Particle decay of proton-unbound levels in ${}^{12}\mathrm{N}$

Background: Transfer reactions are a useful tool for studying nuclear structure, particularly in the regime of low level densities and strong single-particle strengths. In addition, transfer reactions can populate levels above particle decay thresholds, allowing for the possibility of studying the subsequent decays and furthering our understanding of the nuclei being probed. In particular, the decay of loosely bound nuclei such as ${}^{12}\mathrm{N}$ can help inform and improve structure models.

Purpose: To learn about the decay of excited states in ${}^{12}\mathrm{N}$, to more generally inform nuclear structure models, particularly in the case of particle-unbound levels in low-mass systems which are within the reach of state-of-the-art ab initio calculations.

Method: In this follow-up analysis of previously published data [Chipps et al. (JENSA Collaboration), Phys. Rev. C 92, 034325 (2015)], decay particles from excited states populated in ${}^{12}\mathrm{N}$ have been detected in coincidence with tritons from the ${}^{14}\mathrm{N}(p,t){}^{12}\mathrm{N}$ transfer reaction. Specifically, decay protons from proton-unbound levels above $\sim 2$ MeV excitation energy were observed by utilizing the Jet Experiments in Nuclear Structure and Astrophysics (JENSA) gas jet target.

Results: Isotropic proton branching ratios for the $p0$ and $p1$ decay channels are calculated and decay particle spectra for the populated levels from $p0$, $p1$, and $p2$ decay are given.

Conclusions: The current data from ${}^{14}\mathrm{N}(p,t){}^{12}\mathrm{N}$ will help provide nuclear structure and decay information input to models in this mass region.

Figure 1

Energy level diagram for ${}^{12}\mathrm{N}$, from the 1990 evaluation by Ajzenberg-Selove [26]. The particle decay thresholds for ${}^{11}\mathrm{C}+p$ and ${}^8\mathrm{B}+\alpha$ are shown on the right.

Figure 2

Two-dimensional spectrum of triton energy (abscissa, arb. units) versus the energy of any particle detected in coincidence in any other detector telescope (ordinate, arb. units). Both axes are approximately 30 keV per channel. In (a), several lines of coincidences due to particle decay can be clearly seen, trending toward the proton threshold around channel 380. Due to the high rate of random coincidences near the ground and first excited states (to the right of the figure), the coincidence gate could not be reliably extended into this region (see text). (b) shows the same spectrum with the coincidence gates corresponding to $p0$ (red, upper), $p1$ (green, middle), and $p2$ (blue, lower) overlaid.

Figure 3

Top panel: triton singles energy spectrum summed over all angles (black; compare with Fig. 2 of Ref. [11]) overlaid with the summed triton spectrum gated on decay particle coincidences (red for $p0$, farthest right greyscale curve; green for $p1$, middle greyscale curve; and blue for $p2$, farthest left greyscale curve; as in Fig. 2), corrected only for detection efficiency. The result of a similarly sized gate off the coincident particle bands is shown in solid brown. Bottom three panels: the isotropic proton branching ratio strength function (black) derived from the ratio of the gated and ungated spectra shown above for each of the decay branches ($p0$, $p1$, $p2$), as indicated by the axis labels. The grey band indicates the combined uncertainty due to statistics and the efficiency calibration.

Figure 4

Branching ratios extracted for the different ${}^{12}\mathrm{N}$ levels populated in the current work, assuming isotropic proton decay. Some allowed decays are not observed due to detector thresholds and low statistics. Observation of $p2$ decay to any discrete levels could not be confirmed, and hence is not included. Uncertainties include statistics and a 30% systematic uncertainty to account for the possible anisotropy (see text).

Figure 5

The ($p,t$)-coincident $p0$ (top), $p1$ (middle), and $p2$ (bottom) decay spectra (summed over all angles). The brown shaded area in each panel corresponds to the background as determined by a gate outside the area of reaction-decay coincidences (cf. Fig. 2). Prominent peaks are labeled with the excitation energy of the ${}^{12}\mathrm{N}$ level from which they originated; see also Table 2.

Figure 6

The intensity of $p0$ decay protons falling within three energy ranges (cf. Fig. 5, corresponding to the first and second spectrum peaks and an area without obvious structure; arbitrary units) is plotted against calculated intensity curves for isotropic (black solid) and an example anisotropic (maroon dashed) decay, as a function of the relative laboratory angle between the decay proton and reaction triton.