Radiative-capture cross sections for the ${}^{139}\mathrm{La}(n,\gamma )$ reaction using thermal neutrons and structural properties of ${}^{140}\mathrm{La}$

A set of prompt partial $\gamma$-ray production cross sections from thermal neutron capture were measured for the ${}^{139}\mathrm{La}(n,\gamma )$ reaction using a guided beam of subthermal (thermal and cold) neutrons incident on a ${}^{\mathrm{nat}}\mathrm{La}_2{\mathrm{O}}_3$ target at the Prompt Gamma Activation Analysis facility of the Budapest Research Reactor. Absolute ${}^{140}\mathrm{La}$ cross sections were determined relative to the well-known comparator ${}^{35}\mathrm{Cl}(n,\gamma )$ cross sections from the irradiation of a stoichiometric ${}^{\mathrm{nat}}\mathrm{LaCl}_3$ sample. The total cross section for radiative thermal neutron capture on ${}^{139}\mathrm{La}$ from the sum of experimentally measured cross sections observed to directly feed the ${}^{140}\mathrm{La}$ ground state was determined to be ${\sigma }_0^{\mathrm{expt}}=8.58\left(50\right)$ b. To assess completeness of the decay scheme and as a consistency check, the measured cross sections for transitions feeding the ground state from levels below a critical energy of $E_c=285$ keV were combined with a modeled contribution accounting for ground-state feeding from the quasicontinuum to arrive at a total cross section of ${\sigma }_0=9.36\left(74\right)$ b. In addition, a neutron-separation energy of $S_n=5161.001\left(21\right)$ keV was determined from a least-squares fit of the measured primary $\gamma$-ray energies to the low-lying levels of the ${}^{140}\mathrm{La}$ decay scheme. Furthermore, several nuclear structure improvements are proposed for the decay scheme. The measured cross-section and separation-energy results are comparable to earlier measurements of these quantities.

Figure 1

Prompt $\gamma$-ray energy spectra observed following the ${}^{139}\mathrm{La}(n,\gamma )$ reaction with the ${\mathrm{La}}_2{\mathrm{O}}_3$ sample. The upper panel (a) reveals the complete energy range of interest in the capture-$\gamma$ product ${}^{140}\mathrm{La}$. A strong background line at around 2223 keV from ${}^1\mathrm{H}(n,\gamma )$ [22] is clearly visible, as are a few contaminant lines from ${}^{157}\mathrm{Gd}(n,\gamma )$. In the lower panel (b), the same spectrum is expanded around $E_{\gamma }=3000–5350$ keV to highlight many of the intense primary $\gamma$ rays observed in this measurement. Prominent background lines (B) and single-escape peaks (SE) are labeled as is the position of the neutron-separation energy ($S_n$) for ${}^{140}\mathrm{La}$.

Figure 2

Normalization $\gamma$ rays used to standardize the (a) ${\mathrm{LaCl}}_3$ and (b) ${\mathrm{La}}_2{\mathrm{O}}_3$ spectra. The expectation value of the ratio ${\sigma }_{\gamma }/a_{\gamma }$ is indicated by the shaded 1-$\sigma$ range on each plot. The standardized cross sections used in these plots are listed in Table 1 and were deduced from the ${\mathrm{LaCl}}_3$ sample.

Figure 3

Measured isotopic abundances of ${}^{149}\mathrm{Sm}$ and ${}^{155,157}\mathrm{Gd}$ contaminants in the ${\mathrm{La}}_2{\mathrm{O}}_3$ sample. The dark blue (dark gray) and hatched light green (light gray) shaded boundaries correspond to the 1-$\sigma$ uncertainty range on the measured abundances for ${}^{149}\mathrm{Sm}$ and ${}^{157}\mathrm{Gd}$, respectively.

Figure 4

Ratios of standardized partial $\gamma$-ray production cross sections for clean prompt ${}^{139}\mathrm{La}(n,\gamma )$ transitions measured in the thick ${\mathrm{LaCl}}_3$ sample (T) relative to the same transitions in the thin reference ${\mathrm{LaCl}}_3$ sample (S). The ratios were measured at the following $\gamma$-ray energies: 54.9, 63.2, 155.6, 162.6, 209.2, 272.4, 422.7, and 567.4 keV.

Figure 5

Upper panels: plot of the global ${\chi }^2$ values as a function of the target thickness for (a):(i) ${\mathrm{LaCl}}_3·7{\mathrm{H}}_2\mathrm{O}$ and (b):(i) ${\mathrm{La}}_2{\mathrm{O}}_3$. The dashed line drawn at ${\chi }_{\min }^2+2.3$ defines the $1\sigma$ range of acceptable $t$ values for two adjustable parameters [36]. Lower panels: plot of the calculated attenuation factor at the deduced effective sample thicknesses of (a):(ii) $\langle t\rangle =0.842{(}_{-124}^{+125})$ mm for ${\mathrm{LaCl}}_3·7{\mathrm{H}}_2\mathrm{O}$ and (b):(ii) $\langle t\rangle =0.078{(}_{-22}^{+24})$ mm for ${\mathrm{La}}_2{\mathrm{O}}_3$. The experimentally measured attenuation factors [Eq. (3)] for a subset of strong well-resolved $\gamma$ rays are indicated on each plot. The red square data points indicate the $\gamma$-ray standards used to constrain the fits: 54.9-keV (${\mathrm{LaCl}}_3·7{\mathrm{H}}_2\mathrm{O}$ only); 63.2-keV (${\mathrm{La}}_2{\mathrm{O}}_3$ only); 155.6-keV; 162.6-keV; and 272.4-keV transitions. Also shown for comparison are the 63.2- (${\mathrm{LaCl}}_3·7{\mathrm{H}}_2\mathrm{O}$ only), 209.2-, 235.8-, 288.3-, 422.7-, and 567.4-keV transitions (blue circle data points) that were not used to constrain the fitting procedure.

Figure 6

Experimental data overlaid with BA [51, 52] (green dot-dashed line), GLO [56] (blue medium-dashed line), and KMF [55] (black long-dashed line) models describing the $E1$ PSF. The photonuclear ${}^{139}\mathrm{La}(\gamma ,xn)$ data (filled black circles) are taken from Ref. [53], the ${}^{139}\mathrm{La}({}^3\mathrm{He},\stackrel{\prime }{{}^3\mathrm{He}}$) data (filled red squares) and the ${}^{139}\mathrm{La}({}^3\mathrm{He},\alpha$) data (open green circles) are both taken from Ref. [9], and the ${}^{139}\mathrm{La}(d,p)$ data (filled blue triangles) are taken from Ref. [10]. The small-dashed purple curve labeled as Kheswa corresponds to the PSF described in Ref. [9]. The solid orange curve represents a Lorentzian fit to the giant dipole resonance observed over 12–17 MeV used to deduce the adopted parametrizations of the PSF.

Figure 7

Upper panels (a):(i) and (b):(i): The simulated population (per neutron capture) of levels below the critical energy of 285 keV vs their experimental depopulation (per neutron capture) assuming using the GLO [56] model for the PSF and the CTF [43, 44] model for the LD using the parametrization of Ref. [47]. The $J^{\pi }$ distribution of the levels is indicated on the plot and a dashed black line of unit slope is drawn for guidance. Lower panels (a):(ii) and (b):(ii): Residuals ($R$) between simulations and experimental data as a function of the excitation energy. The plots correspond to (a) $J^{\pi }=5^{-}$ and (b) $J^{\pi }=6^{-}$ assignments for the 103.8-keV level.

Figure 8

Partial $\gamma$-ray energy spectrum illustrating the fit for the 3424.8-keV primary $\gamma$ ray (dashed green curve). Representative fits to neighboring $\gamma$ rays (solid red curves) are presented for comparison.

Figure 9

Plot of the total $\gamma$-ray intensity balance $[\mathrm{\Delta }{I}_{\gamma }=\sum I_{\gamma }\left(\mathrm{in}\right)-\sum {\mathrm{I}}_{\gamma }\left(\mathrm{out}\right)]$ for all levels observed in ${}^{140}\mathrm{La}$. A dashed black line at $\mathrm{\Delta }{I}_{\gamma }=0$ is drawn for guidance.

Figure 10

Variation in the total radiative thermal neutron-capture cross section ${\sigma }_0$ and the sum of the experimental cross sections ${\sum }_i{\sigma }_{{\gamma }_{i0}}^{\mathrm{expt}}(1+{\alpha }_{i0})$ feeding the ground state as a function of cutoff energy $E_c$. This plot was obtained with the BSFG-GLO models using the LD parametrization of Ref. [47] and is representative for PSF-LD combinations adopted in this work.

Figure 11

Absolute residuals $\left|R\right|$, in units of standard deviations $\sigma$, between simulated population and experimental depopulation to all levels below $E_c$ for the PSF-LD model combinations adopted in this work (Sec. 4f). The LD parametrizations for the combinations denoted “VE(05)” and “VE(09)” are taken from Refs. [46] and [47], respectively. The average residual $\langle R\rangle$ is listed alongside each model combination.