(See Energy Level Diagrams for 6He)
GENERAL: References to articles on general properties of 6He published since the previous review (1988AJ01) are grouped into categories and listed, along with brief descriptions of each item, in the General Tables for 6He located on our website at (www.tunl.duke.edu/NuclData/General_Tables/6he.shtml).
Ground State Properties:
The interaction radius of 6He, obtained from measurements of the total interaction cross section, is 2.18 ± 0.02 fm (1985TA13, 1985TA18). These authors have also derived nuclear matter, charge and neutron r.m.s. radii.
6He is considered to be a neutron-halo nucleus because its interaction radius, which is deduced from the total interaction cross section in (1985TA13, 1985TA18), is appreciably larger than that of 6Li. A Glauber calculation using proton and neutron densities from an alpha-core valence-neutron model leads to the conclusion that the matter radius is much larger than the charge radius, as predicted by theoretical models of the 6He ground-state wave function. These theoretical models include three-body models (1993ZH1J, 1995HI15), cluster-orbital shell models (1991SU03, 1994FU04), no-core microscopic shell models (1996NA24), and microscopic cluster models for various effective nucleon-nucleon interactions (1993CS04, 1997WU01). See also (1992TA18). The point proton and point neutron radii are often compared in order to enhance the effect, and are found to differ by 0.4 - 0.8 fm. For other typical properties of halo nuclei see (1995HA56).
The half-life is 806.7 ± 1.5 ms (1984AJ01). The decay to the ground state of 6Li (Jπ = 1+) is via a super-allowed Gamow-Teller transition; log ft = 2.910 ± 0.002 (1984AJ01, 1988AJ01). A second beta-decay branch leading to an unbound final state consisting of a deuteron and an α particle was reported (1990RI01) based on the observation of beta-delayed deuterons. The branching ratio for Ed > 350 keV was measured (1993BO24, 1993RIZY) to be (7.6 ± 0.6) × 10-6. Calculations are presented which consider alternative decay routes. (One considers a decay to an unbound state of 6Li which then decays into α + d. In the other route 6He breaks up into an alpha particle plus a di-neutron which β decays to a deuteron). The calculation of (1994BA11) successfully reproduces the deuteron spectrum shape and branching ratios. References to theoretical work on the 6He(β-)6Li decay are presented in 6.2 (in PDF or PS).
Angular distributions for elastic scattering and for 1n and 2n transfer were measured at 25 MeV/A, and spectroscopic amplitudes were extracted by (1999WO13). An analysis of elastic scattering data at 700 MeV/A is described in (1998AL05). See also the analysis (2000DE43) of data at E = 25, 40 MeV and that of (2000GU19) at E = 25 - 70 MeV. The reaction cross section was measured for 36 MeV/A 6He on hydrogen, and a value of σR = 409 ± 22 mb was obtained (2001DE19). Analysis within a microscopic model allowed the 6He density distribution to be explored.
The use of elastic and inelastic scattering with secondary beams to probe ground-state transition densities of halo nuclei has been explored in a theoretical study (1995BE26). Cross sections for E = 151 MeV were calculated by (2000AV02), and density-distribution features were deduced. See also the discussion of (1999EG02).
The cross section for reaction (b) was measured for Et = 30 to 115 keV by (1986BR20, 1985JA16) who also calculated the astrophysical S-factors [the extrapolated S(0) ≈ 180 keV · b] and discussed the earlier measurements. See also (1974AJ01, 1979AJ01) and (1986JA1E). Calculations have also been made within the framework of the two-channel resonating group method (1989VA20), the microscopic multichannel resonating group method (1991TY01) and the generator coordinate method (1990FU1H). For muon-catalyzed fusion see (1988MA1V, 1989BR23, 1989CH2F, 1990HA46). For earlier work see (1988AJ01).
A mechanism for this reaction in astrophysical processes is suggested, and a reaction rate is calculated (1996EF02).
Angular distributions of the protons to 6He*(0, 1.80) have been measured at Et = 22 and 23 MeV. [No L-values were assigned.] No other states are observed with Ex ≤ 4.2 MeV: see (1979AJ01). Cross sections and angular distributions for the reaction products of the 3H(α, p)6He reaction were measured at Eα = 27.2 MeV (1992GO21). A potential description of 3H + 4He elastic scattering is discussed in (1993DU09).
Total cross sections for the production of 6He have been measured (2001AU06) at Eα = 159, 280 and 620 MeV in a study of cosmic ray nucleosynthesis. The resulting cross sections decrease rapidly with energy.
Differential cross sections were measured at E(6He) = 151 MeV. DWBA analysis suggests a spectroscopic factor of ≈ 1 for the di-neutron cluster. (1998TE1D, 1998TE03). Measurements at Ecm = 11.6 and 15.9 MeV (1999RA15) also show evidence for the 2n transfer process in the elastic scattering. However, a couple-discretized-continuum channel analysis discussed in (2000RU03) suggests a smaller 2n transfer process than commonly assumed (2001TE03). See also the analyses and calculations of (1998GO1J, 1999OG06, 1999OG09). A microscopic multicluster model description of the elastic scattering process is discussed in (1999FU03).
See reaction 2 for experimental information on the 6He + 1H system.
Calculations of the elastic scattering of protons from 6He at Ep ≥ 100 MeV are described in (1992GA27). A folding model with target densities which reproduce the r.m.s. radii and a range of electroweak data was used.
A calculation of the expansion of the Glauber amplitude described in (1999AB37) found that a 6He matter radius constant with the analysis is 2.51 fm. Finite-range coupled channel calculations have been performed below the 6He three-body breakup threshold (2000TI02). A theoretical study (2000WE03) with four differential nuclear structure models concluded that elastic scattering at < 100 MeV/A does not provide good constraints on the structure of the 6He ground state. First order optical potentials were studied for 20 - 40 MeV scattering by (2000DE43). A microscopic multicluster calculation of σ(θ) and σ(E) for Ecm = 0 - 5 MeV is reported in (2001AR05).
(1986SH14) report breaks in (e, π+) spectra at Ee = 202 MeV corresponding to Ex = 7, 9, 12, 13.6, 17.7 and 24.0 MeV. Using the shape of the virtual photon spectrum results in groups with angular distributions that suggest that the states at 13.6, 17.7 and 24.0 MeV are spin-dipole isovector states [Jπ = 1-, 2-]. See also (1990SH1I). For the earlier work see (1984AJ01). [Note: The states reported here at 7, 9 and 12 MeV are inconsistent with the work reported in reactions 12, 13, 22 and 23, and with the work on the analog region in 6Be].
The excitation of 6He*(0, 1.8) and possibly of (broad) states at Ex = 15.6 ± 0.5, 23.2 ± 0.7 and 29.7 ± 1.3 MeV has been reported: see (1979AJ01). A study of capture branching ratios to 6He*(0, 1.8) was reported in (1986PE05). For reaction (b) see (1984AJ01).
Angular distributions of the ground state proton group, p0 have been reported at En = 4.7 to 6.8 MeV, at 14 MeV and at 59.6 MeV [see (1979AJ01, 1984AJ01)] and at 118 MeV (1987PO18, 1988HA2C, 1988WA24). At En = 59.6 MeV broad structures in the spectra are ascribed to states at Ex = 15.5 ± 0.5 and 25 ± 1 MeV with Γ = 4 ± 1.5 and 8 ± 2 MeV (1983BR32, 1984BR03) [see for discussions of the GDR strength]. The ground state reaction has also been studied at En = 198 MeV (1988JA01). Proton spectra were measured at En = 118 MeV by (1998HA24).
An angular distribution of the proton group corresponding to population of the Ex = 1.8 MeV Jπ = 2+ state in 6He was also reported (1988WA24). See also (1989WA1F). Angular distributions were measured for p0 at En = 280 MeV in tests of isospin symmetry in (n, p), (p, p') and (p, n) reactions populating the T = 1 isospin triads in A = 6 nuclei (1990MI10). Cross sections for θlab = 1° - 10° for En = 60 - 260 MeV were measured to obtain the energy dependence of the Gamow-Teller strength (1991SOZZ, 1992SO02).
Several theoretical studies have been reported since the previous review. A dynamical multicluster model was used to generate transition densities for 6He and 6Li (1991DA08). A microscopic calculation in the framework of the α + 2N model (1993SH1G) reproduced energy spectra and cross sections reliably. Predictions for the structure of a second 2(+) resonance in the 6He continuum were made with a α + N + N cluster model (1997DA01). Halo excitation of 6He in 6Li(n, p)6He was studied using four-body distorted wave theory (1997ER05); see also (1997VA06). The status of experimental and theoretical research on nuclei featuring a two-particle halo is reviewed in (1996DA31).
The previous review (1988AJ01) notes that at Ed = 55 MeV, 6He*(0, 1.8) [the latter weak] are populated: no other states are observed with Ex ≤ 25 MeV [see (1984AJ01)]. More recently cross sections at 0° were measured at Ed = 260 MeV (1993OH01) and at Ed = 125.2 MeV (1995XU02). In both studies the cross section for (d, 2He) showed a linear relationship with Gamow-Teller strength from β decay or (p, n) reactions.
The ground-state angular distribution has been studied at Et = 17 MeV. At Et = 22 MeV only 6He*(0, 1.8) are populated for Ex ≤ 8.5 MeV: see (1979AJ01). Differential cross sections for the transition to 6He*(1.8) are reported at E(6Li) = 65 MeV (1987AL23). In a more recent experiment at Et = 336 MeV reported in (2000NA35), the 6He ground and 1.8 MeV states were populated. In addition, a broad asymmetric structure around Ex ≈ 5 MeV was observed with an angular distribution which exhibited ΔL = 1 dominance. Another structure at Ex ≈ 14.6 MeV was observed with the angular distribution indicating ΔL = 1.
Angular distributions have been studied for E(6Li) = 32 and 36 MeV for the transitions to 6Heg.s., 6Beg.s. and, in inelastic scattering of 6Li [see 6Li], to the analog state 6Li*(3.56): for a discussion of these see the references quoted in (1979AJ01).
Measurements of differential cross sections at E(7Li) = 82 MeV are reported in (1992GLZX, 1993GLZZ, 1994SAZZ) and at E(7Li) = 78 MeV in (1993SA35, 1994RUZZ). The 6He levels at Ex = 0 Jπ = 0+ and Ex = 1.80 Jπ = 2+ were identified. A maximum at Ex ≈ 6 MeV is interpreted as consistent with a soft-dipole response expected in neutron-halo nuclei. A study (1996JA11, 1999AN13) at E(7Li) = 350 MeV utilized magnetic analysis to observe transitions to the Jπ = 0+ ground state, and the Jπ = 2+ state at Ex = 1.8 MeV, as well as pronounced resonances at ≈ 5.6 MeV, ≈ 14.6 MeV and ≈ 23.3 MeV (1996JA11). See 6.3 (in PDF or PS). In experiments at E = 65 MeV/A with this reaction, isovector spin-flip and spin non-flip resonances were deduced (1998NAZP, 1998NAZR). See also the more recent measurements described in (2000NA22) and (2001NA18).
A theoretical study of 6He structure with an extended microscopic three-cluster model is described in (1999AR08).
At Eγ = 60 MeV, the proton spectrum shows two prominent peaks attributed to 6He*(0 + 1.8, 18 ± 3): see (1979AJ01). Reactions (a) and (b) have been studied by (1985SE17). See also 7Li, (1984AJ01) and (1986BA2G). An analysis of the available experimental data on 7Li photodisintegration at energies up to Eγ = 50 MeV is presented in (1990VAZM, 1990VA16). See also the discussion of reactions involving scattering of polarized electrons from polarized targets (1993CA11). In more recent work a broad excited state was observed (2001BO38) in 6He with energy Ex = 5 ± 1 MeV and width Γ = 3 ± 1 MeV. In experiments with reaction (b) momentum distributions from transitions to the 6He ground and first excited states were measured by (1999LA13, 2000LA17). The deduced spectroscopic factor for both reactions is 0.58 ± 0.05 in agreement with variational Monte Carlo calculations.
The results of measurements of inclusive spectra made with π- mesons with momentum 90 MeV/c are presented in (1993AM09). The yield of one-neutron emission was found to be Y = (1.1 ± 0.2) × 10-3 per stopped π-.
Pion and proton spectra were measured at 0.7, 0.9, 1.25 GeV/c by (2000AB25). Fermi-momentum distributions were deduced.
At En = 60 MeV, the deuteron spectrum shows two prominent peaks attributed to states centered at Ex = 13.6, 15.4 and 17.7 MeV (± 0.5 MeV) and a possible state or states (populated with an lp transfer ≥ 2) at Ex = 23.7 MeV. DWBA analyses of the d0 and d1 groups are consistent with lp = 1 and S(1p3/2) = 0.62 for 6Heg.s. and to S(1p3/2) = 0.37, S(1p1/2) = 0.32 for 6He*(1.8) (1977BR17): see (1979AJ01). Measurements of the cross section as a function of energy for Ex = 10 - 30 MeV were reported in (1989CO22). See also the measurements at En = 14.1 MeV (1989SHZS).
From measurements at Ep = 1 GeV (1985BE30, 1985DO16), the separation energy between 6 - 7 MeV broad 1p3/2 and 1s1/2 peaks is reported to be 14.1 ± 0.7 MeV. See also (1983GO06) and (1979AJ01). Differential cross section measurements at Ep = 70 MeV are reported in (1988PA26, 1998SH33, 2001SH03). Contributions from 1p and 1s nucleons in 7Li were distinguished. Proton spectra measurements for Ep = 1 GeV were reported by (2000MI17, 2001MI07). Effective proton polarizations were deduced. See also the review of experimental and theoretical nucleon and cluster knockout reactions in light nuclei presented in (1987VD1A).
As summarized in the previous review (1988AJ01), angular distributions of the 3He ions to 6He*(0, 1.8) have been measured at Ed = 14.4 and 22 MeV: they have an lp = 1 character and therefore these two states have Jπ = (0 - 3)+. There is no evidence for any other states of 6He with Ex < 10.7 MeV: see (1979AJ01). (1987BO39) [Ed = 30.7 MeV] deduce that the branching ratio of 6He*(1.8) into a dineutron [n2: T = 1, S = 0] and an α-particle is 0.75 ± 0.10. See also (1985BO55) and (1987DA31). More recently, the energy spectrum of neutrons from the 6He excited state at Ex = 1.8 MeV populated in this reaction was measured at Ed = 23 MeV (1994BO46).
As summarized in (1988AJ01), the energy of the first-excited state is 1.797 ± 0.025 MeV, Γ = 113 ± 20 keV. 6He*(1.80) decays into 4He + 2n. The branching ratio Γγ/Γα ≤ 2 × 10-6: for Γcm = 113 ± 20 keV, Γγ ≤ 0.23 eV. Angular distributions of the α0 and α1 groups have been measured at Et = 13 and 22 MeV. No other α-groups are reported corresponding to 6He states with Ex < 24 MeV (region between Ex ≈ 13 and 16 MeV was obscured by the presence of breakup α-particles): see (1979AJ01). Angular distributions were reported at Et = 0.151 and 0.272 MeV (1987AB09; α0, α1) and at E(7Li) = 31 MeV (1987AL23; to 6He*(0, 1.8, 13.6)).
In more recent work, differential cross sections were measured at Et = 38 MeV (1992CL04). DWBA calculations are presented and spectroscopic factors are deduced.
The resonance theory of threshold phenomena was used to analyze differential cross sections for 7Li(t, α)6He*(1.8) for θ < 90° at Et = 80 - 500 keV in a study of 10Be levels (1991LA1D).
At E(3He) = 120 MeV the missing mass spectra show 6He*(0, 1.8) and a strong, broad peak corresponding to 6He*(16) [possibly due to unresolved states]. There is no indication of a state near 23.7 MeV but there is some evidence of structures at Ex = 32.0 and 35.7 MeV, with Γ ≤ 2 MeV (1985FR01).
In reaction (a) at E(6Li) = 93 MeV a broad peak (Γ = 5.5 MeV) was reported at Ex = 14 MeV. A second structure may also be present at 15.5 MeV (1987GLZW, 1988BUZH). 6He*(0, 1.8) are also populated (1988BUZH). For reaction (b) see 8Be in (1988AJ01). See also 7Be, (1984AJ01) and (1988BU1Q, 1984BA53), and see (1996SO17) which involves 10Be excited states. Measurements of differential cross sections at E(7Li) = 22 MeV were reported in (1988BO18).
Angular distributions have been reported for En = 12.2 to 18.0 MeV (α0, α1). No other states are observed with Ex ≤ 7 MeV: see (1979AJ01). For a study of possible dineutron breakup of 6He*(1.8) see (1983OT02). An analysis of the alpha and neutron spectra observed in this reaction for En ≈ 14 MeV is presented in (1988FE06). See also 10Be in (1988AJ01) and (1983SH1J).
Elastic scattering measurements for E(6He) = 8.8 - 9.3 MeV were reported in (1991SM01). The data are well reproduced with calculations using 6Li or 7Li optical model parameters. See also 9Be in (1988AJ01).
Differential cross sections were measured at E(6Li) = 34, 62 MeV, and spectroscopic factors were deduced (1985CO09). Vector and tensor analyzing powers were measured for detection of the 6He nuclei at θcm = 14° - 80° at E(6Li) = 32 MeV (1993RE04). See 9B in (1988AJ01).
Measurements of the energy dependence at E = 100, 190 GeV were reported by (2000HA33).
Peripheral fragmentation of 6He at 240 MeV/A was studied (1997CH24, 1997CH47, 1998AL10) in a kinematically complete experiment. It was found that one-neutron stripping to 5He is the dominant mechanism. A continuation of the anlysis described in (2000AL04) indicates excitiation of the 6He first 2+ state and associates it with E1 dipole oscillation. See also (1993FE02). Model calculations are discussed in (1998BE09, 1998GA37).
Measurements at 240 MeV/A are described in (1998AL10, 1998AN02, 1999AU01, 2000AL04). Fragmentation cross sections of 6He were analyzed in the Glauber theory to investigate the importance of neutron correlation (1994SU02). Fragmentation reaction data and beta-delayed particle emission data are reproduced successfully. Detailed structure is described with a multicluster model and halo-like structure is discussed in (1995SU13). See also (1998BE09, 1998GA37).
Elastic and quasielastic scattering of 6He on 12C was studied at E(6He) = 10.2 MeV (1995WA01). See also (1995PE1D). Measurements of cross sections were made at 41.6 MeV/A (1996AL11). The results were successfully analyzed within a 4-body (α + n + n + 12C) eikonal scattering model.
Potential parameters were deduced and differential cross sections were calculated for 6He scattering at 50 and 100 MeV/A (1993GO06). The possibility of studying the structure of the neutron halo in 6He elastic rainbow scattering is discussed. See also (1989SI02, 1992CL04, 1993FE02, 1995GA24). Calculations of cross sections at E = 20 - 60 MeV/A were reported in (2000BO45). Proton, neutron and matter r.m.s. distributions were also calculated.