First evidence for spin-flip $M1$ strength in ${}^{40}\mathrm{Ar}$

The ${}^{40}\mathrm{Ar}$($\overrightarrow{\gamma },{\gamma }^{\prime }$) photon scattering reaction was used to search for spin-flip $M1$ strength in ${}^{40}\mathrm{Ar}$. The nearly monochromatic, linearly polarized photon beam of HIγS, in an energy region from 7.7 to 11 MeV, was employed in this study. 28 dipole excitations were observed. The azimuthal intensity asymmetry indicated that all of these states were $E1$ except for the state at $E_x=9.757$ MeV. Shell-model calculations were used to interpret this state as one fragment of the spin-flip $M1$ strength in ${}^{40}\mathrm{Ar}$.

Figure 1

Schematic cross section of the arrangement of the argon gas container and the HPGe γ-ray detectors (only one shown, see text).

Figure 2

Spectra of the ${}^{40}\mathrm{Ar}$($\overrightarrow{\gamma },{\gamma }^{\prime }$) reaction at HIγS. Data were taken at a mean polar angle $\overline{\theta }={90}^{°}$ relative to the incident photon beam and azimuthal angles ${\varphi }_{\parallel }$ (top) and ${\varphi }_{\perp }$ (bottom) relative to the polarization plane of the γ-ray beam. The mean γ-ray energy ${\overline{E}}_{\gamma }$ of the incident beam is given in the upper left corners. The $J^{\pi }$ assignments are indicated for the ground state transitions in ${}^{40}\mathrm{Ar}$. The single escape peaks from some ground state transitions are indicated by brackets.

Figure 3

Asymmetry values $A_{\mathrm{expt}}$ of the observed ground state $1^{\pi }\hspace{1em}\to \hspace{1em}{0}_1^{+}$ transitions for the determination of their radiation character. The polarization sensitivity in this experiment amounted to about 50%. Parity quantum numbers of the corresponding dipole excitations were assigned to be $J^{\pi }=1^{+}$ or $1^{-}$ for $M1$ or $E1$ transitions, respectively. Upper limits with centroid values marked as a cross are given for those transitions for which the photon scattering intensity at ${\varphi }_{\parallel }$ was too weak to be observed. We conclude that only one of the observed excited states has $J^{\pi }=1^{+}$.

Figure 4

Electric dipole excitation strength $B(E1)\uparrow$ distribution for ${}^{40}\mathrm{Ar}$ between 7.5 and 11 MeV. The error bars are displayed on top of the data bars (online green).

Figure 5

Experimental and theoretical $M1$ strength distributions in ${}^{40}\mathrm{Ar}$. (top) The single known $1^{+}$ state observed here at 9.757 MeV within an energy region between 7.7 and 11 MeV. Other energy and strength regions, to which our experiment was insensitive, are shaded. The distribution of the magnetic dipole excitation strengths $B(M1)\uparrow$ (middle) and the proton spin contribution $B(M{1}_{{\sigma }_p})\uparrow$ (bottom) of the first ten $1^{+}$ states predicted by the shell model calculation. Proton $d_{5/2}\to d_{3/2}$ spin-flip transition strength dominates the $M1$ matrix elements for the third and sixth $1^{+}$ states at 6.882 and 9.465 MeV, respectively, as labeled.