${}^{145}\mathrm{Ba}$ ${\beta }^{-}$ decay: Excited states and half-lives in neutron-rich ${}^{145}\mathrm{La}$

Background: Neutron-rich nuclei in the $\mathrm{A}\approx 140–160$ mass region provide valuable information on nuclear structure such as quadrupole- and octupole-shape coexistence and the evolution of the collectivity. These nuclei have also a nuclear engineering interest because they contribute to the total decay heat after a fission burst. The information concerning ${}^{145}\mathrm{La}$ is very limited.

Purpose: The study of low-spin states in ${}^{145}\mathrm{La}$ will provide a more detailed level scheme and enable the determination of the half-lives of the excited states.

Methods: Low-spin excited states in ${}^{145}\mathrm{La}$ have been investigated from the ${}^{145}\mathrm{Ba}{\beta }^{-}$ decay. The ${}^{145}\mathrm{Ba}$ nuclei were directly produced by photofission in the ALTO facility or obtained from the ${\beta }^{-}$ decay of ${}^{145}\mathrm{Cs}$ also produced by photofission. Gamma spectroscopy and fast-timing techniques were used.

Results: A new level scheme was proposed including 67 excited levels up to about 3 MeV and 164 transitions. Half-lives in the few-nanosecond range were measured for the first excited states. Configurations for levels up to $\approx 600$ keV were discussed.

Conclusions: The available information on the low-spin states of ${}^{145}\mathrm{La}$ has been modified and considerably extended. The analysis of the properties of the first excited states, such as excitation energies, decay modes, log $\mathit{\text{ft}}$ values, reduced transition probabilities, and Weisskopf hindrance factors, has enabled the identification of the first members of the bands corresponding to the $g_{7/2},$ $d_{5/2},$ and $h_{11/2}$ proton configurations.

Figure 1

Detector array at the BEDO measurement station. The radioactive ions are coming from the left, the $\beta$ detector is placed behind the implanted source and the other detectors are mounted around the source in the plane perpendicular to the beam direction.

Figure 2

Yields of Cs and Ba isotopes released by a ${\mathrm{UC}}_x$ target heated to ${1950}^{\circ }\mathrm{C}$ and bombarded by a 50 MeV $10\mu \mathrm{A}$ electron beam.

Figure 3

Ge(HP) $\gamma$-ray projection of the $\beta \text{−}\gamma$ coincidence matrix. The main transitions are labeled.

Figure 4

Decay curves for: (a) selected transitions belonging to nuclei with $A=145$, where the adopted half-lives of the corresponding parent nuclei [14] are indicated, (b) selected transitions assigned to ${}^{145}\mathrm{La}$.

Figure 5

Level scheme for ${}^{145}\mathrm{La}$ (lower part). Previously reported transitions, levels, spins, and parities are drawn in red. The $Q$ value and $T_{1/2}$ are taken from Refs. [23] and [14], respectively.

Figure 6

Level scheme for ${}^{145}\mathrm{La}$ (upper part). Previously reported transitions, levels, spins, and parities are drawn in red. The $Q$ value and $T_{1/2}$ are taken from Refs. [23] and [14], respectively.

Figure 7

Coincidence spectra in the 0–800 keV energy range. Only transitions belonging to the ${}^{145}\mathrm{Ba}\to {}^{145}\mathrm{La}$ decay are indicated.

Figure 8

Coincidence spectra in the 800–1600 keV energy range. Only transitions belonging to the ${}^{145}\mathrm{Ba}\to {}^{145}\mathrm{La}$ decay are indicated.

Figure 9

Coincidence spectra in the 1600–2600 keV energy range. Only transitions belonging to the ${}^{145}\mathrm{Ba}\to {}^{145}\mathrm{La}$ decay are indicated.

Figure 10

Coincidence spectra gated on the (a) 352 keV and (b) 493 keV doublets. In panel (a) peaks in coincidence with the 351.8 (351.9) keV line are indicated by a full (open) circle. In panel (b) peaks in coincidence with the 492.6 (492.9) keV line are indicated by a full (open) square. Peaks marked with open triangles are in coincidence with the 492.2 keV line corresponding to the ${}^{145}\mathrm{Ce}\to {}^{145}\mathrm{Pr}$ decay.

Figure 11

Time spectra obtained with a ${}^{60}\mathrm{Co}$ calibration source (upper part) and the 1898.6 keV level of ${}^{138}\mathrm{Ba}$ (lower part).

Figure 12

Spectra corresponding to the determination of the half-life of the 65.9 keV level: (a) Two-dimensional energy projection of the $E_A{E}_B{T}_{AB}$ cube, (b) time spectra obtained by gating on the regions indicated in panel (a), (c) time spectra with the G gating conditions, and (d) time spectra from $\beta \text{−}\gamma$ coincidences.

Figure 13

Spectra corresponding to the determination of the half-life of the 97.1 keV level: (a) Two-dimensional energy projection of the $E_A{E}_B{T}_{AB}$ cube showing the 92.4–97.1 keV coincidence, (b) time spectra obtained setting the gate marked in panel (a), (c) two-dimensional energy projection of the $E_A{E}_B{T}_{AB}$ cube showing the 379.0–97.1 keV and the 417.9–97.1 keV coincidences, and (d) sum of time spectra obtained setting the gates indicated in panel (c).

Figure 14

(a) $E_A$ projection of the $E_A{E}_B{T}_{AB}$ cube gated on the 92.4 keV line in the Ge(HP) detectors. (b) Time spectrum corresponding to the 97.1 keV level obtained from the G 92.4 keV-gated $E_A{E}_B{T}_{AB}$ cube. See text for the contribution of the 189.5 keV level. (c) Same as (a) but with a gate on the 97.1 keV line. (d) Time spectrum corresponding to the 189.5 keV level obtained from the G 97.1 keV-gated $E_A{E}_B{T}_{AB}$ cube.

Figure 15

Time spectra corresponding to: (a) 97.1 keV level, (b) 476.1 keV, and (c) 514.9 keV, obtained setting gates on $\beta$ (start) and $A$ (stop).

Figure 16

Prompt centroid of $T_{\beta A}$ as a function of $\gamma$-ray energy measured in $A$ obtained from the 409, 547, and 1010 keV lines of ${}^{138}\mathrm{Ba}$ (open circles) and the linear fit through these data points (dashed line). The centroids corresponding to the 379.0 and 417.9 keV lines of ${}^{145}\mathrm{La}$ are plotted with full circles.

Figure 17

Hindrance factors with respect to the Weisskopf estimate for the $d_{5/2}\to g_{7/2}$ transition in odd ${}^{133-145}\mathrm{La}$ nuclei. The ${\mathrm{F}}_w({}^{133}\mathrm{La}$) value corresponds to the upper limit.

Figure 18

Experimental and calculated negative parity levels.