Radiation-induced Centers Research Articles

Limits of retrospective accident dosimetry by EPR and TL with natural materials

The radiation response of natural materials and domestic articles was investigated by EPR and TL to select suitable materials for retrospective dose assessment in accident dosimetry. The thermal stability and the influence of UV-exposure to the radiation-induced EPR centres were investigated. Based on a required precision of ±20% for dosimetry the lower limits of applicability of the materials were determined. The lowest dosimetry limits of 0.5 Gy were found using sugar, boiler scale and egg shells by EPR and 0.3 Gy by using TL with boiler scale. A list of materials found not to be applicable for retrospective radiation accident EPR dosimetry is also given.

The Observations of Paramagnetic Centers in Tellurite Glasses

AbstractThis is the first report of ESR observations of three distinct paramagnetic centers in TeO29 glasses. One center is intrinsic to the glass and the other two are induced by KrFexcimer-laser radiation. The intrinsic center with a broad ESR spectrum is tentatively identified as an oxygen-associated hole center. One radiation-induced center fades slowly at room temperature; its proposed structure is that of an electron trapped in a diffuse orbital associated with a modifier ion. The other radiation-induced center is stable at room temperature and corresponds to the Vo· center observed in crystalline paratellurite.

The Role of Core Levels in Scintillation Processes

ABSTRACTThe Auger decay of a core hole results in appearance of several strongly correlated excitations. This excited region strongly polarizes the lattice and thus the defect creation is possible. In all cases the core hole causes the strong local perturbation of electronic and lattice subsystems. The creation of such excited region with mutual relaxation of correlated electrons and holes can result in the increase of the efficiency of energy transfer to activators, acceleration of the luminescence kinetics, and the appearance of radiation-induced luminescence centers.

Paramagnetic defect centers in BESOI and SIMOX buried oxides

Electron paramagnetic resonance (EPR) and capacitance-voltage measurements have been combined to identify the chemical nature and charge state of defects in BESOI (bonded and etchback silicon-on-insulator) and SIMOX (separation by implantation of oxygen) materials. The four types of defect centers observed, charged oxygen vacancies, delocalized hole centers, amorphous-Si centers, and oxygen-related donors, are strikingly similar. In the BESOI materials, the radiation-induced EPR centers are located at or near the bonded interface. Therefore, the bonded interface is a potential hole trap site and may lead to radiation-induced back-channel leakage. In SIMOX materials, it is found that all of the defects in the buried oxide are due to excess Si. The results using poly-Si/thermal oxide/Si structures strongly suggest that it is the postimplantation, high-temperature anneal processing step in SIMOX that leads to their existence. The anneal leads to the out-diffusion of oxygen from the buried oxide, creating excess-Si related defects in the oxide and O-related donors in the underlying Si substrates. A relatively new class of defects in SiO/sub 2/, delocalized spin centers, are found to be hole traps in both SIMOX and BESOI materials.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

An ESR study of gamma-irradiated borosilicate glass containing cesium and strontium oxides

Until the mid eighties, it was considered that ionizing ({beta},{gamma})-radiation from radionuclides in vitrified waste from reactor fuel reprocessing did not have a critical effect on the structural, mechanical, or chemical properties of the matrix. That conclusion was based on accelerator experiments using electron beams producing a 10{sup 4}-10{sup 6} times higher irradiation dose rate than under real conditions of internal irradiation. It was, however, shown in 1985 that {sup 60}Co gamma radiation at a comparatively low dose rate formed oxygen-filled microbubbles stochastically distributed through the volume of borosilicate glasses, even at a dose attained after 10 years` storage (and not 1 million years`, as followed from the electron irradiation date). Obviously, the generation of oxygen is accompanied by the growth of internal pressure, change in chemical composition, and increase surface area. From a practical point of view, this can lead to a dangerous deterioration in the isolating function of the material. In the present work, we have studied the dynamics of formation of radiation-induced oxygen centers in a borosilicate matrix containing cesium and strontium oxides.

Identification of a radiation-induced hole center in KTiOPO4.

Electron-paramagnetic-resonance (EPR) and electron-nuclear double-resonance (ENDOR) techniques have been used to determine the structure of a radiation-induced trapped-hole center in potassium titanyl phosphate (KTiOPO 4 or KTP). A single crystal of KTP was irradiated with 60-kV x rays at 77 K to produce the S=1/2 hole center. Data were then taken near 30 K. Spin-Hamiltonian parameters describing the g matrix and three 31 P hyperfine matrices were obtained from the angular dependence of the EPR and ENDOR spectra, respectively. The principal values of the g matrix are 2.0008, 2.0250, and 2.0422 and the principal values of the largest phosphorus interaction are -6.809, -8.125, and -3.848 MHz

Using electron irradiation to control the turn-off time of static induction transistors

Experimental data on the effect of irradiation and anneal treatments on the electric characteristics of high-voltage power static induction transistors (BSITS) are given. The treatments include bombardment by high-energy electrons and a subsequent isothermal treatment. The anneal behaviour of radiation-induced centres in the operating elements of BSITS is discussed. A criterion of preference for the centres, which control lifetimes, is suggested. The criterion of preference fits the A-centre. In addition, it is shown that the irradiation/anneal processing gives the best improvement of the semiconductor device characteristics when the phosphorus impurity concentration greatly exceeds the oxygen concentration in the heavily doped Si substrate and the oxygen content greatly exceeds the carbon content in the epilayer.

Thermally stimulated luminescence studies of a LiYF4:U4+ crystal

The present paper gives the first detailed report on trap level spectroscopic studies of gamma irradiated lithium-yttrium tetrafluoride single crystals doped with tetravalent natural uranium. The work carried out includes the growth of single crystals followed by thermally stimulated luminescence (TSL) and electron spin resonance (ESR) studies on the gamma irradiated crystal. The TSL studies in the 300–600 K temperature range have revealed the presence of four glow-peaks. The trap parameters have been calculated for the two prominent peaks. The predominant radiation-induced colour centre has been identified as an F centre and its growth and annihilation behaviour has been studied in detail. ESR studies on the gamma irradiated crystal at 300 K have shown formation of U 5+ ions in small amounts. Further, the spectral character of the glow-peaks revealed the presence of a strong emission group at 690 nm, characteristic of tetravalent uranium ions. It has been suggested that U 4+ ions play an important role in TSL, taking part in the (U 4+ →U 5+ ) process.

Characterization of 3CSiC epilayers by pulsed electron spin resonance

Electron spin resonance (ESR) centres in as-grown and irradiated 3CSic epilayers were studied using the electron spin echo method for the first time. The spin echo signals of as-grown samples were examined by their decomposition to components with different relaxation times. The spin-spin relaxation times of the radiation-induced ESR centres were measured at various temperatures.

Effects of soft X-ray irradiation on solid-state imagers

The visible effects of soft X-ray damage in solid-state imagers are changes in the flat-band voltages, degradation in charge transfer efficiency and increased dark current. Details of these effects, and possible underlying mechanisms, are discussed. Deep level transient spectroscopy is introduced as a useful tool for identifying existing and radiation-induced trapping centres in charge-coupled devices. The effects of various annealing methods for charge-coupled and photodiode imagers are presented.

In vivo assessment of radiation exposure.

Three workers at a San Salvador 60Co irradiation facility were victims of a radiation accident, resulting in the amputation of a leg from two workers. The absorbed dose to bone was successfully assessed by electron paramagnetic resonance (EPR) quantitation of radiation-induced hydroxyapatite centers.

A review of selected techniques for characterizing radiation-induced defects in solar cells

Abstract A review of three promising defect characterization techniques is presented. It is shown how deep level transient spectroscopy can provide information about energy level, density and capture cross-section of electrically active defects produced by irradiation. Both Doppler and lifetime positron annihilation spectroscopies are reviewed to show how configuration of radiation-induced defects can be identified by these techniques. Finally, the electron paramagnetic resonance technique is briefly reviewed and its usefulness is demonstrated by describing the investigations of radiation-induced Si-A center and positively charged silicon vacancy defect.

A numerical study of cluster center formation in neutron-irradiated silicon

A numerical study of the formation of a radiation-induced defect cluster center in neutron-irradiated silicon has been performed by solving a set of semilinear parabolic reaction-diffusion-coupled equations. It is found that most of the primary displacement defects [interstitial (I) and vacancy (V)] will be annihilated by I-V direct recombination in an extremely short time. In particular, the formation of four-vacancy defects is independent of the concentration of sinks and impurities in a sample, and of the energy of the recoil particle relatively. The threshold energy for the formation of a vacancy cluster has also been studied. The results are discussed with the experimental observations.

Paramagnetic Defect Centers in Hydrothermal Kaolinite from an Altered Tuff in the Nopal Uranium Deposit, Chihuahua, Mexico

Point defect centers in hydrothermal kaolinite have been investigated using electron para- magnetic resonance (EPR). Kaolinite was sampled in petrographically well-defined materials coming from uranium-rich hydrotherrnally altered volcanic tufts (Nopal I uranium deposit, Chihuahua, Mexico), which show extensive kaolinization and an intense redistribution of uranium. Several kaolinite parageneses were defined according to their origin (fissure fillings and feldspar pseudomorphs); their location relative to the U 6+ mineralization at the scale of the deposit (mineralized breccia pipe vs. barren surrounding rhyolitic tufts), and at the scale of mineral assemblages; and their crystal chemistry. Two types of centers of axial symmetry were identified (A- and N-centers) and represent positive holes trapped on apical oxygens (Si-O--centers). A-centers were stable to 400"12, whereas A'-centers annealed at 350"C. A relation between defect-center concentration and U content demonstrates that natural irra- diation was responsible for these centers. On the other hand, defect-center concentration was not directly linked to the origin (fissural or feldspar pseudomorph) or the crystal chemistry (structural order and substitutional Fe 3+ content) of the kaolinite, According to petrographic data, and with respect to the relative thermal stability of A- and A'-centers, two successive irradiations of kaolinite were evidenced: (1) originally during crystallization of kaolinite from radioactive hydrothermal solutions, and (2) per- manently when kaolinite was in contact with secondary U-silicates, which led to the formation of A'- centers. Because of the short half-life of U, these two radiation-induced centers were created by short-lived elements of the U-decay series. As a consequence, variations of defect-center concentration possibly reflect variations in radioactive disequilibrium during the history of the alteration system. This provides a unique tool for tracing the dynamics of the transfer of radionuclides in the geosphere: kaolinite may be used as a sensitive in situ dosimeter, which may be useful in the fields of weathering petrology and nuclear waste management.

Deuterium diffusion and magnetic resonance investigations in LiTaO3 single crystals.

The diffusion coefficients of deuterium in single crystals of ${\mathrm{LiTaO}}_{3}$ have been measured by monitoring the growth of ${\mathrm{OD}}^{\mathrm{\ensuremath{-}}}$ infrared-absorption bands in samples heated in flowing ${\mathrm{D}}_{2}$O vapor. At 950 K, the diffusion coefficient is 1.7\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}8}$ ${\mathrm{cm}}^{2}$/s for diffusion along the crystallographic c axis, and the activation energy is 1.5 eV. Electric-field-enhanced diffusion of deuterium was also demonstrated. Proton replacement and electron-nuclear double resonance experiments have shown that the structure previously reported in the electron paramagnetic resonance spectrum of a radiation-induced center in ${\mathrm{LiTaO}}_{3}$ is not due to a hyperfine interaction with protons.

E.S.R. studies on melinex irradiated with oxygen ions

E.S.R. investigations on Melinex irradiated with high-energy oxygen ions are reported. The angular dependence of radiation-induced active centres is investigated. Tentatively, experimental data are related to high energy ion tracks.

The problem of long-term fading of absorbed palaeodose on ESR-dating of quaternary mollusc shells

Due to relatively low thermal stability of radiation-induced centres in biogenic carbonate of subfossil molluscs (mean life θ = 3.7 × 10 5 y at 10°C, g = 2.0012) correct dating of the latter by means of ESR-spectroscopy without considering natural relaxation of absorbed palaeodose my prove rather problematic. The long-term fading of palaeodose complicates correlation between absorbed natural radiation energy, and the value of the laboratory effect observed restricts the upper dating limit and may result in considerably younger ages on dating of the most ancient Quaternary samples. The present paper deals with the connection between the absorbed palaeodose value and the age of the sample, its dosimetric properties, as well as with the effect of different factors on the upper dating limit. Energetic parameters of CO 3- 3 centres (g = 2.0012) have been determined for the shell's carbonate: trap depth E = 1.52 ± 0.01 eV, frequency factor s = 8 × 10 13 s -1, mean life θ = 1.14 × 10 6y at 5°C. Mathematical models of the palaeodose accumulation in nature are discussed. The author suggests palaeodosimetric equations for calculating the age of shells and some other materials. The equations may be applied both to ESR- and TL-dating.

X-ray and UV influence on the optical absorption spectra of the non-photorefractive lithium niobate

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Gold and Platinum Doped Radiation Resistant Silicon Diode Detectors

Abstract Silicon diode detectors are subject to radiation damage in high energy electron and photon beams, which reduces their sensitivity. This is due to the reduction in minority carrier lifetime and thence diffusion length by the introduction of recombination centres (primarily the divacancy centre in n-type silicon which introduces an acceptor level at Ec-0.41 eV). Gold or platinum doping has the effect of reducing minority carrier lifetime by introducing recombination centres, such that the additional reduction due to radiation-induced centres is small. Groups of undoped, Au-doped, and Pt-doped diodes made from n-base silicon were irradiated with 15 MeV electrons. After a dose of 2000 Gy, the undoped diodes exhibited a sensitivity equal to 60% of the original, whereas the Au and Pt doped diodes retained 92% of their original sensitivity. The Pt-doped diodes appear to be the most useful for dosimetry due to lower leakage current.

Room-Temperature Annealing Effects on Radiation-Induced Defects in InP Crystals and Solar Cells

Remarkable defect annealing in both p-type and n-type InP following 1-MeV electron irradiation has been observed at room temperature, resulting in the recovery of InP solar cell properties. The room-temperature annealing characteristics of radiation-induced defects in InP were studied by measuring InP solar cell photovoltaic properties in conjunction with deep-level transient spectroscopy. The recovery of InP solar cell radiation damage is found to be due mainly to the room-temperature annihilation of radiation-induced recombination centers such as an H4 trap (Ev+0.37 eV) in p-InP. Moreover, the room-temperature annealing rate of radiation-induced defects in InP was found to be proportional to the 2/3 power of the carrier concentration. Additionally, a model has been considered in which point defects diffuse to sinks through impurities so as to annihilate and bind with impurities, thus forming point defect-impurity complexes.