The ISOLDE experiment at CERN

ISOLDE

ISOLDE is part of CERN.

ISOLDE, is CERN:s radioactive beam facility, and provides beams for experiments in nuclear physics, including applications in nuclear astrophysic and fundamental physics, atomic physics, solid-state physics, biophysics and medical physics. The user group consists of ca 500 university physicists with research activities at the facility, and is hosted in the experimental physics department of CERN.

The ISOLDE experiment is located at the PS booster, which is part of the CERN injector chain for the LHC (see Fig. 3). Approximately 50% of the proton pulses produced are dedicated to drive the ISOLDE radioactive beam targets. The protons for ISOLDE start in the LINAC2 (soon to be replaced by the new LINAC 4) and go into the four stacked rings of the PS booster after which they are directed to the ISOLDE targets. The proton beam energy is typically 1.3 GeV and the pulse repetition period is 1.2 s. The nominal intensity is 3E13 protons per pulse and average intensity on the target is typically 1.5 uA.

At ISOLDE the proton beam enters the target zone and bombards either of two target stations (see Fig. 4). A significant part of the work consists in manufacturing proper targets for the radioactive beam production using different target materials, in combination with different types of ion sources, to optimize the production and extraction yield of a certain beam species. In connection to this, work is also done to develop new laser ionization schemes for enhanced ionization efficiency of specific elements of interest for the physics program.

The first steps in the target development work are carried out in a separate offline laboratory that includes an offline separator. In connection to this there is also a mechanical workshop for the production of the target vacuum vessels and target material containers, both for tests and for production runs. The primary targets are operated in a separate vacuum to that of beam lines from the PS booster. The target containers suffer mechanical stress during operation, on the one hand from the impinging high-energy proton beam and on the other from the high operating temperatures (used to enhance diffusion in the target material). Depending on operating conditions a production target can be operated from a few days to a few weeks without significant decline in yield. A separate offline laboratory for the development of new laser ionization schemes exists as well.

In addition to the target zone the experiment involves the operation of two separators, the High-Resolution Separator (HRS) and the General-Purpose Separator (GPS) and their associated pulsed HV beam extraction systems. A radiofrequency beam cooler can be used in order to improve the emittance of the beams. The separated beam is directed via low energy beam lines, either to experiment stations that use beams up to 60 keV energy or, for experiments at higher energy, into the beam cooling stage for further acceleration in the post-accelerator.

The post-accelerator that has been developed at ISOLDE, starting with the REX-ISOLDE project and now latest with the HIE-ISOLDE project, makes the ISOLDE facility world unique. In a cross-fertilization with the mass measurement program, that uses Penning traps to capture low energy beams and measure masses of exotic isotopes, a beam cooling and charge breeding system consisting of a buffer-gas filled Penning trap operated in tandem with an Electron Beam Ion Source (EBIS) was developed. This technology has had significant input of know-how, funding and manpower for its development from Swedish physicists at ISOLDE. After cooling and charge breeding the beam is again mass separated on the fly before being brought into a pre-acceleration stage, consisting of a four-rod RFQ, before it is injected into the new superconducting high-energy stage, HIE-ISOLDE, that brings the beams up to 5 MeV/u and 10 MeV/u in two stages (i.e. for a typical heavy mass beam in the A=200 region, an energy of 1 - 2 GeV). The HIE-ISOLDE post-accelerator was inaugurated the fall 2016 and the first experiment to run in normal production mode was led from Sweden (Lund University).

The description above gives a general overview of the equipment that is operated as part of the infrastructure to produce exotic beams. In the MoU the following parts are given as the components of the infrastructure needed for radioactive beam production (the buildings are located at the CERN Meyrin site):

- The target workshop with target material research laboratory, the off-line separator and target conditioning systems, located in building 3,
- The class A radioactive laboratory with target conditioning units and hot cell in building 179,
- The ISOLDE proton beam line, intermediate irradiated target storage and remote handling area, and the target areas (GPS and HRS) in building 838.
- The two on-line ISOLDE front-ends (GPS and HRS). These front-ends accelerate 1+ ions to 60 keV. There is also one spare front-end.
- The two ISOLDE magnetic separators GPS and HRS,
- The beam distribution system including instrumentation,
- The RFQ cooler for the HRS beams in building 170,
- The experimental area in building 170,
- The bunching, charge breeding and the linear post accelerator system, made up of normal conducting elements (REX-ISOLDE) and new superconducting elements (HIE-ISOLDE), with its three experimental beam lines. The ISOLDE targets and ion sources including the Resonant Ionization Laser Ion Source (RILIS) system located in building 170 and the off-line RILIS.
- Test and prototyping space for the physics experiments and short-term storage for experimental equipment (building 275),
- The ISOLDE control room in building 508,
- The solid-state physics laboratory, the laser laboratories, the workshop and DAQ room in building 508.

The members of the collaboration run experiments using the radioactive beams. Two types of setups exist. On the one hand travelling setups that are at ISOLDE for a shorter period (weeks to months) before being moved to the home institute or potentially for experiments at other laboratories, and fixed setups that are dedicated to experiments at ISOLDE. Experiments using fixed setups follow the same procedure to obtain beam time as travelling setups, i.e. via proposal submission, endorsement by the INTC and approval from the CERN research board, on a case-by-case basis. Currently a number of fixed setups exist at ISOLDE run by members of the collaboration. In alphabetic order these are the following:COLLAPS: Collinear laser spectroscopy setup focusing on nuclear spins, magnetic moments, quadrupole moments and mean-square charge radii of ground states in exotic atomic nuclei. Uses spin polarized beams for beta-NMR studies.
CRIS: Collinear Resonance Ionization Spectroscopy setup, using the high efficiency and selectivity of resonant ionization to measure ground state nuclear properties of isotopes far from stability. CRIS will in the future be the site for studies of negative ions using the GANDALPH detector led by Swedish physicists.HiFi: HIE-ISOLDE Fragment Identifier, is a new project to install a zero-degree separator for experiments at HIE-ISOLDE.
IDS: The ISOLDE decay station used to measure properties of excited states in exotic atomic nuclei using charged-particle detectors, neutron detectors and gamma-ray detectors following beta-decay of isotopes far from stability, with focus on low-spin excited states.
ISOLTRAP: A Penning trap system for mass measurements of short-lived atomic nuclei of interest for nuclear models, fundamental symmetries and nuclear astrophysics.
ISS: The ISOLDE solenoidal spectrometer. A superconducting solenoid spectrometer that uses a novel detection method of charged particles from reactions with beams from HIE-ISOLDE. Swedish physicists are active with experiments at the ISS.
NICOLE: On-line nuclear orientation facility using a He-3/He-4 refrigerator providing temperatures down to or below 10 mK to produce nuclear orientation. The method is used to determine multipolarities/partial wave amplitudes of the decay.
MINIBALL: A segmented Ge-detector array for gamma-ray detection following reactions with radioactive beams from HIE-ISOLDE. Swedish physicists are members of the MINIBALL experiment.
REX-ISOLDE: The low-energy, cooling, charge breeding and first acceleration stage of HIE-ISOLDE. Originally built as an experiment with considerable Swedish involvement and funding. Now part of the CERN-ISOLDE infrastructure.
SEC: A multi-purpose scattering chamber for reactions, using radioactive ion beams from HIE-ISOLDE, involving charged-particle and neutron detection. The scattering chamber has been funded with Swedish grants.
Solid State Physics Collections: Collection points for radioactive probe nuclei used in analysis of material properties in solid-state physics. Includes a dedicated laboratory for offline work.
TAS: A total absorption spectrometer for high-efficiency measurements of the total energy spectrum, i.e. the strength function, following beta-decay.
VITO: An ECR funded project for ultra-high vacuum beta-NMR as well as differential pumping stages for beta-NMR using liquids. Supports a versatile set of experiments from solid state physics to biophysics.
WITCH: A retardation spectrometer used for fundamental physics studies of the weak interaction (currently under re-development).
The Swedish efforts are focused on experiments with HIE-ISOLDE using the MINIBALL array, SEC, ISS and CRIS. A new approach to neutron detection is planned for the high-energy beams at SEC and Swedish physicists are also involved in the effort to introduce a zero-degree magnetic separator for experiments at HIE-ISOLDE.

The infrastructure provides accelerators and associated equipment for physics experiments.

The infrastructure provides radioactive beams for experiments. This includes running and maintaining accelerator infrastructure and associated equipment and developing beams for new experiments.

Concerning all scientific and technical matters, and all resource and legal matters, the ISOLDE Collaboration is governed by the ISOLDE Collaboration Committee (ISCC). This committee comprises one representative of each signatory to the MoU between the funding agencies and CERN, with voting rights. The ISCC elects the Chairperson of the ISCC from among the members of the Collaboration for a period of office of 3 years. The ISOLDE physics leader, the Resource Coordinator, the ISOLDE Technical Coordinator, the ISOLDE Physics Coordinator, and the Chairperson of the INTC are invited to attend (without voting rights unless having been appointed by CERN management to represent the organization as member in the committee). On request of the Chairperson of the ISCC, other coordinators can be invited to attend (without voting rights).

The Spokesperson of the Collaboration represents the Collaboration to the outside, including to CERN as Host Laboratory, and coordinates its work. The ISCC elects the Spokesperson for a period of office of 3 years.
The Resource Coordinator oversees the resource planning and will typically deal with budget and manpower planning, MoUs and the Common Fund. He/she is appointed by the ISCC in consultation with the CERN Management.
The ISOLDE Technical Coordinator, appointed by the CERN Department responsible for ISOLDE operation (SY Department), coordinates the ISOLDE facility operation, maintenance and technical R&D. He/She is the liaison person (a) between the CERN staff in SY Department and those in EP Department, and (b) between SY Department and the rest of the ISOLDE Collaboration. He/she is also the contact person for such technical collaborations concerning the ISOLDE facility as may occasionally form outside the context of the ISOLDE Collaboration.

The ISOLDE Physics Coordinator deals with the details of beam-time scheduling and floor space allocation for the experiments. He/she acts as liaison between EP Department and the ISOLDE users.
The Group Leader in Matters of Safety (GLIMOS) is responsible to the CERN Management for all matters of safety concerning ISOLDE Collaboration personnel, work and equipment on the CERN premises. He/she is appointed by the Leader of EP Department, on the proposal of the Spokesperson.
In addition to its role as program advisory committee (see relevant section) that endorses experiments for decision by the CERN research board, the ISOLDE Neutron Time of Flight Committee (INTC) advises CERN Management on physics issues related to the upgrade of the ISOLDE facility. This to ensure that the committee's evaluation of physics cases in e.g. new letters of intent from users receive a scientific evaluation and that new opportunities for the physics program are brought to the knowledge of the Management.
Concerning the priorities for upgrade, CERN Management is advised by the Standing Group for the Upgrade of ISOLDE, the membership of which comprises the Chairperson of the INTC, the Chairperson of the ISCC, the immediate past Chairperson of the ISCC, the ISOLDE Technical Coordinator, the ISOLDE Physics Coordinator, the ISOLDE Physics Leader, and those responsible at CERN for the relevant activities. It is chaired by the ISOLDE Physics Leader.