In-target production of [¹¹C]CH₄ from a nitrogen/hydrogen gas target as a function of beam current, irradiation time, and target temperature

Background: Production of [C-11]CH4 from gas targets is notorious for weak performance with respect to yield, especially when using high beam currents. Post-target conversion of [C-11]CO2 to [C-11]CH4 is a widely used roundabout method in C-11-radiochemistry, but the added complexity increase the challenge to control carrier carbon. Thus in-target-produced [C-11]CH4 is superior with respect to molar activity. We studied the in-target production of [C-11]CO2 and [C-11]CH4 from nitrogen gas targets as a function of beam current, irradiation time, and target temperature. Results: [C-11]CO2 production was practically unchanged across the range of varied parameters, but the [C-11]CH4 yield, presented in terms of saturation yield Y-SAT((CH4)-C-11), had a negative correlation with beam current and a positive correlation with target chamber temperature. A formulated model equation indicates behavior where the [C-11]CH4 formation follows a parabolic graph as a function of beam current. The negative square term, i.e., the yield loss, is postulated to arise from Haber-Bosch-like NH3 formation: N-2 + 3H(2) -> 2NH(3). The studied conditions suggest that the NH3 (liq.) would be condensed on the target chamber walls, thus depleting the hydrogen reserve needed for the conversion of nascent C-11 to [C-11]CH4. Conclusions: [C-11]CH4 production can be improved by increasing the target chamber temperature, which is presented in a mathematical formula. Our observations have implications for targetry design (geometry, gas volume and composition, pressure) and irradiation conditions, providing specific knowledge to enhance [C-11]CH4 production at high beam currents. Increased [C-11]CH4 radioactivity is an obvious benefit in radiosynthesis in terms of product yield and molar radioactivity.