(See Energy Level Diagram for 18F)
The positron end point is 635 ± 15 (BL49A), 649 ± 9 keV (RU51). The spectrum is simple (see (DR56A)). The half-life is 112 ± 1 min (BL49A), 111 ± 1 min (JA55), 110 ± 1 min (BE58G): log ft = 3.62. The fact that the β transition to the ground state of 18O is allowed indicates J = 1+ for 18F (assumed T = 0) (MO54). The ratio εK/β+ = 0.030 ± 0.002 (DR56A). See also (DZ56) and (RE58; theor.).
Resonances have been observed at Eα = 1.53, 1.62, 2.35, and 2.88 MeV, corresponding to 18F*(5.61, 5.68, 6.25, and 6.651) (PR55, AL58, BR58, PH58): see Table 18.5 [Resonances in 14N + α] (in PDF or PS).
The 5.61 MeV state decays to the 18F ground state (16%), and to states at 1.08 MeV (41%) and 3.06 MeV (45%). Gamma rays with Eγ = 2.12 and 0.94 MeV are also observed (AL58): see Fig. 39 (Gamma-ray transitions in 18F). The radiative widths ωΓs to the ground state and 18F*(1.08) are 0.7 and 2.2 eV, respectively; all others amount to 3 eV (PR55). The branching ratios favor J = 1-; T = 0 for 18F*(5.61) and T = 1 for 18F*(1.08 and 3.06). Angular distributions of the cascade γ-rays are consistent with J = 0+ and 2+ for the states at 1.08 and 3.06 MeV. Angular correlations rule out J = 0 for the 0.94 MeV state. The 3.06 MeV state shows a 25% ground-state branch and a 75% cascade through the 0.94 MeV level (AL58: see also (KU58C)). Delayed coincidence measurements on the γ-decay of the 0.94 MeV state show τ ≤ 5 x 10-9 sec. This implies 1 ≤ J ≤ 3 when taken together with the results of the 16O(3He, pγ)18F reaction (BR58).
The 5.68 MeV state decays to the 1.08 MeV state (1.075 ± 0.010 MeV) with ωΓγ = 2.2 ± 0.3 eV. The ground-state transition is < 0.2 eV; others amount to about 2 eV (PR55). The strength of the transition 5.68 → 1.08 suggests dipole radiation, J(5.68) = 1; T = 0 (WA59).
The 6.25 MeV state decays mainly to 18F*(1.7): Eγ = 4.45 ± 0.05 MeV (ωΓγ = 7.9 ± 1.0 eV), and 18F*(0.94): Eγ = 5.30 ± 0.10 MeV (ωΓγ = 0.8 ± 0.15 eV). Transitions to the ground state and to levels at ~ 3 MeV have upper limits of 5% and 20%. The 1.7 MeV state decays through 18F*(1.05). The (6.25 → 1.7) and (1.05 → 0) γ-γ correlation is isotropic within 10%, consistent with J = 0+ for the 1.05 MeV state. The 1.7 MeV state is most likely to be 1± or (2+), and is assumed to be T = 0 from the 18O level structure (compare 20Ne(d, α)18F). The 6.25 MeV state is then 2± or 3- (PH58). If the 1.7 MeV state is 1+, J(6.25) is 2+ or 2-; the strength of the transition (6.25 → 1.7) indicates T = 1 for 18F*(6.25) (WA59).
The 6.65 MeV state decays with Eγ = 5.80 ± 0.10 (7%), ωΓγ = 0.25 eV and 4.90 ± 0.05 MeV (70%), ωΓγ = 2.1 eV to the 0.94 and 1.70 MeV states, respectively. Transitions with relative intensity ~ 23% are also reported to states with Ex ~ 3 MeV (PH58). See also (BA58H, BR58E, HE58A) and 16O(3He, p)18F.
Observed resonances are displayed in Table 18.5 [Resonances in 14N + α] (in PDF or PS) (HE53, HE58D, KA58C). Absolute cross sections are given by (HE58D) for Eα = 2.7 to 3.6 MeV. The yield of protons to 17O*(0.7) is at least a factor of 10 smaller than the ground-state yield. See also (MA54M, GR55) and (1952AJ38).
Observed anomalies in the elastic scattering are exhibited in Table 18.5 [Resonances in 14N + α] (in PDF or PS) (BR39, DE39A, HE53, HE58D, HE58E, KA58C). The indicated assignments to the narrow resonances at Eα = 2.35, 2.37, 2.77, 2.88, and 3.58 MeV are obtained from qualitative analysis of the excitation functions. The two broad resonances at Eα = 2.870 and 3.080 MeV have been analyzed in detail: both are formed with p-waves, J = 1- and 2-, respectively. For the former, Γα/Γ = 0.85. See also (KA58C). A broad anomaly at Eα = 3.7 MeV requires two levels, and possible admixture of higher l-values (HE58E, KA58C).
The yield shows strong variations with energy, suggesting compound nucleus formation (HE48A, BE55E, ST55C, RO56A, VA56C): see Table 18.6 [Maxima in the yield of 16O + α reactions] (in PDF or PS). On the other hand, angular distribution measurements show strong stripping peaks for both p0 and p1, for Ed = 0.5 to 4.1 MeV (AL55F, BE55E, JU55A, ST55C, GR56, JU56C, KO56F, BA57F, JU57). See also 17O.
Observed maxima in the θ = 135° and 97° cross section are listed in Table 18.6 [Maxima in the yield of 16O + α reactions] (in PDF or PS) (BE56D). Structure in the yield of elastically scattered deuterons is also reported by (BA57F). See also (KH55, KH55A).
Two resonances are reported for ground-state α-particles at Ed = 3.85 MeV, Γ = 35 keV, and 4.0 MeV, Γ = 100 keV (BA57F): see Table 18.6 [Maxima in the yield of 16O + α reactions] (in PDF or PS). Yield curves of the α0, α1 and α2 groups are reported for Ed = 5.5 to 7.5 MeV by (BR56F). The α1-group, leading to the T = 1, 2.31 MeV state of 14N, is greatly inhibited: the average intensity is only a few per cent of the average yield of the α0 group. Broad resonances are observed in the yields of all three α groups. The resonances in the α1-yield occur at ~ 6.2, 6.6, and 7.0 MeV; they are presumably due to predominantly T = 1 states at Ex ~ 13.0, 13.4, and 13.8 MeV (BR56F). At Ed = 7.2 MeV, the intensity ratio α1/α0 is 16% (θ = 35°), at 6.8 and 7.1 MeV, α1/α0 < 6%, and at 8.9 MeV, < 2% (DA58B). The observations are consistent with expected isobaric spin impurities in the states involved (BR56F). Angular distributions for α0, α2 may give evidence of compound nucleus effects (DA58B): see also 14N.
Seventeen proton groups corresponding to excited states of 18F from 0 to Ex = 4.36 MeV are reported by (HI59: E(3He) = 5.7 to 5.9 MeV): see Table 18.7 [Energy levels of 18F from 16O(3He, p)18F, 19F(3He, α)18F and 20Ne(d, α)18F] (in PDF or PS). See also (KU58C). Angular distributions for the first five states, analyzed by direct interaction theory, indicate J = 1+, 2+ or 3+, 0+, (?), and 4+ or 5+ for 18F*(0, 0.94, 1.04, 1.08 and 1.13), respectively (HI59: tentative assignments). Gamma transitions to 18F(0) have been observed from the levels at 0.94, 1.04 and 1.08 MeV; the decay of the 1.13 MeV level has not been detected (KU58C). Proton-gamma coincidence techniques have been used to determine branching ratios and angular correlations involving 18F states at 0.94, 1.08, 1.7, 2.1, 2.5, and 3.07 MeV (KU58: see also (BU56)): see Fig. 39 (Gamma-ray transitions in 18F). All of these states show at least one anisotropic radiation except for 18F*(1.04 and 1.08), excluding J = 0 for all but these two ((KU58) and E. Almqvist, private communication).
The mean life of the 0.94 MeV state is ≤ 5 x 10-9 sec, indicating J ≤ 3 (BR58). The polarization of the γ-radiation indicates positive parity for the state (LI58B). The strong transition from 18F*(3.07: T = 1) suggests T = 0. For the 1.04 MeV state, J = 0+ is indicated by the proton angular distributions (HI59); J = 0-; T = 0 is suggested by the γ-branching (E. Almqvist, private communication). The 1.08 MeV state appears to have J = 0+; T = 1 (see 14N(α, γ)18F). The absence of the ground-state transition from 18F*(1.12) is consistent with its expected J = 5+ character. The 1.7 MeV state is fixed as J = 1+ from the present reaction and 19F(p, d)18F. For 18F*(2.1 and 2.54) assignments of J = 1, 2 and 1, 2, 3 are indicated by the branching ratios (E. Almqvist, private communication). See also (BR58D, BR58E).
A resonance in the capture cross section may be indicated near 1.24 MeV (Ex = 6.79 MeV), σ ~ 1 mb (NI56).
Slow neutron thresholds are reported corresponding to Ex = 960 ± 10, 1065 ± 15, and, possibly, 1245 ± 10 keV; no others appear with Ex < 1.65 MeV (NA56). Gamma rays are observed with Eγ = 940 ± 20 and 1040 ± 20 keV at Ep = 4.3 and 4.4 MeV: the 1040 keV γ-ray exhibits a Doppler shift (τ < 3 x 10-13 sec, consistent with J = 0+), the 940 keV γ-ray does not, which is consistent with J = 3+, 5+. No γ-ray is observed from the 1245 keV state (NA56A, RO56B). See also 16O(3He, p)18F and 19F.
At Ep = 18 MeV, angular distributions have been measured to the ground state of 18F (RE56, BE58A) and to levels at 0.94, 1.05, 1.7, 3.4 and 4.1 MeV (BE58A). The results indicate J = 0+ or 1+ for the ground state (RE56), even parity for the 0.94, 1.04, 1.7 and 4.1 MeV states; probably assignments are stated to be 3+, 0+, 0+ and 2+, respectively, and odd parity, J ≤ 2 for the 3.4 MeV state (BE58A). The reduced width for the ground state reaction is θ2 = 0.009 (RE56). According to (WA59), the distributions permit J = 0+ or 1+ for 18F*(1.7); J = 0 is excluded by the observed γ-decay to 18F*(1.04). See also 19F and (EL58A; theor.).
At Ed = 8.9 MeV proton groups are observed to the ground state and to states at 0.94 ± 0.02 and 1.07 ± 0.02 MeV. The angular distributions are consistent with ln = 0, 1 and 0, respectively (EL57A). The ln = 1, odd parity assignment for the 0.94 MeV state is not in accord with other experiments (see, e.g., 16O(3He, p)18F and 19F(p, d)18F). See also (BO50D, SH51A).
At E(3He) = 5.9 MeV, 41 α-particle groups have been observed, corresponding to the ground state of 18F and to excited states with Ex < 7.5 MeV (HI59): see Table 18.7 [Energy levels of 18F from 16O(3He, p)18F, 19F(3He, α)18F and 20Ne(d, α)18F] (in PDF or PS).