β-delayed α emission of ${}^{18}\mathrm{N}$: Broad $J^{\pi }=1^{-}$ states in the ${}^{14}\mathrm{C}$ $+\alpha$ system

The β-delayed α-spectrum of ${}^{18}\mathrm{N}$ has been measured at the TISOL facility at TRIUMF resolving the structure of a broad α-particle distribution at α-energies of 1.5–5.5 MeV into several states that show a distinct interference pattern. $R$-matrix fits assuming a solely $J^{\pi }=1^{-}$ partial wave confirm this finding. The extrapolated low energy part of this structure constitutes the natural parity background in a possible experiment to observe the parity forbidden α-decay of the $E_x=6.880$ MeV, $J^{\pi }=0^{-}$ state in ${}^{18}\mathrm{O}$ that is estimated from the fit to be $2\times {10}^{-8}$ of the intensity of the $E_{\alpha }=1081$ keV peak. In addition, the tails of the broad $1^{-}$ states observed likely contribute to the astrophysical ${}^{14}\mathrm{C}$ ($\alpha ,\gamma$)${}^{18}\mathrm{O}$ reaction rate though the experimental information available is too incomplete to allow precise predictions. The β-delayed α-spectrum of ${}^{18}\mathrm{N}$ also excludes complex β-feeding amplitudes at a significant level. The states seen in the β-delayed α-decay produce a pattern of approximately equally spaced $1^{-}$ states in ${}^{18}\mathrm{O}$ consistent with $J^{\pi }=1^{-}$ states seen at the same and higher excitation energies of ${}^{18}\mathrm{O}$.

Figure 1

Pulse height correlation spectrum of two opposite detectors ($30\mu$m thickness) for the β-delayed α-decay of ${}^{18}\mathrm{N}$. E${}_{D_i}$ labels the approximate α energy of the events.

Figure 2

β-delayed α-spectrum of ${}^{18}\mathrm{N}$ as obtained in the present experiment. In addition, an $R$-matrix fit to the data is shown with the solid line corresponding to the total convoluted fit, the dashed line to the unconvoluted $p$-wave.

Figure 3

Gamow-Teller strength per unit energy from the β-delayed α-spectrum of ${}^{18}\mathrm{N}$ as obtained in the present experiment versus the center-of-mass energy.