Search for 4-neutron resonant state in the interaction of 11 Li with light targets

At the EXON-2001 Symposium, three events of 4-n coincidences fulfilling the 96% criterion of cross-talk (c-t) rejection have been presented. When reanalyzing the events, it was found that two of them satisfy also the 99% c-t rejection criterion. A table is given where the 4-n event no 59, is exemplified. For c-t rejection a formula by R. Ghetti et al. (quoted in the following as F1) was used. The columns in the given table contain: the event number, the detector number, the neutron time of flight T, the neutron energy E, the light output A and the minimum energies Em calculated in 4 variants, subtracting from t1, up to 3 σt. The resolution σt = 0.4 ns was estimated from the γ-time-of flight peak. At present, the investigation of 4-n coincidences at a level of much higher statistics appears to be possible, by using the pre-emission effect on C instead of Si target. The number of expected 4-n events in an experiment 11 Li + C, can be estimated in the following way: one assumes that the number of n-n coincidences will increase ∼ 7 times in case of C target (s = 3.5 times due to less target-screening and 2 times due to the higher number of carbon nuclei in the target). Considering that 4-n scales with s2, one may expect up to 50 4-n events. The observation of 4-fold coincidences would be an evidence for a 4-n resonant state appearing in the early interaction of 11 Li with the target, probably in the channel 7 Li + 4-n, for which the separation energy S4-n is only 6.5 MeV. In a following contribution the P4-n probability obtained, with a new method, supporting the data of the given table is presented. (authors)