Examination of the low-energy enhancement of the $\gamma$-ray strength function of ${}^{56}\mathrm{Fe}$

A model-independent technique was used to determine the $\gamma$-ray strength function ($\gamma \mathrm{SF}$) of ${}^{56}\mathrm{Fe}$ down to $\gamma$-ray energies less than 1 MeV for the first time with GRETINA using the $(p,p^{\prime })$ reaction at 16 MeV. No difference was observed in the energy dependence of the $\gamma \mathrm{SF}$ built on $2^{+}$ and $4^{+}$ final states, supporting the Brink hypothesis. In addition, angular distribution and polarization measurements were performed. The angular distributions are consistent with dipole radiation. The polarization results show a small bias towards magnetic character in the region of the enhancement.

Figure 1

The ratio $R=f\left(E_i\right)/f\left(E_j\right)$ as a function of excitation energy $E_x$, for several pairs of $2^{+}$ and $4^{+}$ states in ${}^{56}\mathrm{Fe}$. The red-dashed line denotes $R=1$, while the solid red-curve shows the expected ratios from the polynomial fit in Fig. 2. The final states used to construct the ratio are denoted by the fraction in the upper left or right with energies in keV.

Figure 2

$\gamma$-ray strength function for ${}^{56}\mathrm{Fe}$ from the present work (colored squares) in comparison to previous measurements, Refs. [21] (black circles) and [33] (green triangles). The polynomial fit to Voinov et al. [21] is shown by the red-solid curve.

Figure 3

(a) Angular distribution for the 847 keV $2^{+}$ state in ${}^{56}\mathrm{Fe}$. (b) The relative angular distribution for quasicontinuum $\gamma$ rays between 1.0 and 6.0 MeV. The dashed line shows $1/W{\left(\theta \right)}_{847}$. The bottom panels show the polarization asymmetry $A$ for an electric (c) and magnetic (d) transition in ${}^{56}\mathrm{Fe}$ and ${}^{55}\mathrm{Fe}$, respectively, with fits in solid red.

Figure 4

Polarization asymmetry ${\mathcal{A}}_0$ as a function of primary $\gamma$-ray energy. In blue (circles) are the extracted asymmetries from the best-fits in Fig. 3 for electric and magnetic transitions in ${}^{55}\mathrm{Fe}$ and ${}^{56}\mathrm{Fe}$. The red-solid curves denote the expected asymmetry given a linear polarization of $P=0.30$, and the red band (dashed) shows the range of asymmetries given the uncertainty in $P$. The grey band denotes the statistical uncertainty of measuring a uniform distribution.