Research Physicist, 203 publications in books and refereed journals, 114 as first author. Fellow of the American Association for the Advancement of Science. Fellow the American Physical Society. Fellow of the American Ceramic Society. Many awards. New work on the geology of extraterrestrial impacts on the earth.
A b s t r a c t -The i n f l u e n c e s o f s m a l l p a r t i c l e s i z e s are c o n s i d ... more A b s t r a c t -The i n f l u e n c e s o f s m a l l p a r t i c l e s i z e s are c o n s i d e r e d i n a n a l y z i n g t h e t e m p e r a t u r e d e p e n d e n c e s o f t h e a n i s o t r o p y -d e t e r m i n e d FMR l i n e w i d t h a n d t h e i n t eg r a t e d FMR i n t e n s i t y o f m a g n e t i c p r e c i p i t a t e s i n g l a s s . Some o f t h e s p e c t r a u s e d h e r e were obtained by E. J. F r i e b e l e , C. L. M a r q u a r d t , o r R. A. Weeks i n c o n j u n c t i o n w i t h earlier s t u d i e s . Samp l e s 2.1-600-16,FQ-xxx, and S-17-3 were provided by M. P. O'Horo, D. B. Shinn and R. .M. Housley, respect i v e l y .
Induced loss spectra in four Al-coated large-core optical fibers are measured in the range 380-10... more Induced loss spectra in four Al-coated large-core optical fibers are measured in the range 380-1000 nm during and after 60 Co ␥ irradiation. These fibers had F-doped-silica claddings and synthetic silica core materials produced by different technologies. Measurements were carried out independently in Washington and Moscow using differing techniques but yielding very similar results. The highest radiation resistance is exhibited by two fibers with extremely low OH and Cl contents in their cores, one with a core material containing 0.5 mass % of fluorine, fabricated specially for this study, and one based on KS-4V silica, an original technology developed in Russia. The latter fiber demonstrates a rather flat induced loss spectrum in the visible region at a level of 2-2.5 dB/m for ␥-ray doses of ϳ10 MGy͑Si͒.
Direct evidence for the creation of oxygen-vacancy, oxygen-interstitial pairs in SiO2 glasses by ... more Direct evidence for the creation of oxygen-vacancy, oxygen-interstitial pairs in SiO2 glasses by an excitonic mechanism is developed. This finding is based on an electron-spin-resonance study of high-purity fused silicas exposed to highly focused 6.4-eV excimer laser light.
ABSTRACT Influences of oxygen-deficiency on radiation properties of high-purity, low-OH fused sil... more ABSTRACT Influences of oxygen-deficiency on radiation properties of high-purity, low-OH fused silica were studied. It is found that thermostimulated luminescence (TSL) peaks are different for photo (7.7 eV) and X-ray excitation at 77 K. X-ray excitation produces TSL peaks at 125 and 170 K corresponding to the anneal temperatures of two types of self-trapped holes centers STH2 and STH1, respectively, detected by electron spin resonance (ESR). Oxygen-deficiency apparently increases the number of electron traps, stabilizing a larger number of STHs in the continuous defect-free silica network than is observed in similarly X-irradiated stoichiometric silica glasses. Photoexcitation of oxygen-deficient glass at 7.7 eV produces TSL peaks at 105 and 200 K, presently attributed to STHs at perturbed sites in the immediate vicinity of oxygen-deficiency centers (ODCs). High temperature TSL peaks at 240 and 400 K are produced by both types of irradiations and are followed by ESR detection of E′ centers only. All observed TSL and ESR signals were proportional to the level of oxygen-deficiency. The main spectral band in TSL near 2.7 eV is a triplet–singlet transition, ascribable to the twofold-coordinated silicon center (ODC(II)) modified by its nearest structure. It is proposed that this recombination process results when a thermally detrapped STH encounters an electron trapped at the site of ODC(I), and is transmuted into a modified ODC(II):e−. In principle, any such ODC:e− defect should be paramagnetic. However, no trapped-electron centers were detected by ESR in the present experiments. Based on the recent work of others, the centers that were observed are believed to be trapped hole centers. Thus, for reasons unknown, the postulated ODC(II):e− centers, which may be the primary electron traps in oxygen-deficient silicas, appear to be ESR-silent.
Electron spin resonance studies have been carried out at X-band frequencies on a large number of ... more Electron spin resonance studies have been carried out at X-band frequencies on a large number of glasses spanning the lithium, sodium, and potassium borate glass systems, following x irradiation at cryogenic temperatures (T ∼ 25 or ∼ 77°K). A new, intrinsic paramagnetic defect center characterized by a sizeable hyperfine interaction with boron [A (11B) ∼ 100 G] was observed, in addition to the more familiar boron–oxygen hole centers. The new center, which is readily destroyed by visible light or thermal annealing above ∼ 100°K, is shown to be of the trapped-electron type. By means of computer simulations of the spectra, a reasonably complete set of spin–Hamiltonian parameters is derived in spite of the obscuring effects of a substantial distribution in hyperfine coupling constants. Possible models for the new defect are considered and its potential usefulness as a “probe” of the structure of alkali borate glasses is discussed.
An electron-spin-resonance (ESR) study has been carried out on gamma-ray-induced color centers in... more An electron-spin-resonance (ESR) study has been carried out on gamma-ray-induced color centers in a pure-SiO2-core B2O3.3SiO2-clad prototype optical fiber and in bulk glasses of the core and cladding compositions. Defects in the core material included the familiar silicon E' center and other centers tentatively attributed to oxygen-associated structures. Approximately 102 times more centers per unit mass and unit gamma-ray dose were created in the cladding material than in the core glass; these comprised just three defect types: the boron-oxygen hole center (BOHC), the silicon E' center, and a boron analog of the latter, the boron E' center. These same defects were also observed in the irradiated prototype fiber in concentrations comparable to those attained in the bulk glass of the cladding composition. The unexpectedly high radiation sensitivity of the fiber, therefore, would appear to be due to the efficient production of (intrinsic) boron-related defects in the cladding. Considerable insight has been gained into the electronic structures of the BOHC and the boron E' center, by means of careful computer simulations of their ESR line shapes in B2O3.SiO2 glasses, as functions of both frequency and boron isotopic enrichment.
Optical absorption in Si02-Core/Si02:F-Clad fibers was investigated in situ during 6oCo 7 irradia... more Optical absorption in Si02-Core/Si02:F-Clad fibers was investigated in situ during 6oCo 7 irradiation. Bands at 610 and c400 nm were first created, then permanently erased by exposures > 1 MGy(Si) in the presence of injected visible light ( -5 pW).
Sources of the 'characteristic' ferromagnetic resonance spectrum of lunar soils are con... more Sources of the 'characteristic' ferromagnetic resonance spectrum of lunar soils are considered, and some relevant implications of a line shape analysis are presented. It is shown that potential contributions of ferromagnetic phases having axial magnetocrystalline anisotropy can only be ruled out by careful computer line shape simulations, and that positive anisotropy constants are consistent with the presence of both body-centered cubic metallic iron and certain cation-substituted magnetites.
A b s t r a c t -The i n f l u e n c e s o f s m a l l p a r t i c l e s i z e s are c o n s i d ... more A b s t r a c t -The i n f l u e n c e s o f s m a l l p a r t i c l e s i z e s are c o n s i d e r e d i n a n a l y z i n g t h e t e m p e r a t u r e d e p e n d e n c e s o f t h e a n i s o t r o p y -d e t e r m i n e d FMR l i n e w i d t h a n d t h e i n t eg r a t e d FMR i n t e n s i t y o f m a g n e t i c p r e c i p i t a t e s i n g l a s s . Some o f t h e s p e c t r a u s e d h e r e were obtained by E. J. F r i e b e l e , C. L. M a r q u a r d t , o r R. A. Weeks i n c o n j u n c t i o n w i t h earlier s t u d i e s . Samp l e s 2.1-600-16,FQ-xxx, and S-17-3 were provided by M. P. O'Horo, D. B. Shinn and R. .M. Housley, respect i v e l y .
Induced loss spectra in four Al-coated large-core optical fibers are measured in the range 380-10... more Induced loss spectra in four Al-coated large-core optical fibers are measured in the range 380-1000 nm during and after 60 Co ␥ irradiation. These fibers had F-doped-silica claddings and synthetic silica core materials produced by different technologies. Measurements were carried out independently in Washington and Moscow using differing techniques but yielding very similar results. The highest radiation resistance is exhibited by two fibers with extremely low OH and Cl contents in their cores, one with a core material containing 0.5 mass % of fluorine, fabricated specially for this study, and one based on KS-4V silica, an original technology developed in Russia. The latter fiber demonstrates a rather flat induced loss spectrum in the visible region at a level of 2-2.5 dB/m for ␥-ray doses of ϳ10 MGy͑Si͒.
Direct evidence for the creation of oxygen-vacancy, oxygen-interstitial pairs in SiO2 glasses by ... more Direct evidence for the creation of oxygen-vacancy, oxygen-interstitial pairs in SiO2 glasses by an excitonic mechanism is developed. This finding is based on an electron-spin-resonance study of high-purity fused silicas exposed to highly focused 6.4-eV excimer laser light.
ABSTRACT Influences of oxygen-deficiency on radiation properties of high-purity, low-OH fused sil... more ABSTRACT Influences of oxygen-deficiency on radiation properties of high-purity, low-OH fused silica were studied. It is found that thermostimulated luminescence (TSL) peaks are different for photo (7.7 eV) and X-ray excitation at 77 K. X-ray excitation produces TSL peaks at 125 and 170 K corresponding to the anneal temperatures of two types of self-trapped holes centers STH2 and STH1, respectively, detected by electron spin resonance (ESR). Oxygen-deficiency apparently increases the number of electron traps, stabilizing a larger number of STHs in the continuous defect-free silica network than is observed in similarly X-irradiated stoichiometric silica glasses. Photoexcitation of oxygen-deficient glass at 7.7 eV produces TSL peaks at 105 and 200 K, presently attributed to STHs at perturbed sites in the immediate vicinity of oxygen-deficiency centers (ODCs). High temperature TSL peaks at 240 and 400 K are produced by both types of irradiations and are followed by ESR detection of E′ centers only. All observed TSL and ESR signals were proportional to the level of oxygen-deficiency. The main spectral band in TSL near 2.7 eV is a triplet–singlet transition, ascribable to the twofold-coordinated silicon center (ODC(II)) modified by its nearest structure. It is proposed that this recombination process results when a thermally detrapped STH encounters an electron trapped at the site of ODC(I), and is transmuted into a modified ODC(II):e−. In principle, any such ODC:e− defect should be paramagnetic. However, no trapped-electron centers were detected by ESR in the present experiments. Based on the recent work of others, the centers that were observed are believed to be trapped hole centers. Thus, for reasons unknown, the postulated ODC(II):e− centers, which may be the primary electron traps in oxygen-deficient silicas, appear to be ESR-silent.
Electron spin resonance studies have been carried out at X-band frequencies on a large number of ... more Electron spin resonance studies have been carried out at X-band frequencies on a large number of glasses spanning the lithium, sodium, and potassium borate glass systems, following x irradiation at cryogenic temperatures (T ∼ 25 or ∼ 77°K). A new, intrinsic paramagnetic defect center characterized by a sizeable hyperfine interaction with boron [A (11B) ∼ 100 G] was observed, in addition to the more familiar boron–oxygen hole centers. The new center, which is readily destroyed by visible light or thermal annealing above ∼ 100°K, is shown to be of the trapped-electron type. By means of computer simulations of the spectra, a reasonably complete set of spin–Hamiltonian parameters is derived in spite of the obscuring effects of a substantial distribution in hyperfine coupling constants. Possible models for the new defect are considered and its potential usefulness as a “probe” of the structure of alkali borate glasses is discussed.
An electron-spin-resonance (ESR) study has been carried out on gamma-ray-induced color centers in... more An electron-spin-resonance (ESR) study has been carried out on gamma-ray-induced color centers in a pure-SiO2-core B2O3.3SiO2-clad prototype optical fiber and in bulk glasses of the core and cladding compositions. Defects in the core material included the familiar silicon E' center and other centers tentatively attributed to oxygen-associated structures. Approximately 102 times more centers per unit mass and unit gamma-ray dose were created in the cladding material than in the core glass; these comprised just three defect types: the boron-oxygen hole center (BOHC), the silicon E' center, and a boron analog of the latter, the boron E' center. These same defects were also observed in the irradiated prototype fiber in concentrations comparable to those attained in the bulk glass of the cladding composition. The unexpectedly high radiation sensitivity of the fiber, therefore, would appear to be due to the efficient production of (intrinsic) boron-related defects in the cladding. Considerable insight has been gained into the electronic structures of the BOHC and the boron E' center, by means of careful computer simulations of their ESR line shapes in B2O3.SiO2 glasses, as functions of both frequency and boron isotopic enrichment.
Optical absorption in Si02-Core/Si02:F-Clad fibers was investigated in situ during 6oCo 7 irradia... more Optical absorption in Si02-Core/Si02:F-Clad fibers was investigated in situ during 6oCo 7 irradiation. Bands at 610 and c400 nm were first created, then permanently erased by exposures > 1 MGy(Si) in the presence of injected visible light ( -5 pW).
Sources of the 'characteristic' ferromagnetic resonance spectrum of lunar soils are con... more Sources of the 'characteristic' ferromagnetic resonance spectrum of lunar soils are considered, and some relevant implications of a line shape analysis are presented. It is shown that potential contributions of ferromagnetic phases having axial magnetocrystalline anisotropy can only be ruled out by careful computer line shape simulations, and that positive anisotropy constants are consistent with the presence of both body-centered cubic metallic iron and certain cation-substituted magnetites.
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Papers by D. Griscom