(See Energy Level Diagrams for 18F)
Special states: (1978WI1B, 1982ZH01, 1983BI1C, 1983BR29, 1983ME12, 1983KI13, 1984AD1E, 1984HA14, 1984HO1H, 1984MI1H, 1984MI17, 1985AD1A, 1985HA18, 1985LE1K, 1985MI10, 1985SO12, 1985YU1B, 1986AN07, 1986CA27, 1986HA1E, 1986HA1Q, 1986SI1K, 1986TO1D, 1986WI1P, 1986YU1B, 1987ER05).
Astrophysical questions: (1982WI1B).
Complex reactions involving 18F: (1983DE26, 1983JA05, 1983OL1A, 1983WI1A, 1984GR08, 1984HI1A, 1984HO23, 1985BE40, 1985CU1C, 1985DR07, 1985GR1K, 1985PO11, 1986AV07, 1986BA3G, 1986GR1A, 1986HA1B, 1986HO27, 1986PO06, 1986SC28, 1986SC29, 1986VA23, 1987RI03, 1987RO10).
Q1.12 = 0.13 ± 0.03 b [see (1983AJ01)].
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 (1985BR29), (1982KA1C; applications), (1985AR1A, 1985KL1A, 1986HE1C, 1987GOZX; astrophyics) and (1983CA03; theor.) [see for discussions of weak magnetism form factor and of search for heavy neutrinos].
See (1986CU02) for production cross sections of 0.94 MeV γ-rays.
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 while for 9 to 14 MeV it varies from 0.4 to 1.8%. At 20 MeV, the α1 yield is 0.02% of the allowed yield. 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. A resonance is also reported 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. Cross sections for populating 16O*(8.87, 10.36, 11.08, 11.10) are reported by (1981GL02).
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. At E(pol. 6Li) = 20 MeV analyzing power measurements are reported for many deuteron and α groups and for elastically scattered 6Li ions. VAP measurements for elastic scattering are also reported at Epol. d = 9.0 and 19.2 MeV (1983RU09) and at 150 MeV (1986TA1B, 1986KA1C).
For fusion studies see (1982DE30, 1987PA12). For references to earlier work and for additional comments see (1978AJ03, 1983AJ01). See also (1980KR1F), (1984MU1D; theor.) and 12C in (1985AJ01), 14N in (1986AJ01) and 16O in (1986AJ04).
τm = 5.2 ± 0.9 psec for 18F*(4.85) [5-; T = 0]. The E1 strength is (3.4 ± 0.6) × 10-6 W.u. for the transition to 18F*(1.12) [5+; T = 0] and the E2 strength is 14.8 ± 2.6 W.u. for that to 18F*(3.79) [3-; 0]. The latter strength, which is that of a highly collective transition, corresponds to a quadrupole moment Q0 = 395 ± 35 mb and suggests that 18F*(4.85) is the 5- state of a (strongly decoupled) Kπ = 1- band 1982KO24). See also Tables 18.14 (in PDF or PS) and 18.15(in PDF or PS).
The non-resonant S-factor for this reaction, S ≈ 0.7 MeV · b: see (1978AJ03). A number of resonances have been observed for Eα < 3 MeV: see Table 18.16 (in PDF or PS). Studies of these, principally by the Toronto and Queen's groups [see references in (1978AJ03, 1983AJ01)] in conjunction with work on 14N(α, α), 16O(3He, p), 17O(p, γ) and 17O(p, α) [see Tables 18.18 (in PDF or PS), 18.19 (in PDF or PS), 18.20 (in PDF or PS)] have led to the determination of branching ratios, mixing ratios and widths (Table 18.14 (in PDF or PS)), lifetimes (Table 18.15 (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.
No evidence is seen for the excitation of the (forbidden) state at Ex = 4.753 MeV [Jπ = 0+; T = 1] (1981LE1A, 1983LE08). See also (1985GO27; applications) and (1982BA1D, 1985AR1A, 1985KL1A, 1985MA1A, 1986HE1C, 1981LE1A, 1987GOZX; astrophysics).
Observed anomalies in the elastic scattering [reaction (a)] are exhibited in Table 18.16 (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 argeement 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.
The total cross sections for formation of 10B and 6Li have been studied for Eα = 21 to 42 MeV [see (1978AJ03)], as has the cross section for production of 1.64 and 2.31 MeV γ-rays from threshold to Eα = 26 MeV [(1985DY05): see for astrophysical implications.] See also (1984RE14, 1986CO11) and (1986ALZZ; theor.).
At E(6Li) = 36 MeV angular distributions have been measured for the deuteron groups to 18F*(5.34 [4+], 6.56 [5+], 9.58, 11.2, 14.1). Angular correlations lead to Jπ = 6+ and 8+ for 18F*(9.58, 14.1) and the data are consistent with Jπ = 7+ for 18F*(11.2) (1983ET02). For the earlier work see (1978AJ03).
At E(7Li) = 36 MeV the Kπ = 1+ band appears to be selectively populated. States at Ex=9.58 ± 0.02, 11.22 ± 0.03 and 14.18 ± 0.04 MeV are stongly populated. It is suggested that the first two are the 6+ and 7+ members of that band: see reaction 8. [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 (1978AJ03) for the earlier work, and (1986NE1A; E(14N) = 150 MeV). See also (1986CO11; 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: see (1983AJ01).
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) (1983WA05) [see for (2J + 1)Γ3HeΓγ values]. It is suggested that the structures decaying by γ0 have Jπ = 2- (and possibly T = 1) (1983WA05). For analyzing power measurements at E(pol. 3He) = 33 MeV see (1986DR03).
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: see (1978AJ03).
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. Differential cross sections for 18F*(4.40, 6.10, 7.15, 9.45) [Jπ = 4-, (5-), (7-), (6-)], are fitted by FRDWBA: see (1983AJ01).
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: see (1972AJ02).
Excitation functions and polarization studies have been carried out to Ed = 17 MeV [see (1978AJ03, 1983AJ01)] and at Ed ≈ 5.6 to 8.3 MeV (1985GR1B; p0, p3, p4). Structures attributed to states in 18F are displyed in Table 18.17 (in PDF or PS). See also 17O and 17F in (1986AJ04), (1985JOZZ, 1986SE1B) and (1983DA31; applications).
The yield of elastically scattered deuterons and elastic polarization measurements have been reported for Ed = 0.65 to 56 MeV: see (1978AJ03, 1983AJ01). More recent measurements are those by (1985GR1B) [excitation functions for Ed ≈ 5.6 to 8.3 MeV] and the polarization studies at Epol. d = 20.5 MeV (1984FR14; TAP), 56 MeV (1986MA32; VAP, TAP) and 200, 400 and 700 MeV (1987NG01; VAP, TAP). See also (1982AO06, 1983AO03, 1986NG1B, 1986YA1R, 1987MA09; theor.) and 16O in (1986AJ04).
The yields of various groups of α-particles have been measured for Ed ≤ 20 MeV: see (1978AJ03, 1983AJ01). The yield curves have been fitted in terms of a large number of states in 18F: see Tables 18.17 (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 (1985JOZZ). [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, 1983AJ01).
Excitation energies derived from measurements of γ-rays are displayed in Table 18.18 (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 18.14 (in PDF or PS) and 18.15 (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 +(0.56 ± 0.05): see (1983AJ01). The circular polarization of the 1.08 MeV → g.s. γ-ray, Pγ = (-10 ± 18) × 10-4 (1982AH07), (2.7 ± 5.7) × 10-4 (1985BI03), (1.6 ± 5.6) × 10-4 (1985EV03), (1.7 ± 5.8) × 10-4 (1987PA07). The weak pion-nucleon coupling constant deduced from the weighted average of all recent Pγ measurements [(1.2 ± 3.9) × 10-4] is (0.3+1.0-0.3) × 10-7. Together with PNC matrix elements in other experiments this suggests that the isovector weak NN interaction may be strongly suppressed compared with the isoscalar weak NN interaction (1985EV03, 1987PA07). For a measurement of the ICC of the 0.94, 1.02, 1.04 and 1.08 MeV γ-rays see (1986KR04). See also (1978AJ03, 1983AJ01), (1984BE1A; applications), (1987ER05; theor.) and 19Ne.
Angular distributions of the deuteron groups to 18F*(1.12) [Jπ = 5+] have been studied at Eα = 28.0 to 33.6 MeV: see (1983AJ01). At Eα = 65.3 MeV a number of angular distributions are reported to 18F states with Ex ≤ 11.4 MeV: 18F*(9.49, 10.54) are suggested to have Jπ = 6- and 7+ respectively (1986KA36). See, however, reactions 9 and 10.
Angular distributions have been measured at E(6Li) = 5.5 to 34 MeV [see (1983AJ01)] and at E(6Li) = 48 MeV (1984CO05; α0, α1, α4). (1982FR15) report the excitation of a state at Ex = 4848.3 ± 0.5 keV which decays (35 ± 4)% to 18F*(3.79) [Eγ = 1056.8 ± 0.4 keV] and (65 ± 4)% to 18F*(1.12). Alpha-γ angular correlations are consistent with Jπ = 5-, and T = 0 (1982FR05). See also (1986BGL02) and (1986IC01; theor.).
Gamma-ray measurements lead to the very accurate Ex determinations for 18F states below 6.2 MeV: see Table 18.19 (in PDF or PS). Observed resonances are displayed in Table 18.20 (in PDF or PS); branching ratios, radiative widths and multipole mixing ratios are shown in Table 18.14 (in PDF or PS); and τm in Table 18.15 (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. For astrophysical considerations see (1978AJ03, 1983AJ01) and (1982RO1A).
The elastic scattering has been studied for Ep = 0.5 to 13 MeV [see (1978AJ03, 1983AJ01)]: observed anomalies are displayed in Table 18.20 (in PDF or PS). Analyzing powers have been measured at Epol. p = 89.7 MeV (1985VO12; p0).
Analyzing powers have been reported at Epol. p = 89.7 MeV for the triton groups to a number of 15O states (1985VO12).
The yield of α0 shows a number of resonances for Ep = 0.49 to 3.0 MeV: see Table 18.20 (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 (1982RO1A, 1987RO25) and (1986BA89; theor.).
At E(3He) = 15 MeV DWBA analysis of angular distributions of deuteron groups corresponding to states of 18F with Ex < 5 MeV have led to Jπ values and spectroscopic information: see (1972AJ02). See also (1987ER05; theor.).
(1983AN05) have studied the distribution of Gamow-Teller (GT) strength. At Ep = 135 MeV angular distributions have been studied to the 0+ state at 1.04 MeV and to the 1+ states 18F*(0, 1.70, 3.72, 4.36, 6.26) as well as to possible 1+; T = 1 groups at Ex = 9.9 10.9 and 11.9 MeV. 82% of the observed strength lies in the ground state group and 5.5% in the (T = 1) states. The observed GT strength is ≈ 2/3 of that expected from the simple sum rule (1983AN05). See also (1985TAZY), (1985WA24), (1986MA1P, 1987HI1B; applied) and the earlier work in (1978AJ03).
At E(3He) = 16 MeV, the triton spectrum is dominated by strong groups to 18F*(0, 0.94) and to the 0+ and 2+, T = 1 states 18F*(1.04, 3.06). Angular distributions have been studied to these and many other states at this energy and at E(3He) = 17.3 MeV. At E(pol. 3He) = 33 MeV Ay measurements for t0 have been reported. See (1983AJ01) for references.
Cross sections have been reported to 30 MeV for the transitions to 18F*(0.94, 1.04, 1.08, 3.06, 3.13, 4.75): see (1983AJ01).
Angular distributions have been reported to many states of 18F with Ex ≲ 6 MeV: see Table 18.20 (in PDF or PS) in (1983AJ01). See also (1986VAZV [Ep = 18.6 MeV; d0, d2]), (1983BEYY) and (1981CL05, 1983KI13; theor.).
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. Measurements of the TAP for Epol. d = 10.25 to 12.0 MeV leads to assignments of 2-, 1+, 0+, 1-, 1+, 3+, 3+ to 18F*(4.23, 4.36, 4.75, 4.86, 5.603, 6.16, 6.48). See (1972AJ02, 1978AJ03, 1983AJ01) for references and for other results and (1987HI1B; applications).