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

Accelerator mass spectrometry (AMS) represents a complementary approach for precise measurements of neutron capture cross sections, e.g., for nuclear astrophysics. This technique, completely independent of previous experimental methods, was applied for the measurement of the ${}^{54}\mathrm{Fe}(n,\gamma ){}^{55}\mathrm{Fe}$ reaction. Following a series of irradiations with neutrons from cold and thermal to keV energies, the produced long-lived ${}^{55}\mathrm{Fe}$ nuclei ($t_{1/2}=2.744+-0.009)$ yr) were analyzed at the Vienna Environmental Research Accelerator. A reproducibility of about 1% could be achieved for the detection of ${}^{55}\mathrm{Fe}$, yielding cross-section uncertainties of less than 3%. Thus, this method produces new and precise data that can serve as anchor points for time-of-flight experiments. We report significantly improved neutron capture cross sections at thermal energy (${\sigma }_{\mathrm{th}}=2.30\pm 0.07$ b) as well as for a quasi-Maxwellian spectrum of $kT=25$ keV ($\sigma =30.3\pm 1.2$ mb) and for $E_n=481\pm 53$ keV ($\sigma =6.01\pm 0.23$ mb). The new experimental cross sections have been used to deduce improved Maxwellian-averaged cross sections in the temperature regime of the common $s$-process scenarios. The astrophysical impact is discussed by using stellar models for low-mass asymptotic giant branch stars.

Figure 1

Schematic sketch of the setup used for the neutron activations at the Karlsruhe Van de Graaff.

Figure 2

Neutron energy distributions in the irradiations at the Karlsruhe Van de Graaff accelerator obtained with protons of (a) 1912 keV energy and (b) 2284 keV.

Figure 3

Schematic layout of the AMS facility VERA. Negative Fe ions were extracted from the ion source and mass analyzed before the tandem accelerator. After stripping in the terminal the three-fold positively charged ($3^{+}$) ions with an energy of 12 MeV were selected for analysis. The stable ${}^{54,56}\mathrm{Fe}$ nuclei were measured with Faraday cups, and the rare nuclide ${}^{55}\mathrm{Fe}$ was counted in one of the three subsequent particle detectors (see text for details).

Figure 4

Individual ${}^{55}\mathrm{Fe}/{}^{56}\mathrm{Fe}$ ratios as obtained during one AMS beam time. The upper panel (a) gives the data for a test sample irradiated at ATI with thermal neutrons. The two lower panels (b) and (c) represent the data for two unirradiated blank samples assumed to have negligible ${}^{55}\mathrm{Fe}$ content. Measurements in panel (c) show a reduced measurement background (average ${}^{55}\mathrm{Fe}/{}^{56}\mathrm{Fe}$ ratio $<{10}^{-15}$ at detector position 2). The solid and dashed lines represent the weighted mean and the standard deviation of the mean, respectively, for the respective samples.

Figure 5

Mean ${}^{55}\mathrm{Fe}/{}^{56}\mathrm{Fe}$ ratios as obtained during the various AMS beam times. The upper panel (a) gives the data for the two samples irradiated at KIT with a quasistellar Maxwell–Boltzmann spectrum, the lower panel (b) represents the data for the two samples irradiated at KIT with higher energies around 481 keV. The solid and dashed lines represent the weighted mean and the standard deviation of the mean for the respective samples.

Figure 6

Comparison of the thermal cross-section values obtained in this work from two independent activations with thermal (ATI) and cold (BNC) neutrons. Also plotted are the two previous experiments from more than 60 years ago by Brooksbank et al. [60] and by Pomerance [61]. The weighted average (square) of our work is in very good agreement with the value of Ref. [61], which was the basis for the recommend value in Ref. [31]. Also plotted is a new value quoted by Belgya et al. that is based on an improved decay scheme of ${}^{55}\mathrm{Fe}$ [59].

Figure 7

Comparison of the present MACS value at $kT=30$ keV with the recommended value in the compilation of Ref. [10], data obtained in previous TOF measurements [24, 25, 26, 27] and calculated from the evaluated cross sections in the ENDF/B-VII.1 [28], JENDL-4.0 [29], and JEFF-3.2 [30] libraries.

Figure 8

Recommended MACS values between $kT=5$ and 100 keV compared with data obtained with evaluated cross sections from ENDF/B-VII.1 [28] and from TOF-based experimental data [23, 25, 26] (see text for details).

Figure 9

Ratio of the MACS values obtained in previous work to this work using the temperature dependence found at CERN n_TOF [26, 27]. Both, ENDF/B-VII.1 [28] and the KADoNiS v0.3 compilation [9, 10] show a different energy dependence.