1971¹ showed that if single neutrons are added to or removed from the nuclei of mercury isotopes, the shape will change to a "rugby ball". Newer studies, from RILIS, show that this shape staggering also occurs with bismuth isotopes. Contributions to island of inversion measurements and potential discovery of new magic numbers The island of inversion is a region of the chart of nuclides in which isotopes have enhanced stability, compared to the surrounding unstable nuclei. The island is associated with the magic neutron numbers (N = 8, 14, 20, 28, 50, 82, 126), where this breakdown occurs. Various experiments at ISOLDE have determined properties of these island of inversion isotopes, including the first of their kind measurements performed with Miniball on magnesium-32, lying in the island of inversion at N = 20. Furthermore, the ISOLTRAP experiment provided results using calcium-52 to reveal a potential new magic number, 32, which was later disproven by the CRIS experiment. Production of isomeric beams A nuclear isomer is a metastable state of a nucleus, in which one or more nucleons occupy higher energy levels than in the ground state of the same nucleus. In the mid-2000s, REX-ISOLDE developed a technique to select and post-accelerate isomeric beams to use in nuclear-decay experiments, such as at Miniball. Production of gold In 2022, the RILIS, Windmill and ISOLTRAP teams reported producing 18 gold nuclei from proton bombardment of a uranium target. Discovery of beta-delayed multi-particle emission The first observation of beta-delayed two-neutron emission was made at ISOLDE in 1979, using the isotope lithium-11. Beta-delayed emission occurs for isotopes further away from the line of stability, and involves particle emission after beta decay. Newer studies have been proposed to investigate beta-delayed multi-particle emission of lithium-11 using the IDS. Studies on nuclear resonance systems beyond the drip line and existence of halo structure The nuclear drip line is the boundary beyond which adding nucleons to a nucleus will result in the immediate decay of a nucleon (nucleon has 'dripped' out of the nucleus). Accelerated RIBs from REX-ISOLDE are used in transfer reactions which allow for studies of nuclear resonance systems beyond the dripline. Some light nuclei close to the drip line may have a neutron halo structure, due to the tunnelling of loosely bound neutrons outside the nucleus. This proof of the halo structure was made at ISOLDE from a series of experiments analysing the lithium-11 nucleus. First observations of short-lived pear-shaped atomic nuclei Research conducting using the Miniball experimental setup found evidence of pear-shaped heavy nuclei, in particular radon-220 and radium-224. These results were named in the Institute of Physics (IoP) "top 10 breakthroughs in physics" in 2013, and was featured as the cover of Nature 2013. In 2020, due to the HIE-ISOLDE upgrade, radium-222 was also found to have a "stable pear shape". Laser spectroscopy has been performed on a short-lived radioactive molecule, containing radium, which further studies into could reveal physics beyond the Standard Model due to time-reversal symmetry breaking. Measurement of 229m Th transition energy In 2023, ISOLDE made the first 1%-level measurement of the ultralow-energy thorium-229m nuclear isomer, detecting photons at an energy of . This was a key step in the construction of a future nuclear clock. A mini documentary series about ISOLDE by CERN (YouTube playlist) Celebrating 50 years of physics at ISOLDE by CERN (YouTube video) A poster about ISOLDE from ISOLDE website A poster about HIE-ISOLDE and some other upgrades from ISOLDE website

Further reading

References

Text derived from https://en.wikipedia.org/w/index.php?oldid=1373167490 under CC BY-SA 4.0; modifications have been made.