Spectroscopy of low-spin states in Dy157 : Search for evidence of enhanced octupole correlations

Abstract

Low-spin states of 157 Dy have been studied using the JUROGAM II array, following the 155 Gd (α, 2n) reaction at a beam energy of 25 MeV. The level scheme of 157 Dy has been expanded with four new bands. Rotational structures built on the [523]5/2and [402]3/2 + neutron orbitals constitute new additions to the level scheme as do many of the inter-and intra-band transitions. This manuscript also reports the observation of cross I + → (I-1)and I -→ (I-1) + E1 dipole transitions inter-linking structures built on the [523]5/2 -(band 5) and [402]3/2 + (band 7) neutron orbitals. These interlacing band structures are interpreted as the bands of parity doublets with simplex quantum number s = -i related to possible octupole correlations.Two previous experiments conducted with large Ge detector arrays used heavy-ion induced reactions 150 Nd( 12 C,5n) and 124 Sc( 36 S,3n) [71,72,97] to study the medium to high-spin states of 157 Dy. These experiments reported rotational bands built on the h9/2[521]3/2 -, i13/2[651]3/2 + and h11/2[505]11/2neutrons configurations but reported no transitions linking these structures. To this end we have performed in-beam measurements using the JUROGAM II array to study the rotational bands of 157 Dy and to search for evidence of octupole correlations. We report on four new bands as well as many new linking transitions relative to the previously observed structures.The rotational behaviour of the new bands is described in terms of quasi-particle assignments, and the observed alignment properties are analyzed within the framework of the cranked shell model. II. Experimental details and analysisExcited states of 157 Dy were populated using the 155 Gd (α, 2n) reaction at a beam energy of 25 MeV. The 155 Gd target was 0.98 mg/cm 2 thick with a purity of 91%. Gamma-rays following the fusion evaporation reaction were detected with the JUROGAM II multi-detector array [73] in JYFL, Jyväskylä, Finland. The spectrometer setup comprised 39 high-purity germanium detectors, all with BGO escape suppression shields; 15 EUROGAM coaxial detectors (Phase 1 [75] and GASP [74] type) and 24 EUROGAM Phase II clover detectors [76]. Approximately 14 x 10 9 γ-γ events were unfolded from the data in the off-line analysis and replayed into γ-γ Radware [77, 78] coincidence matrices, which were used to construct the level scheme of 157 Dy. The spins and parities for the new rotational structures were successfully assigned using the Directional Correlation from Oriented States (DCO ratios/RDCO) [79, 80] and linear polarization anisotropy (Ap) methods [81]. While Ap is given as used in [82], the DCO matrices were constructed using data from detectors in the rings at 158° and 86°+94°, thus the RDCO for the JUROGAM II array in this work is; RDCO = I γ1(at 158º : gated on γ2 near 90 º) / I γ1(near 90º : gated on γ2 at 158º)

First candidates for γ vibrational bands built on the [505]11/2− neutron orbital in odd- A Dy isotopes

“…Similarly, the same feature is observed between the K π =15/2 + band in 157 Dy (Ex = 928-keV) [see Fig. 2] and the K π =2 + γband band-head in 158 Dy (Ex = 946-keV) [9]. This trend is also evident in the γ-band that was recently identified in 155 Gd [17].…”

“…The level scheme of 157 Dy is well known from the previous in-beam work of Hayakawa et al, [53], Pipidis [41] [19]. 7 The rotational structures shown in Fig.…”

“…Furthermore, since ΔK = 2 for the out-of-band γ-rays decaying out of both bands, M1 components are K-forbidden in the ΔI =0 (from the even-spin members to the ground band) and ΔI = ±1 (from the odd-spin members to the ground band) transitions [5,6]. Similarly, due to the very inhibited M1 components, the ΔI=1 transitions between the even-and odd-spin members of the K π =2 + bands, particularly in the 150 to 170 mass region, are observed to be very weak at most [5][6][7][8][9].…”

“…Consequently, it is challenging to find cases where these bands have been identified beyond spin 14 + . However, wherever they are observed in the transitional rare-earth region, one of their notable features is that they, particularly the odd-spin members, faithfully track their intrinsic configuration (usually the ground state band in even-even nuclei), as a function of spin [7][8][9][10][11][12][13][14].…”

“…It is worth noting that this work focuses mainly on results relating to bands built on the h11/2 orbital. Results pertaining to bands built on other prominent single-particle orbitals close to the Fermi surface in N ≈ 90 nuclei such as i13/2, h9/2 and f7/2, particularly for 157 Dy, have been presented in a separate publication [19]. A previous data set has also been examined in order to identify levels built on top of the [505]11/2bands in 155 Dy [13,20,21].…”