
^{6}He (2002TI10)(See Energy Level Diagrams for ^{6}He) GENERAL: References to articles on general properties of ^{6}He published since the previous review (1988AJ01) are grouped into categories and listed, along with brief descriptions of each item, in the General Tables for ^{6}He located on our website at (www.tunl.duke.edu/NuclData/General_Tables/6he.shtml). See also 6.1 [Table of Energy Levels] (in PDF or PS). Ground State Properties: The interaction radius of ^{6}He, 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. ^{6}He is considered to be a neutronhalo nucleus because its interaction radius, which is deduced from the total interaction cross section in (1985TA13, 1985TA18), is appreciably larger than that of ^{6}Li. A Glauber calculation using proton and neutron densities from an alphacore valenceneutron model leads to the conclusion that the matter radius is much larger than the charge radius, as predicted by theoretical models of the ^{6}He groundstate wave function. These theoretical models include threebody models (1993ZH1J, 1995HI15), clusterorbital shell models (1991SU03, 1994FU04), nocore microscopic shell models (1996NA24), and microscopic cluster models for various effective nucleonnucleon 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 halflife is 806.7 ± 1.5 ms (1984AJ01). The decay to the ground state of ^{6}Li (J^{π} = 1^{+}) is via a superallowed GamowTeller transition; log ft = 2.910 ± 0.002 (1984AJ01, 1988AJ01). A second betadecay branch leading to an unbound final state consisting of a deuteron and an α particle was reported (1990RI01) based on the observation of betadelayed deuterons. The branching ratio for E_{d} > 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 ^{6}Li which then decays into α + d. In the other route ^{6}He breaks up into an alpha particle plus a dineutron 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 ^{6}He(β^{})^{6}Li 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 ^{6}He on hydrogen, and a value of σ_{R} = 409 ± 22 mb was obtained (2001DE19). Analysis within a microscopic model allowed the ^{6}He density distribution to be explored. The use of elastic and inelastic scattering with secondary beams to probe groundstate transition densities of halo nuclei has been explored in a theoretical study (1995BE26). Cross sections for E = 151 MeV were calculated by (2000AV02), and densitydistribution features were deduced. See also the discussion of (1999EG02).
The cross section for reaction (b) was measured for E_{t} = 30 to 115 keV by (1986BR20, 1985JA16) who also calculated the astrophysical Sfactors [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 twochannel resonating group method (1989VA20), the microscopic multichannel resonating group method (1991TY01) and the generator coordinate method (1990FU1H). For muoncatalyzed 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 ^{6}He*(0, 1.80) have been measured at E_{t} = 22 and 23 MeV. [No Lvalues were assigned.] No other states are observed with E_{x} ≤ 4.2 MeV: see (1979AJ01). Cross sections and angular distributions for the reaction products of the ^{3}H(α, p)^{6}He reaction were measured at E_{α} = 27.2 MeV (1992GO21). A potential description of ^{3}H + ^{4}He elastic scattering is discussed in (1993DU09).
Total cross sections for the production of ^{6}He have been measured (2001AU06) at E_{α} = 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(^{6}He) = 151 MeV. DWBA analysis suggests a spectroscopic factor of ≈ 1 for the dineutron cluster. (1998TE1D, 1998TE03). Measurements at E_{cm} = 11.6 and 15.9 MeV (1999RA15) also show evidence for the 2n transfer process in the elastic scattering. However, a couplediscretizedcontinuum 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 ^{6}He + ^{1}H system. Calculations of the elastic scattering of protons from ^{6}He at E_{p} ≥ 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 ^{6}He matter radius constant with the analysis is 2.51 fm. Finiterange coupled channel calculations have been performed below the ^{6}He threebody 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 ^{6}He 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 E_{cm} = 0  5 MeV is reported in (2001AR05).
(1986SH14) report breaks in (e, π^{+}) spectra at E_{e} = 202 MeV corresponding to E_{x} = 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 spindipole isovector states [J^{π} = 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 ^{6}Be].
The excitation of ^{6}He*(0, 1.8) and possibly of (broad) states at E_{x} = 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 ^{6}He*(0, 1.8) was reported in (1986PE05). For reaction (b) see (1984AJ01).
Angular distributions of the ground state proton group, p_{0} have been reported at E_{n} = 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 E_{n} = 59.6 MeV broad structures in the spectra are ascribed to states at E_{x} = 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 E_{n} = 198 MeV (1988JA01). Proton spectra were measured at E_{n} = 118 MeV by (1998HA24). An angular distribution of the proton group corresponding to population of the E_{x} = 1.8 MeV J^{π} = 2^{+} state in ^{6}He was also reported (1988WA24). See also (1989WA1F). Angular distributions were measured for p_{0} at E_{n} = 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 θ_{lab} = 1°  10° for E_{n} = 60  260 MeV were measured to obtain the energy dependence of the GamowTeller strength (1991SOZZ, 1992SO02). Several theoretical studies have been reported since the previous review. A dynamical multicluster model was used to generate transition densities for ^{6}He and ^{6}Li (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 ^{6}He continuum were made with a α + N + N cluster model (1997DA01). Halo excitation of ^{6}He in ^{6}Li(n, p)^{6}He was studied using fourbody distorted wave theory (1997ER05); see also (1997VA06). The status of experimental and theoretical research on nuclei featuring a twoparticle halo is reviewed in (1996DA31).
The previous review (1988AJ01) notes that at E_{d} = 55 MeV, ^{6}He*(0, 1.8) [the latter weak] are populated: no other states are observed with E_{x} ≤ 25 MeV [see (1984AJ01)]. More recently cross sections at 0° were measured at E_{d} = 260 MeV (1993OH01) and at E_{d} = 125.2 MeV (1995XU02). In both studies the cross section for (d, ^{2}He) showed a linear relationship with GamowTeller strength from β decay or (p, n) reactions.
The groundstate angular distribution has been studied at E_{t} = 17 MeV. At E_{t} = 22 MeV only ^{6}He*(0, 1.8) are populated for E_{x} ≤ 8.5 MeV: see (1979AJ01). Differential cross sections for the transition to ^{6}He*(1.8) are reported at E(^{6}Li) = 65 MeV (1987AL23). In a more recent experiment at E_{t} = 336 MeV reported in (2000NA35), the ^{6}He ground and 1.8 MeV states were populated. In addition, a broad asymmetric structure around E_{x} ≈ 5 MeV was observed with an angular distribution which exhibited ΔL = 1 dominance. Another structure at E_{x} ≈ 14.6 MeV was observed with the angular distribution indicating ΔL = 1.
Angular distributions have been studied for E(^{6}Li) = 32 and 36 MeV for the transitions to ^{6}He_{g.s.}, ^{6}Be_{g.s.} and, in inelastic scattering of ^{6}Li [see ^{6}Li], to the analog state ^{6}Li*(3.56): for a discussion of these see the references quoted in (1979AJ01).
Measurements of differential cross sections at E(^{7}Li) = 82 MeV are reported in (1992GLZX, 1993GLZZ, 1994SAZZ) and at E(^{7}Li) = 78 MeV in (1993SA35, 1994RUZZ). The ^{6}He levels at E_{x} = 0 J^{π} = 0^{+} and E_{x} = 1.80 J^{π} = 2^{+} were identified. A maximum at E_{x} ≈ 6 MeV is interpreted as consistent with a softdipole response expected in neutronhalo nuclei. A study (1996JA11, 1999AN13) at E(^{7}Li) = 350 MeV utilized magnetic analysis to observe transitions to the J^{π} = 0^{+} ground state, and the J^{π} = 2^{+} state at E_{x} = 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 spinflip and spin nonflip resonances were deduced (1998NAZP, 1998NAZR). See also the more recent measurements described in (2000NA22) and (2001NA18). A theoretical study of ^{6}He structure with an extended microscopic threecluster model is described in (1999AR08).
At E_{γ} = 60 MeV, the proton spectrum shows two prominent peaks attributed to ^{6}He*(0 + 1.8, 18 ± 3): see (1979AJ01). Reactions (a) and (b) have been studied by (1985SE17). See also ^{7}Li, (1984AJ01) and (1986BA2G). An analysis of the available experimental data on ^{7}Li photodisintegration at energies up to E_{γ} = 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 ^{6}He with energy E_{x} = 5 ± 1 MeV and width Γ = 3 ± 1 MeV. In experiments with reaction (b) momentum distributions from transitions to the ^{6}He 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 oneneutron 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). Fermimomentum distributions were deduced.
At E_{n} = 60 MeV, the deuteron spectrum shows two prominent peaks attributed to states centered at E_{x} = 13.6, 15.4 and 17.7 MeV (± 0.5 MeV) and a possible state or states (populated with an l_{p} transfer ≥ 2) at E_{x} = 23.7 MeV. DWBA analyses of the d_{0} and d_{1} groups are consistent with l_{p} = 1 and S(1p_{3/2}) = 0.62 for ^{6}He_{g.s.} and to S(1_{p}_{3/2}) = 0.37, S(1_{p}_{1/2}) = 0.32 for ^{6}He*(1.8) (1977BR17): see (1979AJ01). Measurements of the cross section as a function of energy for E_{x} = 10  30 MeV were reported in (1989CO22). See also the measurements at E_{n} = 14.1 MeV (1989SHZS).
From measurements at E_{p} = 1 GeV (1985BE30, 1985DO16), the separation energy between 6  7 MeV broad 1p_{3/2} and 1s_{1/2} peaks is reported to be 14.1 ± 0.7 MeV. See also (1983GO06) and (1979AJ01). Differential cross section measurements at E_{p} = 70 MeV are reported in (1988PA26, 1998SH33, 2001SH03). Contributions from 1p and 1s nucleons in ^{7}Li were distinguished. Proton spectra measurements for E_{p} = 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 ^{3}He ions to ^{6}He*(0, 1.8) have been measured at E_{d} = 14.4 and 22 MeV: they have an l_{p} = 1 character and therefore these two states have J^{π} = (0  3)^{+}. There is no evidence for any other states of ^{6}He with E_{x} < 10.7 MeV: see (1979AJ01). (1987BO39) [E_{d} = 30.7 MeV] deduce that the branching ratio of ^{6}He*(1.8) into a dineutron [n^{2}: T = 1, S = 0] and an αparticle is 0.75 ± 0.10. See also (1985BO55) and (1987DA31). More recently, the energy spectrum of neutrons from the ^{6}He excited state at E_{x} = 1.8 MeV populated in this reaction was measured at E_{d} = 23 MeV (1994BO46).
As summarized in (1988AJ01), the energy of the firstexcited state is 1.797 ± 0.025 MeV, Γ = 113 ± 20 keV. ^{6}He*(1.80) decays into ^{4}He + 2n. The branching ratio Γ_{γ}/Γ_{α} ≤ 2 × 10^{6}: for Γ_{cm} = 113 ± 20 keV, Γ_{γ} ≤ 0.23 eV. Angular distributions of the α_{0} and α_{1} groups have been measured at E_{t} = 13 and 22 MeV. No other αgroups are reported corresponding to ^{6}He states with E_{x} < 24 MeV (region between E_{x} ≈ 13 and 16 MeV was obscured by the presence of breakup αparticles): see (1979AJ01). Angular distributions were reported at E_{t} = 0.151 and 0.272 MeV (1987AB09; α_{0}, α_{1}) and at E(^{7}Li) = 31 MeV (1987AL23; to ^{6}He*(0, 1.8, 13.6)). In more recent work, differential cross sections were measured at E_{t} = 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 ^{7}Li(t, α)^{6}He*(1.8) for θ < 90° at E_{t} = 80  500 keV in a study of ^{10}Be levels (1991LA1D).
At E(^{3}He) = 120 MeV the missing mass spectra show ^{6}He*(0, 1.8) and a strong, broad peak corresponding to ^{6}He*(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 E_{x} = 32.0 and 35.7 MeV, with Γ ≤ 2 MeV (1985FR01).
In reaction (a) at E(^{6}Li) = 93 MeV a broad peak (Γ = 5.5 MeV) was reported at E_{x} = 14 MeV. A second structure may also be present at 15.5 MeV (1987GLZW, 1988BUZH). ^{6}He*(0, 1.8) are also populated (1988BUZH). For reaction (b) see ^{8}Be in (1988AJ01). See also ^{7}Be, (1984AJ01) and (1988BU1Q, 1984BA53), and see (1996SO17) which involves ^{10}Be excited states. Measurements of differential cross sections at E(^{7}Li) = 22 MeV were reported in (1988BO18).
Measurements of groundstate cross sections and angular distributions are reported in (1999SH05). See (1999ZHZN) for a compilation and evaluation of cross section data for E_{α} ≤ 130 MeV.
Angular distributions have been reported for E_{n} = 12.2 to 18.0 MeV (α_{0}, α_{1}). No other states are observed with E_{x} ≤ 7 MeV: see (1979AJ01). For a study of possible dineutron breakup of ^{6}He*(1.8) see (1983OT02). An analysis of the alpha and neutron spectra observed in this reaction for E_{n} ≈ 14 MeV is presented in (1988FE06). See also ^{10}Be in (1988AJ01) and (1983SH1J).
Elastic scattering measurements for E(^{6}He) = 8.8  9.3 MeV were reported in (1991SM01). The data are well reproduced with calculations using ^{6}Li or ^{7}Li optical model parameters. See also ^{9}Be in (1988AJ01).
Differential cross sections were measured at E(^{6}Li) = 34, 62 MeV, and spectroscopic factors were deduced (1985CO09). Vector and tensor analyzing powers were measured for detection of the ^{6}He nuclei at θ_{cm} = 14°  80° at E(^{6}Li) = 32 MeV (1993RE04). See ^{9}B in (1988AJ01).
This reaction has been used as a source of ^{6}He beams for elastic scattering experiments at E(^{6}He) = 8.8  9.3 MeV (1991SM01) and at E(^{6}He) = 10.2 MeV (1995WA01).
Angular distributions were measured at E(^{9}Be) = 40 MeV (1992CO05). See ^{9}Be in (1988AJ01) and ^{12}C in (1990AJ01).
At E(^{11}B) = 88 MeV the population of the ground state and the firstexcited state at E_{x} = 1.8 ± 0.3 MeV (Γ ≤ 0.2 MeV) is reported (1987BEYI). See also (1988BEYJ).
Measurements of the energy dependence at E = 100, 190 GeV were reported by (2000HA33).
Peripheral fragmentation of ^{6}He at 240 MeV/A was studied (1997CH24, 1997CH47, 1998AL10) in a kinematically complete experiment. It was found that oneneutron stripping to ^{5}He is the dominant mechanism. A continuation of the anlysis described in (2000AL04) indicates excitiation of the ^{6}He 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 ^{6}He were analyzed in the Glauber theory to investigate the importance of neutron correlation (1994SU02). Fragmentation reaction data and betadelayed particle emission data are reproduced successfully. Detailed structure is described with a multicluster model and halolike structure is discussed in (1995SU13). See also (1998BE09, 1998GA37).
Elastic and quasielastic scattering of ^{6}He on ^{12}C was studied at E(^{6}He) = 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 4body (α + n + n + ^{12}C) eikonal scattering model. Potential parameters were deduced and differential cross sections were calculated for ^{6}He scattering at 50 and 100 MeV/A (1993GO06). The possibility of studying the structure of the neutron halo in ^{6}He 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.
Measurements and analyses of a threebody breakup experiment at 240 MeV/A are described in (1999AU01, 2000AL04). Twoneutron interferometry measurements at 50 MeV/A are discussed in (2000MA12).
