Answer to Question #7462 Submitted to "Ask the Experts"

My question is related to a cyclotron target. Can you tell me about the difference between targets or target materials—e.g., when should we use a gaseous target and when a liquid or solid target? Why are different physical states of targets required? For example, in preparation of ¹⁸F-FDG we need ¹⁸O water as a liquid target, but when ¹⁸F-FDOPA is to be prepared, we need a gaseous target?

In a typical application an accelerator, such as the cyclotron, might be used to prepare a specific radioactive species through a particular nuclear reaction. For example, the ¹⁸F you mention is often prepared via a proton-induced (p,n) reaction on ¹⁸O; in theory the ¹⁸O could be in any physical state, liquid water being one convenient form. Some facilities use a different deuteron-induced reaction—a (d,α) reaction on ²⁰Ne (neon); under usual conditions neon is present as a gas, and this would be a typical gaseous target.

Keep in mind that after the radioactive product is formed it must be isolated from the residual target material and then chemically reacted with appropriate molecules to form the desired radiolabeled end-product, such as the fluorodeoxyglucose, ¹⁸F-FDG. Common chemical routes to production of this compound involve reactions of fluoride ion, F^-, in liquid solutions, and the use of the liquid target is often reasonable for such applications. There are also methods involving fluoride reactions in liquid solutions that can be used to produce the ¹⁸F labeled L-dihydroxyphenylalanine (L-Dopa), but there are other preparation methods that involve bubbling of fluorine gas (F₂) through solutions of appropriate chemicals. The latter methods might favor use of a gaseous target from which the ¹⁸F₂ could be more easily isolated.

Besides the chemistry requirements imposed by the post-irradiation need to incorporate the product radionuclide into particular molecules, other considerations that affect the decision as to the physical state of the target include the following:

1. The atom density of the target species in the target material—e.g., a solid or liquid might provide higher atom densities than a gas and thus produce greater amounts of the desired product.
2. The cross section of the intended target atoms for the specific nuclear reaction of interest—e.g., a small cross section may require a higher target density, often favoring a solid or liquid target over a gas.
3. The preponderance of interfering species in the irradiated target—some target materials may contain nuclear species that produce undesired radioactive products that might be difficult to separate from the desired species, and such considerations can affect the type of target selected; similar considerations may also apply to stable target species that interfere with separation of the desired product.
4. The associated undesired radioactivity of the target  materials following irradiation—high radiation levels of some potential target materials, as a consequence of incidental irradiation of miscellaneous species may mitigate against selection of some target types and/or favor selection of other types.

I hope the above addresses your concerns.

George Chabot, PhD, CHP