M. Piiparinen

High-spin spectroscopy of the 142Eu, 143Eu and 144Eu nuclei

Abstract High-spin level schemes of the 142Eu, 143Eu and 144Eu nuclei have been investigated by in-beam γ-ray spectroscopic methods using the NORDBALL Compton-suppressed multi-detector array. The previously known schemes have been extended to considerably higher spin and excitation energy, up to I ≈ 30, Ex ⩾ 9 MeV in 142Eu, I = 75 2 , Ex = 15.6 MeV in 143Eu and I ≈ 40, Ex ⩾ 16 MeV in 144Eu. The level schemes are very complicated, characteristic of spherical or slightly oblate nuclei. Also, long cascades of stretched E2 transitions have been observed in all three nuclei. Plunger and DSAM lifetime results demonstrate high collectivity values up to ∼100 W.u. (up to ∼200 W.u. in 142Eu) in these E2 cascades and they are proposed to represent several crossing collective bands with the nucleus at triaxial shape of β2 ≈ 0.25 and γ ≈ 30°. Low parts of the level schemes have been discussed within the spherical shell model. Below 5.5 MeV excitation in 144Eu and 3.5 MeV in 143Eu practically all the observed levels are interpreted with shell model configurations of respectively two or four, and one or three quasiparticles.

Structural connections between 148Sm and 149Sm nuclei

Abstract The 146, 148 Nd(α, χn) and 148, 150 Nd( 3 He, χn) reactions at E α = 20–43 MeV and E 3He = 19–27 MeV, are used to study excited states in the 149 Sm 86 and 149 Sm 87 nucleides and consequently the low-spin odd-parity excitation. The mixing ratios and multipolarities of the most prominent transitions are deduced from the combined evidence of angular distribution and electron conversion data. The spin-parity assignments for most of the levels observed are established. In 148 Sm the ground state band extending to I π = 10 + is predominantly populated. A negative-parity odd-spin band extending from I π = 3 − through 11 − is also observed. The bands in 148 Sm are interpreted within the framework of the interacting boson approximation model. In 149 Sm positive-parity levels with spin up to 25 2 and negative-parity levels with spins up to 21 2 are observed. The predominant γ-decay proceeds via transitions associated with i 13 2 , h 9 2 , f 7 2 and h 11 2 intrinsic configurations. The branching ratios B (E1)/ B (E2) are calculated and compared in both 148 Sm and 149 Sm nucleides. The B (E1)/ B (E2) dependence on the value of Z for some N = 86 (as well as 88 and 84) isotones showing a minimum of Z = 64 was noted. A 4 ns high-spin isomer mainly decaying into the positive-parity band based on the i 13 2 state in 149 Sm is found. Experimental evidence is presented to interprete the 1 2 + , 15 2 + , … and 9 2 − , 13 2 − , …, ΔI = 2, sequences in 149 Sm as arising from the coupling of an h 9 2 neutron to the octupole and quadrupole modes of the 148 Sm core nucleus. The absolute reaction cross sections for the 146, 148, 150 Nd( 3 He, χn) reactions have been determined for different bombarding energies. The mixing of the f 7 2 and h 9 2 shells is discussed in the framework of an axial-particle-rotor model calculation.

Lifetimes of yrast states in 110Cd

Abstract Lifetimes or lifetime limits of 20 yrast levels in 110 Cd have been measured using the recoil-distance method and the NORDBALL array of Compton-suppressed Ge detectors. From the reduced transition probabilities competing rotational and vibrational behaviour of 110 Cd is revealed. Also non-collective states, associated with proton configurations, have been observed.

Transition probabilities in negative parity bands of the 119I nucleus

Abstract Lifetimes in four negative-parity bands of 119 I were measured using DSAM and RDM. 119 I nuclei were produced in the 109 Ag( 13 C,3n) reaction, γγ coincidences were collected using the NORDBALL array. RDDSA — a new method of RDM analysis — is described. This method allowed for the self-calibration of stopping power. From 31 measured lifetimes, 39 values of B (E2) were established. Calculations in the frame of the Core Quasi Particle Coupling (CQPC) model were focused on the problem of susceptibility of the nucleus to γ -deformation. It was established that nonaxial quadrupole deformation of 119 I plays on important role. The Wilets–Jean model of a γ -soft nucleus describes the 119 I nucleus in a more consistent way then the Davydov–Filippov model of a γ -rigid nucleus.

Neutron and proton bands in 122Xe at high spins

High-spin bands in 122,123,124Xe and 123Cs were populated using 3He- and 16, 18O-induced reactions on Te, Pd and Ag targets. The subsequent de-excitation was studied using gamma -ray and conversion-electron spectroscopic methods. States with spins up to Jpi =(25-), (35/2-), (21-) and (51/2-), respectively, were identified. In 122Xe four Delta I=2 bands were identified in addition to the ground state and gamma bands, and furthermore a Delta J=1 band with a tentative (K,I)pi =(10, 10)- band head was observed. In 123Xe the previously observed 11/2- and 7/2+ bands were extended to Jpi =(35/-) and (23/2+), respectively. The yrast cascade in 123Cs was extended to spin 51/2-. In 124Xe the recently presented level scheme was completed with some high-lying levels. The band structures were interpreted within the cranked shell model (CSM) framework. A consistent description of band crossing frequencies in both odd and even nuclides was obtained, and configurations were assigned to most of the observed bands. Some relative transition rates were obtained from branching intensities.

Coexisting structures in 119I

Abstract High-spin structures of 119 I have been studied by using 13 C and 15 N induced reactions. In all, fifteen ΔI = 1 or 2 bands belonging to 119 I were found. No evidence was found for bands with collective oblate shape, instead, all the observed rotational bands were interpreted to possess a collective prolate shape. A rich tapestry of noncollective states of both negative and positive parity was observed. Based on TRS calculations various configurations at β 2 ≈ 0.17 and γ = 60° were assigned to these states.

Structure of odd-A Pm nuclei (I). 147Pm

Abstract The level structure of 147 Pm was studied by methods of in-beam γ-ray and electron spectroscopy using the 148 Nd(p, 2n) reaction. Twenty-four new levels with spins between 1 2 and ( 15 2 ) were observed. The 12 ± 2 ns h 11 2 level at 649.3 keV decays by a strongly hindered El transition (1.2 × 10 −6 s.p.u.) to the 9 2 + level at 408.2 keV and by a M2 transition (0.15 s.p.u.) to the ground state.

Competing proton and neutron alignments in neutron-deficient Xe-nuclei

Abstract High-spin structures of the neutron-deficient 117,118,120 Xe isotopes have been studied by in-beam γ-ray spectroscopic techniques. The final nuclei have been identified by means of charged-particle detectors. Collective rotational bands based on the neutron d 5 2 , g 7 2 and h 11 2 configurations have been identified in 117 Xe. In the even 118 Xe and 120 Xe nuclei several new side bands were observed and the previously known bands were extended. The present level schemes include two positive-parity bands constructed to high spin in 118 Xe, whereas in 120 Xe three such bands were observed. In order to explain these bands, both proton and neutron ( h 11 2 ) 2 alignments, as well as the shape degree of freedom, have to be invoked. The possible occurrence of a γS-band is addressed and a pronounced structural change for more heavy Xe-isotopes is discussed. All negative-parity side bands are interpreted in terms of proton two-quasiparticle excitations. The experimental data are compared with total routhian surface calculations.

High spin shell model excitations in149Gd

The high spin level structure of the three-neutron nucleus149Gd has been investigated by in-beamγ-ray and electron spectroscopy with (α, xn) reactions. The observed levels are characterized as members of the shell model multipletsνf72/3,νh9/2f72/2,νf72/3×3−,νh9/2f72/2×3−, and tentativelyνf72/3×(3−)2. The energies of theνf72/3 states agree only moderately with those calculated using empirical two-nucleon interactions taken from148Gd, which indicates the importance of long range contributions already atN=85.

On the decay of compound nuclei following alpha-particle and12C induced reactions

Multiple coincidence rates have been measured using a detector system consisting of a Ge(Li) spectrometer and eight NaI(Tl) or eight liquid scintillators. Reactions induced byα-particles with energies of 51–55 MeV and 118 MeV12C ions are studied. The data are analysed to give the first and second central moments of the distribution of the number ofγ-rays feeding individual levels in the final nuclei. When these numbers are compared to spin distributions calculated with the statistical model code GROGI the relative importance of dipole and quadrupole deexcitation modes can be ascertained. In particular, in the122Te(α, 4n)122Xe reaction theγ-decay prior to the entry into the ground band is well described as a statistical process proceeding to 50% by dipole and 50% by quadrupole radiation. In the166Er(α,4n)166Yb and192Os(α,4n)192Pt reactions the relative amount of quadrupole radiation is larger and it seems that the dipole and quadrupole decay takes place via separate cascades. In the164Dy(12C, 7-8n) reactions the average multiplicity is independent of spin, suggesting that the nucleus forgets the spin of the entry state before the process enters into the ground band. In the176Yb(12C, 8n)180Os reaction, finally, the nucleus definitely retains memory of the entry state during the decay. In this last case the multiplicity measurement is combined with aγ-ray singles measurement to give an average excitation energy prior to theα-decay and the average moment of inertia characterising the decay of the high-spin states.