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5He (1966LA04)

(See Energy Level Diagrams for 5He)

GENERAL: See Table 5.1 [Table of Energy Levels] (in PDF or PS).

See also (1959BA1D, 1959BR1E, 1959MI1C, 1959SA11, 1960PE14, 1960PH1A, 1961BA1E, 1961TA05, 1962DI1B, 1962IN02, 1962IN1A, 1963KU1B, 1964BA1Y, 1964BE1M, 1964GR1J, 1964SA1F, 1964ST1B, 1965BO1M).

1. 3H(d, γ)5He Qm = 16.632

The ratio of the yields of (16.7 MeV) γ-rays and neutrons is ≈ 2 × 10-5 at 0° and 90°, Ed = 0.47 MeV. The yield is of the same order of magnitude as that for the mirror reaction 3He(d, γ)5Li (1959CO57). [This yield would appear to lead to a value of Γγ considerably lower than the Γγ = 11 eV reported for the mirror reaction: see (1955SA52)].

2. (a) 3H(d, n)4He Qm = 17.590 Eb = 16.632
(b) 3H(d, 2n)3He Qm = -2.988
(c) 3H(d, pn)3H Qm = -2.225

Excitation curves and angular distributions for reaction (a) from Ed = 8 keV to 19 MeV are summarized by (1956FO1A, 1957JA37, 1960BR1E, 1960BR1F, 1960ST25, 1961GO02, 1964BR1P). See also (1964PA24). Below Ed = 100 keV, the cross section follows the Gamow function, σ = (A/E)exp(-44.40E-1/2) (1953JA1A, 1954AR02). A strong resonance, σ(peak) = 5.0 b, appears at Ed = 107 keV. There is some evidence of resonant behavior between Ed = 3 and 9 MeV (1960ST25: see also (1956GA51, 1957BA21)).

In the region Ed = 10 to 500 keV, the cross section is closely fitted with the assumption of s-wave formation of a J = 3/2+ state with the parameters given in Table 5.3 (in PDF or PS) (1952AR30, 1952CO35, 1955KU03). Analysis in terms of complex eigenvalue theory is discussed by (1963MA1N, 1964JE1B).

The angular distribution of neutrons is isotropic at and below resonance, and shows increasing forward peaking at higher energies (1957JA37). (1961GO02) report that the distributions in the range Ed = 6.2 to 11.4 MeV are all peaked forward with a second maximum at about 65° which becomes more pronounced with increasing energy, and a rise at back angles. It does not appear that plane wave stripping theory, including heavy-particle stripping can account for the observed distributions (1961GO02: see also (1960ST25)). See also (1951BU1B).

A test of parity violation using polarized 150 keV deuterons yields |Re F| < 4.5 × 10-3 (1964HE1G).

The polarization of neutrons has been studied at Ed = 0.1 and 0.17 MeV by (1961RU1A, 1962SE09), at 0.6 and 1.2 MeV by (1965BO13), from 0.1 to 7.7 MeV by (1961PE13, 1964PE14), from 6 to 11 MeV by (1964WA22), from 8 to 20 MeV by (1964AL1E) and at 10 MeV by (1961TR05). See also (1959GO1G, 1960WI1B, 1963HA1G).

For Ed > 3.7 MeV, deuteron breakup (reaction (c)) is possible, and above Ed = 5.0 MeV production of 3He (reaction (b)) may occur. Time-of-flight neutron spectra observed for Ed = 5 to 12 MeV exhibit two maxima, the lower corresponding to the three-body distribution arising from 3H(d, 2n)3He and the upper to 3H(d, n)4He* → p + 3H with formation of an excited state of 4He at 20.1 ± 0.06 MeV (1962LE12, 1962PO04, 1963PO02: see also (1959SM97)). Analysis of the observed distributions by stripping theory including final state interaction in the (t + p) channel indicates a 1S0 resonance at Ec.m. = 0.4 MeV, γ2p = 4.2 MeV, R = 3.0 fm. The assignment T = 1 would imply a bound 4H (1962WE1E, 1963WE10, 1964WE1B). The apparent non-existence of a stable 4H or of a corresponding 4Li state argues for T = 0 (1963KA28, 1964NE02): see also 3H(d, p)4H.

See also (1960BR10, 1960JU04, 1961DI1B, 1963RU1A, 1964TR1C).

3. 3H(d, p)4H

If 4H is particle-stable, its mass excess lies in the range 23.0 to 18.5 MeV; Qm for this reaction is -2.2 to +2.3 MeV. Over this range, the cross section for formation of 4H is < 4 × 10-3 of the corresponding cross section in 3H(d, n)4He*(20.1) (1951MC37, 1960ST25, 1964IM03, 1964RO08). There have also been attempts to observe 4H in the bombardment of 6Li by 160 MeV protons (1965BE1P), of Li by 250 MeV bremsstrahlung (1964NE02), of 7Li by 14 MeV neutrons (1964PO03, 1964PO1B), of 12C by 300 MeV protons (1955RE44) and in the 10B(7Li, 4H)13N reaction (1959NO40): all these attempts have given negative results. Some evidence is reported by (1962AR05, 1963AR06) for the formation of 4H in the 4He(γ, π+)4H with 1 GeV bremmstrahlung: the atomic mass excess of 26.5 to 30 MeV would correspond to Ex = 24.1 to 27.6 MeV in 4He. See, however, (1963LO1C, 1964SM1B, 1964VO1D). Evidence for a bound 4H is reported in π- + 6Li and 7Li by (1965CO1D). See also (1964CA05, 1964GO1B, 1964GO25, 1964WE1B, 1965AJ03).

4. 3H(d, d)3H Eb = 16.632

Differential cross sections are tabulated for Ed = 0.96 to 3.2 MeV (1952ST69), Ed = 5.6, 5.9, 8.3, 12.3 and 14.4 MeV (1960BR10) and Ed = 10 MeV (1952AL36). The angular distributions at the higher energies are characterized by minima on either side of θ = 90°; the central maximum moves towards higher angles as the energy is increased. Distributions are closely similar to 3He(d, d)3He (1960BR10).

Elastic scattering at θ = 90°, observed from Ec.m. = 100 to 260 keV can be closely fitted by the one-level Breit-Wigner formula (1958BA82, 1960BA1M). See also (1960LA1B).

5. 3H(t, n)5He Qm = 10.374

This reaction has been studied for Et = 0.95 to 2.10 MeV (1958JA06: see also (1957BA10)). In addition to the neutron group corresponding to 5Heg.s., the spectrum contains an excess of medium energy neutrons, attributed to direct three-body reaction or to a broad excited state of 5He. The alpha particles show a double peaking, reflecting the influence of the P3/2 ground state, superimposed on a distribution arising from the P1/2 state and direct three-body decay (1958JA06). See also 6He.

6. 3He(t, p)5He Qm = 11.138

Proton and alpha-particle spectra have been studied at Et = 1.9 MeV by (1963SM03). The protons exhibit a conspicuous peak at the high-energy end of a continuous distribution: the peak corresponds to the ground state of 5He with a binding energy of -0.79 ± 0.03 MeV and has a width Γ(lab) = 525 ± 30 keV; see Table 5.2 (in PDF or PS). In the α-distribution a sharp peak is seen, corresponding to 3He(t, d)4He, plus a broad distribution with considerable structure. A knee at the high-energy limit is ascribed to 3He(t, α)n + p with the neutron and proton interacting in the 1S0 state; a binding energy of -0.1 ± 0.05 MeV is obtained, in good agreement with the value -0.074 deduced from n - p scattering. The structure observed in the proton and alpha continua is quantitatively accounted for by appropriate superposition of the processes 3He(t, p)5Heg.s. → α + n, 3He(t, n)5Lig.s. → α + p, and 3He(t, p + α + n) in the ratios σ = 1.37/0.90/2.40. The experiment does not distinguish the direct three-body process from those involving 5He*, 5Li*(2P1/2) (1963SM03). See also (1953AL1A, 1953MO61, 1960YO06, 1961BA40).

7. 4He(n, n)4He Eb = -0.958

The coherent scattering length (thermal, bound) is 3.0 fm (1961WI1A). The thermal scattering cross section is 0.73 ± 0.05 b (1964ST25). Total cross sections for En = 0.0004 eV to 20 MeV are given in (1958HU18, 1960HU08, 1964ST25): recent measurements have been made at En = 0.1 to 6.2 MeV and 12 to 20 MeV (1960VA04), En = 1 to 20 MeV (1959BA48: ± 1% to ± 5%), En = 7 to 12 MeV (1960FO09, 1962AU03: ± 3%), En = 10 to 25 MeV (1965HO1D), En = 20 to 29 MeV (1963SH06, 1964SH30: ± 1% to ± 2%), and En ≈ 147 MeV (1964PA19).

The total cross section has a peak of 7.8 b (1960VA04) at En = 1.15 ± 0.05 MeV, Ec.m. = 0.92 ± 0.04 MeV, with a width of about 1.2 MeV (1964ST25). A second resonance is observed at En = 22.15 ± 0.12 MeV, corresponding to the 16.7 MeV J = 3/2+ level (1959BO54, 1964SH30): Γc.m. = 100 ± 50 keV, Γn = Γd = 50 ± 35 keV (1960HU08). The change in cross section and in the angular distributions at resonance is consistent wih J = 3/2, Γd ≈ Γn (1964SH30). Attempts to detect additional resonances at En = 5.5 to 16 MeV (1959BA02) and at En = 20 to 29 MeV (1964SH30), 22 to 29 MeV (1965BE03) have been unsuccessful. If the "19 MeV" excited state of 5He exists, the change in total cross section is less than a few percent if its width is greater than about 100 keV; it is pointed out that a T = 3/2 level would be isospin forbidden in the present reaction (1964SH30).

Information on angular distributions is summarized in (1963GO1M). Recent measurements of differential cross sections are reported for En = 2.0 to 20.9 MeV (1962AU03), En = 1.79 MeV (1963YO05), En = 2.37 to 2.87 MeV (1962DE01), En = 6.4 to 6.9 MeV (1959MA1E), En = 14.9 MeV (1963MA1M) and En = 16 to 26 MeV (1964SH30). Both the total cross sections and the angular distributions are well accounted for, below 15 MeV, by the phase shifts determined by (1952DO30, 1953SE29) for 4He(p, p)4He with a shift in Eλ of about 1 MeV (DGS phase shifts). The s-wave phase shift decreases monotonically with increasing energy, and can be accounted for by hard-sphere scattering with R = 2.6 fm. The P3/2 shift shows strong resonance behavior near 1 MeV, while the P1/2 shift changes more slowly, possibly indicating a broad P1/2 level at several MeV excitation (1952DO30). For En = 16 to 22 MeV, the GTP phase shifts involving d- and f-waves (1958GA13) are preferred (1963MA29, 1964SH30, 1964SH1E: see, however, (1962AU03)). See also (1965RO1Q). At En = 23.7 MeV, the angular distribution determined from phase shifts that are consistent with polarization and cross section data of (1963MA29) does not agree with the experimental curve (1964SH30).

Polarization of neutrons scattered by 4He has been discussed by (1952AD09, 1953SE29, 1953SI1A, 1960SA07), and asymmetry in elastic scattering using partially polarized neutrons has been studied for En = 2 to 24 MeV by (1961TR05, 1963MA29, 1963OT01, 1964PE14, 1964WA22). See also (1963HA1G, 1965DA1F).

Theoretical discussions of n, α scattering are given by (1958HO1B, 1959NA1A, 1959PI42, 1959SA1D, 1960BU1F, 1960MC1D, 1960MI1B, 1960NA1B, 1960SA1L, 1960SI1C, 1961TA1E, 1962LA1E, 1962MI1B, 1963FA1A, 1963PI03, 1964CR1B).

8. 4He(n, d)3H Qm = -17.590 Eb = -0.958

See (1960YO06, 1962AU03, 1964SH30).

9. (a) 4He(d, p)5He Qm = -3.182
(b) 4He(d, pn)4He Qm = -2.225

The proton spectrum observed at Ed = 14.8 MeV shows a prominent peak, of width Γc.m. = 550 ± 30 keV, and a monotonic continuum of lower energy protons, attributed to reaction (b). There is no evidence of structure corresponding to possible sharp excited states of 5He (1956WA1B, 1957WA01: see also (1960AR1A)). At Ed = 8 and 14 MeV, θ = 21°, the proton peak is well fitted with DGS phase shifts for the α - n final state interaction (1964RO1D). The ground state group, analyzed by stripping theory, gives θ2 = 0.05, more than a factor of 10 smaller than is indicated by 4He(n, n)4He (see 5Li: 4He(p, p)4He) (1956WA1B, 1957WA01). At Ed = 7.7 to 11 MeV, proton spectra indicate Q = 850 keV for 5He → 4He + n. Correction to the energy corresponding to an (n, α) phase shift of 90° gives Q = 930 ± 70 keV, Γ = 570 ± 20 keV. Similar treatment of (n, α) data yields Q = 900 ± 40 keV (1964OH01): see Table 5.2 (in PDF or PS).

Neutron time-of-flight spectra have been obtained at Ed = 7.9, 8.9 and 10 MeV (1962LE12) and 18.6 MeV (1961RY01). Two maxima are observed in the distribution, attributed to 4He(d, n)5Lig.s. and 4He(d, p)5Heg.s. → α + n (1962LE12). The peak shapes have been analyzed in terms of α + n, α + p final state interactions by (1961RY01). See also (1963ER02, 1965IS1D, 1965NA1D, 1965NA1E).

10. 4He(t, d)5He Qm = -7.215

Not reported.

11. 6Li(γ, p)5He Qm = -4.655

See 6Li.

12. 6Li(n, d)5He Qm = -2.430

At En = 14 MeV, a well-defined ground-state group (Γc.m. = 0.8 MeV) is observed, as is a continuum extending to Ex ≈ 4 MeV in 5He. Both angular distributions are consistent with lp = 1 (1954FR03). See also 7Li, and (1964SL1A, 1964TO1C).

13. 6Li(p, 2p)5He Qm = -4.655

At Ep = 155 to 450 MeV, the summed proton spectra show two peaks, with Q = -4.5 ± 1.5 and -20.3 ± 1.5 MeV (1962GA09), -4.8 ± 0.3 and -22.4 ± 0.7 MeV (1964TI02), -4.9 ± 0.3 and -22.7 ± 0.3 MeV (1965TY1A). The higher energy peak corresponds to ejection of an l = 1 proton: 6Li → 5Heg.s. + p, while the lower peak results from ejection of an l = 0 proton, presumably leaving 5He in the 16.7 MeV, 3/2+ state. See 6Li.

14. 6Li(d, 3He)5He Qm = 0.839

At Ed = 14.5 MeV, the ground state group is observed: Γc.m. = 0.69 ± 0.2 MeV (1955LE24). The 3He spectrum has been measured at several angles at Ed = 14.8 MeV; the shape is analyzed in terms of an (n + α) model of 5He, using the known (n - α) phase shifts. The angular distribution of ground state neutrons fits the Butler formula at forward angles with l = 1, R = 6 fm; a value θ20 = 0.15 is obtained (1960HA14). See also (1959HA29).

15. 6Li(t, α)5He Qm = 15.160

The width of the ground state Γc.m. = 0.7 ± 0.2 MeV (1956CR47). See also (1961HO01) and (1959AJ76).

16. 7Li(n, t)5He Qm = -3.425

See (1954AL24, 1964SL1A, 1964VA19, 1964VA1E) and 8Li.

17. (a) 7Li(p, 3He)5He Qm = -4.189
(b) 7Li(p, pd)5He Qm = -9.683

These reactions have not been reported. For reaction (b) see (1964BA1C).

18. 7Li(d, α)5He Qm = 14.164

The angular correlation of ground-state α-particles and those resulting from the breakup of 5He is consistent with Jπ = 3/2- (1951FR1A, 1956RI37) as is the (α - n) correlation (1957FA10). See also (1964BR31).

High resolution spectra (Ed = 1.0 MeV) show only the ground state peak, superposed on a continuous distribution (1958WE27). The ground state has a width of 0.66 ± 0.2 MeV (1955LE24). At Ed = 0.15 to 0.20 MeV, α - α coincidence studies indicate a group corresponding to Ex = 2.6 ± 0.4 MeV, Γ = 4.0 ± 1.0 MeV with intensity 50% greater than the ground state group. No other excited states with Ex < 7 MeV are seen (1964FE01); see also (1960HA09, 1964JO1D, 1964MA60, 1964SA1G, 1965BI1F, 1965IM01, 1965JO19).

19. 9Be(γ, α)5He Qm = -2.528

See 9Be.