(See the Energy Level Diagram for 7Li)
GENERAL: See also Table 7.1 [Table of Energy Levels] (in PDF or PS).
Theory: See (1955AU1A, 1955DA1A, 1955LA1D, 1956AB1A, 1956FE1A, 1956KU1A, 1956ME1A, 1957FE1A, 1957FR1B, 1957LE1E, 1957MA1E, 1957MA57, 1957SO1A, 1958HA1D, 1958SK1A).
For Eα = 0.5 to 1.9 MeV, capture radiation is observed to 7Li(0) and 7Li*(0.48), with intensity ratio 5 : 2. The smooth rise of the cross section suggests a direct capture process. The angular distribution is not isotropic, indicating l > 0 (1958RI34); see also (1958LI1A). In the range Eα = 0.48 to 1.13 MeV, the cross section increases from 0.6 to 1.2 μb (1959HO03).
Differential cross sections have been measured for Et = 1.2 to 2.2 MeV (1956HE16). At Et = 1.677 MeV, θ = 30°, dσ/dΩ (lab) = 875 mb/sr (1958AL05).
At Eα = 38.5 MeV, two groups of protons are observed, corresponding to the ground and 0.48-MeV states of 7Li. Absolute c.m. differential cross sections are given as (1.53 ± 0.09) + (1.93 ± 0.15)cos2θ and (1.32 ± 0.07) + (0.66 ± 0.23)cos2θ mb/sr for the ground state and excited state groups, respectively. The cross sections are in fair agreement with those calculated from 7Li(p, α)4He by the principle of detailed balancing. The partial inelastic cross sections are estimated to be 11 ± 1 mb for s-waves, 33 ± 3 mb for d-waves (1958BU38).
Two γ-rays with Eγ = 7.26 ± 0.03 and 6.78 ± 0.05 MeV, and relative intensities 10 and 7.5 ± 2.0, corresponding to transitions to the first two states of 7Li are observed. The total radiative capture cross section is 28 ± 8 mb (1957BA18).
The total cross section has been measured from En = 4 eV to 10 MeV (1954JO17, 1954NE1A, 1955HU1B, 1956GO62, 1957HU1D, 1957KA1B, 1958HU18), from En = 7 to 14 MeV by (1958BR16) and at 14 MeV by (1952CO41, 1957KA1B). A pronounced resonance occurs at En = 255 keV (see Table 7.2 (in PDF or PS)) with a peak cross section of 10.3 b (1954JO17). Angular distributions for En = 0.2 to 0.4 MeV have been studied by (1956WI04): all observations are consistent with p-wave formation of a J = 5/2- level, 7Li*(7.47). The s-wave potential scattering has a statistical channel spin mixture with negative phase shift (1956WI04). Resonance parameters are compared with those of the mirror level (7Be*) in Table 7.2 (in PDF or PS). The 7.47-MeV level has been assumed to be the upper member of the 2F7/2, 5/2 doublet of which 7Li*(4.6) is the lower. It is pointed out by (1956ME1A, 1957MA57, 1957SO1A) that it may in fact be a component of the 4P multiplet and that the F5/2 is to be found at a lower energy. This assumption would explain the large values of θ2p and θ2n and would make the large difference of θ2α for the mirror levels less disturbing (1957MA57). See also (1954LA1A, 1956DA1A, 1958HA1D; theor.).
No other clearly defined resonance is observed, although the cross section exhibits a broad maximum near En = 5 MeV (1954JO17, 1957HU1D, 1958HU18).
The large coherent thermal scattering length suggests the existence of at least one bound s-level (1954LA1A).
The isotropic thermal capture cross section is 945 b (1958HU18); up to En ≈ 70 eV, the cross section varies as 1/v. Cross sections and angular distributions are summarized in (1955HU1B, 1956HU1A, 1957HU1D, 1958HO1C, 1958HU18). A strong resonance is observed at En = 0.25 MeV with a peak cross section of 2.9 b, attributed to p-wave formation of a J = 5/2- level (see 6Li(n, n)6Li and Table 7.2 (in PDF or PS)). In the range En = 9 to 90 keV the cross section is higher than the 1/v law would indicate (1956GO1E, 1959BA1H). Above the resonance, the cross section appears to exhibit a broad hump, σ ≈ 250 mb, at En = 1.5 to 2 MeV, and then decreases smoothly to 26 mb at 14 MeV. The decrease at 2 MeV is attributed to competing reactions (1956RI34, 1958MU07; see, however, (1958HO1C)). (1958KE30) find an almost linear decrease from 34.3 mb at En = 12.5 MeV to 17.5 mb at En = 18.3 MeV. For En ≳ 2 MeV the reaction appears to proceed by a pickup process. See 6Li(n, t)4He in 6Li. See also (1955PE1B, 1956DA1A, 1957BE71, 1957EL1C, 1957KO1A) and (1959BA1H).
The cross section at En = 14 MeV is 6.7 mb (1953BA04, 1954FR03).
The total cross section is about 200 mb at En = 5.5, 6.5 and 14 MeV (1956RI34). For the 5He ground-state peak, σ = 89 mb for the continuum, σ = 77 mb at En = 14 MeV (1954FR03). The reaction appears to proceed by pickup: see 6Li(n, d)5He in 6Li.
The weighted mean value of the excitation energy of the first excited state is 477 ± 2 keV (see (1955AJ61)). Levels are also observed at Ex = 4.630 MeV, Γ = 93 ± 8 keV (1957BR97), 4.61 ± 0.02 MeV (1952GE07), 4.46 ± 0.02 MeV (1955KH35) and Ex = 6.54 ± 0.02 MeV (1955KH31, 1955KH35), 6.56 MeV (1955LE24: see, however, (1958HA22)) and Ex = 7.457 MeV, Γ = 91 ± 8 keV (1957BR97). A search for a level at 5.5 MeV was unsuccessful at θ = 30°, 70° and 90°, Ed = 7.0 and 7.5 MeV (1957BR97). Angular distributions of the protons to the ground state and the 0.48-MeV level, at Ed = 8 MeV (1953HO48) and Ed = 14.5 MeV (1955LE24), analyzed by stripping theory, show odd parity, J ≤ 5/2 for both states: see also (1954NI10). The ratio of reduced widths θ2(g.s.)/θ2(0.48) = 0.69 and 0.66 from these two experiments (1956RE04); θ2(g.s.) ≈ 0.060 (1957FR1B). The angular correlation between the protons and the 0.48-MeV γ-rays is isotropic (see (1955AJ61)) indicating J = 1/2 for 7Li*. The mean life of this state as determined in the present reaction and in 9Be(d, α)7Li is (7.7 ± 0.8) × 10-14 sec (1956BU83). At Ed = 15 MeV, the angular distribution of the protons to the 4.6-MeV state has been measured for θ = 30° to 100° and can be reasonably accounted for by stripping theory with ln = 2, J ≤ 7/2- (1956HA90); on the other hand, (1958HA22) report the distribution nearly isotropic at Ed = 15 MeV. At Ed = 24 MeV the distribution is in poor agreement with stripping theory (1957CO68). For the 7.5-MeV level, the proton distribution at Ed = 15 MeV indicates ln = 1 and a reduced width equal to that of the ground state (1958HA22). A search for 4.1 MeV γ-rays from a possible (4.6 → 0.48) transition was unsuccessful: at Ed = 1.8 MeV, I4.1 < 0.1 (I0.48) (1957WI24). See also 8Be, (1955AU1A, 1955LA1D, 1957FR1B; theor.) and (1952AJ38).
At Et = 240 keV, the reaction has been observed to the ground state and to the 0.48-MeV level (1954AL35). See also (1955CU17).
The total cross section for neutron production (including the (γ, np) and (γ, 2n) processes) has a broad maximum (σ = 2.3 mb) at Eγ = 16.8 MeV, Γ = 9.3 MeV (1958RY77: natLi). For the (γ, n) process alone, the peak cross section is estimated as ≈ 2.7 mb at Eγ ≈ 14 MeV (1954GO1A). Discontinuities reported in the integral yield curve are listed in Table 7.3 (in PDF or PS). Integrated cross sections are tabulated by (1954GO39). See also (1953MO1B, 1955HE51, 1958BE1C, 1958TI1A) and (1953PE1A, 1957BA1H; theor.). (It is to be noted that the total cross section in 6Li(n, n)6Li exhibits no evidence of sharp structure in the region Ex = 9 to 12 MeV: see (1954JO17, 1957HU1D, 1958HU18).)
According to (1953TI02) and (1954TI16), the cross section in the range Eγ = 12 to 20 MeV exhibits a single broad maximum at ≈ 15.6 MeV with a width of ≈ 4 MeV. At Eγ = 14.8 MeV the cross section is 2.2 ± 0.25 mb; according to (HA55A), the integrated cross section is 11 MeV-mb. There appears to be some disagreement on the shape of this excitation function. It is suggested that a γ-emitting state of 6He at ≈ 1.6 MeV could account for the discrepancy between the Canberra and Saskatoon results (1954TI16, 1955TI1A). Some evidence for fine structure is reported by (HA55A). See also (1953TU1A, 1954RU27, 1954TU1A, 1958SM1A, 1958ST1A, 1958TI1A, 1958WH35).
See (1958BA1C, 1958PA1B, 1958TI1A, 1958WH35).
Reported peaks in the excitation function are listed in Table 7.3 (in PDF or PS). Observations are made in nuclear emulsions, mainly under continuous-spectrum irradiation. It is pointed out by (1953PE1A) that T = 3/2 levels of 7Li will not be observed in the present reaction in the region 0 to 15 MeV above threshold: compare 7Li(γ, n)6Li. See also (1952NA1A, 1953TI1A, 1953TU1A, 1954ER1A, 1954TU1A, 1955TI1A, 1958TI1A) and (1955CZ1A, 1956CZ1A; theor.).
Measurement of resonance scattering and absorption in Li yield τ(mean) = (1.1 ± 0.3) × 10-13 sec (1958BE10), (1.15 ± 0.14) × 10-13 sec (1958SW65, 1959SW63) for the 0.48-MeV state.
Elastic and inelastic (0.47-MeV state) scattering have been studied at Ee = 187 MeV by (1955ST85). Elastic scattering results indicate an r.m.s. radius of about 2.1 × 10-13 cm (1957HO1E: see also (1956HO93, 1957ME1B)).
At En = 14 MeV, there is evidence for 7Li states at 4.6 ± 0.25, ≈ 6.5 ± 0.25, 7.5 ± 0.25 and 9.25 (?) MeV (1954AL24: Li-loaded photoplates). See also (1954FR03).
Elastic scattering at Ep > 10 MeV is characterized by direct interaction processes: see (1956KI54, 1957HI56). Inelastic proton groups corresponding to the states at 0.48, 4.63, 6.54 and 7.46 MeV have been observed at bombarding energies up to 18.3 MeV: see (1952AJ38). A careful check on the isotropy of the 477-keV radiation from the J = 1/2- level yields W(θ) = 1 + αcosθ, with α = (2.9 ± 6.2) × 10-4; the result implies an upper limit for the intensity of a parity non-conserving part of the wave function F2 < 1 × 10-4 (1958WI38).
At Ep = 12 MeV, the angular distribution of the protons leaving 7Li in the 4.63-MeV state can be accounted for by a mixture of direct interaction with l = 0, and compound nucleus formation, indicating J = 1/2-, 3/2-, or 5/2-. It is also possible to fit the data under the assumption of compound nucleus formation with s, p, and d-waves, permitting J = 7/2- (1957CO53). Angular distributions at Ep = 17.5 MeV are reported by (1957MA04). The relative intensities observed in this work for protons leading to the 4.6, 6.6, and 7.5-MeV states are consistent with assignments 22F7/2, 22F5/2, 24P5/2; it is presumed that the 6.6-MeV state in question here is not the broad even-parity state at 6.54 MeV (1957LE1E). At 31.8 MeV, angular distributions of protons leading to the 4.6-MeV and 6.6-MeV levels fit the direct interaction theory with l = 1 (J = 1/2+, 3/2+, 5/2+, 7/2+). A weak group to a 9.6-MeV level is also reported (1957SI36). Levels at 18 ± 1.4 (?) and 22 ± 1.4 (?) MeV are observed with 96-MeV protons (1956ST30). See also (1957TY35, 1958CH26, 1958MA1B, 1958TY46, 1958TY49).
The ground-sate reduced width θ2n = 0.05 and 0.035 for 6Li(0) and 6Li*(2.2), respectively: see 6Li.
θ2n = 0.11 and 0.061 for 6Li(0) and 6Li*(2.2), respectively: see 6Li.
θ2p = 0.055 and 0.017 for 6He(0) and 6He*(1.7), respectively: see 6He.
Inelastic deuteron groups have been observed to the states at 0.48 and 4.6 MeV: see (1952AJ38) and (1955KH31, 1955KH35). (1955LE24) report Q = -4.62 ± 0.04, Γ = 0.3 ± 0.1 MeV, for the latter state; θ2α = 0.5 ± 0.2 (1957MA57). At Ed = 14.4 MeV, the angular distribution of the deuterons corresponding to the 0.48-MeV state is moderately well accounted for by the theory of (1952HU1B), with l = 2 (1955LE24). Some discrepancies appear to exist between reported angular distributions of deuterons corresponding to the 4.6-MeV level at Ed = 14 to 15 MeV: compare (1955LE24) and (1956HA90). See also (1956SO21, 1956SO33).
7Li states at 0.48 and 4.6 MeV have been observed at various energies up to 48 MeV: see (1955AJ61). At Eα = 31.8 and 48 MeV, the angular distribution of the α-particles to the 4.6-MeV state exhibits a prominent peak in the forward direction, suggesting a direct interaction process (1956CO61, 1957SI36). (1954ZH1A) finds Eγ = 478 ± 2 keV and a lifetime for the first excited state < 1.3 × 10-13 sec (see also 6Li(d, p)7Li and 9Be(d, α)7Li). At Eα = 1.9 MeV, the angular distribution of the 0.48-MeV radiation is isotropic within 10% (1954LI48). See also (1955BR1A, 1957NE1B).
† The symbol (ε) denotes orbital electron capture.
The decay proceeds to the ground and 0.48-MeV states. The fraction to the excited state is 0.115 ± 0.01 (see (1955AJ61)). The γ-ray energy is 477.8 ± 0.3 keV (weighted mean of values quoted in (1955AJ61), including the value 477.3 ± 0.4 reported by (1957DU37)). The weighted mean value of the half-life is 53.37 ± 0.11 days (1949SE20, 1953KR16, 1956BO36, 1957WR37), log ft = 3.25 for the ground state transition and 3.44 for the excited state. Both transitions are super-allowed (1954MA1D, 1956CH1B). A measurement of the internal conversion coefficient for the 0.48-MeV γ-radiation indicates that the radiation is 55% to 100% M1 (1958LE20). See also (1955LA1D; theor.).
See (1955AJ61) and reactions 11 and 12 in 9Be.
See (1957SC12, 1957VA12) and 10Be.
A number of α-groups have been observed with deuteron energies of up to 14 MeV. These correspond to levels at 480 ± 2 keV (1948BU31, 1953CO02), 4.62 ± 0.02 MeV (1953GE01: see also (1951GO47, 1952AS1A, 1956JU1D)), and 7.5 ± 0.17 (?) MeV (1951GO47). There is no evidence in this reaction of a state at 5.5 MeV suggested in 7Li(γ, t)4He (see (1955AJ61)): at Ed = 0.5 and 0.7 MeV (θ = 70°), the intensity of a group leading to such a state is less than 5% of the intensity of the ground-state alpha group (1956GE1A: see also (1955CU16, 1956JU1D)). Observation of the continuous distribution of alpha-particles and tritons indicates production and breakup of 7Li levels at 4.6 and 6.6 MeV (1955CU16, 1956JU1D).
The (α, γ) angular correlation has been observed for Ed = 0.40 and 0.84 MeV (1953UE01, 1954CO17). There is no significant departure from isotropy, in agreement with J = 1/2 for the 0.48-MeV level. The mean life of this state is (7.7 ± 0.8) × 10-14 sec (1956BU83: Doppler shift; mean of observations in this reaction and in 6Li(d, p)7Li); compare 7Li(γ, γ)7Li. This value is considerably shorter than that given by a shell-model calculation (1955LA1D). See also 10B(p, α)7Be in 7Be.
See also (1956GR37, 1956TU1A) and 11B.
With thermal neutrons, two groups of α-particles are observed, corresponding to 7Li*(0, 0.48); the fraction of transitions leading to the ground state is about 6%; see (1952AJ38, 1954DE38, 1958BU02) and 11B. The γ-ray energy is 478.5 ± 1.5 keV (1948EL1A), 478 ± 4 keV (1956DA23); the mean life is (7.5 ± 2.5) × 10-14 sec (1949EL07). The (α - γ) correlation is isotropic (1950RO1A: pile neutrons). See also (1955JA18).
For neutrons in the range En = 5.6 to 19.5 MeV, the reaction appears to proceed mainly through the 4.6-MeV level or through direct three-body disintegration (1956FR18). See also (1955AJ61).