Strong and durable fluorine-implanted targets developed for deep underground nuclear astrophysical experiments

Nine fluorine targets of various types were developed for the ¹⁹F($p$, $\alpha$)¹⁶O experiment to be performed at the Jinping Underground Nuclear Astrophysics Experiment (JUNA), China. Two targets were produced by evaporating CaF₂ on Ta backings: one was produced by sputtering MgF₂ on the Cr+Fe backings, and six were produced by implanting 20–80 keV ¹⁹F ions into pure Fe and Cu backings. Thin Cr protective layers were covered over these targets. We assessed each target's stability by monitoring the $\gamma$-ray yields of the ¹⁹F($p$, $\alpha \gamma$)¹⁶O reaction over the well-known 340 keV resonance. Our results indicate that the traditional evaporated and sputtered targets exhibit 0.6%–6% deterioration (or ¹⁹F material loss) per Coulomb, while the implanted targets exhibit relatively small deterioration due to the proton beam bombardment. For the optimum target #8, the ¹⁹F target material loss is only approximately 0.05% per Coulomb proton beam bombardment, and such target is much more stable than the traditional targets. The fluorine depth distribution of the implanted target #8 was precisely analyzed by the atom probe tomography (APT) technique. The obtained depth distribution can well reproduce the experimental $\gamma$-ray yield curve. Furthermore, as test experiment, the ¹⁹F($p$, $\alpha \gamma$)¹⁶O cross section was measured with the implanted target #8 in a center-of-mass energy region of $E_{\mathrm{c.m.}}$ = 174–358 keV, which is consistent with the previous results. In conclusion, traditional fluorine targets produced with the evaporation and sputtering techniques are not suitable for the high-current experiment even with a protective layer. For the upcoming high-current JUNA experiment, this work offers an optimum target scheme: first, implanting ¹⁹F ions into the pure Fe backings with an implantation energy of 40 keV, and then sputtering a 50 nm thick Cr layer to further prevent the fluorine material loss.