(See Energy Level Diagrams for 18O)
GENERAL: See 3 [Electromagnetic Transitions in A = 18-19] (in PDF or PS), 18.8 [General Table] (in PDF or PS), 18.9 [Table of Energy Levels] (in PDF or PS) and 18.10 [Radiative decays in 18O] (in PDF or PS).
< r2 >1/2 = 2.784 ± 0.020 fm: see reaction 25.
g = -0.287 ± 0.015 [see (1983AJ01)].
Reactions (a) and (b) have been studied by (1993CU01, 1993DA17) in low energy heavy-ion fusion reactions. It is reported that the ≈ 3 MeV wide resonance observed at Ex(18O) ≈ 28 MeV in the 7Li + 11B → 18O → 16O + nn and 9Be + 9Be → 18O → 16O + nn reactions overlap with the higher part of the T< = 1, 18O GDR observed in photonuclear excitation.
See (1986CU02) for production cross sections of 1.98 MeV γ-rays.
In a recent experiment, the 4+ state at 7117 keV in 18O was studied by (1994ME02) and an E2 strength for the 7117 - 5060 branches of B(E2) = 6.4 ± 1.6 W.u. was deduced in agreement with results of (1989GA01). It was concluded that it is highly improbable that the 7117 keV state is energetically degenerate with a state of different decay properties.
See (1986SM01) and 18.5 (in PDF or PS) in (1987AJ02). It is noted there that existing data indicate that when σtot to a particular state in 18O is large in this reaction, it is also large in the 12C(7Li, p) reaction. More recent data are reported in (1988SM01) (see 18.11 (in PDF or PS) here). Differential cross sections were measured and compared with results of Hauser-Feshbach calculations. The results suggest the presence of an additional non-statistical mechanism.
Resonances in the yield of capture γ-rays are observed at Eα = 1.14, 1.79, 2.09, 2.33, 2.44, 2.55, and 2.64 MeV: see 18.12 (in PDF or PS) here and 18.5 (in PDF or PS) in (1978AJ03). Gamma-ray angular distribution and correlation measurements lead to Jπ = 4+, 1-, 1-, and 5- for 18O*(7.11, 7.62, 8.04, 8.13), as well as to Jπ assignments for lower states involved in the cascade decay. See also references in (1987AJ02) and see the cross section measurements of (1993DA17). The speculated presence of enhanced E1 γ de-excitation in 18O (1983GA02) was followed by further experimental and theoretical investigations of collective band structure in 18O (1989FU08, 1993RE03). See however (1986HA1J). See also (1989FU1H, 1989KAZH). The 4+2 → 2+2 (7117 → 5260) keV γ branching ratio of 0.30 ± 0.08% was measured by (1989GA01) and an E2 transition strength B(E2) = 5.7 ± 1.9 W.u. was deduced. This result is conformed by the (ΓαxΓγ)/(Γα + Γγ) and (7117 → 5260) keV γ branching (0.24 ± 0.08%) measurements of (1992GO14) and the Γγ/Γα measurement of (1994ME02).
The 14C(α, γ) reaction is important in astrophysical processes and the details of the cross section are relevant to the process of heavy element formation in inhomogeneous big bang nucleosynthesis (1988AP1A, 1989FU06, 1990WIZP, 1992GA11, 1992GO14). See also (1988BU01, 1988MA1U, 1989KA1K, 1989NO1A, 1989TH1C) and the review of thermonuclear reaction rates in (1988CA26).
Observed anomalies in the scattering [reaction (a)] for Eα = 2 to 8.2 MeV and the resonances in the relative neutron yield [reaction (b)] for Eα = 2.3 MeV are displayed in 18.12 (in PDF or PS). See also (1978AJ03).
The α-cluster structure of 18O has been investigated in theoretical work of (1989FU08, 1993RE03) based on 14C(α, α) scattering, and the results do not support the existence of proposed negative-parity molecular dipole states. See (1989GA01).
At E(6Li) = 34 MeV angular distributions have been measured for the deuteron groups to many states of 18O (1981CU07) [see also (1983AJ01)] including 18O*(17.6 ± 0.2) (1982CU01). Jπ = 4+, 2+, 2+, (4+), and (4+) are suggested for 18O*(7.86, 8.9, 12.04, 14.6, 17.0) (1981CU07). The 2+, 4+, 6+ and 8+ members of the Kπ = 0+2 rotational band based on 18O*(3.62) are 18O*(5.26, 7.12, 11.69, 17.6) (1982CU01).
Angular correlations have been measured at E(6Li) = 34 MeV; these lead to the assignment of Jπ = 8+ to 18O*(17.6) (1982CU01) and to the assignment of Jπ = 4+, 5-, 6+, 7- and 8+ to sixteen states in 18O with 11.4 ≤ Ex ≤ 23.1 MeV (1983CU03) [see (1983CU03) for assignment of 18O states to bands]. At E(6Li) = 32 MeV (1983AR11) find that the strongest groups are those to (unresolved) structures at Ex = 17.05 and 18.95 MeV [each Γ ≈ 0.35 MeV] dominated by Jπ = 7-. 18O*(11.6, 12.6) with Jπ = (6+, 5-) and 5- are also observed (1983AR11) [see, however, the density of states]. See also (1987AJ02, 1990OS03).
At E(7Li) = 20.4 MeV, triton groups are observed corresponding to a number of states of 18O with Ex < 12.6 MeV. Angular distributions were obtained for some of these, including 18O*(0, 1.98, 7.12, 11.69) with Jπ = 0+, 2+, 4+, 6+. The latter two are the most strongly populated in this reaction: they appear to be part of the ground-state rotational band: see (1972AJ02). See also (1987AJ02).
In more recent work at E(7Li) = 15 MeV, 18O gamma de-excitation modes for all natural parity states up to the alpha-particle threshold at Ex = 6.227 MeV were studied (1991GA08). See 18.14 (in PDF or PS).
Proton groups corresponding to states of 18O are displayed in 18.15 (in PDF or PS) (1981CO13). See (1976LA13) for a general discussion of the properties of the states of 18O. Lifetime measurements are reported in 18.4 (in PDF or PS) of (1978AJ03). See also reaction 19 and (1982AN12, 1985AN17, 1985BA1A).
At Eα = 65 MeV, the angular distribution to 18O*(3.55) [Jπ = 4+] has been studied. 18O*(8.04, 9.15, 10.3) are also populated: see (1983AJ01).
At E(10B) = 100 MeV, 18O*(3.55) [first (d5/2)24+ state] is preferentially populated. 18O*(1.98, 5.26, 7.12, 8.0, 8.3, 9.1) are also observed. The angular distribution to 18O*(3.55) has been measured at E(13C) = 105 MeV. See (1983AJ01, 1983OS07).
For reaction (a) see (1983AJ01). [The work reported there has not been published.] The scattering amplitude (bound) a = 5.62 ± 0.45 fm, σfree = 3.55 ± 0.25 b. The thermal cross section for reaction (c) is 235 ± 10 mb. See (1983AJ01) for references. See also (1988MCZT).
In more recent work, the cross section for 17O(n, α) has been measured from En = 25 × 10-3 eV to 1 MeV (1991KO31). An evaluation of the cross sections from En = 10-5 eV to 20 MeV has been carried out by (1991HI15). Results are given in tabular and graphical form. See also (1991KO1P).
Observed proton groups are displayed in 18.16 (in PDF or PS). A strong asymmetric peak is observed at Ed = 12 MeV corresponding to Ex = 9.0 MeV. On the basis of this work and the measurement of the cross section at a peak at about the same energy observed in the 16O(t, p) reaction, (1985FO11) assign Jπ = 4+ and a (1d5/2) (1d3/2) configuration to 18O*(9.0). Proton-γ coincidence measurements are shown in 18.10 (in PDF or PS).
Differential cross sections were measured at Eα = 65 MeV (1992YA08) for 18O states up to Ex = 15 MeV. DWBA analysis led to proposed spin parity and isospin assignments, and spectroscopic factors. See 18.17 (in PDF or PS).
Angular distributions involving 18O*(0, 1.98, 3.55) have been studied at E(12C) = 115 MeV: see (1983AJ01).
The γ-decaying states were measured by (1982OL01) and estimated 15 ± 6% branching to non-γ-decaying states in 18O was assumed. At least 12.2 ± 0.6% of the β-decay branching ratio has been measured to feed 1- alpha-particle emitting states (1989ZH04). See also the measurements of (1987GAZW, 1987ZHZV, 1988MI1G). A β-delayed neutron emission probability of 14.3 ± 2.0% has been measured (1991RE02). The β- branchings to γ-emitting states of (1982OL01) has been renormalized to take in account the 26.5 ± 2.1% branches to particle emitting states. The γ-ray intensities of (1982OL01) also need to be renormalized by this factor, see 18.19 (in PDF or PS). (1994SC01) has measured β decay branching ratios to 9 neutron emitting states in 18O listed in 18.18 (in PDF or PS) for a total of 2.2 ± 0.4%.
The cross sections for the (γ, p), (γ, n), (γ, 2n) and (γ, tot) [tot = total absorption] have been measured with monoenergetic photons to 42 MeV: observed resonances are displayed in 18.20 (in PDF or PS). All three of the partial cross sections have substantial strength in the giant resonance region; the (γ, 2n) cross section is a significant fraction of σ(γ, tot) and is even larger than σ(γ, p). Above the GDR the partial cross sections decrease. The integrated σ(γ, tot) between 29 and 42 MeV is about one-third of the value integrated from threshold to 42 MeV. The relative strengths of partial cross sections leads to the T assignments shown in 18.20 (in PDF or PS). The T< and T> components of the 18O photo absorption cross section are also derived (1979WO04).
In a related, but more recent, experiment the cross section for reaction (e) was measured (1991MC01) and it was determined that the cross section rises to a maximum of 1.2 mb at 27.5 MeV, approximately one-tenth of the total (γ, n) cross section there. The cross section integrated to 43 MeV is only 11.8 mb · MeV, and as a result the isospin assignments of (1979WO04) are unaffected by neglect of this channel. A recent extensive study of isospin effects in the photodisintegration of light nuclei (1993MC02) used a collection of data on (γ, p), (γ, n), (γ, 2n) and (γ, n0) cross sections and separated the T> and T< isospin components of the GDR in several light nuclei including 18O. The relative strengths were extracted. See also the atlas of photoneutron cross sections with monoenergetic photons (1988DI02), and see (1988BE1T, 1989NO1C). Structures in the (γ, α0) cross section are reported at Ex = 18.2, 20.9, 22.1, and 24.2 MeV (1982BA03; Ebrems.). The decay of the GDR to 14C, 15N, 16O, 17N and 17O states has been studied: see (1983AJ01). Less than 20% of the decay of states with 14.5 < Ex < 20 MeV goes via the n0 channel (1987JU07). See (1978AJ03, 1987AJ02) for the earlier work.
For 18O*(6.20) Γγ0 = 0.18 ± 0.03 eV, assuming Γγ0/Γ = 0.88; Ex = 6202.7 ± 0.8 keV: see (1978AJ03).
The 18O charge radius, < r2 >1/2 = 2.784 ± 0.020 fm, based on studies of the elastic charge form factors for Ee = 70 to 370 MeV, the resulting determinations of the difference in the 18O and 16O radii, and the rms radius of 16O: see (1983AJ01).
Inelastic scattering has been reported to many states of 18O: see (1983AJ01, 1987AJ02) and 18.21 (in PDF or PS) here which also includes the recent work reported in (1995SE02). See also the comment (1987MI25) and reply (1987MA40) on the work reported in (1986MA48). Recent measurements are reported for 4- and 6- states at Ee = 140 - 275 MeV (1990SEZZ), and for 1-, 3-, 5- states (1991MA14). Form factor measurements for the 2+ level at Ex = 8.21 MeV and the (2+) level at Ex = 9.3 MeV at momentum transfer 0.9 < q < 2.1 fm-1 (1990MA06) and for the 1-, 3- and 5- levels at 0.6 < q < 2.7 fm-1 (1991MA14) are reported.
Several theoretical studies of inelastic electron scattering to states of 18O have been carried out. A microscopic calculation for scattering to 2+ states is reported in (1988HAZZ) and to 0+ and 2+ states in (1988KU17). See also the calculations of transition charge densities described in (1988GU03, 1988GU12, 1992GU11) and see (1987GU1D, 1988GU1B, 1989AJ1A).
Angular distributions for the scattering to 18O*(0, 1.98, 5.10) have been reported at Eπ± = 29.2 to 230 MeV [see (1983AJ01)] and at 50 MeV (1984TA1A; 18O*(0, 1.98)) at 140, 180, and 220 MeV (1984SE1A; 18O*(1.98)), at 164 MeV (1987CH14; 18O*(0, 1.98, 4.46, 5.10)) and (1988SE04; 18O*(1.98, 3.92, 5.26 MeV)). See also (1989GR1M, 1990WI1K). Measurements and analysis work reported in (1983AJ01) determine < rn2 >1/2 = 2.81 ± 0.03 fm, < rn2 >1/2 - < rp2 >1/2 = 0.03 ± 0.03 fm. For a discussion of proton matter distribution in 18O see (1985BA27). Total reaction cross sections at Eπ = 50 MeV have been determined by (1987ME12). At E = 165 MeV, the cross section for reaction (c) is larger for 18O than for 16O while reaction (b) has a lower cross section (1982PI06). For the (π+, 2p), (π+, pn) and (π-, pn) reactions at Eπ = 165 MeV see (1984AL20, 1986AL22).
Results of Glauber model calculations of pion scattering from 18O at energies above the Δ33 resonance are presented in (1991OS01). A microscopic study of inelastic scattering to the 2+ states in 18O is reported in (1988HAZZ). See also the review of pion-nucleus physics in (1991MO13).
Angular distributions have been measured for Ep = 0.84 to 135 MeV [see (1978AJ03, 1983AJ01)], at Ep = 135 MeV (1986KE05; p1) and at Ep = 800 MeV (1982GL08; p to 18O*(0, 1.98, 7.12).) At Ep = 24.5 MeV (1974ES02) have studied the angular distributions of the proton groups to 18O*(1.98, 3.55, 3.63, 3.92, 4.46, 5.10, 5.26, 5.53, 7.12): a modified DWBA analysis leads to Jπ = 2+, 4+, 0+, 2+, 1-, 3-, 2+, 2- and 4+ for these states. A coupled-channels calculation suggests β2 = 0.37 ± 0.03, 0.56 ± 0.06 and 0.18 ± 0.04 for 18O*(1.98, 5.10, 7.12). Such calculations also support evidence for a rotational band involving 18O*(0, 1.98, 7.12). The 3- state at 5.10 MeV is strongly excited and collective in nature: B(E3) = 1120 e2 · fm6. For 18O*(1.98, 3.92, 5.26), B(E2) = 45, 8.3 and 24 e2 · fm4 (1974ES02). The 800 MeV data indicates that 18O*(7.12) can be described only if a large hexadecapole deformation is assumed (1982GL08). At Ep = 201 MeV, σ(θ) at forward angles has been measured to 18O*(8.21, 8.82, 16.40): it is proposed that 18O*(8.82) has Jπ = 1+ and that additional 1+ strength is located in a group centered at Ex ≈ 10.1 MeV as well as in the region Ex = 12.4 to 15 MeV. The 1+; T = 2 state 18O*(18.87), reported in (e, e'), is not observed (1987DJ01). See also (1988CR1B).
A Dirac optical model analysis of 18O(p, p) cross section and analyzing power at 800 MeV is described in (1990PH02). A coupled-channels analysis was presented in (1988DE31). The intrinsic radial sensitivity of nucleon inelastic scattering was studied by (1988KE01) and a comparison of electromagnetic and hadronic probes of nuclear structures is described in (1986KE1C).
Differential cross sections for elastic and inelastic scattering of 180 MeV antiprotons by 18O were calculated in the eikonal and Glauber approaches by (1992TA08).
The elastic scattering has been studied at E(3He) = 11.0 to 41 MeV [see (1972AJ02, 1983AJ01)] and at 14 MeV (1982AB04), at 25 MeV (1982VE13) [the matter radius, < r2 >1/2m = 2.59 ± 0.12 fm] and at 33 MeV (1983LE03; also Aγ; and also to 18O*(1.98)). A strong-absorption model analysis of angular distributions at 2.5 and 41 MeV is described in (1987RA36). See also (1985HA11, 1987CO07).
Recent elastic scattering cross sections at Eα = 44.8 MeV were reported by (1992AR18). Angular distributions of many α-groups have been measured in the range Eα = 21 to 40.5 MeV [see (1978AJ03)], at 23.5 MeV (1984SA28; to 18O*(1.98, 3.56 + 3.63, 3.92, 4.45, 5.1 - 5.53)) and at 54.1 MeV (1987AB03; g.s.). The transitions to 18O*(4.46, 5.10) are L = 1 and 3, respectively, fixing Jπ = 1- and 3- for these states. Measurements of α-groups near 180° for Eα = 20.0 to 23.4 MeV confirm assignments of natural parity for 18O*(1.98, 3.55, 3.63, 3.92, 4.46, 5.10, 5.26, 5.34, 6.20, 6.40, 7.12, 7.62, 7.86, 8.22, 8.29, 8.82, 8.96, 9.03, 9.10, 9.36, 9.41, 9.67, 9.72 ± 0.03, 9.88, 10.12, 10.30, 10.40, 11.62, 11.69). [See, however, 18.9 (in PDF or PS).] Levels at Ex = 5.38, 8.48 and 8.64 MeV were not observed, and those at 5.53, 6.35 and 6.88 MeV were populated weakly indicating unnatural parity; Jπ = 3+ and 2- respectively for 18O*(5.38, 5.53).
Alpha-gamma correlation measurements involving 18O states below Ex = 6.4 MeV [see 18.10 (in PDF or PS)] lead to Jπ = 1- and 3- for 18O*(6.20, 6.40). Other Jπ values agree with previous assignments. The transitions 3.92 → 1.98 and 5.26 → 1.98 are almost pure M1. For τm measurements, see 18.4 (in PDF or PS) in (1978AJ03). For references see (1983AJ01, 1987AJ02). A microscopic investigation of the α + 18O system in a three-cluster model is discussed in (1988DE37).
A recent search for a bound system of π- and neutrons in the fragmentation region of 18O + 9Be collisions at 100 A · MeV is reported in (1993SU08). Upper limits were obtained.
Elastic angular distributions have been studied at E(18O) = 32.3 to 57.5 MeV for reaction (a) [as well as at E(18O) = 70, 100, and 140 MeV (1982HE07)] and at E(18O) = 31 MeV for reaction (b). Yields and fusion cross sections are reported by (1982BA49, 1982HE07, 1985BE40, 1985CA01, 1986GA13). For reaction (c) see (1986STZY). See also (1983AJ01, 1987AJ02).
Angular correlations (reaction (d)) have been studied at E(18O) = 82 MeV. 18O*(7.10, 7.62, 7.86, 8.04, 8.22, 10.30, 11.59, 12.55) are observed: the first seven of these have Jπ = 4+, 1-, 5-, 1-, 2+, 4+, 5- (1984BH01, 1984RA07). In addition 18O*(9.33, 9.65) are also populated [Γ ≈ 0.3 MeV]: a possible interpretation of the data is that these two are 3- states and that there is in addition a very wide (> 1 MeV) 2+ state at ≈ 9.5 MeV (1984RA17). See also (1987AJ02).
Giant dipole decays in nuclei excited by 18O + 12C collisions were discussed in (1989BEZC, 1990SN1A). Competition between p2n, dn and t emissions in the 12C + 18O reaction was studied in an experiment reported in (1990XE01).
Predictions of possible resonant behavior in medium-mass colliding systems are discussed in (1989CI1C). Molecular single particle effects for 12C + 18O are explored in calculations described in (1987MO27).
Angular distributions have been measured at many energies for E(16O) = 24 to 54.5 MeV and E(18O) = 25 to 52 MeV, involving besides 18Og.s., 18O*(1.98, 3.55 + 3.63, 3.92, 4.46, 5.10, 7.12). At E(18O) = 126 MeV 18O*(9.0) is relatively strongly populated. See (1983AJ01). For yields and fusion cross sections, including the effect of 18O*(1.98), see (1985TH03, 1985WU03, 1986GA13, 1986TH01). See also (1987AJ02). Competition between p2n, dn and t emissions in 18O + 16O reactions was studied in an experiment reported in (1990XE01).
A unified description of sub-barrier interactions of oxygen isotopes is discussed in (1987PO11); see the coupled-channels calculations reported in (1992LI1K). See also the review of sub-barrier fusion in (1988BE1W). A semi-classical analysis of two particle transfer in 16O + 18O reactions is discussed in (1987MA22).
Angular distributions involving 18O*(0, 1.98) are reported at E(17O) = 36 MeV. Angular distributions [reaction (b)] have been studied at E(18O) = 20 to 52 MeV. 18O*(3.55 + 3.63, 4.46, 5.10, 7.12) are also populated; see (1978AJ03, 1983AJ01). See also (1987AJ02) and see (1990XE01) reporting on p2n, dn and t emissions in 18O + 18O reactions.
The effect of high spin states on fusion in 18O + 18O systems has been studied in the framework of a statistical theory (1987RA28).
The elastic scattering has been studied at E(19F) = 27, 30, and 33 MeV: see (1983AJ01). See also (1987AJ02). An experiment reported in (1990XE01) studied p2n, dn and t emission in 18O + 19F reactions.
Angular distributions are reported for reaction (a) at E(18O) = 29 and 35 MeV to 18O*(0, 1.98). See (1987AJ02).
Angular distributions have been measured at E(18O) = 62.1 MeV [reaction (a)] for the transitions to 18O*(0, 1.98, 5.10) (1982RE14). For a fusion study [reaction (b)] see (1984DE38). See also (1987AJ02, 1987SC34).
Experimental and theoretical studies of knockout reactions are reviewed in (1987VD1A).
Many states of 18O (Ex < 14.6 MeV) have been populated in this reaction: see 18.8 (in PDF or PS) in (1978AJ03). [Comment: Note, however, density of states.] Analyzing powers for the ground-state transition are reported at Ed = 12.4 MeV (1983EN02). See also (1983KI13).
At Ed = 80 MeV angular distributions have been measured for the 6Li groups to the ground state of 18O and to excited states at 1.98, 3.57, 5.10, 6.30, 7.8, 9.4 [ ± 0.04] MeV (1984OE02) [see also for Srel.]. For the earlier work see (1983AJ01).