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The Daresbury On-Line Isotope Separator-Cryogenic On-Line Device (DOLIS-COLD), consisting of a 3He-4He dilution refrigerator coupled to an on-line isotope separator, has proved to be a powerful system for the study of nuclei far from stability. The design and performance of DOLIS-COLD is discussed. To illustrate the capabilities of the system, examples of magnetic moment measurements by static low-temperature nuclear orientation and by NMR on oriented nuclei (NMR/ON) are presented.

The AKr(α, 2nγ)A+2Sr reactions have been studied by in-beam γ-ray spectroscopy for A = 78, 80, and 82. States with spins up to 10+ in 82,84Sr and 6+ in 80Sr are identified. The Sr nuclei undergo a transition from spherical to deformed ground states as neutrons are removed from the N = 50 shell. The systematics of these nuclei are compared with calculations based upon the shell model and interacting boson model.

A sideband in 170 Yb has been identified up to I π =17 − . It has an isomeric I =4 bandhead, yet the odd-spin states have prompt branches to the ground-state band. The contrasting properties are explained by the Coriolis effects on the quasi-particle configuration.

The magnetic moment of116Sbg(t1/2=16 m, Iπ=3+) has been measured to be |μ|=2.716(7)μN using the technique of NMR/ON on nuclei oriented at low temperatures in an iron host, on-line at the DOLIS-COLD facility, Daresbury. The measured moment can be well interpreted using empirical magnetic moments of neighbouring odd-A antimony and tin nuclei. The change in Iπ=3+ moments between116-120Sb is well accounted for by the variation in the odd neutron contribution, the odd proton acting as a spectator. © 1987 J.C. Baltzer A.G., Scientific Publishing Company.

Decay sequences based on the 1 2 + [660] proton orbital have been identified in 171 Ta and 177 Re based on spin, parity, and large alignment. This decay sequence is observed higher in energy than predicted in cranking calculations based on modified oscillator potentials. Similarly known 1 2 − [541] decay sequences in these and other neighbouring isotopes are observed lower in energy than predicted. A reduction in the strength of the spin-orbit potential for protons is suggested as a solution to these problems.

The crossing of an octupole band by a rotation-aligned two-quasiparticle band has been observed in detail in 162Dy. Both the yrast and yrare states are identified from the band heads (I =2 and I = 5) to high spin (I ∼ 14), with band crossing in both the even-spin and odd-spin sequences.

At I π =22 + , the yrast band of 174 Hf is observed to have a second rotational anomaly, which may be linked with the high-spin extension of the K π =0 + “β-band”. A possible explanation of the anomaly is that the nucleus is more deformed in the excited band than in the yrast band.

The hyperfine field of Cs Fe has been measured to be (+)40.8(0.7)T, using the technique of on-line low temperature nuclear orientation. The time dependent relaxation of121CsmFe has also been investigated using a new technique, giving a Korringa constant Ck of 0.059 (15) sK. From this, an effective relaxation time of 3.0 (0.8)s can be deduced for the short lived118Csm(T1/2=17s). This value has then been used to extract the magnetic moment of118Csm to be |μ|=5.4 (1.1) nm. © 1987 J.C. Baltzer A.G., Scientific Publishing Company.

Members of the octupole band up to spin 14− have been identified with the 170 Er (α,2 n ) 172 Yb reaction. The band is strongly perturbed by Coriolis effects and has a high moment of inertia with variations characteristic of particle alignment. Vabrational alignment is insufficient to explain the data.

High-spin data are presented for 170 Ta and 171 Ta for the first time, and the known level scheme of 173 Ta is extended to higher spins. Anomalies found in the crossing frequencies for the alignment of a pair of neutrons are considered. At high spin a large and constant value of J (2) , with J (2) > J (1) , is found for two 171 Ta bands and the implications of this are discussed. The π i 13/2 [660]1/2 + , α = + 1/2 orbital is identified as it crosses the rotational band built on the [402]5/2 + , α = + 1/2 orbital in 171 Ta. Magnetic transition probabilities obtained from measured branching ratios show the effects of the aligning pair of neutrons and the spectator orbital. The data are interpreted using cranked shell-model calculations and a semiclassical vector-coupling scheme developed by Donau and Frauendorf.

The magnetic dipole mooments of 117 I, 118 I g , 118 I m , 119 I, 120 I g , 120 I m , 121 I and 122 I have been measured by the technique of on-line low-temperature nuclear orientation. The ground-state moments show the influence of N = 64 subshell closure, with near-spherical configurations, while the moments of the metastable states suggest large prolate deformations.

Rotational side-bands in 162Dy have been studied using the 160Gd(α, 2nγ)162Dy reaction. Seven side-bands are observed, with Kπ = 2+, 2−, (0)−, 0+, 5−, 4+ and (6−). Four of these bands have collective structure at low spin: the Kπ = 2+ γ-vibrational band, the Kπ = 0+ β-vibrational band, and the Kπ = 2− and (0)− octupole vibrational bands. Of the remaining bands, the 4+ band is deformation coupled while the 5− and (6−) bands are rotation-aligned. Several bandcrossings are observed in this nucleus. The β and γ-bands are crossed at I = 6 h and 12 h , respectively, by a highly aligned ( i 13 2 )2 S-band; extrapolation of this S-band to higher spin suggests that it crosses the g.s.b. between I = 18 h and 20 h . The 2− octupole band is crossed by the 5− band at I = 9 h and again by the (6−) band at I = 12 h . The latter bandcrossings are discussed in terms of two-quasiparticle plus rotor calculations.

The interactions between a K/sup ..pi../ = 5/sup -/ aligned two-quasineutron band and two octupole bands have been observed in /sup 164/Er. Band crossings occur in both even- and odd-spin sequences. The members of all three bands are observed from the bandheads (L/sup ..pi../ = 1/sup -/, 2/sup -/, and 5/sup -/) to about 12/sup -/.

Collective octupole bands with K π = 0 − , 1 − have been observed to high spin in 152 Sm. The difference in their level spacings is interpreted to be a consequence of the contributing quasiparticle configuration.

Rotational band structures in 156Dy have been populated via the reactions 148Nd(12C, 4n) and 124Sn(36S, 4n) at beam energies of 65 MeV and 155 MeV, respectively. High-spin states (I ≳ 40kh) have been observed and a detailed level scheme is presented. Two positive-parity bands have been observed to spins I = 42 and 36. Four negative-parity bands have been identified up to or above spin I = 36. New levels are also observed in octupole and gamma vibrational bands. The band structures are understood in terms of quasiparticle configurations using the cranked-shell model. The second and third i 13 2 neutron alignments, the systematics of the first h 11 2 proton alignments in even-even N = 90 isotones and the observation of a third discontinuity in the lowest energy positive- and negative-parity bands near spin 40 in 156Dy are discussed.

The magnetic dipole moments of 118 I g (t 1 2 =12 min ) and 118 I m (t 1 2 = 8 min ) have been measured to be |μ| = 2.4±0.4 nm and 4.7±0.7 nm, respectively, using the technique of non-linear orientation. In order to understand the high moment of the Iπ=(7−) metastable state, it is necessary to invoke the excitation of a g 9 2 proton hole across the Z = 50 shell gap, together with a considerable quadrupole deformation. In the daughter nucleus, 118Te, a low-lying Kπ = 0+ state is established, and the possible influence of shape coexistence is discussed.

The g-factors for Kπ = 6+,8− and 232− isomers, with half-lives in the range 5 ns to 163 ns, have been measured in 172, 173, 174, 178Hf, using the method of perturbed angular distributions. The results imply significant two-quasineutron admixtures in the Kπ = 6+ isomer in 174Hf, while the other two-quasiparticle isomers have pure two-quasiproton configurations. The g-factor for the Kπ = 232− isomer in 173Hf is consistent with the recently suggested two-quasiproton plus one- quasineutron configuration.

The level structure of 152Sm has been studied with various spectroscopic techniques using the (α, 2nγ) reaction. Seven rotational bands were identified up to spin values of 9 to 14ħ, viz. the ground-state band, the β- and γ-bands, and four negative-parity bands (NPB's). In the β-band a beginning of backbending or upbending was observed at I ⋍ 14 h . Two of the NPB's are based upon collective states; the lowest can be interpreted as an aligned octupole band, the other one shows two-quasiparticle characteristics at high spin. The properties of the other NPB's are consistent with Kπ = 5− and Kπ = 7− two-quasiparticle configurations of the bandheads. At high spin these bands are strongly mixed. Excitation energies for all bands with a collective bandhead, as well as B(rmE1) and B(E2) branching ratios and (for the β-band) X(E0/E2) ratios, were compared with IBA calculations.

Six side bands have been identified up to high spin in 172 Yb. Four of these, with K π = 1 − , 4 − , 6 − and possibly 9 − , involve an i 13 2 neutron, and have high moments of inertia. The properties of these bands are reproduced in a two-quasiparticle-plus-rotor Coriolis coupling calculation. A well-known band with K π = 3 +; is extended to I = 14; its properties suggest a mixed neutron-proton configuration. The first excited K π = 0 + band is extended to I = 14. It has a rapidly rising moment of inertia, and is predicted to cross the g.s.b. at about I = 20 + . These K = 0 + states could be the low-spin members of an ( i 13 2 ) 2 band.

At least seven side bands, with Kπ = (0+), (1−), (2+), (3−), 4−, 6−, and 7−, have been identified in 170Yb up to high spin, using γ-ray techniques following (α, 2n) and (α, 4n) reactions. Backbending in the positive-parity yrast band is confirmed, and candidates for low-spin members of the S-band are found. The negative-parity yrast band has behaviour characteristic of both strong coupling and decoupling. These properties, together with those in the other negative-parity side bands, are interpreted as arising from Coriolis effects on the quasiparticle configurations, and the experimental results are compared with a two-quasipartiele-plus-rotor calculation. Evidence of ΔK = 2 mixing is found in the K = (3) side band. The relationship between the moments of inertia of the positiveparity and negative-parity yrast bands in the N = 100 isotones is discussed in the framework of a simple empirical model, giving insight into the presence or absence of backbending. The negativeparity yrast bands in the N = 100 isotones are compared. Their significant differences are qualitatively reprodued in two-quasiparticle-plus-rotor calculations.

Two 3-quasiparticle isomers with spins, parities and half-lives of 19 2 + , 1.1 μs and 23 2 − , 1.2 ns have been identified at 1433 and 1766 keV in 175 Hf. A third isomer, possibly 35 2 − with a 1.2 μs half-life, is found at 3015 keV. The first two are characterised as a 7 2 + [633] neutron coupled to the known 6 + and 8 − 2-proton isomers of the core nuclei. Rotational bands based on the 3-qp isomers are highly perturbed, due to Coriolis mixing, and their structure is reproduced in a band mixing calculation. The energy depression of the 3-quasiparticle states relative to the 2-quasiproton core states is attributed mainly to the residual proton-neutron interaction, and possibly also to blocking effects through neutron admixtures.

The two signatures of a band built on a π h 11 2 shell model state have been established to high spin (I π = 65 2 − ) in 165 Lu. An observed second band crossing at ℏ ω ∼ 0.40 MeV is interpreted as the alignment of a second pair of i 13 2 neutrons. M1 transitions are observed through both crossing and the data utilized in connection with the observed signature splitting of energies and alignments to interpret the measured values of B( M1 , I → I − 1) Q 2 0 . Alignment effects and shape changes are discussed.

The high spin states of 162 Yb and 164 Hf have been studied using 122 Sn( 44 Ca, 4n) and 120 Sn( 48 Sn( 48 Ti, 4n) reactions respectively. The resulting new level schemes for 162 Yb and 164 Hf are presented and compared with existing data for 160 Er. The systematic behaviour of these three N = 92 isotones at large angular momentum yields information both on modifications of the single-neutron spectrum of states resulting from the polarisation of the nuclear field and on the single-proton spectrum of states.

The resonant destruction of gamma-ray anisotropy from 116 Sb g (t 1 2 =16 m , I π =3 + ) has been observed at 161.65 ( MHz, using the technique of NMR/ON. The deduced magnetic moment is |μ|=2.175(9) μ N. The change in the Iπ=3+ magnetic moments between 116Sb and 120Sb can be accounted for by variation inthe odd-neutron contribution, the odd proton in these antimony states behaving as a spectator.

On-Line measurements of magnetic dipole moments of117-122I are interpreted using coupling of the odd particles to a deformed core. The results show interesting effects of g7/2, d5/2 orbital admixtures in the odd-A isotopes, which are close to spherical. The odd-odd isotopes118, 120I show clear examples of shape co-existence. © 1987 J.C. Baltzer A.G., Scientific Publishing Company.

High spin states in /sup 178/W have been studied using the /sup 170/Er(/sup 12/C,4n) reaction and ..gamma..-ray techniques. The yrast band in /sup 178/W has been identified to spin 18. At this spin the moment of inertia curve shows backbending. The systematics of backbending in the tungsten isotopes is discussed.

Gamma rays associated with the decay of giant dipole resonance (GDR) modes and emitted in competition with the first evaporated particle from the compound system 166 Er ∗ have been isolated by a difference technique. It is found that the measured gamma ray yield from the very highest excitation energy populated in the reaction, accounts for ≈ 25% of the total gamma ray cross section for transition energies in the range 10–20 MeV, and this agrees well with the prediction of the statistical model for compound nuclear decay.

Calculations using the IBA-2 model code are presented for even-even tellurium isotopes118Te to124Te. Three parameter sets are used, two from previous work and a third chosen to give optimum fit to experimental positive parity energy levels and B(E2) ratios. With only two additional parameters, each with simple physical interpretation, the three parameter sets are shown to give excellent fits to the known collective negative parity states in these isotopes. Failure to fit the O 2 + ,O 3 + and 2 3 + levels suggests these may be 4p-2h intruder states, although their Behaviour with varying neutron number is not as clearly characteristic of such configurations as is found for the 2p-4h states in Cd isotopes.