${}^{49}\mathrm{Cr}$: Towards full spectroscopy up to 4 MeV

The nucleus ${}^{49}\mathrm{Cr}$ has been studied by analyzing $\gamma \text{−}\gamma$ coincidences in the reaction ${}^{46}\mathrm{Ti}$($\alpha ,n$)${}^{49}\mathrm{Cr}$ at the bombarding energy of 12 MeV. The level scheme has been greatly extended at low excitation energy, and several lifetimes have been determined by means of the Doppler shift attenuation method. Shell model calculations in the full pf configuration space reproduce well the negative parity levels. Satisfactory agreement is obtained for positive parity levels by extending the configuration space to include a nucleon-hole in either the $1d_{3/2}$ or $2s_{1/2}$ orbitals. A nearly one-to-one correspondence is found between experimental and theoretical levels up to an excitation energy of 4 MeV. Experimental data and shell model calculations are interpreted in terms of the Nilsson diagram and the particle-rotor model, showing the strongly coupled nature of the bands in this prolate nucleus. Nine values of $K^{\pi }$ are proposed for the levels observed in this experiment. As a secondary result, it is shown that the values of the experimental magnetic moments in $1f_{7/2}$ nuclei are well reproduced without quenching the nucleon g factors.

Figure 1

Spectra obtained by gating on the 272 keV transition in the forward and backward matrices.

Figure 2

Experimental ${}^{49}\mathrm{Cr}$ level scheme. All experimental levels are reported up to about 4 MeV, while only levels observed in this work are reported at higher energies. Levels previously known are represented with thicker lines. Only observed transitions are reported apart for the ones of 2613, 2979, 3251, and 2504 keV, taken from Ref. [2]. The suggested assignments of the levels $9/2^{-}$, $11/2^{-}$, and $13/2^{-}$ to the bands $K^{\pi }=1/2^{-}$ and $7/2^{-}$ may be interchanged.

Figure 3

Examples of DSAM lineshape analysis along the $K^{\pi }=3/2^{+}$ band.

Figure 4

First two panels show the lineshape analyses of transitions from the $15/2^{-}$ and $17/2^{-}$ levels of the $K^{\pi }\hspace{1em}=\hspace{1em}13/2^{-}$ band, respectively. The last panel refers to a transition from the ($9/2^{+}$) level attributed to the $K^{\pi }\hspace{1em}=\hspace{1em}$($7/2^{+}$) band.

Figure 5

Particle-rotor predictions in ${}^{49}\mathrm{Cr}$ assuming axial symmetry and $\beta =0.26$.

Figure 6

Comparison of experimental negative parity levels with SM predictions.

Figure 7

Comparison of positive parity levels with SM predictions.

Figure 8

Comparison of positive parity levels with SM predictions in ${}^{45}\mathrm{Ti}$.