The stability of 8He (1968BA48, 1968ME03, 1973FI04, 1988AJ01) sets an upper limit to the total binding energy of 4n, because the decay 8He → 4He + 4n does not occur (1964GO1B, 1964GO25). The most precisely determined mass excess of 8He (1988WA18) yields B(4n) ≤ 3.1 MeV. Noting that in all known nuclei the proton binding energy increases when two neutrons are added, (1964VL1A) show that B(4n) < -Q, where Q is the decay energy for 5H → 3H + 2n. Since Q > 0 (1988AJ01), 4n must be unbound.
If bound 4n exists, a T = 2 state should be found in 4He (see 1968ME03, 1973FI04) at 26 < Ex < 29 MeV. Also a T = 2 resonance should occur in n + 3H at 6 < Ecm < 9 MeV. Resonances have been found in n + 3H (see 4H), but there is no evidence to support a suggestion (1963AR06) of T = 2. No low-lying T = 2 states of 4He have been found (see 4He); systematics give Ex ( T = 2) ≈ 34 MeV (1963FR14), ≥ 32 MeV (Kurath, quoted in 1963SC35) and Ex > 50 MeV based on shell-model calculations by (1966KR06, 1970SZ03, 1972SZ07). All experimental searches for 4n have failed [see (1965BA1A) and recent work on reaction 2 below].
Variational calculations of the energy of the 4n system, in which the trial wave function assumes a relative s-wave motion of two di-neutron clusters fail to produce either a bound state or a low-lying resonance (1965TA14, 1970TH12), although a similar theoretical technique successfully reproduces the binding energies of 3H, 3He and 4He (1965TA15) and the 2H(d, d)2H differential cross section (1970TH12). A K-harmonic approach also finds that the 4n state is absent (1968BA20). A hyperspherical-basis study of A = 3 - 8 multineutron systems (1989GO18) indicates that these systems have no bound states. However, a similar theoretical study (1989GU16) reports that 4n has a resonance state because of the existence of a kinematical barrier.
Arguments based on pairing-energy systematics (1960GO36, 1960ZE03) would require the stability of 4n, if 3n is stable (1965TA14), but see (1968KA35). Suspicion of the stability of 3n (1965AJ03) has not been confirmed (see 1968KA35 and 1968ME03, 1975FI08, 1987TI07).
In the following reactions, Q-values have been computed assuming B(4n) = 0.
The previous compilation (1973FI04) notes one experimental search for neutrons from the above reaction with negative results: σ ≤ 1.7μb.
Measurements of the 4He(π-, π+)4n reaction carried out in search of evidence for 4n and extending from Eπ- = 100 - 215 MeV are summarized in the previous compilation (1973FI04). Neither bound nor unbound 4n was detected in this early work. More recently, the 0° momentum spectrum from the double-charge-exchange reaction at Eπ- = 165 MeV was measured (1984UN02) in a search for 4n, and an upper limit of 22 nb/sr was set for the cross section. Note, however, that the theoretical study of (1986KE20) reports that the final-state interaction in the four-neutron system is so strong that the tetraneutron could not have been observed in the kinematic region explored in (1984UN02). Pion spectra were also measured (1986KI20) for incident pion energies of 180 and 240 MeV and found to be qualitatively consistent with two sequential single-charge-exchange processes. No evidence for 4n was obtained. Total cross sections for pion double-charge exchange at 180 and 240 MeV were measured and compared with a phenomenological model in which successive charge-exchange processes complete with quasi-free scattering were included. A very recent search for the tetraneutron was carried out at Eπ- = 80 MeV and θπ+(lab) = 50° - 100° (1989GO17) and set an upper limit σ(θ) ≤ 13 nb/sr. Several theoretical studies of pion double-charge exchange on 4He have been reported. In (1977GI04) cross sections were calculated for E = 0 - 500 MeV in a model in which two single pn charge-exchange scatterings occur. In the work of (1980JI03, 1981JI02) the reaction was studied in the framework of a four-body hyperspherical basis method.
The previous compilation (1973FI04) includes only two experiments involving this reaction and no indication of the formation of 4n. Since that time the only investigation reported (1977BA47) utilized a nuclear emulsion loaded with 7Li. An upper limit of 1.2 × 10-3 was determined for the relative probability of forming 3n and 4n compared to all other π- + 7Li reactions.
A measurement (1988AL11) carried out at θ lab = 10° for 82 MeV 7Li ions found no 4n resonances. An upper limit of 4 nb/sr for the cross section was determined.
A measurement (1988BE02) of the spectrum of outgoing nuclei for incident 7Li energies of 72 - 90 MeV found no evidence of a bound state of the four-neutron system, but an upper limit was reported.
This reaction was studied (1988BE02) for incident 9Be energies of 72 - 90 MeV. No evidence for bound or quasi-stationary states of 4n was obtained.