Xe Nuclei Research Articles

Fast timing: Lifetime measurements with LaBr3 scintillators

In order to perform half-life measurements at Alto, we developed a fast-timing setup using fast scintillators : LaBr3. The main components of our setup and their characteristics (timing properties) are presented. The resulting time resolution, obtained after optimization, and measured off-line with radioactive sources is discussed. This set-up was tested on-line by measuring using the slope method half-lives of 137–139Cs levels fed by the β− decay of 137–139Xe. The neutron-rich Xe nuclei were produced by 238U photofission using, at low intensity, the electron beam and the line of the future facility ALTO. Our results show the ability of such a fast-timing system to measure half-lives lower than the total time resolution. Off-line and on-line results are discussed.

Evolution of the mixed-symmetry 21,ms+ quadrupole-phonon excitation from spherical to γ-soft Xe nuclei

Evolution of the mixed-symmetry 21,ms+ quadrupole-phonon excitation from spherical to γ-soft Xe nuclei

Study of the Xe–NH3 van der Waals complex: High-resolution microwave spectra and ab initio calculations

An ab initio potential energy surface of the Xe-NH(3) van der Waals complex was constructed at the coupled cluster level of theory with single, double, and pertubatively included triple excitations. The small-core pseudopotential and augmented correlation-consistent polarized valence quadruple-zeta basis set was used for the Xe atom and Dunning's augmented correlation-consistent polarized valence triple-zeta basis set for the other atoms. The basis sets were supplemented with midbond functions. Rotational spectra of the Xe-NH(3) van der Waals complex were recorded using a pulsed-nozzle Fourier transform microwave spectrometer. Rotational transitions within two internal rotor states, namely, the Sigma0(0) and Pi1(1) (lower) states, were measured and assigned to the Xe-(14)NH(3) and Xe-(15)NH(3) isotopologues. For the deuterated isotopologues, only the Sigma0(0) states were observed. Two inversion components were observed for each state except for the "s" component of the Sigma0(0) state of the Xe-(14)NH(3) and Xe-(15)NH(3) isotopologues, which has a spin statistical weight of zero. Nuclear quadrupole hyperfine structures arising from the (14)N (nuclear spin angular momentum quantum number I=1) and (131)Xe (I=32) nuclei were detected and analyzed. The observed spectra suggest that the Pi1(1) (lower) state has lower energy than the unobserved Sigma1(1) state, in contrast to the case of Ar-NH(3).

INFLUENCE OF ENTRANCE-CHANNEL SHELL EFFECTS ON FUSION HINDRANCE

A study is presented to what extent a striking difference in the evaporation-residue cross sections for two reactions, 86 Kr + 130 Xe and 86 Kr + 136 Xe , originates from different shell effects in 130 Xe and 136 Xe nuclei. A classical dynamical model with one-body dissipation was used to calculate the injection point to the "diffusion" stage of the Fusion-by-Diffusion model of Światecki, Siwek-Wilczyńska and Wilczyński. Only a factor of 6 in the ratio of the fusion hindrance factors in these two reactions can be associated with the entrance-channel shell effects.

Coulomb excitation of neutron-rich 138,140,142Xe at REX-ISOLDE

We report on “safe” Coulomb excitation of neutron-rich 138,140,142Xe nuclei. The radioactive nuclei have been produced by ISOLDE at CERN and post-accelerated by the REX-ISOLDE facility. The γ-rays emitted by the decay of excited states have been detected by the MINIBALL array. Recent results are presented.

Evidence of rigid triaxiality in some xenon nuclei

The present study deals with a topic of current interest, getting clear evidence for triaxiality in the 120–130 Xe nuclei near A = 130. The odd–even staggering in a so-called gamma-vibrational band may be treated using the asymmetric rotor model (ARM) on employing the Lipas parameter. The Hilbert space is too limited to describe the full variation of the moment of inertia and, therefore, the Lipas Ansatz ismade to correct the ARM energies, which conserve the good result for B(E2) values.PACS Nos.: 21.10.Re, 21.60.Ev, 27.60.+j

Further evidence for formation of a narrow baryon resonance with positive strangeness in K + collisions with Xe nuclei

We have continued our investigation of the charge-exchange reaction K^+ Xe --> K^0 p Xe' in the bubble chamber DIANA. In agreement with our previous results based on part of the present statistics, formation of a narrow p K^0 resonance with mass of 1537+-2 MeV/c^2 is observed in the elementary transition K^+ n --> K^0 p on a neutron bound in the Xenon nucleus. Visible width of the peak is consistent with being entirely due to instrumental resolution and allows to place an upper limit on its intrinsic width: \Gamma < 9 MeV/c^2. A more precise estimate of the resonance intrinsic width, \Gamma = 0.36+-0.11 MeV/c^2, is obtained from the ratio between the numbers of resonant and non-resonant charge-exchange events. The signal is observed in a restricted interval of incident K^+ momentum, that is consistent with smearing of a narrow p K^0 resonance by Fermi motion of the target neutron. Statistical significance of the signal is some 7.3, 5.3, and 4.3 standard deviations for the estimators S/sqrt{B}, S/sqrt{S+B}, and S/sqrt{S+2B}, respectively. This observation confirms and reinforces our earlier results, and offers strong evidence for formation of a pentaquark baryon with positive strangeness in the charge-exchange reaction K^+ n --> K^0 p on a bound neutron.

The XENON dark matter search experiment

The XENON experiment aims at the direct detection of dark matter in the form of Weakly Interacting Massive Particles (WIMPs) via their elastic scattering off Xe nuclei. A fiducial mass of 1000 kg, distributed in 10 independent liquid xenon time projection chambers will be used to probe the lowest interaction cross-section predicted by SUSY models. The TPCs are operated in dual (liquid/gas) phase, to allow a measurement of nuclear recoils down to 16 keV energy, via simultaneous detection of the ionization, through secondary scintillation in the gas, and primary scintillation in the liquid. The distinct ratio of primary to secondary scintillation for nuclear recoils from WIMPs (or neutrons), and for electron recoils from background, is key to the event-by-event discrimination capability of XENON. A dual phase xenon prototype has been realized and is currently being tested, along with other prototypes dedicated to other measurements relevant to the XENON program. As part of the R&D phase, we will realize and move underground a first XENON module (XENON10) with at least 10 kg fiducial mass to measure the background rejection capability and to optimize the conditions for continuous and stable detector operation underground. We present some of the results from the on-going R&D and summarize the expected performance of the 10 kg experiment, from Monte-Carlo simulations. The main design features of the 100 kg detector unit(XENON100), with which we envisage to make up the 1 ton sensitive mass of XENON1T will also be presented.

Hadron formation and attenuation in deep inelastic lepton scattering off nuclei

We investigate hadron formation in deep inelastic lepton scattering on N, Kr and Xe nuclei in the kinematic regime of the HERMES experiment. The elementary electron–nucleon interaction is described within the event generator PYTHIA while a full coupled-channel treatment of the final state interactions is included by means of a BUU transport model. We find a good agreement with the measured charged hadron multiplicity ratio RMh for N and Kr targets by accounting for the deceleration and absorption of the primarily produced particles as well as for the creation of secondary hadrons in the final state interactions.

βdecay of odd-ACs isotopes in the interacting boson-fermion model

The structure of odd-mass isotopes of Cs and Xe is described in the framework of the proton-neutron interacting boson-fermion model. The model provides a consistent description of $\ensuremath{\beta}$ decay from Cs to Xe nuclei of mass number $A=125,$ 127, and 129.

Observation of a baryon resonance with positive strangeness in K + collisions with Xe nuclei

The status of our investigation of low-energy K +Xe collisions in the xenon bubble chamber DIANA is reported. In the charge-exchange reaction K +Xe → K 0 pXe′, the spectrum of K 0 p effective mass shows a resonant enhancement with M=1539±2 MeV/c 2 and Γ≤9 MeV/c 2. The statistical significance of the enhancement is near 4.4σ. The mass and width of the observed resonance are consistent with expectations for the lightest member of the antidecuplet of exotic pentaquark baryons, as predicted in the framework of the chiral soliton model.

Recoil distance lifetime measurements in118Xe

Lifetimes of the excited states of the ground state band in ${}^{118}\mathrm{Xe}$ are newly measured using the recoil-distance Doppler-shift technique. The reaction ${}^{93}\mathrm{Nb}{(}^{29}\mathrm{Si}{,p3n)}^{118}\mathrm{Xe}$ at a beam energy of 135 MeV was used for this experiment. The lifetimes of the ${2}^{+},$ ${4}^{+},$ ${6}^{+},$ ${8}^{+},$ and ${10}^{+}$ states of the ground state band were extracted using the computer code LIFETIME which includes the corrections due to the side feeding and the nuclear deorientation effects. The present $B(E2)$ values are in good agreement with the extracted $B(E2)$ values from the Hartee-Fock-Bogoliubov calculations. The measured $B(E2)$ values are also compared with the standard algebraic and the geometrical models. The $B(E2)$ values for the ${2}^{+}$ state for this nucleus and the other Xe nuclei as a function of the neutron number are well reproduced in the framework of the algebraic model IBA-1 with O(6) symmetry and the geometrical finite range droplet model.

Nucleon-pair shell model calculations of the even–even Xe and Ba nuclei

The nucleon-pair shell model, truncated to an SD subspace, is applied to 128–134Xe and 130–136Ba. The spectra, eigenstates, and electromagnetic transition strengths of these nuclei, which are known to display O(6) symmetry, are described in terms of a three-parameter Hamiltonian. Agreement with experiment is good, in particular, mixed-symmetry configurations and the upward shift and fragmentation of the M1 strength with increasing neutron numbers emerges naturally. The results show that the SD subspace approximation improves with the number of pairs.

Exploring Surfaces and Cavities in Lipoxygenase and Other Proteins by Hyperpolarized Xenon-129 NMR

This paper presents an exploratory study of the binding interactions of xenon with the surface of several different proteins in the solution and solid states using both conventional and hyperpolarized (129)Xe NMR. The generation of hyperpolarized (129)Xe by spin exchange optical pumping affords an enhancement by 3-4 orders of magnitude of its NMR signal. As a result, it is possible to observe Xe directly bound to the surface of micromolar quantities of lyophilized protein. The highly sensitive nature of the (129)Xe line shape and chemical shift are used as indicators for the conditions most likely to yield maximal dipolar contact between (129)Xe nuclei and nuclear spins situated on the protein. This is an intermediate step toward achieving the ultimate goal of NMR enhancement of the binding-site nuclei by polarization transfer from hyperpolarized (129)Xe. The hyperpolarized (129)Xe spectra resulting from exposure of four different proteins in the lyophilized, powdered form have been examined for evidence of binding. Each of the proteins, namely, metmyoglobin, methemoglobin, hen egg white lysozyme, and soybean lipoxygenase, yielded a distinctly different NMR line shape. With the exception of lysozyme, the proteins all possess a paramagnetic iron center which can be expected to rapidly relax the (129)Xe and produce a net shift in its resonance position if the noble gas atom occupies specific binding sites near the iron. At temperatures from 223 to 183 K, NMR signals were observed in the 0-40 ppm chemical shift range, relative to Xe in the gas phase. The signals broadened and shifted downfield as the temperature was reduced, indicating that Xe is exchanging between the gas phase and internal or external binding sites of the proteins. Additionally, conventional (129)Xe NMR studies of metmyoglobin and lipoxygenase in the solution state are presented. The temperature dependence of the chemical shift and line shape indicate exchange of Xe between adsorption sites on lipoxygenase and Xe in the solvent on the slow to intermediate exchange time scale. The NMR results are compared with N(2), Xe, and CH(4) gas adsorption isotherms. It is found that lipoxygenase is unique among the proteins studied in possessing a relatively high affinity for gas molecules, and in addition, demonstrating the most clearly resolved adsorbed (129)Xe NMR peak in the lyophilized state.

High spin structures of 122Xe

High — spin states in 122Xe have been investigated by γ − γ coincidence measurements using the EUROGAM array. The reaction 96Zr(30Si,4n)122Xe at a beam energy of 135 MeV was used to populate states of 122Xe. The level scheme has been extended up to ∼ 13.5 MeV and also enriched as compared to previous studies. New structures of competing dipole to quadrupole transitions have been observed. Experimental B(M1)/B(E2) ratios have been extracted for these structures and compared with theoretical estimates. They have also been compared with similar structures in Xe, Ba and Ce nuclei.

Search for octupole deformation in neutron-rich Xe isotopes

A search for octupole deformation in neutron-rich Xe isotopes has been conducted through prompt gammaray spectroscopy of secondary fragments produced in the spontaneous fission of248Cm. The spectrometer consisted of the Eurogam 1 array and a set of 5 LEPS detectors. Level schemes were constructed for Xe isotopes with mass number ranging from 140 to 144 and excited states for143,144Xe nuclei were observed for the first time. None of the level schemes exhibit an alternating parity quasimolecular band, a feature usually expected in nuclei in which octupole correlation effects are strong enough to produce stable octupole deformation. For several isotopes, structures observed in the level schemes are consistent with an octupole softness of the nuclei.

Effective gamma deformation near A=130 in the interacting boson model.

{open_quote}{open_quote}Effective{close_quote}{close_quote} {gamma}-deformation parameters are derived for even-even Xe, Ba, and Ce nuclei from the matrix elements of ({ital Q}{times}{ital Q}){sub 00} and ({ital Q}{times}{ital Q}{times}{ital Q}){sub 00}. Interacting boson model calculations are performed with the quadrupole operator {ital Q} determined from the {ital E}2 branching ratios of the 2{sub 2}{sup +} decay and compared to results obtained with the rigid triaxial rotor model. {copyright} {ital 1996 The American Physical Society.}

Suppressing axial oscillations of the energy distribution in a VVÉR-1000 reactor without half-length control rods

Nonequilibrium redistribution of the concentration of {sup 135}Xe nuclei in the active zone of the VVER-1000 reactor may lead to xenon oscillations, that is, relatively powerful axial oscillations of the energy distribution, the prevention and suppression of which is one of the goals of reactor control. Xenon oscillations are described by means of the axial offset (AO); this quantity, which characterizes the nonuniformity of the energy distribution, expresses the difference in the rates of energy liberation in the upper and lower halves of the active zone as a percentage of the total rate of energy liberation. In the ascending phase of the xenon oscillations, energy liberation is redistributed from the lower to the upper half of the active zone; the offset increases. In the descending phase, conversely, energy liberation is redistributed from the upper to the lower half, and the offset decreases.

Tilted four-quasiparticle bands in even-even Xe and Ba nuclei

Rotational band structures with distinct properties found in several even-even Xe and Ba nuclei are investigated by means of the conventional cranked shell model and by the newly developed tilted axis cranking model. Analysing the band energies and electromagnetic branching ratios a nearly oblate configuration ( π h 11 2 d 5 2 ) ( ν h 11 2 2 ) with a tilted i.e. non-principal axis spin orientation is found to give a fair description of the experimental features.

New insights from studies of spontaneous fission with large detector arrays

The new insights into a) the rapidly varying structures of neutron-rich nuclei up to spins as high as 20 + and 19 −, b) the spontaneous fission process itself, and c) modes of cluster radioactivity that have come out of studies of spontaneous fission of 242Pu, 248Cm and 252Cf with large detector arrays are presented. The studies include γ-γ-γ coincidences with the second generation (14–20 Compton-suppressed Ge arrays) and new, third generation (35–45 Compton-suppressed Ge arrays, Gammasphere and Eurogam) gamma-ray detector arrays. The neutron-rich nuclei observed span the full range from superdeformed ( β 2 ≥ 0.4) double-magic ground states to spherical double-magic nuclei and competing shapes in between. New structure insights include the following: New regions of identical ground state bands to spins of 10 + to 16 + were discovered; one at the sudden onset of large deformation at N=60,62 in both even- and odd-A 98–101Sr, one at midshell N = 64,66 in 108,110Ru, one for N = 88–90 144,146Ba, one for N = 92–94 152,154Nd, and at high spin in 156Nd and also in excited bands in these regions. New shape coexisting structures in both even- and odd-A for A = 96–102 Sr and Zr nuclei are found. Evidence is found in both even-even and odd-A nuclei for a new region of octupole deformation around Z = 56 and N = 86–88. Rapid changes in moments of inertia are observed with changes in Z and N of two units with sudden changes in the moments of inertia associated with band crossings. The levels in many neutron-rich nuclei are observed for the first time. Earlier, plunger studies yielded lifetimes of low spin states and recently the Doppler Shift Line Shape Analysis yielded lifetimes of high spin states. For the first time, direct measurements of yields and neutron multiplicities have been made for five correlated pairs of fission fragments of SrNd, ZrCe, MoBa, RuXe and PdTe nuclei. Neutron multiplicities from 0–10 v emission (10v for the first time) were observed in MoBa correlated pairs with the 0 and 7–10 neutron-emission yields enhanced compared to gross yields for all fragments. The MoBa data provide evidence for two different fission modes. By unfolding the observed MoBa yields, the masses and excitation energies and mass distributions at scission were extracted. These data revealed a new mode involving the high neutron multiplicities that occurs essentially through one pair, 108Mo 144Ba, 107Mo 145Ba, or 106Mo 146Ba or some combination where the 144Ba, 145Ba and/or 146Ba at scission are hyperdeformed with a long-to-short axis ratio of 3:1. The zero-neutron emission channels provide new examples of the cold rearrangements of nucleons in a new type of cluster radioactivity. The cluster radioactivity model predicted the observed enhancements of the zero-neutron channels for odd-A-odd-A nuclei as well as the observed fine structure, that is, the spin distributions of the two fragments. The measurements of intensities and γ-γ-γ coincidences in SF with large detector arrays open a new era in the determination of previously inaccessible properties of neutron-rich nuclei and the fission process.