(See the Energy Level Diagram for 9Be)
The differential cross section at 90° for reaction (a) rises steeply from 8.8 mb/sr at Et = 0.72 MeV to 19 mb at 0.90 MeV, and then more slowly to 21 mb at Et = 1.15 MeV. For reaction (d), the differential cross section at 90° rises from 0.75 mb/sr at 0.62 MeV to 5.7 mb/sr at 1.8 MeV and then decreases slowly to 5.3 mb/sr at 2.2 MeV (R.W. Crews, quoted in (1957JA37)).
Cross sections have been measured for Ed = 0.4 to 1.8 MeV by (1952BA64), and for Ed = 0.7 to 3.3 MeV by (1954BA46); see (1957JA37). The yield for Ed = 1.1 to 4.0 MeV has been measured by (1956BE1A). Resonances are observed at 0.80 and 1.04 MeV (1952BA64, 1954BA46), 1.4 MeV (1952BA64: see, however, (1954BA46)), 2.0, 2.5 and 3.7 MeV (1956BE1A: see, however, (1954BA46)). See also (1955AJ61).
The cross section for (a) has been measured for Ed = 70 to 110 keV by (1955RA14) and that for (b) has been measured for Ed = 30 to 250 keV by (1953SA1A). Resonances for neutron production occur, in the range Ed = 0.2 to 4.8 MeV, at Ed = 0.68 (Γ = 250 keV), 0.98 (Γ = 60 keV) and 1.8 MeV (Γ = 400 keV) (1952BA64, 1957SL01). At Ed = 0.90 MeV, the α-particles in reaction (b) are isotropic to within 2%, consistent with formation by s-wave deuterons. No evidence is found for direct interaction effects at this energy (1957RI39). The angular correlation of ground-state α-particles with those resulting from breakup of 5He indicate J = 5/2- (1956RI37), J = 3/2- (1957FA10), for the 9Be level mainly responsible for this reaction at Ed = 0.9 MeV. Reactions (a) and (c) account for less than 10% of the disintegrations at this energy (1956RI37). See also 5He, 8Be and (1956CA1B).
At Ed = 0.7 and 1 MeV, proton groups are observed corresponding to excited states of 9Be at 1.83 ± 0.04 MeV (Γ < 0.4 MeV), 2.39 ± 0.08 MeV (Γ < 0.2 MeV), 3.10 ± 0.04 MeV (Γ < 0.3 MeV), 4.74 ± 0.08 MeV (Γ = 1.25 ± 0.25 MeV) and (9.1 ± 0.2 MeV) (Γ = 1.2 MeV) (1954MO92, 1955AL57, 1958MO99). The 1.8-MeV level admits a description in terms of an s-wave n - 8Be interaction with a scattering length a = 23 × 10-13 cm (1958MO99: see 11B(d, α)9Be).
The photo-neutron cross section has been studied from threshold (Eγ = 1.664 ± 0.004 MeV: (1956CO56)) to 320 MeV: see Table 9.2 (in PDF or PS). A sharp peak, Γ < 50 keV, σ ≈ 100 mb, occurs at Eγ = 1.70 MeV (1956CO56). A further broad maximum appears at Eγ ≈ 10 MeV, followed by the giant resonance at 20 - 22 MeV (1953JO1B, 1953NA1A: see (1956CO59)). Measurements from 6 to 18 MeV show pronounced peaks at 11.3 and 13.3 MeV and a broad, low peak at 16 MeV, immediately preceding the giant resonance (1959SP1B). At Eγ = 6.1 MeV the main processes appear to involve 9Be(γ, n)8Be*(2.9) and 9Be(γ, α)5He (1954CA1A). Angular distributions for Eγ = 1.7 and 1.8 MeV are spherically symmetric; at Eγ = 2.76 MeV, W(θ) = 1.2 + sin2θ (1949HA1A: see also (1954NI1B, 1956FA30)).
Calculations of (1949GU1A) and (1956MA1M) envisage E1 transitions to p4s and p4d configurations, with levels at ≈ 1.8 and ≈ 5 MeV (assuming a fixed well depth). The angular distributions and general trend of the cross section are well accounted for, as is the ratio of σ(γ, n)/σ(e, e'n) (1958BA60: 6 to 17 MeV). (1956CO56) state, on the other hand, that the 1.70-MeV peak is to be attributed to an M1 transition.
The yield has a broad maximum, Γ = 4.7 MeV, at Eγ = 22.2 MeV where σ = 2.72 mb (1953HA1A). The angular and energy distributions of photoprotons produced by bremsstrahlung with Emax = 25 to 80 MeV has been studied by (1956CO59, 1956KL19). The angular distributions can be accounted for by the direct interaction of γ-rays with individual nucleons (1956KL19). The energy distributions indicate that transitions to levels in the residual nucleus play an important part in the direct photo effect (1956KL19). (1956CO59) suggest, on the other hand, that the relatively large proportion of low-energy protons indicates predominance of a (γ, n) process, followed by proton emission from 8Be levels at high excitation. See also (1953HE1B, 1953NA1A, 1957CH24, 1958CH31, 1958PA1B, 1958ST1A, 1958WH35).
Elastic scattering has been studied at Ee = 125, 150 and 190 MeV (1953HO79, 1954MC45). An r.m.s. radius of (2.2 ± 0.2) × 10-13 cm is obtained (1957HO1E: see also (1956HO93)). Inelastic peaks corresponding to levels at 2.54 and 6.96 MeV are reported by (1954MC45). Comparison of σ(γ, n) and σ(e, e'n) for Ee = 6 to 17 MeV indicates that the transitions are mainly E1 (1958BA60). See also (1957EH1A, 1958EH1A).
The neutron spectrum observed when 9Be is bombarded with 3.7-MeV neutrons exhibits a structure which is consistent with the excitation of the known states at 0, 1.5, 2.4 and 3.1 MeV, with subsequent neutron emission from the latter two. It is concluded that the (n, 2n) process at this energy proceeds mainly via discrete states of 9Be (1957HU14, 1958WA05). At the same bombarding energy (1955FO1B) observe two discrete groups in coincidence, suggesting the process 9Be(n, n')9Be*(2.43) → n + 8Be: see, however, (1957BO83: 9Be(α, α')9Be*). Using monochromatic neutrons with En = 2.6 to 3.25 MeV, (1957FI52) finds a sharp threshold for (n, 2n) at En = 2.7 MeV, corresponding to excitation of 9Be*(2.43). On the other hand, (1958MA22) find evidence for (n, 2n) at En = 2.6 MeV, below the threshold for (n, n'); at En = 5 to 6 MeV, the spectra show neutrons from the direct (n, 2n) process. At En = 14 MeV, the cross section for production of 9Be* is 170 ± 30 mb; comparison with σ(n, 2n) = 530 ± 40 mb indicates that about 1/3 of the (n, 2n) processes proceed via 9Be*(2.4) (1958AN32). See also (1957RO57) and (1958BE1E; theor.).
A search for γ-transitions from the 1.8 and 2.4-MeV levels yields upper limits of 0.3 and 0.2 mb, respectively, for En = 2.56 MeV and 1.8 and 0.3 mb for En = 2.74 MeV (1956DA23).
Elastic scattering has been studied at Ep = 14.5, 20 and 31.5 MeV by (1956KI54), at 10 MeV by (1956RA32), at 12 MeV by (1958SU14), at 17 MeV by (1956DA03), at 31 MeV by (1953WRZZ, 1954FI35, 1956BE14): see also 10B. All angular distributions show pronounced diffraction maxima characteristic of the optical model. Analysis in terms of the diffuse-surface optical model is discussed by (1957ME21). See also (1956KL55, 1956SH1C).
Inelastic scattering is observed corresponding to levels at (1.8), 2.43, 3.1, 5.0, 6.8, 7.9, 11.3, (14.5), (17.5), (19.9) and (21.7) MeV: see Table 9.3 (in PDF or PS). It is not clear whether the structure observed near 1.8 MeV is properly to be attributed to a level at this energy or to a three-body breakup, modified by the proximity of the neutron threshold. If the former is correct, the level width is 0.15 MeV and J = 1/2+ (1955GO48, 1958SU14). If a three-body breakup is involved, some interaction between the neutron and the 8Be must be involved: (1956BO18) find a satisfactory fit to the observed shape with an s-wave scattering length of 20 × 10-13 cm; see also (1958MI1C) and 11B(d, α)9Be. It is suggested by (1956SU67, 1958SU14) that the observed structure is to be understood in terms of a heavy particle stripping process and it is conjectured that the 4.8-MeV level may reflect the same effect, involving 8Be*(2.9); see, however, (1958MI1C).
A continuous distribution of neutrons, observed for Ep > 4.5 MeV is attributed to formation of 9Be*(2.4) with subsequent breakup into n + 8Be*(2.9) (1959MA20). Breakup into (8Be + n) occurs in 12 ± 5 % of transitions (J.B. Marion, private communication).
The energy of the 2.4-MeV level is given as 2433 ± 5 (1951BR72), 2434 ± 5 (1956BO18), 2432 ± 4 keV (1955GO48); the width is ≤ 1 keV (1955GO48). Analysis of angular distributions at Ep = 12 MeV in terms of direct interaction plus compound nucleus formation indicate J = 1/2- or 5/2- (1958SU14). At Ep = 31 MeV, analysis indicates J = 1/2+, 3/2+ or 5/2+ (1956BE14: see, however, (1958SU14)). Results at Ep = 10 MeV are consistent with odd parity (1956RA32). See also 10B(n, d)9Be and (1952DA1B).
The 3.03 ± 0.03 MeV level has a width > 0.28 MeV; the Wigner limit then restricts ln to 0 or 1, J ≤ 3/2 (1956BO18: see, however, 11B(d, α)9Be). Angular distributions for the higher levels have been studied by (1956BE14); the resulting spin assignments are given in Table 9.3 (in PDF or PS). See also (1954FI35, 1955GR12, 1958TY46) and (1956BL1B; theor.).
Inelastic groups are reported corresponding to levels at (1.7), 2.4, 3.01 ± 0.1 (Γ ≈ 0.3), 4.8 and 6.8 MeV (1955RA41, 1956GR37, 1958MI1C, 1958SU14). The structure at Q = -1.7 MeV may be accounted for as a three-body breakup with an s-wave (n - 8Be) interaction characterized by a scattering length of 20 × 10-13 cm, or by a resonance of about single-particle width. In either case a J = 1/2+ level of 9Be is indicated near the binding energy (1955RA41, 1958MI1C: see also 11B(d, α)9Be). The angular distribution of the Q = -2.43 MeV group has been studied at Ed = 15 MeV (1956HA90) and Ed = 24 MeV (1958SU14). Analysis by direct interaction theory yields l = 2, J = 1/2-, 5/2- or 7/2- (1958SU14: see, however, (1956HA90)). See also (1956SU1A).
Elastic scattering has been studied at Eα = 48 MeV by (1958SU14).
Inelastic groups are observed corresponding to 9Be* = (1.8), 2.4, (3.1), 6.8 and 11.3 MeV (1955RA41, 1956FA02, 1958SU14). The group corresponding to the 1.8-MeV "level" has a peak at Q = -1.83 ± 0.03 MeV, Γ ≈ 200 keV. It is suggested, however, that just such a structure would be expected from a heavy particle stripping process in which the proton also escapes, preferentially with the maximum possible energy. In this case, the 4.8-MeV "level" might arise from a similar process, in which the final nucleus is now 8Be*(2.9). The angular distribution of the Q = -2.4 MeV group at Eα = 48 MeV indicates l = 2, J = 1/2-, 5/2- or 7/2- (1958SU14). Measurement of the momentum and angular distributions of α-particles from the breakup of 9Be*(2.4) indicate that the decay proceeds mainly via 4He + 5He, or by direct three-body breakup. Gamma decay is < 1%, neutron emission to 8Be(0) is < 10% (1957BO83). See also (1956BL1B, 1958PI1B; theor.).
At Ep = 95 MeV, two groups of deuterons are observed from reaction (a), corresponding to 8Beg.s. and to states near 17 MeV. It is suggested that these reflect the "snatching" of the loosely bound neutron or of one of the tightly bound "alpha-particle" neutrons, respectively. Angular distributions lend some support to the α-particle model of 9Be; the occurrence of high momenta for the tightly bound neutrons indicates the operation of strong short-range two-body forces (1956SE1A).
Angular distributions at Ep = 6.5 and 22 MeV, and at Ed = 7.7 MeV are analyzed in terms of pickup theory, using a square-well (n - 8Be) interaction to represent the ground state of 9Be (1955DA1D, 1955DA1E). See also 10B, 11B.
At En = 14.4 MeV, the ground state and the level at 2.43 MeV are observed. No other deuteron groups were detected below Ex ≈ 5.5 MeV. The angular distribution of the deuterons, analyzed by pickup theory, indicate odd parity, 1/2 < J ≤ 9/2 for both states (1954RI15). See also (1956FR18), (1955FR1F; theor.) and 11B.
At Et = 1 MeV, α-groups are observed corresponding to 9Be levels at 2.39 and 3.06 MeV (1955AL57).
Alpha-particle groups are reported to states at (1.75), 2.4 and 3.0 MeV. The structure corresponding to the 1.75-MeV state can be explained in terms of an (n - 8Be) interaction with a scattering length a of 20 × 10-13 cm or in terms of a resonance near threshold, with θ2 ≈ 1 (1956BO18, 1958MI1C). (1958KA31) find, on the other hand, that a value of a ≥ 80 × 10-13 cm is required to fit their distribution and suggest that the (n - 8Be) interaction must be very near resonance on this model. The energy of the 2.4-MeV state is given as 2422 ± 5 keV by (1951VA08), 2431 ± 6 keV (1954EL10), 2424 ± 5 keV (1956BO18). The next state is located at 3.02 ± 0.03 (1955LE36), 3.05 ± 0.03 MeV (1956BO18). The width is ≈ 0.3 MeV (1956BO18), Γc.m. = 161 ± 15 keV (1958KA31). See also (1955HO48).
At En = 14 MeV, the cross section to the ground state of 9Be is 80 ± 20 mb; that to the 2.43-MeV level, σ2.43, is 10 ≤ σ2.43 ≤ 210 mb (1955GR21).