The N-Ring neutron detector array


The N-Ring array consists of 6 individual detectors. The detectors replace the 6 most forward elements of the BGO inner ball and therefore they are placed on a ring at 20o against the beam axis. The distance from the target to the front face of the neutron detectors is of about 30 cm. Each detector is filled with 1 lit. of BC501A liquid scintillator that is characterized by good timing properties (~ 3 ns.). The inner walls of the detectors were painted with reflector paint BC622A. The light produced in the liquid scintillator is collected by standard 3 in. photomultiplier tubes through a quartz window. The 10 cm. depth of the detectors yields an intrinsic efficiency for evaporation neutrons of about 25%. Each detector subtends a solid angle of 1%. The resulting total efficiency of the detector system is of the order of 3-5 % depending on the reaction kinematics. To reduce the large counting rate produced by the X-rays emitted following the reactions, in front of each detector were placed composite absorbers made of Pb/Cu/Cd. The signals from the PMT's are analyzed by CAMAC modules that produced three output signals: energy, zero crossing (Z/C) and time. The neutron energy accepted by these modules ranges from 200 keV to 10 MeV. The Z/C signal is used to discriminate neutrons from gammas. The modules allow for the multiplexing of both the analogue and digital signals. The programming and control of the neutron modules could be easily integrated in the GASP's computer control system.


The good suppression of the channels with charged particles evaporation makes such a device a unique filter for the selection of proton-rich nuclei close to the N=Z and proton drip-line, populated with extremely low cross sections. In recent experiments with this detector we have pushed the observational limit of GASP down to 10-5, as in the case of the N=Z nucleus 88Ru populated with only ~3 microbarn cross section.