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

Background: Production of [ ¹¹C]CH ₄ from gas targets is notorious for weak performance with respect to yield, especially when using high beam currents. Post-target conversion of [ ¹¹C]CO ₂ to [ ¹¹C]CH ₄ is a widely used roundabout method in ¹¹C-radiochemistry, but the added complexity increase the challenge to control carrier carbon. Thus in-target-produced [ ¹¹C]CH ₄ is superior with respect to molar activity. We studied the in-target production of [ ¹¹C]CO ₂ and [ ¹¹C]CH ₄ from nitrogen gas targets as a function of beam current, irradiation time, and target temperature. Results: [ ¹¹C]CO ₂ production was practically unchanged across the range of varied parameters, but the [ ¹¹C]CH ₄ yield, presented in terms of saturation yield Y _SAT( ¹¹CH ₄), had a negative correlation with beam current and a positive correlation with target chamber temperature. A formulated model equation indicates behavior where the [ ¹¹C]CH ₄ 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 NH ₃ formation: N ₂ + 3H ₂ → 2NH ₃. The studied conditions suggest that the NH ₃ (liq.) would be condensed on the target chamber walls, thus depleting the hydrogen reserve needed for the conversion of nascent ¹¹C to [ ¹¹C]CH ₄. Conclusions: [ ¹¹C]CH ₄ 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 [ ¹¹C]CH ₄ production at high beam currents. Increased [ ¹¹C]CH ₄ radioactivity is an obvious benefit in radiosynthesis in terms of product yield and molar radioactivity.