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11N (2012KE01)

(See Table 11.45 preview 11.45 (in PDF or PS) and Energy Level Diagram for 11N and Isobar Diagram )

Experimental evidence supporting states in 11N have produced a generally consistent picture of the 11N structure. However, sizeable inconsistencies persist amongst measured values for the ground state energy (mass excess) and the widths of states. There are essentially three high resolution measurements of the 11N ground state mass. They do not have overlap in their uncertainties. The measured values, in the 10C + p relative energy system, are Eres = 1.54 ± 0.02 MeV from 1H(10C, 1H) (2006CA05), 1.63 ± 0.05 MeV from 10B(14N, 13B) (2000OL01) and 1.31 ± 0.05 MeV from 14N(3He, 6He) (2003GU30). In the present evaluation, we have taken the unweighted average and assigned an uncertainty of 60 keV; this yields 11Ng.s. = Eres = 1.49 ± 0.06 MeV which is in reasonable agreement with each value and the average. This corresponds to a mass excess of 24477 ± 60 keV for 11N, and compares with ΔM = 24303 ± 46 (Eres = 1315 ± 46 keV) from (2011AUZZ). See theoretical analysis relevant to 11N and other A = 11 nuclei in (1995FO11, 1996BA13, 1997GR18, 2000AO06, 2001MU35, 2001SH39, 2002LE40).

1. 1H(10C, p)10C

The 1H + 10C resonant scattering excitation function was measured (1996AX01, 2000MA62). In (1996AX01) the 0° cross sections were evaluated in an R-matrix analysis which found evidence for three states at Eres = 1.3, 2.04 and 3.72 MeV. Weak evidence was found for a state at 4.32 MeV [Γ = 70 keV, (3/2-)] and higher-lying states above 5 MeV (not fully analyzed because of a limited sensitivity). In (2000MA62) the data of (1996AX01) are combined with additional measurements at θlab = 12°; the analysis of low-lying resonances is shown in Table 11.46 preview 11.46 (in PDF or PS). The resonance at 4.33 MeV is thought to be the analog of 11Be*(2.69). A separate R-matrix analysis of the high-energy part of the resonant spectrum, which only distinctly shows the Eres = 4.33 MeV resonances, was well fit by including resonances at Eres = 3.94 MeV [Γ = 580 keV, 3/2+], 4.81 MeV [Γ = 400 keV, (5/2+)] analogs of 11Be*(3.41) and 5.4 MeV [Γ = 250 keV, (7/2-)]. Additional measurements at E(10C) = 25.5 and 32 MeV (2004AN28, 2006ANZV, 2006CA05) find Eres = 1.54 and 2.27 MeV for the 1/2+ and 1/2- states; see Table 11.46 preview 11.46 (in PDF or PS). See also (1998AO01) for a discussion of the Thomas-Erman effect and a comparison of the low-lying 11N and 11Be states.

2. 9Be(12N, 10C + p) Qm = -6.9086

Complete kinematics decay spectroscopy was used to determine the excitation energies of 11N states populated in the fragmentation of 40 MeV/A 12N on a 9Be target (1998AZ01). In addition to a strong contribution apparently from the Jπ = 1/2- first excited state, an enhancement at low relative energies was attributed to 11N at Eres = 1.45 ± 0.40 MeV with Γ > 400 keV. Further interpretation, involving predicted higher-lying Jπ = 3/2- and 5/2- states that decay to 10C + p0 and 10C*(3.36) + p1 channels is also given.

3. 10B(14N, 13B)11N Qm = -26.1250

At E(14N) = 30 MeV/A the 11N ground state and multiple excited states are observed (2000OL01, 2003LE26), see Table 11.47 preview 11.47 (in PDF or PS). The mass excess of 24618 ± 50 keV is deduced for the 11N ground state. Shell model predictions suggest the peaks observed at 2.16, 4.33 and 5.98 MeV decay energy could be the 1/2-, 3/2- and 5/2- members of the K = 1/2 rotational band.

4. 12C(π+, π-p) Qm = -31.7660

A search for evidence of Δ components in normal nuclear matter was carried out at Eπ+ = 500 MeV (1998MO09).

5. 12C(14N, 15C)11N Qm = -31.4867

At E(14N) = 30 MeV/A five levels in 11N have been observed at θlab = 2.5° (1998BO38, 1998LE06, 1999LE37, 2003LE26). The observed resonances were evaluated in an R-matrix analysis to determine the probable Jπ values, see Table 11.48 preview 11.48 (in PDF or PS). The Jπ = 1/2+ ground state was not observed, though its population was expected to be strongly hindered.

6. 14N(3He, 6He)11N Qm = -24.2752

At E(3He) = 70 MeV a 6He group was observed which was interpreted as the first observation of a 11N state with an atomic mass excess of 25.23 ± 0.10 MeV and Γ = 740 ± 100 keV (1974BE20). The cross section for forming this state is 0.5 μb/sr at 10°. The observed state was interpreted as the Jπ = 1/2- mirror of 11Be*(0.32) because of its width; the 1/2+ mirror of 11Beg.s. was expected to be much broader (1974BE20). A subsequent investigation (1995GU08) at E(3He) = 70 MeV and at 6.8 to 25.0 degrees reported two resolved states thought to be the g.s. and first excited state, possibly having Jπ = 1/2+ and 1/2-. Further measurements at E(3He) = 73.4 MeV (2003GU06, 2003GU30), resolved the g.s. and first excited states and found evidence for several additional states, see Table 11.49 preview 11.49 (in PDF or PS). A DWBA analysis of the angular distributions was used to evaluate spin assignments for the resonances up to Eres = 5.9 MeV.