Results

A partial level scheme of $^{156}$Gd, established in this work, is displayed in Fig. 1. For the odd-spin negative-parity band we confirm that the E2 transitions vanish below the $I^\pi=9^{-}$ state. The intensity of the $11^{-} \rightarrow 9^{-}$ transition is very weak and could not be firmly established. In fact, this transition is part of a doublet (400-402 keV) in coincidences with another doublet at 470 keV both present in the odd-spin band and the even spin band. Therefore gating on the 470 keV line to extract the 402 keV intensity would bring in any case residuel contamination from the 400 keV line.

Figure 1: Partial decay scheme of ${}^{156}$Gd showing the ground-state band, odd- and and even-spin negative parity bands and the newly established interconnecting transitions are shown.

The E1 transitions de-exciting the $I^\pi=1^{-}$ state and the E2 transition connecting the $4^{-} \rightarrow 2^{-}$ (reported in previous experiments) from the even-spin band cannot be confirmed by our results. However, $\gamma\gamma\gamma$ coincidences allowed us to clarify a number of uncertainties caused by the presence of doublet- and even triplet-lines in the spectrum of this nucleus. Moreover, we were able to examine the transitions in the medium spin range and firmly establish new inter-band transitions with $E_\gamma$ of 538, 469 and 390 keV. Angular distributions will be analysed in the near future to determine the character (stretched M1 or non-stretched E2) of these transitions. Table 1 shows some preliminary $B$(E2)/$B$(E1) ratios that we have found, compared to the results of the previous studies of Refs. [4,5]. For the $15^{-}$ and $13^{-}$ states of the odd-spin negative-parity band, the transition strength ratios are of the same order of magnitude as previously reported, while for higher spin states they could not be determined because of the cut-off in angular momentum due to the use of the $\alpha$-beam.

Table 1: Branching ratios $B$(E2)/$B$(E1) in units $10^{\,6}$fm$^2$. (a) Established in the present work - in comparison with: (b) Previous results from Refs. [4,5].

$I^\pi$	$B$(E2)/$B$(E1)			$I^\pi$	$B$(E2)/$B$(E1)
	(a)	(b)$\;\;$			(a)	(b)
$17^-$	--	16(3)		$12^-$	--
$15^-$	4.5(1.0)	6(2)		$10^-$	640(100)	240
$13^-$	5.5(0.6)	7(2)		$8^-$	330(10)	700
$11^-$	$<$9(-2)	15(7)		$6^-$	210(15)	350
$9^-$	$<$26(-5)			$4^-$	--
$7^-$	$<$92(-11)
$5^-$	--

Only upper limits are established for the lowest spins, however this information represents already a progress since no earlier publication quotes any estimates for the $(9^-\to7^-)$ and $(7^-\to5^-)$ transitions. For the even-spin negative-parity band the B(E2)/B(E1) ratios decrease with decreasing spin and are up to two orders of magnitude higher than those of the odd-spin negative-parity band.