(See the Energy Level Diagram for 16O)
Theory: See (1954DE1E, 1954FL1A, 1955HE1E, 1955JA1D, 1955MA1K, 1955MA1L, 1955SC1B, 1955WI1F, 1956EL1C, 1956FE1E, 1956JA1C, 1956KA1D, 1956MO1D, 1956PE1A, 1956RE1C, 1956WI1D, 1957EL1B, 1957FE1D, 1957GR1G, 1957HE1B, 1957RE1A, 1957TA1C, 1957TO1A, 1958CA1G, 1958DA1E, 1958DA1F, 1958FE1C, 1958FE1D, 1958HA1F, 1958MO17, 1958RA1F, 1958UM1A, 1958WI1E).
Resonant capture radiation to 16Og.s. is observed at Eα ≈ 3.24 MeV, corresponding to the known J = 1- state at 9.58 MeV: see 12C(α, α)12C. The radiative width of 6 × 10-3 eV implies a T = 1 admixture of the order of 3 × 10-4, an amount slightly lower than usual for 16O states. It is suggested that the T = 1 admixture may derive from 16O*(13.09), J = 1-; T = 1 (1957BL01): see also (1953JO1A, 1956WI1D). This state does not arise in any natural way from p-1s, or p-1d (1957EL1B). The integrated capture cross section to Eα = 1.60 MeV is < 30 μb-MeV (1955AL16). See also (1958PH37). The relevance of the capture of alpha particles by 12C to the buildup of the elements in stars is discussed by (1956CA1F, 1956HA1C, 1956HA1D, 1957BU66, 1957SA1B).
The angular distribution at Eα = 42 MeV shows three peaks and three valleys in the range θc.m. = 40° to 150°. The location of peaks and the absolute cross sections agree closely with those obtained in the inverse reaction at Ed = 20 MeV. The experiment provides a test of the hypothesis of invariance under time reversal and places an upper limit of a few per cent on possible T - R odd forces (1958BO71).
Resonances derived from a phase-shift analysis of the elastic scattering are exhibited in Table 16.4 [Resonances in 12C(α, α)12C] (in PDF or PS) (1953HI05, 1954BI96). At the upper limit of these experiments (16O*(12.5)), the existence of higher J = 0+ and 2+ levels is indicated by a pronounced increase in the l = 0 and l = 2 phase shifts (1953HI05, 1954BI96). The inelastic scattering for Eα = 20.4 to 22.6 MeV indicates a resonance at Eα = 21.85 ± 0.1 MeV, Γ ≈ 0.4 MeV (16O*(23.54)). The asymmetry of the angular distribution is said to indicate at least one other level, of opposite parity (1955RA1B). See also (1954DE1E; theor.) and 12C.
The yields of ground state protons have been studied at several angles in the region E(3He) = 1.4 to 4.8 MeV. There are indications of resonances at E(3He) ≈ 3.6 and 4.5 MeV (16O*(25.7 and 26.4)) (1956SC01, 1957IL01). See also 15N.
Excitation functions have been measured for Ed = 0.6 to 4.5 MeV (1955MA85, 1956JO1D, 1957JO1C, 1957NO1C, 1958MO14, 1958WE31). The yield of ground-state neutrons shows broad but well-defined peaks at Ed = 1.52, 2.62, and 4.19 MeV; Ex = 22.05, 23.05, and 24.38 MeV. It is not clear whether the structure is to be attributed to resonances or to surface interaction (1958WE31). See also 15O.
Excitation functions and angular distributions are reported by (1954JO1F: Ed = 0.4 to 0.6 MeV) (1956KO26, 1956VA17: Ed = 0.2 to 0.7 MeV), and (1958BO18: Ed = 0.6 to 1.0 MeV): see also (1957JA37). At the lower energies, both stripping and compound nucleus effects are reported, although the distributions for Ed = 0.6 to 1.0 MeV (1958BO18) appear to be explicable entirely on the basis of overlapping resonance levels. See also 15N and (1954ST1C).
The cross section rises gradually from Ed = 0.45 to 0.90 MeV (1954CA1D). Angular distributions for Ed = 0.6 to 1.0 MeV indicate that the stripping contribution is small in this range (1958BO18). The angular distribution at Ed = 20 MeV agrees closely with that of the inverse reaction at Eα = 42 MeV (1958BO71): see 12C(α, d)14N, (1957FI1C) and 12C.
At E(3He) = 2.1 MeV, proton groups corresponding to 16O levels up to Ex = 13.6 MeV have been identified. In the region Ex ≈ 11 MeV, four groups are resolved, corresponding to 16O*(10.94, 11.087 ± 0.020, 11.25, 11.51). The first two are presumably those observed in 15N(d, n)16O at 10.937 and 11.063 MeV. The gamma decay of 16O*(8.88, 10.94 and 11.07) has been studied with coincidence techniques: branching ratios are given in Fig. 33 (Gamma-ray transitions in 16O). From the observed transition intensities and from (p - γ) and (γ - γ) correlations, the assignment J = 2- for 16O*(8.88) is confirmed, and assignments of J = 0- and 3+ are fixed for 16O*(10.94 and 11.07), respectively. For the 10.94 MeV state, Γα/Γγ < 0.2, indicating an upper limit for the intensity of possible opposite-parity admixture of ≈ 2 × 10-9 (1959BR68, 1959KU78). See also (1957BR17, 1957LI1D, 1958BR1D).
Ground-state capture radiation resonances occur at Ep = 190, 338 keV (1958HE52) and 1050 keV (1952SC28, 1957HA98, 1958HE52): see Table 16.5 [Levels of 16O from 15N(p, p)15N, 15N(p, γ)16O and 15N(p, α)12C] (in PDF or PS). The large radiative width, Γγ = 150 eV, of the latter indicates E1 radiation and J = 1-; T = 1 for 16O*(13.09); on the other hand, the large α-width speaks for a strong admixture of T = 0 (1953WI1B, 1956WI1D: see also (1957BA03)). The 338 keV resonance is relatively weak, Γγ = 8 eV. The Breit-Wigner formula with destructive interference between these two J = 1- states gives a good account of the γ0 yield from Ep = 200 to 1200 keV (1958HE52). Cascade radiation, via 16O*(6 and 7), is weakly resonant at Ep = 340 keV (1958HE52): at the higher resonance, the relative amount of cascade radiation is < 1.3 × 10-3 (1953DE1A, 1954GO1F). No further resonances for ground-state radiation appear for Ep < 3.3 MeV with intensity > 2% of that at Ep = 1050 keV (1957BA03). Within a few microbarns, the cross section for Ep < 3 MeV is completely accounted for by the 13.09 MeV state; a small rise from Ep = 3 to 4 MeV may reflect higher resonances. This result is in disagreement with that of (1955SP1B) on the inverse reaction in which a level at 16O*(14.7) (Ep = 2.8 MeV) is reported. The isotropy of the radiation over the entire range from Ep = 1 to 4 MeV places an upper limit of 3% on the relative intensity of p-1d in the 13.09 MeV state (1957WI1H); see also (1957EL1B).
Elastic scattering studies are reported for Ep = 600 to 1800 keV by (1957HA98) and for Ep = 950 to 3960 keV by (1956BA1H); see Table 16.5 [Levels of 16O from 15N(p, p)15N, 15N(p, γ)16O and 15N(p, α)12C] (in PDF or PS). The Ep = 710 keV state, (16O*(12.78)) having J = 0-, does not appear in the (p, α) reaction or in the ground-state capture: see 15N(p, γ)16O. The assignment J = 3- to the state at Ep = 1210 keV (16O*(13.25)) is in disagreement with the observed angular distribution in 15N(p, α1)12C* but is confirmed by the α1 - γ correlation. The reason for this discrepancy is not known (1957HA98).
By comparison of Jπ assignments and reduced widths, it is concluded that the 16O levels formed at Ep = 898, 710, 1210, and 1028 keV are the analogues of the first four states of 16N (1956WI1G, 1956WI1H, 1957EL1B, 1957HA98). It is pointed out that the Ep = 340 and 429 keV states are also possible candidates, and that in any case appreciable isobaric spin mixing is to be expected (1957HA98). See also (1956WI1D).
Two groups of α-particles occur, to 12C(0) (α0) and to 12C*(4.43, J = 2+) (α1, γ). Observed resonances are exhibited in Table 16.5 [Levels of 16O from 15N(p, p)15N, 15N(p, γ)16O and 15N(p, α)12C] (in PDF or PS). The cross section for (p, α0) is 5 × 10-7 b at Ep = 100 keV (1950SC1A: see also (1952SC28, 1957BA03)). See (1957JA37).
Angular distributions (p - α0) have been studied in the range Ep = 230 to 960 keV by (1953CO1D), Ep = 500 to 1000 keV by (1953NE1B), Ep = 920 to 1260 keV by (1957HG01). At and below the 338 keV resonance, the distribution is isotropic. Strong cos θ terms develop for Ep > 400 keV, with cos2θ terms gradually increasing above 700 keV, and higher-order terms above 1 MeV. The terms in cos θ indicate interference between states of opposite parity. Analysis of this effect led (1953CO1D) to the conclusion that the resonances in question were Ep = 338 keV, assigned J = 0+, and Ep = 1028 keV, J = 1-. Subsequent work has shown, however, that the lower state is J = 1-, formed by s-wave protons: evidently the interfering even parity state or states remain to be identified; see also 12C(α, α)12C. The 1028 keV resonance has a large width both for α-particles and E1 radiation and appears to have a mixed character as regards isobaric spin (1953WI1A, 1953WI1B, 1956WI1D, 1957HA98).
Analysis of the (p - α0) distribution at the Ep = 1210 keV resonance indicates J = 3-, although J = 4+ is not clearly excluded (1957HG01). Angular distributions of (p, α1) and the 4.4 MeV γ-ray have been studied at the Ep = 429, 898, and 1210 keV resonances by (1952BA1C, 1952SE1B, 1953KR1B). Channel-spin ratios have been interpreted in terms of L - S and j - j coupling models by (1953CH1A). The α1-distribution at Ep = 1210 keV appears to require J = 4+ (1953KR1B, 1957HA98), but the (α1 - γ) correlation requires J = 3-, in agreement with elastic scattering results; the reason for this discrepancy is not clear (1957HA98). Angular distributions of the 4.4 MeV γ-rays are reported for Ep = 1050 and 1640 keV by (1954KR1C) and for various energies from Ep = 1210 to 3900 keV by (1957BA03). See also (1958CA13).
The excitation function has been measured from threshold to 6.4 MeV, and angular distributions have been studied at several energies: see Table 16.6 [Resonances in 15N(p, n)15O] (in PDF or PS). Only four levels are found with Γ ≲ 40 keV, as contrasted with the ten or more reported in 16O(γ, n)15O (1958JO28).
Slow neutron thresholds have been observed at Ed = 1.192 and 1.335 MeV, corresponding to 16O*(10.937 ± 0.010, 11.063 ± 0.015) (1957WE1A). At Ed = 5.1 MeV, in this reaction, and at Ep = 5.77 MeV in 19F(p, α)16O, four γ-ray lines are observed which are assigned to 16O: Eγ = 2.73 (8.87 → 6.14), 3.86 ± 0.04 (10.94 → 7.12), 6.14 (6.14 → 0), and 7.1 MeV (6.9 + 7.1 → 0). The 10.94 MeV state is not observed to decay in any way other than through the J = 1- state at 7.12 MeV; upper limits to transitions to 16O*(0, 6.06, 6.14, 6.92, 8.87) are, respectively, 5, 1, 6, 20, and 40% of the observed cascade. The strong transition to 16O*(7.1), J = 1-, suggests J = 0- for 16O*(10.94), although J = 1+ is not ruled out. The γ - γ correlation strongly favors J = 0-. It is suggested that this γ-emitting state is to be identified with the first neutron threshold: 16O*(10.94), above, and is distinct from either the (doubtful) 16O*(11.1) reported in 12C(α, α)12C or the 16O*(11.08) reported in 16O(p, p')16O*. Possibly both 16O*(10.94 and 11.08) are involved in 14N(3He, p)16O (1957BE61). It is noted that the 19F(p, α)16O spectrum of (1956SQ1A) shows three groups in this range, of which one is definitely attributed to 16O*(11.085) (1957WE1A). The 8.87 MeV state has a 7 ± 2 % direct transition to 16Og.s. (1957BE61). Angular distributions of ground-state neutrons for Ed = 1.1 to 5.2 MeV are well accounted for by the exchange stripping theory of (1957OW03) (1958WE31: see 17O). See also (1955AJ61) and (1957EL1B; theor.).
16N decays to several states of 16O: reported branching fractions are listed in Table 16.7 [Beta-decay of 16N] (in PDF or PS). The ground-state transition, with Eβ(max) = 10.33 ± 0.08 MeV (1957MO1A), 10.40 ± 0.05 MeV (1958BR95), has the unique first-forbidden shape corresponding to ΔJ = 2, yes, fixing Jπ of 16N as 2- (1957MO1A, 1958BR95, 1959AL1M). This assignment is also indicated by the fact that the transitions to 16O*(6.1 and 7.1) are both allowed (1951MI1B). There appears to be some discrepancy between the branching ratios of 16O*(6.1 and 7.1) as determined by decomposition of the β-spectrum (1958BR95) and by comparison of the γ-ray intensities (1951MI1B, 1957BO04: see, however, (1959AL1M)). The low ft-value for 16O*(7.1) presents some difficulty for the theory (1957EL1B). Transitions to the nuclear pair emitting state are < 1.5 × 10-4: log ft > 8.2 (1958AL13: see also (1956EL1C, 1957EL1B)). A 1.1% branch leads to 16O*(8.87) which decays via the 7.1, 6.9, and 6.1 MeV levels in the ratio 3 : 1 : 30. Since the β-transition is allowed, Jπ of 16O*(8.8) is 1-, 2- or 3-; the first and last would permit α-decay, so Jπ = 2-. The γ-branching and the γ - γ correlation (8.88 → 6.1 → g.s.) are consistent with this assignment (1956WI1A).
The cross section exhibits a slow rise for ≈ 3 MeV above threshold followed by the usual giant resonance. Characteristics of the giant resonance are: Eγ = 24 MeV, Γ = 3.5 MeV, σmax = 10 mb; ∫ σdE = 40 MeV-mb (1951JO1B, 1953MO1B, 1954FE16, 1957CA1D).
Discontinuities in the activation curve are said to indicate absorption into discrete levels of 16O: see Table 16.8 [Levels in 16O from 16O(γ, n)15O] (in PDF or PS) (1955BA1P, 1955CO1C, 1955PE1D, 1957BA27, 1958BE74, 1958KA1D). From the cross section it would appear that a substantial fraction of the absorption takes place through discrete levels (1954KA1A, 1955PE1D). However, in a measurement of the absorption in water for Eγ = 15 to 25 MeV, using various detectors including 16O, (1958SI1A) conclude that the absorption is generally continuous, with only a small contribution from discrete, narrow levels. See also (1954BI04, 1955CA1E, 1955SA1F, 1957SP1A, 1957SV1A, 1958LI1D, 1958WO51), (1955MO1C, 1956WI1D, 1957BA1H, 1957EL1B, 1958FE1C, 1958FE1D, 1958WI1E; theor.) and (1955AJ61).
Resonances are observed at Eγ = 19.6, 20.6, 22.4 MeV; ∫ σdE = 2, 2, and 20 MeV-mb for ground-state protons (1956CO59), Eγ = 20.7, 21.9, 24.0 MeV (1956LI1D). See also (1955JO1D). The 22 MeV resonance appears to be the giant resonance. Proton angular distributions have been measured by (1956CO59, 1956LI1D, 1957MI1C). A resonance reported at Eγ = 14.7 MeV (1955SP1B: see also (1956CO59)) does not appear in the inverse reaction 15N(p, γ)16O (1955WI1F, 1957WI1H). (1958SI1A) find that the absorption in the range Eγ = 15 to 25 MeV is generally continuous, with only small contributions from narrow resonances: see also 16O(γ, n)15O. See also (1955ST1D, 1956JO1C, 1957BR55, 1957MI1B, 1957SV1A, 1958LI1D, 1958MI89, 1958PE1A) and (1955MO1B, 1956GO1G, 1957BA1K, 1957WI1J; theor.).
The cross section for production of 12C exhibits a maximum near 17.5 MeV (Γ ≈ 5 MeV), σ(max) ≈ 50 μb (1953MI31). Evidence is also reported for excited states of 16O*((14.2), 16.75, 17.3, 22.6, (23.15), and 24.6) with J = 2+; T = 0 (1954ST89). See also (1955HA1D, 1955TI1A, 1956DA1C) and (1955AJ61).
The cross section for production of 4-pronged stars shows a number of maxima: see Table 16.9 [Maxima in 16O(γ, 4α)] (in PDF or PS) (1952GO1A, CO54I, 1956DA1C). An appreciable fraction of the stars appear to involve excited states of 12C and 8Be (1955AJ61, 1956DA1C: see, however, (1953MI31)). See also (1955RA1E, 1955HA1D) and (1955AJ61).
Measurement of the resonance scattering cross section for 6.9 and 7.1 MeV γ-rays from 19F(p, α)16O yields mean lifetimes of (1.2 ± 0.3) × 10-14 and (1.0 ± 0.3) × 10-14 sec for 16O*(6.92 and 7.12), respectively. Values obtained from self absorption measurements are consistent with these. The observed life of the 7.1 → g.s., E1 transition is consistent with an isobaric spin inhibition of the order of 300 (see (1955MA1K)). The lifetime of the 6.9 MeV state is longer than that expected from a collective model but can be accounted for by one version of the α-particle model (1957SW17). See also (1956KA1D, 1957GR1G; theor.) and 19F(p, α)16O.
Elastic scattering angular distributions at Ee = 240, 360, and 420 MeV strongly favor a shell-model charge distribution based on a harmonic well with a length parameter a = (1.76 ± 0.02) × 10-13 cm. The rms radius of the charge distribution is 2.70 × 10-13 cm (1958EH1B); see also (1956FE1B, 1957HO1E, 1958FE1D, 1958RA43).
At Ep = 19 MeV, proton groups are observed corresponding to 16O*(6.14, 7.02, 8.87, 9.85, 10.34, 11.08, 11.51, 12.02, 12.53, 13.06, and (13.39) MeV) (± 30 keV). Of these, three have not been reported elsewhere: 16O*(11.08, 12.02, 13.39). The level at 11.08 MeV decays via cascade through the 6 to 7 MeV states; it is presumably not to be identified with the (doubtful) 11.10 MeV state reported by (1954BI96) in 12C(α, α)12C since the 10 keV width of the latter would preclude observation of the γ-decay. If it is a pure γ-emitting state, J = 2- is favored. The 12.02 MeV state appears to give weak γ-radiation; the state at 12.53 MeV yields 4.4 MeV γ-rays via 16O(p, p'α)12C* as expected from its behavior in 15N + p. The 8.87 MeV state cascades 80% via 16O*(6.1) and 20% via 16O*(6.9 - 7.1) (1955HO68).
At higher bombarding energies, evidence is reported for the excitation of states at 6 to 7, 12.5, and ≈ 20 MeV (1956ST30, 1957TY36: Ep = 96 and 177 MeV). The elastic scattering at high energies is generally characterized by direct interaction and may be described in terms of the optical model: see (1953BU72, 1955FU1A, 1955KI43, 1956BU95, 1956KI54, 1957CH32, 1957GI14, 1957VA1B). Polarization of scattered protons has been studied at Ep = 173 MeV by (1957AL39, 1957HI98, 1957MA58). See also (1955KO1A, 1957JA1B, 1958MA1B, 1958TY49).
Levels derived from observed α-particle groups are listed in Table 16.10 [16O levels from 19F(p, α)16O] (in PDF or PS) (1956SQ1A, 1957YO04: see also (1952AJ38)). There is some evidence for a broad level near 9.58 MeV (compare 12C(α, α)12C) and for two additional unidentified groups which may represent levels near 11 MeV (see 15N(d, n)16O); no other groups appear with intensity > 5% of that corresponding to 16O*(8.87) (1956SQ1A). (1957YO04) find no other groups corresponding to 16O*(6 - 9) with dσ/dΩ > 0.4 mb/sr. The indicated assignments for the first five states derive from a variety of evidence; see (1955AJ61).
Wπ = 6.065 ± 0.009 MeV (1956AL1G). The angular correlation of the monopole pairs is given by W(θ) = 1 + (0.955 ± 0.007)cos θ, consistent with J = 0+ → 0+ (1955GO1E, 1958AR1B: see also (1954DE36)). The mean life is (7.2 ± 0.7) × 10-11 sec, considerably longer than is given by the α-model (1954DE36). Calculations on various models are summarized by (1957GR1G); see also (1956EL1C, 1957EL1B, 1957FE1D).
The mean life is (1.2 ± 0.6) × 10-11 sec (1958KO63), 0.5 to 1 × 10-11 sec (1955DE51). This transition rate is somewhat faster than that predicted on the α-particle model (1956KA1D) and is an order of magnitude faster than indicated by shell-model calculations (1957EL1B). It accounts for about one third of the total E3 width to the ground state from T = 0 states (1958KO63). See also (1958KN1B).
The mean life is ≤ 1.2 × 10-14 sec (1955DE51); compare 16O(γ, γ')16O*: (1957SW17). A search for circular polarization of the γ-ray yields an upper limit of 2.0 × 10-3 indicating a maximum of F2 = 3 × 10-8 for the intensity of any parity non-conserving part of the wave function (1958WI41).
The direct ground state transition is 7 ± 2 % (1957BE61), 9 ± 4 % (1957MC35), 0.6% (1957WA1B) of the cascade decays. The cascade decays take place via the 6.14, 6.92, and 7.12 MeV states with relative intensities 27 : 1 : 3. The three γ-rays have energies 2.75 ± 0.02, 1.90 ± 0.03, and 1.72 ± 0.03 MeV, fixing the energy of the state as 8.87 ± 0.02 MeV. The observed branching ratios, together with the absence of α-decay fix the assignment as Jπ = 2- (1956WI1A, 1957MC35). The separation of this state from the nearest 2+ state (16O*(9.85)) is too large to be accounted for in the α-model of (1954DE1E) (1956WI1A: see also (1956KA1D)).
At Ep = 5.77 MeV, a γ-ray of energy 3.86 ± 0.03 MeV is observed in coincidence with the 7.1 MeV radiation, indicating a level energy of 10.98 ± 0.04 MeV. It is presumed that this level is to be identified with one of the unassigned groups observed by (1956SQ1A, 1957BE61). The direct ground state decay is < 5% and cascades via 16O*(6.0, 6.1, 6.9, and 8.9) are less than 1, 6, 20, and 40%, respectively. The strong branch to 16O*(7.1), J = 1-, and the weakness of other branches suggests J(10.94) = 0-, although 1+ is not excluded. The γ - γ correlation favors J = 0- (1957BE61: see also 15N(d, n)16O). ((1957WA1B) find the γ-ray energy to be 3.86 ± 0.02 MeV and report a weak 4.9 MeV γ-ray). A search for possible nuclear pairs from the 0- → 0+ ground-state transition yielded an upper limit of < 2 × 10-5 for the pair branch (1958EK36). The importance of a definitive assignment of Jπ for this state is stressed by (1957EL1B).
The influence of isobaric spin selection rules on cascade decays in 16O has been discussed by (1953WI1E, 1956WI1D: see also (1955MA1K, 1955MA1L)). At Ep = 18.5 MeV, the α0 angular distribution has been studied by (1956LI37): see 19F.