Search

Gypsum is an authigenic precipitate that forms under periods of accentuated aridity and occurs widely in arid zones. However its use in quantitative paleoclimatology has been limited due to the absence of a method to determine the timing of its formation. We present here the results of a feasibility study that demonstrates that the timing of the formation event of gypsum can be estimated using Electron Spin Resonance (ESR) analysis. We used well documented samples from White Sands in New Mexico, USA, the Thar Desert, India and lakes in the Simpson Desert and Mallee Region, Australia and found that ESR ages could be obtained using radiation sensitive SO 4 − , SO 3 − radicals and a photobleachable signal O 3 − . ESR signals were consistent with control ages based on contextual information. These suggest that the dating signals (SO 4 − , SO 3 − ) are stable over time scales >100 ka. We propose that this stability of the SO 4 − signals over geological time scales arises due to hydrogen bonding between the water proton and the SO 4 − radical and that the suitability of these radiation-induced radicals comes from their being a part of the host matrix. Further, ESR along with Fourier Transform Infrared (FT-IR) Spectroscopy methods additionally inform on the geochemical pathways for gypsum formation and help elucidate complex formation processes even in samples that appeared unambiguous gypsum precipitates. Thus, the presence of Hannebachite (CaSO 3 .½H 2 O) and Mn 2+ in Thar and Australian samples suggested a reducing environment such that low valence sulfur reacted with CaCO 3 to form hannebachite and eventually gypsum. The presence of sulfur, partially as sulfite in Thar gypsum samples suggested that redox cycles were mediated by microbial activity. Absence of these features in White Sands samples suggested oxic conditions during gypsum precipitation.

Polycrystalline ammonium dithionate has been examined for its radiation response in the low dose range (< 5 Gy) using EPR technique. The •SO3- radical ion was detected as a single EPR line with a peak-to-peak derivative width of ca. 0.44 mT in irradiated samples and its intensity was found to vary linearly with dose. At equal and moderate settings of microwave power and modulation amplitude ammonium dithionate was at least 7 times more sensitive than L-alanine which is the most common EPR dosimeter standard. Pulse experiments were performed on the powder samples to obtain the longitudinal relaxation time. These and microwave saturation experiments served to indicate the optimal microwave power to be applied during measurements as an EPR dosimeter for best sensitivity of this material. It is thus claimed that ammonium dithionate has excellent potential to become an EPR dosimeter with a low limit of the measurable dose for cases where tissue equivalence is not required or can be corrected for. Original publication: M. Danilczuka, H. Gustafsson, M.D. Sastry, E. Lund and A. Lund, Ammonium Dithionate – a New Material for Highly Sensitive EPR Dosimetry, 2008, Spectrochimica Acta - Part A: Molecular and Biomolecular Spectroscopy, (69), 1, 18-21. http://dx.doi.org/ 10.1016/j.saa.2007.03.001. Copyright: Copyright: Elsevier B.V., http://www.elsevier.com/

EPR dosimetry employing L-alpha-alanine has been in vogue during the past few years, due to its tissue equivalence and linear dose response. However, L-alpha-alanine dosimetry has been improved during the past years, the sensitivity of this material is still too low for clinical applications. Polycrystalline lithium formate doped with NiCl2 was therefore examined for radiation response in the dose range of clinical interest (5 Gy) using CW EPR and pulse EPR techniques. At equal and moderate settings of microwave power and modulation amplitude lithium formate doped with 1.6 wt% of NiCl2 was almost four times more sensitive compared to L-alpha-alanine, which is the most common EPR dosimeter standard. It was shown that the nickel-doped lithium formate has an excellent radiation response with a low limit of the measurable dose, and a linear dose response in the range 1-5 Gy. The relaxation and power saturation studies showed that high microwave power can be applied during measurements to improve the sensitivity of this material as an EPR dosimeter. These results show that lithium formate doped with Ni(II) exhibits promising properties required for further development of an EPR dosimeter in the dose range typical for clinical dosimetry.

Three-pulse electron spin echo envelope modulation (ESEEM), hyperfine sublevel correlation spectroscopy (HYSCORE) investigations and two-pulse electron spin echo (ESE) measurements of phase memory time TM, were carried out, in the 20-200 K temperature range, on an AsO44- paramagnetic probe stabilized in RbH2PO4 (RDP), NH4H2PO4 (ADP), and dipolar glass Rb0.5(NH4)0.5H2PO4 (RADP). The results obtained on ADP revealed hyperfine interaction of the probe ion with the 14N of the ammonium ion, the coupling constant satisfying the condition of 'cancellation' at a field of 480 mT. The ammonium ion was found to be in two different sites in ADP, which became indistinguishable on the formation of dipolar glass RADP. These results were confirmed by HYSCORE spectral measurements. The fast Fourier transform (FFT) spectra of three-pulse ESEEM decays have clearly revealed the interaction with protons in the bond both in ADP and RDP, and in RADP with an averaged coupling constant. The phase memory times in RADP exhibited strong temperature dependence and were found to be dependent on the nuclear spin quantum number mI of 75As. The temperature dependence of TM exhibited a well-defined maximum around 90 K, coinciding with the temperature of onset of 'freezing' in Rb0.5(NH4)0.5H2PO 4. This is symptomatic of dynamic fluctuations in the dipolar glass phase, with onset around 150 K, going through a maximum around 90 K and slowing down on further cooling. These results suggest that in RADP, a dynamical mechanism with progressive slowing down below 90 K is operative in the glass formation. This implies that the RADP system, with x = 0.5, exists in an ergodic relaxor (R)-state in the 20-200 K temperature range wherein every fluctuating monodomain can be viewed as statistically representative of the whole sample. © IOP Publishing Ltd.

Electron paramagnetic resonance (EPR) studies of X-irradiated lithium dithionate, Li 2 S 2 O 6 ·2H 2 O, doped with Ni and Rh have shown that these impurities enhance the yield of radicals formed by X-irradiation at room temperature. The signal in the doped samples, measured peak-to-peak of the single EPR derivative line attributed to the SO 3 − anion was about 3–4 times that of the pure lithium dithionate and more than 10 times stronger than the alanine signal. These impurities also shortened the spin-lattice relaxation time, T 1 , which gives the possibility to measure the doped samples at a higher microwave power. This implies that sensitivity could be further enhanced in the already sensitive EPR dosimeter material lithium dithionate.

Polycrystalline, formates and dithionates are promising materials for EPR dosimetry, as large yields of radiation induced stable radicals are formed with a linear dose response. Rapid spin relaxation rates were detected in many of the substances, indicating that a high microwave power can be applied during EPR acquisition in order to improve sensitivity. Different techniques used to further improve the sensitivity, such as the replacement of Li-7 with Li-6 or exchange of protons with deuterons in the corresponding crystalline matrices and metal ion doping are discussed. It is concluded that formates and dithionates may be up to 10 times as sensitive as L-alpha-alanine. (C) 2004 Elsevier Ltd. All rights reserved.

With a view to monitoring the changes in coloration caused by the nuclear reaction 51V(n,β)52Cr in solution of vanadyl sulphate and using it for neutron dosimetry, electronic absorption spectra of vanadyl sulphate solutions were investigated at different concentrations of chromate impurity in micromolar range. It was observed that the presence of chromate enhances the absorptivity over a wide wavelength range serving essentially as a colouring agent for vanadium matrix, presumably due to charge transfer process. The absorbance at 380nm varied linearly over a wide concentration range. The limit of detection of chromate obtained is shown to be adequate for detecting neutron-induced chemical transmutation of vanadium to chromium under standard reactor conditions, when used with long path length cells. It was observed that the absorbance does not change on electron irradiation, suggesting that radiolytic effects due to beta decay, if any, do not interfere in the measurement of neutron-induced changes. In addition to its potential for neutron dosimetry, this is the first report of a simple and direct method of estimation of Cr in vanadium matrix at sub ppm level.

Electron paramagnetic resonance (EPR) investigations were conducted on [Cu(II) (1-phenylamidino-O-n-butylurea) en (H2O)]2(2+) (1) and [Cu(II) sulphato-mono (1-phenylamidino-O-methylurea)]2 (2) respectively, in the temperature range 300-77K. Fine structure characteristics of S = 1 system, was observed in both complexes with zero field splitting of 0.0525 and 0.0225 cm(-1), respectively, suggesting the formation of dimeric complexes. The presence of the half-field signal (DeltaMs= +/-2), in the complex 1, further confirmed the formation of dimer. The temperature dependence of EPR signal intensity has given evidence for the ferromagnetic (FM) coupling between the two Cu2+ ions. The isotropic exchange interaction constants J, were evaluated from this and were found out to be approximately 57 and approximately 27 cm(-1), respectively, for the complexes 1 and 2. The photoacoustic spectra of these complexes had shown a band around 26,400 cm(-1) characteristic of metal-metal bonding giving an independent support for the existence of dimeric Cu2+ species. The high magnetic moment values at room temperature for complex 1 (2.68 microB) and complex 2 (2.00 microB), obtained from the magnetic susceptibility measurements, support the formation of ferromagnetically coupled Cu2+ dimers.

Photo-luminescence studies of Pr3+ activated thorium oxide phosphor have revealed that mainly 3P→3H4 and 1D2→3H4 transitions with life-time of 30 and 600 μs are observed in this sample. An exponential reduction in the emission intensity of Pr3+ ions was observed on following continuous excitation with 275 nm corresponding to the f–d transition band of Pr3+ ions. Such a reduction in emission intensity was observed at all temperatures investigated in the range 90–330 K. The emission intensity recovered partially on dark storage only above 180 K. The recovery of emission intensity was also observed on the illumination of pre-exposed sample to light in the wavelength region 300–430 nm. Following illumination with 275 nm, Pr3+ activated thorium oxide phosphor has displayed a weak thermally stimulated luminescence. These results thus suggest that the optical excitation dynamically changes the state of the system under observation, and that changes are occurring in the valence state of Pr ions due to e/h transfer process on 275-nm exposure. On dark storage and also on 365-nm illumination of the pre-exposed sample, e/h traps recombine to cause emission signal recovery. The analysis of data on reduction in intensity obtained with exposure to 275 nm suggests the likelihood of the of Pr3+ ions existing at three different sites. The activation energies associated with the release of electrons from excited Pr3+ ions at different sites were determined from the temperature dependence of the photo-induced charge transfer process.

Absorption spectrophotometric studies indicated that in chloroform β-carotene reacted with NO2 with a stoichiometry of 1:2. To understand the reaction mechanism, free radical intermediates produced during their reaction were trapped by spin traps 2-methyl-2-nitroso propane (MNP) and 4-pyridyl-1-oxide-N-1-butyl nitrone (POBN). The resulting stable spin adducts were detected by electron spin resonance (ESR) spectroscopy. NO2 alone did not interact with spin traps, but introduction of β-carotene-generated ESR signals with different nitrogen and hydrogen hyperfine coupling constants (AN and AH) characteristsic of the trapped radical. Two possible pathways exist for the reaction of NO2 with β-carotene, viz., electron transfer and addition to the polyene chain. Our ESR results supported an addition mechanism, where formation of carbon-centered [NO2 · · · β-carotene] free radical adduct was observed.

Rare earth and actinide doped alkali and alkaline earth sulphates, exhibit a variety of point defects stabilized due to the charge imbalance/self-irradiation and they play an important role in luminescence properties. In this paper, we report the point defects formed in the mixed sulphate K3Na(SO4)2 on doping with the radio-active rare earth element 147 Pm as studied by TSL and EPR techniques. EPR studies on self and gamma irradiated samples showed the signal due to the radical pair, SO4−–SO3− in addition to the signals from SO4−, SO3− and O2− ions. However EPR spectra of electron beam irradiated (undoped) K3Na(SO4)2 samples did not show the presence of the radical pair SO4−–SO3−, indicating the importance of dopant per se, in stabilizing the radical pair. EPR studies of 147 Pm doped samples annealed at different temperatures after gamma irradiation showed that SO4−–SO3− radical pair gets destroyed around 550 K , whereas SO4− ion gets annealed in the temperature range 375– 450 K . SO3− ion was found to be stable upto 600 K . From TSL–EPR correlation, it is inferred that the release of trapped hole at SO4− ion on heating leads to the formation of SO 4 2− ∗ and energy transfer to Pm3+ ion, results in the glow peak at 415 K .

Thermally stimulated luminescence (TSL) and electron paramagnetic resonance (EPR) studies on uranium doped calcium phosphate yielded mechanistic information on the observed glow peaks at 365, 410 and 450 K. TSL spectral studies of the glow peaks showed that UO2 2+ acts as the luminescent center. Electron paramagnetic resonance studies on gamma-irradiated samples revealed that the predominant radiation induced centers are H0, PO4 2-, PO3 2- and O- ion. Studies on the temperature dependence studies of the EPR spectra of samples annealed to different temperatures indicate the role of H0 and PO4 2- ions in the main glow peak at 410 K.

Electron paramagnetic resonance studies were conducted on synthetic calcium hydroxy apatite samples co-doped with 239Pu and carbonate ion. These investigations were carried out to assess the self-irradiation effects in bone and teeth on exposure to plutonium, as calcium hydroxy apatite is the major constituent of bone and teeth. On self-irradiation, in addition to the signal from O- ion arising from the radiolysis of hydroxide ion, EPR signals due to CO2-, PO22- and another signal assigned to surface O- ions were observed in the samples. In freshly quenched gamma irradiated samples, signals from CO3-, O- , PO22- and O2- ions were observed. The EPR signal of O2- ion shows a doublet splitting suggesting that O2- ion gets preferentially stabilized close to Pu4+. The radiation damage due to Pu4+ at Ca2+ sites, in the sample appears to be lower as compared to that due to external gamma-irradiation. Moreover, the alpha-dose in 239Pu doped samples has self-annealing effects. These are attributed to localized radiation damage due to alpha-particles compared to evenly distributed radical ions produced due to gamma-irradiation.

EPR studies of VOSO4·5H2O revealed that, on reactor neutron irradiation, it exhibits magnetic order and regions of spin glass behavior which is persistent even up to 473 K. This is attributed to dehydration of the compound together with homogeneous incorporation, in the lattice, of chromium produced due to nuclear reaction 51V (n,β) 52Cr. Experiments with gamma irradiated and heat treated samples doped with chromate, have given strong support to the role of chromium in inducing the magnetic order.

Thermally stimulated luminescence (TSL) and electron paramagnetic resonance (EPR) studies were carried out on cerium doped and cerium–uranium co-doped K 3 Na(SO 4 ) 2 samples after γ -irradiation. Three glow peaks around 352, 415 and 475 K were observed and their spectral characteristics have shown that Ce 3+ and UO 2 2+ act as the emission centres in K 3 Na(SO 4 ) 2 :Ce and K 3 Na(SO 4 ) 2 :Ce, U, respectively. In Ce–U co-doped sample, energy transfer from cerium to uranium takes place. The commonly occurring radiation-induced centres in sulphates, viz SO 3 − and SO 4 − were observed by EPR and SO 4 − radical ion was found to take part in the TSL emission at 415 K . The hitherto unknown information, however, is the formation of SO 4 − –SO 3 − radical pair creating deep traps in these lattices, apparently assisted by the dopants. This is the first observation of such radical pair formation leading to the identification of deep traps in this lattice. The radical pair, (SO 3 − –SO 4 − ) which is stable up to 970 K , decreases the intensity of the peak at 415 K due to the depletion of SO 4 − centres.

Thermally stimulated luminescence of natural mineral petalite (LiAlSi4O10) crystals was investigated and the possible electron/hole traps responsible for thermoluminescence (TL) emission were identified using optical absorption and electron paramagnetic resonance (EPR) measurements. The glow curves for natural samples, obtained with a heating rate of 4C s-1, show two glow peaks at 160°C and 330°C. Pre-annealed and subsequently irradiated samples give rise to three glow peaks at 175°C, 340°C and 435°C. An isochronal thermal study established correlation between the first glow peak and Ti3+-like trap centre and E'1-centre and Al-O--Al-like recombination centres. The second TL peak at 340°C may be related to the E'1 centres formed on irradiation. A mechanism for the observed TL is suggested.

Thermally stimulated luminescence (TSL) and electron paramagnetic resonance (EPR) studies were conducted on self-irradiated as well as gamma irradiated BaCO 3 co-doped with 237 Np 4+ and NO 3 − ions. EPR studies of the irradiated samples revealed that the electron transfer process between iso-electronic carbonate and nitrate groups is the dominant effect of ionizing radiation, forming CO 3 − and NO 3 2− ions. This facilitated the radiation stability of the valence of Np in the tetravalent state. TSL spectral studies showed that Np 4+ ion acts as the luminescence centre. TSL–EPR correlation studies revealed the role of CO 3 − ion in the glow peak at 413 K .

Electron paramagnetic resonance (EPR) investigations were carried out on two bimetallic molecular magnets (NBu 4 ) 2 Co 2 [Cu(opba)] 3 ·S, where S=3DMSO·3H 2 O and 2H 2 O, known to exhibit long-range magnetic order below 32 and 34 K, respectively, in the temperature range 300–15 K. Temperature dependence of the intensity of the EPR signal arising from Cu 2+ in these compounds showed a rapid decrease in intensity upon cooling below 65 K, which was accompanied by a disappearance of the super-hyperfine structures, observed on the Cu 2+ signal at higher temperatures, pointing to a Co 2+ Cu 2+ spin–spin interaction following the slowing down of Co 2+ spin–lattice relaxation. The appearance of a new signal at zero field (ZFS) below 40 K is attributed to the onset of long-range magnetic ordering in both the compounds.

EPR investigations on La 0.7 Ca 0.3 MnO 3 were carried out in the temperature range from 14 K to 300 K. The important observations are: (i) a coincidence of resistivity maxima with the disappearance of paramagnetic EPR line I, which suggests that there exists a strong electron–phonon interaction, (ii) the appearance of an "out of phase" line near the Curie temperature T c due to the formation of spin clusters consisting of Mn3+ and Mn4+ ions, and (iii) the appearance of an intense EPR line III and the splitting of FC and ZFC magnetization curves below 160 K, ascribed to local magnetic frustrations. The above EPR observations provide experimental evidence for the co existence of spin fluctuations and magnetic disorders in this sample, which leads to the formation of spin-glass features below T c .

Photo-EPR measurements carried out on Nd3+ -doped polyvinyl alcohol (PVA) films have shown that nearly 100% reduction occurs in the intensity of EPR of Nd3+ under in situ copper vapor laser (CVL) illumination (510.5 nm and 578.2 nm). The kinetics of decay and recovery were investigated. Photoacoustic (PA) spectra, observed under CVL pump condition had shown that the CVL induced changes were not due to photoinduced valence change, and that the CVL pumping creates highly favorable conditions for non-equilibrium population distribution in the excited electronic states. The complete disappearance of EPR under CVL pumping is attributed either to the possible equalization of population of |+〈 and |−〈 Zeeman components, through the decay of many excited states in the presence of magnetic field or configurational changes around Nd3+ shifting the resonance frequency. The former appears less probable in view of the relatively slower recovery of EPR signal.

The effects of partial substitution of La by a smaller Lanthanide element (R) in La 0.7-x R x Ca 0.3 MnO 3 ( R = Ce , Pr, Nd Sm, Gd, Dy with x = 0 and 0.1) ceramic pellets on the electron paramagnetic resonance has been investigated. The line width increased with decrease in ionic radius of the A site A > in paramagnetic phase, which is explained on the basis of tolerance factor. The observed increase in g-value and line width with increase in A > in ferromagnetic phase explained qualitatively by Mn-O-Mn bond angle distortion and Mn-O orbital overlap which in turn is governed by the size of the substitution ion.

The CO2- radical ion, detected by ESR technique in bones and teeth enamel, was proved to be invaluable in high level and retrospective dosimetry. In these matrices, impurity carbonate (at phosphate sites) was the precursor to CO2-. With a view to investigate the possibility of using inorganic materials such as lithium carbonate as ESR dosimeters, studies were carried out on gamma-irradiated Li2CO3. The intensity of radiation-induced ESR signals of Li2CO3 at g = 2.0036 (CO3-) and g = 2.0006 (CO2-) was followed as a function of gamma dose in the low dose range of 1-1350 Gy. It was observed that the intensity of the ESR signal at g = 2.0036 (CO3-) was in a linear relation with the radiation dose in the dose range 10-800 Gy and the signal at g = 2.0006 (CO2-) showed linear response in the dose range 5-800 Gy. The lowest dose that could be detected in the present studies using the signal of CO2- in Li2CO3 powder samples (approximately 50 mg) is 3.2 Gy. ESR studies were also carried out on the widely used TL dosimetric material CaSO4:Dy and in pure CaSO4 after gamma irradiation. The TL materials were used in powder as well as pellet forms. The linearity of ESR response with dose for powder and pellet forms of CaSO4: Dy was also studied using the signals at g = 2.0030 (SO3-) and at g = 2.0139 (SO4-). It was observed that the range of linearity of dose response extended between 20 and 1200 Gy, for SO3- signals. The results of dosimetric study indicate that the ESR-Li2CO3 system could be used in dosimetric applications in radiotherapy. However, for the actual applications further advancement is needed to lower the detection limit. The TL phosphor, CaSO4:Dy in powder and pellet forms, could be used as ESR dosimeter in the dose range 20-600 Gy.

Polycrystalline MgB2, synthesized using a conventional solid-state reaction route, has been investigated for its magnetic field dependent microwave absorption using a standard X-band EPR spectrometer. The changes in microwave absorption as a function of temperature and/or magnetic field were recorded by following the microwave reflectivity of a sample-loaded cavity. The modulated low-field microwave absorption signal was found to be similar to that observed in high-temperature superconducting materials. The field dependent direct microwave absorption in the 0-10 kG range was used to determine Hc1 values at different temperatures, and the value of Hc1(0) was found to be 250 G. The absorbed microwave power has been found to obey a (H)1/2 dependence with a change of slope indicating a transition from a strongly pinned flux lattice to the flux flow regime.

With a view to understanding the role of photo-induced valence changes of impurities in BaTiO3 in the phenomena of photorefraction, EPR experiments were conducted under in situ He-Ne laser illumination. These experiments gave evidence for photoinduced valence change of Fe in BaTiO3 at room temperature. The EPR signal due to trivalent iron was found to reduce in intensity with laser illumination The kinetics of the valence change has been investigated. Under large fringe width condition, the time constant of the decay is identified as the dielectric relaxation time τd. The changes in line shape on laser illumination to Dysonian form, appeared most predominantly in mechanically poled crystal compared to electrically poled single domain crystals. This demonstrated the possible role of domain walls and the defects there, as source or sinks of charge carriers on photo excitation. It is observed, that there is transient growth of Fe3+, when the laser illumination was put on, before its decay. This was attributed to charge transfer between electrons in oxygen vacancies and Fe4+. This predicted the growth of a transient grating under depleted pump condition in a two beam coupling experiment. This was experimentally proved by following the diffracted signal of the reading beam under the depleted pump condition.

Photo-EPR investigation of the Fe3+ ion in a BaTiO3 single-crystal at 15 K using in situ copper vapor laser (CVL) illumination has shown that the electron transfer at the Fe site is symmetry dependent. It was seen that relaxation of the photo-induced electron transfer to the pre-illumination condition occurred even at 15 K very quickly. The growth and decay kinetics were found to be nearly identical. These investigations have shown that the history of the sample and the wavelength of light have an important role in the photo induced charge transfer process.

With a view to investigating the nature of weak links in granular superconductors at very dilute level, we have studied the low-field microwave absorption signal (below about 50 G) in the electron paramagnetic resonance mode of detection in samples of Tl2BaCa2Cu2O8+δ and YBa2Cu3O7-δ at different dilution levels in the Al2O3 matrix. The signal has been monitored as a function of temperature as well as concentration. The temperature dependence of the signal intensity has been understood by applying a model in which microwave absorption is assumed to be caused by the field- and tempera ture-dependent currents in the Josephson links. It is shown that the parameter ηo quantifying the strength of coupling between superconducting grains can be calculated from the value of temperature at which the intensity I pp(T) (the subscript pp indicates a peak-to-peak value), goes through a maximum. Using the ηo value so obtained, the fitting for I pp(T) fhroughout the temperature range, was used to distinguish sup...

Electron magnetic resonance (EMR) studies of intercalation ferromagnet 2, 2’-bipyridine-MnPS3 (Tc = 40 K) in the temperature range 300–14 K have revealed many interesting features across the (magnetic) order–disorder transition. The exchange narrowed line in the paramagnetic phase exhibited sudden reduction in its intensity concomitant with a g-shift to lower values (line shifted to higher fields) and increased line width. These changes took place in the 40–25 K range. In the 25–22 K range of temperature, the paramagnetic line disappeared and the FMR signal appeared at lower fields. It is significant that two closely lying FMR signals appeared in the magnetically ordered regime, suggesting the possibility of existence of two different Mn-sites having different g-values, in this state. This may be responsible for the reported magnetic moments values of less than 5.9 BM from bulk magnetisation studies.

E.p.r. and optical absorption studies have been conducted on dichloromono(1-phenylamidino-o-alkylurea)copper(II) complexes (Alkyl = Me, Et, Pr, Bu or Pe). The e.p.r. spectra of solids, recorded at 300 K, confirmed the square-planar geometry with the unpaired electron in the dx2−y2 orbital of copper. Magnetic field-induced partial molecular alignment has been observed in some of the polycrystalline samples when cooled in a magnetic field of 1 T at 77 K. E.p.r. spectra at 77 K in pyridine and DMF has shown axial ligation of solvent molecules (pyridine and DMF), whereas in MeOH and DMSO at least three structurally different CuII species have been identified. These features are consistent with differences in electronic absorption spectra in the powder and in solution.

EPR and ENDOR investigations have been carried out on polycrystalline samples of self(α)-irradiated PuO2C2O4·3H2O (cumulative dose = 1 MGy). The 239Pu (I=1/2) hyperfine structure on the EPR signal of the CO2− radical ion has given clear evidence for participation of 5f electrons of 239Pu in chemical bonding. The ENDOR spectra were used to identify unambiguously the hyperfine interaction between the unpaired electron on the CO2− radical ion and the nuclear spin of 239Pu. In addition to the CO2− radical ion, EPR evidence was also obtained for the formation of O2− during the autoradiolysis of PuO2C2O4·3H2O.

The electrothermal vaporization (ETV) mode of sample introduction into inductively coupled plasma (ICP) has been examined for its suitability for the analysis of trace metals in uranium and thorium compounds without prior chemical separation of the matrices using atomic emission spectrometry (AES). The ETV interfaced with an ICP torch adapted for glove box operation, has led to determination of sub-nanogram amounts of Al, Be, Ca, Cd, Co, Cr, Cu, Mg, Mn, Na, Ni, Zn, Dy, Eu, Gd and Sm using 10 μl of the sample aliquot containing 200 μg of U/Th matrix with a precision of 1–2%RSD. The results obtained here especially for rare earths in the presence of the matrix are of significance.

Electron magnetic resonance (EMR) studies of intercalation ferromagnet 2,2′-bipyridine-MnPS3 (Tc = 40 K) in the temperature range 300 – 14 K have revealed many interesting features across the (magnetic) order ← → disorder transition. The exchange narrowed line in the paramagnetic phase exhibited sudden reduction in its intensity concomitant with a g-shift to lower values (line shifted to higher fields) and increased line width. These changes took place in the 40–25 K range. In the 25–22K range of temperature, the paramagnetic line disappeared and the FMR signal appeared at lower fields. It is significant that two closely lying FMR signals appeared in the magnetically ordered regime, suggesting the possibility of existence of two different Mn-sites having different g-values, in this state. This may be responsible for the reported magnetic moments values of less than 5.98 BM from bulk magnetisation studies.

EPR spectra of some powdered metallomesogens cooled in magnetic fields of 0.3/1.2 T showed at 77 K, a distinctive change in line shape from that at room temperature (RT), indicating appearance of field-induced alignment. However, conformation and length of the chains have to match the packing requirement. Thus, the closed packed C7Cu shows 100% alignment, whereas C11OCu/C9OCu show only partial alignment. These have a tendency to self-assemble into random domains in the polycrystalline solid at RT, which tend to orient themselves along the magnetic field at 77 K. This is the first report of a dry paramagnetic powder, showing a single crystal-like alignment along H at the high temperature of 77 K.

We report Electron Paramagnetic Resonance (EPR) studies on intercalation ferromagnets Mn 0.79 PS 3 (4,4'-bipy) 0.42 and Mn 0.84 PS 3 (1,10-phen) 0.64 and their antiferromagnetic host MnPS3, in the temperature range 300 - 15 K. The nature of the organic guest cation appears to have a strong influence on the magnetic behaviour of this inorganic-organic hybrid system. The continued appearance of the exchange-narrowed sharp line, characteristic of the paramagnetic phase, even at 15 K suggests that the bulk of the intercalate ferromagnets in this system do not attain a completely ordered slate.

An Electron Spin Resonance (ESR) study of radiation induced free radicals in the commonly used detergent powder (Surf), was conducted to examine its potential application for retrospective/high level dosimetry, including possible dating of the accidents. The radical ions present in the irradiated detergent powder were identified as arising predominantly from sodium tri poly phosphate (Na3P5O10) and sodium sulphate (Na2SO4). The physical mixture of sodium tri poly phosphate and sodium sulphate in 4:1 ratio was found to give an ESR response, similar to that of the detergent powder. These results suggest that the detergent powder can be used as a dosimeter in the 20 Gy to 18 kGy dose range, spanning over 3 orders of magnitude. The second derivative ESR spectra of the detergent powder irradiated to different doses showed that the relative yield of the radicals is independent of irradiation dose. The decay pattern of radicals when followed as a function of post irradiation days, lead to the idea about the contrasting decay characteristics of the radicals. From the ESR signals of the irradiated detergent samples, it is shown that radiation dose can be evaluated and the radiation incident can be dated with an accuracy of ±10%.

Low and high field dependent microwave absorption is studied in various sintered and melt textured HTSs. The low field loss is found to originate from weak links and the high field loss is due to thermally assisted flux flow. Melt textured superconductors are found to be best suited for low field device applications due to the drastic reduction in low field induced loss when compared to sintered samples. Melt textured superconductors with 211 inclusions are found to show reduced high field losses due to pinning of field at the secondary defects created by 211 particulates. Various parameters of the critical state such as Jc and the pinning force constant, Kx, are determined from the microwave absorption study.

In order to understand the mechanism of optical image storage in photorefractive polyvinyl alcohol (PVA) films, photo-EPR and photoacoustic spectral investigations were carried out on PVA films doped with Cr3+ and VO2+. The EPR spectrum of Cr3+ has shown reduction in intensity onin situ illumination with copper vapor laser (CVL). The decay and recovery of Cr3+ signal, with and without CVL illumination respectively, was monitored at different temperatures in 10–300 K region. These were found to obey a double exponential, with one time constant independent of temperature, and the other showing significant temperature dependence. From Τ(T), activation energy for the charge carrier transport in one of the processes was estimated to be 0.016 eV. The PA spectra showed shift towards lower wavelength side on consecutive runs. On the other hand, VO2+ doped PVA film has not shown any significant changes in intensity on laser illumination. These observations suggest (i) interaction of PVA matrix with excited Cr3+ and (ii) predominant non-radiative relaxation in VO2+: PVA system with no change in the oxidation state.

The PAS cell with carbon black absorber is used as a detector for spectrophotometry in an open room. This has immense potential of obtaining absorption speciation information from remote samples.

EPR studies of (NBu4)2Mn2[Cu(opba)]3·6DMSO·1H2O, in the 10–300 K range with in-situ illumination by copper vapour laser radiation (510.5 and 578.2 nm) have given evidence for the following: (i) The EPR results obtained on these compounds as a function of temperature are not predominantly due to demagnetization effects, (ii) the magnetic order is affected by light most likely due to the decoupling of spins followed by electronic excitation. This enables the possibility of photo-induced switching of magnetic order.

Summary The effect of gamma-irradiation doses of 1, 2, 4 and 5 kGy on ESR signal intensity in pre-packed irradiated lamb meat chunks containing bones was studied. Irradiation-induced a characteristic ESR signal in bone that was not detected in non-irradiated samples and its intensity was proportional to irradiation dose up to 5 kGy. In samples irradiated to 2.5 kGy for shelf-life enhancement, ESR signal intensity was monitored immediately after irradiation and subsequently at weekly intervals during storage at 0–3 °C and after cooking by different methods. Hind leg bones (femur) displayed a higher signal intensity than rib bones. The ESR signal intensity faded by 30 and 42% during 4 weeks at 0–3 °C in hind leg and rib bones, respectively. The magnitude of ESR signal in hind leg bone declined by a maximum of 30% upon pressure cooking (for 15 min), 10% on microwave cooking (for 5 min) and 13% after boiling (for 30 min). However, in the case of rib bones, all cooking methods reduced the ESR signal by about 40%. Storage of irradiated freeze-dried bone powder samples at ambient temperature also resulted in slight reduction in ESR signal intensity. Results point to the suitability of ESR technique for detection of irradiated lamb meat even after storage or cooking, and also in samples that had been stored and then cooked.

EPR investigations using Cu2+ ion as a probe have been performed on supersaturated sucrose solution with percent concentration c = 66 as a function of temperature T, and at room temperature as a function of c. The motionally averaged spectrum of [Cu(H2O)6]2+ was used to monitor the changes in intermolecular interactions that occur as a function of [c, T]. A drastic increase in the line width, symptomatic of increase in the rotational correlation time of [Cu(H2O)6]2+, is observed between 293 and 288 K. The motionally averaged spectrum disappears below 281 K. The motionally averaged spectrum is also absent in the room temperature spectra of the solution with c= 85. Even in the [c, T] range where [Cu(H2O)6]2 is found to be nearly static, these molecules appear to have an orientational fluctuation manifesting in the m 1 dependence of the line width of the parallel component.

Temperature dependence of power absorption of HTSCs is found to follow the London two fluid model at both radio and microwave frequencies. The two fluid model is shown to account for the Josephson penetration depth at radio frequencies (MHz) and grain penetration depth at microwave frequencies in sintered HTSCs. The Josephson penetration depth determined is of the order of 100 μm. Also, it is discussed that reduced superelectron screening due to short mean free path in HTSC cannot account for the large penetration depth determined from the radio frequency absorption.

We have observed anomalous microwave absorption (MWA) representative of paramagnetic Meissner effect in GdBCO powder. The results are discussed in terms of π junction theory. The phase change in the MWA gives evidence for the change in behaviour of the π junctions from para to dia state. The intragrain weak links with high J c are likely to be forming π junctions in the sample. It is expressed that the high frequency absorption is a better tool than the magnetization measurements to investigate the π junctions. Thermally induced vortex–antivortex pair formation and the flux compression caused by an inhomogeneous superconducting transition are ruled out as the possible mechanisms of the PME, from the microwave absorption study.

EPR measurements on (NBu 4 ) 2 Mn 2 [Cu(opba)] 3 ·6DMSO·1H 2 O in second harmonic mode in the 300–13 K range have shown a strong 1/ T dependence of line position in the paramagnetic phase, evolution of local ordering below 40 K and a new zero-field signal in the long-range ordered phase. These features suggest: (i) existence of spin correlations from room temperature resulting in local short-range order below 40 K; and (ii) coexistence of ordered and disordered phases at 14 K. The new EPR signature at zero-field obtained at and below 15 K, is attributed to domain walls.

The photoacoustic spectra of Pu(IV) and Pu(VI) as PuO2 2+ ions in fluoride, oxalate and hydroxide in polycrystalline samples are reported for the first time. The photoacoustic spectra of Pu(IV) in these compounds showed good agreement with that of electronic absorption spectra of Pu(IV) in 1 m HClO4 solution. The photoacoustic spectrum of Pu(IV) hydroxide is, however, conspicuous with the absence of the 540 nm band which was prominently seen in the normal absorption of Pu(IV). Further, from the similarity of the observed spectra with Pu4+ in ZrSiO4, ligand-field splitting of Pu4+ electronic states in these compounds could be identified. The photoacoustic spectra of PuO2 2+ fluoride, oxalate and hydroxide samples was correlated with the reported spectra of PuO2 2+ in 1 m HClO4. The intense absorption at 816 nm in PuO2(OH)2 · H2O is attributed to an electronic transition with strong coupling to the Pu–O stretching mode. The evidence for a weak vibronic coupling of electronic states of PuO2 2+ is presented f...

EPR investigations have been carried outboth in the first and second harmonic modes on the organic ferromagnet (NBu4)2Mn2[Cu(opba)]3 with Tc = 22 K. The data was collected in the temperature range 300 – 15 K. At temperatures much higher than Tc (T ≥ 100 K), the spectrum consists of a single line at g = 2.00 exhibiting Curie behaviour. Upon cooling below 100 K, the line shows strong temperature dependent g - shift in resonance field to higher value by 50 G at 15 K. This is taken as evidence for the onset of uncompensated antiferromagnetic spin correlations well above Tc. Evidence was also obtained for spin fluctuations which got slowed down with lowering of temperature. The correlation time at 15 K is greater that 10−10 sec.

This paper describes the results of the photoacoustic spectral investigations of fluoride, oxalate and hydroxide of Pu(III) in powder form. The spectra obtained are compared with known absorption spectra of Pu(III) ions in aqueous medium and the matrix-dependent non-radiative relaxations are identified. It is seen that the OH- ligand has a strong influence on the J = [equation;5/2 → 7/2, 5/2 transition.

Our electron paramagnetic resonance (EPR) investigation of the intrinsic semiconductor UMo6S8 in the temperature range 10-100 K has revealed the resonance due to U4+ with a strong temperature-dependent g shift. This is due to the dynamic averaging of EPR signals between the static 5f2 state of U4+ and the conduction electrons. The energy gap ΔE g, between the U 5f levels and the conduction band in UMo6S8 can be estimated from the temperature dependence of the g shift to be 0.0165eV.

Photo-induced charge transfer and its kinetics were investigated in Bi12SiO20 in 10–300 K temperature range, using EPR of Fe3+ centre, underin situ illumination with copper vapour laser (CVL). The decay kinetics was found to follow double exponential behaviour. Relaxation of the photo-induced electron transfer to the preillumination condition occurred even at 10 K. Shallow traps were, therefore, associated with the electron trapping, leading to a better understanding of the fast photorefractive response of BSO.

The EPR spectrum of KO2 is re-examined over a wide temperature range through the sequence of phase transitions. It is shown that these transitions are essentially associated with slowing down of O− 2 molecular motion.