Stellar $\mathit{s}$-process neutron capture cross section of Ce isotopes

Stellar abundances of cerium are of high current interest based both on observations and theoretical models, especially with regard to the neutron-magic ${}^{140}\mathrm{Ce}$ isotope. A large discrepancy of $s$-process stellar models relative to cerium abundance observed in globular clusters was highlighted, pointing to possible uncertainties in experimental nuclear reaction rates. In this work, the stellar neutron capture cross section of the stable cerium isotopes ${}^{136}\mathrm{Ce}$, ${}^{138}\mathrm{Ce}$, ${}^{140}\mathrm{Ce}$, and ${}^{142}\mathrm{Ce}$, were remeasured. A ${}^{\text{nat}}\mathrm{Ce}$ sample was irradiated with quasi-Maxwellian neutrons at $kT=34.2$ keV using the ${}^7\mathrm{Li}(p,n)$ reaction. The neutron field with an intensity of $3\text{–}5\times {10}^{10}$ n/s was produced by irradiating the liquid-lithium target (LiLiT) with a mA proton beam at an energy (1.92 MeV) just above the threshold at Soreq Applied Research Accelerator Facility (SARAF). The activities of the ${}^{\text{nat}}\mathrm{Ce}$ neutron capture products were measured using a shielded high purity germanium detector. Cross sections were extracted relative to that of the ${}^{197}\mathrm{Au}(n,\gamma )$ reaction and the Maxwellian-averaged cross section (MACS) of the Ce isotopes were derived. The MACS values extracted from this experiment are generally consistent with previous measurements and show for ${}^{140}\mathrm{Ce}$ a value $\approx 15$% smaller than most recent experimental values.

Figure 1

Part of the nuclide chart showing the $s$-process nucleosynthesis path in the Ce-Pr-Nd mass around the branching at $\mathrm{A}=141\text{–}142$.

Figure 2

Cross-section diagram of the liquid-lithium target (LiLiT) and activation target assembly. The proton beam (dashed thick arrow) impinges on the free-surface lithium film; inlet and outlet of the liquid lithium, circulating in a closed loop at $\approx 200{}^{\circ }\mathrm{C}$, are indicated by thick downward arrows (see Ref. [39] for details). The activation target sandwich (Au-Ce-Au) is mounted on a circular target holder and positioned in the outgoing neutron field (dotted lines) at a distance of 6–8 mm from the lithium surface in a vacuum chamber separated from the LiLiT chamber by a 0.5 mm stainless steel wall convex to the beam. The retractable shaft (at left) is used to load and unload rapidly the target assembly.

Figure 3

Time profile of proton beam intensity during irradiation ($E_p=1.924$ MeV). The left vertical axis represents the count rate of ${}^{235}\mathrm{U}$ fission events, produced in the fission detector (see text). The right vertical axis displays the corresponding proton beam intensity, calibrated at low intensity against an electron-suppressed Faraday cup located in front of the LiLiT chamber. The low-intensity groups near $\mathrm{time}=0$ and $\mathrm{time}=6750s$ correspond to the calibration runs of the fission detector. Other gaps are periods of SARAF accelerator instability.

Figure 4

Simulated neutron spectrum $\frac{d{n}_{\mathrm{sim}}}{d{E}_n}$ impinging on the ${}^{\text{nat}}\mathrm{Ce}$ target (histogram) fitted to a Maxwell-Boltzmann flux distribution at 34.2 keV (solid line).

Figure 5

Full-energy peak efficiency of the HPGe detector using standard multi-$\gamma$ sources at 5 cm apart from the end cap.

Figure 6

$\gamma$-ray spectrum obtained after irradiating ${}^{\text{nat}}\mathrm{Ce}$ with Maxwellian neutron flux at SARAF-I above threshold energy. $\gamma$ transition of all the isotopes of ${}^{\text{nat}}\mathrm{Ce}(n,\gamma$) reactions are identified along with the isomeric ${}^{137m}\mathrm{Ce}$ state. The figure is reproduced from Ref. [37].

Figure 7

$\gamma$ -ray spectrum obtained after irradiating ${}^{\text{nat}}\mathrm{Ce}$ with $\gamma$ rays from the ${}^7\mathrm{Li}(\mathrm{p},\gamma$) reaction (under-threshold energy). Only ${}^{139}\mathrm{Ce}$ and ${}^{141}\mathrm{Ce}\gamma$ transitions are observed and used for correction of the $(n,\gamma )$ yields. The figure is reproduced from Ref. [37].

Figure 8

$\gamma$-ray spectrum obtained for the upstream Au(1) monitor (above-threshold irradiation). The ${}^{198}\mathrm{Au}$ transitions (411.8 keV, and 675.9 keV) are labeled.

Figure 9

Decay curves of ${}^{137,139,141,143}\mathrm{Ce}$ and ${}^{137m}\mathrm{Ce}$ (irradiation above neutron threshold). Data are fitted using the half-life adopted in the literature.

Figure 10

Decay curves of ${}^{139,141}\mathrm{Ce}$ (irradiation under neutron threshold). Data are fitted using the half-life adopted in the literature.

Figure 11

Simplified decay scheme of ${}^{137}\mathrm{Ce}$, showing the combined decay of ${}^{137(m+g)}\mathrm{Ce}$.

Figure 12

Comparison of the ${}^{138}\mathrm{Ce}(n,\gamma )$ excitation function evaluated in the different libraries.