New isomers in83Br,85Rb,85Kr and86Kr

Using methods of in-beam spectroscopy in connection with 35 MeV⁷Li bombardment of⁸²Se targets four new isomers have been identified. They are observed in⁸³Br at 3070 keV (T_1/2=0.6±0.2 μs), in⁸⁵Rb at 2826 keV (T_1/2=12.5±0.6 ns), in⁸⁵Kr at 1991 keV (T_1/2=1.2+ _0.4 ^1.0 μs) and in⁸⁶Kr at 2250 keV (T_1/2=3.1±0.6 ns). Spin and parity 19/2^? have been assigned to the new isomer in⁸⁵Rb that is interpreted as the coupling of a p_3/2 proton to the g ₉ ^2/?2 two-neutron excitation of the N=48 neutron system. Using the TDPAD method an upper limit for the g-factor of ¦g¦≦0.17 has been estimated for this isomer. The same two-neutron excitation is also believed to be responsible for the new isomer in⁸³Br. The new isomers in⁸⁵Kr and⁸⁶Kr are interpreted as configurations containing the two-proton excitation (f ₅ ^2/?1 , p ₃ ^2/?1 ) where in the isomeric 17/2⁺ level of⁸⁵Kr an additional g_9/2 neutron hole is involved.     

High-spin structure of yrast-band in 78Kr

Lifetime of levels up to 22⁺, have been measured in ⁷⁸Kr and an oblate shape is assigned to the ground state using the CSM and the configuration dependent shell correction calculations. Calculations further show that ⁷⁸Kr is highly γ-soft nucleus. The experimental Q _t values coupled with theoretical calculations indicate an oblate shape for ⁷⁸Kr at low spins and triaxial shape at higher spins     

Shape evolution in <Superscript>76,78</Superscript>Kr nuclei at high spins in tilted axis cranking Hartree-Fock-Bogoliubov approach

A two-dimensional tilted axis cranking Hartree-Fock-Bogoliubov (CHFB) calculation is performed for ⁷⁶Kr and ⁷⁸Kr nuclei up to high spins J = 30 employing a pairing-plus-quadrupole (PPQ) model interaction Hamiltonian. Intricate details of the evolution of single particle structures and shapes as a function of spin have been investigated. The results show the existence of energy levels with high K quantum numbers lying close to the yrast line in both the nuclei. Such high K states should exhibit isomeric characteristics due to the K-selection rules for the γ-decays. Moreover, in ⁷⁸Kr a new band with J = 20–30 lying below the observed ground band is predicted.      

A new sequence of high-spin states built on the 17/2+ isomer in85Kr

High-spin states in⁸⁵Kr have been studied via in-beam spectroscopy by bombarding a⁸²Se target with 32 MeV⁷Li ions. Since⁸⁵Kr is only formed with a small relative cross section proton-— coincidence techniques have been applied to enhance the rays of⁸⁵Kr, even with respect to those of⁸⁶Kr, in the measurements. A new sequence of high-spin states with excitation energies up to 4.8 MeV and tentative spins up to (23/2) has been established on top of the 17/2⁺ s isomer. A tentative assignment of the configurationvg ₉ ^2/–1 (g _9/2 p ₃ ^2/–1 orf ₅ ^2/–1 ) to the new levels is proposed.Supported by Deutsche Forschungsgemeinschaft     

Der Zerfall des77Kr

⁷⁷Kr (T _1/2=1.2 h) was produced by bombarding⁷⁶Se with 48 MeV-α-particles. After irradiation the radioactive⁷⁷Kr gas was collected in glass ampoules filled with charcoal and cooled by liquid air. The decay of⁷⁷Kr has been investigated using Ge(Li)-counters and a Ge(Li)-Ge(Li)-coincidence circuit. 27 γ-transitions, 13 of them unknown up to the present, have been found and their relative intensities have been determined. A level scheme for⁷⁷Kr→⁷⁷Br having 14 levels is proposed. This accounts for all the observed γ-transitions.     

Experimental determination of the potential energy curves of the I(3/2u) and I(3/2g) states of Kr+ 2

The I(3/2u) and I(3/2g) states of Kr⁺ ₂ have been investigated by pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectroscopy following (2 + 1′) resonance-enhanced multiphoton excitation via the 0⁺ _g Rydberg state located below the Kr?([4p]⁵5p[1/2]₀) + Kr(¹S₀) dissociation limit of Kr₂. From the positions of a large number of vibrational bands in the spectra of the ⁸⁴Kr₂ and ⁸⁴Kr-⁸⁶Kr isotopomers, the adiabatic ionization potentials (IP(I(3/2u)) = 112672.4 ± 0.8cm^?1, IP(I(3/2g)) = 111 395.0 ± 1.4cm^?1), the dissociation energies (D ⁺ ₀(I(3/2u)) = 368.8 ± 2.0cm^?1, D ⁺ ₀(I(3/2g)) = 1646.2 ± 2.3cm^?1) and vibrational constants for both ionic states have been determined. Potential energy curves have been extracted which perfectly reproduce all experimental observations and are accurate over a wide range of energies and internuclear distances. The equilibrium internuclear distances (R ⁺ _e(I(3/2u)) = 4.11 ± 0.04 Å, R ⁺ _e(I(3/2g)) = 3.35 ± 0.10 Å) have been derived by comparing the intensity distribution in the PFI-ZEKE photoelectron spectra to calculated Franck-Condon factors. The dissociation energy of the I(3/2g) state and the equilibrium internuclear distance of the I(3/2u) state differ markedly from previously reported values.     

Shapes of the neutron-rich 88-94Kr nuclei

Neutron-rich, ^88-94Kr nuclei, populated in spontaneous fission of ²⁴⁸Cm, have been studied with EUROGAM 2, by measuring prompt γ-rays. Many new excited states in even-even Kr isotopes have been identified. For the first time spins and parities were determined experimentally in these nuclei. Our results indicate that the quadrupole deformation of Kr isotopes will appear only above N = 58, as observed in Sr and Zr nuclei. The newly found 3^- level at 1506.4 keV in ⁹⁰Kr suggests the exsistence of a new region of increased octupole correlations, probably associated with the ν(d _5/2 h _11/2) pair of Δl = Δj = 3 orbitals. Received: 18 September 2000 / Accepted: 18 October 2000     

In-beam study of74Br and a first cascade in75Kr

An in-beam study of⁷⁴Br has been carried out using γ-ray spectroscopy in connection with the reactions (³He,p2in) and (d,2n) on⁷⁴Se targets. Separate level schemes on top of the two β-decaying isomers of⁷⁴Br are proposed. Two cascades connecting highspin states are assigned to populate theI ^π=4⁽⁺⁾ isomer. In this level scheme, the excited states at 72.1, 188.3 and 472.0 keV are found to have half-lives of 13.3, 0.7 and 0.7 ns, respectively. The low-spin isomer of⁷⁴Br is tentatively assigned asI ^π=(0^?). This state is populated by two other cascades. For the low-spin levels two-particle configurations of proton and neutron Nilsson orbitals are proposed. The high-spin states are tentatively interpreted as arising from the configurations (πg _9/2,νg _9/2) and (πg _3/2,νg _9/2). As a by-product, a cascade of four transitions has been observed during the³He bombardment of⁷⁴Se and tentatively assigned to negative-parity states in⁷⁵Kr.     

Simultaneousg 9/2 proton and neutron alignments in76Kr

Yrast states with spins as high as 22 ? have been identified in⁷⁶Kr with the reactions⁵⁸Ni(²⁴Mg,α2p) at 110 MeV and⁴⁰Ca (⁴⁰Ca, 4p) at 155 MeV. Simultaneous band crossings due to g_9/2 proton and neutron quasiparticles have been observed in the frequency range 0.60≦?ω≦0.75 MeV. The moment of inertia increases rapidly at the crossing frequency. The systematics of proton and neutron alignments in the Kr isotopes are discussed.     

Beyond the N = 50 shell closure: High-spin excitations of 87Kr and ground-state spin of 87Br

The ⁸⁷Kr nucleus has been produced as fission fragment in the fusion reaction ¹⁸O + ²⁰⁸Pb at 85MeV bombarding energy and studied with the Euroball IV array. High-spin states of this neutron-rich isotope have been identified for the first time. Its level scheme has been obtained up to 6.3MeV excitation energy and spin I ∼ 23/2ℏ. Its structure is interpreted by analogy with those of the heavier isotones. The proposed configurations involve both proton and neutron excitations from several sub-shells located close to the Fermi levels, particularly νd_5/2, πp_3/2f_5/2 and πg_9/2. Moreover, a revised spin value of 5/2^- for the ⁸⁷Br ground state is proposed.     

New stage of search for 2K(2ν) capture of 78Kr

A technique to search for 2K capture of ⁷⁸Kr with a large low-background proportional counter filled with a sample of krypton enriched in ⁷⁸Kr up to 99.8% at a pressure of 4.51 atm is described in this paper. The results of first measurements are presented. An analysis of data collected for 159 h yielded a new limit to the half-life of ⁷⁸Kr with regard to 2K capture (T _1/2 ≥ 1.5 × 10²¹ yr (90% C.L.)). The sensitivity of the facility to the process for one year of measurement was evaluated to be S = 1.0 × 10²² yr (90% C.L.). The text was submitted by the authors in English.     

Infrared spectra of the Kr—CO and Xe—CO van der Waals complexes

The infrared spectra of the weakly bound complexes Kr—CO and Xe—CO have been studied in the region of the CO stretching vibration (4.7 μm) using a high-resolution tuneable diode laser probe. The complexes were observed in a long path (200 m) low temperature (76 K) gas cell (Kr—CO) and in a pulsed supersonic jet expansion (Kr—CO and Xe—CO). Previous long path cell measurements on these complexes at lower resolution analysed only the K = 0 and 1 stacks of rotational levels in the ground intermolecular vibrational state. The new data extend up to K = 3 (Xe—CO) or 4 (Kr—CO), and also include K = 0 and 1 stacks in the excited bending state, ν₂ = 1. The bending frequencies for Kr—CO and Xe—CO (in the ν_co = 1 upper state) were determined to be 13.156cm_?1 and 13.794cm^?1, respectively. Detailed molecular parameters were determined to describe the rotational energy levels of each complex using a simple empirical Hamiltonian. These results enable parameters to be compared for the entire series of rare gas—carbon monoxide complexes, from He—CO to Xe—CO. Also they will guide the future development and evaluation of accurate intermolecular potential energy surfaces for Kr—CO and Xe—CO.     

Anomalous behaviour of transition probabilities in75Kr

Two collective bands of⁷⁵Kr have been extended up to spin 21/2 using the compound reactions⁶⁴Zn(¹⁴N,p2n)⁷⁵Kr and⁵⁰Cr(²⁸Si, 2pn)⁷⁵Kr. Spins and parities were assigned from neutron-gatedγ-ray angular distributions and excitation functions using the OSIRIS anti-Compton spectrometer. The bands are interpreted to be built on the well-deformed Nilsson states: [442] 5/2 and [301] 3/2. Energies for both bands and the order of magnitude of the mixing ratios in thef _5/2 band can be reproduced within the single-particle-plus-rotor model, while the experimentalQ(I→I?1)/Q(I→I?2) ratios, deduced from mixing ratios and branching ratios, exhibit large deviations by a factor 4 to 6 from theoretical values (which are around one). An explanation of this effect may be found by treating the two rotational bands each as a result of mixing between rotational bands of oblate and prolate states; thus explaining the large difference between B(E2,I→I?1) andB(E2, I →I?2) in the bands of⁷⁵Kr.     

Subshell closure effects on the collectivity of 36 88 Kr52 and the beta decay of88Br to levels in88Kr

The level structure of⁸⁸Kr has been investigated by measurements on chemically separated bromine samples. A total of 146γ-rays have been assigned to the decay of⁸⁸Br, and a level scheme with 58 levels is proposed. The collectivity of⁸⁸Kr has been investigated within the framework of the Interacting Boson Approximation (IBA-2 model). Comparison of our detailedγ-ray data and calculations have led to the establishment of a protonhole boson desciption rather than protonparticle nature. This supports recent works establishingZ=40 as a major subshell closure.     

Microscopic study of yrast bands and backbending anomaly in 78-82Kr isotopes

The yrast spectra of ^78-82Kr are studied by using the projected shell model (PSM) approach. The energy states are obtained by taking oblate as well as prolate quadrupole deformations for ^78-82Kr. The structure of yrast states and backbending phenomena are investigated. The theoretical results predict low-lying states in ^{78, 82}Kr to be oblate and coexistence of oblate-prolate shapes for ⁸⁰Kr. The B(E2) transition probabilities and g-factors are obtained and compared with the available experimental data.     

Hyperpolarized 83Kr MRI of lungs

Hyperpolarized (hp) ⁸³Kr (spin I = 9/2) is a promising gas-phase contrast agent that displays sensitivity to the surface chemistry, surface-to-volume ratio, and surface temperature of the surrounding environment. This proof-of-principle study demonstrates the feasibility of ex vivo hp ⁸³Kr magnetic resonance imaging (MRI) of lungs using natural abundance krypton gas (11.5% ⁸³Kr) and excised, but otherwise intact, rat lungs located within a custom designed ventilation chamber. Experiments comparing the ⁸³Kr MR signal intensity from lungs to that arising from a balloon with no internal structure inflated to the same volume with krypton gas mixture suggest that most of the observed signal originated from the alveoli and not merely the conducting airways. The ⁸³Kr longitudinal relaxation times in the rat lungs ranged from 0.7 to 3.7 s but were reproducible for a given lung. Although the source of these variations was not explored in this work, hp ⁸³Kr T₁ differences may ultimately lead to a novel form of MRI contrast in lungs. The currently obtained 1200-fold signal enhancement for hp ⁸³Kr at 9.4 T field strength is found to be 180 times below the theoretical upper limit.     

Near-threshold photoionization from the excited mp<Superscript>5</Superscript>(m + 1)p J = 0−3 levels of Ar,Kr and Xe (m = 3−5)

Using the configuration interaction Pauli-Fock including core polarization (CIPFCP) approach we have calculated absolute total and partial cross sections for photoionization of excited mp ⁵(m + 1)p J = 0−3 levels in Ar, Kr and Xe (m = 3−5) near threshold. Particular emphasis is paid to the lineshapes of the odd mp _1/2⁵ n(s/d)′ autoionizing resonances and their variation with the character of the intermediate excited J = 0−3 state. For selected intermediate levels of Ar and Kr the computed cross sections are compared with laser spectroscopic measurements; good agreement between the theoretical and experimental lineshapes and branching ratios for different resonances is observed.     

Lifetime measurement in 74Kr and 76Kr

Lifetimes of excited states in the ground-state bands of ⁷⁴Kr and ⁷⁶Kr were measured using the recoil-distance Doppler-shift and the differential decay curve methods. The states were populated in the ⁴⁰Ca(⁴⁰Ca, α2p) and ⁴⁰Ca(⁴⁰Ca, 4p) reactions. Gamma rays were detected with the GASP array which was coupled to the Cologne Plunger device. The results resolve discrepancies between earlier lifetime measurements and a recent Coulomb excitation experiment. Experimental transition rates are compared to theoretical calculations. The results support a strong mixing between prolate and oblate configurations for the low-spin states, and represent an important basis for the interpretation and understanding of the shape coexistence phenomenon in this mass region.     

Temperatures in 3D optical lattices influenced by neighbouring transitions

A detailed experimental study of the steady-state temperature in a 3D optical lattice for cesium has been performed for a wide range of detunings. Specifically, we have investigated the situation with the cooling and trapping light detuned far red of a ( J _g↦J _e = J _g + 1)-transition, where the blue detuned interaction with a ( J _g↦J _e = J _g)-transition can not be neglected. We find that the temperature scales with the optical potential due to the interaction with just the ( J _g↦J _e = J _g + 1)-transition. This indicates that blue Sisyphus cooling has essentially no effect on the dynamics of the system, when there exists a neighbouring red detuned transition. Received 6 June 2000 and Received in final form 26 September 2000