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USNDP

7He (2002TI10)


(See Energy Level Diagrams for 7He)

GENERAL: References to articles on general properties of 7He published since the previous review (1988AJ01) are grouped into categories and listed, along with brief descriptions of each item, in the General Tables for 7He located on our website at (www.tunl.duke.edu/NuclData/General_Tables/7he.shtml).

See also Table 7.1 preview 7.1 [Table of Energy Levels] (in PDF or PS).

Mass of 7He: The atomic mass excess of 7He is 26.11 ± 0.03 MeV: 7He is then unbound with respect to decay into 6He + n by 0.44 MeV: see (1984AJ01). The ground state is calculated to have Jπ = 3/2- and to be unstable with respect to decay into 6He + n by about 1 MeV (1985PO10). See (1988AJ01).

7He excited-state calculations: In addition to the (0 + 1)ℏω and (0 + 2)ℏω model space calculation of (1985PO10) cited in (1988AJ01), a number of calculations have been reported since the previous evaluation. They include calculations: in a (0 + 2)ℏω model space (1990WO10); in a two-frequency shell model (2001CO21); in a large-basis shell model (1998NA17); with Green's Function Monte-Carlo methods (1997PU03, 2000WI09, 2001PI07); and in a refined resonating group model (1997WU01).

1. 1H(8He, d)7He Qm = -0.36

This reaction was studied at E(8He) = 50 MeV/A (1999KO14, 2000KO46). Deuterons, neutrons, 4He and 6He were detected. Spectra indicate a level in 7He at Ex = 2.9 ± 0.3 MeV, Γ = 2.2 ± 0.3 MeV which decays mainly into 3n + 4He, Γα + 3n/Γtot = 0.7 ± 0.2. Arguments are given for a tentative assignment Jπ = (5/2-).

2. 7Li(π-, γ)7He Qm = 128.37

Capture γ-rays from the transition to 7Heg.s. are reported by (1986PE05).

3. 7Li(n, p)7He Qm = -10.42

The proton group corresponding to 7Heg.s. has Γ < 0.2 MeV: see (1979AJ01). At En = 60 MeV broad bumps in the spectra are ascribed to states at Ex ≈ 20 ± 1 MeV [Γ = 9 ± 2 MeV] and, possibly, at ≈ 6 MeV (1983BR32, 1984BR03; see for discussion of the GDR). See also (1987HE24) and (1987BR32). Analyzing powers measured at En = 0.88 GeV are reported in (1996BB23).

4. 7Li(t, 3He)7He Qm = -11.18

The 3He particles leading to the ground state of 7He have been observed at Et = 22 MeV. The width of the ground state given in (1988AJ01) is 160 ± 30 keV; for a radius of 2.2 fm and ln = 1, this width is 0.22 of the Wigner limit. The angular distribution is peaked in the forward direction. No other states of 7He were observed for Ex < 2.4 MeV: see (1979AJ01).

5. (a) 6Li(14C, 13N)7He Qm = -14.36
(b) 7Li(7Li, 7Be)7He Qm = -12.06
(c) 7Li(11B, 11C)7He Qm = -13.19
(d) 9Be(6Li, 8B)7He Qm = -23.60
(e) 9Be(9Be, 11C)7He Qm = -14.07
(f) 9Be(11B, 13N)7He Qm = -11.44
(g) 9Be(14C, 16O)7He Qm = -7.01
(h) 9Be(15N, 17F)7He Qm = -16.61

Reaction (a) was investigated at E(14C) = 24 MeV/A (1995BO10, 1995VO05). The 7He ground state was populated strongly, but no excited states were observed. At E(6Li) = 72 MeV and at E(7Li) = 70 MeV (reactions (b) and (d)) there is no evidence for excited states with Γ ≤ 2 MeV for Ex < 10 MeV (1985AL1G, 1985AL29, 1985ALZM). The ground state of 7He is strongly populated. Reactions (c), (e), (f) and (g) have been investigated at E(11B) = 88, E(9Be) = 106.7 and E(14C) = 152.6 MeV. The ground state of 7He is populated. There is some evidence for a second state in reaction (g) at Ex = 2.9 ± 0.5, Γ = 1.5 ± 0.5 MeV (1988BE34). See also (1979AJ01) and (1988BEYJ). At E(15N) = 240 MeV, reaction (h) shows evidence for a level at Ex = 3.2 ± 0.2 MeV, Γ = 1.5 ± 0.2 MeV (1998BO1M, 1998BO38, 1999BO26).

A more recent experiment with incident energies Elab = 240 MeV and 318.5 MeV is described in (2001BO35). The width of the 7He ground state resonance was measured to be 0.14 ± 0.02 MeV. An excited state with Ex = 2.95 ± 0.10 MeV, corresponding to a resonance energy ER = 3.39 ± 0.10 and width ΓR = 1.9 ± 0.2 MeV, was observed. There was no indication of a state at Ex = 0.8 MeV reported by (2001MA05) [See reaction 7]. Evidence for a broad excited state at Ex = 5.8 ± 0.3 MeV with Γ = 4 ± 1 MeV was also obtained.

6. 10B(π-, pd)7He Qm = 105.10

Stopped negative pion absorption on 10B was studied with detection of protons and deuterons (1998GO30). The missing-mass spectra show evidence for a level in 7He with Ex = 2.8 ± 0.2 MeV, Γ ≈ 2.0 MeV.

7. natC(8He, 6He + n + n) Qm = 105.10

The relative energy spectra of fragments from 227 MeV/A 8He on a carbon target were measured (2001MA05). The 6He + n spectra show a structure which is interpreted as being due to a Jπ = 3/2- resonance (the 7He ground state) with ER = 0.44 MeV, Γ = 0.16 MeV with about equal contribution from a Jπ = 1/2- resonance with ER = 1.2 ± 0.2 MeV, Γ = 1.0 ± 0.2 MeV (Ex = 0.8 ± 0.2 MeV).