(See Energy Level Diagrams for 18F)
Special states: (1977AN1P, 1977GR16, 1977HE18, 1977SH18, 1978AN1P, 1978BU03, 1978EN1D, 1978ER1C, 1978KO1W, 1978KR1G, 1978MA2H, 1978PI1E, 1978SA15, 1978TA1A, 1979BA35, 1979BU12, 1979DA15, 1979SA31, 1979WI1Q, 1979ZA07, 1980BR21, 1980GO01, 1980HA41, 1980MA18, 1980RO19, 1980SH1K, 1981AD1E, 1981AR1D, 1981BI03, 1981EL1D, 1981ER03, 1981SO03, 1982KI02, 1982NA03).
Other topics: (1977AN1P, 1977GR16, 1977SH18, 1977SO1C, 1978AN1P, 1978CO08, 1978DE1K, 1978EN1D, 1978GO1U, 1978JO1C, 1978KR1G, 1978MA2H, 1978SH1B, 1979BA35, 1979BE2L, 1979CO09, 1979DE18, 1979GO1W, 1979HW01, 1979KU04, 1979WI1Q, 1980BR21, 1980DE1F, 1980GO01, 1980TA1L, 1981AR1D, 1981BI03, 1981CH24, 1981EL1D, 1981ER03, 1981SO03, 1982FI1C, 1982KI02, 1982NA03).
The positron decay is entirely to the ground state of 18O [Jπ = 0+, T = 1]; the half-life is 109.77 ± 0.05 min [see Table 18.11 (in PDF or PS) in (1972AJ02)]; log ft = 3.554. The fact that the β+ transition to 18Og.s. is allowed indicates Jπ = 1+ for 18Fg.s.. The ratio εK/β+ = 0.030 ± 0.002; see (1978AJ03). See also (1976BE1E, 1981BE2N, 1982PI1B) and (1980AF1A; theor.).
Cross-section measurements have been carried out in the range E(6Li) = 1.9 to 36 MeV: see (1978AJ03) and (1976PO02; 4 to 13 MeV; 6Li elastic), (1978SO01; 9 to 14 MeV and 20 MeV; α0, α1, α2), (1980FU06; 20 to 36 MeV; 6Li to 12C*(0, 4.43)) and (1978CU03; 20 to 34 MeV; d to 16O*(0, 6.13, 6.92, 7.12, 8.87, 10.36, 11.09, 16.3)).
The cross section for the isospin-forbidden α1 group [to 14N*(2.31), 0+, T = 1] is 1 to 2% of the cross section to the allowed α0 and α2 groups for E(6Li) = 3.2 to 6 MeV (1981SC13) while for 9 to 14 MeV it varies from 0.4 to 1.8% (1978SO01). At 20 MeV, the α1 yield is 0.02% of the allowed yield (1978SO01). Structures are reported at E(6Li) = 11.0 and 13.0 MeV in the α0 yield, at 11.5 and (13.0) MeV in the α1 yield and at ≈ 11.7 and 12.8 MeV in the α2 yield (1978SO01), while (1981SC13) report a resonance in the α1 yield at E(6Li) = 4.2 MeV: Ex = 15.99 ± 0.02 MeV, Γc.m. = 290 ± 30 keV, Jπ = 2+ (one-level BW fit). It is suggested that this resonance is due to 2+ states with T = 0 and T = 1 which are unresolved (1981SC13). The excitation functions for the 6Li ions to 12C*(0, 4.43) shows a single isolated structure at E(6Li) = 22.8 MeV, in the range 20 - 36 MeV, with Γ ≈ 0.8 MeV. It is unlikely to be due to an isolated state in 18F (1980FU06). At E(pol. 6Li) = 20 MeV analyzing power measurements are reported for many deuteron and α groups and for elastically scattered 6Li ions (1978MA13, 1979MA1T). Breakup processes have been studied by (1980NE05), cross sections at very high energies by (1980SK1A), fusion by (1981DE1H, 1981DE2K). See also 16O, 17O in (1982AJ01), and 14N, 15N in (1981AJ01) and 12C in (1980AJ01).
See (1978AJ03). The work on reaction (b) displayed there has not been published.
A number of resonances have been observed for Eα < 3 MeV: see Table 18.14 (in PDF or PS). Studies of these, principally by the Toronto and Queen's groups [see references in (1978AJ03) and (1977BE46, 1979KI12, 1980MA26)] in conjunction with work on 14N(α, α), 16O(3He, p), 17O(p, γ) and 17O(p, α) [see Tables 18.16 (in PDF or PS), 18.17 (in PDF or PS), 18.18 (in PDF or PS)] have led to the determination of branching ratios, mixing ratios and widths (Table 18.12 (in PDF or PS)), lifetimes (Table 18.13 (in PDF or PS)) and the Ex, Jπ and Kπ assignments for 18F states with Ex < 6.9 MeV. The reader is referred to the series of papers by the Toronto group for the most complete and definitive arguments on the parameters of the low-lying states of 18F.
Observed anomalies in the elastic scattering are exhibited in Table 18.14 (in PDF or PS). Resonances in the α1 isospin-forbidden yield are displayed in Table 18.14 (in PDF or PS) of (1978AJ03). In the α1 study, carried out for Eα = 7.6 - 16.9 MeV, a partial-wave analysis involving a method of removing ambiguities and parametrizing S-matrix elements gives the level parameters of 151 isospin mixed, natural-parity states in 18F with 10.4 < Ex < 17.5 MeV. Many of these states have also been reported in the 16O(d, α1) reaction [Table 18.16 (in PDF or PS) of (1978AJ03)]. The agreement is best for low-lying 2+ or 4+ states, and is quite good for 3- and 5- states, while for high-J states the greater centrifugal barrier for 16O + d at the same Ex relatively suppresses high-J states in the 16O + d work. A study of the energy dependence of averaged intensities of the partial waves shows some indication that the lower partial waves reconserve isospin as Ex increases (1976CH24).
The total cross section for formation of 10B and 6Li have been studied for Eα = 21 to 42 MeV by (1974JA11) who discuss the astrophysical importance of these processes. For spallation studies see (1979VI05, 1980RE1B, 1981GO1E). See also (1980BA2K; theor.).
Angular distributions have been measured for E(6Li) = 5.3 to 6 MeV: see (1978AJ03).
At E(7Li) = 15 MeV, triton groups are observed to the known T = 0 states with Ex < 7.4 MeV: the T = 1 states are not excited although such transitions are not forbidden in principle, suggesting a direct α-transfer mechanism. The transitions to 18F*(1.70, 2.53, 3.36, 4.23, 5.30, 6.57) account for more than one-half of the summed cross section at 15°. It is proposed that these states (which are only weakly excited in 16O(3He, p)18F and 17O(3He, d)18F) are predominantly of a 4p - 2h nature and are excited by the transfer of four nucleons into the (2s, 1d) shell (1968MI09). At E(7Li) = 36 MeV the Kπ = 1+ band also appears to be selectively populated. States at Ex = 9.58 ± 0.02, 11.22 ± 0.03 and 14.18 ± 0.04 MeV are strongly populated. It is suggested that the first two are the 6+ and 7+ members of that band (1977CO09). [Angular distributions are reported for 18F*(1.70, 2.10, 2.52, 3.36, 4.40, 5.30, 6.57, 9.58, 11.22, 14.18).] See also (1979SA31; theor.).
These reactions have been studied at E(11B) = 115 MeV and E(13C) = 105 MeV. Differential cross sections at three angles are reported for the transitions to 18F*(9.58, 10.57 ± 0.07, 11.2) in reaction (a) and to 18F*(5.30, 6.57, 9.58, 10.60 ± 0.08, 11.2) in reaction (b). In addition to these states 18F*(14.18) is strongly excited in both reactions, and transitions to 18F*(15.79 ± 0.10, 18.62 ± 0.12) are also reported (1979BR04).
Excitation functions have been measured for E(3He) = 2.5 to 16 MeV for the γ0 and γ1→4 yields. Resonances are observed corresponding to Ex = (19.00 ± 0.15) [γ1→4], 20.1 ± 0.2 [γ0, γ1→4], 22.7 ± 0.2 [γ0, γ1→4] and (24.1 ± 0.2) MeV [γ1→4], with Γc.m. = 0.5 ± 0.15, 1.6 ± 0.1, 1.2 ± 0.1 and 1.4 ± 0.3 MeV, respectively. The γ0 yield is dominated by 18F*(20.1) (1981WA1R) [see for (2J + 1)Γ3HeΓγ values]. It is suggested that the structures decaying by γ0 have Jπ = 2- (and possibly T = 1) (1981WA1R, and J. Lowe, private communication).
See 17O in (1982AJ01).
At E(6Li) = 30 MeV preferential excitation of odd-parity states of 18F below Ex = 5 MeV is reported. Angular distributions of the tritons to 18F*(0, 0.94, 2.10, 4.40) [Jπ = 1+, 3+, 2-, 4-] are all strongly forward peaked (1972LI24).
These reactions have been studied with E(11B) = E(12C) = 115 MeV. Reaction (a) is dominated by the transitions to 18F*(1.12) [presumably the Jπ = 5+ state, although the group is unresolved] and to 18F*(7.15, 9.45) [Jπ = (7-) and (6-)]. In reaction (b) no single state is strongly preferentially populated (1979RA10, 1981GO11). Differential cross sections for 18F*(4.40, 6.10, 7.15, 9.45) [Jπ = 4-, (5-), (7-), (6-)], fitted by FRDWBA, are discussed by (1981GO11). See also (1977FO1E).
The capture cross section rises from 0.1 μb at Ed = 0.4 MeV to 25 μb at 3.5 MeV: Γγ over this range is ≈ 2 eV. The results can be interpreted satisfactorily in terms of compound nucleus formation (1965OW01).
Excitation functions have been measured for the n0 and n1 groups from threshold to 17 MeV: see (1978AJ03). Some structure is observed: that which is attributed to states of 18F is displayed in Table 18.15 (in PDF or PS). Polarization measurements are reported for Ed = 3 to 15 MeV: see (1978AJ03) and at Ed-bar = 5 to 15 MeV (1981LI23; pol. n0+1). See also 17F in (1982AJ01), (1980HU1J, 1980HU1D) and (1981NO1B).
Excitation functions and polarization studies have been reported for several proton groups for Ed = 0.3 to 17 MeV: see (1978AJ03) and (1978RO1M; Ed-bar = 14 to 17 MeV). Some of the maxima in the yield measurements are interpreted in terms of resonances: these are shown in Table 18.15 (in PDF or PS). See also 17O in (1982AJ01).
The yield of elastically scattered deuterons and elastic polarization measurements have been reported for Ed = 0.65 to 56 MeV: see (1978AJ03) and (1982AOZZ; Ed = 13.8 to 16.5 MeV), (1981FR1C; TAP; Ed-bar = 20 MeV), (1981CL1C; VAP; Ed-bar = 20.5 MeV; also to 16O*(6.13)), (1980MA10; VAP; Ed-bar = 52 MeV), (1980HA14; VAP; Ed-bar = 56 MeV). For total cross sections at high energies see (1979DE31). See also 16O in (1982AJ01), (1978RO1M) and (1979NI1B, 1980AY01, 1981AO01, 1982YA1A; theor.).
The yields of various groups of α-particles have been measured for Ed ≤ 20 MeV: see (1978AJ03) and (1979MA2Q; 0.65 to 1.1 MeV; α0), (1978RI05; 6.7 to 12.5 MeV; α3) and (1982AOZZ; 13.8 to 16.5 MeV; α0). The yield curves have been fitted in terms of a large number of states in 18F: see Tables 18.15 (in PDF or PS) here and 18.16 (in PDF or PS) in (1978AJ03).
A detailed study by (1973JO13) of the isospin-forbidden α1 yield, analyzed by S-matrix theory, identifies a large number of isospin mixed states in 18F, possibly as many as 138 with 9.2 < Ex < 19.4 MeV. The reaction mechanism appears to be almost entirely compound nuclear. The isospin impurity, averaged over 1 MeV intervals, is 3 - 10% for the above Ex range. The average coherence width increases from ≈ 100 keV at Ex = 14 MeV to ≈ 500 keV at Ex = 20 MeV. The level densities appear to be consistent with predictions of the Fermi-gas model (1973JO13). See also (1981JO1D). [For mixed isospin states observed in 14N(α, α1) see Table 18.14 (in PDF or PS) in (1978AJ03)].
Polarization measurements are reported for Ed = 6.8 to 16 MeV: see (1978AJ03). The T21 analyzing power of the transition to 14N*(4.91) [0-] has been measured at Ed = 8.06 MeV (θ = 45°) to 0.5% (1981KA21). See also (1979TO1A, 1980PR1A, 1981TO1G), (1979MA2Q: applications), (1978SE01, 1979SE04; theor.) and 14N in (1981AJ01).
Excitation energies derived from measurements of γ-rays are displayed in Table 18.16 (in PDF or PS), together with l-assignments obtained from distorted-wave analyses, and Jπ; T and Kπ assignments from branching ratios, radiative widths, linear polarization, γ-ray angular distributions and τm measurements [see also Tables 18.12 (in PDF or PS) and 18.13 (in PDF or PS)]. Studies of this reaction, together with the work on 14N(α, γ) and 17O(p, γ), have defined the low-lying states of 18F.
The g-factor of 18F*(0.94) [Jπ = 3+] is +0.54 ± 0.06 (1981ST21), 0.58 ± 0.07 (1978GO1T). τm of 18F*(1.04) = 2.7 ± 0.7 fsec. The weighted mean "best" value [see Table 18.12 (in PDF or PS)] is 2.7 ± 0.4 fsec. This leads to |M|2 = 10.3 ± 1.5 W.u., and using the E1 strength of the 18F*(1.08 → g.s.) decay leads to a value for the circular polarization of the 1.08 → g.s. γ-ray [Pγ] [0-; T = 0 to 1+; 0] = ± (5.8 ± 0.4) keV-1< 1.04|VPNC|1.08 > (1981KE04). The experimental value for Pγ = (-0.7 ± 2.0) × 10-3 (1978BA08). (1981AD1E) reports a best value of Pγ = (-0.1 ± 1.2) × 10-3. The importance of the decays of 18F*(1.04, 1.08) [Jπ = 0+, T = 1; Jπ = 0-, T = 1, respectively] lies in testing the parity non-conserving (PNC) component of nuclear forces: see also (1980BI11), (1981HA1H) and (1980HA41; theor.). The magnetic moment of 18F*(1.13) = +2.855 ± 0.030 nm, in agreement with shell-model predictions for a (1d25/2)5+ state: see (1978AJ03). See also (1979HA1M), (1977FI13, 1980HE06; applied) and 19Ne.
Angular distributions for α0 are reported at E(6Li) = 5.5 to 13.3 and 26 MeV: see (1978AJ03). At E(6Li) = 34 MeV angular distributions are reported to 18F*(0, 0.94, 1.1 [unres.]) and forward angle data has been obtained for 18F*(2.10, 3.72, 3.79, 3.84). The angular distributions for the first three groups are forward peaked and show little structure at back angles. 18F*(1.04, 3.06) are not observed, as expected from isospin conservation. States at Ex = 4.12, 4.23, 4.4, 5.61, 6.11, 6.20, 6.53, 6.80 and 6.88 were populated (1976MO24). See also (1981MA26; theor.).
Angular distributions are reported at E(14N) = 76.2 MeV to 18Fg.s. (1977MO1A).
Gamma-ray measurements lead to the very accurate Ex determinations for 18F states below 6.2 MeV: see Table 18.17 (in PDF or PS). Observed resonances are displayed in Table 18.18 (in PDF or PS); branching ratios, radiative widths and multipole mixing ratios are shown in Table 18.12 (in PDF or PS); and τm in Table 18.13 (in PDF or PS).
The direct capture cross section has been studied for Ep = 0.3 to 1.9 MeV: 18F*(5.603, 5.605, 5.668, 5.786) have Jπ = 1+, 1-, 1- and 2-. The 1- states have mixed isospin. Astrophysical considerations are discussed in (1978AJ03) and (1975ZI1A, 1977RO1H, 1978RO1D).
The yield of α0 shows a number of resonances for Ep = 0.49 to 3.0 MeV: see Table 18.18 (in PDF or PS). The R-matrix fit of (1979KI13), obtained from data from Ep = 400 to 1400 keV, confirms the earlier result [see, e.g., reaction 31 in (1978AJ03)] that a significant quantity of 17O is burned up in the (p, γ) rather than in the (p, α) reaction for a wide range of stellar temperatures (1979KI13). See also (1977RO1H, 1978RO1D; astrophys.).
At E(3He) = 15 MeV, DWBA analysis of angular distributions of deuteron groups corresponding to the ground state of 18F [l = 2] and to the excited states at 0.94 [l = 0 + 2], 1.04 , 1.12 , 2.53 [0 + 2], 3.06 [0 + 2], 3.84 [0 + 2], 4.12 [0 + 2], 4.66  and 4.96 [l = 0 + 2] have been obtained by (1969PO11) who also report spectroscopic information. Thus all these states have even parity and 18F*(4.11) may be assigned Jπ = (2+) or 3+. Since l = 2 for 18F*(4.65), Jπ ≤ 5+, with 4+ most likely (1969PO11). See also (1979SA31; theor.).
Forward angle differential cross sections are reported for the transition to 18F*(1.12) at E(12C) = 115 MeV (1980PR09).
Angular distributions of a number of neutron groups are reported for Ep = 5.5 to 13.5 MeV: see (1978AJ03). The τm of 18F*(1.04) [Jπ = 0+; T = 1] is 2.7+0.6-0.4 fsec (1980KE05): see Table 18.13 (in PDF or PS). See also 19F.
At E(3He) = 16 MeV, the triton spectrum is dominated by strong groups to the ground and 0.94 MeV excited states and to the 0+ and 2+, T = 1 states at Ex = 1.04 and 3.06 MeV. Angular distributions have been measured and analyzed by DWBA for the tritons corresponding to these states and to 18F*(1.08, 1.12, 1.70, 2.10, 3.13, 3.36, 3.72, 3.79, 3.84, 4.12, 4.23, 4.36, 4.40, 4.65, 4.75). The angular distributions are consistent with the Jπ assignments shown in Table 18.11 (in PDF or PS), except for the distribution to 18F*(1.04) (1970DU08). At E(3He) = 17.3 MeV, angular distributions to 18F states with Ex < 4 MeV have been analyzed using DWBA and a two-body interaction between the incident and target nucleons. An exact coupled-channel calculation was also made for the transition to 18F*(1.04) (1968HA30). At E(pol. 3He) = 33 MeV the analyzing power angular distribution for t0 has been measured by (1981BA1G).
At E(6Li) = 34 MeV angular distributions have been obtained for the transitions to 18F*(0, 0.94): there appears to be a sizeable contribution due to two-step processes: see (1978AJ03).
Angular distributions of the d0 group have been measured at Ep = 16 to 155.6 MeV: ln = 0: see (1972AJ02) and (1977GU14: 17.7 MeV). At Ep = 19.3 MeV angular distributions are also reported to excited states of 18F: see Table 18.20 (in PDF or PS). The angular distributions associated with 0h - 2p states in 18F are only qualitatively accounted for by DWBA. Those corresponding to 1h - 3p negative-parity states generally cannot be accounted for by a single-step mechanism, while the distributions to states arising from 2p - 4h configurations do not show the features associated with direct interaction (1980DE05). See also (1982MA1H) and (1978AJ03).
At Ep = 45 MeV, 18F*(0, 1.04, 1.70, 3.06, 6.27) are populated: see (1978AJ03).
At Ed = 11 MeV α-groups are observed to many states of 18F with Ex < 7 MeV. Weak or absent (each ≤ 0.3% of the total yield at 30°) are the groups corresponding to 18F*(1.04, 3.06, 4.66, 4.74, 4.96): T = 1 for these states (1969PO11). Angular distributions at Ed = 6.66, 7.29, 7.93 and 12.95 MeV have been obtained for the α-groups to 18F*(2.10 [not 12.95 MeV], 2.52, 3.06 [2+; 1], 3.13, 3.36, 4.12). The ratio of the total cross section for the isospin-forbidden transition to 18F*(3.06) to the total cross section of four T = 0 states ranges from 6% to 40%. The average is 21% suggesting isospin mixing in the compound nucleus: the yield curves show strong structures particularly in the range 5 to 9 MeV (1973HR03). Angular distributions have also been measured at Ed = 2 to 14.7 MeV: see (1978AJ03). Measurements of the tensor analyzing power for Ed-bar = 10.25 to 12.0 MeV leads to assignments of unnatural parity for 18F*(0, 0.94, 1.08 + 1.12, 1.70, 2.10, 3.36, 3.72, 4.12, 4.23, 4.36, 4.40, 5.603 + 5.605 + 5.67, 5.79, 6.10 + 6.10, 6.14 + 6.16, 6.31, 6.48) and to natural parity for 18F*(3.79, 3.84, 4.86): assignments of Jπ = 2-, 1+, 0+, 1-, 1+, 3+, 3+ are made for 18F*(4.23, 4.36, 4.75, 4.86, 5.603, 6.16, 6.48) [all T = 0 except 18F*(4.75)] (1978DA02).