Electric and magnetic dipole strength in ${}^{54}\mathrm{Fe}$

The dipole strength of the $N=28$ closed-shell nuclide ${}^{54}\mathrm{Fe}$ was studied in photon-scattering experiments using bremsstrahlung produced with electron beams of kinetic energies of 7.5 and 13.9 MeV at the $\gamma \mathrm{ELBE}$ facility as well as using quasimonoenergetic and linearly polarized photon beams of 26 different energies within the range from 5.5 to 11.4 MeV at the $\mathrm{HI}\gamma \mathrm{S}$ facility. About 100 $J=1$ states were newly identified, out of them 19 with $1^{+}$ and 30 with $1^{-}$ assignments. The quasicontinuum of unresolved transitions was included in the analysis of the spectra and the intensities of branching transitions were estimated on the basis of simulations of statistical $\gamma$-ray cascades. As a result, the photoabsorption cross section up to the neutron-separation energy was determined and compared with predictions of the statistical reaction model. The experimental $M1$ strengths from resolved $1^{+}$ states are compared with results of large-scale shell-model calculations.

Figure 1

Part of a spectrum of photons scattered from ${}^{54}\mathrm{Fe}$ combined with ${}^{11}\mathrm{B}$, measured during the irradiation with bremsstrahlung produced by electrons of an energy of $E_e^{\mathrm{kin}}=13.9$ MeV. This spectrum is the sum of the spectra measured with the two detectors placed at ${127}^{\circ }$ relative to the beam.

Figure 2

Absolute photon flux on the target deduced from intensities of known transitions in ${}^{11}\mathrm{B}$ for the measurements at $E_e=7.5$ MeV (squares) and $E_e=13.9$ MeV (circles). The solid lines represent the calculated flux curves described in the text.

Figure 3

Parts of spectra of photons scattered from ${}^{54}\mathrm{Fe}$, measured during the irradiation with a quasimonoenergetic polarized photon beam of 10.6 MeV at $\mathrm{HI}\gamma \mathrm{S}$. The spectrum plotted in red was measured with the vertical detectors and contains $E1$ radiation, whereas the spectrum plotted in blue was measured with the horizontal detectors and contains $M1$ radiation. For comparison, the corresponding section of the spectrum measured with bremsstrahlung of $E_e=13.9$ MeV at $\gamma \mathrm{ELBE}$ (cf. Fig. 1) is shown in black.

Figure 4

Response-corrected spectrum of the two detectors placed at ${127}^{\circ }$ (blue) and simulated spectrum of photons scattered from the target to the detectors by atomic processes (black).

Figure 5

Average branching ratios (solid line) with their uncertainty ranges (dashed lines) resulting from the simulations of statistical $\gamma$-ray cascades up to $S_n$ as described in the text.

Figure 6

Photoabsorption cross sections of ${}^{54}\mathrm{Fe}$ resulting from the present $(\gamma ,{\gamma }^{\prime })$ experiments at $E_e=7.5$ MeV (blue triangles) and $E_e=13.9$ MeV (red circles), from $(\gamma ,n)$ data taken from Ref. [54] (green squares), from calculations using the talys code as given in the TENDL-2019 library (black solid line), and from the TLO with parameters given in the text (black dashed line).

Figure 7

Photoabsorption cross section of ${}^{54}\mathrm{Fe}$ (black line) together with the partial cross sections for the reaction channels $(\gamma ,{\gamma }^{\prime })$ (red line), $(\gamma ,n)$ (green line), and $(\gamma ,p)$ (blue line) taken from the TENDL-2019 library.

Figure 8

Experimental $B(M1,1^{+}\to 0_1^{+})$ values (red circles) and calculated values $B(M1,1^{+}\to 0_1^{+})>0.005{\mu }_N^2$ from the lowest 120 $1^{+}$ states (black squares).

Figure 9

Running sums $\sum B(M1,1^{+}\to 0_1^{+})$ of experimental (red circles) and calculated values (black squares). Also shown are the values obtained in a truncated model space (blue triangles) discussed in the text.

Figure 10

Energies of the lowest 40 states with $J^{\pi }=2^{+}$ calculated in a model space with limited occupation numbers and in the full $fp$ space.

Figure 11

Energies of the lowest 40 states with $J^{\pi }=1^{+}$ up to 10.6 MeV calculated in a model space with limited occupation numbers and the 120 states up to 10.6 MeV in the full $fp$ space.