Lifetime measurements and evidence for triaxial nuclear shapes in ${}^{127}\mathrm{Cs}$

Rotational bands built on single quasiproton excitations in ${}^{127}\mathrm{Cs}$ have been investigated using the ${}^{121}\mathrm{Sb}({}^{12}\mathrm{C},\alpha 2n$) reaction and the ROSPHERE detector array at IFIN-HH, Bucharest. The lifetimes in the yrast negative-parity band based on the proton in $h_{11/2}$ orbital have been measured by applying the Doppler-shift attenuation method. The lifetime of the ${7/2}^{+}$ 273-keV state, member of the rotational band associated with the proton $g_{7/2}$ orbital, has been determined as $\tau$ = 1.22(5) ns by the fast timing technique. The experimental data were compared with the calculations performed using the quasiparticle plus triaxial rotor model and deformation parameters ${\epsilon }_2$ and $\gamma$ have been derived for the positive- and negative-parity structures. Evidence for triaxial nuclear shapes in the states described by the proton $h_{11/2}$ orbital was provided by both signature splitting and $B(E2,\mathrm{\Delta }J=2)$ transition strengths.

Figure 1

Partial level scheme of ${}^{127}\mathrm{Cs}$ investigated in the present work. The transition energies are given in keV and the widths of the arrows indicate their relative intensities.

Figure 2

Coincidence spectra from the symmetric $\gamma \text{−}\gamma$ matrix showing transitions in the $g_{9/2}$ band (a), $g_{7/2}$ band (b), and $h_{11/2}$ band (c). Transitions denoted by a solid circle belong to ${}^{127}\mathrm{Cs}$ (band 1 of Ref. [23]) and those denoted by a star belong to ${}^{130}\mathrm{La}$ (Ref. [35]).

Figure 3

Experimental and calculated lineshapes for transitions de-exciting the states in the favored branch of the negative-parity yrast band in ${}^{127}\mathrm{Cs}$, with $J^{\pi }$ = ${23/2}^{-}$(a), ${27/2}^{-}$(b), and ${31/2}^{-}$(c), respectively. The fitted DSAM spectra are shown in red dashed lines.

Figure 4

Experimental and calculated lineshapes for transitions de-exciting the states in the unfavored branch of the negative-parity yrast band in ${}^{127}\mathrm{Cs}$, with $J^{\pi }$ = ${21/2}^{-}$(a), ${25/2}^{-}$(b), and ${29/2}^{-}$(c), respectively. The fitted DSAM spectra are shown in red dashed lines.

Figure 5

(a) ${\mathrm{LaBr}}_3$(Ce) energy spectrum obtained by gating on the 616-keV $\gamma$ ray in HPGe detectors and on the 434-keV $\gamma$ ray in ${\mathrm{LaBr}}_3$(Ce) detectors. Peaks belonging to the $g_{7/2}$ favored signature band are marked by energy. (b) Experimental background-subtracted time spectrum for the 206.7-keV and the exponential fit. The gates used for the peak and background time spectra are illustrated in panel (a) with red and blue dotted lines, respectively.

Figure 6

Experimental signature splitting $S\left(J\right)$ for the band based on the $g_{7/2}$ proton orbital (empty black circle) compared with the theoretical values calculated for a quadrupole deformation ${\epsilon }_2$ = 0.18 and $\gamma$ = $0^{\text{o}}$ (red full circle) and $\gamma$ = ${22}^{\text{o}}$ (blue full square).

Figure 7

Experimental transition probabilities $B\left(E2\right)(5/2^{+}\to 1/2^{+})$ (full black circle) and $B\left(E2\right)(7/2^{+}\to 3/2^{+})$ (full black triangle) compared with QTR model calculations for the proton $d_{5/2}$ band (red empty circle) and $g_{7/2}$ band (red empty triangle)

Figure 8

Experimental signature splitting $S\left(J\right)$ for the band based on the proton $h_{11/2}$ orbital (empty black circle) compared with the theoretical values calculated for a quadrupole deformation ${\epsilon }_2$ = 0.11 and $\gamma$ = $0^{\text{o}}$ (red full circle), $\gamma$ = ${22}^{\text{o}}$ (blue full square), and $\gamma$ = ${28}^{\text{o}}$ (magenta full triangle).

Figure 9

Experimental transition probabilities for the yrast band based on the proton $h_{11/2}$ orbital (full black circle) compared with theoretical values calculated for a quadrupole deformation ${\epsilon }_2$ = 0.11 and $\gamma$ = $0^{\text{o}}$ (red open circle) and $\gamma$ = ${22}^{\text{o}}$ (blue full square).