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 [¹¹C]CH₄ from gas targets is notorious for weak performance with respect to yield, especially when using high beam currents. Post-target conversion of [¹¹C]CO₂ to [¹¹C]CH₄ 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]CH₄ is superior with respect to molar activity. We studied the in-target production of [¹¹C]CO₂ and [¹¹C]CH₄ from nitrogen gas targets as a function of beam current, irradiation time, and target temperature.

Results

[¹¹C]CO₂ production was practically unchanged across the range of varied parameters, but the [¹¹C]CH₄ yield, presented in terms of saturation yield Y_SAT(¹¹CH₄), had a negative correlation with beam current and a positive correlation with target chamber temperature. A formulated model equation indicates behavior where the [¹¹C]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: N₂ + 3H₂ → 2NH₃. 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 ¹¹C to [¹¹C]CH₄.

Conclusions

[¹¹C]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 [¹¹C]CH₄ production at high beam currents. Increased [¹¹C]CH₄ radioactivity is an obvious benefit in radiosynthesis in terms of product yield and molar radioactivity.