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5He (1959AJ76)

(See the Energy Level Diagram for 5He)

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

1. 3H(d, γ)5He Qm = 16.629

At Ed = 160 keV, the capture cross section is less than 0.5 mb. This limit is not inconsistent with Γγ ≈ 11 eV as estimated from the mirror reaction 3He(d, γ)5Li (1955SA52).

2. (a) 3H(d, n)4He Qm = 17.586 Eb = 16.629
(b) 3H(d, 2n)3He Qm = -2.991
(c) 3H(d, pn)3H Qm = -2.226
Q0 = 17.580 ± 0.025 (1957MA1C).

Excitation curves and angular distributions for reaction (a) from Ed = 8 keV to 10 MeV are summarized by (1956FO1A, 1957JA37). Additional data are given for Ed = 0.04 to 0.73 MeV by (1957BA1F, 1957BA1G), for Ed = 1.0 to 5.8 MeV by (1956GA51) and for Ed = 0.25 to 7.0 MeV by (1957BA21). 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. A precision measurement (± 5%) of the cross section at Et = 1.50 MeV gives 20.0 and 19.6 mb/sr (c.m.) at 30° and 60° (lab), respectively (1958AL05). There is some question as to whether a broad maximum exists between 4 and 8 MeV (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 5.2 (in PDF or PS). For a given interaction radius, two sets of parameters are obtained, depending upon whether Γnd is assumed > 1 or < 1 (1952AR30, 1952CO35). Agreement with the mirror reaction, 3He(d, p)4He, is obtained with the second choice (1955KU03). The fact that the proton width is relatively small suggests that this level arises from excitation of the 4He core. See also (1951FL1A, 1954JO1C, 1955JO1A, 1957DA1B, 1958LE1A) and (1955HA1B; theor.).

The angular distribution of neutrons is isotropic at and below resonance, and shows increasing forward peaking at higher energies (1957JA37). Angular distributions at Ed = 6 MeV are almost identical to those of 3H(d, p)4H (1957BR23). At Ed = 10 MeV, the distribution is dominated by the stripping process, with lp = 0 (1951BU1B). Again, close correspondence is found with the mirror reaction. See also (1955HE89, 1956BA1D, 1958LE1A, 1958PA09).

At Ed = 12 to 14 MeV, reactions (b) and (c) are observed (1956BO44, 1958BR14). The 3He distributions from (b) show no evidence for a bound dineutron or for a well-defined virtual state, although some interaction between the neutrons does appear to occur. Absolute differential cross sections are reported (1958BR14).

For Ed > 3.71 MeV, deuteron breakup (reaction (c)) is energetically possible. The cross section for this process has been studied for Ed = 3.8 to 6.0 MeV by (1955HE90). At Ed = 14 MeV, the number of low-energy (4 to 10 MeV) neutrons is about three times as large as observed in the corresponding reaction 3He(d, pn)3He. The difference may indicate formation of a T = 0, 22-MeV excited state of 4He via 3H(d, n)4He* (1956BO1F, 1956BO43, 1956BO44: see, however, (1958BR14)). See also (1954CO1B, 1955BA1G, 1955BE1B, 1956FO1A, 1957BL1A) and (1955LA1B, 1956BL1A, 1958PO1A; theor.).

3. 3H(d, p)4H

Assuming the atomic mass excess of 4H = 26 MeV (if first T = 1 state in 4He has Ex = 22 MeV), Qm for this reaction would be -4 MeV. This reaction has not been observed (1951MC37). An attempt has also been made by (1955RE44) to observe the β-decay of 4H formed in the 300-MeV proton bombardment of 12C. The results are negative: if τ1/2 = (2 to 4) × 10-3 sec, σ < 1 μb; if τ1/2 = (4 to 10) × 10-3 sec, σ < 10 μb. See also (1957NO17: 10B(7Li, 4H)13N).

4. 3H(d, d)3H Eb = 16.629

Differential cross sections for Ed = 0.96 to 3.2 MeV are tabulated by (1952ST69) and for Ed = 10 MeV by (1952AL36); see also (1957BR23, 1957JA37). The distributions are closely similar to those for 3He(d, d)3He. See also (1958BA82).

5. 3H(t, n)5He Qm = 10.371

The ground state of 5He has been observed at Et = 1.48 MeV (1957BA10) and 1.9 MeV (1958JA06). The neutron spectrum contains an excess of medium-energy neutrons, attributed to direct three-body reaction or to a broad excited state of 5He. An earlier reported peak corresponding to a 2.6-MeV excited state (1951LE1A) is not confirmed (1957BA10). The alphas show a double peaking, reflecting the influence of the P3/2 ground state, superposed on a distribution arising from the P1/2 state and direct three-body disintegration (1958JA06); see 6He.

6. 3H(3He, p)5He Qm = 11.136
Q0 = 11.18 ± 0.07 (1953AL1A);
Q0 = 11.13 ± 0.07 (1953MO61).

The spectrum shows a well-defined proton peak corresponding to the ground state, superposed on a background attributed to the three-body breakup (1953AL1A, 1953MO61). See also (1954GO18).

7. 3H(α, d)5He Qm = -7.215

Not observed.

8. 4He(n, n)4He Eb = -0.957

Total cross sections for En = 0.0004 eV to 20 MeV are given in (1955HU1B, 1957HU1D, 1958HU18); angular distributions of (1952AD09) and (1953SE29) are given in (1956HU1A): additional data for En = 2.6 to 4.1 MeV are given by (1957ST1B), for En = 14 MeV by (1954SM97, 1955SH1D) and at En = 15.7 MeV by (1954AL28). The current experimental and theoretical situation is surveyed by (1958HO1B). The total cross section has a peak of 7.4 b at En = 1.15 ± 0.05 MeV, Ec.m. = 0.95, with a width of ≈ 1.7 MeV (1951BA85), Γc.m. = 1.4 ± 0.2 MeV (1958HU18). The thermal cross section is 0.78 b (1951HI1A), 0.71 ± 0.1 b (1951MC63), 0.74 ± 0.04 b (1955SO56).

Both the total cross sections and the angular distributions are well accounted for by the phase shifts determined by (1949CR1A, 1952DO30) for 4He(p, p)4He with a shift in Eλ of about 1 MeV: see also (1955CL1A; theor.). Earlier discrepancies in the range 3 to 4 MeV (1952HU1A) appear to have been resolved by (1957ST1B). In a polarization measurement, (1957LE1B, 1957LE1C) find δ(P1/2) = 12 ± 1° at En = 2.45 MeV, in disagreement with the value δ = 20° derived from 4He(p, p)4He, but agreeing with a low cross section point at En = 2.61 MeV reported by (1953SE29). The s-wave phase shift decreases monotonically with increasing energy, and can be accounted for by hard-sphere scattering, with R = 2.9 × 10-13 cm (1952AD09, 1952DO30: see, however, (1954BR1B, 1954HO1B, 1955HO1C, 1956VA1C)). 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). At En = 15.7 MeV the angular distribution is best accounted for with δ(D3/2) = -14°, δ(D5/2) = -7°, the latter being somewhat less than the hard-sphere value, suggesting a higher resonance (1954AL28). Theory: see 5Li. (For present purpose, the ground state of 5He is assumed to correspond to the maximum in the total cross section; Ec.m. = 0.95 MeV).

A resonance is reported at En = 22.15 ± 0.13 MeV, Γ ≤ 120 keV, corresponding to the 16.7-MeV level (see 3H(d, n)4He) (1957BO14). See also (1957IN1A).

Polarization of neutrons scattered in He has been discussed by (1952AD09, 1953SE29, 1953SI1A, 1955LE1D, 1956LE1B, 1957LE1B, 1957LE1C, 1957WH1A, 1958PA09) and others.

9. (a) 4He(d, p)5He Qm = -3.184
(b) 4He(d, pn)4He Qm = -2.226
Q = -3.10 ± 0.05 (1954FR22) for reaction (a).

The proton spectrum observed at Ed = 14.8 MeV, θ = 19.5°, 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. Cross sections for the ground-state group are dσ/dΩ = 25 ± 5 mb/sr at θc.m. = 18° and 15 ± 4 mb/sr at θc.m. = 24°. The dimensionless reduced width of the ground-state group, analyzed by stripping theory is θ2 = 0.05, more than a factor of 10 smaller than is indicated by 4He(n, n)4He (see 4He(p, p)4He in 5Li) (1956WA1B, 1957WA01). For reaction (b), see 6Li.

10. 6Li(γ, p)5He Qm = -4.655

The threshold is Eγ = 4.64 ± 0.08 MeV (1958RY77); see 6Li.

11. 6Li(n, d)5He Qm = -2.428

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. Angular distributions of the ground-state group and of the continuum indicate lp = 1 for the ground state transition and are not inconsistent with lp = 1 for the continuum (1954FR03): see 6Li and reaction 8 in 7Li.

12. 6Li(d, 3He)5He Qm = 0.839

At Ed = 14.5 MeV, the ground-state group is observed: Q0 = 0.91 ± 0.09 MeV, Γc.m. = 0.69 ± 0.2 MeV (1955LE24).

13. 6Li(t, α)5He Qm = 15.158
Q0 = 15.15 ± 0.04 (1956CR47).

The width of the ground state Γc.m. = 0.7 ± 0.2 MeV (1956CR47). See also (1954AL35, 1956BA1E, 1956MA09).

14. 7Li(n, t)5He Qm = -3.423

The angular distribution exhibits a forward maximum at En = 14 MeV. The total cross section is 55 ± 8 mb (1954FR03). At En = 14 MeV, events corresponding to transitions to the ground state and possibly to a level at 2.4 ± 0.6 MeV are observed in Li-loaded photoplates; the latter group may actually be due to a level at 9.25 MeV in 7Li (1954AL24): see 7Li(n, n')7Li*.

15. 7Li(p, 3He)5He Qm = -4.188

Not reported.

16. 7Li(d, α)5He Qm = 14.163
Q0 = 14.26 ± 0.09 (1955LE1C);
Q0 = 13.719 ± 0.02 (1955KH31, 1955KH35);
Q0 = 14.15 ± 0.22 (1957FA10);
Q0 = 14.11 ± 0.08 (1958WE27).

The angular correlation of ground state alpha particles and those resulting from the break up of 5He is W(θ) = 1 + 1.2sin2θ, excluding a J = 1/2 assignment, bur consistent with J = 3/2 (1951FR1A). (1956RI37) reports W(θ) = sin2θ, also consistent with J = 3/2-. The (α - n) correlation observed at Ed = 0.16 MeV, yields W(θ) = 1 + (0.75 ± 0.05)sin2θ and is again consistent with the assignment J = 3/2- for the ground state of 5He (1957FA10). The coincidence α-spectrum agrees in shape with a computed spectrum based on the 4He(n, n)4He cross sections (1956RI37).

The width of the ground state of 5He is 0.3 ± 0.1 MeV (1953CU20), 0.66 ± 0.2 MeV (1955LE24). The work of (1953CU20, 1956JU1B) at Ed = 0.6 to 1.5 MeV appears to indicate an excited state at Ex = 2.5 ± 0.2 MeV, Γ ≈ 1.5 ± 0.3 MeV. High-resolution magnetic spectra, observed for Ed = 1.0 to 2.2 MeV show only the ground state peak, superposed on a continuous distribution, with no evidence of an excited-state group. The shape of the ground state peak is well accounted for with the parameters γ2 = 17.6 × 10-13 MeV-cm, R = 2.9 × 10-13 cm, taken from (n, α) scattering data (1958WE27).

17. 9Be(γ, α)5He Qm = -2.529

See 9Be.