[11C]CH4 from gas targets: the effects of forced convection and adsorption on target walls

[11C]CH4 from gas targets: the effects of forced convection and adsorption on target walls

Nicholas Zacchia, Trevor Uittenbosch, Ken Buckley, Thomas J. Ruth, Mark Martinez, Cornelia Hoehr
TRIUMF, University of British Columbia

Oral Presentation

None

Obtaining theoretically predicted [11C]CH4 activity from a gas target has traditionally been challenging. Two phenomena thought to work against higher production are heat distribution in the target and wall interactions which render the activity unrecoverable.
Heating of the target gas increases internal target pressure, limiting the beam current that can be applied to the target system, prompting efforts to optimize target cooling. To this end we have produced a forced-convection gas target for the production of [11C]CH4 on a 13 MeV cyclotron. The target was tested and heat transfer coefficients as well as target gas velocities are estimated. We show that the bottleneck in cooling the target gas lies in transferring heat from the target gas into the target body. Forced convection greatly enhances this transfer.
Using Langmuir kinetics, a theoretical model of isotope adsorption has been created to describe detrimental wall effects in gas target systems and applied to this forced-convection target. The act of mixing the target gas seems to slightly inhibit adsorption losses to the walls. This may be attributed to disruption of adsorption pathways or to increased formation rates of [11C]CH4. Interpretations of these results and model implications will be discussed.