(See Energy Level Diagram for 20Ne)
An unsuccessful attempt has been made to observe the isobaric spin-forbidden transition between the T = 0 states at 7.19 MeV (J = 3-) and 1.63 MeV (J = 2+). The radiative width is < 6 × 10-3 eV, indicating an admixture of T = 1 of < 1.3 × 10-3 in 20Ne*(7.19) (1957TO1B). The relevance of alpha capture in 16O to element synthesis has been discussed by (1956CA1F, 1956HA1C, 1956HA1D, 1957BU66, 1957LI1A, 1957SA1B).
The elastic scattering has been studied in the range Eα = 0.9 to 4.0 MeV by (1953CA44) and from Eα = 3.9 to 5.5 MeV by (1958MC61): see Table 20.7 [Resonances in 16O(α, α)16O] (in PDF or PS). See also (1940FE01, 1957CO1H, 1958CO59) and 16O.
Resonances for capture radiation are listed in Table 20.8 [Resonances in 19F(p, γ)20Ne] (in PDF or PS) (1954SI07, 1955FA1A). The level formed at Ep = 0.67 MeV decays predominantly to the 1.63 MeV state: Γγ = 2.2 eV (1951CA1A, 1952JO1D, 1955CL1B). The angular distribution of the hard radiation is isotropic within two per cent (1955FA1A). The γ-γ angular correlation shows that the transition to the 1.63 MeV state is predominantly M1. The direct ground state transition is < 0.02 eV which implies different nucleon configurations for the ground and first excited states of 20Ne (1955CL1B). At Ep = 0.67 and 1.09 MeV, cascades through the 4.97 MeV state have been observed. The 4.97 MeV level decays with > 95% probability through the 1.63 MeV state. Triple γ-correlations (C → 4.97 → 1.63 → 0) show J ≠ 0 for the 4.97 MeV state. Since a cascade takes place to it from the J = 1+ state at the 0.67 MeV resonances, J ≤ 3 (1958GO03). See also (1953WI1F, 1957WA1B, 1958BR1D).
The elastic scattering has been studied in the range Ep = 500 to 2000 keV by (1954DE1A, 1954PE1A, 1955BA1C, 1955WE44, 1956DE33). Parameters for the observed resonances are exhibited in Table 20.9 [Levels of 20Ne from 19F(p, p)19F] (in PDF or PS), taken mainly from (1955BA1C); values given by (1956DE33) are in good agreement with these. Orbital angular momentum assignments derive from angular distributions; Jπ from the anomaly shape, and Γp/Γ from σ(p, p), σ(p, p') and σ(p, α) cross sections. At Ep = 340 keV, 480 keV (1955WE44), and 598 keV (1956DE33), the elastic scattering anomaly is too small to be detected: it is concluded that Γp/Γ is small for these resonances. Some unresolved structure is observed at Ep = 900, 1092, and 1137 keV, in addition to a broad structure near Ep = 1700 keV (1955WE44). It is of interest to note that two of the resonances, at Ep = 669 and 843 keV have comparatively large proton reduced widths (1955BA1C). For high-energy scattering, see 19F.
Resonances for inelastic scattering involving the 110 keV, J = 1/2- and 197 keV, J = 5/2+, states of 19F are listed in Table 20.10 [Resonances in 19F(p, p')19F*] (in PDF or PS) (1955BA94). Resonance structure in the yield of 109, 197, 1240, and 1360 keV γ-radiation is reported by (1958RA15) up to Ep = 4.5 MeV. In general, the resonances observed are identical with those reported from other 19F + p reactions, although the relative intensities differ greatly. The nonresonant yield of the 197 keV radiation appears to be mainly due to Coulomb excitation, while that of the 110 keV radiation suggests contributions from broad, unresolved s-wave resonances (1954PE1A, 1955BA94). See also (1954PE1C) and 19F.
For Ep ≈ 1 to 3 MeV, five α-particle groups are reported. All show resonance effects with relative intensities varying greatly with bombarding energy. The long-range group (α0) leaves 16O in the ground state (J = 0+); the next longest (απ) results in the formation of the J = 0+ nuclear pair-emitting state at 6.06 MeV, while the three remaining groups (α1, α2, α3) lead to γ-ray emitting states at 6.14 (J = 3-), 6.91 (J = 2+), and 7.12 MeV (J = 1-). At Ep > 3 MeV, excitation of higher 16O levels occurs: see 16O. Resonances for α0 and απ (Tables 20.11 [Resonances for ground state α-particles (α0) in 19F(p, α0)16O] (in PDF or PS) and 20.12 [Nuclear pair resonances (απ) in 19F(p, απ)16O] (in PDF or PS)) are generally identical and different from those for α1, α2, α3 (Table 20.13 [Resonances for 6 - 7 MeV γ-rays (α1, α2, α3) in 19F(p, α)16O] (in PDF or PS)). The resonances for α0 and απ are required to have even J, even π or odd J, odd π, while the α1, α2, α3 resonances, insofar as their assignments are known, are all odd-even or even-odd.
Studies of the α0 yield and of angular distributions have been made by (1957CL42, 1957TA1D, 1958BR1K, 1958FR03, 1958IS10, 1958IS11, 1958RA15): see Table 20.11 [Resonances for ground state α-particles (α0) in 19F(p, α0)16O] (in PDF or PS). Assignments of (1958IS11) and (1957CL42) are based on detailed analysis of angular distributions. It is of interest that the reduced widths of these levels, where known, are generally less than a few per cent of the single-particle limit. There is some disagreement on the assignment of the prominent Ep = 843 keV resonance (compare 19F(p, p)19F). The 1358 keV resonance reported here appears to be distinct from the Ep = 1372 keV (J = 2-) resonance seen in 19F(p, p)19F and 19F(p, αγ)16O. (1958BR1K) find from angular distribution studies that the 720 keV resonance has J = 2+, and is formed in channel spin 0. Near Ep = 400 keV, two resonances are required, with Jπ = 1- and 0+, both with Γ ≈ 100 keV, Jc = 0. A third resonance occurs at Ep = 650 keV, with J = 1-, Γ = 200 keV (1958BR1K).
A special study has been made to detect a possible resonance for 19F(p, α0)16O at the Ep = 340 keV, J = 1+, resonance. An upper limit of 2% resonant/nonresonant yield is found, indicating a maximum admixture of 2 × 10-4 of odd-parity component in the wave functions involved (1957TA1D: see also (1958WI41)).
Resonances in the 19F(p, απ)16O yield have been investigated by (1950CH53, 1951PH1A, 1954DE36, 1955IS1A, 1956IS1A, 1958IS11, 1958RA15): see Table 20.12 [Nuclear pair resonances (απ) in 19F(p, απ)16O] (in PDF or PS). Resonance locations and absolute reduced widths appear to correspond closely to those for (p, α0), although some exceptions occur. In the work of (1958RA15) only 6 of 23 (p, α0) resonances have no clear counterpart in σ(p, απ). For resonances at Ep = 1.35, 1.72, 1.88, and 2.33 MeV, θ2α0 = θ2απ within about 10%; at Ep = 2.17 MeV, a large difference occurs, possibly to be ascribed to superposition of several resonances (1958RA15). Below Ep = 1.3 MeV, several fairly large differences occur (1958IS11).
Resonances in the yield of 6 - 7 MeV γ-rays have been studied by (1948BO21, 1949HE1A, 1950AR1A, 1950BA1A, 1950CH1A, 1950CH53, 1952HU1C, 1952WI27, 1953FA18, 1953HU18, 1953WI1F, 1955BU1A, 1955HU1A, 1955KI28, 1956BU27): see Table 20.13 [Resonances for 6 - 7 MeV γ-rays (α1, α2, α3) in 19F(p, α)16O] (in PDF or PS).
At Ep = 224 keV, the angular distribution of 6 MeV radiation is anisotropic (1954NE1C). The 6 - 7 MeV radiation produced at the Ep = 340, 483, 669, and 935 keV resonances is isotropic or nearly so (1949DE1A, 1951DA1B, 1952SA1A, 1953SA1B, 1957GA1B, 1957GO1E). (1958RE24) finds a 3.5% anisotropy at Ep = 340 keV, indicating a 1% admixture of d-waves: θ2d ≈ θ2s. At Ep = 340, 669, and 935 keV, the α1-γ correlations establish that J = 1+ for the corresponding 20Ne levels (1950AR1A, 1950BA1A, 1952SE1C, 1957MA1A): this assignment is confirmed for the 935 keV resonance by the α1 angular distribution (1954PE1C). Correlations and angular distributions at the Ep = 874 keV resonance establish J = 2- for this level (1952SE1C, 1954PE1C, 1957MA1A), and the observed polarization of the γ rays is consistent with this assignment (1953FA1B). The level corresponding to Ep = 598 keV is assigned J = 2- by (1950CH1A) on the basis of γ-ray angular distributions: W(θ) = 1 + 0.17cos2θ (1958RE24): see also (1953WI1F). Levels corresponding to resonances at Ep = 1283 and 1348 keV are assigned J = 3+ by (1950CH1A). Angular distributions of the α groups at Ep = 1283 keV are not inconsistent with J = 3+, but lead to J = 2- for the 1348 keV resonance (1954PE1C). Gamma-ray angular distributions at Ep = 1375 keV indicate a pronounced anisotropy and are consistent with J = 2- (1953SA1B: see also 19F(p, p)19F). This is evidently not the same as the 1.36 MeV α0 resonance. (α1-γ1) angular correlations have been studied by (1957MA1A) at Ep = 873, 935, 1250, 1280, 1346, and 1372 keV. For the J = 2- resonances at Ep = 873, 1346, and 1372 keV, the p-wave and f-wave proton reduced widths are comparable in magnitude: for the first and last, g-wave α1-emission has a surprisingly large probability.
An analysis of certain 20Ne levels into partial widths, based on information from 19F(p, p), (p, p'), (p, γ) and (p, α) is given by (1955BA1C, 1955BA94, 1958IS11): see Table 20.14 [Resonance parameters in 19F + p] (in PDF or PS). It is noted that a number of T = 1 levels may be expected in this region (compare 20F).
Levels of 20Ne derived from reported neutron groups are listed in Table 20.16 [Neutron groups from 19F(d, n)20Ne] (in PDF or PS): see also (1956BA1F, 1956TO1C). The earlier reported 2.2 MeV state appears to be spurious. The g.s. and 1.6 MeV groups show a clear stripping pattern, with lp = 0 and 2, respectively. No clear evidence is found for the 4.2 and 5.4 MeV levels at Ed = 9 MeV (1955CA1F). Levels at Ex = 9.7 MeV (1950FR1C) (9.97 ?), and Ex = 11.85 MeV (1952WA1A) decay by α-particle emission.
Thresholds for production of γ radiation with Eγ ≈ 10 - 12 MeV, are listed in Table 20.17 [Levels of 20Ne from 19F(d, n)20Ne thresholds] (in PDF or PS) (1955BU1E, 1958BU12). Observed γ-ray energies are exhibited in Table 20.18 [Gamma radiation from 19F(d, n)20Ne] (in PDF or PS) (1951TE1B, 1955BE81, 1957KR1B)†. It is noted that levels yielding appreciable γ-radiation must be presumed not to decay by α-emission. The 9.97 and 10.61 MeV levels may be the T = 1 levels corresponding to 20F(0) and *(0.65). The 10.6 MeV level has not been reported in any other reaction.
† Note added in proof: Recent studies of the γ-spectra at Ed = 3.6 MeV confirm the work of (1955BE81) and are not consistent with the fine structure reported in Table 20.17 [Levels of 20Ne from 19F(d, n)20Ne thresholds] (in PDF or PS) (T.W. Bonner, private communication).
The decay is to the 1.6 MeV state of 20Ne: Eβ(max) = 5.413 ± 0.013 MeV. The energy of the subsequent γ-ray is 1.629 ± 0.005 MeV (1952AL26, 1952AL30, 1954WO23). The relative intensity of the ground-state transition is < 3.2 × 10-4 (1954WO23). The Fermi plot is straight from the end point to Eβ = 1 MeV. With a half-life of 11.4 sec, log ft (1.63 MeV state) = 4.99, log ft (ground state) ≳ 9 (1954WO23). A search for γ-γ coincidences from the cascade decay of the 4.97 MeV state was unsuccessful: log ft > 6.5 (1958KA14). The (β-γ) angular correlation has the form W(θ) = 1 + acos2θ, where a = (0.94 ± 0.28) × 10-2. The anisotropy is attributed to a "weak magnetic" interaction associated with the anomalous nucleon moments. The sign of a indicates J = 2+ for 20F (1958BO65, 1958GE1C).
20Ne levels derived from observations of proton groups are listed in Table 20.19 [Levels of 20Ne from 20Ne(p, p')20Ne*, 20Ne(α, α')20Ne* and 23Na(p, α)20Ne] (in PDF or PS) (1954FR43, 1956SC1F). A 2.6 MeV γ-ray has been observed in coincidence with the p2 group (4.2 MeV state) at Ep = 5.85 MeV (1957KR1B). At Ep = 185 MeV, inelastic peaks corresponding to levels near 5 and 20 MeV are observed (1958TY1D).
Elastic scattering angular distributions have been studied in the range Ep = 1.8 to 4.3 MeV by (1955HA1F), for Ep = 4.7 to 5.5 MeV by (1958KO58), at 9.5 MeV by (1954FR43, 1956BU95, 1957GI14). Angular distributions of inelastic groups are reported by (1954FR43, 1956SC1F, 1957GI14, 1958KO58). See also (1957BU52; theor.).
At Ed = 7.8 MeV, an inelastic deuteron group is observed corresponding to a state at 1.66 ± 0.02 MeV (1951MI1A). The angular distribution, analyzed by the direct interaction theory (of (1952HU1B)), indicates l = 2, J = 1, 2, 3+; the observed distributions can also be ascribed to electric excitation, but the required moments are about two orders of magnitude too large (1958VA06). See also (1952MI1B).
At Eα = 18.0 MeV, inelastic groups are observed to 20Ne states at 1.63, 4.25, 4.97, 5.81, and 7.2 MeV: see Table 20.19 [Levels of 20Ne from 20Ne(p, p')20Ne*, 20Ne(α, α')20Ne* and 23Na(p, α)20Ne] (in PDF or PS). Angular distributions for α0(g.s.) and α1 show sharp diffractions maxima characteristic of direct interaction (1958SE51). See also (1957MO1C; theor.) and (1957EN01).
The decay proceeds to excited states of 20Ne between 6.8 and 10.8 MeV which decay by α-particle emission (1950AL57). The half-life is 0.23 ± 0.08 sec (1951SH38), 0.385 ± 0.01 sec (1953HO01). See also (1957EN01).
At Ep = 7.04 to 7.45 MeV, five α-groups are observed, corresponding to the ground state and to levels at 1.635, 4.248, 4.969, and 5.631 MeV (1953DO04, 1957BU36): see Table 20.19 [Levels of 20Ne from 20Ne(p, p')20Ne*, 20Ne(α, α')20Ne* and 23Na(p, α)20Ne] (in PDF or PS). The first excited state decays with a mean life of (7.6 ± 3.3) × 10-13 sec (1956DE22), emitting a γ-ray with Eγ = 1.63 ± 0.02 (1954ST91), 1.629 ± 0.008 MeV (1954NE1D, 1955NE1B). Over 90% of the decays of the 4.25 and 4.97 MeV states are by cascades through the 1.63 MeV state (1957KR1B: see also (1958KR67)). At a resonance located at Ep = 1.255 MeV, the α-γ correlation permits the unique assignment of J = 1+ to the 24Mg state and J = 2+ to the 1.63 MeV state of 20Ne (1953SE1C). See also (1954ST92, 1955RU1B, 1956SQ1A, 1957EN01).