
^{20}F (1998TI06)(See Energy Level Diagrams for ^{20}F) 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 ^{20}F] (in PDF or PS).
Q = 0.042 (3) b (1989RA17)
The halflife of ^{20}F is (11.163 ± 0.008) s (1992WA04), (11.11 ± 0.04) s (1995ITZY). For earlier measurements see (1987AJ02). ^{20}F decays principally to ^{20}Ne*(1.63): see ^{20}Ne, reaction 37.
Excitation functions were measured for incident energies E_{i} = 10  30 MeV (1989CO22).
Activation cross sections were measured for E_{i} = 10  40 MeV by (1987DI07).
Excitation functions have been measured at E_{i} = 6  32 MeV (1988CO12).
For excitation curves involving ^{20}F*(0, 1.82 + 1.84 + 1.97 + 2.04 + 2.19) see (1982HU06, 1983JA09). At E(^{9}Be) = 12 to 27 MeV angular distributions are reported for p_{0} and p_{1+2+3+4}: see (1983AJ01).
For fusion cross sections see (1982DE30). See also ^{13}C in (1991AJ01).
See (1983AJ01).
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 ^{20}F*(1.97, 0.82) [J^{π} = (3^{}), 4^{+}]: this is consistent with J^{π} = (4^{}) for ^{20}F*(2.968) (1978LE19, 1996RA04). The reactions ^{13}C(^{11}B, α)^{20}F and ^{11}B(^{13}C, α)^{20}F were used to populate ^{20}F states up to E_{x} = 10.1 MeV by (1988LI28). Comparisons with ^{14}N(^{7}Li, p)^{20}F were discussed.
20.7 (in PDF or PS) here and 20.6 (in PDF or PS) in (1983AJ01) display ^{20}F 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 E_{d} = 52 MeV (reaction (e)), see (1982MA25). See also ^{19}O and ^{19}F in (1995TI07), and ^{16}N and ^{17}N in (1993TI07). See also (1986SE1B).
In earlier work, proton groups have been observed to states of ^{20}F with E_{x} < 4.1 MeV: see 20.8 (in PDF or PS) in (1978AJ03). Angular distributions, γray polarization data and branching ratios lead to the J^{π} values shown in that table. A state at E_{x} = 6519 ± 3 keV is also populated. It decays primarily (> 90%) to ^{20}F*(1.06) [J^{π} = 1^{+}]: the γrays are isotropic. ^{20}F*(6.52) is the 0^{+}; T = 2 analog of the ground state of ^{20}O: see (1978AJ03, 1977BA50). More recently, the reaction was studied at E(^{18}O) = 18 MeV (1992CH39). Energy levels were measured up to E_{x} ≈ 8 MeV. See 20.8 (in PDF or PS). See also (1987SE17).
At E_{α} = 64.4 MeV angular distributions have been reported to ^{20}F*(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 E_{x} ≈ 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 γrays resulting from capture at thermal energies (^{20}F*(6.60); J^{π} = 1^{+}) and at E_{n} = 27, 44, and 49 keV (^{20}F*(6.63, 6.643, 6.647); J^{π} = 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 largebasis shellmodel calculation. It appears that the thermal capture [^{20}F*(6.60)] is dominated by two intense transitions (E1) to ^{20}F*(5.94, 6.02) [both J^{π} = 2^{}]. If the groundstate 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 ^{20}F*(6.63, 6.64, 6.65) [J^{π} = 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 J^{π} 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 ^{20}F 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 ^{20}F states involved in cascade and in primary γtransitions 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, σ_{free} = 3.641 ± 0.010 b (1979KO26). The difference in the spindependent bound scattering lengths, b^{+}  b^{} = 0.19 ± 0.02 fm (1979GL12). The total cross section has been measured for E_{n} = 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 E_{n} = 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 ^{19}F*(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 ^{20}F 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 ^{20}F*(1.06, 3.49), J^{π} = 1^{+}: see ^{20}O. For ^{20}F*(1.06) E_{x} = 1056.848 ± 0.004 keV. The β branch to ^{20}F*(3.17) (0^{},1^{+}) is < 0.012%, log f_{0}t > 5.1 (1987AL06).
The branching ratio to ^{20}F*(1.06) [J^{π} = 1^{+}] is compared to the analogous M1 decay width ^{20}Ne*(11.26) [J^{π} = 1^{+}] → ^{20}Ne_{g.s.}. The M1 amplitude contains (47 ± 16)% spinflip, in agreement with shellmodel calculations. The population of ^{20}F*(0, 1.31, 1.84) [J^{π} = 2^{+}, 2^{}, 2^{}] is also reported (1981MA04). See also (1986BA16) and (1983KN05).
Differential cross sections were measured at E_{n} = 198 MeV to study GamowTeller strength up to E_{x} ≈ 10 MeV in ^{20}F (1990HE1G, 1991PO14). See also the measurement of groundstate 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 spinisospin excitations by (1988HE1I).
Measurements at E_{t} = 33.4 MeV (1990CL06) reveal a strongly excited state in ^{20}F at E_{x} = 6.75 ± 0.04 MeV with an angular distribution suggesting (3 < J < 6). In more recent work by the same authors (1993CL09), the reactions ^{20}Ne(t, ^{3}He)^{20}F and ^{20}Ne(^{3}He, t) were studied at E_{t} = 33.4 MeV. Evidence was obtained that the J^{π} = 3^{+}, E_{x} = 2.966 MeV state in ^{20}F should be identified as the analog of the E_{x} = 2.646 MeV state in ^{20}Na.
Measurements at 900 MeV/nucleon for studies of spinisospin excitations were reported by (1988RO1H).
The ^{20}F states observed at E_{d} = 26 MeV in this reaction and analog [T = 1] states observed in ^{20}Ne 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 ltransfer.
Angular distributions were measured at E_{t} = 15.0 MeV by (1988LI10). States in ^{20}F up to E_{x} = 4.0 MeV were observed and analyzed with DWBA calculations. Spectroscopic factors were deduced.
At E_{p} = 43.7 to 45.0 MeV analog states have been studied in ^{20}F and ^{20}Ne [the latter via ^{22}Ne(p, t)^{20}Ne]. Angular distributions for the ^{3}He ions and the tritons corresponding to the first T = 2 states (J^{π} = 0^{+}) [^{20}Ne*(16.7329 ± 0.0027) and ^{20}F*(6.519 ± 0.003)] have been compared. There is indication also for the excitation of the 2^{+}; T = 2 states [at E_{x} = 8.05 MeV in ^{20}F and at 18.430 ± 0.007 MeV in ^{20}Ne]: see (1978AJ03).
Angular distributions have been obtained at E_{d} = 10 MeV to ^{20}F states with E_{x} < 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 paritynonconserving coupling in ^{20}F by observing the asymmetry in the gamma rays from the ^{20}F E_{x} = 0.983 MeV 1^{} state has been proposed (1993HO14, 1993HO1N).
Reactionmodel 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 preequilibrium emission, constant temperature evaporation models were reported in (1993KH09).
The Δ resonance is very strongly excited in this reaction at E(^{20}Ne) = 950 MeV/A (1986BA16).
