SA-CERN – ISOLDE
ISOLDE, CERN, Switzerland
ISOLDE is the CERN radioactive beam facility. The high energy, 1.4 GeV, and high average intensity of 2 µA of the proton beam on different thick targets produces high quality intense beams of rare isotopes. The accumulated target and ion source knowledge allow the extraction and separation of more than 1000 different isotopes of 75 chemical elements; this is by far the highest number of isotopes available for users at any ISOL facility worldwide. A substantial fraction of the radioactive isotopes was accelerated up to 3 MeV/u with the REX-ISOLDE post-accelerator until 2012 and can be re-accelerated presently up to 5.5 MeV/u with a combination of the normal conducting REX-ISOLDE and the superconducting linac post-accelerator. In order to broaden the scientific opportunities of the facility, the on-going HIE-ISOLDE (High Intensity and Energy) project was approved by CERN in September 2009 to provide major improvements in energy range, beam intensity and beam quality. The first stage boosting the beam energy of the post-accelerator to 5.5 MeV/u is fully operative delivering the first radioactive post-accelerated beams in September 2016. In the new energy regime, the Coulomb excitation cross sections are strongly increased with respect to the previous 3 MeV/u and many transfer reaction channels become accessible. The second stage allows energies of the beam up to 10 MeV/u for A/q = 4. ISOLDE offers thus the largest variety of post-accelerated radioactive beams near and above the Coulomb barrier in the world. The intensity and beam quality upgrade address many aspects that are implemented on a wider time scale.
Nuclear Physics Experiment
Present experiments mainly deal with nuclear structure questions, explored via measurements of ground state properties (mass, radii, electromagnetic properties), via decay studies or Coulomb excitation and transfer reaction studies. A sizeable part of the programme is devoted to other fields, such as nuclear astrophysics, and fundamental physics. Around 20% of the beam time is devoted to solid state physics and life sciences with broad societal benefits. An average of 50 different physics experiments are performed per year. The facility includes two target-ion source units connected to a mass separator each delivering beams of radioactive nuclei to a dozen beamlines. The beamlines at the low energy part (beams of 30 – 60 keV) host permanent devices dedicated to ground state studies.
- Mass measurements are carried out at ISOLTRAP: a precision Penningtrap-based mass spectrometer. It also includes a multi-reflection time of flight (MR-TOF) spectrometer, used for mass measurements of very exotic beams and characterising the ISOLDE beam prior to measurements.
- Relative charge radii and electromagnetic properties are determined using CRIS/ COLLAPS: laser spectroscopy experiments which probe the hyperfine properties of radioactive ions. CRIS is more sensitive while COLLAPS offers higher precision.
- Decay studies are served by IDS, the ISOLDE decay station, a polyvalent setup which can be used for all types of beta decay studies. Interchangeable setups allow for charged particle, beta and gamma spectroscopy. A recently added neutron detector array also allows neutron time of flight measurements to be performed. In addition, total absorption spectroscopy can be carried out at a dedicated setup (TAS).
- Nuclear orientation measurements can be performed at the NICOLE setup, a dilution refrigerator.
- Weak interaction studies to probe the existence of scalar currents, was first done with WITCH. Its upgrade, WISARD, will allow for the study of beta-delayed protons emitted in the decay of 32Ar with the aim to reach a precision limit of 0.1% for the correlation coefficient “a” between positron and neutrino.
For specific projects contact:
Dr Rob Bark – iThemba LABS (email@example.com)
Dr Pete Jones – iThemba LABS (firstname.lastname@example.org)
Professor Sifiso Senzo Ntshangase (UNIZULU) NtshangaseS@unizulu.ac.za
Professor Nico Orce – UWC ( email@example.com)
Dr Christine Steenkamp – SU (firstname.lastname@example.org)
Professor Mathis Wiedeking – iThemba LABS (email@example.com)
Materials Science to Biophysics
- For applications, which range from materials science to biophysics there are numerous setups available:
- A dedicated beamline for applications (VITO) allows beta-NMR to be applied to biophysics, where liquid samples can be studied online.
- ASPIC houses an ultra-high vacuum chamber which allows for surface science experiments online. – EC-SLI is an online setup for the measurement of emission channelling using short-lived isotopes in single crystal materials.
- Two beamlines are free for “travelling” systems.
- In addition, two other beamlines are used for collections dedicated to studies of material science, biophysics or medicine.
Mössbauer spectroscopy is an ideal tool for measuring local variations of charge density and symmetry on a local atomic scale around the Mössbauer active probe through the hyperfine interaction parameters. The Mössbauer collaboration at ISOLDE/CERN applies implantation of short-lived parent isotopes (57Mn→57Fe, T½ = 1.45 min. and 119In→119Sn, T½ = 2.4 min.) for emission Mössbauer spectroscopy (eMS) within material sciences. eMS allows for measurements in the extremely dilute regime (~10-4 at. %), to make use of the chemical nature of the (implanted) parent atom and to make use of recoil to create/study interstitial defects. Radioactive 57Mn+/119In+ beams are produced at ISOLDE/CERN, following 1.4 GeV proton-induced nuclear fission in a uranium carbide (UC2) target and selective ionization by a multi-stage laser irradiation system. Singly charged ions are then accelerated to energies of 50-60 keV before they are mass separated to yield pure 57Mn+/119In+ ion beams with intensities 108 ions/s. The broad categories of materials investigated include semiconductors, magnetic semiconductors, Heusler Alloys, Chalcogenides, Soft and Hard metals by performing temperature dependent, angle dependent, time-delayed, magnetic and quenching measurements. The resulting Mössbauer spectra give wealth of information on the environment of the Mössbauer isotope, such as on the charge state of the Mössbauer probe atom, site symmetry through the electric quadrupole interaction, magnetic interactions and binding properties of the probe atom through the Debye-Waller factor. In addition, the eMS group also participates in Emission Channelling and Perturbed Angular Correlation experiments at ISOLDE.
A new building has been built in the northern part of the experimental hall hosting enlarged laser laboratories, a new dedicated laboratory for material science studies, and a new chemistry laboratory also used by biochemistry groups and in general open to the whole user community. The post-accelerated beams have presently two operative beamlines. The first one hosts the high resolution Miniball germanium detector array and the alternative ancillary detectors CD, T-REX and the new electron spectrometer SPEDE. A zero-degree spectrometer is also considered to ease the characterisation of the ejectiles. The second beamline is dedicated to reaction studies with the main focus on detection of the fragments. With the present successful programme and ongoing projects, the number of users at ISOLDE is continuously increasing currently being of the order of 500 users per year. The member states are 16, among them South Africa.