Stretched E2 Transitions Research Articles

Investigation of γ-γ coincidence counting using the National Nuclear Array (NANA) as a primary standard

The National Physical Laboratory has recently been in the process of commissioning a multi-detector γ ray array - the National Nuclear Array (NANA). In this study we have sought to exploit the NANA and the excellent timing characteristics of its intrinsic LaBr3(Ce) scintillation detectors for use as a primary standardisation system. For this initial investigation, the absolute standardisation of 60Co has been performed by the γ-γ coincidence technique using NANA and the result compared to the established 4π(LS)-γ Digital Coincidence Counting (DCC) system. The effect of the angular correlation of the stretched E2 transitions emitted from the 4+→2+→0 states of 60Ni on the activity determined by NANA was observed between the pairs of detectors. Corrections for these angular correlations were derived through Monte Carlo simulations. An activity per unit mass by NANA of 330.8 (10) kBqg−1 for the 60Co solution was determined. There was no significant statistical difference between the results of NANA and the 4π(LS)-γ DCC, with a relative difference of 0.04% observed. This study shows that NANA can be used as a primary standard.

High Spin States in 143Gd

The level scheme of 143Gd has been extended to E = 8.7MeV (I = 53/2+) in an experiment with the GASP γ-ray spectrometer and the 100Mo(48Ti,5n) fusion reaction. Most of the level scheme shows single particle character. A strongly populated cascade consisting of ten stretched E2 transitions and another structure with weak M1 and strong E2 shows an indication of a collective character.

Lifetimes of high-spin states in theπi13/2band of173Re

Lifetime measurements on the i{sub 13/2} band built on the proton configuration 1/2{sup +}[660] in {sup 173}Re have been carried out by the Doppler-broadened line shape method with the use of the GAMMASPHERE array in an early-implementation arrangement. Lifetimes of the stretched E2 transitions from states of spin 41/2{sup +}{endash}69/2{sup +} in this band were measured, and the transition quadrupole moments calculated from these lifetimes are consistent (average Q{sub t}=8.1{plus_minus}0.5eb) with a large and near-constant deformation ({beta}{sub 2}{approx}0.29) over the frequency range of {h_bar}{omega}=0.29{endash}0.47 MeV. This result attests to the strong deformation-driving effect of the intruder {pi}i{sub 13/2} orbital when it lies just above the Fermi surface. From thin-target measurements, it was possible to add three new levels to this band and to determine that the experimental aligned angular momenta gradually evolve over a large frequency range and level off with a total gain characteristic of alignment by i{sub 13/2} neutrons. Calculations carried out for diabatic configurations give a satisfactory accounting of the alignment gain. However, because of the large interaction between crossing bands, a more meaningful description of the deformation pattern in this band was achieved through calculations of the spin-adiabatic type. Both calculations are discussed and compared withmore » the experimental results. {copyright} {ital 1997} {ital The American Physical Society}« less

High spin states in 109Sn.

High spin states in $^{109}\mathrm{Sn}$ have been established up to 9.1 MeV in excitation energy and to a spin of 43/${2}^{\mathrm{\ensuremath{-}}}$\ensuremath{\Elzxh} with the reaction $^{94}\mathrm{Mo}$${(}^{19}$F,p3n${)}^{109}$Sn at a beam energy of 88 MeV. In-beam measurements involved \ensuremath{\gamma}-\ensuremath{\gamma} coincidences and directional correlation of oriented states. A negative-parity band consisting of stretched E2 transitions has been established. This band has been identified as the favored signature branch of the decoupled ${\mathrm{h}}_{11/2}$ neutron coupled to the deformed \ensuremath{\pi}(${\mathrm{g}}_{9/2}^{\mathrm{\ensuremath{-}}2}$\ensuremath{\bigotimes}${\mathrm{g}}_{7/2}^{2}$) core configuration. \textcopyright{} 1996 The American Physical Society.

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

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 = 752, 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.

Collectivity in "spherical" 143,144Eu nuclei.

Long cascades of stretched E2 transitions have been observed in $^{143}\mathrm{Eu}$ and $^{144}\mathrm{Eu}$ nuclei which at low excitation exhibit spherical shape. Lifetime experiments demonstrate high collectivity values up to B(E2)\ensuremath{\approxeq}100 W.u. among these E2 cascades. The total Routhian surface calculations indicate many crossing rotational bands of the nucleus with triaxial shape at ${\mathrm{\ensuremath{\beta}}}_{2}$\ensuremath{\approxeq}0.25 and \ensuremath{\gamma}\ensuremath{\approxeq}30\ifmmode^\circ\else\textdegree\fi{}. The E2 cascades in $^{143}\mathrm{Eu}$ are proposed to represent the \ensuremath{\nu}${\mathit{h}}_{11/2}^{2}$\ensuremath{\pi}${\mathit{h}}_{11/2}^{\mathit{n}}$ configurations with an additional \ensuremath{\nu}${\mathit{d}}_{3/2}^{\mathrm{\ensuremath{-}}1}$${\mathit{i}}_{13/2}$ excitation at the highest states.

Poisson and Porter-Thomas fluctuations in off-yrast rotational transitions

Fluctuations associated with stretched E2 transitions from high-spin levels in nuclei around 168Yb are investigated by a cranked shell model extended to include residual two-body interactions. In the cranked mean-field model without residual interactions, it is found that gamma-ray energies behave like random variables and the energy spectra show Poisson fluctuation. With two-body residual interactions included, the discrete transition pattern with unmixed rotational bands is still valid up to around 600 keV above yrast, in good agreement with experiments. At higher excitation energy, a gradual onset of rotational damping emerges. At 1.8 MeV above yrast, complete damping is observed with GOE-type fluctuations for both energy levels and transition strengths (Porter-Thomas fluctuations).

First evidence for the hyperdeformed nuclear shape at high angular momentum.

A ridge structure consisting of stretched E2 transitions and extending from \ensuremath{\Elzxh}\ensuremath{\omega}\ensuremath{\simeq}0.6 to 0.75 MeV has been found in a proton-selected \ensuremath{\gamma}-\ensuremath{\gamma} matrix in the reaction $^{120}\mathrm{Sn}$${(}^{37}$Cl, xn${)}^{152,153}$Dy at 187 MeV. The ridge spacing corresponds to a dynamic moment of inertia, ${\mathit{scrI}}^{(2)}$, of about 130${\mathrm{\ensuremath{\Elzxh}}}^{2}$ ${\mathrm{MeV}}^{\mathrm{\ensuremath{-}}1}$ and suggests the existence of a hyperdeformed prolate shape with a quadrupole deformation ${\mathrm{\ensuremath{\beta}}}_{2}$\ensuremath{\ge}0.9. Furthermore, a cascade of 10 discrete transitions with an average energy spacing of 30\ifmmode\pm\else\textpm\fi{}3 keV has been found. This result is consistent with calculations indicating conditions favorable to hyperdeformed structures.

Test of predicted deformation-driving effects in 173Re

Lifetime measurements on the 52+[402] band in 173Re have been carried out by the recoil-distance technique. Lifetimes of the stretched E2 transitions in both signatures of this band were measured up to a spin of 352+ as were the lifetimes of most of the M1 transitions between the two signatures. Large deformation increases were found in both signatures at low rotational frequencies (h̷ω≈160 keV) as predicted by TRS calculations and, thus, offer new insight into the behavior of the 52+[402] band in 173Re and perhaps into other light nuclei in this region. The B(M1) values also show an increase at about the same rotational frequency. Particle-rotor calculations can account qualitatively for the observed transition rates.

High spin level structure of 63 143 Eu80

The level scheme of143Eu has been extended to I=75/2 in an experiment with the NORDBALL Compton-suppressed Ge detector array and the110Pd(37Cl, 4n) reaction. Most of the scheme shows irregular structure of multiparticle excitations. A strongly populated straight cascade of more than 10 stretched E2 transitions suggests the onset of collectivity.

Discrete levels in 154Dy above spin 30ħ: Evidence for terminating band structures

High-spin states in 154Dy have been studied using the TESSA2 γ-rays spectrometer following the 110Pd( 48Ca,4n) 154Dy reaction at a beam energy of 210 MeV. States up to 44 + and 37 − have been observed. Below spin 30 the data display regular rotational behaviour which can be interpreted in terms of the cranked shell model. Above spin 30, sequences of levels connected by stretched E2 transitions, which show large gains in energy when compared to a rotating liquid drop reference, are lowest in energy for both parities. Particularly low energy levels are observed for spin I π = 36 + and 42 + and in addition dipole transitions are found connecting negative-parity states around spin I = 35. The experimental data for I ≳ 30 are compared with calculations, based on the Nilsson-Strutinsky cranking formalism, in which it is possible to trace fixed configurations through a sequence of spins. For the high-spin positive-parity sequence, the similarity with the 156Er spectrum is discussed.

Spectroscopy of221Th

A positive and a negative parity band in221Th have been investigated via the208Pb(16O, 3n)221Th reaction. Strong E1 transitions compete with stretched E2 transitions deexciting levels of alternating parity. The measured averaged =2.6 · 10−6 fm−2 is similar to that observed in neighbouring nuclei. The level scheme may be interpreted assuming an octupole deformed core.

High spin states in 128Ce.

Properties of high spin states in /sup 128/Ce have been measured following the reaction /sup 112/Cd(/sup 20/Ne, 4n)/sup 128/Ce induced by 103-MeV /sup 20/Ne ions. High efficiency ..gamma..-..gamma.. coincidence measurements and angular distribution measurements showed a series of stretched E2 transitions that form the yrast sequence up to I = 24/sup +/, six stretched E2 transitions which form one sideband, and four transitions of undetermined multipolarity forming a second sideband.

Yrast levels of 220Ra populated in the 208Pb(14C, 2n) reaction

Abstract Yrast states in the nucleus 220 Ra were studied by means of the 208 Pb( 14 C, 2n) reaction at 61 and 64 MeV. A staggering sequence of levels of positive and negative parity has been observed up to spin and parity I π = 16 + (18 + ) and from I π = 5 − to I π = 17 − , respectively. These states are connected by strong E1 transitions competing with the stretched E2 transitions, the B( E 1) B( E 2) ratio being ~ 10 −6 fm −2 . The ratio of the excitation energy of the 4 + state to that of the 2 + state is close to the vibrational limit. The moment of inertia associated with the negative-parity yrast states is slightly increasing with the rotational frequency ω. It is considerably higher than that of the positive-parity states at lower spins, the difference decreasing monotonically with increasing ω. The data are discussed with reference to the octupole vibrational picture as well as to the results of recent models predicting reflection-asymmetric shapes in the Ra-Th region.

Angular momentum alignment in the reaction154Sm+214 MeV32S

Angular momentum alignment in the reaction154Sm+214 MeV32S has been investigated using discrete-lineγ-ray techniques. The anisotropies of stretchedE2 transitions from the target-like fragments reach approximately three at the largest energy losses, implying rather strong alignment. The results are compared with the predictions of a simple model.

Direction of the angular momentum in the entry state and the crystal ball

In a crystal ball all continuum gamma rays of a cascade from the entry state to a yrast state can in principle be observed. If one assumes a preference for stretched E2 transitions one expects a 'doughnut'-shaped angular distribution where the symmetry axis gives the direction of the entry-state angular momentum approximately. The computer code ICARUS, which simulates gamma -ray cascades after heavy-ion fusion reactions, is extended to include angular distributions. By Monte-Carlo simulation one tests how accurately one can extract the direction of the entry angular momentum as a function of excitation energy and angular momentum of the entry state. For 158Er the mean error varies from 20 degrees -40 degrees depending on excitation energy and angular momentum. The entry-state weighted mean error is 25 degrees .

Side feeding times to yrast states of158Dy in26Mg(136Xe, 4n)158Dy

Side feeding times to the yrast states of158Dy were analyzed and compared with Monte Carlo calculations. It is found that B (E2) value's in pre-yrast stretched E2 transitions are about 80% of the rigid rotor prediction. Average multiplicities and corresponding transition energies are also calculated and compared with the experimental data.

The [formula omitted] bands in the odd- A = 19–31 nuclei

The systematics of excitation energies, single-particle spectroscopic factors and electromagnetic transition rates of levels belonging to the K π = 1 2 − bands based on the Nilsson orbits 4 and 14 in the odd- A = 19–31 nuclei have been studied and compared to weak-coupling and particle-rotor model predictions. Both bands are characterized by large decoupling parameters. The E1 transition rates are found to be typically of the order of 10 −4 to 10 −3 W.u., which is normal for this mass region. The transitions within the 1 2 −[101] band are enhanced, stretched E2 transitions. The over-all agreement with the model predictions is good.

Study of high spin states in 78Kr

High spin states in 78Kr have been excited in oxygen, fluorine and carbon induced reactions on 65Cu, 64Ni and 68Zn targets. On the basis of γ-γ coincidences, angular distributions and excitation functions, the extension of the level scheme of 78Kr is proposed; four bands connected by stretched E2 transitions have been identified. The lifetimes of some twenty levels have been determined by the recoil distance Doppler shift and Doppler shift attenuation method. The β-decay half-lives of the 103 keV isomeric state and ground state of 78Rb have been remeasured. Very good agreement of the level scheme along the experimental E2 and E1 transition strengths with the interacting boson model IBA-1 and IBA-2 has been established.

Observation of a decoupled band in123Cs

The methods of in-beamγ-ray spectroscopy have been used to study123Cs produced by the115In(12C, 4n) reaction. Five coincident stretchedE2 transitions, previously assigned in the literature to123Ba, have been identified as members of a decoupled band in123Cs.