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.[1] 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.[2] 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.[3][4]
ISOLDE experimental setups
Colinear Laser Spectroscopy
Collinear Resonance Ionization Spectroscopy
Emission Channeling with Short-Lived Isotopes
ISOLDE Decay Station experiment
ISOLDE Solenoidal Spectrometer
ISOLTRAP
LUCRECIA
Miniball
Multi Ion Reflection Apparatus for Collinear Spectroscopy
Scattering Chamber Experiments
Versatile Ion Polarisation Technique Online
Weak Interaction Studies with Radioactive Ion-Beams
Medical Isotopes Collected from ISOLDE
Solid State Physics Laboratory
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.[5]
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.[6] 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.[7]
Solid-state physics laboratory[edit]
Attached to ISOLDE in building 508, is CERN's solid-state physics laboratory.[99] 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.[100] The laboratory uses the technique of Time Differential Perturbed Angular Correlation (TDPAC) to probe the large quantity of available radioactive elements provided by ISOLDE.[101] 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.[102] Additional methods used for SSP are tracer diffusion, online-Mössbauer spectroscopy (57Mn) and photoluminescence with radioactive nuclei.[103]
Below is the list of some physics activities done at ISOLDE facility.[114][115]
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.
The ISOLTRAP experimental setup Is able to make high precision measurements of nuclear masses by using a series of Penning traps.[116] The experiment has been able to measure isotopes with very short half-lives (<100 ms) with a precision of below 10−8.[117][118] 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.[119][120][121]
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 1971 showed that if single neutrons are added to or removed from the nuclei of mercury isotopes, the shape will change to a "rugby ball".[122] Newer studies, from RILIS, show that this shape staggering also occurs with bismuth isotopes.[123][124]
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.[125][126] 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.[127][128]
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.[129][130]
The first observation of beta-delayed two-neutron emission was made at ISOLDE in 1979, using the isotope lithium-11.[131] Beta-delayed emission occurs for isotopes further away from the line of stability, and involves particle emission after beta decay.[132] Newer studies have been proposed to investigate beta-delayed multi-particle emission of lithium-11 using the IDS.[133]
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).[134] Accelerated RIBs from REX-ISOLDE are used in transfer reactions which allow for studies of nuclear resonance systems beyond the dripline.[135]
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.[136] This proof of the halo structure was made at ISOLDE from a series of experiments analysing the lithium-11 nucleus.[137]
Research conducting using the Miniball experimental setup found evidence of pear-shaped heavy nuclei, in particular radon-220 and radium-224.[88] 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.[138][139] In 2020, due to the HIE-ISOLDE upgrade, radium-222 was also found to have a "stable pear shape".[140][141] 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.[142]