Preparation of Be-7 targets for nuclear astrophysics research

This work describes the preparation of three Be-7 targets which were used in two independent measurements of the Be-7(n,alpha)He-4 cross section in the energy range of interest for the Big-Bang nucleosynthesis at the n_TOF-CERN facility and at Soreq-SARAF. A more precise value of this cross section could shed light on the long lasting "Cosmological Lithium problem". Two methods for target preparation were used. A target was obtained by deposition and subsequent air-drying of (24.50 +/- 0.54) GBq of Be(NO3)(2) droplets precisely positioned onto a stretched low density polyethylene film 0.635 mu m thick. The thickness of the deposited Be(NO3)(2) layer was deduced using Monte-Carlo simulations to be 0.36 mu m. The energy loss of 8500 keV alpha particles passing through the target obtained by air-drying of Be-7(NO3)(2) droplets was estimated to be 88 keV. Two other targets were prepared via molecular plating onto similar to 5 mu m and 1 mm thick aluminium backings respectively. The first was obtained by molecular plating (24.47 +/- 0.53) GBq of Be-7 resulting in a deposited layer of Be(OH)(2) 1.04 mu m thick. The second molecular plated target was obtained depositing (3.95 +/- 0.08) GBq of Be-7. The mean energy loss of 8500 keV alpha particles passing through the molecular plated target with 5 mu m thick aluminium backings was estimated as 814 keV. The energy loss by 8500 keV alpha particles in all the obtained targets is considered tolerable for the envisaged cross section measurements. The preparation and characterization of the targets is here described.