On-line separation of163Ta and162Ta

A new 12s activity has been produced via the³⁵C1+¹¹²Sn reaction at 191 MeV and ascribed to the¹⁴³Tb EC/β⁺ decay. A decay scheme is derived from γ-γ and γ-X-ray coincidence measurements.     

NMR-on of182Ta and183Ta in Fe

Nuclear magnetic resonance has been observed on radioactive¹⁸²Ta and¹⁸³Ta oriented at low temperature in an Fe host, by detection of the change in spatial anisotropy of γ-rays emitted during nuclear decay. By measuring the resonant frequencies of¹⁸³Ta in four different applied magnetic fields the nuclear magnetic moment and hyperfine field have been deduced. These are: $$|\mu \left( {{}^{183}Ta; I = \tfrac{\user2{7}}{\user2{2}}} \right)| = 2.28(3)\mu _{\rm N} and B_{hf} \left( {Ta\underline {Fe} at 0 K} \right) = - 67.2(1.3)T$$ . The spin of the ground state of¹⁸²Ta has been determined asI=3 by comparing resonance results with those obtained in a thermal equilibrium nuclear orientation study. The ratio of the resonant frequencies observed for¹⁸²Ta and¹⁸³Ta at one applied field value yields a magnetic moment for the former of $$|\mu \left( {{}^{182}Ta; I = \user2{3}} \right)| = 2.91(3)\mu _{\rm N} $$ . The spin lattice relaxation time for¹⁸³TaFe (0.12 at% Ta) at 18 mK in an applied field of 0.5 T has been found to be 40(10) s.     

Ta/Ni/Ta multilayered ohmic contacts on n-type SiC

The interface formation, electrical properties and the surface morphology of multilayered Ta/Ni/Ta/SiC contacts were reported in this study. It was found that the conducting behavior of the contacts so fabricated is much dependent on the metal layer thickness and the subsequent annealing temperature. Auger electron spectroscopy (AES) and X-ray diffraction analyses revealed that Ni₂Si and TaC formed as a result of the annealing. The Ni atoms diffused downward to metal/SiC interface and converted into Ni₂Si layer in adjacent to the SiC substrate. The released carbon atoms reacted with Ta atoms to form TaC layer. Ohmic contacts with specific contact resistivity as low as 3 × 10⁻⁴ Ω cm² have been achieved after thermal annealing. The formation of carbon vacancies at the Ni₂Si/SiC interface, probably created by dissociation of SiC and formation of TaC during thermal annealing, should be responsible for the ohmic formation of the annealed Ta/Ni/Ta contacts. The addition of Ta into the Ni metallization scheme to n-SiC restricted the accumulation of carbon atoms left behind during Ni₂Si formation, improving the electrical and microstructure properties.     

Intrinsic states and rotational bands in 176Ta and 178Ta

High-spin states in the deformed, doubly odd nuclei, ¹⁷⁶Ta and ¹⁷⁸Ta have been studied by time-correlated γ-ray and electron spectroscopy techniques following (HI,xn) reactions. The new results independently confirm and extend in spin the previously known two- and four-quasiparticle states, albeit with some differences. Many high-seniority structures, most of them with associated rotational bands, have been identified for the first time. Several high-K isomers with half-lives ranging from a few nanoseconds up to hundreds of milliseconds have been discovered. Observation of rotational bands, built upon the intrinsic states has allowed characterization of the configurations, through both the in-band decay properties and alignments. The excitation energies of the observed multi-quasiparticle structures are generally reproduced using calculations based on the Lipkin-Nogami pairing model with inclusion of the effects of blocking and empirical nucleon-nucleon interactions. Some discrepancies in the energies, such as for the 14⁻ and 15⁻ isomers in ¹⁷⁶Ta and ¹⁷⁸Ta, respectively, are attributed to specific configuration mixing. Several anomalously fast K-forbidden transitions are discussed.     

Orientation of the oxygen-induced non-axially symmetric electric field gradient at181Ta in Ta

The interaction of dilute¹⁸¹Ta in Ta with interstitial oxygen has been investigated via the γ-γ TDPAC-technique applied to the 482 keV state in¹⁸¹Ta. The trapping of Oxygen leading to a quadrupole interaction frequency ν_Q=580(5) MHz with an asymmetry parameter of η-0.37 (1) has been observed after melting the radioactive parent isotope¹⁸¹Hf with Ta. The temperature dependence of the quadrupole interaction frequency between 17 K and 293 K was found to be very weak whereas η varied by about 10%. In a single crystal experiment the orientation of the three principal axes of the electric field gradient leading to ν_Q was determined. The V_zz-axis points in <110>, the V_yy-axis in <100> and the V_xx-axis in <110> direction. These results can be understood in the charge cloud model, suggested by Wrede et al. /1/ for a similar situation found in the HfNb system.     

Rotational bands in 181Ta

High-spin states in ¹⁸¹Ta have been studied via the ¹⁷⁶Yb(¹¹B,α2n) reaction at 52 MeV using the PEX array and at 57 MeV using the NORDBALL array, with α-particle detection. The previously known, K^π=(7/2)⁺ ground state band and K^π=(9/2)⁻ band have been extended to spins (29/2)⁺ and (31/2)⁻, respectively. Two new one-quasiparticle bands, the K^π=(5/2)⁺ band built on the known (5/2)⁺ isomer and a K^π=((1/2)⁻) band have been observed. Two other rotational bands with three-quasiparticle structure, K^π=(15/2)⁻ and ((19/2)⁺ with π(7/2)[404]ν²(1/2)[510](9/2)[624] and π(9/2)[514]ν²(1/2)[510](9/2)[624] configurations, respectively, have been newly observed. The half-life of the K^π=((19/2)⁺) bandhead which decays to the head of the (15/2)⁻ band has been measured to be 140(36) ns. However, transitions from the ((19/2)⁺) state to the (15/2)⁻ band have not been observed. Received: 26 August 1998     

The decay of174Ta

Levels in¹⁷⁴Hf excited in the decay of¹⁷⁴Ta have been studied. Measurements of gamma-rays, conversion electrons and gamma-gamma-time coincidences were performed. The ground state band, the beta-vibrational band, the gamma-vibrational band with its head at 1226.81 keV, aK=3⁺ band at 1303.42 keV and aK=2^? octupole band at 1308.67 keV were observed. Several 2⁺ levels withK=0 are excited. The mainβ ⁺-branches proceed through allowed or first-forbidden transitions to the 2⁺ and 4⁺ levels of the ground state band. The character of theI ^π=3⁽⁺⁾ ground state of¹⁷⁴Ta is discussed.     

Study of deuterium and hydrogen distributions in Ta|CD<Subscript>2</Subscript>|Ta,Ta|Ta|CD<Subscript>2</Subscript>|Ta|Ta,and Nb|CD<Subscript>2</Subscript>|Nb assemblies after exposure to high-temperature argon plasma

Assemblies made of Ta|CD₂|Ta, Ta|Ta|CD₂|Ta|Ta and Nb|CD₂|Nb foils are irradiated with pulses of high-temperature argon plasma created by means of a “Plasma Focus” setup. The irradiated foil samples are investigated by recording the recoil nuclei of hydrogen and deuterium. It is found that hydrogen and deuterium are redistributed in foil stacks. The ultradeep penetration of light gas impurities (hydrogen and deuterium) can be explained by the influence of shock waves on the foils and accelerated diffusion under an external force.     

New rotational bands in 174Ta

Three new bands in ¹⁷⁴Ta have been identified by using the ¹⁶⁰Gd(¹⁹F,5n) reaction at beam energies of 87 MeV and 96 MeV. Nilsson configurations are assigned to these bands. In the 9/2⁻[514]_p+5/2⁻[512]_n band, the AB neutron crossing occurs at a rotational frequency of 0.30 MeV. This is indicative of the disappearance of the evidence for a reduction in neutron pair correlations. Received: 30 March 1998     

Hydrogen in Nb/Ta superlattices

The structural and thermodynamic properties of hydrogen dissolved in Nb/Ta superlattices are studied by in-situ x-ray scattering techniques. From the x-ray satellite intensities, it is found that H induces a strain modulation exhibiting a Curie-Weiss temperature dependence. The intensities are analyzed in terms of a mean field model of the modulated lattice gas, yielding quantitative information on the hydrogen-metal and hydrogen-hydrogen interaction energies. Critical behavior associated with a gas-liquid transition is also observed. Hydrogen density fluctuations with wavelengths shorter than a superlattice period are, however, suppressed by the superlattice, which represents a novel manifestation of a coherent phase transition. These experiments provide new and fundamental insight into the role of spacially varying two body interactions in critical phenomena.     

The identical bands in177Ta

The [402]5/2 bands in¹⁷⁷Ta which are identical to the neighboring even-even¹⁷⁶Hf groundstate band have been extended to higher angular momentum. These bands in the two nuclei are seen to diverge from each other in the region of the first i_13/2 neutron alignment. The lower observed crossing frequency for the [402]5/2 bands indicates a lower deformation for these bands compared to¹⁷⁶Hf. Extensions to theh _9/2 [541]1/2 yrast band are also reported.Special thanks to D.C. Radford, It. MacLeod, W.T. Milner and R. Books for their invaluable software support. Thanks also to F.K. McGowan and R. Darlington for their help with the targets. This work was supported by the National Science Foundation and the State of Florida and by the UK Science and Engineering Research Council. MAR and JS acknowledge the receipt of a NATO Collaborative Research Grant.     

Transfer reactions of B with Ta

Excitation functions and mean recoil ranges are reported for the formation of ¹⁸²Os, ¹⁸³Re, ¹⁸²Re, ¹⁸¹Re and ¹⁸²Ta in ^{10, 11}B interactions with ¹⁸¹Ta. The stacked foil technique was used. The results are indicative of formation through a transfer reaction. We have considered a nucleon group transfer mechanism in a surface interaction followed by de-excitation via nucleon emission from the excited intermediate. Thus the energy dependence of recoils and cross sections of single nucleon and multinucleon transfer products have been calculated and found in good agreement with experimental results.     

Ta,Ta/Cu缓冲层对NiFe/Fe Mn双层膜交换偏置场的影响

采用磁控溅射方法制备了分别以Ta和Ta/Cu作为缓冲层的一系列NiFe/FeMn双层膜.实验发现,以Ta为缓冲层的NiFe/FeMn双层膜的交换偏置场比以Ta/Cu为缓冲层的NiFe/FeMn双层膜的交换偏置场大.测量了这两种双层膜的织构、表面粗糙度和表面成分.结果表明,以Ta/Cu为缓冲层时,Cu在NiFe层的上表面偏聚是造成NiFe/FeMn双层膜交换偏置场降低的重要原因. 关键词： NiFe/FeMn 交换偏置场 织构 表面粗糙度     

The nuclear magnetic moment of182Ta

The nuclear magnetic moment of¹⁸²Ta has been measured with the aid of the NMR-ON technique, using the hyperfine fieldB=–65.6±1.3T in iron. Its value =298±0.06 _N is in excellent agreement with that calculated for a Nilsson configuration, using proper parametersg _K andg _R derived from neighbouring odd-A nuclei. Agreement is also found between experimental and calculated nuclear magnetic moments of¹⁸⁴Re, which has the sameK=1/2 neutron state as¹⁸²Ta.     

Hydrogen distribution in Nb/Ta superlattices

The distribution of hydrogen in Nb/Ta superlattices has been investigated by combined neutron reflectivity and x-ray scattering. We provide evidence to support that strain modulations determined with x-ray diffraction can be interpreted as modulations in hydrogen content. We show that the hydrogen concentration is modulated and favors Nb, in agreement with previous studies. We measure the concentration directly using neutron reflectivity and demonstrate no detectable change in the distribution of hydrogen with temperature, in stark contrast to previous studies.     

Band structures and proton-neutron interactions in Ta

Six rotational bands in the odd-odd nucleus ¹⁷⁴Ta have been populated with the ¹⁶⁰Gd(¹⁹F,5n) reaction. High-spin states were identified using the NORDBALL array. Both signatures of the doubly decoupled π1/2⁻ [541] ν1/2⁻ [521] band and semi-decoupled π1/2⁻ [541] ν7/2⁻ [633] band are observed, in addition to the high-K couplings of the π9/2⁻ [514] ν7/2⁺ [633], π9/2⁻ [514] ν5/2⁻ [512], π7/2⁺ [404] ν7/2⁺ [633], and π5/2⁺ [402] ν5/2⁻ [512] configurations. The signature splitting of the π1/2⁻ [541] ν7/2⁺ [633] band is inverted from the expected splitting, and this is interpreted as being due to a residual proton-neutron interaction. It is shown empirically that this interaction, together with deformation changes, can account for the increased crossing frequency associated with the alignment of i_13/2 neutrons in the π1/2⁻ [541] bands of odd-Z nuclei.     

Hyperfine interaction of 181Ta in nickel

Nuclear gamma resonance spectroscopy was used to study the hyperfine interaction of ¹⁸¹Ta impurities in nickel at room temperature and above the Curie point. Both the hyperfine field for ¹⁸¹Ta in nickel at room temperature and the isomer shift relative to Ta metal have been derived.     

Quadrupole interaction at181Ta in selenium

The nuclear quadrupole interaction of Ta-181 in selenium was measured using perturbed angular correlation technique. The electric field gradient at room temperature is 4.56(32) × 10¹⁷ V/cm². The temperature dependence of EFG follows the usual T^3/2 behaviour.     

Quadrupole interaction at181Ta in tellurium

The quandrupole interaction at¹⁸¹Ta in tellurium measured at various temperatures revealed two unique sites with an axially symmetric electric field gradient. These are interpreted as a near substitutional site with almost octahedral symmetry and a pure interstitial site in the tellurium lattice. The temperature dependence observed for QI frequencies corresponding to these sites supports this conclusion.