Photoneutron cross sections for samarium isotopes: Toward a unified understanding of $(\gamma ,n)$ and $(n,\gamma )$ reactions in the rare earth region

Photoneutron cross sections were measured for the seven stable samarium isotopes ${}^{144,147,148,149,150,152,154}\mathrm{Sm}$ near the neutron threshold with quasi-monochromatic laser-Compton scattering $\gamma \mathrm{rays}$. Our photoneutron cross sections are found to be low by 20%–37% relative to existing data. The photoneutron data are analyzed with the talys reaction code by considering the Skyrme Hartree-Fock-Bogoliubov (HFB) plus quasiparticle random phase approximation (QRPA) model and the axially symmetric deformed Gogny HFB plus QRPA model of the $E1$ $\gamma \text{−}\mathrm{ray}$ strength. Using the $\gamma \text{−}\mathrm{ray}$ strength function constrained by the present photoneutron data, we made a thorough analysis of the reverse $(n,\gamma )$ cross sections including the radioactive nucleus ${}^{151}\mathrm{Sm}$ with a half-life of 90 yr. The radiative neutron capture cross section for ${}^{153}\mathrm{Sm}$ with the half-life of 1.928 d is deduced with the $\gamma \text{−}\mathrm{ray}$ strength function method.

Figure 1

The chart of nuclei depicting our systematic analysis of $(\gamma ,n)$ and $(n,\gamma )$ cross sections for Sm isotopes in the context of the $\gamma \text{−}\mathrm{ray}$ strength function method. Photoneutron cross sections measured in the present experiment are shown by left-pointing arrows. Radiative neutron capture cross sections discussed in the present systematic analysis are shown by right-pointing arrows. The radiative neutron capture cross section of the radioactive nucleus ${}^{153}\mathrm{Sm}$ is deduced with the $\gamma \text{−}\mathrm{ray}$ strength function method.

Figure 2

The $\gamma \text{−}\mathrm{ray}$ beamline at the NewSUBARU synchrotorn radiation facility.

Figure 3

Energy calibration of the ${\mathrm{LaBr}}_3:\mathrm{Ce}$ detector with ${}^{137}\mathrm{Cs},{}^{60}\mathrm{Co}$, and ${}^{138}\mathrm{La}$ and the maximum energies of LCS $\gamma \text{−}\mathrm{ray}$ beams produced using a $\mathrm{Nd}:{\mathrm{YVO}}_4$ laser and electron beams at energies between 573 and 850 MeV.

Figure 4

A typical spectrum of the $\gamma \text{−}\mathrm{ray}$ beam recorded with the ${\mathrm{LaBr}}_3:\mathrm{Ce}$ detector (solid line) and the simulations of the response function (dotted line) and of the incident $\gamma \text{−}\mathrm{ray}$ beam (gray line). A single collimation with a C2 collimator of 2-mm aperture was used.

Figure 5

Typical spectra of the $\gamma \text{−}\mathrm{ray}$ beams recorded with the ${\mathrm{LaBr}}_3:\mathrm{Ce}$ detector (solid lines) and the simulations of the response function (dotted lines) and of the incident $\gamma \text{−}\mathrm{ray}$ beam (gray lines). A double collimation with a C1 collimator of 6-mm aperture and a C2 collimator of 2-mm aperture was employed.

Figure 6

Experimental pile-up energy spectrum of the LCS $\gamma \text{−}\mathrm{ray}$ beam obtained with a $6^{\prime \prime }\times 5^{\prime \prime }$ NaI(Tl) detector. A single-photon spectrum is also shown by the dashed line. The maximum energy of the LCS $\gamma \text{−}\mathrm{ray}$ beam is 13.03 MeV (electron beam energy of 860.8 MeV). The average number of photons per beam pulse is 1.78.

Figure 7

The monochromatic cross section and the nonmonochromatic cross section of ${}^{148}\mathrm{Sm}$. The arrow-indicated $S_n$ gives the neutron threshold of this nucleus.

Figure 8

Comparison between the present photoneutron emission cross sections and previously measured ones [26] for ${}^{144}\mathrm{Sm}$. Also included are the predictions from Skyrme HFB+QRPA (based on the BSk7 interaction) [27] and axially deformed Gogny HFB+QRPA models (based on the D1M interaction) [28].

Figure 9

Same as Fig. 8 for ${}^{147}\mathrm{Sm}$.

Figure 10

Same as Fig. 8 for ${}^{148}\mathrm{Sm}$.

Figure 11

Same as Fig. 8 for ${}^{149}\mathrm{Sm}$.

Figure 12

Same as Fig. 8 for ${}^{150}\mathrm{Sm}$.

Figure 13

Same as Fig. 8 for ${}^{152}\mathrm{Sm}$. Experimental $\left(\gamma ,n\right)$ data from Ref. [29] are also included.

Figure 14

Same as Fig. 8 for ${}^{154}\mathrm{Sm}$. Experimental photoabsorption data from Ref. [30] are also included.

Figure 15

Comparison between the Sm measured radiative neutron capture cross sections [13, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53] with a talys calculation making use of the D1M+QRPA calculation for the $E1$ strength. The hashed area corresponds to the sensitivity to the NLD.

Figure 16

Prediction of the ${}^{153}\mathrm{Sm}\left(n,\gamma \right){}^{154}\mathrm{Sm}$ cross section. The dotted, dashed, and dashed-dotted curves correspond to the Japanese JENDL-4.0, American ENDF/B-VII.1, and Russian ROSFOND-2010 evaluations [57], respectively.