Analysis of decay data from neutron-rich nuclei

The β-decays of the neutron-rich nuclei ¹²Be and ²⁹Ne have been studied. The statistical correlations between the almost identical half-lives of ¹²Be and its daughter ¹²B are analysed for a large sample of ¹²Be decay data. Stringent mutual bounds are obtained on the parameter set, leading to a precise determination of the ¹²Be half-life of 21.50±0.04 ms. From a simultaneous detection of β-particles and neutrons from the decay of ²⁹Ne the neutron emission probability, P_n, is determined to 17±5%. No indication of two-neutron emission is seen from this nucleus. An upper limit of 2.2% (90% confidence level) is established for P_2n. Received: 3 May 2001 / Accepted: 6 August 2001     

Large electron polarization of H-like20Ne-ions traversing gd foils at very high velocities

It is shown that transient magnetic fields in Gd-host for²⁰Ne ions at a mean velocity of 12.5v₀(v₀=c/137), considerably beyond the Bohr velocity of 1s electrons of Ne ions (v_1s=10v₀), are dominated by the Fermi contact field of these electrons. The first excited 2⁺-state of²Ne was used as probe. The derived value for the mean degree of polarization p_1s=0.32(13), is surprisingly large, though still consistent with values obtained at lower velocities. Present calculations of spin exchange cross sections severely underestimate the observed polarization at this high velocity.The authors are grateful to the operating staff of the UNILAC for providing excellent beam conditions. They also thank Dr. P. Maier-Komor for preparing the delicate targets. Support by the BMFT is acknowledged.     

Measurement of evaporation residue cross-sections of the reaction 30Si + 238U at subbarrier energies

The reaction ³⁰Si + ²³⁸U → ²⁶⁸Sg^* was studied at beam energies close to the Coulomb barrier. At a center-of-mass energy of E _c.m. = 144.0MeV for reactions at half thickness of the target we measured three decay chains of ²⁶³Sg produced by evaporation of five neutrons. The cross-section was ( 67⁺⁶⁷ _-37) pb. At E _c.m. = 133.0MeV we measured three spontaneously fissioning nuclei which we assigned to the isotope ²⁶⁴Sg. The production cross-section was ( 10⁺¹⁰ _-6) pb and a half-life of ( 120⁺¹²⁶ _-44) ms was determined. This half-life is a factor of twenty shorter than theoretical predictions. At E _c.m. = 128.0MeV an upper cross-section limit of 15pb was measured. The cross-section data reveal a strong influence of the orientation of the deformed target nucleus on the production yield. Compared to excitation functions measured for the lighter system ¹⁶O + ²³⁸U → ²⁵⁴Fm^*, a reduction of the fusion probability was observed at low beam energies indicating increasing competition from quasifission processes.     

A new alpha-particle-emitting isotope 259Db

An isotope of the element 105 with mass number 259 has been produced via the reaction ²⁴¹Am(²²Ne, 4n)²⁵⁹Db at E _lab = 118 MeV. The reaction products were transported and collected using the helium-jet technique and the rotating wheel apparatus. The α-decays of the products and their daughter nuclides were detected by a set of Si(Au) detectors arranged ingeniously. The Z and A of the nuclide have been unambiguously identified by the genetic relationship between the new activity and the known nuclide ²⁵⁵Lr established by α-recoiled milking measurement. The new nuclide ²⁵⁹Db has a half-life of 0.51±0.16 s and decays by alpha-particle emission of E _α = 9.47 MeV. Furthermore, the nuclide ²⁵⁸Db and its daughter ²⁵⁴Lr have also been clearly observed using the same projectile-target combination. Their half-lives and α-particle energies determined in this work are in agreement with previous known data, thus also proving the reliability of our assignment of ²⁵⁹Db. Received: 7 December 2000 / Accepted: 6 February 2001     

The 198Au βhalf-life in the metal Au⋆

For the β^--decay of ¹⁹⁸Au in a Au metallic environment the half-life was observed to be longer by 0.4±0.7% at room temperature (T = 293K) and by 4.0±0.7% when the metal was cooled to T = 12K, both compared to the literature value of T _1/2 = 2.6943±0.0008d.     

Change of the 7Be electron capture half-life in metallic environments

For the electron capture of ⁷Be in the metallic environments Pd and In the ⁷Be half-life was observed to increase by 0.9±0.2 and 0.7±0.2%, respectively, while in the insulator Li₂O it was unchanged within experimental error (all samples cooled to T = 12K). The observations are consistent with the predictions of the Debye plasma model applied to the quasi-free electrons in the metals.     

One-neutron knockout from Ne isotopes

One-neutron knockout reactions of ^24–28Ne in a beryllium target have been studied in the Fragment Separator (FRS), at GSI. The results include inclusive one-neutron knockout cross-sections as well as longitudinal-momentum distributions of the knockout fragments. The ground-state structure of the neutron-rich neon isotopes was obtained from an analysis of the measured momentum distributions. The results indicate that the two heaviest isotopes, ²⁷Ne and ²⁸Ne, are dominated by a configuration in which a s_1/2$s_{1/2}$ neutron is coupled to an excited state of the ²⁶Ne and ²⁷Ne core, respectively.     

The half-life of the 1437 keV 11/2− state in97Mo

Using the (,n) reaction, the half-life of the first 11/2^– level in⁹⁷Mo has been determined as T_1/2(1437.0 keV)=2.5±0.3 ns by means of the pulsed beam method. The experimentally obtainedB(M2) value between theh _11/2 andg _7/2 configurations in⁹⁷Mo has been satisfactorily described within the quasiparticle-phonon model usingg _s ^eff =0.6g _s ^free . Similar calculations and comparisons with experminental data have been performed also for^95,99Mo as well as for⁹⁹Ru and¹⁰¹Pd.This work is partly supported by the Bulgarian National Research Foundation     

Femtosecond fiber laser based methane optical clock

An optical clock based on an Er³⁺ fiber femtosecond laser and a two-mode He–Ne/CH₄ optical frequency standard (λ=3.39 μm) is realized. Difference-frequency generation is used to down convert the 1.5-μm frequency comb of the Er³⁺ femtosecond laser to the 3.4-μm range. The generated infrared comb overlaps with the He–Ne/CH₄ laser wavelength and does not depend on the carrier–envelope offset frequency of the 1.5-μm comb. In this way a direct phase-coherent connection between the optical frequency of the He–Ne/CH₄ standard and the radio frequency pulse repetition rate of the fiber laser is established. The stability of the optical clock is measured against a commercial hydrogen maser. The measured relative instability is 1×10⁻¹² at 1 s and for averaging times less than 50 s it is determined by the microwave standard, while for longer times a drift of the He–Ne/CH₄ optical standard is dominant.     

Study of N=20 shell gap with 1H(28Ne,27,28Ne) reactions

This paper reports on the ¹H(²⁸Ne,²⁸Ne) and ¹H(²⁸Ne,²⁷Ne) reactions studied at intermediate energy using a liquid hydrogen target. From the cross section populating the first 2⁺ excited state of ²⁸Ne, and using the previously determined B(E2) value, the neutron quadrupole transition matrix element has been calculated to be M_n=13.8 ±3.7 fm². In the neutron knock-out reaction, two low-lying excited states were populated in ²⁷Ne. Only one of them can be interpreted by the sd shell model while the additional state may intrude from the fp shell. These experimental observations are consistent with the presence of fp shell configurations at low excitation energy in ^27,28Ne nuclei caused by a vanishing N=20 shell gap at Z=10.     

Nuclearg-factors of the 1229 and 2911 keV isomers in143Nd

Nuclearg-factors of the 1229 keV 13⁺/2 and 2911 keV 21⁺/2 level and the half-life of the 1229 keV level have been measured to beg(1229)=0.058(5),g(2911)=0.69(12) andT _1/2 (1229)=6.79(2) ns. The experimental g-factors show that the 1229 keV level has a dominant configuration of (f _7/23^–), and the 2911 keV level has an almost pure (f _7/27^–), configuration.The authors would like to thank the crew of CYRIC for the operation of the cyclotron.     

Rotational spectrum of propylene oxide-neon

The free jet millimetre-wave absorption spectrum of the propylene oxide-Ne complex is reported for both the ²⁰Ne and ²²Ne isotopomers. The Ne atom lies on the same side as the methyl group (syn conformer) with respect to the plane of the COC ring. From the D_J centrifugal distortion constant the dissociation energy has been estimated to be about 1.1 kJ mol^?1.     

Possible candidates for discrete gamma rays linking superdeformed and yrast states in an odd nucleus:193Pb

The francium isotopes^200–202Fr were produced in the reaction³⁵Cl+¹⁷⁰Yb using bombarding energies of 4.9–5.3 MeV/nucleon. Fusion products were separated in-flight from the primary beam using a gas-filled recoil separator. An alpha line with the alpha particle energy and half-life of (7468±9) keV and (19 ₋₆ ⁺¹³ ) ms, respectively, was assigned to²⁰⁰Fr. Previously reported decay properties of^201,202Fr were confirmed. Communicated by V. Metag     

Systematics of half-lives for proton radioactivity

Half-life measurements for both ground-state and isomeric transitions in proton radioactivity are systematized by using a semiempirical, one-parameter model based on tunneling through a potential barrier, where the centrifugal and overlapping effects are taken into account within the spherical nucleus approximation. This approach, which has been successfully applied to alpha decay cases covering ∼ 30 orders of magnitude in half-life, has shown, in addition, very adequate at fitting all existing data on partial half-life, T _1/2p , of proton emission from nuclei. Nearly 70 measured half-life values have been analysed, and the data could be described by two straight lines relating the pure Coulomb contribution to half-life with the quantity (Z _d is the atomic number of the daughter nucleus, μ₀ is the reduced mass, and Q _p is the total nuclear energy available for decay). These straight lines are shown to correspond to different degrees of deformation, namely, very prolate ( δ ≳ 0.1 , and other shaped ( δ ≲ 0.1 parent nuclei. The goodness in reproducing the data attained in the present systematics allows for half-life predictions for a few possible cases of proton radioactivity not yet experimentally accessed.     

High-resolution emission spectroscopy of the AΠ–XΣ system of AlH

The emission spectrum of the A²Π–X²Σ⁺ system of the AlH⁺ ion was investigated in the range of 27 000–29 000 cm⁻¹ by using a conventional spectroscopic technique. The AlH⁺ molecules were formed and excited in an aluminium hollow-cathode lamp with two anodes, filled with a mixture of Ne carried gas and a trace of NH₃. The emission from the discharge was observed with a plane grating spectrograph and recorded by a photomultiplier tube. The full rotational structure of the 0-0 and 1-1 bands has been observed for the first time (12 branches up to J″ = 36.5) and many new constants of the X²Σ⁺ state have been derived from the analysis. For the A²Π, v = 0 and 1 state a considerable irregularities of the Λ-doubling have been observed. The most reasonable explanation for this anomaly is an interaction with unstableness rotational levels of X²Σ⁺ state and perturbing of the A²Π state by the nearly lyingB²Σ⁺ state.     

C( ,pn) photon asymmetry for Eγ=180–340 MeV

The photon asymmetry (Σ) of the ¹²C( ,pn) reaction has been measured using linearly polarised tagged photons at the Mainz microtron MAMI for E_γ=180–340 MeV. The data have been analysed in separate missing energy (E_m) regions corresponding to the ejection of nucleons from (1p)² and (1p)(1s) shells. The measured Σ values for both E_m regions are smaller in magnitude than corresponding ²H data, but the (1p)² results have a similar E_γ dependence. Calculations of direct two-nucleon emission overestimate the magnitude of Σ at all photon energies.     

Solid-state NMR investigations of Si-29 and N-15 enriched silicon nitride

We compare ²⁹Si magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra from the two modifications of silicon nitride, α-Si₃N₄ and β-Si₃N₄, with that of a fully (²⁹Si, ¹⁵N)-enriched sample ²⁹Si₃¹⁵N₄, as well as ¹⁵N NMR spectra of Si₃¹⁵N₄ (having ²⁹Si at natural abundance) and ²⁹Si₃¹⁵N₄. We show that the ¹⁵N NMR peak-widths from the latter are dominated by J(²⁹Si–¹⁵N) through-bond interactions, leading to significantly broader NMR signals compared to those of Si₃¹⁵N₄. By fitting calculated ²⁹Si NMR spectra to experimental ones, we obtained an estimated coupling constant J(²⁹Si–¹⁵N) of 20 Hz. We provide ²⁹Si spin-lattice (T₁) relaxation data for the ²⁹Si₃¹⁵N₄ sample and chemical shift anisotropy results for the ²⁹Si site of β-Si₃N₄. Various factors potentially contributing to the ²⁹Si and ¹⁵N NMR peak-widths of the various silicon nitride specimens are discussed. We also provide powder X-ray diffraction (XRD) and mass spectrometry data of the samples.     

The β-decay of 22Al

In an experiment performed at the LISE3 facility of GANIL, we studied the decay of ²²Al produced by the fragmentation of a ³⁶Ar primary beam. A β-decay half-life of T_1/2 = 91.1±0.5ms was measured. The β-delayed one- and two-proton emission as well as β-α and β-delayed γ-decays were measured and allowed us to establish a partial decay scheme for this nucleus. New levels were determined in the daughter nucleus ²²Mg. The comparison with model calculations strongly favours a spin-parity of I^π = 4⁺ for the ground state of ²²Al.     

Excited states of Ne isoelectronic ions: SAC-CI study

Excited states of the s, p, and d symmetries up to principal quantum number n = 4 are studied for the first eight members of Ne isoelectronic sequence (Ne to Cl⁷⁺) by the SAC-CI (symmetry-adapted-cluster configuration-interaction) method. The valence STO basis sets of Clementi et al. and the optimized excited STO are used by the STO-6G expansion method. The calculated transition energies agree well with the experimental values wherever available. Received 26 July 2000 and Received in final form 14 September 2000     

Isomeric states in 214Th and 213Th

Isomeric states in ²¹⁴Th and ²¹³Th were identified by means of γ -rays measured in delayed coincidence with the implanted evaporation residues. These were produced in irradiations of ¹⁶⁴Dy with ⁵⁴Cr projectiles and separated in-flight by the velocity filter SHIP. An isomeric state of I ^π = 8⁺ with a half-life of (1.24±0.12) μs was identified in ²¹⁴Th . The configuration π[1h _9/2 ⊗ 2f _7/2] was assigned to this state. An isomeric state with a half-life of (1.4±0.4) μs was observed in ²¹³Th . Tentatively it was assigned to an I ^π = 13/2⁺ state.