Measurement of the ${}^{151}\mathrm{Sm}$($n,\gamma$) cross section from 0.6 eV to 1 MeV via the neutron time-of-flight technique at the CERN n_TOF facility

The ${}^{151}\mathrm{Sm}$($n,\gamma )$ cross section was measured with the time-of-flight technique from 0.6 eV up to 1 MeV relative to the Au standard with an overall uncertainty of typically 6%. Neutrons were produced by spallation at the innovative n_TOF facility at CERN; the $\gamma$ rays from capture events were detected with organic ${\text{C}}_6{D}_6$ scintillators. Experimental setup and data analysis procedures are described with emphasis on the corrections for detection efficiency, background subtraction, and neutron flux determination. At low energies, resonances could be resolved up to about 1 keV, yielding a resonance integral of $3575\pm 210$ b, an average s-wave resonance spacing of $\langle D_0\rangle =1.49\pm 0.07$ eV, and a neutron strength function of $\langle S_0\rangle =(3.87\pm 0.33)\times {10}^{-4}$. Maxwellian-averaged capture cross sections are reported for thermal energies between 5 and 100 keV. These results are of relevance for nuclear structure studies, nuclear astrophysics, and nuclear technology. The new value of the Maxwellian-averaged cross section at $\mathit{kT}=30$ keV is $3.08\pm 0.15$ b, considerably larger than previous theoretical estimates, and provides better constraints for the thermodynamic conditions during the occurrence of the slow neutron capture process in low-mass stars during their asymptotic giant branch phase.

Figure 1

Reaction network illustrating the s-process branchings in the Sm-Eu-Gd region. Neutron capture, β decay, and electron capture are indicated by arrows. The s-only isotopes are marked by double boxes. Unstable isotopes are given with their terrestrial half-lives.

Figure 2

Schematic sketch of the experimental setup used in the capture measurement.

Figure 3

Samarium capture yields (a) before and (b) after background subtraction. (c) Background-corrected Au yield. The spectrum of the Ti can in (a) exhibits the Ti resonances at 3, 8, and 13 keV.

Figure 4

(Color online) The capture yield measured with the ${\text{Sm}}_2{O}_3$ sample (red dots) and the sammy fit (black solid line) compared with the capture yield calculated with the resonance parameters of Kirouac and Eiland [6] (blue dashed line). The very small resonances are due to isotopic impurities.

Figure 5

The ${}^{151}\mathrm{Sm}$ capture cross section in the unresolved resonance region compared with the recent measurement at FZK [29] (open squares) and with the JEFF-3.1 [30] evaluated data (dashed line). The corrections for isotopic impurities, self-shielding, and multiple scattering are indicated by full squares.

Figure 6

The cumulative number of levels as a function of neutron energy (top) compared to the previous data [6] (open circles). The levels sequence seems to be almost complete up to 400 eV. The bottom panel shows the cumulative sum of $g{\Gamma }_n^0$ values for deducing the neutron strength function. In both plots the dashed lines represent a best fit to the levels up to a neutron energy of 400 eV.

Figure 7

Histogram representing the cumulated number of levels having value larger than x. The dotted line indicates the fit of the histogram by means of Eq. (4) (see text for details). The levels start to disappear at very low values of x. In this analysis, only the levels up to 400 eV neutron energy are considered.

Figure 8

Level density parameters of the samarium isotopes with and without corrections for shell effects vs the mass of the compound nucleus. The curves are intended to guide the eye.

Figure 9

Variation of $k_{\infty }$ as a function of burnup (1 pcm$\hspace{1em}=10{}^{-5}$) calculated for an EADF [45] by using the JENDL-3.3 [50] and JEFF-3.1 [30] nuclear data libraries (full circles). Significantly smaller variations are achieved by using experimental data for five important isotopes (empty circles; see the text for details). The black line indicates the evolution of the neutron multiplication $k_{\mathrm{src}}$ (right axis) for the same system.

Figure 10

The product of one-group capture cross sections $\langle {\sigma }_{n,\gamma }\rangle$ and fission yields fy, as a function of the atomic mass number for an EADF [45] device. The one-group cross sections are calculated using the neutron flux of the EADF and the capture cross sections of JEFF-3.1 [30]. The uncertainties are estimated by comparison with other data libraries.

Figure 11

The abundance evolution of isotopes in the Sm-Eu-Gd region and of the average neutron density (right axis) in the He intershell during the 15th convective thermal pulse calculated for a star of 1.5 solar masses with half the solar metallicity [60]. All values are normalized to the final ${}^{150}\mathrm{Sm}$ abundance. Arrows indicate the decay of radioactive isotopes and the in-growth of their daughter nuclei in the interpulse phase.