Isotope Production Methods
There are numerous strategies for producing isotopes for research and applications.
Production with Reactors
Radioisotopes can be produced in reactors by exposing suitable target materials to the intense reactor neutron flux for an appropriate time. In light-water moderated, swimming pool-type reactors, the compact core is accessible from the top of the pool. Target materials to be irradiated are sealed in capsules, loaded in simple assemblies and lowered into predetermined core locations for irradiation. Afterwards, the irradiated targets are loaded in appropriate shielding containers and transported to hot chemistry labs for processing. In uranium, heavy-water moderated, tank-type reactors, sophisticated assemblies containing numerous target capsules are used for target irridiations. For both approaches, the quality and specific activity of the radioisotopes produced depends on both the target and the irradiation conditions.
A wide range of isotopes are made at reactors, from as light as Carbon-14 to as heavy as Mercury-203, with irridiations lasting minutes to weeks. For example, Mo-99 — the parent to the widely used medical diagnostic radioisotope Tc-99m — is usually produced via neutron-induced fission of targets with U-235 using a 4 to 8 day irradiation time.
The High-Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory for DOE uses highly-enriched uranium fuel elements to generate a power of 85 MW and a maximum neutron flux of about 2.6x1015 neutrons/cm2s2. Irradiation cycles of 26 days are used to produce isotopes such as Se-75, Cf-252, W-188/Re-188, and Ni-63.
Production via Accelerators
Accelerators are used to bombard production targets with beams of charged nuclei impinge on targets to produce a wide range range of isotopes, including many proton-rich nuclei (F-18, C-11) that are not available at reactors. Beams of protons and deuterons are primarily used, but alpha particles and heavier ion beams can also in principle be used. Possible alternatives involve bombarding a primary target to produce neutrons or photons, which them impact the production target to form the isotopes of interest. The range of particle energies and intensities vary between facilities — 10 – 100 MeV for commercial cyclotrons dedicated for isotope production, with higher energies available at some research accelerators. For example, the Brookhaven Linac Isotope Producer (BLIP) at Brookhaven National Laboratory uses a 200 MeV, 150 microAmp proton beam from the Alternating Gradient Synchrotron to bombard samples for the production of Ge-68/Ga-68, Sr-82/Rb-82, as well as Zn-65, Mg-28, Fe-52, Rb-83. Another is the Isotope Production Facility (IPF) at Los Alamos National Lab that uses the 100 MeV, 250 microAmp proton beam from the LANSCE linac to produce Ge-68/Ga-68 and Sr-82/Rb-82, as well as smaller amounts of Al-26 and Si-32.
Production via Chemical Separation
Even though Isotopes have nearly identical chemical behavior, chemical methods have been used for over 60 years to provide significant quantities of separated stable isotopes. Some of the earliest examples include the separation of Uranium isotopes by gaseous diffusion, chemical exchange processes to produce C-13 and N-15, and thermal diffusion and distillation to produce O-18, S-34, S-36, and some isotopes of the rare gases. Major separation techniques include: those that directly exploit the atomic mass of the isotopes; those that exploit slight differences in chemical reaction rates due to different atomic masses; and those based on the [often significantly different] atomic properties of different isotopes.
Distillation is a popular approach based on mass differences. It is effective for separating isotopes with large relative mass differences between isobars — and therefore is only practical for the light elements such as He, Li, B and C. Gaseous diffusion using a centrifuge is a cost effective means to separate isotopes based on mass differences that are too heavy for distillation. However, it is necessary to have a suitable gaseous compound of the element for this approach, limiting the possible isotopes. Isotopes such as Fe, Ni, Zn, Cd, Ge, Se, Te, W, and U are made via gaseous diffusion. Lasers tuned to certain energies can be used o raise an isotope of interest to an excited atomic state — and not effect other isobars because of their quantum properties. The excitation is followed by a variety of mechanisms to sweep away the other, non-excited isotopes.
To produce commercial quantities of separated isotopes, it is often the case that multiple separation stages are required where the output of one stage feeds the input of a subsequent stage.
Electromagnetic Enrichment and Purification
Electromagnetic separation exploits the mass difference of isotopes to change their deflection in a magnetic field. This low-throughput technique is quite costly, but can yield some of the highest purities of separated samples. It is often used in conjunction with other approaches — such as to increase the purity of samples obtained from gaseous diffusion. Devices called calutrons were historically used for electromagnetic purification. This approach can work for almost all elements, and is typically used for isotopes of Tl, Pd, Sr, Ca and the Lanthanide group.
Please check back here for links to more information on the electromagnetic separation of isotopes.