Charge Breeding of Radioactive Ions

Charge breeding is a technique to increase the charge state of ions, in many cases radioactive ions. The singly charged radioactive ions, produced in an isotope separator on-line facility, and extracted with a low kinetic energy of some tens of keV, are injected into a charge breeder, where the charge state is increased to Q. The transformed ions are either directed towards a dedicated experiment requiring highly charged ions, or post-accelerated to higher beam energies. In this paper the physics processes involved in the production of highly charged ions will be introduced, and the injection and extraction beam parameters of the charge breeder defined. A description of the three main charge-breeding methods is given, namely: electron stripping in gas jet or foil; external ion injection into an electron-beam ion source/trap (EBIS/T); and external ion injection into an electron cyclotron resonance ion source (ECRIS). In addition, some preparatory devices for charge breeding and practical beam delivery aspects will be covered.Comment: 30 pages, contribution to the CAS-CERN Accelerator School: Ion Sources, Senec, Slovakia, 29 May - 8 June 2012, edited by R. Bailey, CERN-2013-00

Experiments on the Pulsed Afterglow Operation of an ECR Ion Source

Various experiments have been performed on the 14.5 GHz ECR4 in order to improve the beam yield. The source operates in pulsed "afterglow" mode, and provides currents >120 emA of Pb27+ to the Heavy Ion Facility on an operational basis. In the search for higher beam intensities, the effects of a pulsed biased disk on axis at the injection side were investigated with different pulse timing and voltage settings. Different plasma electrode geometries were also tested, including running the source without a plasma electrode. The use of CF4 as mixing gas was investigated, and high secondary electron emission materials, such as LaB6 and Al2O3, were inserted inside the plasma chamber in an attempt to increase the cold electron density. No proof for higher intensities was seen for any of the tested modifications. On the contrary, several of the modifications resulted in lower source performance, and less stability. Although the source has previously proved to have very stable modes of operation, during the last physics run, after the above tests, the stability decreased and the source settings were very different from the normal operation values

Transverse emittance investigation of the ISOLDE target ion sources

In order to produce target-ion sources allowing for a high isotopic resolution in the separator, CERN/ISOLDE (Isotope Separator On Line) has purchased a commercial emittance metre, capable of measuring transverse phase-space emittances for ion-beam intensities down to approximately 1 nA. It was installed at the ISOLDE off-line separator where targets are tested with respect to material purity and the ion-source efficiencies are determined. Now, also the emittance and brightness are measured for different stable elements. An extensive programme has been launched aiming at a complete survey of the emittance dependence on the various ion-source parameters. Results from the measurements on the different ISOLDE ion-source types, with associated analysis, are presented

Studies on ECR4 for the CERN ion programme

The CERN heavy ion community, and some other high energy physics experiments, are starting to demand other ions, both heavy and light, in addition to the traditional lead ions. Studies of the behaviour of the afterglow for different operation modes of the ECR4 at CERN have been continued to try to understand the differences between pulsed afterglow and continuous operation, and their effect on ion yield and beam reproducibility. The progress in adapting the source and ion beam characteristics to meet the new demands will be presented, as will new information on voltage holding problems in the extraction

REXEBIS the Electron Beam Ion Source for the REX-ISOLDE project

The REXEBIS is an Electron Beam Ion Source (EBIS) developed especially to trap and further ionise the sometimes rare and short-lived isotopes that are produced in the ISOLDE separator for the Radioactive beam EXperiment at ISOLDE (REX-ISOLDE). By promoting the single-charged ions to a high charge-state the ions are more efficiently accelerated in the following linear accelerator. The EBIS uses an electron gun capable of producing a 0.5 A electron beam. The electron gun is immersed in a magnetic field of 0.2 T, and the electron beam is compressed to a current density of >200 A/cm2 inside a 2 T superconducting solenoid. The EBIS is situated on a high voltage (HV) platform with an initial electric potential of 60 kV allowing cooled and bunched 60 keV ions extracted from a Penning trap to be captured. After a period of confinement in the electron beam (<20 ms), the single-charged ions have been ionised to a charge-to-mass ratio of approximately ¼. During this confinement period, the platform potential is decreased to about 20 kV, and an axial potential barrier is lowered to allow the now highly charged ions to be extracted from the EBIS at an energy matching the requirement of the Radio Frequency Quadrupole (RFQ). Several different topics are presented in this report, all connected with the design and construction of an EBIS. Old 'truths' have also been scrutinised, for instance alignment tolerances. A large part is devoted to the description of a novel EBIS simulation implementation. A complete injection, breeding and extraction cycle has been simulated to certify high injection and extraction efficiencies. The entire EBIS was modelled in an ion-tracing program called SIMION, and the accepted and emitted phase spaces were determined. Beam optics parameters such as lens positions, voltages, accepted beam-tilt and displacement tolerances at the focal points were also settled using SIMION. An analytically derived acceptance formula was verified with simulations, and general conclusions on acceptance, emittance and energy spread of an EBIS are presented in this report. Any possible correlation between the two transverse phase spaces was shown to be insignificant. Furthermore, continuous injection, and maximal obtainable efficiency for such an injection mode were studied theoretically. The electron reflection and back-scattering in the collector was simulated using a combination of EGUN and SIMION. The result showed that a much lower degree of electron back-scattering may be obtained with this design as compared to previously published estimations. Furthermore, the Penning trapping of electrons at the trap barrier (or the post anode) was addressed, and techniques to avoid it were evaluated. Vacuum considerations for residual gas in the warm-bore magnet chamber, and the back-flow of Ar cooling gas from the Penning trap, have also been addressed since there is a risk of outnumbering the small number of radioactive ions. Simulated extraction spectra for different pressure scenarios are presented. All different REXEBIS elements (magnet, electron gun, inner structure, collector etc) are described from a design and performance perspective, and preliminary investigations of the platform high voltage switching and the beam diagnostics are included as well. A very elegant and simple method to align the solenoid within the iron yoke was developed and used. The high experimental emittance value obtained for electron beam ion source at MSL in Stockholm (4 times larger than the absolute upper theoretical value) was reproduced in simulations and could be justified by aberrations in the small einzel lens following the collector. The result of this simulation also verified the validity of the developed EBIS code

Effect of a Biased Probe on the Afterglow Operation of an ECR4 Ion Source

Various experiments have been performed on a 14.5 GHz ECR4 in order to improve the ion yield. The source runs in pulsed afterglow mode, and provides currents ~120 emA of Pb27+ to the CERN Heavy Ion Facility on an operational basis. In the search for higher beam intensities, the effects of a pulsed biased disk on axis at the injection side were investigated with different pulse timing and voltage settings. No proof for absolute higher intensities was seen for any of these modifications. However, the yield from a poorly tuned/low-performing source could be improved and the extracted pulse was less noisy with bias voltage applied. The fast response on the bias implies that increases/decreases are not due to ionisation processes. A good tune for high yield of high charge states during the afterglow coincides with a high plasma potential

First detection and energy measurement of recoil ions following beta decay in a Penning trap with the WITCH experiment

The WITCH experiment (Weak Interaction Trap for CHarged particles) will search for exotic interactions by investigating the beta-neutrino angular correlation via the measurement of the recoil energy spectrum after beta decay. As a first step the recoil ions from the beta-minus decay of 124In stored in a Penning trap have been detected. The evidence for the detection of recoil ions is shown and the properties of the ion cloud that forms the radioactive source for the experiment in the Penning trap are presented.Comment: 9 pages, 6 figures (9 figure files), submitted to European Physical Journal

Preparation of cooled and bunched ion beams at ISOLDE-CERN

Abstract.: At ISOLDE a new RadioFrequency Quadrupole ion Cooler and Buncher (RFQCB) is being constructed to improve ion optical properties of low-energy RIBs. The new features of the mechanical design and the status of the test bench, which will serve to test the device, will be presented in this contributio

Calibration of a solid state nuclear track detector (SSNTD) with high detection threshold to search for rare events in cosmic rays

We have investigated a commercially available polymer for its suitability as a solid state nuclear track detector (SSNTD). We identified that polymer to be polyethylene terephthalate (PET) and found that it has a higher detection threshold compared to many other widely used SSNTDs which makes this detector particularly suitable for rare event search in cosmic rays as it eliminates the dominant low Z background. Systematic studies were carried out to determine its charge response which is essential before any new material can be used as an SSNTD. In this paper we describe the charge response of PET to 129Xe, 78Kr and 49Ti ions from the REX-ISOLDE facility at CERN, present the calibration curve for PET and characterize it as a nuclear track detector