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9He (2004TI06)

(See the Isobar Diagram for 9He)

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 Jπ = 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 Eπ- = 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 Elab = 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 Ex = 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 Jπ = 1/2+ or Jπ = 1/2- (1999KA67). (2001CH31) obtain the Jπ = 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 Jπ = 1/2- state (2002PI19), if it is identified with the state at Sn = -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).