Speaker
Description
The CNO cycle plays a paramount role in the evolution of massive stars. Above 1.3 solar masses, the CNO cycle becomes the dominant energy-generation mechanism, overtaking the role of the proton-proton chain. In the CNO cycle, carbon and nitrogen isotopes act as catalysts, helping the fusion of hydrogen into helium.
The reaction rate of the entire cycle is governed by its slowest reaction, the 14N (p, γ) 15O, which has the smallest cross section among the reactions in the cycle. Since several astrophysical parameters are depending on the rate of the cycle – including the ages of the oldest globular clusters and indirectly, the age of the Universe – its precise determination is of particular importance in nuclear astrophysics.
At astrophysically relevant energies, this reaction has an extremely low probability. Thus, reducing the natural background in the detection system is critical for a precise measurements, as the radiation produced by the reaction would be undetectable in the presence of a high background. For this reason, previous in-beam gamma-ray spectroscopy measurements have been carried out at ultralow-background facilities, including the underground laboratory of the LUNA collaboration beneath the Gran Sasso Mountain in Italy and a similar facility used by the JUNA collaboration in Jinping, China. In addition, valuable information can be obtained at the Earth's surface using the activation technique, which measures the radioactivity of the produced 15O. The feasibility of this approach has already been demonstrated at ATOMKI.
Building on the results of previous ATOMKI experiments, our measurements employ a new, alternative approach to determine the reaction cross section: inverse kinematics, in which a hydrogen-containing target is bombarded with 14N ions. This method avoids high-energy proton activation, thus preventing the unwanted radioactivity of the measurement environment, and substantially reduces the in-beam gamma-ray background.
To date, we have used thick targets, pressed from titanium hydride powder, in our measurements. Cyclic irradiation was employed, where the prompt gamma-rays from the reaction was detected during the irradiations, while the radioactivity of the produced 15O was measured independently in the off beam periods. This demonstrates the feasibility and usefulness of cross-validation of the two methods. To improve measurement accuracy, we are refining the preparation techniques for thick targets. In parallel, the production of thin targets, with thicknesses of a few tens of nanometres, is underway using a plasma sputtering system.
Following our preliminary measurements at ATOMKI, we plan to continue our research at an underground laboratory to reach even smaller bombarding energy and cross section regions. This would enable a precise determination of the cross section at astrophysical energies and, consequently, the CNO-cycle reaction rate. The improved accuracy may help refine estimates of the age of the Universe and our understanding of the Sun's core composition.