Parity assignments in ${}^{140}\mathrm{Ce}$ up to 7 MeV using Compton polarimetry

Parity quantum numbers of $J=1$ states up to 7 MeV in the region of the Pygmy Dipole Resonance in ${}^{140}\mathrm{Ce}$ were determined model independently by combining the methods of Nuclear Resonance Fluorescence and Compton polarimetry. For the first time the well-established method of Compton polarimetry was applied at such high energies. The experiment was performed using a fourfold segmented HPGe clover detector for the detection of the scattered photons. For all investigated dipole transitions asymmetries are found which correspond to negative parity of the excited states.

Figure 1

High-Intensity Photon Setup at the superconducting linear accelerator for electrons S-DALINAC. The electron beam coming from the left produces bremsstrahlung photons in a copper radiator.

Figure 2

Principles of Compton polarimetry. The primary photon beam $\gamma$ is resonantly scattered at the target and shows a linear polarization depending on the parity of the excited state. The linear polarization of the scattered photons ${\gamma }^{\text{'}}$ is determined with a system of scatterer and analyzers. On the right-hand side a schematic front view of the Clover detector is shown. The arrows symbolize the coincidences between neighboring segments which define the Compton scattering plane.

Figure 3

Intrinsic asymmetry of the detection system. Asymmetries of decay lines of ${}^{56}\mathrm{Co}$ and ${}^{60}\mathrm{Co}$ sources and of excitations of ${}^{11}\mathrm{B}$ were fitted with a constant function. An intrinsic-asymmetry parameter of $\alpha =0.995(6)$ was obtained from the fit.

Figure 4

Polarization sensitivity of the clover detector. Dipole and quadrupole transitions of excited states of known parity of ${}^{32}\mathrm{S}$, ${}^{208}\mathrm{Pb}$, ${}^{140}\mathrm{Ce}$ were analyzed in terms of their asymmetry. The function from Eq. (8) was fitted to the absolute values. The fitting parameters $a_Q=0.38(12)$ and $b_Q=-1.9(22)\times {10}^{-5}\hspace{0.5em}{\mathrm{keV}}^{-1}$ are obtained.

Figure 5

Spectra of ${}^{140}\mathrm{Ce}$ between 6 and 7 MeV of coincidences perpendicular and parallel to the reaction plane. The duration of the measurement was 290 h. Only peaks marked with number symbols were evaluated. See Fig. 6 and Table for the results.

Figure 6

Asymmetries of dipole transitions in ${}^{140}\mathrm{Ce}$ between 6 and 7 MeV versus the values of the asymmetries expected for $E1$ and $M1$ transitions, respectively. The bands are obtained from the polarization-sensitivity analysis, see Sec. 3a. For all transitions magnetic character can be excluded with confidence levels of at least $1\sigma$, see text and Table .