A perturbed angular correlation spectrometer for material science studies

A four-detector perturbed angular correlation (PAC) spectrometer has been developed with ultra-fast BaF₂ detectors to acquire four coincidence spectra simultaneously, two at 180° and two at 90°. This spectrometer has double efficiency compared to that of a three-detector set-up. Higher efficiency is desirable for PAC studies in solid state physics where large number of coincidences are required to obtain the PAC spectra with good statistics and is particularly useful when the half-lives of the parent probe nuclei used for PAC measurements are ∼2–3 days or less as in ¹¹¹In (2.8 d), ⁹⁹Mo (2.7 d) and ¹⁴⁰La (1.7 d). The performance of the spectrometer has been tested for the HfO₂ monoclinic crystal in the temperature range from 77 to 873 K and for the HfF₄·3H₂O crystal at room temperature. The polycrystalline HfO₂ has been synthesized from Hf metal by heating in air. The hydrated hafnium fluoride has been crystallized by dissolving Hf metal in 40% HF and drying slowly at room temperature.      

Mössbauer Characterization of Iron-Based Nanogranular Films

The electric field gradients (EFG) of the Hf₂Fe intermetallic compound were calculated using the full-potential linearized augmented plain-wave (FP-LAPW) method as embodied in the WIEN 97 code. The obtained values are compared with other ab-initio calculations and on a qualitative basis with the previously reported experimental data obtained from TDPAC. The calculated results, −23.1·10²¹ V/m² and 2.7·10²¹ V/m² for Hf 48f and Fe 32e position, respectively, are in excellent agreement with experimental data (23.4·10²¹ V/m² and 2.7·10²¹ V/m²), better than those reported in earlier calculations. The calculated EFG for Hf 16c position (4.2·10²¹ V/m²) is stronger than the experimental one (1.1·10²¹ V/m²).     

The EFG at cadmium in chlorides of indium

Using¹¹¹In as a probe, TDPAC measurements have been performed on the 173–247 keV γ-γ cascade in¹¹¹Cd. Information on nuclear quadrupole interaction parameters is reported for α-In Cl, In Cl₂ and In Cl₃.     

A simple digital TDPAC spectrometer

We present a simplified digital time differential perturbed γ???γ angular correlation (TDPAC) spectrometer that demonstrates that such instruments can be built using primarily commercial components and with relatively modest coding effort. The system handles data rates of 70 kcps/detector with a timing resolution of better than 500 ps, and has been used with both ¹¹¹In and ¹⁸¹Hf.     

Electric field gradients at Ta in Zr and Hf inter-metallic compounds

Here we calculate the electric field gradient (EFG) at the nucleus of the substitutional Ta impurity site in Zr₂T and Hf₂T (T=Cu, Ag, Au, and Pd) C11_b inter-metallic compounds. We use the ab initio FP-LAPW method as embodied in the Wien97 code in a super-cell approach and include lattice relaxations around the impurity. Our results are compared with EFG values inferred from measurements of the quadrupole coupling constants at the ¹¹¹Ta probe in these compounds performed with the time differential perturbed angular correlation (TDPAC) technique. We also performed EFG calculations for the pure inter-metallic compounds. Through the comparison of theoretical and experimental EFGs in these cases, we elucidate the role played by the Ta probe in the TDPAC measurements of Hf and Zr compounds. Our results show that, although the EFGs at the Hf site are very similar to the EFGs at the Ta impurity, there is no direct correlation between the Zr and Ta EFGs.     

Magnetic and electric hyperfine interactions of181Ta implanted in thulium

The magnetic and electric hyperfine interaction at the site of dilute¹⁸¹Ta impurities in the rare earth metal Tm has been investigated as a function of temperature by TDPAC measurements. The samples were prepared by ion implantation of radioactive¹⁸¹Hf. In the paramagnetic phase between 100 K and 700 K the electric fieldgradient is a linear function of temperature: V_zz(T)=V_zz(O)·(1-A·T) with A=4.6·10^?4K^?1 and V_zz(293K)=6.4 (4)·10¹⁷v/cm². The TDPAC spectrum observed at 4.2 K reflects the 4 magnetically non-equivalent sites for an impurity in magnetically ordered Tm. The relative values and amplitudes of the corresponding 4 magnetic hyperfine fields are consistent with the predictions of the RKKY theory.     

Investigation of hyperfine interactions in GdCrO3 perovskite oxide using PAC spectroscopy

Perturbed angular correlation (PAC) measurements have been carried out in the antiferromagnetic GdCrO₃ perovskite oxide using ¹¹¹In (¹¹¹Cd) and ¹⁸¹Hf(¹⁸¹Ta) nuclear probes. The radioactive parent nuclei ¹¹¹In and ¹⁸¹Hf were introduced in the compound through a chemical process during sample preparation. The PAC measurements were carried out in the temperature range 20–300 K. Measurements with the ¹⁸¹Ta indicated a unique quadrupole interaction above 170 K and a combined electric quadrupole and magnetic dipole interactions below this temperature. The observed interaction was assigned to the probe nuclei substituting Cr sites. Measurements with ¹¹¹Cd showed two quadrupole interactions. Only one of the fractions however, showed a combined electric and magnetic interaction in the temperature rage 20–170 K which was assigned to ¹¹¹Cd probe substituting Cr site. The other fraction was attributed to the Gd site. The present results are compared with those of LaCrO₃ and NdCrO₃.     

The TDPAC study of quasicrystalline and T-phases

Time differential perturbed angular correlation (TDPAC) spectra of the quasicrystalline phase Al₇₉Mn₂₀Si₁ (800 ppm Hf) and the decagonal phase (T-Phase) Al₈₀Mn₂₀ (800 ppm Hf) were measured using the¹⁸¹Hf probe. It is obtained that the structure around the center in Mackay icosahedra in the decagonal phase seems to be distorted in comparison with that in the quasicrystalline phase.     

111In(111Cd) quadrupole interaction in the chalcogenide spinel CdCr2Se4

The¹¹¹Cd quadrupole coupling in the spinel CdCr₂Se₄ is investigated by TDPAC after different sample treatments. The experiments are consistent with Se vacancies causing the non-cubic probe environments and with the assumption that the majority of the¹¹¹In atoms are positioned at A-sites.     

Measurement of parity admixtures in nuclear forces in180Hf,181Ta and203Tl

The parity mixing in nuclear states was studied by measuring the circular polarizationP of the γ-radiation emitted from unpolarized nuclei. The analyzer was a “radial transmission magnet” of high symmetry. With 4 NaI scintillation counters and the current integration method 4 independent results were obtained simultaneously. The polarimeter efficiency was determined via the Quadratic Compton effect. Studies of the polarized bremsstrahlung with¹⁷⁰Tm,¹⁷⁷Lu and¹⁹⁸Au sources served as tests for the calculated corrections due to the bremsstrahlung effect. Our results for the circular polarization are:P=?(2.0±0.4) · 10^?3 for the 501 keV transition in¹⁸⁰Hf,p=+(1±4) · 10^?6 for the 482 keV transition in¹⁸¹Ta andP=+(1±7) · 10^?6 for the 279 keV transition in²⁰³Tl. Thus, in the case of¹⁸⁰Hf, the existence of the parity mixing in nuclear states was confirmed.     

Measurement of theβ-decay branch in180Lu to180m Hf for Investigation of the r-process nucleosynthesis of180m Ta

Theβ-decay branch of¹⁸⁰Lu to^180m Hf is measured by an integral method tracing the amount of^180m Hf daughter activity, to be 0.46 (15)%. Samples of¹⁸⁰Lu are prepared by on-line mass separation of¹³⁶Xe-on-tungsten transfer reaction products. The amount of isobaric^180m Hf in the samples is strongly suppressed by the employed separator ion source; its directly produced fraction is derived from comparison with^177m Hf activity. The properties of lutetium and hafnium release from the ion source are separately determined. With this measurementr-process nucleosynthesis is estimated to account for at most 22% of the solar^180m Ta abundance.     

Study of surface-bulk mass transport and phase transformation in nano-TiO2 using hyperfine interaction technique

Phase transition from anatase to rutile for the 70 nm TiO₂ crystallite has been investigated by the time differential perturbed angular correlation (TDPAC) technique. The study involved the annealing of the TiO₂ nanocrystals, adsorbed with the nuclear probe (¹⁸¹Hf/¹⁸¹Ta) at trace level, at different temperatures for different durations. The TDPAC measurement was also supported by XRD measurement where the width of the peaks increases with the increase in annealing temperature indicating a crystal growth. The samples annealed up to 823 K for 4 h showed no phase transition, except for the growth of the crystallites. However, it showed phase transition at the same temperature (823 K), when annealed for longer duration, indicating the slower kinetics of the phase transition process. Further the sample, when annealed at 1123 K for 4 h, showed phase transition. It has also been observed that the ¹⁸¹Hf tracer, adsorbed on 70 nm anatase TiO₂, diffuses from surface to bulk during the phase transition process and the extent of diffusion in anatase differs from that in rutile phase. However, surface to bulk mass-transfer is found to play a significant role in the phase transition process.     

Investigation of gamma radiation induced changes in local structure of borosilicate glass by TDPAC and EXAFS

Gamma radiation induced changes in local structure around the probe atom (Hafnium) were investigated in sodium barium borosilicate (NBS) glass, used for immobilization of high level liquid waste generated from the reprocessing plant at Trombay, Mumbai. The (NBS) glass was doped with ¹⁸¹Hf as a probe for time differential perturbed angular correlation (TDPAC) spectroscopy studies, while for studies using extended X-ray absorption fine structure (EXAFS) spectroscopy, the same was doped with 0.5 and 2 % (mole %) hafnium oxide. The irradiated as well as un-irradiated glass samples were studied by TDPAC and EXAFS techniques to obtain information about the changes (if any) around the probe atom due to gamma irradiation. TDPAC spectra of unirradiated and irradiated glasses were similar and reminescent of amorphous materials, indicating negligible effect of gamma radiation on the microstructure around Hafnium probe atom, though the quaqdrupole interaction frequency (ω _Q) and asymmetry parameter (η) did show a marginal decrease in the irradiated glass compared to that in the unirradiated glass. EXAFS measurements showed a slight decrease in the Hf-O bond distance upon gamma irradiation of Hf doped NBS glass indicating densification of the glass matrix, while the cordination number around hafnium remains unchanged.     

The Nuclear Quadrupole Interaction of 204mPb in Lead Oxides

The nuclear quadrupole interaction of ^204mPb in lead oxides has been measured by γ–γ time differential perturbed angular correlation. Ab-initio calculations of the electric field gradients and X-ray diffraction allowed the assignment of the detected nuclear quadrupole interactions to the different Pb sites in the PbO phases litharge and massicote as well as in Pb₃O₄. The TDPAC probe ^204mPb was produced with a ²⁰⁴Bi/^204mPb-generator at the home laboratory at the University of Leipzig. The use of a high performance liquid chromatography system increased significantly the yield, the specific activity of ^204mPb, and reduced the acidic concentration of the eluate.     

Time differential perturbed angular correlation studies of hafnium hydrides

The dependence of the electric field gradients (EFG) in Hafnium-Hydride systems as a function of the composition H/Hf in the cubic δ and tetragonal ε phases were investigated using the time-dependent perturbed angular correlation (TDPAC) technique, with¹⁸¹Ta as the nuclear probe. Two EFGs were found in the ε phase, indicating the existence of two major symmetries surrounding the Ta probe. The results indicate that the trend of the EFGs, in the ε phase, are due to the changes in the lattice parametersa _o,c _o as hydrogen is added to the Hf-H system. In the δ phase, only one major symmetry was found. Both phases are characterized by broad frequency distributions and large anisotropies.     

Measurement of the parity mixing of the 501 keV transition in180Hf by use of a backscattering polarimeter

The circular polarization of the 501 keV gamma radiation in¹⁸⁰Hf was remeasured by use of a Compton backscattering polarimeter with the single pulse counting technique. The resultP _c=?(1.9±0.6) · 10^?3 confirms the strong parity mixing which was previously reported by other groups.     

Comparative mössbauer and TDPAC studies on the after-effects of the ec decays of119Sb and111In in α-Fe2O3

Mössbauer emission spectra of (¹¹⁹Sb→)¹¹⁹Sn and TDPAC spectra of (¹¹¹In→)¹¹¹Cd in α-Fe₂O₃ are measured. The origin of apparetly much pronouced after-effects of the EC decay observed in the TDPAC spectra, as compared with those in the Mössbauer spectra is clarified.     

Quadrupole interaction at Ta and lattice location of Hf in Be

The time-differential perturbed angular correlation technique (TDPAC) has been applied to the 482 keV excited state of¹⁸¹Ta, to determine the nuclear electric quadrupole interaction (QI) at Ta in Be. Sources were prepared by ion-implantation of¹⁸¹Hf in Be. Particle channeling measurements have revealed that. Hf impurities implanted in Be reside primarily at the tetrahedral interstitial site. The fundamental QI frequency obtained for Ta at this interstitial site in Be is |v _Q| =e ² qQ/h = 227.0 ± 2.2 MHz which corresponds to an electric field gradient |eq|=(3.71±0.15)×10¹⁷ V/cm². This result is discussed in terms of results for Cd impurities in the same system and the systematics of the impurity QI in metals.     

Electric quadrupole interaction measurements in YBa2Cu3O x and related compounds

In this work we present Electric Quadrupole Interaction (EQI) measurements, made by Time Differential Perturbed Angular Correlation (TDPAC), on¹¹¹Cd in YBa₂Cu₃O _x and some related compounds. These studies were intended to determine the relationship between the EQI and the actual probe site. The probes were introduced into the materials as a diluted¹¹¹In-complex or via In(¹¹¹In)₂O₃. Our observations indicated that there is no need to suppose the presence of many probe sites in YBa₂Cu₃O _x to explain the experimental results.