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The detection of long-lived radionuclides through ultra-sensitive single atomcounting via accelerator mass spectrometry (AMS) offers opportunities forprecise measurements of neutron capture cross sections, e.g. for nuclearastrophysics. The technique represents a truly complementary approach,completely independent of previous experimental methods. The potential of thistechnique is highlighted at the example of the $^{54}$Fe($n, \gamma$)$^{55}$Fereaction. Following a series of irradiations with neutrons from cold andthermal to keV energies, the produced long-lived $^{55}$Fe nuclei($t_{1/2}=2.744(9)$ yr) were analyzed at the Vienna Environmental ResearchAccelerator (VERA). A reproducibility of about 1% could be achieved for thedetection of $^{55}$Fe, yielding cross section uncertainties of less than 3%.Thus, the new data can serve as anchor points to time-of-flight experiments. Wereport significantly improved neutron capture cross sections at thermal energy($\sigma_{th}=2.30\pm0.07$ b) as well as for a quasi-Maxwellian spectrum of$kT=25$ keV ($\sigma=30.3\pm1.2$ mb) and for $E_n=481\pm53$ keV ($\sigma=6.01\pm0.23$ mb). The new experimental cross sections have been used to deduceimproved Maxwellian average cross sections in the temperature regime of thecommon $s$-process scenarios. The astrophysical impact is discussed usingstellar models for low-mass AGB stars.

Precise measurement of the thermal and stellar $^{54}$Fe($n, \gamma$)$^{55}$Fe cross sections via AMS