Investigation of $J=1$ states and their $\gamma$-decay behavior in ${}^{52}\mathrm{Cr}$

Background: In the $A\approx 50$ mass region $M1$ spin-flip transitions are prominent around 9 MeV. An accumulation of $1^{-}$ states between 5 and 8 MeV generating additional $E1$ strength, also denoted as pygmy dipole resonance, has been established in many nuclei with neutron excess within the last decade.

Purpose: The $\gamma$-decay behavior of $J=1$ states has been investigated in an NRF experiment. $M1$ excitations have been compared to shell model calculations.

Methods: $J=1$ states were excited by quasi-monoenergetic, linearly polarized $\gamma$-ray beams generated by laser-Compton backscattering at the $\mathrm{HI}\gamma \mathrm{S}$ facility, Durham, NC, USA. Depopulating $\gamma$ rays were detected with the multidetector array ${\gamma }^3$.

Results: For eleven beam-energy settings the $\gamma$-decay behavior of dipole states was analyzed by a state-to-state analysis and average $\gamma$-decay branching ratios have been investigated. 34 parity quantum numbers were assigned to $J=1$ states.

Conclusions: Six $1^{-}$ states and two $1^{+}$ states have been investigated in NRF experiments for the first time. The $M1$ strength distribution is in good agreement with shell-model calculations.

Figure 1

The sum spectrum of all HPGe detectors for ${}^{52}\mathrm{Cr}$ is depicted for a beam energy of 9.2 MeV. The $\gamma$-ray beam profile (dashed) and resolved peaks of ${}^{52}\mathrm{Cr}$ with known cross sections to determine the absolute photon-flux distribution are illustrated. Additional $J=1$ states that have not been observed in previous bremsstrahlung experiments are shown (*). The state at $E_x=9009.4\mathrm{keV}$ was observed for the first time in NRF experiments.

Figure 2

Experimental asymmetry values and upper limits obtained for $1^{-}$ states (squares) and lower limits for $1^{+}$ states (circles). Experimental sensitivities $q$ are indicated by horizontally dashed lines ($+0.82$ and $-0.88$).

Figure 3

HPGe spectra at 7.1 MeV $\gamma$-ray beam energy are shown for detectors positioned horizontal (a), vertical (b), and under backward angles ($\varphi =\pm 45$ and $\theta =135$) (c) with respect to the polarization plane. Transitions in ${}^{52}\mathrm{Cr}$ at 6922.6 keV, 7013.4 keV, and 7091.1 keV are clearly of $E1$ nature.

Figure 4

The low-energy part of the HPGe sum spectrum is shown at a $\gamma$-ray beam energy of 9.2 MeV. All events detected in the HPGe detectors are shown in red. The black spectrum shows only beam-pulse (bp) correlated events, the suppression of uncorrelated background is obvious. The depopulation of the first excited states ($2_1^{+},2_2^{+},2_3^{+}$) are clearly visible.

Figure 5

The $B\left(M1\right)\uparrow$-strength distribution for shell-model calculations using the interactions KB3G (green), GPFX1A (blue), and GPFX1C (red) and experiments from this work (5.5 MeV to 9.5 MeV, solid black) and from Shizuma et al. (above 9.5 MeV, dashed black) are shown.

Figure 6

Summed $M1$ strength is shown for the data of this work (5.5 MeV to 9.5 MeV) together with the data from Shizuma et al. (9.5 MeV to 12 MeV) and the shell-model calculations using the KB3G interaction. The experimental data are indicated in red and the shell-model calculations in gray. Both are shown in bins of $0.1{\mu }_N^2$.