Determination of the ${}^{142}\mathrm{Ce}$$(\gamma ,n)$ cross section using quasi-monoenergetic Compton backscattered $\gamma$ rays

Background: Knowing the energy dependence of the $(\gamma ,n)$ cross section is mandatory to predict the abundances of heavy elements using astrophysical models. The data can be applied directly or used to constrain the cross section of the inverse ($n$,$\gamma$) reaction.

Purpose: The measurement of the reaction ${}^{142}\mathrm{Ce}$${(\gamma ,n)}^{141}$Ce just above the reaction threshold amends the existing experimental database in that mass region for $p$-process nucleosynthesis and helps to understand the $s$-process branching at the isotope ${}^{141}\mathrm{Ce}$.

Method: The quasi-monoenergetic photon beam of the High Intensity $\gamma$-ray Source (HI$\gamma$S), TUNL, USA, is used to irradiate naturally composed Ce targets. The reaction yield is determined afterwards with high-resolution $\gamma$-ray spectroscopy.

Results: The experimental data are in agreement with previous measurements at higher energies. Since the cross-section prediction of the ${}^{142}\mathrm{Ce}$$(\gamma ,n)$ reaction is exclusively sensitive to the $\gamma$-ray strength function, the resulting cross-section values were compared to Hauser-Feshbach calculations using different $\gamma$-ray strength functions. A microscopic description within the framework of the Hartree-Fock-BCS model describes the experimental values well within the measured energy range.

Conclusions: The measured data show that the predicted $(\gamma ,n)$ reaction rate is correct within a factor of 2 even though the closed neutron shell $N=82$ is approached. This agreement allows us to constrain the ($n$,$\gamma$) cross section and to improve the understanding of the $s$-process branching at ${}^{141}\mathrm{Ce}$.

Figure 1

(a) Photon flux measured at $0^{\circ }$ with respect to the beam axis. (b) Photon flux $N_{\gamma }$ obtained from deconvolution of the spectrum (histogram). The dashed curve is a function fitted to the histogram as described in the text.

Figure 2

Experimentally determined cross-section results for the reaction ${}^{142}\mathrm{Ce}$$(\gamma ,n)$ are depicted as a function of photon energy.

Figure 3

The experimentally determined cross-section data of the reaction ${}^{142}\mathrm{Ce}$$(\gamma ,n)$ are compared with statistical-model calculations using the talys code [41, 43]. The calculations were performed using the default settings, but the $\gamma$-ray strength function was varied. In total, five $\gamma$-ray strength functions were used: standard Lorentzian [44], microscopic Hartree-Fock BCS [45], microscopic Hartree-Fock-Bogoliubov [46], microscopic hybrid model [47], and generalized Lorentzian [48].