Isospin-forbidden Resonance Research Articles

Spin dependence of the isospin-forbidden decay 8Be(Ex=27.5 MeV)-->d

The first T-italic = 2 state in /sup 8/Be (E-italic/sub x-italic/ = 27.483 MeV) has been studied by means of the isospin-forbidden resonance in the reaction d+ /sup 6/Li..--> cap alpha..+..cap alpha... Angular distributions of the cross section and of the four analyzing powers for the reaction /sup 6/Li(d-arrow-right,..cap alpha..) /sup 4/He have been measured at energies above, below, and on resonance. Off-resonance analyzing power measurements have also been obtained for /sup 2/H(/sup 6/Li-arrow-right,..cap alpha..) /sup 4/He. Because of the symmetry properties of the reaction, the number of independent collision matrix elements is fairly small, and this makes it possible to determine the matrix elements directly by fitting the data. By comparing the matrix elements, one can determine partial widths (GAMMA/sub 0/ and GAMMA/sub 2/) for decay of the /sup 8/Be state into d+ /sup 6/Li states with channel spin 0 and 2. The measured ratio of the partial widths is GAMMA/sub 2//GAMMA/sub 0/ = 0.322 +- 0.091.

Isospin and Isobar Components in Light Nuclei

Isospin mixing in light nuclei is examined through isospin-forbidden particle decay channels. From data on ($\stackrel{\ensuremath{\rightarrow}}{\mathrm{p}}$,$p$) isospin-forbidden resonances, isospin-mixing amplitudes are extracted. They show definitively a periodicity of 8 in $A$ for $8l~Al~40$. These results are combined with neutron- and alpha-decay data in an isospin analysis that estimates isovector and isotensor amplitudes and nucleon isobar contributions to isospin mixing.

The α-width of the lowest T = 2 state in 28Si

We have studied the E x = 15.22 MeV, J π = 0 + T = 2 level in 28Si as an isospin forbidden resonance in 24Mg(α, α 0) 24Mg and obtained Γ α, c.m. = 222 ± 22 eV. This corresponds to a reduced width ≈ 1.4 × 10 −4 times the Wigner limit, which is the largest known reduced α 0 width of a 0 + T = 2 state in the A = 4 n T z = 0 nuclei. We have also established the existence of a J π = 0 + state at E x = 15.12 MeV with Γ c.m. = 11 ± 2 keV and Γ x/ Γ = 0.7 ± 0.1. Narrow resonances at E x = 15.08, 15.29 and 15.31 MeV are also observed.

The [formula omitted] resonance at 6.44 MeV

The first T = 3 2 level in 13C has been studied as an isospin-forbidden resonance in the 9Be(α, n 0), 9Be(α, n 1) and 9Be(α, α 0) reactions at an incident energy of 6443.5±2.0 keV, corresponding to an excitation energy of 15108.9±1.5 keV in 13C. Total and partial decay widths were deduced as Γ = 5.49±0.25 keV and Γ α0/Γ ≧ 0.017 . The resonance parameters are discussed along the lines of similar investigations in 17O and 21Ne and with respect to the isobaric mass multiplet equation. Combining the present value for Γ with previous measurements of Γ γ0 / Γ yields the ground-state γ-decay width Γ γ0 = 22.8±1.9 eV, which compares well with the most precise value until now known from inelastic electron scattering data. The weighted combination of all available information on the ground-state-γ-decay of the first T = 3 2 states in 13C and 13N leads to B( 13 C)/B( 13 N)−1 = −0.15±0.07 for the reduced M1 transition strengths. The charge asymmetry of the reduced M1 transition strengths is discussed with respect to a charge-dependent mixing in the initial and final states and a possible isotensor component of the M1 transition operator.

Mass of lowestT=2state ofC12

A precise measurement of the excitation energy of the lowest T = 2 state in /sup 1/2C has been made via the /sup 1/4C(p,t)/sup 1/2C reaction. The value obtained, 27.5950 +- 0.0024 MeV, is in agreement with earlier (less precise) measurements and therefore does not explain the failure to observe the T = 2 state in isospin-forbidden resonance reactions. Also reported in this work are an upper limit of 30 keV for the total width of the T = 2 state and an excitation energy of 3.3492 +- 0.0012 MeV for the first excited state of /sup 1/0C.

Isospin-forbidden particle decays in light nuclei: (III). Decay widths of the lowest [formula omitted] level of 21Ne

The lowest T = 3 2 level of 21Ne has been observed as an isospin-forbidden resonance in the 17O(α,n) 20Ne(1.63 MeV) reaction at an incident energy of 1862.2±2.6 keV, corresponding to an excitation energy of 8856.7±2.2 keV in 21Ne. Total and partial decay widths were determined as follows: Γ = 2.8±0.5 keV; Γ n(0.0)/ Γ < 0.3; Γ α Γ n(1.63)/ Γ = 0.42±0.16 eV. No resonant structure was observed for the second T = 3 2 level in 21Ne and upper limits of 0.4 eV and 1.0 eV were determined for Γ α Γ n(0.0)/ Γ and Γ α Γ n(1.63)/ Γ, respectively. All available information on partial widths for isospin-forbidden nucleon decay of T = 3 2 levels from mass-9 to mass-41 have been compiled. Trends observable in these data are discussed.

Isospin Forbidden Resonances in the ReactionNb93(p,α)Zr90

Isospin forbidden resonances ($\ensuremath{\Delta}T=1$) have been observed in the reaction $^{93}\mathrm{Nb}(p,\ensuremath{\alpha})^{90}\mathrm{Zr}$. Angular distributions to the ${0}^{+}$ ground state measured at the ${4}^{+}$ (59-keV) and ${2}^{+}$ (334-keV) analog resonances indicate that the ${2}^{+}$ isospin analog state is formed via a pure ${d}_{\frac{5}{2}}$ capture whereas the ${4}^{+}$ isospin analog state is formed by a coherent admixture of ${d}_{\frac{5}{2}}$, ${s}_{\frac{1}{2}}$, and ${d}_{\frac{3}{2}}$ components.