The combined AGATA-DSSD set-up for the measurement of very small heavy-ion fusion cross sections

We describe the data analysis methods and the characteristics of the combined set-up of the γ-ray tracking spectrometer AGATA with two annular double sided silicon strip detectors (DSSD) (4” diameter) placed 5.65 cm upstream and downstream of the target, for the measurement of γ-light particle coincidences in near- and sub-barrier heavy-ion fusion reactions. The DSSD cover the angular ranges θlab=139.6o–162.7o and =23.0o–40.4o, corresponding to a total solid angle ≃26% of 4π. The ohmic side of the DSSD is turned towards the target, so protons and α particles can be clearly identified. We show the relevant results obtained for the two systems 28Si+12C and 16O+12C down to very small cross sections in the tenths of nb range. Particle identification allows us to observe well-separated groups of protons and α’s populating low-energy levels of the various fusion-evaporation residues (ER). Relative fusion cross sections are obtained with a procedure described in detail, and the absolute cross section scale is fixed by complementary measurements using an electrostatic beam deflector set-up where the ER are directly detected down to a few μb. The AGATA+DSSD set-up is subject to significant improvements thanks to the scheduled increase of the number of γ detectors, and to the possibility of installing silicon detector arrays covering a larger solid angle. This will open the possibility of measuring fusion cross sections near, and possibly below, 1nb.