Perturbed angular correlation measurements on the 40.9 keV level of62Cu. A preliminary study of 2Zn-insulin

Time-differential PAC measurements have been made using⁶²Zn, both in aqueous solution at pH 6.0 and also incorporated into crystalline 2Zn-insulin. Three Nal(Tl) detectors were used — a 2.54 cm thick scintillator detecting the stop 41 keV gamma ray and two 5.0 cm thick scintillators at 90 and 135 detecting the start 597 keV gamma rays. The need to separate the 597 keV line from the 511 keV background was found to require very careful setting-up conditions in the case of a fixed-detector spectrometer. The solution data produced a fast relaxation time of (75±25) ns, while the three 2Zn-insulin batches gave data which were analyzed using a static interaction with an assumed axial symmetry to give an electric quadrupole frequency of (77±8) MHz.     

Perturbed angular correlation experiments on the rehydration of ZnTiF6

The rehydration process at room temperature, after the dehydration of ZnTiF₆· 6H₂O to ZnTiF₆, was studied by means of the Time Differential Perturbed Angular Correlations technique. The time evolution of the hyperfine parameters related to the hyperfine interaction observed is reported and interpreted in terms of a simple model for the growth of the phases. In the framework of this model, the time evolution of the relative fractions of each interaction is interpreted. This revised version was published online in July 2006 with corrections to the Cover Date.     

Perturbed angular correlation study of radiation-induced defects in Rh metal

Radiation-induced defects are studied in cubic rhodium metal, using the local probe technique ‘Time differential perturbed angular correlation’ (TDPAC) at liquid N₂ temperature. Isochronal annealing was done at 300, 1073 and 1473 K temperatures. The irradiated sample showed two quadrupole interaction frequencies at 1150 and 93 MHz. The low frequency disappeared at room-temperature annealing, which was assigned to In trapped at a vacancy, whereas the higher frequency remained up to high temperatures and was attributed to In trapped at Rh–C complexes in the Rh matrix.     

Perturbed angular correlation study of surface magnetization in iron single crystals

The behaviour of closure domains at the surface of iron single crystals in an external magnetic field was studied by DPAC on samples implanted with¹¹¹In. It is observed that the surface magnetization does not follow that of the bulk. The movement of both the 90 and 180 walls of the closure domains is blocked up to a certain starting value H_s of the external field that is related to the demagnetization factor D of the sample and also depends on the precise orientation of the crystal axes and on the implanted indium dose.On leave from the Institue of Nuclear Physics, Cracow, PolandOn leave from the Institue of Physics, Jagiellonian University, Cracow, PolandThis work forms part of the research program of the Foundation for Fundamental Research of Matter (FOM) subsidized through the Netherlands Organization for Pure Research (ZWO).     

Perturbed angular correlation study of a nanostructured HfO2 film

The hyperfine field at ¹⁸¹Ta lattice sites in a nanostructured HfO₂ thin film was studied by the perturbed angular correlation (PAC) technique. The thin oxide film was deposited by pulsed laser ablation on a silicon substrate kept at 673 K. The thickness was about 25 nm. The radioactive ¹⁸¹Hf ions were produced by neutron activation of the very thin film in the Portuguese research reactor by the reaction ¹⁸⁰Hf(n,γ)¹⁸¹Hf. PAC measurements were carried out at room temperature after annealing at different temperatures up to 1,473 K in air. The PAC technique allows determining the electric field gradient at the ¹⁸¹Ta probe sites. The ¹⁸¹Ta isotopes appear in the sample as disintegration product of ¹⁸¹Hf.     

Perturbed alpha-gamma angular correlation studies of radon recoils into aluminium

The quadrupole interaction of ²²²Rn and ²²⁰Rn implanted by α-particle-decay in Al metal and Al-oxide has been investigated. The experimental results indicate an electrical interaction in cubic Al metal not in agreement with earlier measurements using isotope implanted sources.     

Measurement of the magnetic dipole moment of the 57 keV state of143Pr by time-differential perturbed angular correlation technique

The magnetic dipole interaction of the 57 keV state of¹⁴³Pr in an environment of metallic cerium and in an aqueous solution of Ce(NO₃)₃ has been investigated by time-differential perturbed angular correlation experiments. The measurements were performed in external magnetic fields between 0.94 T and 2.24 T and at temperatures between 77 K and 1200 K. From the temperature dependence of the observed spin rotation the ionicity of the Pr ion is determined as Pr⁴⁺ in the Ce(NO₃)₃ and as Pr³⁺ in Ce metal in the intermediate nuclear state. The magnetic dipole moment of the 57 keV state is derived as = +(3.4±0.1)_K. From the room temperature relaxation constants ₂ = (0.19±0.03)10⁹s^–1 for Ce(NO₃)₃ and ₂= (0.07±0.02)10⁹s^–1 for Ce metal the correlation times _J = (3.8±0.7)10^–13s and _J = (1.4±0.4)10^–13s respectively are deduced. The angular correlation coefficientA ₂=+0.17±0.02 is in good agreement with the result of a decoupling experiment.     

Perturbed angular correlation experiments on111In in oxidized AgCd alloy

The hyperfine interaction of¹¹¹Cd probes in preoxidized and α-irradiated Ag_0.94Cd_0.06 alloy has been investigated by PAC measurements and compared with the results for pure Ag. Several probe atom sites were distinguished and characterized by their hyperfine parameters and temperature evolution. The different behavior of In-oxygen and In-oxygen-vacancy complexes in AgCd and Ag was found and discussed. A considerable influence of the irradiation temperature and sample thickness on the different In-oxygen-vacancy complexes formation was evidenced. The parameters of¹¹¹Cd quadrupole interaction measured during isochronal annealing studies in preoxidized and deuteron irradiated AgCd were very similar to the ones observed after α-irradiation.     

Perturbed angular correlation study of the ion exchange of indium into silicalite zeolites

Two indium-containing silicalite zeolites (In/H–ZSM5) catalysts prepared by wet impregnation and ionic exchange were characterized by the Perturbed Angular Correlation (PAC) technique using ¹¹¹In as probe to determine the nature of the indium species. Some of these species take part in the catalytic reaction of the selective reduction (SCR) of NO_x with methane. PAC experiments were performed at 500oC in air before and after reduction–reoxidation treatments on the catalysts in order to determine the origin of the different hyperfine interactions and then the degree of ionic exchange. Complementary catalytic activity characterizations were also performed. PAC experiments performed on the catalyst obtained by wet impregnation showed that all In-atoms form In₂O₃ crystallites while almost 70% of In-atoms form In₂O₃ in the catalyst obtained by ionic exchange. The PAC experiments of both catalysts performed after the reduction–reoxidation treatment revealed the presence of two hyperfine interactions, different from those corresponding to indium in In₂O₃. These hyperfine interactions should be associated to disperse In species responsible of the catalytic activity located in the ionic exchange-sites of the zeolites. This revised version was published online in August 2006 with corrections to the Cover Date.     

Perturbed angular correlation studies of Ta hyperfine interactions in Hf-Ni and Zr-Ni compounds

The hyperfine interaction experienced by ¹⁸¹Ta nuclei in the intermetallic compounds ZrNi₅, HfNi₅, and Hf₂Ni₇ has been investigated by perturbed angular correlation (PAC) spectroscopy. At temperatures T≥15 K the ¹⁸¹Ta angular correlation of appropriately annealed ZrNi₅ and HfNi₅ is unperturbed, indicating the absence of a magnetic hyperfine interaction. This observation rules out the possibility of spontaneous magnetic order of ZrNi₅ and HfNi₅ recently proposed in the literature. The temperature dependence of the electric quadrupole interaction of ¹⁸¹Ta in Hf₂Ni₇ suggests the existence of a reversible phase transformation at T≥500 K.     

Perturbed angular correlation study of order– disorder transition in HfW2O8

The perturbed angular correlation (PAC) technique has been used to characterize the degree of atomic order in the neighbourhood of radioactive ¹⁸¹Hf isotopes in HfW₂O₈. PAC measurements were carried out at temperatures between 14 and 723 K. The compound was synthesized starting with the oxides HfO₂ and WO₃, using a method involving ball milling, high temperature annealing and quenching in liquid nitrogen. Fast cooling allows to have the compound at temperatures below 1050 K. The compound has a high degree of stability below such temperature and around 430 K atomic ordering occurs. This transition order–disorder is reversible. ^aAlso at Comisión de Investigasiones Científicas de la Provincia de Buenos Aires, Argentina.     

Measurement of the electric quadrupole moment of the 482 keV state of 181Ta by the time differential perturbed angular correlation technique

The electric quadrupole interaction of the 482 keV state of ¹⁸¹Ta in an environment of metallic rhenium has been investigated by time differential angular correlation measurements. For the field gradient calibration a recent observation^{1, 2}) of the quadrupole splitting of the 6.3 keV line of ¹⁸¹Tain the same environment has been used. Adopting the quadrupole moment of the ground state which was derived from muonic X-ray data ³) we obtain $\text{Q}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}\text{(482}\text{keV}\text{) = 2.51 ± 0.15}\text{b}$. A comparison of the intrinsic quadrupole moment Q₀ with that of the ground state revealed that the excited state is slightly less deformed: $\text{Q}{}_0\text{(}\text{5}\text{2}{}^{\mathrm{+}}\text{)/Q}{}_0\text{(}\text{7}\text{2}{}^{\mathrm{+}}\text{) = 0.936 ± 0.014}$. A measurement of the temperature dependence between liquid nitrogen and room temperature showed that the electric quadrupole interaction remains essentially constant.     

On theg-factor shift of the relaxed excited state ofF-centres in alkali halides

An interpretation of the large shifts of the electronicg-factors of the relaxed excited state (RES) ofF-centres relative to theg-factor of the free electron is given. Theseg-shifts are quantitatively explained by admixture of higher excited states to the RES through the magnetic interaction. This interpretation avoids the serious difficulties and contradictions which arise when one tries to explain theg-factor shifts exclusively within the framework of the vibronic model. As a byproduct new values for the mean radius of the RES wavefunctions are given.     

Sub-Coulomb transfer and fission in collisions of129Xe,132Xe and136Xe with238U

Integral cross sections for fission and for one- and two-neutron transfer reactions in the system¹³²Xe+²³⁸U were measured radiochemically in the energy range 0.7≦E/E _Coul≦1. The excitation functions for fission and transfer are found to be essentially parallel below 0.85×E _Coul. Even at the lowest energies the transfer cross sections exceed the fission cross section by more than one order of magnitude. With the other projectiles¹²⁹Xe and¹³⁶Xe different transfer cross sections illustrating their sensitivity for the ground stateQ-values,Q _gg , are observed while the fission cross sections are the same as in the¹³²Xe +²³⁸U reaction. The fission data are interpreted in terms of a continuous transition between Coulomb fission and several transfer-induced fission processes.     

Perturbed angular correlations of187Re in nickel

The internal field at¹⁸⁷Re in Ni has been measured by the TDPAC technique to beH _int=?105±3 kG at 293 °K.     

Perturbed angular distributions in the presence of isotropic paramagnetic relaxation

The effect of paramagnetic relaxation on perturbed angular distributions is treated for nuclei interacting with their electronic shells via isotropic hyperfine interaction. The conditions are given under which Blume's analytical stochastic-model result for the nuclear perturbation factorsG _k (t) can be derived quantum mechanically. Systems with arbitrary nuclear spin, but electronic spinS=1/2 may be calculated without resorting to the assumption necessary forS>1/2. Explicit closed expressions forG _k (t) can be found for this particular case.     

In situ studies of solid state reactions by perturbed angular correlation

We report on three examples of in situ studies of solid state reactions by time differential perturbed angular correlation of¹⁸¹Ta: (i) the oxidation of hafnium metal and the doping of ZrS₂ with Hf during iodine vapour transport crystal growth; (ii) the observation of sublattice melting during polymorphic phase transitions in TaS₂; (iii) the electrointercalation of 2H-TaS₂ with silver.     

Subsurface detection of a buried object using angular correlation function measurement

Abstract

A new technique is proposed for subsurface detection of buried objects using the angular correlation function (ACF) measurement of scattered waves. Compared with the traditional detection technique which relies on radar cross section (intensity) measurement, this new ACF-based technique results in better signal-to-clutter ratio and thus higher target visibility. Laboratory experiments were conducted at millimetre-wave (80–105 GHz) and X-band (7–13 GHz) frequencies to illustrate the potential effectiveness of this new correlation approach over the traditional cross section approach.