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GENERAL: References to articles on general properties of ⁹He published since the previous review (1988AJ01) are grouped into categories and listed, along with brief descriptions of each item, in the General Tables for ⁹He located on our website at (nucldata.tunl.duke.edu/nucldata/General_Tables/9he.shtml).

Mass of ⁹He: Although the value adopted in the 2003 Atomic Mass Evaluation (2003AU02) for the ⁹He ground state is 40.939 ± 0.029 MeV based on the results of (1999BO26), an experiment (2001CH31) suggests that the ground-state of ⁹He has J^π = 1/2⁺ and lies within 0.2 MeV of the ⁸He + n threshold. In light of this result, the Atomic Mass Evaluation center [private communication from Audi, Wapstra, and Jokinen] has adopted an atomic mass excess of 39.770 ± 0.060 MeV for ⁹He. See the discussion below.

The experimental data on states of ⁹He, all from double-charge exchange reactions on ⁹Be, was reviewed in (1999KA67) and compared with results from shell-model calculations. From the ⁹Be(π^-, π⁺)⁹He reaction at E_π^- = 180 and 194 MeV (1987SE05) an atomic mass excess of 40.80 ± 0.10 MeV was obtained, implying that ⁹He is unstable with respect to decay into ⁸He + n by 1.13 MeV. (1987SE05) also reported the population of excited states of ⁹He at 1.2, 3.8 and 7.0 MeV. In the ⁹Be(¹⁴C, ¹⁴O)⁹He reaction at E_lab = 337 MeV, (1999BO26) find a state of ⁹He at 1.27 ± 0.10 MeV above the ⁸He + n threshold with Γ = 0.10 ± 0.06 MeV. Assuming this to be the ground-state of ⁹He, the measurements of (1999BO26) indicate ⁹He excited states at E_x = 1.15 ± 0.10 MeV (Γ = 0.7 ± 0.2 MeV), 3.03 ± 0.10 MeV, and 3.98 ± 0.12 MeV. See also (2001PE27). Analogs of the lowest three states have been observed in ⁹Li via ⁸He + p elastic scattering (2003RO07).

Evidence has been obtained from neutron-fragment velocity difference measurements in the two-proton knockout reaction ⁹Be(¹¹Be, ⁸He + n)X that the ground-state of ⁹He is a virtual s-wave state within 0.2 MeV of the ⁸He + n threshold (2001CH31). Most structure calculations predict that the two lowest states of ⁹He have J^π = 1/2⁺ or J^π = 1/2^- (1999KA67). (2001CH31) obtain the J^π = 1/2⁺ state lowest but both states are underbound by several MeV with respect to the experimental candidates. Quantum Monte Carlo calculations have a similar problem for the J^π = 1/2^- state (2002PI19), if it is identified with the state at S_n = -1.27 MeV. Both (2001CH31) and (2002PI19) suggest that the promotion of neutrons to the sd shell could play an important role. The narrow width from (1999BO26) also argues against the simple p-shell structure because the single-particle width for a p-wave resonance at the observed energy is ≈ 2 MeV and a typical p-shell spectroscopic factor is 0.74 ± 0.10. Similar conclusions were reached in (2003RO07).

Attempts have also been made to assign spins to excited states by comparing calculated two-step transfer angular distributions with those measured for the ⁹Be(¹³C, ¹³O)⁹He and ⁹Be(¹⁴C, ¹⁴O)⁹He reactions (1999BO26, 1999KA67).