Magnetic dipole strength in ${}^{128}\mathrm{Xe}$ and ${}^{134}\mathrm{Xe}$ in the spin-flip resonance region

The magnetic dipole strength in the energy region of the spin-flip resonance is investigated in ${}^{128}\mathrm{Xe}$ and ${}^{134}\mathrm{Xe}$ using quasimonoenergetic and linearly polarized $\gamma$-ray beams at the High-Intensity $\gamma$-Ray Source facility in Durham, North Carolina, USA. Absorption cross sections were deduced for the magnetic and electric and dipole strength distributions separately for various intervals of excitation energy, including the strength of states in the unresolved quasicontinuum. The magnetic dipole strength distributions show structures resembling a resonance in the spin-flip region around an excitation energy of 8 MeV. The electric dipole strength distributions obtained from the present experiments are in agreement with the ones deduced from an earlier experiment using broad-band bremsstrahlung instead of a quasimonoenergetic beam. The experimental magnetic and electric dipole strength distributions are compared with phenomenological approximations and with predictions of a quasiparticle random phase approximation in a deformed basis.

Figure 1

Spectrum of incident photon energies for the beam setting for $E_{\gamma }=8.7$ MeV. The solid black curve shows the beam distributions corrected for detector response and attenuator effects, while the dashed curve is the uncorrected spectrum. The lower solid (red) curve is the calculated beam distribution in relativistic Compton backscattering following Ref. [28]. The horizontal (orange) bar indicates the region of analysis for these beam parameters.

Figure 2

Spectra measured using $E_{\gamma }=7.5$ MeV photons for the two Xe isotopes [lighter (red) lines and black lines, respectively). Prominent $M1$ transitions from the steel container are labeled with the corresponding isotopes. The horizontal (orange) bar indicates the analysis region chosen for this beam energy. The shaded (gray) area in the bottom panel shows the incident beam profile as described in the text. Vertical detectors are sensitive to $E1$ transitions. The combination of horizontal and backward detector allows identification of the $M1$ strength. Spectra measured in the backward and horizontal detectors are corrected for their efficiency relative to the pair of vertical detectors.

Figure 3

Transitions in ${}^{128}\mathrm{Xe}$ used to estimate the branching ratios of inelastic transitions. The dashed arrow stands for the bunch of ground-state transitions. Solid arrows mark transitions depopulating known low-lying states assumed to collect most of the inelastic transitions from higher-lying levels. Dashed gray lines indicate known states not decaying directly to the ground state and therefore not considered in the analysis.

Figure 4

Branching ratios deduced from the present experiments [(red) diamonds] and results of $\gamma$-ray cascade simulations using the code $\gamma$dex (solid black curves) and their uncertainty bands (dashed black curves).

Figure 5

Photoabsorption cross sections for ${}^{128}\mathrm{Xe}$ deduced from the present experiments at $\mathrm{HI}\gamma \mathrm{S}$ for the $E1$ part [(red) diamonds] and the $M1$ part (black diamonds). For comparison, data from our earlier experiment at $\gamma \mathrm{ELBE}$ [10] including both $E1$ and $M1$ contributions [open (green) circles], in 0.3-MeV energy bins, are shown. In addition, predictions of phenomenological expressions are given. RIPL3 [6] recommendations for the $E1$ and $M1$ parts are plotted as lighter (red) and black solid curves, respectively. The TLO model [9] for $E1$ and the triple-Gaussian model [36] for $M1$ are shown as lighter (red) and black dashed curves, respectively. Lighter (red) and black $X\mathrm{s}$ represent the result of QRPA calculations for the $E1$ and $M1$ strength, respectively.

Figure 6

Photoabsorption cross sections for ${}^{134}\mathrm{Xe}$. For the definition of the symbols and curves see the caption to Fig. 5.