Three-body correlations in the ground-state decay of ${}^{26}\mathrm{O}$

Background: Theoretical calculations have shown that the energy and angular correlations in the three-body decay of the two-neutron unbound ${}^{26}\mathrm{O}$ can provide information on the ground-state wave function, which has been predicted to have a dineutron configuration and $2n$ halo structure.

Purpose: To use the experimentally measured three-body correlations to gain insight into the properties of ${}^{26}\mathrm{O}$, including the decay mechanism and ground-state resonance energy.

Method: ${}^{26}\mathrm{O}$ was produced in a one-proton knockout reaction from ${}^{27}\mathrm{F}$ and the ${}^{24}\mathrm{O}+n+n$ decay products were measured using the MoNA-Sweeper setup. The three-body correlations from the ${}^{26}\mathrm{O}$ ground-state resonance decay were extracted. The experimental results were compared to Monte Carlo simulations in which the resonance energy and decay mechanism were varied.

Results: The measured three-body correlations were well reproduced by the Monte Carlo simulations but were not sensitive to the decay mechanism due to the experimental resolutions. However, the three-body correlations were found to be sensitive to the resonance energy of ${}^{26}\mathrm{O}$. A $1\sigma$ upper limit of 53 keV was extracted for the ground-state resonance energy of ${}^{26}\mathrm{O}$.

Conclusions: Future attempts to measure the three-body correlations from the ground-state decay of ${}^{26}\mathrm{O}$ will be very challenging due to the need for a precise measurement of the ${}^{24}\mathrm{O}$ momentum at the reaction point in the target.

Figure 1

(a) Three-body decay energy spectrum, (b) Jacobi relative energy in the T system, (c) Jacobi relative energy in the Y system, (d) relative velocity spectrum, (e) Jacobi angle in the T system, and (f) Jacobi angle in the Y system from the decay of ${}^{26}\mathrm{O}$. The experimental data are compared with simulations by using three different decay modes and a half-life of 4 ps. All results are gated on $E_{\mathrm{decay}}<0.7$ MeV and have the causality cuts applied. The remaining false $2n$ component in the spectra, based on the geant4 simulation, is shown as the solid gray area.

Figure 2

Illustration of the T and Y Jacobi coordinate systems used to define the energy and angular correlations of the three-body decay.

Figure 3

Input $cos\left({\theta }_k\right)$ distributions from the Y Jacobi coordinate system for the Monte Carlo simulations. The distributions from the phase-space, dineutron, and Hagino decay models are presented.

Figure 4

$cos\left({\theta }_k\right)$ distributions from the Y Jacobi system for simulations using the Hagino decay mode with varying Be target thicknesses and decay energies for ${}^{26}\mathrm{O}$. The results are shown panels (a) with and (b) without the inclusion of the experimental incoming-beam characteristics in the simulations.

Figure 5

Reduced $\chi$ squared as a function of ${}^{26}\mathrm{O}$ ground-state decay energy for (a) $t_{1/2}=0$ ps and (b) 4 ps with the three different decay modes used in the simulation.