Development of 12C-enriched irradiation-resistant diamond targets for astrophysical 12C(α,γ)16O reaction measurements

The ¹²$C$ $(\alpha ,\gamma )$¹⁶$O$ reaction plays a pivotal role in nuclear astrophysics research. The direct measurement of this reaction remains particularly challenging due to its extremely low cross section (approximately $10^{-17}$ barn at 300 keV) within the Gamow window. This study addresses the critical need for irradiation-resistant ¹²$C$-enriched targets to enable accurate measurements. We successfully fabricated a ¹²$C$-enriched diamond target on molybdenum substrate through Microwave Plasma Chemical Vapor Deposition (MPCVD), demonstrating remarkable stability under high-intensity proton beam irradiation. Experimental results revealed only a 1.8% decrease in the ¹²$C$ $(p,\gamma )$¹³$N$ reaction yield in the ¹²$C$ layer following proton bombardment at 270 keV with 2 mA beam current and a total accumulated charge of 124.2 C, indicating a significant improvement compared to conventional carbon targets. Isotopic analysis confirmed a ¹³$C$/¹²$C$ ratio of $(9.7\pm 1.3)\times 10^{-5}$, verifying the absence of contamination during MPCVD processing. Additionally, we developed a novel nuclear-reaction-based methodology for quantifying hydrogen content in thin films, establishing an upper limit of 0.058% (95% confidence level) for hydrogen concentration in the diamond target. The combined irradiation resistance and isotopic purity of this target meet the stringent requirements for direct ¹²$C$ $(\alpha ,\gamma )$¹⁶$O$ reaction measurements in astrophysical environments.