[en] Evidence for neutron pair pre-emission in the fusion of 11 Li halo nuclei with Si targets was reported by us for the first time in 1996. For further testing of this effect, an array detector covering (36x36) cm2, has been built. To have also good position resolution a 4 x 4 cm2 area of a single scintillation element was chosen. As concerns the thickness of the scintillation element, a compromise between efficiency, cross-talk between adjacent detectors and cost was established. For example, for 6 cm thickness of the plastic crystal, the detection efficiency for 13 MeV neutrons is only 20% and the cross-talk 1.7%. For 12 cm thickness the detection efficiency significantly raises to 35% and the cross-talk becomes 2.8%. For larger thickness the efficiency only slightly increases. For 18 cm thickness the efficiency is expected to be 41%, and the cross-talk 3.3%. Therefore, by choosing the 12 cm thickness, one expects good efficiency with a reasonable increase of the cross-talk. The above mentioned efficiency and cross-talk values were obtained by Monte-Carlo calculations. The 81 BC400 plastic scintillation elements were cut from two 1000 x 300 x 40 mm3 pieces. After polishing, the quality of the surfaces was tested by optical methods. The BC400 crystal detects charged particles, γ rays, and neutrons. The γ-ray and neutron separation will be done at RIKEN by the time of flight technique. The Philips XP2972 photomultiplier was selected for building the array detector. This selection was based on the following considerations: 1.the diameter is suited to the granularity of the detector; 2. the phototube sensitivity matches the plastic crystal emission; 3. the output pulse has very good characteristics (pulse height close to 1 volt and a rise time of 2.1 ns). The voltage divider has been designed for large multiplication and stability, in conditions of counting rates up to 104 pulses/s. The total resistance is 2.6 MΩ. The following testings on the new array detector have been performed: 1. testing of scintillation crystal quality; 2. testing of time characteristics of the detector; 3. testing of response to β and γ rays; 4. testing of response to neutrons. A detailed description of these testings and of the obtained results, will be published elsewhere. (authors)
Source
Carstea, Stefan; Dragulici, Felicia; Enescu, Sanda-Elena; Oancea, Margareta; Preda, Mihaela; Prodan, Lucia; Raduta, Adriana; Sandu, Doina; Schiaua, Claudiu (Horia Hulubei National Institute for Physics and Nuclear Engineering, PO Box MG-6, RO-76900 -Bucharest-Magurele (Romania)); Horia Hulubei National Institute for Physics and Nuclear Engineering, IFIN-HH, PO Box MG-6, RO-76900 Bucharest-Magurele (Romania); 175 p; ISSN 1454-2714; 
; 2000; p. 56; Available from author(s) or Office of Documentation, Publication and Printing, Horia Hulubei National Institute for Physics and Nuclear Engineering, PO Box MG-6, RO-76900 -Bucharest-Magurele (RO); Available from Office of Documentation, Publication and Printing, Horia Hulubei National Institute for Physics and Nuclear Engineering, PO Box MG-6, RO-76900 Bucharest-Magurele (RO); Short communication. 5 refs., 1 fig.
; 2000; p. 56; Available from author(s) or Office of Documentation, Publication and Printing, Horia Hulubei National Institute for Physics and Nuclear Engineering, PO Box MG-6, RO-76900 -Bucharest-Magurele (RO); Available from Office of Documentation, Publication and Printing, Horia Hulubei National Institute for Physics and Nuclear Engineering, PO Box MG-6, RO-76900 Bucharest-Magurele (RO); Short communication. 5 refs., 1 fig.