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

Nine fluorine targets of various types were developed for the F-19(p, alpha)O-16 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 F-19 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 F-19(p, alpha gamma)O-16 reaction over the well-known 340 keV resonance. Our results indicate that the traditional evaporated and sputtered targets exhibit 0.6%-6% deterioration (or F-19 material loss) per Coulomb, while the implanted targets exhibit relatively small deterioration due to the proton beam bombardment. For the optimum target #8, the F-19 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(19)F(p,alpha gamma)O-16 cross section was measured with the implanted target #8 in a center-of-mass energy region of E-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(19)F 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.

Automated summary

The study found that traditional fluorine targets are prone to material loss in high-current experiments, while targets prepared by implantation method are more stable. The optimal target scheme is to first implant F ions into pure Fe backing, then sputter a Cr layer to further prevent material loss.