Determination of $\gamma$-ray widths in ${}^{15}\mathrm{N}$ using nuclear resonance fluorescence

Background: The stable nucleus ${}^{15}\mathrm{N}$ is the mirror of ${}^{15}\mathrm{O}$, the bottleneck in the hydrogen burning CNO cycle. Most of the ${}^{15}\mathrm{N}$ level widths below the proton emission threshold are known from just one nuclear resonance fluorescence (NRF) measurement, with limited precision in some cases. A recent experiment with the AGATA demonstrator array determined level lifetimes using the Doppler shift attenuation method in ${}^{15}\mathrm{O}$. As a reference and for testing the method, level lifetimes in ${}^{15}\mathrm{N}$ have also been determined in the same experiment.

Purpose: The latest compilation of ${}^{15}\mathrm{N}$ level properties dates back to 1991. The limited precision in some cases in the compilation calls for a new measurement to enable a comparison to the AGATA demonstrator data. The widths of several ${}^{15}\mathrm{N}$ levels have been studied with the NRF method.

Method: The solid nitrogen compounds enriched in ${}^{15}\mathrm{N}$ have been irradiated with bremsstrahlung. The $\gamma$ rays following the deexcitation of the excited nuclear levels were detected with four high-purity germanium detectors.

Results: Integrated photon-scattering cross sections of 10 levels below the proton emission threshold have been measured. Partial $\gamma$-ray widths of ground-state transitions were deduced and compared to the literature. The photon-scattering cross sections of two levels above the proton emission threshold, but still below other particle emission energies have also been measured, and proton resonance strengths and proton widths were deduced.

Conclusions: Gamma and proton widths consistent with the literature values were obtained, but with greatly improved precision.

Figure 1

Schematic top view of the photon scattering facility [11] at the ELBE superconducting electron accelerator of HZDR. Detectors at $1.7{}^{\circ }$ with respect to the $\gamma$ beam are below and above the beam line.

Figure 2

Comparison of the experimental (black histogram) and calculated (red dashed curve) bremsstrahlung energy distribution. For details see text.

Figure 3

Average bremsstrahlung flux experienced by target No. 4 during the different runs. The previously calculated distributions matching the experimental spectrum from the D($\gamma$,p)n reaction are scaled to the measured ${}^{11}\mathrm{B}$ values, and are potted by red (blue) lines for end point of 12.6 MeV (9.8 MeV) with a $1\sigma$ uncertainty bands, respectively.

Figure 4

(a) Level scheme of ${}^{15}\mathrm{N}$ from the first excited state up to the proton separation threshold at 10.21 MeV. The spin and parities of the levels, and the level energies in MeV are taken from [9]. The investigated levels are marked by bold lines in red. (b) Spectra recorded by the HPGe detectors placed at different angles, during the irradiation of target No. 4 with bremsstrahlung created by electrons of 12.6-MeV kinetic energy. (c) Same as (b), but with bremsstrahlung endpoint energy of 9.8 MeV. Peaks correspond to ${}^{15}\mathrm{N}$ (blue solid), ${}^{11}\mathrm{B}$ (green dashed), and ${}^{16}\mathrm{O}$ (orange dotted) transitions, and the respective single escape peaks are marked. A few weak double escape peaks are also observed, but not marked in the spectra. The spectra recorded at $127{}^{\circ }$ were multiplied by a factor of 10 for clearer view.