There is no experimental evidence for either bound states or narrow resonances of the three neutron system. Theoretical studies in the 3n system using the Faddeev method and fairly realistic two-body interactions have been carried out for complex energies looking for evidence of resonances; see (1999WI08, 2002HE25). The conclusion in each case is that such resonances probably do not exist close to the physical region. However, a similar study reported in (1996CS02) concluded that a resonance exists in the Jπ = 3/2+ channel with an energy of 14 MeV and a width of 13 MeV. A study of a Jπ = 1/2- subthreshold state in the 3n system using the hyperspherical method with simplified NN interactions is reported in (1997SO27).
To date, there have been two types of experimental approaches that have been used to look for bound or resonant 3n states. One approach is to use negative pions either in the capture reaction 3H(π-, γ)3n, in the double charge exchange reaction 3He(π-, π+)3n, or in knockout reactions such as 4He(π-, p)3n and 7Li(π-, 4He)3n. The second approach makes use of heavy ion reactions such as 7Li(7Li, 11C)3n and 2H(14C, 13N)3n.
In a series of experiments reported in (1995BO10) with E(14C) = 336 MeV, this reaction was used to look for 3n states. None were found.
A theoretical estimate of the total width of the 1s level in the 3H pionic atom is 2.2 ± 0.4 eV (1988WE01). However, the measured total width of the 1s level in the 3H pionic atom is reported to be 28 ± 7 eV; see (1984SC09, 1995DA16).
This reaction was studied at E(7Li) = 65 and 78 MeV (1987AL10). No evidence of 3n states was found.
This pionic double charge exchange reaction on 3He has been studied with Eπ = 65 - 295 MeV. For Eπ- = 5, 75, 120 MeV, see (1999GR01); see also (1999GR31) for seven energies between 65 and 120 MeV. For Eπ- = 120, 180, 210, 240 MeV, see (1997YU01). For Eπ- = 140, 200, 295 MeV, see (1986ST09). In the missing mass spectra, enhancements resembling resonances have been seen at forward angles. However, it appears that the best explanation for these enhancements is that they are due to final state interactions and not due to true resonances of the 3n system; see (1986ST09, 1997YU01). The authors of (1999GR01) conclude that there is no evidence for either a bound state or a resonance in the 3n system. A discussion of previous pion double charge exchange work on 3He (and 4He) is included in (1997YU01) along with comparisons of experimental results with model calculations.
No studies of this specific reaction have been reported since the previous evaluation. Studies of absorption of zero energy negative pions in gaseous 4He leading to emission of nn, np, nd and nt pairs are reported in (1995DA16).
This reaction was studied at E(7Li) = 79.6 MeV and no evidence for either a bound or resonance state was found (1974CE06). Studies of this reaction have been reported in (1987AL10, 2005AL15); in both reports, 7Li ions with E(7Li) = 82 MeV were used to look for evidence of 3n states, but none were found.