New Determination of the Astrophysical $S$ Factor $S_{E1}$ of the ${}^{12}\mathrm{C}(\alpha ,\gamma ){}^{16}\mathrm{O}$ Reaction

A new measurement of the $\beta$-delayed $\alpha$ decay of ${}^{16}\mathrm{N}$ has been performed using a set of high efficiency ionization chambers. Sources were made by implantation of a ${}^{16}\mathrm{N}$ beam, yielding very clean $\alpha$ spectra down to energies as low as 400 keV. Our data are in good agreement with earlier results. For the $S$ factor $S_{E1}$, we obtain a value of $74\pm 21\mathrm{keV}\mathrm{b}$. In spite of improvements in the measurement, the error in $S_{E1}$ remains relatively large because of the correlations among the fit parameters and the uncertainties inherent to the extrapolation.

Figure 1

Schematic of the experimental setup used for a measurement of the $\beta$-delayed $\alpha$ decay of ${}^{16}\mathrm{N}$.

Figure 2

(Left) coincident energy spectrum, measured with one of the twin-ionization detectors for a foil with implanted ${}^{16}\mathrm{N}$ particles. (Right) same as above, but for a nonimplanted foil.

Figure 3

(a) The $\alpha$ spectrum measured in one of the twin-ionization chambers filled with 150 Torr of isobutane. (b) Same as (a), but at a pressure of 195 Torr. The solid line is the spectrum obtained at 150 Torr, scaled in height.

Figure 4

Summed $\alpha$ spectrum obtained in this experiment in comparison with an $R$-matrix fit. The insert shows the low-energy part of spectrum together with the previous results (solid [14] and dashed lines [5]). See text for details.