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12O (2017KE05)

(See Table 12.52 preview 12.52 (in PDF or PS) and Energy Level Diagrams for 12O and Isobar Diagram)

Theoretical studies on 12O have focused on the location and width of the ground state (1988CO15, 1997KR10, 1999BA03, 1999SH43, 2001LE22, 2002GR03, 2003FO14, 2010FO08, 2013FO10, 2014ROZZ). In several approaches the analog states in 12Be are utilized to gain an understanding of the expected 12O structure (1999SH43, 2006FO11, 2011FO04). In most cases the expected width (≈ 100 keV) is significantly smaller than the experimentally observed widths. The possibility for 2He decay has been discussed in (2001BA31, 2001GR29, 2002GR03, 2002GR25, 2003BA99, 2003FO14, 2006CA05, 2009LE22). See also (2005JI04, 2006SA29, 2009BA41, 2011SH26, 2016PA05).

1. 9Be(13O, 12O)10Be Qm = -10.0590

At E(13O) = 33.4 MeV/A population of 12O states was measured by stripping a single neutron from 13O on a 9Be target and measuring the 10C + 2p products resulting from decay of the unbound 12O nuclei (1995KR03, 1995KR24, 1998AZ03). The decay energy spectrum was deduced by reconstructing the 10C + 2p relative energy spectrum; a peak corresponding to the 12O ground state is found at Ecmrel = 1.77 ± 0.02 MeV with Γ = 578 ± 205 keV. Evidence for 12O excited states is present in the spectrum, however no resonance peaks are resolved. Analysis of the p-p angle correlations indicates that equal energy protons are emitted isotropically; this is consistent with direct three body breakup, though sequential decay through the 11N ground state appears likely (1998AZ03). An upper limit of Γ2He ≤ 7% is deduced for diproton decay (1995KR03).

The reaction was also measured at E(13O) = 30.3 MeV/A, the beam impinged on a 9Be target where 1n and 1p knockout reactions occurred yielding 12N and 12O reaction products (2012JA11). The 12O nuclei decayed into 10C + 2p, which were detected in a position sensitive ΔE-E array. Kinematic reconstruction of the relative energies found evidence for 12O*(0, 1968 ± 52 keV) at Ecmrel = 1.638 ± 0.024 MeV and 3.606 ± 0.060 MeV with Γ = < 72 keV and 475 ± 110 keV, respectively. For 12Og.s., ΔM = 31914 ± 24 keV. This mass excess is smaller than the previously reported values, but the measurement seems more reliable since the ground state peak is well separated and since the experimental resolution is improved. Comparison using the IMME formula finds the T = 2 quintet for A = 12 can be fit with a quadratic form. See also 12N reaction 5 and see discussion in (2016FO19).

2. 12C(π+, π-)12O Qm = -31.9150

At Eπ+ = 180 MeV the mass excess of 12O was measured as ΔM = 32059 ± 48 keV (1980BU15). In addition to observing the 12O ground state, at Eπ = 164 MeV (1985MO18) found additional cross section strength at Ex = 1.7 MeV and viewed this as evidence of the Jπ = 2+ first excited state. At Eπ = 59.4 MeV, weak evidence for 12O*(0, 1.3, 2.8) is found (1987FA05). See Table 12.53 preview 12.53 (in PDF or PS) for a summary of relevant (π+, π-) double charge exchange measurements. The table also includes some results that explored (π+, π-) reaction dynamics; also see theoretical analysis in (1979HU02, 1981DZ03, 1985OS08, 1986CH39, 1986FO03, 1986GI13, 1988MA27, 1989AU05, 1993CL03, 1992BI04, 1993GI03, 1994MO44).

3. 14O(p, t)12O Qm = -31.5684

An E(14O) = 50 MeV/A beam impinged on a cryogenic hydrogen target and the residual triton products were detected in a set of four position sensitive ΔE-E telescopes (2016SU05). The 12O products decayed in flight to 10C, which were detected in coincidence with the tritons using a ΔE-ΔE-E at θ < 1.6°. Excitation energies with ΔE = ± 100 keV were determined by constructing the missing mass spectrum. The ground state properties were assumed from (2012JA11). Peaks at Ex = 0, 1620 ± 30 (stat.) ± 100 (sys.), 4200 and 7000 were observed with Γ < 72 keV (2012JA11), 1.2 ± 0.1 (stat.) +0.3-0.7 (sys.) MeV, 2.2 MeV and 2.2 MeV, respectively. A DWBA analysis of the angular distributions found L = 0, 0, 1 and (0, 1, 2) for these states, which implies Jπ = 0+, 0+ and 1- for the first three states. An earlier result in (2009SU14, 2012SU21) suggested L = 0, 2 in the Ex = 1.8 MeV region that could imply presence of a doublet; however the present result does not support this interpretation.

4. 16O(α, 8He)12O Qm = -65.8368

At Eα = 117.4 MeV the Q-value for the 16O(α, 8He) reaction was measured by detecting the 8He recoils at θlab = 8° (1978KE06). The energy spectrum indicated Q = 66.02 ± 0.12 MeV and Γ = 400 ± 250 keV for the reaction (ΔM = 32.10 ± 0.12 MeV). In addition five counts are attributed to an excited state at Ex = 1.0 ± 0.1 MeV, using the Wigner limit for reduced widths, the diproton decay branching ratio is 30 to 90 %.