(See Energy Level Diagrams for 5He)
General reviews: (1966DE1E).
Other topics: (1967BO1C, 1967EL03, 1968GO01, 1968HE1D, 1969GI1A, 1969HE1L, 1969SC1J, 1970SU1B, 1970ZO1A, 1971BA2Y, 1971CH50, 1971ZA1D, 1972CA37, 1972GI11, 1972JA14, 1972KA38, 1972LE1L, 1972MA57, 1972RI10, 1973MA20, 1973RA1E, 1973RO1R).
b Similar considerations prevail for 5Li.
Ground state: The ground and first excited states of 5He comprise the components 2P3/2 and 2P1/2 formed by l = 1 scattering of a neutron by an α-particle. Shape parameters of the state are best described by tabulated phase shifts, but it is sufficiently narrow to appear as a distinct group in many reactions. The central energy of this group is generally taken as defining the "mass" of 5He.
First excited state: Ordinarily appears as a broad continuum in particle spectra, centered several MeV (3 - 5) above the ground state. The resonant phase shift goes through 1/2π at Ec.m. = 6.43 MeV, about 5.5 MeV above the ground state.
16.8 MeV: Conspicuous in the 3H + d reactions, the state is almost entirely of a 3 + 2 configuration, with relative angular momentum zero, and a quite small reduced width for neutron (ln = 2) emission. Properties of the state are listed in Table 5.2 (in PDF or PS).
19.9 MeV: In the range Ex = 20 - 25 MeV, there appears to be a number of broad overlapping states, principally of 3 + 2 configuration, with even parity. There is some indication from 7Li(p, 3He), [7Li(p, t)] that they have mainly S = 3/2. Among this manifold, a level at 19.9 ± 0.4 MeV is reported to form a well-defined group (Γ = 3 MeV) in 7Li(p, 3He). There are indications of another structure near Ex = 24 - 25 MeV.
At low energies the reaction is dominated by a resonance at Ed = 107 keV; the mirror reaction shows resonance at Ed = 430 keV. (1970BE1A) have measured the cross section for emission of 16.7 MeV γ-rays for Ed = 25 to 100 keV: the ratio σ(d, γ)/σ(d, n) is approximately constant at (2.1 ± 0.6) × 10-4, leading to Γγ = 14 ± 4 eV, where Γn is taken as 66 keV. (1963BU07) have measured thick target yields from Ed = 150 to 1300 keV. The derived cross sections are analyzed into resonant and direct-capture contributions: the cross section at resonance is here reported as 60 μb [vs. 1 mb reported by (1970BE1A)]. At Ed = 1025 ± 47 keV, the differential cross section id 0.44 ± 0.12 μb/sr (90°) and the γ to n branching ratio is an order of magnitude smaller: 2.3 × 10-5. The angular distribution of the γ-rays is forward peaked and the total cross section is estimated to be 4.8 μb (1970KO09). A cluster model calculation of major even parity states predicts a broad peak in dipole strength due mainly to a 5/2+ state in a cluster of positive parity states some 4 MeV above the 3/2+ state (1971WA08).
Below Ed = 100 keV, the cross section for reaction (a) follows the Gamow function, σ = (A/E)exp(-44.40 E-1/2) (1953JA1A, 1954AR02). A strong resonance, σ(peak) = 5.0 b, appears at Ed = 107 keV: see Table 5.2 (in PDF or PS). From Ed = 10 to 500 keV, the cross section is well fitted with the assumption of s-wave formation of a Jπ = 3/2+ state (1952AR1A, 1952CO35, 1955KU03). Excitation curves and angular distributions for reaction (a) have been measured from Ed = 8 keV to 21 MeV: see (1966LA04) for earlier references and (1966KO19: differential cross section (90°); Ed = 0.115 to 1.65 MeV), (1968IV01: excitation functions at 3 angles; Ed = 3.97 to 10.99 MeV), (1973MC05: angular distributions, at ≈ 1 MeV intervals; absolute 0° cross sections to within ± 3.1%; Ed = 5.0 to 15.7 MeV) and (1968SI1B: differential cross sections (0°), angular distributions; Ed = 7.0, 11.4, 15.0 and 19.0 MeV; relative cross sections at 1 MeV intervals, Ed = 7.0 to 21.0 MeV). There is some evidence of resonant behavior between Ed = 3 and 9 MeV (1960ST25). See also (1968SI1B, 1973MC05).
A study of reaction (a) with polarized deuterons at Ed = 0.2 to 1.0 MeV indicates intervention of the s-wave, Jπ = 1/2+ channel, as well as possible p-waves above Ed = 0.3 MeV (1965TR01, 1971GR32). This effect is evidently very small below Ed = 100 keV (1971OH1B). At higher energies, the neutron polarization P1(θ1) shows an angular distribution that peaks typically at θ1 = 30° lab, then goes negative with increasing angle and has a minimum between 90° and 130°, depending upon energy. (1971MU04) have made an extensive study of P1(30°) for Ed = 5 to 15 MeV and, with deuterium as target, for Et = 4.5 to 19.5 MeV (neutrons near 135° lab). The polarization increases monotonically from 0.03 at Ed = 3 MeV to ≈ 0.5 at Ed = 6.5 MeV and then with a lower slope to 0.69 at Ed = 13 MeV. The change in the slope may be caused by excited states of 5He near 20 MeV. Comparison with the 3He(d, p)4He mirror reaction at corresponding cm energies shows excellent agreement between the polarization values in the two reactions up to Ed = 6 MeV, but then the proton polarization becomes ≈ 15% higher, converging back to the neutron values at Ed ≈ 12 - 13 MeV. This may be due to experimental factors (1971MU04). Using polarized deuterons (1971HI07) find that the average ratio of vector analyzing power of 3He(d, p) to 3H(d, n) is 1.016 ± 0.015 at Ed = 6 MeV and 1.035 ± 0.020 at 10 MeV. The vector analyzing power of the two reactions agree to within ± 0.025 at all angles. This agreement between the mirror reactions is in apparent conflict with the result of (1971MU04). See also reaction 3 in 5Li. For other polarization measurements see Table 5.3 (in PDF or PS).
(1968BO35) have examined the parity non-conserving effects inn this strong reaction with polarized deuterons (Ed = 0.14 MeV). The experiment measured the ratio of the number of neutrons emitted parallel and anti-parallel to the direction of the initial polarization vector. The magnitude of the real part of the parity violating amplitude, F, is ≲ 3.8 × 10-3.
The energy spectrum of 3He particles in reaction (b) has been studied at Ed = 10.9 MeV at several angles: no evidence was found for a bound dineutron [ < 5 μb/sr at 6° and 7.5°]. Strong variations of the spectral shape with angle indicate that the Watson-Migdal approximation is inadequate and that several other first-order processes must be included. The determination of the nn scattering length is ambiguous (1970LA10, 1970LA1K). This problem is also discussed by (1971GR45: Ed = 13.4 MeV), (1970SL1A: Ed = 31.9 MeV), (1966BA1H: Ed = 29.8, 32.5 MeV), (1971GR12: Et = 22 MeV) and (1972BA32: Ed = 83 MeV). A discussion and comparison of attempts to measure the 1S0 nn scattering length is presented by (1971GR45): based on kinematically complete measurements and using the Watson-Migdal treatment, they find ann = -16.0 ± 1.0 fm. Intervention of n - 3He final state interactions is believed to be small in this work (1971GR45). See also (1973GR1L, 1973JE02). At Et = 22 MeV no clear evidence is seen for sequential processes via excited states of 4He (1971GR12).
Analysis of reaction (c) at Ed = 20 MeV is consistent with the existence of a broad resonance in the 3H + p system correspoding to 4He*(22.) (1968NE1D): see (1973FI04) for a general discussion of the states of 4He. n-p final-state interaction enhancement has been studied by (1973SL03) at Ed = 35 MeV. They find that only when En,p ≤ 1 MeV is agreement with the Watson-Migdal theory obtained, even if the data far from dominant n-t final-state interaction and p-t quasi-free scattering is chosen. See also (1966LA04).
See also (1965FL1A, 1966BR02, 1966BR1F, 1966BR1E, 1966BU09, 1966VA1A, 1967BO1D, 1967NE1B, 1968OH1A, 1969BU19, 1969ME1B, 1970HA1L, 1971MC1L, 1971OH1A, 1971PO1C, 1971SC1L, 1973GE1G), (1968TO1F, 1970SL1B, 1971HA2G, 1971WA1D, 1972OH1D, 1972SC1R) and (1965DU1A, 1965GO1B, 1966BA1J, 1966HU16, 1967OH1A, 1968HA1F, 1969HA1M, 1969HE1K, 1970PO1A, 1970SE1B, 1971DE20, 1971OH1C, 1971SI30, 1972SE09; theor.).
The elastic scattering has been studied for Ed = 2.6 to 11.0 MeV. The excitation curves show an interference at Ex ≈ 19 MeV and a braod (Γ > 1 MeV) resonance corresponding to Ex = 20.0 ± 0.5 MeV, also seen in 3He(d, d) [see 5Li]. Together with data from 3H(d, n)4He, this work favors an assignment D3/2 or D5/2 with a mixture of doublet and quartet components (channel spin 1/2 and 3/2) if only one state is involved (1967TO02, 1968IV01) [any appreciable doublet component would, however, be in conflict with results from 7Li(p, 3He)5He]. Measurements of differential cross section and analyzing power using polarized deuterons with Ed = 3.2 to 12.3 MeV show resonance-like behavior in the vector analyzing power near Ed = 5 MeV. The anomaly appears in the odd Legendre coefficients and is interpreted in terms of a (1/2, 3/2)- excited state of 5He with Ex ≈ 19.6 MeV. Broad structure in the differential cross section near 6 MeV, principally in the even Legendre coefficients, corresponds to an even parity state 5He*(20.0) (1971KI02)c. See also (1966BA1J, 1966BA1M, 1966DE1F, 1969BE1M, 1970HE1C, 1973CH27, 1974CH02; theor.) and (1966LA04).
c Tensor analyzing powers have been measured for Ed = 5.0 to 11.5 MeV by (1973DE51).
At Et = 0.5 MeV, the reaction appears to proceed via three channels: (i) direct breakup into 4He + 2n, the three-body breakup shape being modified by the n-n interaction; (ii) sequential decay via 5He(0); (iii) sequential decay via a broad excited state of 5He. The branching ratios at θ = 90° are 0.7: 0.2: 0.1. The width of 5He(0) is estimated to be 0.74 ± 0.18 MeV. Some evidence is also shown for 5He* at Ex ≈ 2 MeV, Γ ≈ 2.4 MeV (1965WO03). For reaction (b), see reaction 2 in 6He.
Some evidence is reported at Et = 22.25 MeV in reaction (a) for a broad state of 5He at Ex ≈ 20 MeV, in addition to a sharp peak corresponding to 5He*(16.7) (1968YO06). For reaction (b) see reaction 1 in 6Li. See also (1966LA04).
The coherent scattering length (thermal, bound) is 3.0 ± 0.1 fm (1973MU14). The thermal scattering cross section is 0.773 ± 0.009 b and the absorption cross section at 2200 m/sec σ0nγ, is < 0.05 b (1969RO16). (1973MU14) adopt 0.76 ± 0.01 b for σ-bars. Total cross sections for En = 4 × 10-4 eV to 147 MeV are summarized in (1958HU18, 1960HU1A, 1964ST25, 1966LA04). Recent measurements include those by (1969RO16: En = 0.187 to 6.195 eV), (1973GO38: En = 0.7 to 20.0 MeV), (1966ME14: 77.2, 88.2, 110.0, 129.4, 150.9 MeV) and (1968EN1A: 10 GeV/c).
Earlier angular distribution studies are summarized in (1970GA1A). Other recent work is reported by (1968MO26, 1970MO31: En = 0.2 to 7.0 MeV), (1969CR1B, 1972CR01: En = 0.55, 0.84, 1.16, 1.33 MeV) and (1971NI06: 17.6, 20.9 and 23.7 MeV). Recent polarization studies are listed in Table 5.4 (in PDF or PS).
The total cross section has a peak of 7.6 b (1973GO38) [see also (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 Jπ = 3/2+ state (1959BO54, 1964SH1A): Γc.m. = 100 ± 50 keV, Γn = Γd = 50 ± 35 keV (1960HU1A) [(1966HO07) find that the data are fitted best by Γn < Γd although Γn > Γd is not excluded]. Attempts to detect additional resonances in the total cross section have been unsuccessful: see (1966LA04).
The P3/2 phase shift shows strong resonance behavior near 1 MeV, while the P1/2 shift changes more slowly, indicating a broad P1/2 level at several MeV excitation (1952DO30, 1966HO07, 1970AR1B, 1972ST01). (1966HO07) have constructed a set of phase shifts for En = 0 to 31 MeV, l = 0, 1, 2, 3 using largely p-α phase shifts. They have measured differential cross sections from 6 to 30 MeV, with special attention to the region near 22.15 MeV and have fitted the data with the assumed phase shifts. At the 3/2+ state the best fit to all data is given by Eres = 17.669 MeV ± 10 keV, γ2d = 2.0 MeV ± 25%, γ2n = 50 keV ± 25% (see Table 5.2 (in PDF or PS)). The polarization has been calculated from the phase shifts and is presented as a contour plat (1966HO07).
(1968MO26) have carried out accurate angular distribution measurements for En = 0.2 to 7.0 MeV and deduced an independent set of p-wave phase shifts, assuming that s-wave shifts are those those of a hard sphere, R = 2.44 fm. The derived total cross sections fit existing data satisfactorily, and polarization results of (1968SA25) and (1963MA29) are well accounted for. Both single-level dispersion formula and Woods-Saxon potential parameters are deduced. A more extensive optical model analysis for En = 0 to 20 MeV has been carried out by (1968SA1B).
(1970AR1B, 1973AR1N, 1973AR1P) have analyzed existing differential cross sections and polarization data with phase shifts constrained to analytic (effective range) energy dependence, for En = 0 to 21 MeV. As compared with the (1966HO07) values, the P1/2 phase shift is higher in the range 3 to 10 MeV, and both S1/2 and P3/2 are larger for En = 10 to 16 MeV. D- and F-wave phase shifts are also determined (1973AR1P).
A single-level-with-background R-matrix parameterization has been carried out by (1972ST01), fitting differential cross section data for En = 0 to 20 MeV and polarization analyzing power for 0 to 15 MeV. Parameters for P3/2 and P1/2 levels are Eres = 0.97 and 6.43 MeV, and γ2 = 7.55 and 12.30 MeV, respectively. See also (1973NI1B). Polarization measurements at six energies from En = 11 to 30.3 MeV are reported by (1972BR10) together with derived phase shifts. Partial waves as high as l = 4 are required, but none shows resonance, other than the D3/2. See also (1969RO20).
See also (1966BR1E, 1969MA1H, 1973GA17), (1966BA1K, 1971RH1A, 1971ST1J, 1971WA1D, 1972SC1T) and (1966BA1J, 1966DE1F, 1966PE1C, 1966TO04, 1967SC1E, 1967SW1B, 1969ER1B, 1968SE1A, 1968BA1U, 1969BE1M, 1969CO1E, 1969GE1A, 1969HE1L, 1969KU1D, 1968TH1C, 1969VO1E, 1970ER1A, 1970FE1C, 1970GN1A, 1970HE1C, 1970HO32, 1970OM1A, 1970RE1A, 1971BA1Y, 1971BA74, 1971EF01, 1971GI1B, 1971PI04, 1971PL06, 1971TH09, 1972AD02, 1972IK01, 1972KI1F, 1972LO1K, 1972ZH1B, 1973AD01, 1973PL02, 1974CH02; theor.).
A typical proton spectrum consists of a peak corresponding to formation of the ground state of 5He, plus a lower continuum of protons ascribed to deuteron breakup (reaction (b)). Ground-state protons show pronounced azimuthal asymmtery when the reaction is induced by 8.5, 10 and 11 MeV vector polarized deuterons. A DWBA calculation is in only qualitative agreement (1967TR05, 1971KE16). See also (1972AV1E).
Coincidence studies of the final state interactions (α + p) and (α + n) have been carried out at Ed = 14.2 MeV (1967FU1D), Eα = 18.0 and 24.0 MeV (1970AS02), 42 MeV (1967WA08, 1968WA01), Eα = 70 MeV (1973TR04) and Ed = 78 MeV (1968BO1M). Spectra of α-particles in coincidence with protons exhibit peaks corresponding to 5Heg.s. as well as broad spectator peaks which occur where the neutron laboratory energy is a minimum (1967WA08, 1968WA01)): see 5Li. [For earlier work see (1966LA04)]. See also (1971LE02).
At Eα = 70 MeV, a kinematically complete experiment shows evidence for sequential decay in reaction (a), proceeding through excited states of 5He. Peaks in the coincident yield of protons and deuterons are ascribed to narrow states at Ex = 16.7 ± 0.1 MeV, Γ = 80 ± 30 keV, at Ex = 18.6 ± 0.1, 18.8 ± 0.1 and 19.2 ± 0.1 MeV, all with Γ = 180 ± 60 keV (1973TR04).
At Ee = 1180 MeV, two values of Q are observed: Q = -3.5 ± 1.0 MeV, corresponding to 5He(0), and Q = -19.0 ± 1.0 MeV, corresponding to 5He*(16.7). Angular distributions at Ep = 1158 MeV show that the first represents ejection of a proton with l > 0 and the second, an s-proton (1972AN24, 1972AN27, 1972AN29).
The angular distribution of ground state deuterons has been studied at En = 14.4 MeV (1965VA05). DWBA analysis does not reproduce either its shape or the cross section: this may be due to neglect of the deuteron knock-on process (1971MI12). At En = 152 MeV, two broad structures in the forward spectrum are interpreted as being due to transitions to 5He*(0, 14). The Q = -17 MeV peak is the stronger of the two (1966ME03, 1967ME11). For reaction (b) see (1967VA12). See also (1970VA1L, 1971HE1M, 1973BO1Y) and (1966WE1B). See also (1966LA04).
At Ep = 100 to 460 MeV, the summed proton spectra show two peaks [Q = -4.9 ± 0.3 and -22.7 ± 0.3 MeV (1966TY01)]. The lower peak corresponds to ejection of an l = 0 proton, presumably leaving 5He in the 3/2+ state at 16.7 MeV. See also (1966LA04). At Ep = 100 MeV, the 1/2- first excited state of 5He is weakly excited (1972MA61). At Ep = 100 and 155 MeV, transitions to states above 20 MeV are reported by (1965RO15, 1967RO06, 1972MA61). See also (1965CO1E, 1966BE1B, 1968ZU1A, 1969RU1A, 1973BH1A), (1967KO1B, 1967SH1C, 1968SA1C, 1972KO13, 1973CH1Q; theor.) and 6Li.
At Ed = 14.5 MeV, the ground state group is observed (1955LE24). See also (1960HA14, 1971IN1C). A study of the proximity process 3He(n, p)3H in the final state of reaction (b) has led to an estimate of the lifetime of 5Heg.s., τ = 1.4 × 10-21 sec (to within a factor of 2 or 3) (1972KA44). See also (1968LE15).
The two-proton spectrum shows broad structures attributed to the ground state of 5He, and to p-1s-1 and s-2 states at ≈ 20 and (≈ 35) MeV excitation (1965CH12).
The angular distriibution of ground state tritons has been measured at Et = 14.4 MeV: it is fairly well reproduced by DWBA (1970MI05). Reaction (b) mainly proceeds as a sequential process through 5Heg.s. (1967VA12). See also (1972AN1Q, 1973LI02), (1967BA1E; theor.) and (1966LA04).
At Ep = 43.7 MeV, angular distributions of the 3He groups to the ground state of 5He (Γ = 0.80 ± 0.04 MeV) and to levels at 16.7 and 19.9 ± 0.4 MeV (Γ = 2.7 MeV) have been determined. The angular distribution of the 5He ground state group indicates substantial mixing of L = 0 and L = 2 transfer. The distribution to 5He*(16.7) is consistent with L = 1. Since no transitions are observed in the 7Li(p, t)5Li reaction to the analogue 20 MeV state in 5Li [see 5Li], the transition is presumably S-forbidden and the putative states in 5He - 5Li near 20 MeV are 4D3/2 or 4D5/2 [compare 3H(d, d)] (1966CE05). Particle-particle coincidence data have been obtained at Ep = 43.7 MeV. They suggest the existence of 5He*(20.0) with Γ = 3.0 ± 0.6 MeV and of a broad state at ≈ 25 MeV. No T = 3/2 states decaying via T = 1 states in 4He were observed (1968MC02). See also (1967JO1B, 1969DE04, 1970CO1M).
At Ed = 24 MeV, the α-particle spectrum from reaction (a) shows structures corresponding to the ground and 16.7 MeV states and to states at Ex ≈ 20.2 and 23.8 MeV with Γ ≈ 2 MeV and ≈ 1 MeV, respectively (1972BA30). See also (1966BI1A, 1966MI09, 1966PO1D) and 9Be.
Spectra measured in reaction (b) suggest the involvement of a 5He state with Ex = 2.6 ± 0.4 MeV, Γ = 4.0 ± 1.0 MeV (1964FE01), Ex = 5.2 ± 0.2 MeV, Γ = 5.6+0.3-0.6 MeV (1965AS06, 1966AS04), Ex = 2.6 ± 0.2 MeV, Γ = 7 ± 2 MeV (1972GI10). See also (1972ST08, 1973HE26). Study of n-α coincidences for Ed = 2.6 to 4.0 MeV shows that reaction (b) proceeds mainly by sequential decay, primarily via 5He(0) and excited states of 8Be (1967VA11). See also (1966AS04, 1966MI09, 1967JE01, 1972DE44, 1973HE26). Attempts to observe n-α rescattering effects, following formation of 8Be*(16.63, 16.91) have been unsuccessful at Ed = 1.90 to 2.10 MeV (1972BR08), 2.07 and 2.09 MeV (1971SW10) and 3.0, 3.2 and 3.6 MeV (1968VA12). The upper limit for the rescattering yield is 1% of the yield from sequential decay via intermediate states in 5He and 8Be (1968VA12). Positive results are reported for Ed = 2.07 to 2.25 MeV by (1969TH02). See also (1967BE13, 1967FL12, 1967WI1C, 1968GA1E, 1968WI1E, 1971HU1H, 1973HE06, 1973KA32), 8Be, 9Be and (1966LA04).
Studies of this reaction have been carried out at Ep = 26.0, 35.0 and 46.8 MeV (1972QU01) and at 57 MeV (1968RO19). Observation of protons and alphas in coincidence at selected angles shows quasifree scattering of the incident protons by S-state α-clusters in 9Be (1968RO19, 1972QU01): see 9Be. See also (1969YA1B, 1970GO12).
At Ed = 10.4 and 12.0 MeV, this reaction involves 5He(0) and states in 8Be and 9Be (1971RE19).
See 12C in (1968AJ02).