CHICO2, a two-dimensional pixelated parallel-plate avalanche counter

摘要

CHICO2 (Compact Heavy Ion COunter), is a large solid-angle, charged-particle detector array developed to provide both θ and φ angle resolutions matching those of GRETINA (Gamma-Ray Energy Tracking In-beam Nuclear Array). CHICO2 was successfully tested at the Argonne National Laboratory where it was fielded as an auxiliary detector with GRETINA for γ-ray spectroscopic studies of nuclei using a ~(252)Cf spontaneous fission source, stable beams, and radioactive beams from CARIBU. In field tests of the ~(72.76)Ge beams on a 0.5 mg/cm~2 ~(208)Pb target at the sub-barrier energy, CHICO2 provided charged-particle angle resolutions (FWHM) of 1.55° in θ and 2.47° in φ. This achieves the design goal for both coordinates assuming a beam-spot size ( ＞ 3 mm) and the target thickness ( ＞ 0.5 mg/cm~2). The combined angular resolution of GRETINA/CHICO2 resulted in a Doppler-shift corrected energy resolution of 0.60% for 1 MeV coincident de-excitation γ-rays. This is nearly a factor of two improvements in resolution and sensitivity compared to Gammasphere/CHICO. Kinematically-coincident detection of scattered ions by CHICO2 still maintains the mass resolution (ΔM/M) of ～5% that enhanced isolation of scattered weak beams of interest from scattered contaminant beams.