A Study of electromagnetic properties of excited nuclear states

Gamma decays of excited states of 11B, 14C, 18Ne, 19Ne, and 26Si have been studied experimentally and the results used to test the predictions of various nuclear models. The phase-consistent Rose and Brink treatment of electromagnetic transitions was used throughout.
States of 11B and 14C were studied using the reactions 12C(t,α)11B and 12C(t,p)14C in conjunction with a conventional charged particle-gamma ray coincidence angular correlation technique. For the 11B, 4.44 MeV, J=5⁄2 level previous measurements of the ground state decay mixing ratio were confirmed (δ = −0.19 ± 0.015). For the 11B, 5.02 MeV level the spin was found to be 3⁄2; the mixing ratio of the (88 ± 2.5%) transition to the ground state was measured to be δ = 0.03 ± 0.05, and that of the 5.02 → 2.12 MeV transition δ = −0.05 ± 0.2. The results for both 11B levels are in substantial agreement with the predictions of intermediate coupling calculations.
The results for 14C levels are summarised in the table (where parities and final state spins nave been taken from the literature).
14C Results
Transition (MeV) Initial Jπ Final Jπ Branch (%) Mixing Ratio δ
6.72 → g.s. 3− 0+ 97.3 ± 1.0 0
6.72 → 6.09 3− 1− 2.7 ± 1.0 −
7.01 → g.s. 2+ 0+ 98.6 ± 0.7 0
7.01 → 6.09 2+ 1− 1.4 ± 0.7 −
7.35 → g.s. 2− 0+ 14 ± 4 0
7.35 → 6.09 2− 1− 52 ± 5 −0.04 ± 0.09
7.35 → 6.72 2− 3− 34 ± 4 0.07 ± 0.3
The results are compared to predictions of True's model of 14C, in which the active particles are two neutrons in the 1p1⁄2, 2s1⁄2, 1d3⁄2 and 1d5⁄2 shells. For the 6.72 MeV level agreement is good, but the 7.01 → 6.09 MeV and the 7.35 MeV → g.s. decays are predicted too strong. The explanation probably lies in excitation of the 1p3⁄2 core particles, as considered by Sebe.
The bound states of 18Ne were studied using the 16O(3He,n)18Ne reaction in conjunction with a neutron-gamma ray coincidence angular correlation technique. Neutrons were detected in a large liquid scintillator using pulse shape discrimination. Gamma ray detection in large-volume Ge(Li) counters enabled separation of the level-decays under study. The missing Jπ = O+ level was found at 3.58 MeV and its origin in 18Ne confirmed. The decay schemes and mixing ratios are shown below. The level spins and parities above 2 MeV have been deduced by synthesis of the present results with published 18Ne level lifetimes and double stripping studies, and by analogies with the mirror nucleus 18O.
18Ne Results
Transition (MeV) Initial Jπ Final Jπ Branch (%) Mixing Ratio δ
3.38 → 0 4+ 0+ < 4  
  → 1.89 4+ 2+ 100 +0.06 ± 0.07
3.58 → 0 0+ 0+ < 17  
  → 1.89 0+ 2+ 100  
3.62 → 0 2+ 0+ < 9  
  → 1.89 2+ 2+ 100 −1.1 + 1.0
− 0.3
The 18Ne results are shown to be in good accord with the predictions of the Benson-Irvine model which includes a deformed 4-particle-2-hole component in the wavefunctions along with the conventional two-proton (sd)2 configurations.
Decays of excited states of 19Ne below 3 MeV were similarly studied using the 19(F(p,n)19Ne reaction. By synthesis with published lifetimes, and by analogy with levels of the mirror nucleus 19F, the results in the table were obtained:
19Ne Results
Transition (MeV) Initial Jπ Final Jπ Branch (%) Mixing Ratio (δ)
1.51 → g.s. 5⁄2− 1⁄2+ < 3 −
1.51 → 0.24 5⁄2− 5⁄2+ 12 ± 7 −
1.51 → 0.28 5⁄2− 1⁄2− 88 ± 3 0.1 ± 0.2
1.54 → g.s. 3⁄2+ 1⁄2+ < 7 −
1.54 → 0.24 3⁄2+ 5⁄2+ 100 −
1.54 → 0.28 3⁄2+ 1⁄2− < 7 −
1.62 → g.s. 3⁄2− 1⁄2+ 20 ± 2 −
1.62 → 0.24 3⁄2− 5⁄2+ 10 ± 2 −
1.62 → 0.28 3⁄2− 1⁄2− 70 ± 2 −
2.79 → g.s. 9⁄2+ 1⁄2+ < 10 −
2.79 → 0.24 9⁄2+ 5⁄2+ 100 0.0 ± 0.1
2.79 → 0.28 9⁄2+ 1⁄2− < 10 −
2.79 → 1.51 9⁄2+ 5⁄2− < 12 −
2.79 → 1.54 9⁄2+ 3⁄2+ < 10 −
2.79 → 1.62 9⁄2+ 3⁄2− < 10 −
Levels of 26Si were studied using the reaction 24Mg(3He,n)26Si. Decay schemes and level energies were determined, and the Doppler-shift attenuation method was used to measure level lifetimes. The results are shown in the table.
26Si Results
Level Energy (keV) Lifetime (ps) Transition to (keV) Branch (%) Initial Jπ
1795.9 ± 0.2 1.38 + 0.7 g.s. 100 2+
− 0.5
2783.5 ± 0.4 0.20 + 0.20 g.s. 30 ± 5 2+
− 0.12
  1796 70 ± 5  
3332.5 ± 0.3 2.7 + 2.3 1796 100 0+
− 1.0
3756 ± 2 < 0.7 1796 70 ± 10 (3+)
  2784 30 ± 10  
3842 ± 2 − 1796 100 (4+)
4093 ± 3 − g.s. 100  
4138 ± 1 0.11 + 0.11 g.s. 20 ± 10  
− 0.10
  1796 80 ± 10  
4445 ± 3 < 0.5 1796 100  
4805 ± 2 < 0.1 1796 30 ± 10  
  2784 70 ± 10  
Jπ for the 3756 and 3842 keV levels have been deduced from the observed decay widths and from mirror level arguments; others are from the literature.
The results are compared to shell-model calculations for ten particles in the 2s1⁄2-1d5⁄2 orbitals using the two~nucleon matrix elements of Wildenthal (from an effective interaction fit to level energies) and of Kuo (based on the realistic Hamada-Johnston potential). Neither calculation gives a good description), though the Kuo predictions are better overall. Discrepancies underline the importance of including the 1d3⁄2 orbital in such calculations.