K isomers in 254No: probing single-particle energies and pairing strengths in the heaviest nuclei

We have identified two isomers in 254No, built on two- and four-quasiparticle excitations, with quantum numbers K pi = 8- and (14+), as well as a low-energy 2-quasiparticle Kpi = 3+ state. The occurrence of isomers establishes that K is a good quantum number and therefore that the nucleus has an axial prolate shape. The 2-quasiparticle states probe the energies of the proton levels that govern the stability of superheavy nuclei, test 2-quasiparticle energies from theory, and thereby check their predictions of magic gaps.     

Decay of isomers stimulated by laser radiation

A theory is presented for the induced decay of isomer levels for nuclei exposed to a laser field, which couples the initial isomer level to intermediate higher lying levels with short lifetimes. We took into account the electric dipole transitions between these levels in a one-photon approximation. The time-dependent Schrödinger equation with a periodic Hamiltonian is solved by the method of the composite Hilbert space. A simple expression for the broadening of the isomer level ΔΓ, caused by the laser radiation, is derived. As an example, we considered the decay of the isomer level 970.17 keV with the spin I ^π = 23/2^? of ¹⁷⁷Lu, coupled by the laser wave to the virtual level 1352.33 keV with I ^π = 21/2⁺. The ratio of the broadening ΔΓ to the natural width of the isomer level Γ is found to be about 40% for a 10¹⁷ W/cm² laser. Any influence of the electronic environment is not taken into account.     

Evidence for non-yrast states in 254No

Evidence for the decay of non-yrast states in ²⁵⁴No has been observed for the first time in an experiment performed at the University of Jyv?skyl?. The experiment employed the JUROGAM array of germanium detectors coupled to the gas-filled recoil separator RITU and the focal-plane spectrometer GREAT. The ground-state rotational band has been tentatively extended up to a spin of 24ℏ and has a smoothly behaving dynamical moment of inertia. It is speculated that the observation of high-energy γ-rays is due to the decay of a K = 3 band-head state.     

Decay of high-spin isomers in Os nuclei by barrier penetration

A decay scheme for the 130±20 ns high-spin isomer in ¹⁸²Os has been established. The excitation energy of the isomer is 7049±1 keV and it has I^π = 25⁽⁺⁾. A 2.4% decay directly to the yrast 24⁺ level at 5988 keV is observed. In ¹⁸⁴Os a 20±5 ns isomer is observed at 2366±1 keV excitation energy with I^π = 10⁺. Again, direct transitions into the yrast 8⁺ and 10⁺ levels are observed. Contrary to previous speculations, there is no compelling evidence for stable triaxial shapes in the structure of the levels through which the isomers decay. The abnormally short half-lives observed, as well as the unusual decay patterns, are best understood in terms of a γ-soft nuclear potential. Motion in the γ-direction allows the isomer to decay via barrier penetration from an axially symmetric prolate shape with the angular momentum along the nuclear symmetry axis (deformation aligned state) via oblate shapes to another prolate shape with the angular momentum perpendicular to the nuclear symmetry axis (rotation-aligned state).     

In-beam spectroscopy of 253, 254No

In-beam conversion electron spectroscopy experiments have been performed on the transfermium nuclei ^{253, 254}No using the conversion electron spectrometer SACRED in nearly collinear geometry in conjunction with the gas-filled separator RITU at the University of Jyv?skyl?. The experimental setup is discussed and the spectra are compared to Monte Carlo simulations. The implications for the ground-state configuration of ²⁵³No are discussed. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: rdh@ns.ph.liv.ac.uk RID="b" ID="b"Present address: GANIL, F-14021 Caen, France. RID="c" ID="c"Permanent address: IReS Strasbourg, IN2P3-CNRS, F-67037-Strasbourg, France. RID="d" ID="d"Present address: CEA/DIF DCRE/SDE/LDN F-91680 Bruyeres-le-Chatel. RID="e" ID="e"Present address: Daresbury Laboratory, Daresbury WA4 4AD, UK. RID="f" ID="f"Permanent address: IPN Lyon, IN2P3-CNRS, F-69037 Lyon, France.     

High-K multi-particle bands and pairing reduction in 254No

The multi-particle states and rotational properties of the two-particle bands in 254No are investigated by the cranked shell model with pairing correlations treated by the particle number conserving method.The rotational bands on top of the two-particle K^π=3^+,8^− and 10^+ states and the pairing reduction are studied theoretically in 254No for the first time.The experimental excitation energies and moments of inertia of the multi-particle states are reproduced well by the calculations.Better agreement with the data is achieved by including the high-order deformation ε6,which leads to enlarged Z=100 and N=152 deformed shell gaps.An increase of J¹ in these two-particle bands compared with the ground state band is attributed to the pairing reduction due to the Pauli blocking effect.     

Conversion electron cascades in 254(102)No

The spectrum of prompt conversion electrons emitted by excited 254No nuclei has been measured, revealing discrete lines arising from transitions within the ground state band. A striking feature is a broad distribution that peaks near 100 keV and comprises high multiplicity electron cascades, probably originating from M1 transitions within rotational bands built on high K states.     

Decay of the 55 min and 12 min Te isomers

Gamma rays emitted in the decay of the ¹³³Te isomers, 55.4 min ^133mTe and 12.5 min ^133gTe, have been observed with Ge(Li) detectors. Sources were prepared by separating Sb from fission products, allowing several minutes for decay of 2.7 min ¹³³Sb and then separating the Te daughter activities. A total of 29 γ-rays between 312 and 2541 keV were attributed to decay of ^133gTe and 30 from 74 to 2050 keV to that of ^133mTe. Gamma-gamma coincidences in the decay of ^133gTe were observed with a NaI(Tl)---Ge(Li) set-up. The decay scheme of ^133gTe was constructed, involving placement of 25 of the γ-rays emitted. The decay of ^133gTe populates levels in ¹³³I at 0, 312, 720, 787, 1313, 1333, 1374, 1565, 1718, 2137, 2194 and 2541 keV. The decay of ^133mTe is very complicated, and we have not attempted construction of a complete decay scheme for it; however, levels at 913 and 1561 keV in ¹³³I are definitely populated in ^133mTe decay. The value for Q_β of ¹³³Te was measured as 3.52±0.10 MeV. The low-lying levels of the odd-mass iodine isotopes vary smoothly through the series ¹²⁵I through ¹³³I. The levels of ¹³³I are fitted rather well by predictions based on quasi-particle-phonon coupling.     

Decay properties of the 19/2− isomers in113,117Sb

By means of the generalized centroid-shift method, the following half-lives were determined:T _1/2(3045 keV)=3.7±0.3 ns in¹¹³Sb using the reaction¹⁰⁴Pd(¹²C,p2n) as well asT _1/2 (1322.8 keV)=3.5±0.3 ns,T _1/2(2779.8 keV) =0.50±0.15ns,T _1/2(2874.9 keV)<0.2 ns andT _1/2(3072.9 keV)<0.1 ns in¹¹⁷Sb using the¹¹⁶In(α, 2n) reaction. Three-quasiparticle configurations of the type πd _5/2ν(h_11/2 d _3/2) in^113,117Sb are found to dominate in the wave functions of the 19/2^? states at 3045 and 2780 keV in¹¹³Sb and¹¹⁷Sb, respectively. TheB(E2, 19/2^? → 15/2^?) values in^113–119Sb are discussed.     

Decay studies of131Ba

Bombarding a⁸⁵Rb target by 27 MeVα-particles the following half-lives of excited states in different nuclei have been determined by means of the generalized controid shift method: in⁸⁷Y,T _1/2(2827.1 keV)=0.75±0.10 ns,T _1/2(2675.9 keV)=0.25±0.10 ns,T _1/2(3595.3 keV)=0.5±0.2 ns andT _1/2(2986.9, 3094.4 keV)<0.1 ns; in⁸⁸Y,T _1/2(232.1 keV)=0.8±0.1 ns and in⁸⁵Rb,T _1/2(151.2 keV)=0.6±0.1 ns. In⁸⁷Y, the members of the doublet (vg ₉ ^2/?2 )₈₊ (πp_1/2) are identified. E2 transition rates 21/2⁺? 17/2⁺ in odd A≈ 90 nuclei are shown to represent essentially core transitions. Evidence for the existence of seniority-five configurations is found.     

Decay studies for neutrino physics

Both ¹⁰⁰Mo and ¹¹⁶Cd are zero-neutrino double-β-decay candidates with J ^π ?=?0^?+? and the ground states of the respective intermediate nuclei, ¹⁰⁰Tc and ¹¹⁶In, have J ^π ?=?1^?+?. This makes it possible to measure these matrix elements, which turn out to provide a valuable benchmark for testing models. The electron-capture (EC) decay of ¹⁰⁰Tc can also be used to directly determine the efficiency of a potential neutrino detector. However, the experimental determinations of the EC decay branches of interest are challenging because they are only small fractions of the main decays and co-produced radioactivities contribute significant background counts in the regions of interest. This article summarizes our work at Jyväskylä, where we have taken advantage of the high-purity beams produced at IGISOL in combination with JYFLTRAP to achieve significant improvements in the determinations of the EC branches of ¹⁰⁰Tc and ¹¹⁶In.     

Decay heat studies for nuclear energy

The energy associated with the decay of fission products plays an important role in the estimation of the amount of heat released by nuclear fuel in reactors. In this article we present results of the study of the beta decay of some refractory isotopes that were considered important contributors to the decay heat in reactors. The measurements were performed at the IGISOL facility of the University of Jyväskylä, Finland. In these studies we have combined for the first time a Penning trap (JYFLTRAP), which was used as a high resolution isobaric separator, with a total absorption spectrometer. The results of the measurements as well as their consequences for decay heat summation calculations are discussed.     

Determination of the partial electron capture- and spontaneous-fission half-lives of254No

The isotope²⁵⁴No was produced in the fusion reaction⁴⁸Ca +²⁰⁸Pb. Using the velocity filter SHIP and radiochemical techniques it was found that the nuclide²⁵⁴No with a half-life of 55 s decays byα, EC, and spontaneous-fission. Deduced partial half-lives are (61±2) s forα-decay, (550 _?160 ⁺³⁷⁰ ) s for EC and [2.2 _?1.0 ^+2.0 ]×10⁴ s for spontaneous fission.     

DSC studies of neohexanol and its isomers

DSC method has been used to establish a solid state polymorphism of four dimethyl butanols having CH₃ side groups: 2,2-dimethyl-1-butanol [CH₃CH₂C(CH₃)₂CH₂OH)], 3,3-dimethyl-1-butanol [(CH₃)₃CCH₂CH₂OH], 3,3-dimethyl-2-butanol [(CH₃)₃CCH(OH)CH₃] and 2,3-dimethyl-2-butanol [(CH₃)₂CHCOH(CH₃)₂]. Three isomers appear to be glass formers. Glass of liquid phase has been observed for 3,3-DM-1-B while glass of plastic phase for 2,2-DM-1-B and 3,3-DM-2-B. For 2,3-DM-2-B on cooling only crystallization has been found. Influence of location of the OH group in molecule on polymorphism is discussed. IR-spectra, measured in the liquid phase, have revealed hydrogen bonds in all isomers.     

Hartree-Fock-Bogoliubov calculations of the rotational band of the very heavy 254No nucleus

We report on Hartree-Fock-Bogoliubov (HFB) calculations of the ground-state rotational band of the heavy nucleus ²⁵⁴No recently observed experimentally. The calculated quadrupole deformation is consistent with the experimental value of β = 0.27 and is almost constant over the whole band. We also reproduce fairly well the excitation spectra and moments of inertia of this isotope up to the maximal experimentally observed state of spin 20. The rather high stability of this nucleus against fission is illustrated by the deformation energy curve providing very high fission barriers at zero spin within the HFB and HFB plus Lipkin-Nogami formalisms. The variation of these barriers with increased angular velocities is also studied. Received: 23 November 2000 / Accepted: 24 October 2001     

Magnetic moments of K isomers as indicators of octupole collectivity

The relation between the quadrupole-octupole deformation and the structure of high-K isomers in heavy even-even nuclei is studied through a reflection asymmetric deformed shell model including a BCS procedure with constant pairing interaction. Two-quasiparticle states with K ^?? = 4^?, 5^?, 6^?, 6⁺ and 7^? are considered in the region of actinide nuclei (U, Pu and Cm) and rare-earth nuclei (Nd, Sm and Gd). The behaviour of two-quasiparticle energies and magnetic dipole moments of these configurations is examined over a wide range in the plane of quadrupole and octupole deformations (?? ₂ and ?? ₃. In all considered actinide nuclei, the calculations show that there is pronounced sensitivity of the magnetic moments to the octupole deformation. In the rare-earth nuclei, the calculations for ^{154, 156}Gd show stronger sensitivity of the magnetic moment to the octupole deformation than in the other considered cases.     

Identification of a K isomer in 252No

A K isomer with a half-life of (110±10) ms at an excitation energy of 1254keV has been identified in the nucleus ²⁵²No . The isomer is interpreted as a two-quasineutron state with K ^π = 8^- . The isotope ²⁵²No was produced in the fusion-evaporation reaction ²⁰⁶Pb(⁴⁸Ca, 2n)²⁵²No . The experiment has been performed at the velocity filter SHIP of GSI Darmstadt.     

Decay of 1+ states as a new probe of the structure of 0+ shape isomers

The nuclides 98Mo and 100Mo have been studied in photon-scattering experiments by using bremsstrahlung produced from electron beams with kinetic energies from 3.2 to 3.8 MeV. Six electromagnetic dipole transitions in 98Mo and 19 in 100Mo were observed for the first time in the energy range from 2 to 4 MeV. A specific feature in the two nuclides is the de-excitation of one state with spin J = 1 to the 0+ ground state as well as to the first excited 0+ state, which cannot be explained in standard models. We present a model that allows us to deduce the mixing coefficients for the two 0+ shape-isomeric states from the experimental ratio of the transition strengths from the J = 1 state to the 0+ ground state and to the 0+ excited state.     

Decay studies of101m Rh and101g Rh

The level structure of¹⁰¹Ru has been investigated from the decays of^101m Rh and^101g Rh. For the gamma-ray measurements Ge(Li) detectors with effective volumes of 24 and 43 cm³ were used for singles and coincidence work, and several new transitions were observed. Conversion electron spectra from the^101m Rh decay were investigated with a 50 cm double-focussing beta spectrometer.K-conversion coefficients were determined by combining internal conversion and gamma-ray intensities. For the stronger transitions probable multipolarity assignments have been obtained from these coefficients and also fromK/L andL-subshell ratios. All transitions observed were fitted into a decay scheme, in which tentative spin-parity assignments of all states are given.