Targets and ion sources

At its core, a rare isotope production target is a material, such as uranium carbide, that when irradiated undergoes nuclear reactions that produce rare isotopes.

TRIUMF’s two target drivers, accelerated protons and electrons, can each be used with a variety of target materials to induce different kinds of nuclear reactions and produce different types and abundances of rare isotopes. The target and ion source form a combined unit in which rare isotopes are stripped of an electron so that the resulting positively charged ions can be electrically and magnetically steered and purified as a rare isotope beam (RIB).

ISAC has two target stations fed with protons from the 520 MeV cyclotron, with one target operational while the other is being prepared. In ARIEL, there will be two Isotope Separation On Line (ISOL) targets: a proton target and an electron target fed by the e-linac. There will also be a more passive symbiotic target behind the proton target, using protons passing through the ISOL target to create rare isotopes for medical research.

TRIUMF’s Target and Ion Source Operation Team specializes in target manufacturing, operation, maintenance, handling and research and development of new and improved targets for the production of more exotic, more intense, and purer RIBs.

The ISOL targets and ion sources are part of nested series of structures that contain, support and control the production of rare isotope beams. Beginning with the largest, these are the target halls, target stations, target containers and ions sources and the target materials themselves.

Target Halls

The ISAC and ARIEL targets 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 weight mostly from steel radiation shielding. 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. Only about 20% of the proton beam’s energy is used in the target, the remainder becoming waste heat in the beam dump, which is cooled by recirculating water. (The beam dump might seem inefficient but serves a critical purpose: the 500 MeV protons are required to maximize rare isotope production but stopping more protons would only produce more heat in the target, not more rare isotopes.) 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. Rather than ISOL, the symbiotic target can be transferred through to an ARIEL hot cell in mere seconds, where the rare isotopes within are extracted and processed.

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. The target container is surrounded by a copper heat shield which is cooled by a recirculating water system that can extract 50kw of energy (which is comparable to a compact car engine – all from a volume as small as a cigar).

While proton target tubes are parallel to the beam, the e-linac target tube is upright. This is because in the proton targets most of the protons pass right through the target into the beam dump. However, with the ARIEL electron target, the high energy photons produced by the impinging of 30 MeV electrons in the photo-converter) stop within a few millimeters of the target’s surface, depositing all of their energy. Thus, to optimize the use of the incident photos, the target container is upright, providing a larger surface area perpendicular to the driver beam, and the electron beam is scanned across the converter surface, distributing the beam’s energy.

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.

Targets

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.

The target material’s diffusion properties are a critical characteristic since most of the rare isotopes required by TRIUMF researchers have half-lives of less than a few seconds. After a nuclear reaction producing a rare isotope, the new element stops within several micrometers, with the rest of its motion out of the target material driven by thermal diffusion. For example, uranium carbide is an effective target in part because it has good diffusion characteristics, enabling the rare isotopes to escape the target, and it can also be heated to very high temperatures without disintegrating, thereby speeding rare isotope diffusion.

In some cases, only one-in-a-million of the rare isotopes produced diffuse out of the target, so improving diffusion rates is an ongoing part of TRIUMF target research and development. For example, target scientists are creating targets composed of nanometre-sized fibres whose microscopic structure facilitates the rapid diffusion of rare isotopes.

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.

Types of nuclear reactions

TRIUMF’s high power and electron targets involve different types of nuclear reactions, providing the ability to customize targets for the production of specific types and yields of rare isotopes.

Nuclear reactions in the proton targets can involve spallation or a combination of spallation and fission reactions. In spallation, the impact of a high-energy proton explodes a target nucleus into smaller nuclei, emitting a number of protons and neutrons. Nuclear fission is a similar process except that it involves a fissile heavy nucleus (such as uranium) that, when it absorbs a high-energy proton, quickly splits into two nuclei while also releasing neutrons and gamma rays.

With the ARIEL e-linac, high-energy electrons have a different mechanism to induce nuclear reactions, photo-reactions.

At the front of the ARIEL electron target container is a V-shaped photo converter, a material that rapidly decelerate high-energy electrons, a process that generates high-energy gamma rays. It’s these gamma rays which drive photo nuclear reactions in the target material. In these reactions, gamma rays enter a nucleus energizing it to the point that it spits-out one or several two neutrons or protons (either a gamma-p reaction, or a gamma-n reaction), or fissions the nucleus (photo fission).

Photo fission of uranium results in neutron-rich rare isotopes with relatively low isobaric (same atomic weight) contamination in comparison with proton-induced spallation. These neutron-rich rare isotopes are critical for astrophysics research focused on the r-process, the step-wise element formation reaction process that occurs in supernovae and merging neutron-stars in which short-lived, neutron-rich isotopes are critical steps. Using photonuclear reactions on beryllium-9 (⁹Be), the electron target will also be used for the production of intense beams of lithium-8 (⁸Li) for use in TRIUMF’s CMMS beta-NMR program.

The e-linac delivers 30 MeV electrons because this energy level results in an optimized ratio for photo fission, which in turn optimizes the production of neutron-rich rare isotopes and power deposited in the target by electrons. The ARIEL photo converter will operate at the highest power (100kW) of any ISOL facility in order to generate high rates of rare isotopes. The resulting higher beam intensity will turn what might otherwise be an impossible or months-long experiment into one that can be accomplished in days.

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.