Towards a comprehensive study of the 14N(p,g)15O astrophysical key reaction: Description of the experimental technique including novel target preparation

TL;DR

This work addresses the critical need for a precise $^{14}$N(p,$\gamma$)$^{15}$O reaction rate by detailing an underground cross-section measurement program at the Bellotti Ion Beam Facility. It introduces two complementary solid-state nitrogen target fabrication methods—ion implantation and reactive sputtering—and thoroughly characterizes the targets using Rutherford Backscattering, Nuclear Reaction Analysis, and Particle-Induced Gamma Emission to ensure accurate thickness, stoichiometry, and stability. The experimental setup employs multiple high-purity HPGe detectors to measure gamma yields across several angles, with careful efficiency and summing corrections validated via calibration resonances. The preliminary results at $55^\circ$ show measurable yields for several weak transitions, and the framework is prepared for comprehensive angular-distribution measurements and multi-channel R-matrix analyses to improve astrophysical extrapolations of the reaction rate.

Abstract

While the 14N(p,g)15O reaction plays a key role in the hydrogen-burning processes in various stellar conditions, its reaction rate is not known with sufficient precision. Therefore, the first scientific project at the recently launched Bellotti Ion Beam Facility of the Laboratori Nazionali del Gran Sasso was the measurement of the 14N(p,g)15O reaction cross section in the proton energy range between 250 and 1500 keV. In this paper, the experimental techniques are summarized with special emphasis on the description of solid state nitrogen target production and characterization. The first results of the reaction yield measured at 55 deg detection angle are also presented.

Figures (13)

• Figure 1: Predicted $^{14}$N depth profiles for two different nominal fluences. The $^{14}$N implantation energy is 40 keV.
• Figure 2: Tantalum (Ta) discs mounted on the rotating holder inside the vacuum chamber of the ion implanter, prior to implantation with $^{14}$N ions.
• Figure 3: Magnetron sputtering system used to produce the TaN targets. b) a picture of the sample holder with Ta backing disks mounted and the snow jet cleaner nozzle used to clean their surface prior to deposition. The equipment is positioned in a filtered air laminar flow enclosure to reduce dust uptake.
• Figure 4: On the left, the $^4$He$^+$ backscattering spectra (black points) for the implanted target ($D_n = 6.0e17$ atoms/cm$^2$) measured at $\theta_{\textup{lab}}=\ang{165}$, for three different energies. The NDF simulated spectra are represented with solid lines. On the right, resonance scans measured at the LUNA-400 accelerator using the 278.0 keV resonance in $^{14}$N(p,$\gamma$)$^{15}$O for test targets produced with sputtering and implantation, before irradiation at BIBF. ERYA-Profiling simulations MANTEIGAS2022108307 for each target are also plotted as solid lines.
• Figure 5: Nitrogen depth profiles and layer structure obtained by NDF+PIGE analysis and by Eq. \ref{['eq:impl']}.