
^{5}H (1966LA04)(Not illustrated) The possible existence of a particlestable ^{5}H is discussed by (1957BL1A, 1960GO36, 1960ZE03, 1961BA1C, 1961YA04, 1963AR06, 1964AN06, 1964GO25, 1964GO1B, 1964VL1A, 1965BA1A). According to (1957BL1A), a T = 3/2 level of ^{5}He  ^{5}Li might be formed by ^{3}H or ^{3}He 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 ^{5}H is 0.6 MeV stable against ^{3}H + 2n. Consideration of empirical pairing energies indicate that ^{5}H is particlestable only if the first T = 1 state of ^{4}He is at E_{x} < 22 MeV (1960GO36) (^{4}H unstable by < 1.4 MeV). On the other hand, if ^{8}He is particlestable, it is unlikely that the tetraneutron is bound by as much as 1 MeV and hence unlikely that ^{5}H is stable (1964GO1B, 1964GO25). In the Q_{m} below, the mass excess of ^{5}H is taken to be 31.1 MeV, i.e., ^{5}H is assumed to be just unbound.
^{†} All mass excesses are given in terms of the ^{12}C standard (M  A ≡ 0).
A search for delayed neutrons with E_{t} = 15.6 MeV yielded negative results: σ ≤ 1 mb (1963EC1A).
With bremsstrahlung E_{max} = 320 MeV, a βemitter is reported with τ_{1/2} = 110 msec, E_{β} > 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 τ_{1/2} = 10 msec, < 0.09 μb if τ_{1/2} = 110 msec ((1962CE03), and C.N. Waddell, private communication). ^{7}Li 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 ^{11}B(p, 3p)^{9}Li reaction is at least 100 times less than that found by (1963NE02) for the corresponding photon induced reactions (1964SC02). At E_{p} = 160 MeV, the upper limit to the cross section is ≈ 0.2 μb, assuming τ_{1/2} is in the range 0.1 to 100 sec (1965BE1P). See also (1964NE02, 1964VO1D) and (1964BO1N: ^{7}Li(π^{}, pn)^{5}H).
