Lifetime measurements in ${}^{44}\mathrm{Ti}$

Lifetimes of excited nuclear states were determined in ${}^{44}\mathrm{Ti}$ using the recoil distance Doppler-shift technique and the Doppler-shift attenuation method. Results from the $K^{\pi }=3^{-}$ band confirm isospin-symmetry breaking for the $3_1^{-}\to 2_1^{+}E1$ transition. The lifetime of the $4_1^{-}$ state differs considerably from the previously known value. Good agreement is found for the $4_1^{+}$ and $6_1^{+}$ level lifetimes with respect to previous values. The experimental values are compared with large-scale shell-model calculations employing established interactions in the $0f1p$ shell, as well as a modern effective Hamiltonian including multiparticle multihole cross-shell configurations. Extended configuration spaces of this shell-model calculation allow for a detailed comparison with newly determined negative-parity states.

Figure 1

Partial level scheme of ${}^{44}\mathrm{Ti}$. Dominant decay branches observed in both experiments as well as obtained level lifetimes are shown. All energies are given in keV.

Figure 2

Exemplary $\gamma$-ray energy spectra for the (a), (b) $3_1^{-}\to 2_1^{+}$ transition at 2093 keV and the (c), (d) $4_1^{-}\to 3_1^{-}$ transition at 470 keV for two different target-to-stopper distances. Spectra are produced by a gate on the shifted part of the direct feeders at 885 and 1506 keV, respectively. Unshifted (US) and shifted (SH) components as well as a contamination of ${}^{41}\mathrm{Ca}$ to the $4_1^{-}\to 3_1^{-}$ transition are labeled. The Gaussian fits to the experimental spectra are given in a red solid lines, and the background is marked as dashed red (see text for details).

Figure 3

Exemplary $\tau$ curves of the (a) $3_1^{-}$ state and (d) $4_1^{-}$ state at backward and forward angles, respectively. The weighted mean value is marked with a black solid line, the black dashed line indicates the statistical uncertainty. The respective (b), (e) unshifted and (c), (f) shifted intensities are presented with the corresponding polynomial-fit functions (dashed red curves). Lifetimes and $\gamma$-ray intensities are plotted against the offset-corrected relative target-to-stopper distances. Note the logarithmic distance scale.

Figure 4

$\gamma$-ray spectrum of the (a) $4_1^{+}\to 2_1^{+}$ transition with a $\gamma$-energy gate on the purely shifted part of the $6_1^{+}\to 4_1^{+}$ transition at 1541–1553 keV at backward angles. Similar data for the (b) $6_1^{+}\to 4_1^{+}$ transition gated on the $8_1^{+}\to 6_1^{+}$ transition at 2466–2480 keV. The $\gamma$-energy gates onto the direct feeding transitions are shown in the insets. The experimental data are shown as black crosses. The simulated lineshapes are given in open red circles.

Figure 5

(a) Experimental and theoretical excitation energies and (b) $B\left(E2\right)$ values of the ground-state band in ${}^{44}\mathrm{Ti}$. $B\left(E2\right)$ values from this work are given by red squares. Previously adopted experimental transition strengths (black filled triangles) are taken from the corresponding lifetimes given in Refs. [13, 15, 17, 18, 19]. Shell-model calculations employing the GXPF1A (blue open triangles), KB3G (purple open circles), FPD6 (orange open diamonds), and ZBM2M (green filled circles) interactions are shown. $B\left(E2\right)$ values are compared with results from an $\alpha$-cluster model (dark red stars) taken from Ref. [24].