Rare Isotope Beam Delivery

From their origins in targets and ion sources, TRIUMF’s rare isotope beams (RIBs) travel on to navigate two important technologies: mass separators and charge breeders.

The singly-ionized rare isotopes enter a High-Resolution Mass Separator (HRS) in order to create a purified, high-intensity RIB of ideally just a single selected isotope.

From the mix of products emerging from a target, the ISAC and ARIEL mass separators use the same underlying technique to sift-out the desired rare isotope: magnets that send ions into a U- or 120-degree turn in which only the ion of a specific chosen mass exits the HRS to form a RIB.

When a charged particle enters a magnetic field the degree to which its direction of motion is bent by the field varies with its mass and energy. For nuclei with the same energy, the lighter the nucleus, the tighter the turn it makes in the magnetic field. Thus, for example, nitrogen with atomic mass 14 will make a tighter turn than oxygen with atomic mass 16.

ARIEL’s new HRS, developed in as part of the CANREB CFI project, will provide the world’s highest-resolution mass separation for an ISOL facility while maintaining a large transmission. It uses a pair of custom magnets with a measured magnetic field flatness accurate to better than five-parts-in-100,000; at any location the magnetic field only varies by this amount. This provides a mass separation resolution of 1:20,000; able to distinguish, and separate out isotopes with a mass difference of just 1:20,000.

Ions entering the ARIEL HRS will be bent by a first magnet to create an initial level of dispersion (spreading out based on a mass), followed by a second magnet doubling the dispersion, the ions now having made a 180-degree turn. The HRS is tuned so that isotopes of the desired mass form a beam about the thickness of a sheet of paper (0.1 mm), and exit through a 0.1 mm-wide selection slit while ions of all other masses are stopped.

Between the two dipole magnets, the ARIEL HRS also incorporates a unique, multipurpose electrostatic lens (a multipole) used to tune the mass separator and correct separation problems.

Leaving the mass separator, the purified RIB is directed to experiments through an elaborate beamline switchyard, the route depending on the experiment to which the RIB is sent.

A RIB is first transported in the Low-Energy Beam Transport (LEBT) electrostatic beam line and sent via a switchyard to either the low-energy experimental area in ISAC I, or to a series of room-temperature accelerating structures in the ISAC-I medium-energy experimental area.

There are three different possible beamline modes of operation:

1. If the RIB is required for a low-energy experiment, such as TITAN or GRIFFIN, the RIB of any ion mass is channeled through the LEBT in ISAC I.
2. If the RIB has an atomic mass of less than 30, it can be directly accelerated for ISAC I medium-energy experiments, such as DRAGON and TUDA, and also further accelerated in ISAC II to higher energies for the TIGRESS, EMMA and IRIS experiments.
3. Finally, if the RIB nuclei have an atomic mass higher than 30 and are required for medium or high-energy experiments, the RIB is sent through a charge breeder to kick-off more electrons, generating a higher charged state that enables its acceleration.

One of the most challenging beamline aspects is precisely turning a RIB. TRIUMF’s beam physics group has designed unique spherical electrodes (“spherical benders”) for bending the beam at 45- or 90-degrees, as is often required for delivery to experiments. This specialized electrostatic optics design conserves a RIB’s shape as the beam turns.

ARIEL LEBT includes a new 200 m long beamline to guide RIBs from ARIEL’s targets to the existing beamlines in the ISAC low energy area, from which they are directed to experiments in ISAC I and II. To reduce rare isotope loss, the ARIEL beamline will operate under harder vacuum conditions of about 10^-9 Torr, a hundred times lower pressure than in ISAC, delivering 95% beam to experiments, as compared to the 70% transported in ISAC beamlines.

The ions leaving the mass separator are primarily singly charged (+1). But for ions with an atomic mass greater than 30, for example zirconium (Zr) and barium (Ba), destined for TRIUMF’s medium- and high-energy experiments, the ions must be further ionized with a charge breeder to increase their charge to +2 or higher.

This is because the key characteristic that determines how a charged particle is accelerated in a radio frequency accelerator is its mass-to-charge ratio. For example, oxygen 16 (¹⁶O) stripped of four electrons, has a +4 charge and a mass-to-charge ratio of 16/4, or 4. Similarly, neon 20 (²⁰Ne), stripped of 5 electrons, has a +5 charge, and a mass-to-charge ratio of 20/5, or also 4. In the accelerator beam, both these isotopes will be accelerated in the same way because they have the same mass-to-charge ratio; to the accelerator, they are identical.

TRIUMF uses two types of charge breeders, each with distinct strengths, enabling flexibility and choice in the charge breeding used for different RIBs for particular experiments. In both types, electrons are stripped from the ions through impact ionization collisions with high-energy free electrons.

ISAC ECRIS

In ISAC, for RIBs of atomic mass 31 and higher that will be accelerated, an electrostatic bender, or beamline off-ramp, is added into the beamline directing the RIB to the charge breeder, the Electron Cyclotron Resonant Ion Source (ECRIS).

In ECRIS, microwaves are beamed into a central chamber holding a very dilute gas, usually oxygen or helium, embedded in a magnetic field. The microwaves energize the gas atoms’ electrons to the point that they escape and form a hot plasma, a mix of ions and high-energy electrons, which is confined by the strong magnetic field.

The RIB is sent through this plasma and electrons are stripped from rare isotopes through collisions with the high-energy electrons, producing rare isotopes with a mass-to-charge ratio below six. Then the highly-charged rare isotopes are extracted and sent through another electromagnetic mass separator which further purifies the RIB based on the desired mass-to-charge ratio.

ARIEL Electron Beam Ion Source (EBIS)

As researchers seek to experiment with shorter-lived and new rare isotopes often first produced at very low intensities, as low as a beam of just several isotopes-per-second, it’s necessary to ionize the isotopes very rapidly with ultra-low levels of contaminants.

ARIEL’s unique Electron Beam Ion Source (EBIS) achieves this using an ultra-high vacuum environment in which ions can be charge bred in less than 10-thousands-of-a-second.

In this technique, a tightly magnetically focused high energy electron beam is fired at the RIB (which is precisely contained in a small target volume) to super-charge the RIB ions via electron impact ionization.

In the ARIEL EBIS, an electron gun (similar to that used in the e-linac) is used to generate an electron beam which is magnetically focused by a powerful cryogenically cooled superconducting magnet. The electron beam interacts with the RIB ions in a tiny space, the high-energy electrons knocking-off electrons from the isotopes, producing ions with a mass-to-charge ratio below seven. These highly charged ions are extracted, and as in the ISAC ECRIS, sent through an electromagnetic mass separator which further purifies the RIB based on the desired mass-to-charge ratio. The control of the electron-beam energy enables operators to maximize the yield of the isotope charge state of interest and further suppress isobaric (same atomic weigth) contamination by the choice of the charge state.

ARIEL’s unique Electron Beam Ion Source was funded through the CANREB CFI project and designed through an international collaboration between TRIUMF researchers and those at the University of British Columbia, Max-Planck-Institut fur Kernphysik, and the Heidelberg Graduate School of Fundamental Physics. It leverages TRIUMF’s expertise in EBIS technology developed through the TITAN experiment, which is adapting the technology to make mass measurements for the first time with highly charged rare isotopes.