(See Energy Level Diagrams for 20F)
GENERAL: See 3 [Electromagnetic Transitions in A = 20] (in PDF or PS), 20.4 [General Table] (in PDF or PS), 20.5 [Table of Energy Levels] (in PDF or PS) and 20.6 [Radiative transitions in 20F] (in PDF or PS).
Q = -0.042 (3) b (1989RA17)
The half-life of 20F is (11.163 ± 0.008) s (1992WA04), (11.11 ± 0.04) s (1995ITZY). For earlier measurements see (1987AJ02). 20F decays principally to 20Ne*(1.63): see 20Ne, reaction 37.
Excitation functions were measured for incident energies Ei = 10 - 30 MeV (1989CO22).
Activation cross sections were measured for Ei = 10 - 40 MeV by (1987DI07).
Excitation functions have been measured at Ei = 6 - 32 MeV (1988CO12).
For excitation curves involving 20F*(0, 1.82 + 1.84 + 1.97 + 2.04 + 2.19) see (1982HU06, 1983JA09). At E(9Be) = 12 to 27 MeV angular distributions are reported for p0 and p1+2+3+4: see (1983AJ01).
For fusion cross sections see (1982DE30). See also 13C in (1991AJ01).
The upper of the two states at 2.97 MeV has an excitation energy of 2968 ± 2 keV and γ branching ratios of (40 ± 10) and (38 ± 10)%, respectively, to 20F*(1.97, 0.82) [Jπ = (3-), 4+]: this is consistent with Jπ = (4-) for 20F*(2.968) (1978LE19, 1996RA04).
The reactions 13C(11B, α)20F and 11B(13C, α)20F were used to populate 20F states up to Ex = 10.1 MeV by (1988LI28). Comparisons with 14N(7Li, p)20F were discussed.
20.7 (in PDF or PS) here and 20.6 (in PDF or PS) in (1983AJ01) display 20F states reported in this reaction.
For reported states see 20.6 (in PDF or PS) in (1983AJ01).
See (1983AJ01) for a listing of the polarization measurements. For VAP measurements at Ed = 52 MeV (reaction (e)), see (1982MA25). See also 19O and 19F in (1995TI07), and 16N and 17N in (1993TI07). See also (1986SE1B).
In earlier work, proton groups have been observed to states of 20F with Ex < 4.1 MeV: see 20.8 (in PDF or PS) in (1978AJ03). Angular distributions, γ-ray polarization data and branching ratios lead to the Jπ values shown in that table. A state at Ex = 6519 ± 3 keV is also populated. It decays primarily (> 90%) to 20F*(1.06) [Jπ = 1+]: the γ-rays are isotropic. 20F*(6.52) is the 0+; T = 2 analog of the ground state of 20O: see (1978AJ03, 1977BA50).
More recently, the reaction was studied at E(18O) = 18 MeV (1992CH39). Energy levels were measured up to Ex ≈ 8 MeV. See 20.8 (in PDF or PS). See also (1987SE17).
At Eα = 64.4 MeV angular distributions have been reported to 20F*(0, 0.66, 0.82, 1.06, 1.82, 2.20, 2.97, 4.24, 5.07, 5.44, 5.80, 6.67, 7.29, 7.75, 8.34, 8.75, 9.00, 9.24, 9.78, 10.01, 10.51, 10.85, 11.56, 12.32, 12.72): L assignments are made [the groups above Ex ≈ 2.9 MeV are probably unresolved] (1986KA36).
The thermal capture cross section is 9.51 ± 0.09 mb. A number of resonances have been observed: see 20.9 (in PDF or PS). The primary γ-rays resulting from capture at thermal energies (20F*(6.60); Jπ = 1+) and at En = 27, 44, and 49 keV (20F*(6.63, 6.643, 6.647); Jπ = 2-, (3, 4) and 1-) have been studied by several groups: see (1972AJ02) and 20.7 (in PDF or PS) in (1987AJ02). For more recent high precision work see (1987KE09) and the comprehensive study of (1996RA04), which included measurements of excitation energies and lifetimes and comparison of level properties with a large-basis shell-model calculation. It appears that the thermal capture [20F*(6.60)] is dominated by two intense transitions (E1) to 20F*(5.94, 6.02) [both Jπ = 2-]. If the ground-state transition is mainly M1, these two E1 transitions are about 150 times stronger (in terms of W.u.) than the M1 transition (1968SP01). See also (1983HU12). It appears also that at 20F*(6.63, 6.64, 6.65) [Jπ = 2-, (3, 4) and 1-] the E1 transitions to the ground state are very weak, even though other E1 transitions in the decay of these three states have approximately normal strengths. The strongest transitions from the 27 keV resonance appear to be M1. On the basis of the Jπ values of the final states involved in the decay of the 44 keV resonance, it appears that J = 3 or 4 for this resonance, assuming dipole transitions. Branching ratios for other 20F states involved in this reaction are shown in 20.6 (in PDF or PS).
See also 20.10 (in PDF or PS) here. 20.11 (in PDF or PS) displays excitation energies for 20F states involved in cascade and in primary γ-transitions from the recent work of (1996RA04). For earlier references see (1978AJ03). See also (1991IG1A, 1991HI23).
The scattering length (bound) b = 5.654 ± 0.010 fm, σfree = 3.641 ± 0.010 b (1979KO26). The difference in the spin-dependent bound scattering lengths, b+ - b- = -0.19 ± 0.02 fm (1979GL12). The total cross section has been measured for En = 0.5 to 29.1 MeV: see (1978AJ03). Observed resonances are displayed in 20.12 (in PDF or PS).
Average cross sections for the region En = 0.55 - 5.5 MeV were measured by (1988KO18). See also the neutron cross section tables and curves of (1988MCZT, 1990NAZH).
Observed resonances in the excitation functions involving 19F*(0.11, 1.5(u)) are displayed in 20.13 (in PDF or PS). For reaction (b) see (1983CSZX). See also (1986BAYL, 1986SA40).
Reported resonances are shown in 20.14 (in PDF or PS). See also the neutron cross section curves and tables of (1990NAZH).
States of 20F observed in this reaction are displayed in 20.15 (in PDF or PS). See (1978AJ03) for a discussion of the earlier work. See also (1983JI04, 1988RO10, 1992WA04, 1994GO16).
The decay is to 20F*(1.06, 3.49), Jπ = 1+: see 20O. For 20F*(1.06) Ex = 1056.848 ± 0.004 keV. The β branch to 20F*(3.17) (0-,1+) is < 0.012%, log f0t > 5.1 (1987AL06).
The branching ratio to 20F*(1.06) [Jπ = 1+] is compared to the analogous M1 decay width 20Ne*(11.26) [Jπ = 1+] → 20Neg.s.. The M1 amplitude contains (47 ± 16)% spin-flip, in agreement with shell-model calculations. The population of 20F*(0, 1.31, 1.84) [Jπ = 2+, 2-, 2-] is also reported (1981MA04). See also (1986BA16) and (1983KN05).
Differential cross sections were measured at En = 198 MeV to study Gamow-Teller strength up to Ex ≈ 10 MeV in 20F (1990HE1G, 1991PO14). See also the measurement of ground-state correlations described in (1988MA53). Cross sections for 14 MeV neutrons are presented for use in activation analysis by (1989PE04).
Angular distribution measurements with polarized deuterons (E ≈ 2 GeV) were made in a study of spin-isospin excitations by (1988HE1I).
Measurements at Et = 33.4 MeV (1990CL06) reveal a strongly excited state in 20F at Ex = 6.75 ± 0.04 MeV with an angular distribution suggesting (3 < J < 6). In more recent work by the same authors (1993CL09), the reactions 20Ne(t, 3He)20F and 20Ne(3He, t) were studied at Et = 33.4 MeV. Evidence was obtained that the Jπ = 3+, Ex = 2.966 MeV state in 20F should be identified as the analog of the Ex = 2.646 MeV state in 20Na.
Measurements at 900 MeV/nucleon for studies of spin-isospin excitations were reported by (1988RO1H).
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 20.16 (in PDF or PS) of (1978AJ03). The spectroscopic factors of analog states are consistent to within 20% for states excited by a single l-transfer.
Angular distributions were measured at Et = 15.0 MeV by (1988LI10). States in 20F up to Ex = 4.0 MeV were observed and analyzed with DWBA calculations. Spectroscopic factors were deduced.
At Ep = 43.7 to 45.0 MeV analog states have been studied in 20F and 20Ne [the latter via 22Ne(p, t)20Ne]. Angular distributions for the 3He ions and the tritons corresponding to the first T = 2 states (Jπ = 0+) [20Ne*(16.7329 ± 0.0027) and 20F*(6.519 ± 0.003)] have been compared. There is indication also for the excitation of the 2+; T = 2 states [at Ex = 8.05 MeV in 20F and at 18.430 ± 0.007 MeV in 20Ne]: see (1978AJ03).
Angular distributions have been obtained at Ed = 10 MeV to 20F states with Ex < 4.4 MeV: they are generally featureless. Observed states are displayed in 20.17 (in PDF or PS) of (1978AJ03).
An experiment which would use this reaction to investigate the weak parity-nonconserving coupling in 20F by observing the asymmetry in the gamma rays from the 20F Ex = 0.983 MeV 1- state has been proposed (1993HO14, 1993HO1N).
Reaction-model calculations for cross sections are described in (1993ST10). The use of this reaction in connection with neutron detection is discussed in (1987LE1G).
Cross sections calculated with pre-equilibrium emission, constant temperature evaporation models were reported in (1993KH09).
The Δ resonance is very strongly excited in this reaction at E(20Ne) = 950 MeV/A (1986BA16).