Thermally stimulated luminescence and electron paramagnetic resonance studies of actinide doped calcium chloro phosphate

Electron paramagnetic resonance [EPR] and thermally stimulated luminescence [TSL] studies were conducted on self [α]-irradiated²³⁹Pu doped calcium chloro phosphate andγ-irradiated²³⁹Pu/²³⁸UO ₂ ²⁺ doped calcium chloro phosphate to elucidate the role of the electron/hole traps in thermally stimulated reactions and to obtain trap parameters from both TSL and EPR data. TSL glow peaks around 135 K (# peak 1), 190 K (# peak 2), 435 K (# peak 5) and 490 K (# peak 7) were observed and their spectral characteristics have shown that Pu³⁺ and UO ₆ ⁶⁻ act as luminescent centres in calcium chloro phosphate with respective dopants. EPR studies have shown the formation of the radical ions H⁰, PO ₄ ²⁻ , O⁻, O ₂ ⁻ and [ClO]²⁻ under different conditions. Whereas the [ClO]²⁻ radical being stable up to 700 K, was not found to have any role in TSL processes, the thermal destruction of other centres was found to be primarily responsible for the TSL peaks observed. The trap depth values were determined both by using the TSL data and also the temperature variation of EPR spectra of these centres.     

Electron paramagnetic resonance and thermally stimulated luminescence studies of uranyl doped calcium sulphate

Electron paramagnetic resonance (EPR) and thermally stimulated luminescence (TSL) studies were conducted onγ-irradiated CaSO₄:UO ₂ ²⁺ to elucidate the role of the electron/hole traps in thermally stimulated reactions and to obtain the trap parameters (trap depth and frequency factor). Intense TSL glow peaks around 140, 375, 400 and 438±2K are observed and their spectral characteristics have shown that UO ₂ ²⁺ and UO ₆ ⁶⁻ act as luminescent centres. EPR studies have shown the peaks at 140 and 400/438K to be associated with the thermal destruction of O⁻ and SO ₄ ⁻ radical ion in two stages respectively. The maximum rate of thermal destruction of SO ₄ ⁻ ions (as seen by EPR) in various alkaline earth sulphate matrices investigated in our laboratory is also summarized. The activation energy which characterizes the electron transfer reaction between SO ₄ ⁻ and the dopant ion lies in the range of (0.95±0.15 eV). This value is independent of the dopant and therefore seems to be characteristic of the binding energy of hole in the SO ₄ ⁻ radical ion.     

Internal irradiation effects in239Pu doped K2Ca2(SO4)3): EPR,TSL and PAS investigations

The electron/hole trapped centres created during internal irradiation in²³⁹Pu doped K₂Ca₂(SO₄)₃ were investigated using electron paramagnetic resonance (EPR), thermally stimulated luminescence (TSL) and photoacoustic spectroscopic studies (PAS). These techniques were used to identify the defects and characterize the thermally induced relaxation processes. TSL studies of self (α)/γ-irradiated²³⁹Pu doped K₂Ca₂(SO₄)₃ revealed two glow peaks around 400 and 433K. Plutonium introduced as Pu⁴⁺ was partly reduced to Pu³⁺ due to self irradiation. This was ascertained from PAS studies. EPR studies carried out on these samples showed the formation of radical ions SO ₄ ⁻ , SO ₃ ⁻ , O ₃ ⁻ , etc. The thermal destruction of SO ₄ ⁻ ion was found to be associated with the prominent glow peak around 433K. Pu³⁺ was found to act as luminescent centre for the observed TSL glow. The trap depth for the glow peak at 433K has been determined from TSL and EPR data.     

Optically stimulated luminescence in doped K3Na(SO4)2 phosphors

Optically stimulated luminescence (OSL) in Cu and Eu doped K₃Na(SO₄)₂ is reported for the first time. The Cu-doped sample shows OSL sensitivity which gets enhanced by co-doping with Mg²⁺ ions. The Cu-doped and quenched sample shows better sensitivity which is almost double than that of the slowly cooled sample, whereas the sensitivity of Mg co-doped sample remains nearly same irrespective of the thermal treatment. The Cu-doped sample shows TL peak around 200 °C and moderate OSL sensitivity. Doping of Mg shifts the TL peak to around 160 °C and is correlated with good OSL sensitivity. Eu-doped sample does not show OSL sensitivity. However, relatively good OSL sensitivity is observed in Aluminium co-doped and slowly cooled sample, which is about 15% of the commercial Al₂O₃:C(Landuer Inc.). A near fully optically sensitive TL peak around 155 °C is observed. The dose response is linear and practically no OSL fading is observed in first five days of storage in slowly cooled sample. This study on conventional sulphate-based TL phosphors will be useful in developing OSL phosphors for radiation dosimetry.     

Radiation stabilization of U5+ in CaO matrix and its thermal stability: Electron paramagnetic resonance and thermally stimulated luminescence studies

Electron paramagnetic resonance (EPR) evidence is presented for the radiation stabilization of pentavalent uranium in CaO matrix. From the theoretical predictions ofg value for U⁵⁺ in axial symmetries, it was concluded that U⁵⁺ at Ca²⁺ site is associated with a second neighbour charge compensating Ca²⁺ vacancy. EPR measurements also revealed the presence of Mn²⁺, Mn⁴⁺ and Cu²⁺ impurities in the samples. The thermal stability of U⁵⁺ was investigated using EPR and thermally stimulated luminescence (TSL) techniques. The TSL and EPR studies on gamma irradiated uranium doped calcium oxide samples had shown that the intense glow peak at 540 K is associated with the reduction in the intensity of EPR signal of U⁵⁺ ion around this temperature. This peak is associated with the process U⁵⁺+hole→U^6+*→U⁶⁺+hv. The activation energy for this process was determined to be 1.4eV.     

Thermally stimulated luminescence studies of undoped,Cu- and Mn-doped CaSO4 compounds

Thermally stimulated luminescence (TSL) of undoped and doped CaSO₄ with activators such as Cu and Mn has been investigated. The polycrystalline samples of undoped and doped CaSO₄ are prepared by the melting method. The formation of CaSO₄ compound is confirmed by X-ray diffraction and Fourier transform infrared studies. Scanning electron microscopic studies of CaSO₄ are also carried out.

The TSL glow curves of undoped CaSO₄, Cu- and Mn-doped CaSO₄ are studied. Comparison of the thermoluminescence (TL) intensity of the most intensive glow peak of Cu-doped CaSO₄ compound with that of undoped CaSO₄ shows that addition of Cu impurity in CaSO₄ compound enhances the TL intensity by about four times. However, the addition of Mn impurity to undoped CaSO₄ increases the TL intensity by about three times when compared with that of undoped CaSO₄. The TL-dose dependence of all three samples was studied and was observed to be almost linear in the studied range of irradiation time. Among the samples studied, namely undoped CaSO₄ and Cu- and Mn-doped CaSO₄, Cu-doped CaSO₄ is found to be the most sensitive. The trap parameters, namely order of kinetics (b), activation energy (E) and frequency factor (s) associated with the most intensive glow peaks of CaSO₄:Mn, CaSO₄:Cu and CaSO₄ phosphors were determined using the glow curve shape (Chen's) method.     

Decay kinetics and thermally stimulated luminescence of Mn4+ in SrTiO3

Abstract

Decay kinetics of Mn⁴⁺ luminescence in SrTiO₃ and thermal stimulation of this luminescence (TSL) after low-temperature irradiation with light from the 355—520 nm region, have been investigated in the temperature range of 4.2—150K and 12—110K, respectively, for the first time. It is concluded that TSL glow peaks are associated with thermal release of carriers from shallow traps followed by Mn⁵⁺→Mn⁴⁺(²E) and/or Mn³⁺→Mn⁴⁺ (²E) charge transfer decay kinetics has been explained by considering trap energy levels taking part in the TSL process.     

Cryostat for low temperature x-irradiation and electron paramagnetic resonance

A cryostat for x-irradiation of solid samples at liquid air temperature is described. The design of the sample mount in the cryostat enables quick transfer of the sample after x-irradiation into EPR cavity to facilitate EPR study without warming the sample.     

An X-band time domain electron paramagnetic resonance spectrometer

A computer-controlled X-band time domain electron paramagnetic resonance (EPR) spectrometer, with a time resolution of the order of 0.5μsec, has been constructed with many of the crucial microwave components designed and fabricated by the Microwave Engineering Group of TIFR. The spectrometer operates either in a microwave power pulsed mode for determination of spin-lattice relaxation times by the saturation recovery technique or in the kinetic mode for determination of the time dependence of EPR signal after laser excitation. It has an automatic frequency control, an automatic phase control and, most importantly, a field-frequency lock which ensures good stability of the EPR line positions enabling signal averaging for extended periods. The constructional details of the spectrometer and its performance in both the modes are described here by reporting results on certain typical systems.     

An electron paramagnetic resonance study of LDPE irradiated with high-energy electron beam

Cross-linking of the polyethylene was performed with a high-energy electron beam. Electron paramagnetic resonance spectroscopy was used to study the lifetime of unpaired electron in the irradiated samples. Time-dependent electrical parameters are investigated for the cross-linked low-density polyethylene. Both dielectric constant and dielectric strength almost remained unchanged, but for short times, the volume resistivity and loss factor increased and decreased, respectively. It is predicted that for lifetime longer than 48 h, the electrical parameters were constant. It is believed that the variation of some electrical properties during time is due to the effects of trapped electrons.     

Thermally stimulated luminescence of bismuth germanate ceramics with the benitoite,eulitine, and sillenite structures

Thermally stimulated luminescence (TSL) of Bi₂Ge₃O₉, Bi₄Ge₃O₁₂, and Bi₁₂GeO₂₀ and the primary components Bi₂O₃ and GeO₂ was studied under x-ray excitation. Thermal activation energies and frequency factors of trapping centers in the studied ceramics were determined. The relationships of TSL bands of the studied ceramics with maxima at 141–145 and 166–170 K and damage to the Ge sublattice and of TSL bands with maxima at 104–110 and 180–190 K and recombination processes in the Bi sublattice were demonstrated. Recombination processes causing luminescence upon nonequilibrium charge carrier release from trapping centers occur in structural complexes of similar configuration that contain the Bi ion in a nearest environment of O atoms. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 3, pp. 359–364, May–June, 2008.     

Thermally and optically stimulated radiative processes in LiBaF3 crystals

In LiBaF₃ crystals both valence–core transitions (5.4–6.5 eV) and so-called self-trapped exciton luminescence (about 4.3 eV) are important for practical application. Here, we present a study of 4.3 eV luminescence under photo- and thermostimulation after X-irradiation of undoped LiBaF₃ crystals at various temperatures. Optically stimulated luminescence as a result of electron recombination with both self-trapped holes and holes localized at some defects, were observed after X-irradiation below 130 K and that of electron recombination with defect-localized holes was observed after X-irradiation above 130 K. The spectra of thermo-stimulated luminescence (TSL) contain a broad band about 4 eV related to the electron (high-energy side) or hole (low-energy side) recombination depending on TSL peak temperature.     

Al2O3:C optically stimulated luminescence droplets: Characterization and applications in medical beams

Optically stimulated luminescence (OSL) detectors, which are widely used in radiation protection, offer a number of potential advantages for radiotherapy dosimetry. In this study we characterized 1-μl of OSL droplets consisting of a mixture of Al₂O₃:C powder and a photo-curable polymer, in addition to results described in a previous work (Nascimento et al., 2013). The concentration test showed that droplets have a higher spatial resolution than other commonly used Al₂O₃:C-based detectors. Our results from the dose response, reproducibility and dependence with accumulative dose were obtained for droplets with a powder/polymer concentration that showed a high Signal to Noise Ratio (SNR) without compromising the droplet malleability. Additional test results show the response of such droplets in percentage depth dose curves and dose profiles of clinical beams.     

Electron paramagnetic resonance studies of photorefractive crystals II: Fe3+ doped Bi12SiO20 with copper vapour laser illumination in 10–100 K range

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.     

High order electron paramagnetic resonance transitions of Gd3+ in Pr2Zn3(NO3)12·24H2O single crystals

Electron paramagnetic resonance of Gd³⁺ in Pr₂Zn₃(NO₃)₁₂ · 24H₂O single crystals has been studied at ∼ 9.45 GHz and at 285 K. In addition to the allowed fine structure lines (ΔM=± 1) some weak low field lines identified as ΔM = ± 2 transitions have been observed.     

Electron paramagnetic resonance investigation of K3H(SO4)2 proton conductor doped with Cr ion

The proton arrangement around SO₄ units in K₃H(SO₄)₂ (KHS) was studied in detail by X-band CW EPR spectra of CrO₄³⁻ paramagnetic centre incorporated into KHS during the crystal growth process. The EPR data prove the theoretical model of coherent motion of protons and SO₄ units at the fast-proton conducting phase proposed by Ito and Kamimura.     

Non-equilibrium effects in copper vapor laser pumped Nd3+ doped PVA film: Photo-electon paramagnetic resonance and photoacoustic spectral investigations

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.     

Thermally stimulated current and high temperature resistivity measurements of 4H semi-insulating silicon carbide

High purity semi-insulating 4H SiC single crystals have potential applications for room temperature radiation detectors because of the wide band gap and radiation hardness. To control carrier lifetime, a key parameter for high performance radiation detectors, it is important to understand the nature of the deep traps in this material. For this purpose, we have successfully applied thermally stimulated current (TSC) and high temperature resistivity measurements to investigate deep level centers in semi-insulating 4H SiC samples grown by physical vapor transport. High temperature resistivity measurements showed that the resistivity at elevated temperatures is controlled by the deep level with an activation energy of 1.56 eV. The dominant traps revealed by TSC measurements were at 1.1-1.2 eV. The deep trap levels in 4H-SiC samples, the impurity and point defect nature of TSC traps peaked at ∼106 K (0.23 eV), ∼126 K (0.32 eV), ∼370 K (0.95 eV), ∼456 K (1.1-1.2 eV) are discussed.     

Measurements of the 10B (n,He) reaction rate in zppr

Direct measurements of the ¹⁰B(n, He) reaction rate have been made in ZPPR. The measurements were made by isotope-dilution mass spectrometric analysis of stainless-steel encapsulated samples of enriched ¹⁰B. The results demonstrate that direct measurements of the ¹⁰B reaction rate in ZPPR are obtainable to an accuracy of ～1%. Comparisons with calculations give a predicted-to-experiment value of ～0.93. This value is consistent with earlier Rockwell measurements in benchmark neutron fields and further supports a need for revision in the ENDF/B cross section for ¹⁰B above ～0.1 MeV.     

Identification of intra‐grain and grain boundary defects in polycrystalline Si thin films by electron paramagnetic resonance

We investigate the characteristics of intra‐grain and grain boundary defects in polycrystalline Si films, by employing quantitative electron paramagnetic resonance measurements on liquid phase crystallized layers with an average grain size of 200 µm and tailored solid phase crystallized Si layers with similar intra‐grain morphology but systematically varied grain sizes between 0.25 µm and 1 µm. The defect characteristics are found to be composed of two distinctive g ‐values of g = 2.0055 and 2.0032, which are attributed to grain boundary defects and intra‐grain defects, respectively. Additional hydrogenation leads to a reduction of the overall defect concentration, while a rapid thermal annealing process primarily heals intra‐grain defects.