Array detector for neutron pre-emission investigations

It was predicted that in a fusion experiment induced by 11 Li on light targets, due to the very large dimension of 11 Li, the valence neutrons will not be absorbed together with the 9 Li core, but will be emitted in the early stage of the reaction process. The experiment aiming to check this expectation was performed at the RIKEN-RIPS facility. It was found from neutron energy spectra measurements that an important number of fusion events, about 40 %, are preceded by the pre-emission of 1 or 2 neutrons. In the position spectra measurements, a very narrow neutron component has been found. The recent results of time-position coincidence measurements show that within the narrow component, the neutrons are pre-emitted predominantly as neutron pairs. The Program Advisory Committee of RIKEN has approved a new experiment at the RIKEN Ring Cyclotron aiming the investigation of neutron-neutron coincidences by using a new neutron array detector. This detector has been recently achieved within the collaboration existing between IFIN-HH, Romania and RIKEN-Japan. It consists of 81 plastic scintillation crystals, 4 x 4 x 12 cm3, each being viewed by a Philips XP2972 photomultiplier. At IFIN-HH, Bucharest the scintillation BC400 crystal quality, the time characteristics of the detector, the response to γ rays and to neutrons, have been tested with good results. It is expected that up to ∼ 900 n-n coincidences could be investigated with the new array detector, within the approved beam time. (authors)