(See Energy Level Diagrams for 20F)
Muon and pion capture and reactions: (1974LI1D).
Q = 0.064 ± 0.012 b (1974ST10).
Recent values of the half-life of 20F are 11.03 ± 0.06 (1970WI05), 10.996 ± 0.020 (1975AL27) and 11.18 ± 0.01 sec (1976GE08). We adopt τ1/2 = 11.00 ± 0.02 sec. See also (1970AS1C, 1974AL11, 1975SA1D) and Table 20.5 (in PDF or PS) in (1972AJ02) for the earlier values. 20F decays principally to 20Ne*(1.63): see 20Ne and Table 20.36 (in PDF or PS). See also (1976CA1E) and (1971WI1C, 1972BE1E, 1972EM02, 1972WI1C, 1973BE35, 1973LA03, 1973WI11, 1974WI1L, 1975NA20, 1975NA21, 1975WI1E, 1977AZ02, 1977CA11; theor.).
Table 20.7 (in PDF or PS) shows the 20F states observed in this reaction at E(7Li) = 16 MeV. The cross sections for forming states of known Jπ are linear with 2Jf + 1 with slopes which are different for the even- and the odd-parity states. Extrapolation of these relationships to states of unknown Jπ leads to the assignments shown in Table 20.7 (in PDF or PS): 20F*(2.86, 2.97, 3.59, 3.68, 4.52, 4.76) and one or both of the unresolved states at 4.2, 4.6 and 4.9 MeV, must, on the basis of the σt for their population, have large values of J (1975BI04). See also (1973FO1A, 1975HO1L).
Vector analyzing power measurements have been carried out at Ed-bar = 14.8 MeV for the proton groups to 19O*(0, 1.47, 3.15, 3.95, 4.12, 4.58, 4.71, 5.00 + 5.09, 5.15, 5.46, 5.71, 6.28) (1974SE01): see 19O. Measurements at Ed-bar = 10 MeV are reported by (1973ST1B; abstract). See also (1974FO1J, 1976DA1K).
Vector analyzing power measurements involving the elastic group have been carried out at Ed-bar = 14.8 MeV (1974SE01).
Excitation functions have been measured for Ed = 0.9 to 3.0 MeV and 9.6 to 11.5 MeV: see (1972AJ02). At the lower energies a number of sharp structures are reported.
Proton groups have been observed to states of 20F with Ex < 4.1 MeV (1970RO06, 1974CR04): see Table 20.8 (in PDF or PS). Angular distribution measurements at E(3He) = 18 (1974CR04) and 19 MeV (1976ME14), γ-ray polarization data (1973LO13) and branching ratio and life-time measurements (1972AL26, 1973PR01) lead to the Jπ values shown in Table 20.8 (in PDF or PS) [see also Tables 20.5 (in PDF or PS) and 20.6 (in PDF or PS)]. At E(3He) = 5 MeV, a state is populated at Ex = 6519 ± 3 keV (1976MI01), which decays principally (> 90%) to 20F*(1.06): the γ-rays are isotropic [Γγ = 3.6 ± 0.6 eV, based on the analog decay in 20Ne]. 20F*(6.52) is the 0+; T = 2 analog of the ground state of 20O (1976MI01, 1977BA50). See also (1971NE1E).
The thermal capture cross section is 9.8 ± 0.7 mb (1974SH1E). See also (1974MU14). A number of resonances have been observed for En ≤ 1.65 MeV: see Table 20.9 (in PDF or PS) (1959GA08, 1971NY02, 1973MA14). The primary γ-rays resulting from capture at thermal energies (20F*(6.60); Jπ = 0+, 1+) and at En = 27, 44 and 49 keV (20F*(6.63, 6.643, 6.647); Jπ = 2-, (3, 4) and 1-, respectively) have been studied by several groups: see (1972AJ02) and Table 20.10 (in PDF or PS) here (1967BE36, 1968SP01, 1969HA04, 1974KE18). It appears that the decay of 20F*(6.60) is dominated by two intense transitions (probably E1) to 20F*(5.94, 6.02) [thus Jπ = 1-, 2-]. If the ground-state transition is mainly M1, these two E1 transitions are (in terms of W.u.) about 150 times stronger than the M1 transition (1968SP01). It appears also that at 20F*(6.63, 6.64, 6.65) [Jπ = 2-, (3, 4) and 1-, respectively] the E1 transitions to the ground state are very weak, even though other E1 transitions in the decay of these two states have approximately normal strengths (1967BE36, 1974KE18). The strongest transitions from the 27 keV resonance appear to be M1. On the basis of the Jπ of the final states involved in the decay of the 44 keV resonance, J = 3 or 4, assuming dipole transitions (1974KE18). Branching ratios for other 20F states involved in this reaction are shown in Table 20.5 (in PDF or PS).
The scattering amplitude (bound) is a = 5.66 ± 0.02 fm (1975KO29). The value of the coherent scattering cross section recommended by (1973MU14) is 4.0 ± 0.1 b. (1974DI1D) find 3.58 ± 0.02 b and a = 5.60 ± 0.01 fm. The spin-dependent part of the scattering amplitude β ≡ a+ - a- = -0.135 ± 0.002 fm (1972AB20).
The total cross section has been measured for En = 0.5 to 29.1 MeV: see the listings in (1972AJ02, 1973MU14), the display in (1976GAYV) and (1974SI27: En = 0.5 to 200 keV), (1974JO1H: En = 20 keV to 10 MeV; abstract). Observed resonances are displayed in Table 20.12 (in PDF or PS). For angular distribution measurements see 19F. For polarization measurements see (1972AJ02). See also (1971KO1W) and (1973BA1U, 1974HU07; theor.).
Recent excitation functions have been measured for the 0.11 and 0.20 MeV γ-rays for En from threshold to 19.4 and 18.6 MeV, respectively: the differential cross section at 92° for the 0.11 MeV γ-ray shows a number of resonances the most pronounced of which are displayed in Table 20.13 (in PDF or PS). The 0.20 MeV γ-ray yield does not show clear resonance structure (1976MO13). See also (1974RO03: En = 0.2 to 1.7 MeV). (1973MA14) has studied the inelastic yield involving the states of 19F near 1.5 MeV: observed resonances are also displayed in Table 20.13 (in PDF or PS). See also (1969RO1F, 1977DI11), (1975FU1D) and (1972AJ02) for the earlier work.
Cross sections have been measured for En = 10 to 37 MeV: see (1972AJ02) for the earlier work, (1972MO15: En = 14.1 MeV) and (1973RO29: En = 14.78 MeV) and the summary in (1976GAYV). See also (1973BO1K, 1974BO1E, 1974CA1J, 1974KO35).
The differential cross section at 92° for production of the 96 keV γ-ray has been studied by (1976MO13: En = 4.0 to 18.6 MeV): the cross section increases sharply at En = 6 MeV and then gradually decreases beyond En = 12 MeV. Cross sections have also been measured for En = 12.6 to 21 MeV: see (1972AJ02) and the summary in (1976GAYV). See also (1972BA1T, 1973BA1T; abstracts) and (1972ED01, 1974BO1E, 1974CA1J, 1976SL2A, 1977DI11).
Measurements of proton groups and of γ-rays have led to the identification of a number of 20F states: see Table 20.15 (in PDF or PS). Angular distributions have been measured for Ed = 0.6 to 13 MeV (see (1972AJ02)) and at Ed = 1.00 - 1.50 MeV (1976BE51), 1.20 to 2.20 MeV (1975CO18: p1), 3 MeV (1972LA18; ln and (2J + 1)S values for a number of states) and at 12 (1974FO21) and 16 MeV (1972FO11). The Jπ and (2J + 1)S values derived from the experiments at 12 and 16 MeV are displayed in Table 20.15 (in PDF or PS). (1975MC1J) have studied the ln = 2 transitions to 20F*(0.66, 2.04, 2.19, 2.97) with Ed-bar = 11 MeV while (1973JO10) report a study at Ed-bar = 12.3 MeV involving 20Fg.s.. (1970QU1A), also using polarized deuterons, have looked at the protons corresponding to 20F*(0.66, 2.04). These two states are found to be populated by a j = 5/2 neutron transfer. This result, together with (p - γ) correlation data, provides a unique Jπ = 3+ assignment for 20F*(0.66) and is in agreement with 2+ for 20F*(2.04). Transitions to 20F*(2.19, 2.97) appear to involve admixed j = 3/2, 5/2 transfer implying J = 2 for both these states (1970QU1A).
Branching ratio, angular correlation and γ-ray polarization measurements, as well as τm determinations, permit a unique choice of Jπ in many cases from among the Jπ values stemming from direct interaction analyses of angular distributions: see Table 20.5 (in PDF or PS) and (1973HA14, 1973WA10). See also (1971BE2F, 1972BA1V) and (1973DO02; theor.).
Total cross section measurements for formation of 20F*(0, 0.66) (1974LA27) and of 20F*(0.66) (1975SE03) have been measured for E(13C) = 10 to 25 MeV and E(19F) = 13 to 24 MeV, respectively. In the latter experiment σt is compared with DWBA assuming S1S2 = 0.54; the population of 20F*(2.04) is also reported (1975SE03). τm for 20F*(0.66) = 390 ± 40 fsec (1975SE04): see Table 20.6 (in PDF or PS).
The 20F states observed, at Ed = 26 MeV, in this reaction and analog [T = 1] states observed in 20Ne in the (d, t) reaction are displayed in Table 20.16 (in PDF or PS). The spectroscopic factors of analog states are consistent to within 20% for states excited by a single l-transfer (1974MI13).
At Ep = 43.7 to 45.0 MeV analog states have been studied in 20F and 20Ne [the latter via 22Ne(p, t)20Ne]. The experimental cross-section ratio, R, for the transitions to 20Ne*(10.28) and 20F(0) [2+; T = 1] is 2.00 ± 0.20; R for the transitions to 20Ne*(12.25 ± 0.03) and 20F*(1.82) [5+; T = 1] is 1.40 ± 0.15 (1969HA19). Angular distributions for the 3He ions and the tritons corresponding to the first T = 2 states (Jπ = 0+) [20Ne*(16.722 ± 0.025) and 20F*(6.513 ± 0.033)] have also been compared. There is indication also for the excitation of the 2+; T = 2 states [at Ex = 8.05 MeV in 20F and at 18.5 MeV in 20Ne (estimated errors ± 0.1 MeV)] (1964CE05, 1969HA38).
Angular distributions have been obtained at Ed = 10 MeV to all 20F states with Ex < 4.4 MeV: they are generally featureless. The cross sections for 20F*(3.68, 3.97) are consistent with their being the second 4+ and the third 1+ states in 20F. A listing of all the states observed in this experiment is displayed in Table 20.17 (in PDF or PS) (1976FO16). See also (1971MO40: Ed = 2 and 3 MeV; α0 → α7).