Experimental study of the ${}^{91}\mathrm{Zr}$($n,\gamma$) reaction up to 26 keV

The neutron capture cross sections of the Zr isotopes are relevant to studies in nuclear structure, nuclear astrophysics, and nuclear technology. The valence neutron of ${}^{91}\mathrm{Zr}$ with respect to the neutron magic nucleus ${}^{90}\mathrm{Zr}$ has interesting implications for the statistical analysis in the proximity of shell closures. In stellar nucleosynthesis, the Zr isotopes are important for the $s$-process reaction flow between the Fe seeds and the heavier isotopes. Because of its relatively small ($n,\gamma$) cross sections, Zr represents also an interesting structural material for nuclear reactors. For the same reason, these cross sections are difficult to measure and reliable data are sparse. Therefore, the ($n,\gamma$) cross sections of the Zr isotopes have been remeasured at the CERN n_TOF facility. Thanks to its high instantaneous flux, good energy resolution, and low background, this facility is particularly suited for the determination of small, resonance-dominated cross sections. In this work, results for the ${}^{91}\mathrm{Zr}$($n,\gamma$)${}^{92}\mathrm{Zr}$ reaction are reported in the neutron energy range from thermal to 26 keV. In this region, accurate data of 157 resonances could be obtained, 33 of these resonances are not present in the main databases and/or were observed for the first time.

Figure 1

$s$- and $p$-wave strength functions vs mass number. The position of ${}^{91}\mathrm{Zr}$ is indicated by the vertical line. Data are taken from Ref. [3].

Figure 2

(a) Particle spectrum from ${}^6\mathrm{Li}$($n,\alpha$)${}^3\mathrm{H}$ reactions taken with the neutron flux monitor. (b) Number of recorded events in the ${}^3\mathrm{H}$ peak vs the number of protons hitting the spallation target.

Figure 3

Capture yield (black) and overall background (grey) between 1 eV and 100 keV. Most of the resonances below 150 eV are due to Hf and Sn contaminations in the sample.

Figure 4

Examples for fits with the $R$-matrix code SAMMY and residuals to the fit (in percentage). The resonance at 7.26 keV is not reported in the databases of Refs. [3, 16].

Figure 5

Upper panel: Capture kernels from Ref. [12] compared to the present results. Lower panel: Ratio of the capture kernels from Ref. [12] and the present results versus resonance energy. The average value is indicated by the dotted line.

Figure 6

Upper panel: Capture widths ${\Gamma }_{\gamma }$ from Ref. [3] compared with the present results as a function of resonance energy. Lower panel: Ratio of capture kernels from Ref. [3] and the present results. Average values are indicated by dotted lines.

Figure 7

Comparison of present MACS values (full circles) with those from Ref. [34] (open circles). To avoid the overlap of error bars, values were displaced along the abscissa axis: $+$0.5 keV (this work), −0.5 keV (Ref. [34]).