The possible existence of a particle-stable 5H is discussed by (1957BL1A, 1960GO36, 1960ZE03, 1961BA1C, 1961YA04, 1963AR06, 1964AN06, 1964GO25, 1964GO1B, 1964VL1A, 1965BA1A). According to (1957BL1A), a T = 3/2 level of 5He - 5Li might be formed by 3H or 3He and a deuteron in the singlet state, at an energy ≈ 2.3 MeV above the 16.7 MeV level. If this is the case, a mass excess, M - A = 30.5 MeV † is indicated, and 5H is 0.6 MeV stable against 3H + 2n. Consideration of empirical pairing energies indicate that 5H is particle-stable only if the first T = 1 state of 4He is at Ex < 22 MeV (1960GO36) (4H unstable by < 1.4 MeV). On the other hand, if 8He is particle-stable, it is unlikely that the tetraneutron is bound by as much as 1 MeV and hence unlikely that 5H is stable (1964GO1B, 1964GO25). In the Qm below, the mass excess of 5H is taken to be 31.1 MeV, i.e., 5H is assumed to be just unbound.
† All mass excesses are given in terms of the 12C standard (M - A ≡ 0).
A search for delayed neutrons with Et = 15.6 MeV yielded negative results: σ ≤ 1 mb (1963EC1A).
With bremsstrahlung Emax = 320 MeV, a β-emitter is reported with τ1/2 = 110 msec, Eβ > 15 MeV, ascribed to reaction (a): the activation cross section is 1.8 ± 0.6 μb (1963NE02). However, other investigations with the same reaction led to a negative result (1958TA03, 1962CE03, 1964SH18): the activation cross section is < 0.03 μb if τ1/2 = 10 msec, < 0.09 μb if τ1/2 = 110 msec ((1962CE03), and C.N. Waddell, private communication).
7Li has been bombarded with 2 GeV protons in an attempt to produce reaction (b). No evidence is found for the 110 msec activity: the activation cross section is ≤ 1 μb. The ratio of reaction (b) to the 11B(p, 3p)9Li reaction is at least 100 times less than that found by (1963NE02) for the corresponding photon induced reactions (1964SC02). At Ep = 160 MeV, the upper limit to the cross section is ≈ 0.2 μb, assuming τ1/2 is in the range 0.1 to 100 sec (1965BE1P).