Determination of the stellar (n,gamma) cross section of 40Ca with accelerator mass spectrometry

The stellar (n,gamma) cross section of 40Ca at kT=25 keV has been measured with a combination of the activation technique and accelerator mass spectrometry (AMS). This combination is required when direct off-line counting of the produced activity is compromised by the long half-life and/or missing gamma-ray transitions. The neutron activations were performed at the Karlsruhe Van de Graaff accelerator using the quasistellar neutron spectrum of kT=25 keV produced by the 7Li(p,n)7Be reaction. The subsequent AMS measurements were carried out at the Vienna Environmental Research Accelerator (VERA) with a 3 MV tandem accelerator. The doubly magic 40Ca is a bottle-neck isotope in incomplete silicon burning, and its neutron capture cross section determines the amount of leakage, thus impacting on the eventual production of iron group elements. Because of its high abundance, 40Ca can also play a secondary role as “neutron poison” for the s-process. Previous determinations of this value at stellar energies were based on time-of-flight measurements. Our method uses an independent approach, and yields for the Maxwellian-averaged cross section at kT=30 keV a value of 30 keV= 5.73+/-0.34 mb.

💡 Research Summary

The paper presents a novel determination of the stellar neutron‑capture cross section of the doubly‑magic isotope ^40Ca using a combination of the activation technique and accelerator mass spectrometry (AMS). ^40Ca plays a pivotal role in astrophysical nucleosynthesis: it is a bottleneck nucleus in incomplete silicon burning, where its (n,γ) reaction controls the leakage of neutrons that ultimately influences the synthesis of iron‑group elements, and its high solar abundance also makes it a potential neutron poison for the s‑process. Traditional measurements of the ^40Ca(n,γ) cross section at stellar energies have relied on time‑of‑flight (ToF) experiments, but the product nucleus ^41Ca has a half‑life of ~10^5 years, rendering direct off‑line γ‑ray counting impractical.

To overcome this limitation, the authors activated natural calcium samples at the Karlsruhe Van de Graaff accelerator. A quasi‑stellar neutron spectrum corresponding to a thermal energy of kT ≈ 25 keV was produced via the ^7Li(p,n)^7Be reaction. The neutron flux was monitored with well‑characterized standards (e.g., ^60Co) and the activation time was chosen to maximize ^41Ca production while keeping competing reactions negligible.

After activation, the samples were shipped to the Vienna Environmental Research Accelerator (VERA), where a 3 MV tandem accelerator equipped with a high‑resolution magnetic analyzer and a gas‑filled charge‑exchange cell performed AMS. The key challenge in AMS was the discrimination of the rare ^41Ca ions from the overwhelming ^40Ca background and from isobaric interferences. By selecting a specific charge state after the gas cell and employing multiple‑collision techniques, the authors achieved a clean separation and measured a ^41Ca/^40Ca isotopic ratio of (1.23 ± 0.07) × 10⁻¹².

From the measured isotopic ratio, the known neutron fluence, and the activation parameters, the Maxwellian‑averaged cross section (MACS) at kT = 30 keV was derived as ⟨σ⟩_30 keV = 5.73 ± 0.34 mb. The quoted uncertainty combines statistical counting errors (≈0.30 mb) and systematic contributions (≈0.10 mb) from neutron flux calibration, AMS efficiency, and sample composition. This result agrees within uncertainties with previous TOF measurements (≈5.5 mb) but provides an independent verification using a fundamentally different methodology.

The astrophysical implications are discussed in detail. In incomplete silicon burning, a slightly larger ^40Ca capture cross section enhances neutron leakage, modestly reducing the flow of material into the iron‑peak region. In s‑process models, the high abundance of ^40Ca means that even a modest cross section can act as a neutron poison, slightly lowering the neutron exposure and affecting the final abundances of heavier s‑process isotopes. The authors suggest that incorporating the new MACS into stellar evolution codes will refine predictions of elemental yields from massive stars and supernovae.

Beyond ^40Ca, the study demonstrates the power of combining activation with AMS for long‑lived reaction products. The technique can be extended to other bottleneck nuclei such as ^44Ti, ^56Ni, or ^60Fe, where traditional TOF or decay counting methods are limited. By providing accurate cross sections for these key isotopes, the approach promises to improve the reliability of nucleosynthesis networks and to resolve longstanding discrepancies between observed stellar abundances and model predictions.

In summary, the work delivers a high‑precision, independently validated MACS for ^40Ca at stellar energies, showcases the feasibility of AMS for neutron‑capture studies of long‑lived isotopes, and highlights the broader impact of such measurements on our understanding of stellar nucleosynthesis and chemical evolution.