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

Abstract

Abstract Nine fluorine targets of various types were developed for the 19F( p , α )16O experiment to be performed at the Jinping Underground Nuclear Astrophysics Experiment\xa0(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\xa0keV 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 γ -ray yields of the 19F( p , α γ )16O reaction over the well-known 340\xa0keV 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\xa0#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\xa0#8 was precisely analyzed by the atom probe tomography\xa0(APT) technique. The obtained depth distribution can well reproduce the experimental γ -ray yield curve. Furthermore, as test experiment, the 19F( p , α γ )16O cross section was measured with the implanted target\xa0#8 in a center-of-mass energy region of E c.m. = 174–358\xa0keV, 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\xa0keV, and then sputtering a 50\xa0nm thick Cr layer to further prevent the fluorine material loss.