Radiation Damage Centers Research Articles

Radiative recombination in Ce-, Pr-, and Tb-doped barium fluoride

Open f-shell rare-earth ions doped into a solid state matrix often easily change their charge state via interactions with charge carriers generated by ionizing radiation. This feature promotes a desired efficient radiative recombination of separated charge carriers at rare-earth ion sites but it may also help to stabilize various radiation defects that destructively interfere with the scintillation. The well-known effect responsible for scintillation light loss due to absorptions introduced by these so-called ‘radiation damage’ centers in alkali halides has been identified and studied for a long time. In this communication we concentrate on a different and much less-known and studied effect in which radiation induced centers directly and actively participate in the scintillation process itself. We present and discuss some selected recent results that illustrate the importance of competition between the prompt radiative recombination via rare-earth ions and generation of radiation damage centers in barium fluoride crystals activated with Ce, Pr and Tb. We demonstrate that results of such measurements as radioluminescence spectra, VUV spectroscopy, low temperature thermoluminescence glow curves, isothermal decays, and scintillation time profiles can be consistently explained in the frame of a simple model that includes one recombination center (RE ion) and a number of charge traps. We find that the trap model of radiation damage centers such as V k centers describes reasonably well their participation in the scintillation process that includes creation (equivalent to charge carrier trapping) and thermally activated decomposition (charge carrier release). These effects are shown to account quantitatively for important characteristics of the scintillation process such as large variations in the scintillation light yield with temperature and longer decay times in the scintillation time profiles that effectively lower the scintillation light yield at ambient temperatures.

Investigation of the x-ray storage phosphors Cs2NaYF6:Pr3+ or Ce3+

We present Cs2NaYF6:Ce3+ or Pr3+ as new x-ray storage phosphors. The radiation damage centers upon x irradiation were investigated using magneto-optical and magnetic resonance spectroscopy. F centers are the photostimulable electron trap centers. For the activator Pr3+ it could be shown that upon room temperature x irradiation only F centers and Pr4+ hole trap centers are involved in the information storage and readout process. There are indications that the process is the same for Ce3+-doped Cs2NaYF6.

Investigation of Radiation-Induced Defects in Cs2NaYF6

We present a detailed study of the radiation damage centres produced in Cs2NaYF6 upon X-irradiation at 10 and 300 K. The defects were investigated using magnetooptical and magnetic resonance spectroscopy. As hole trap centres at low temperatures VK centres and HA-type centres were identified by EPR and their optical transitions by optical absorption and the magnetic circular dichroism of the absorption. As electron trap centres F centres were investigated in detail by ENDOR.

Electron paramagnetic resonance of gamma-irradiated single crystals of two isobutyric acid derivatives

The electron paramagnetic resonance spectra of γ-irradiated α-hydroxy isobutyric acid, (CH 3) 2C(OH)COOH and αa-amino isobutyric acid hydrochloride, (CH 3) 2 C(NH 2)COOH·HCl, single crystals and powders were investigated at 300 K and 113 K. The radiation damage centers in both compounds were attributed to the (CH 3) 2ĊCOOH radical. The hyperfine coupling constants of the six hydrogens of the two methyl groups were found to be equivalent, isotropic and 22.2 G at room temperature. However, at 113 K the hyperfine coupling constants of the methyl groups displayed a slightly anisotropic behavior. The g values were found to be isotropic and g = 2.0028 and g = 2.0032 respectively for the α-hydroxy isobutyric acid and α-amino isobutyric acid hydrochloride between 300 and 113 K.

ESR spectroscopics study of radicals formed from methoxycarbonylcholine picrate hemihydrate

The electron spin resonance of γ-irradiated single crystals of methoxycarbonylcholine picrate hemihydrate, C7H16NO3 + · C6H2N3O7 − · ½ H2O has been observed and analyzed for different orientations of the crystal in the magnetic field. The crystals have been investigated between 70 and 350 K. The spectra were found to be temperature independent and the radiation damage centers are attributed to – ĊŌOCH3 and –CH2CH2OĊ radicals. The g and hyperfine coupling constants were found to be almost isotropic with an average, g = 2.0060, a H1 = 4.4G for –CH2CH2OĊ, g = 2.0050, a H2 = 3.5G for –CH2CH2OĊ and g = 2.0045, a H = 3.5 G for –ĊŌOCH3. These values indicate a long-range coupling between the unpaired electron and H protons.

Isoelectronic hydrogen-related defects in silicon

We report data for two hydrogen-related radiation-damage centres, known as the and centres, and compare their properties with the other `B' centres in silicon. We show that the radiative lifetimes of the luminescence are long, for the centre at 6.5 K and for the centre at 2 K, consistent with the luminescence occurring from excitons bound to centres which are isoelectronic with the silicon lattice. The energies of the bound-exciton multiplet states and their response to magnetic fields can be fitted precisely when the primary axis of each centre is close to a axis in the silicon. The excitons have spectroscopic binding energies of 12 and 18 meV for the two centres. However, the vibronic sidebands indicate that relaxations of meV occur when the excitons are bound. The centres are thus excellent examples of defects whose excited states are resonant with the crystal band states until stabilized by the relaxation. Comparison of the properties of the known shallow hydrogen-related radiation-damage centres in silicon shows that they form a unified class of similar defects.

Hole-trapping sites and the mechanism of the photostimulated luminescence of the x-ray storage phosphor RbI:Tl+

Radiation damage centers generated by x irradiation at 80 and 300 K in the storage phosphor RbI:Tl+ were investigated with optically detected magnetic resonance and optical measurements. It is shown that the intensity of the photostimulated luminescence depends on the concentration of Tl2+ centers. Bleaching into any of the seven identified Tl2+ absorption bands destroys the Tl2+ centers and leads to a Tl+ emission as well as to VK centers. Proportionally to photodestruction of Tl2+ centers, the photostimulated luminescence is decreased. The traps for information storage in RbI:Tl+ are F centers and Tl2+ centers.

High Dose Dosimetry Using Optically Stimulated Luminescence

Very precise high level dosimetry is currently being accomplished using electron spin resonance (ESR) of the radiation damage centres in alanine. While the technique works well, the complications and cost of using ESR machines could be simplified by using optically stimulated luminescence (OSL) techniques. Battelle, in collaboration with Landauer, Inc., has been developing low cost OSL polymer detectors manufactured by either injection moulding or extrusion. LiF and Al2O3 are the OSL active materials that are uniformly mixed with a polymer prior to dosemeter fabrication. Grain sizes of the LiF are usually kept below 10 µm in order to disperse the material uniformly. Plastic manufacturing techniques readily produce uniform detectors. The aim of the project is to produce an inexpensive detector that covers the range of 10 R-106 R. Readout is accomplished using a laser or light source to excite the OSL radiation damage centres. Luminescence of the OSL radiation damage centres is measured by optical filtration of the laser or light source. The method of fabrication, readout techniques, and dosimetric response of the new detectors are presented.

X-ray storage phosphors

Abstract In X-ray storage phosphors an image is formed and stored by generation of room temperature stable radiation-induced electron and hole trap centres. The image is read-out by recording a photo-stimulated luminescence (PSL) from a doped activator, generally stimulating the electron trap centres. The best-known and hitherto most efficient X-ray storage phosphor is BaFBr:EuZ+. However, the exact mechanism of its functioning is not yet understood. The present discussion of the storage and read-out mechanisms is critically reviewed. New results about the role of oxygen Contamination of BaFBr:Eu2+ are presented: if there is much less oxygen present than Edc, then the PSL efficiency decreases and the stimulation energy increases. A new efficient X-ray storage phosphor is presented: Cs2NaYF6 doped with trivalent rare earth activators. Its properties are described and preliminary results on X-radiation-induced radiation damage centres in undoped Cs2NaYF6 are presented.

Carbon- and oxygen-related point defects created by annealing the dicarbon radiation-damage center in crystalline silicon.

Thermal annealing can destroy the dicarbon center with its zero-phonon line at 969 meV (7818 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$). After the destruction of the dicarbon center, we observed many previously unreported photoluminescence lines. The point defects providing these lines consist of carbon and oxygen atoms. Finally, the annealing of the dicarbon centers is not completely understood. The breakup process is not the only way to destroy the dicarbon centers and the presence of other defects in the crystal may also affect the annealing process.

ESR, optical absorption, and luminescence studies of the peroxy-radical defect in topaz.

Fast-neutron irradiation of natural topaz crystals produces a single paramagnetic radiation damage center in high concentration. ESR of this center shows a holelike spectrum with S=1/2 and a strongly anisotropic g tensor: ${\mathit{g}}_{\mathit{x}\mathit{x}}$=2.0027, ${\mathit{g}}_{\mathit{y}\mathit{y}}$=2.0055, and ${\mathit{g}}_{\mathit{z}\mathit{z}}$=2.0407. We identify this defect as an intrinsic ${\mathrm{O}}_{2}^{\mathrm{\ensuremath{-}}}$ center in the form of a peroxy radical. The orientation of the g tensor helps confirm this assignment, as does the extraordinary thermal stability; annealing temperatures near 800 \ifmmode^\circ\else\textdegree\fi{}C are required for complete removal. Two uv absorption bands are associated with the peroxy radical, each with oscillator strength near 0.09. Pumping in the higher energy band leads to a polarization-sensitive 2.5-eV luminescence; the other uv band apparently relaxes nonradiatively.

The characteristics of defects in hydrogen ion irradiated MIS

The dominant divacancies, distributions of H atoms and displaced Si D atom-type defects related radiation damage center in metal-insulator-semiconductor (MIS) p-type Si were mainly studied. Two “hole traps” and new Dit (interface states density) introduced by 2.0 MeV, 3 × 10 12 − 3 × 10 14 H + -ions cm −2, at room temperature implantation, were also investigated in detail by using DLTS (Deep Level Transient Spectroscopy) technique. One of traps at E v + E T = E v + 0.18eV ( σ p = 2.8 × 10 −16cm 2) is assigned as the divacancy [V 2], and the other at E V + E T = E V + 0.36 eV ( σ p = 3.3 × 10 −17cm 2) is assigned as the combination of the [SiH-V F] complex and displaced Si D. It was found that the concentration of these defects is correlated to the total dose implanted into MIS.

Radiative recombination in MBE-prepared epitaxial Si and Si 1- xGe x layers

Examination of near-gap low-temperature photoluminescence (PL) from epitaxial, MBE-prepared Si and Si 1- x Ge x alloys, prepared commensurately to (0 0 1) Si, failed to show either free excitons or dopant- bound-excitons in as-prepared layers. Significant luminescence was, however, successfully induced by the selective introduction of two well-known, and moderately shallow, radiation-damage centers ( I 1 and G) into these epi-layers. Both centers exhibit alloy broadened spectra which, together with detailed PL intensity vs etch depth, indeed demonstrate that the radiative defects are contained well-within the epitaxial layers.

Laser-Induced Optically Stimulated M Centre Luminescence in LiF

Abstract Highly purified LiF crystals have long been used as optical windows. Optical grade LiF is known for its excellent transmission from the deep ultraviolet through the infrared. Radiation dosimetry applications have so far been restricted to high level Mega-Rad gamma dosimetry using radiation-induced absorption peaks measured using a spectrophotometer. However, absorption measurements are a very insensitive way to measure these radiation damage centres. A more sensitive method of measuring radiation damage can be accomplished by measuring the M centre luminescence. At the Pacific Northwest Laboratory, excitation with a 442 nm He-Cd laser has significantly amplified the luminescence yield of the M centre. The He-Cd laser was the preferred excitation laser because absorption measurements in LiF indicate the peak of the M centre absorption occurs at 443 nm. Laser stimulation produces an excited state of the M centre, which undergoes a very strong Stokes' shift. The peak of the M centre emission spectrum occurs at 665 nm with a half-width of 0.36 eV. Since the excitation wavelength differs significantly from the emission wavelength, measurement of the deep red emission can be done simultaneously with the excitation. Therefore, readout of the LiF can be accomplished in a fraction of a second.

Carbon-related radiation damage centres and processes in p-type Si

The paper is concerned with carbon interstitial (Ci), Ev+0.28 eV, and a carbon-related centre, Ev+0.37 eV, growing almost simultaneously with the removal of the Ci atoms in electron-irradiated p-type Cz-grown Si. An observed inverse annealing stage of the former defect at approximately 275 K most likely indicates the existence of internal processes taking place below room temperature which liberate Ci as a final product. These processes are in accord with other experimental findings showing that the concentration of the Ev+0.37 eV defect is larger than that of the Ci. The identity of the two centres found to contribute to the creation of the Ev+0.37 eV defect state is discussed.

Electron spin resonance of gamma irradiated single crystals of acetylcholine iodide and bromide

The electron spin resonance spectra of γ-irradiated acetylcholine iodide, [CH 3COOCH 2CH 2N(CH 3) 3]I, and acetylcholine bromide, [CH 3COOCH 2CH 2N(CH 3) 3Br single crystals and powders have been investigated between 120 and 300 K. The spectra are found to be temperature independent and the radiation damage centers are attributed to CH 3&ċ;ŌO- radicals. The hyperfine coupling constants of CH 3 protons were found to be isotropic and 17 gauss for both compounds. Although some unclear bumps were observed in the spectra of acetylcholine bromide, only one site could be traced at all orientations of the magnetic field. The isotropic values of the g factors are found to be 2.0044 and 2.0045 respectively for acetylcholine iodide and bromide. Principal values of the g tensors and their direction cosines are given.

A spin dependent recombination study of radiation induced defects at and near the Si/SiO/sub 2/ interface

A novel electron spin resonance technique, spin-dependent recombination (SDR), permits extremely rapid, high-signal-to-noise-ratio electron spin resonance (ESR) measurements of electrically active radiation damage centers in (relatively) hard MOS transistors in integrated circuits. SDR was used to observe the radiation-induced buildup of P/sub bo/ and E' centers at relatively low concentration in individual MOSFETs in integrated circuits with

Pressure Dependence of a Deep Excitonic Level in Silicon

AbstractCertain optically active defects in silicon provide a unique opportunity to observe, in detail, the effect of hydrostatic pressure on a deep level. We present a photoluminescence (5 – 100K) study of one such defect, the I, radiation-damage center, under high hydrostatic pressures (1–50 kbar). While the energy variation of this level indicates the expected mutli-band nature typical of a deep level, a severe and continuous reduction in the observed luminescence intensity was also observed. Temperature dependence, time resolved photoluminescence, and photoluminescence excitation spectroscopy are employed to attempt to discern the mechanism involved.

Defect Induced Luminescence from MBE Prepared Si/Si1-xGex Superlattices

AbstractWe report the first definitive observation of photoluminescence from Si/Si1-xGex superlattice heterostructures. Excitons bound to a deep-level radiation damage center (I1) are observed in a series of low Ge content (x = 0.05) unrelaxed structures. We also present preliminary results on photoluminescence from as-grown narrow period (Si6Ge4) superlattices.

Correlation of the concentration of the carbon-associated radiation damage levels with the total carbon concentration in silicon

The dominant carbon-related radiation damage center in silicon was studied in detail by deep level transient spectroscopy. Samples with different carbon and oxygen content were implanted with gradually increasing proton fluence. Two energetically closely spaced levels were revealed and tentative identities were assigned. One at ET+EV=0.344 eV (σp=1.1×10−16 cm2) is assigned as the C+Oi complex, and that at ET+EV=0.370 eV (σp=8×10−18 cm2) is assigned as the Cs-Sii-Cs complex. It was shown that the concentration of these defects is correlated to the total concentration of carbon in the crystal.