C. C. Dey

Characteristics of a Compton suppressed Clover detector up to 5 MeV

The Clover detectors in their addback mode are excellent tools for detecting high-energy gamma rays ðX2 MeVÞ: The characteristics ofthese detectors, at energies above 2 MeV ; are usually determined from simulation data or from extrapolation ofthe empirical data. This is the first time that the characteristics ofa Compton suppressed Clover germanium detector have been studied up to 5 MeV using a radioactive 66 Ga ðT1=2 ¼ 9:41 hÞ source. r 2002 Elsevier

Comparative studies of YAG(Ce) and CsI(Tl) scintillators

Abstract The performances of YAG(Ce) and CsI(Tl) scintillators have been compared using photomultiplier tube (PMT) readout for γ rays and α particles. It is found that the energy resolution of YAG(Ce) is inferior to that of CsI(Tl). With Philips XP2971 PMT, we have obtained improved energy and time resolution for YAG(Ce), compared to the same, obtained by earlier workers using XP2020/Q. The best values of energy resolution (FWHM), obtained in the present work, for 662 keV γ ray, are 6% and 7%, respectively, for CsI(Tl) and YAG(Ce), whereas for 5.48 MeV α particles, the observed values are 6% and 8.4%, respectively. The pulse height response of both the scintillators has been found to be reasonably linear up to 1.3 MeV γ energy. A prompt time resolution of 1.3 ns (FWHM) has been obtained with a BaF 2 – YAG(Ce) combination against 511– 511 keV photopeak selection, which compares well with that obtained for a BaF 2 –CsI(Tl) combination, used by us.

Performance of extrapolated leading edge timing for fast coincidence with a large-volume HPGe detector

Abstract The performance of the extrapolated leading edge type of time pickoff used with a large HPGe coaxial detector is studied. The time resolution using such a type of time pickoff has been found to be comparable with that obtained by employing the commonly used amplitude and rise-time compensation technique. For a BaF2−HPGe (27% efficiency) coincidence setup, the values of FWHM and FWTM are 3.2 and 7.0 ns, respectively, when 60Co photopeaks are selected.

Hafnium oxide thin films studied by time differential perturbed angular correlations

We report on the study of hafnium oxide thin films grown by pulsed laser deposition at various partial oxygen pressures by Time Differential Perturbed Angular Correlations using the nuclear probe 181Hf(β−)181Ta to determine the nuclear quadrupole interaction (NQI), and by x-ray diffraction. The samples were neutron activated and measured at room temperature as received as well as after annealing in air. All spectra exhibited two to three inequivalent probe sites, even after annealing. At 0.3 mbar oxygen partial pressure and annealing for 5 hs at 1073 K the majority (88%) of the sites exhibited NQI parameters as reported for the bulk monoclinic phase [ωQ = 125.4(2) Mrad/s, η = 0.335(5)]. We can exclude amorphous as well as cubic and tetragonal hafnium oxide phases in the annealed samples. There was no indication of room-temperature ferromagnetism.

Measurement of quadrupole moment of the 134 keV level in 131Cs

Abstract: The time differential as well as time integral perturbed angular correlations of the 486-134 keV cascade in 131Cs have been studied in a polycrystalline BaSO4 powder source using a HPGe-BaF2 detector system. The fundamental quadrupole frequency for the 134 keV level has been found to be ω0= (4.4 ± 0.4) × 107 rad/s. From a comparison of the quadrupole frequencies of this level and that of the 81 keV level in 133Cs, measured earlier in the same crystalline environment of the source, the absolute value of the quadrupole moment of the 134 keV level in 131Cs has been found to be 0.022 ± 0.002 b. This value of the quadrupole moment is reported for the first time and compared with the value predicted theoretically within the unified nuclear model with intermediate coupling.

0+ analogue state in118Sb from117Sn(p,nγ) reaction

The analogue of the 0+ ground state in118Sn has been observed in the compound nucleus118Sb through117Sn(p,nγ)117Sb reaction. The neutron decays of this analogue resonance have been studied from the deexcitingγ-rays of the residual nucleus117Sb. From off resonance excitation functions, spin assignments have been made to states in117Sb, on the basis of Hauser-Feshbach formalism. The resonance parameters of the isobaric analogue resonance have been determined, including the total, proton and neutron decay widths.

Recoil Induced Room Temperature Stable Frenkel Pairs in a-Hafnium Upon Thermal Neutron Capture

Ultrapure hafnium metal (110 ppm zirconium) was neutron activated with a thermal neutron flux of 6:6 · 1012 cm-2s-1 in order to obtain 181Hf for subsequent time differential perturbed angular correlation (TDPAC) experiments using the nuclear probe 181Hf(β-) 181Ta. Apart from the expected nuclear quadrupole interaction (NQI) signal for a hexagonal close-packed (hcp) metal, three further discrete NQIs were observed with a few percent fraction each. The TDPAC spectra were recorded for up to 11 half lives with extreme statistical accuracy. The fitted parameters vary slightly within the temperature range between 248 K and 373 K. The signals corresponding to the three additional sites completely disappear after ‘annealing’ at 453 K for one minute. Based on the symmetry of the additional NQIs and their temperature dependencies, they are tentatively attributed to Frenkel pairs produced by recoil due to the emission of a prompt 5:694 MeV -ray following thermal neutron capture and reported by the nuclear probe in three different positions. These Frenkel pairs are stable up to at least 373 K.

Reply to Comment of Professor T. Butz on “Oxidation of Hafnium and Diffusion of Hafnium Atoms in Hexagonal Close-Packed Hf; Microscopic Investigations by Perturbed Angular Correlations” by Chandi C. Dey, Z. Naturforsch. 67a, 633 (2012)

Prof. Butz in his comment basically argued that the fluctuation observed in hafnium metal after heating at 873 K for two days in air is due to oxygen atoms and not due to hafnium atoms as wrote in the paper. But I don’t agree with his views on this point because of the two following arguments: i) In fact, the relaxation effect arises due to fluctuation of atoms, ions or molecules near to the probe. From the fluctuating electric field gradient (EFG), one can determine directly the source of fluctuation. It is shown in the paper that the fluctuating quadrupole frequency matches quite well with the quadrupole frequency of hexagonal close-packed (hcp) hafnium. This point has been discussed in the text as – ‘The quadrupole frequency corresponding to fluctuating EFG can be determined using the relation λ max 2 (μs −1)≈ 3ω f Q Mrad/s [18]. This has been found to be ≈ 53 Mrad/s for the value of λ max 2 = 160 μs−1 found at a temperature of 573 K. This value of ω f Q matches well with the value of quadrupole frequency of hcp hafnium (∼50 Mrad/s) and supports again that spin relaxations observed at 873 K and lower temperatures are due to fluctuation of hafnium atoms’. ii) It is shown that during initial heating of hafnium at 873 K, fluctuation also arises. But this time the static quadrupole frequency was found to be only due to hcp hafnium. If the fluctuation would be due to oxygen atoms as pointed out by Prof. Butz, a quadrupole frequency corresponding to HfO2−x would be expected. However, no such quadrupole frequency was observed during initial heating at 873 K or during long time heating. This point was also clearly discussed in the text. The relevant portion of the text describes as – ‘The strong fluctuating signal can again be attributed to diffusion of hafnium atoms and hopping of oxygen can be ruled out as no signal due to oxygen deficient HfO2−x is observed at reduced temperatures. The quadrupole frequency corresponding to HfO2−x is expected to be closer to HfO2 as found from earlier measurement [4]’. It is shown that at room temperature and above, a weak frequency component (∼10%) appears which is probably due to absorption of oxygen. But this does not change much with temperature. This point has been written in the text as – ‘Values of quadruopole frequency and asymmetry parameter for the weak component have been found to be ωQ = 43.8(7) Mrad/s and η = 0.65(3) with δ = 0. This signal arises possibly due to trapping of oxygen atoms in the interstitial site of hafnium lattice [16]’. I think the absorption of oxygen in the interstitial site and fluctuation of hafnium at 873 K after heating for two days in air are two different phenomena.

Oxidation of Hafnium and Diffusion of Hafnium Atoms in Hexagonal Close-Packed Hafnium; Microscopic Investigations by Perturbed Angular Correlations

Time-differential perturbed angular correlation (TDPAC) studies in hafnium metal (~5%Zr) have been carried out at different temperatures. It is found that hafnium metal on heating at 873 K continuously for two days in air, transforms partially and abruptly to HfO2 while no component of oxide has been observed for heating up to 773 K and during initial heating at 873 K for 1 day. This result is strikingly different to that expected from the Arrhenius theory. Also, a strong nuclear relaxation effect has been observed at 873 K due to rapid fluctuation of hafnium atoms in hexagonal closepacked (hcp) hafnium. At this temperature, ~ 5% probe nuclei experience static perturbation due to monoclinic HfO2, ~ 50% experience fluctuating interaction, and ~ 5% produce static defect configuration of hcp hafnium. With lowering of temperature, defect configurations of hafnium increase at the cost of fluctuating interaction. An almost total fluctuating interaction observed in hcp hafnium at a temperature much lower than its melting point is another interesting phenomenon.

Chemical Transformation of Crystalline Hafnium Tetrafluoride Studied by Perturbed Angular Correlation Spectroscopy

The chemical transformation of the trihydrate hafnium tetrafluoride crystal has been studied with varying temperature using the time-differential perturbed angular correlation technique. The 133 - 482 keV γ -γ cascade of 181Ta after the β −-decay of 181Hf has been selected and a four detector BaF2-BaF2 coincidence set up has been used for measurements. The crystal was produced by evaporating a solution of HfF62−complex in HF at room temperature. Contrary to the earlier report, it has been found that the trihydrate hafnium tetrafluoride compound dehydrates directly to HfF4 without producing any intermediate monohydrate and present results do not support the earlier idea that two water molecules of HfF4·3H2O are loosely bound. Present investigations exhibit a superheated state for the hafnium tetrafluoride crystal. In dehydrated HfF4, two different Hf sites have been observed which suggests two different structures for the anhydrous HfF4.