Spectroscopic study of low-lying ${}^{16}\mathrm{N}$ levels

The magnitude of the ${}^{15}\mathrm{N}(n,\gamma ){}^{16}\mathrm{N}$ reaction rate in asymptotic giant branch stars depends directly on the neutron spectroscopic factors of low-lying ${}^{16}\mathrm{N}$ levels. A new study of the ${}^{15}\mathrm{N}(d,p){}^{16}\mathrm{N}$ reaction is reported populating the ground and first three excited states in ${}^{16}\mathrm{N}$. The measured spectroscopic factors are near unity as expected from shell model calculations, resolving a long-standing discrepancy with earlier measurements that had never been confirmed or understood. Updated ${}^{15}\mathrm{N}(n,\gamma ){}^{16}\mathrm{N}$ reaction rates are presented.

Figure 1

(a) Energy of detected particles observed in the various SIDAR strips. The laboratory angles range from ${169}^{°}$ to ${155}^{°}$ for strips 1 to 16, respectively. (b) Same as (a) but in coincidence with a ${}^{16}\mathrm{N}$ recoil transported through the DRS. The band arising from ${}^{15}\mathrm{N}(d,p){}^{16}\mathrm{N}$ is clearly identified.

Figure 2

Detected energy vs. position spectrum for a portion of the events observed in an ORRUBA detector placed near ${90}^{\circ }$. The scattering of target constituents is noted at positions corresponding to laboratory angles forward of ${90}^{°}$. These scattering rates provide a sensitive measure of target composition changes—none of which were observed. The line from the $\alpha$-calibration source is clearly visible since the energy of these events show no position dependence. The reduction of counts backwards of ${90}^{\circ }$ arises from target-frame shadowing.

Figure 3

Singles energy spectrum observed in the inner strip of SIDAR. Lines show the expected positions of peaks from the ${}^2\mathrm{H}({}^{15}\mathrm{N}$,$p){}^{16}\mathrm{N}$ reaction.

Figure 4

Differential cross sections for the ${}^{15}\mathrm{N}(d,p){}^{16}\mathrm{N}$ reaction as a function of center-of-mass angle. The inset shows an expanded view of the SIDAR data. Since resolution of the closely-spaced levels was not possible, composite DWBA curves have been fitted to the data. The dashed curves show the contributions that were summed to fit the g.s.+120-keV cross sections.

Figure 5

${}^{15}\mathrm{N}(n,\gamma ){}^{16}\mathrm{N}$ rate as a function of stellar temperature in GK. The rate is dominated by $p$-wave direct capture.