Установка ISOLDE в ЦЕРН: производство и исследование радиоактивных ионов.

Physics facility at CERN

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 ProtonSynchroton 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.

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.

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. 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. 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. 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. 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 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.

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. 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 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.

The ISOLDE COOLer (ISCOOL) is located downstream from the HRS, and extends up to the merging switchyard joining the two mass separator beams. ISCOOL is a general purpose Radio Frequency Quadrupole Cooler and Buncher (RFQCB), with the purpose of cooling (improving the beam quality) and bunching the RIB from the HRS. Incoming ions collide with the neutral buffer gas, losing their energy, and then are radially confined. The beam is then extracted from ISCOOL.

The magnetic mass separators are able to separate isobars by mass number, however they are unable to sort isotopes of the same mass. If an experiment requires a higher degree of chemical purity, it will need the beam to have an additional separation, by proton number. RILIS provides this separation by using step wise resonance photo ionisation, involving precisely tuned laser wavelengths matched exactly to a specific element's successive electron transition energies. Ionisation will only occur of the desired element, and the other elements within the ion source will remain unchanged. This process of laser ionisation takes place in a hot metal cavity to provide the spatial confinement needed for the atomic vapour to be illuminated. A high frequency laser system is needed to ionise the atom before it leaves the cavity. All in all, the ISOLDE facility provides 1300 isotopes from 75 elements in the periodic table.

The post accelerator REX ISOLDE is a combination of different devices used to accelerate radioisotopes to boost their energy to 10 MeV per nucleon, increased from 3 MeV per nucleon due to HIE ISOLDE upgrades. The incoming RIBs have enough energy to overcome the first potential threshold of the Penning trap, REXTRAP, but within the trap the ions lose energy through collisions with buffer gas atoms. This cools the ions and their movement is dampened by a combination of a radio frequency (RF) excitation and a buffer gas. The ion bunches are extracted from REXTRAP and injected into REXEBIS. A mass separator is required to separator the subsequent ions, due to the small intensity after being extracted from EBIS. The next stage of REX ISOLDE consists of a normal conducting (room temperature) linac, where the ions are accelerated by an RFQ. An interdigital H type (IH) structure uses resonators to boost the beam energy up to its maximum value.

To be able to satisfy the ever increasing needs of higher quality, intensity, and energy of the production beam is very important for facilities such as ISOLDE. As the latest response to satisfy these needs, HIE ISOLDE upgrade project is currently ongoing. Due to its phased planning, the upgrade project is being carried out with the least impact on the experiments continuing in the facility. The project included an energy increase for the REX ISOLDE up to 10 MeV as well as resonator and cooler upgrades, enhancement of the input beam from PSB, improvements on targets, ion sources, and mass separators. Following the completion of the phase two upgrade in 2018 for the HIE ISOLDE which included installing four high beta cryomodules, the next and final phase will replace REX structures after the IH structure (IHS) with two low beta cryomodules. This will improve the beam quality and allow a continuously variable energy between 0.45 and 10 MeV per nucleon. As a state of the art project, HIE ISOLDE is expected to expand the research opportunities in ISOLDE facility to the next level. When completed, the upgraded facility will be able to host advanced experiments in fields like nuclear physics and nuclear astrophysics.

ISOLDE contains both temporary and fixed experimental setups. Temporary setups in the ISOLDE facility are there for shorter time periods, and generally focus on detecting specific decay modes of nuclei. The fixed experimental setups have a permanent position at the facility. They include:

The COLinear LAser SPectroScopy (COLLAPS) experiment has been operating at ISOLDE since the late 1970s and is the oldest active experiment at the facility. COLLAPS studies ground and isomeric state properties of highly unstable (exotic), short lived nuclei, including measurements of their spins, electro magnetic moments and charge radii. The experiment uses the technique of collinear spectroscopy using lasers to access necessary atomic transitions.

The Emission Channeling with Short Lived Isotopes (EC SLI) experiment uses the emission channelling method to study lattice locations of dopants and impurities in crystals and epitaxial thin films. This is done by introducing short lived isotope probes into the crystal and measuring the electron intensity affected to determine whether they have been affected by the decay particles emitted.

The ISOLDE Decay Station (IDS) experiment is a setup that allows different experiment systems to be coupled to the station, using spectroscopy techniques such as fast timing or time of flight (ToF). The station, operational since 2014, is used to measure decay properties of a wide range of radioactive isotopes for a variety of applications. Results from the IDS have been useful for astrophysics, as they measured the probability of a particular decay seen in red giant stars.

The ISOLDE Solenoidal Spectrometer (ISS) experiment uses an ex MRI magnet to direct RIBs at a light target. Conditions produced by this reaction replicate those present in astrophysical processes, and measuring the properties of the atomic nuclei will also provide a better understanding of nucleon nucleon interactions in exotic nuclei. The experiment was commissioned in 2021 and finished construction during the Long Shutdown 2. Since the start of its operation, ISOLTRAP has measured the mass of hundreds of short lived radioactive nuclei, as well as confirming the existence of doubly magic isotopes. The setup was upgraded in 2011 to include a multi reflection time of flight mass spectrometer (MR ToF), allowing the detection of more exotic isotopes.

The LUCRECIA experiment is based on a Total Absorption gamma Spectrometer (TAS), which measures the gamma transitions in an unstable parent nucleus. From these measurements, nuclear structure is analysed and used to confirm theoretical models and make stellar predictions.

The Miniball experiment is a gamma ray spectroscopy setup consisting of a high resolution germanium detector array. The experiment is used to analyse the decays of short lived nuclei involved in Coulomb excitation and transfer reactions. Results from Miniball at ISOLDE that found evidence of pear shaped heavy nuclei was named in the Institute of Physics (IoP) "top 10 breakthroughs in physics".

The Multi Ion Reflection Apparatus for CoLlinear Spectroscopy (MIRACLS) experiment determines properties exotic radioisotopes by measuring their hyperfine structure. MIRACLS uses laser spectrometer on ion bunches trapped in a MR ToF, to increase the flight path of the ions. Currently, the experiment is being designed and constructed.

The Scattering Experiments Chamber (SEC) experiment facilitates diversified reaction experiments, and is complimentary to the ISS and Miniball, due to SEC not detecting gamma radiation. The station is used to study low lying resonances in light atomic nuclei through transfer reactions.

The Versatile Ion polarisation Technique Online (VITO) experiment is a beamline used to investigate the weak interaction and determine properties of short lived unstable nuclei. The experiment uses the technique of optical pumping to produce laser polarised RIBs allowing for versatile studies. There are three independent studies on the VITO beamline including a β NMR spectroscopy station.

The Weak Interaction Studies with 32Ar Decay (WISArD) experiment investigates the weak interaction to search for physics beyond the Standard Model (SM). The WISArD setup reuses some of the WITCH experiment's infrastructure, as well as its superconducting magnet. 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 (57Mn) and photoluminescence with radioactive nuclei.

The HIE ISOLDE project introduced a network of High Energy Beam Transfer (HEBT) beamlines to the ISOLDE facility. The common section beamline, XT00, joins to three bending beamlines (XT01, XT02, XT03) leading to different experiment setups. The three identical beamlines are independent of each other, for example, if the first XT01 dipole magnet is off, the beam will continue to the XT02 and XT03. They all bend the beam by 90 degrees and focus it using two dipole magnets and a doublet quadrupole. The XT01 beamline leads to Miniball, the XT02 beamline leads to the ISS, and the XT03 beamline leads to movable setups, such as the SEC scattering chamber. The facility includes the beamline enclosed in a Faraday cage as well as a laser laboratory and control station. The offline facility is designed for target test studies, and upgraded to include potential for the production and study of molecular ion beams.

Below is a list of improvements needed for the ISOLDE facility, considering both medium and long term goals. Some of these improvements have been proposed by the EPIC project.