Targets and Ion Sources

The ISOL targets and ion sources are part of nested series of structures that contain, support and control the production of rare isotope beams.  

The target material is chosen to optimize the production of a particular rare isotope requested by TRIUMF users, and to efficiently release them. 

TRIUMF uses about 10 different target materials, all of which are made at TRIUMF in target material laboratories. The three most frequently used proton-beam target materials are uranium carbide, silicon carbide, and tantalum; others include tantalum carbide, tantalum, nickel oxide, niobium, zirconium carbide and titanium carbide. TRIUMF target scientists are researching the best target materials for the electron target station using photo fission, including beryllium oxide for instance. 

Target materials are chosen based on three key characteristics: the types and abundances of rare isotopes produced; and the rate at which they diffuse out of the target; and the beam power they can withstand without melting or decomposing. 

At ISAC, each target operates for two to five weeks, at which point the target module is removed using an overhead crane and remotely moved to a target hall hot cell, or radioactive handling area. Here, TRIUMF staff use telemanipulator arms to remove the target ion source assembly from the module and install a new target container before re-inserting the target module. With the advent of the ARIEL operation, TRIUMF will move to a 3-week production cycle for each target, introducing a heartbeat-like operating module in which one of the three ISOL targets is exchanged each week. 

The Target Container 

The heart of the target system is a ten-to-30-kilogram, shoebox-sized target assembly that holds the target material in a cigar-shaped tantalum tube, 19 cm long and 2 cm in diameter. The tube is also known as the target oven because it’s electrically heated to more than 2000 ºC inside of a high-vacuum environment. 

The tube is oriented parallel to the beam so that protons traverse its entire length and induce nuclear reactions resulting in the creation of short-lived rare isotopes which, because of the high temperature in the target oven, diffuse out of the target to the attached ion source. 

Building on the design of the ISAC target assembly, the ARIEL target will be contained in a new hermetically sealed (air-tight) vacuum container. This aluminium, briefcase-sized container will enable the use of modern target and ion source technology and a much faster and easier ‘plug-and-play’-style exchange of spent targets. Instead of remotely moving the 10-ton ISAC target module to a hot cell for the target exchange, a process that takes two weeks, the hermetically sealed ARIEL target containers will be directly exchanged, reducing the target downtime to just a few days, significantly increasing the amount of RIB time for TRIUMF experiments. 

Target Halls 

The ISAC and ARIEL targets and target stations are contained within warehouse-sized target halls designed for the remote handling of the targets, including overhead cranes and hot cells, and all of the related equipment for target and infrastructure servicing, exchanging and short-term storage of the irradiated target-ion source units.   

The Target Stations 

The target stations are the heavily radiation-shielded areas where the proton or electron beam hits the target material. In ISAC, the two target stations consist of five distinct modular components. Each module is a ten-ton rectangular box. The first module is for diagnostics, containing equipment to monitor the incoming proton beam’s intensity, position and shape. The second module contains the target ion source itself, followed by the beam dump module. The fourth and fifth modules are for the RIB diagnostics and the initial beam optics to characterize and steer the beam prior to the pre-separator, acting as the first step in ISAC’s high resolution mass selection system. 

ARIEL will have both a proton and electron target station, and a third “symbiotic” target for the production of rare isotopes for medical research. Situated behind the main ARIEL proton target, this symbiotic target is powered by protons that have passed through the proton target. The symbiotic target produces rare isotopes that are different than those generated in a small-cyclotron or nuclear reactor commonly used for medical isotope production and thus offers the potential to explore exotic radioisotopes for new medical applications.  

The ion source is a device used to rapidly remove an electron from a neutral isotope and thus produce positively charged ions that can be electromagnetically steered and accelerated. In some cases, the ion source is also used as an initial step in beam purification. 

Depending on the rare isotope, TRIUMF uses one of three kinds of ion sources: a surface ionization; laser ionization; or Force Electron Beam Induced Arc Discharge (FEBIAD) ionization. 

In each case, the ion source is directly attached to the target cylinder via a transfer line, a three-centimeter tube that transports rare isotopes diffusing out of the target. 

The simplest ion source is thermal ionization on the 2000 ºC hot surface of a small tube. Ionization is achieved by the combination of the high temperature and suitable chemical properties, which results in the transfer of an electron from the rare isotope to the hot surface. 

The second ion source is the TRIUMF Resonant Ionization Laser Ion Source (TRILIS). In laser ionization, a tunable, pulsed titanium-sapphire laser located about 30 m from the target generates laser beams of different colours, or wavelengths, which, via a series of mirrors and prisms, hit the rare isotopes in the ion source. The laser light is absorbed by a rare isotope’s outermost electron resonantly exciting it through multiple excitation steps and causing it to be ejected, ionizing the element. 

The advantage of laser ionization is that each element has electrons at different energy levels and thus that will resonantly absorb different wavelengths of laser light. As a result, the TRILIS laser energy is tuned to excite the electrons of only the rare isotope of interest. For example, the production of rare isotopes of neutron-rich magnesium for TRIUMF experiments also produces isotopes of sodium with the identical mass. So, target scientists tune the laser light to preferentially ionize the magnesium atoms over the sodium ones thus providing an initial mode of beam purification. 

The third ion source, the FEBIAD, is used for ionizing elements, particularly noble gases such as argon and xenon, that aren’t ionized via surface ionization or laser ionization. With FEBIAD ion sources, electrons are fired into a plasma chamber along with the rare isotopes, physically stripping electrons from the atoms and thereby ionizing them. The now positively charged rare isotopes are attracted to a series of electrodes that extract them from the plasma chamber to form the initial RIB. Notably, with noble gases, this system can be combined with a pre-purification step using a water-cooled transfer line from the target which causes contaminants to condense-out on the transfer line while the more volatile noble gases pass through.