Ion-source development at the off-line LERIB test-facility at iThemba LABS

The Low Energy Radioactive Ion Beam (LERIB) facility [1] will be used to produce low-energy radioactive-ion beams (RIBs) with energies up to 60 keV. Radioactive reaction products will be created by a 66 MeV proton-beam impinging on a target made of carbide disks, such as SiC [2]. These reaction products will then be ionized in a target-ion-source (TIS) and extracted as beam. The TIS design allows three ion-sources: a surface ion-source [3], a forced electron-beam induced arc-discharge (FEBIAD) ion-source [4], and a resonance-ionization laser ion-source, or RILIS. The surface-ionization source was commissioned with stable beams in October 2021. The production of ions from Group-1 elements was accomplished with beams of 39K+, 41K+ and 23Na+ where currents were measured in the μA range. This source may be advantageous for producing stable pilot-beams for future radioactive-beam experiments. The FEBIAD is still in development at present.


Introduction
The Low Energy Radioactive Ion Beam (LERIB) facility is presently undergoing development at iThemba LABS [1].This will be the first Isotope Separation On-Line (ISOL) facility used to create radioactive ion beams (RIBs) at iThemba LABS.The LERIB facility will be housed in a dedicated laboratory on site at iThemba LABS where it will be coupled to a proton beam-line from the K200 Separated Sector Cyclotron.
Radioactive-beams will be produced at the LERIB facility by impinging 66 MeV protons, at currents of a few micro-amps, onto a target-ion-source (TIS) containing carbide disks [2].The resulting nuclear reaction products will then diffuse through a hot transfer-line where some of them -namely the Group 1 and 2 elements, can be ionized by surface-ionization to the 1+ charge-state [3].Most reaction products will not be ionized in this way however; therefore a universal (but not very selective) method of ionization known as a Forced Electron-Beam Induced Arc-Discharge (FEBIAD) ion-source will also be incorporated into the TIS [4].Radioactive ions produced in this way can then be accelerated as RIBs with energies of up to 60 keV for experimental studies.Figure 1 shows a detailed floor-plan envisioned for phase-zero of the LERIB facility, or LERIB-0.LERIB-0 is part of the larger frame-work of ongoing development for new accelerator-science facilities at iThemba LABS, which also includes the South Africa Isotope Facility (SAIF) [5].
LERIB-0 is to be housed in a re-purposed experimental vault.A 66 MeV proton-beam of a few micro-amps -produced by the K200 Separated Sector Cyclotron (SSC) -will enter the Front-End Vault through the Southern wall (left of Figure 1).The proton-beam will impinge on the carbide target inside the Target-Chamber where it will induce nuclear reactions, and thus create reaction-products which should include some rare radioactive-isotopes.A large percentage of these reaction products will be ionized by the TIS, followed by extraction as a combined beam with up to 60 keV beam energies.
A double-focusing analyzing-magnet, with 1:3000 mass resolution, can then be used to separate out isotopes of interest from the beam of reaction-products, thus selecting a RIB of interest, which in turn will be studied at the experimental station (top right of Figure 1).A tape-station is being developed for this purpose at present.Target chambers will be positioned and removed (once exhausted) by means of an overhead crane.Spent chambers will be moved via this gantry-system for storage and decommissioning to a designated waste area.
Consulting engineers have already provided detailed civil engineering plans for the facility.These include important safety features such as shielding to ensure that outside neutron levels are kept at a minimum -FLUKA simulations were done in-house for UCx targets in order to verify the safety of this design in a worst-case-scenario.HVAC considerations such as a negative pressure differential will ensure that no radioactive air will leak out of the vault.Operation of LERIB-0 will take place in an adjacent control-room (right of Figure 1) that is separated by over 4 m of shielded concrete walls.To ensure that the Front-End will be ready for use in the LERIB-0 facility, certain objectives must be achieved: • Completion of remote-control operation of all sub-systems: Systems including vacuum, heating, cooling, pneumatic-motors, power-supplies, beam-optics, beam-diagnostics, etc. must all be accessible from a control-room that is a safe distance away from the experiment.
No personnel can be in proximity of the Front-End once production of RIBs begins due to the dangerously high radiation levels expected.The full system must therefore be completely remotely-controlled and fail-proof, as health and safety are of primary concern.• Installation and commissioning of both surface-ionization and FEBIAD ion-sources: These two ion-sources must also be operational via remote-control, and together should be able to produce a wide variety of stable beams.
The calibration of all ion-optical elements, the development and testing of all diagnostic tools, and test-runs of the entire LERIB Front-End system -should firstly be performed with stable beams produced by the two aforementioned ion-sources.• Commissioning of a Micro-Channel-Plate (MCP): It is essential that rare isotopes will be detectable at LERIB-0.Such isotopes will either have extremely low abundances, or very limited efficiencies and expected yields.Therefore a very sensitive MCP, which is capable of detecting beam currents of less than 10 7 particles/s, is being commissioned at present.

First stable beams using surface-ionization
A surface ion-source was developed and installed in the LERIB Front-End at the test facility in October 2021.Shortly afterwards, beams of 39 K + and 41 K + were produced.
A double-focusing analyzing magnet was used to separate these two isotopes, and their measured abundance agreed with the expected values within experimental uncertainty.
There was also an observation of a very small peak in the mass-spectrum between the two stable isotopes.This corresponds to the radioisotope 40 K + , which has an expected abundance of 0.0117%.However, the signal-to-noise ratio on the faraday-cup used to measure beam current was too large to adequately resolve this peak, which was only a few pA.This may however be the first "radioactive-ion" beam produced at LERIB, and will therefore be investigated in the near future using the MCP.
The 39 K + beam was used to calibrate the beam-optics and diagnostic tools installed on the beam-line: transverse beam emittance could be measured for the surface-ionization source using an electrostatic-quadrupole-triplet lens and a multi-wire harp profilometer: for a beam energy of 30 keV, normalized transverse emittance values of ε x = 1.3 ± 0.4 and ε y = 2.1 ± 0.4 mm.mrad.√ V were obtained.Later, a sodium sample was introduced into the surface-ionization source and beams of 23 Na + were also produced.

Outlook
The design of the FEBIAD is complete.Assembly will commence upon the arrival of materials which were ordered in early 2022.Once the FEBIAD construction is complete, initial tests will be performed with the goal of producing stable beams of the residual gasses: H, C, N, O and other molecular ions from air.
Modifications will be made inside the target-chamber in order to install and operate this source.A magnetic solenoid may also later be incorporated, as this should improve the ionization efficiency due to increased electron-beam focusing [6].
The Front-End is then expected to be completely prepared for installation at LERIB-0 provided that: 1) stable beams have been commissioned with both surface-ionization and FEBIAD ion-sources, and; 2) all sub-systems are safely, remotely, and flawlessly operable.

3 .Figure 2 .
Figure 2. The LERIB Front-End at the off-line test facility in S-Block, iThemba LABS