Unirradiated Crystals Research Articles

Influence of neutron-irradiation-induced defects on the flux pinning inHgBa2Ca2Cu3O8+xsingle crystals

The influence of fast neutron irradiation on flux pinning in ${\mathrm{HgBa}}_{2}{\mathrm{Ca}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{8+x}$ single crystals ${(T}_{c}=120\mathrm{K})$ subjected to a fluence of $5\ifmmode\times\else\texttimes\fi{}{10}^{17}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ was studied. Magnetic measurements were performed using a commercial superconducting quantum interference device magnetometer and a miniaturized torque magnetometer. In the unirradiated state, the irreversibility line (IL), plotted as $\mathrm{ln}{(H}_{\mathrm{irr}})$ vs $\mathrm{ln}(1\ensuremath{-}{T}_{\mathrm{irr}}{/T}_{c}),$ shows two slopes. At higher temperatures (85--100 K) the IL is described by a power-law dependence ${H}_{\mathrm{irr}}{(T)=H}_{\mathrm{irr}}(0)$ $(1\ensuremath{-}{T}_{\mathrm{irr}}{/T}_{c}{)}^{\ensuremath{\alpha}}$ with $\ensuremath{\alpha}\ensuremath{\approx}2.1.$ At lower temperatures (25--60 K), a more rapid change of ${H}_{\mathrm{irr}}$ with temperature is observed, with the exponent $\ensuremath{\alpha}\ensuremath{\approx}4.8.$ Irradiation shifts the IL to significantly higher magnetic fields/temperatures, where it is rather well described by a single power-law dependence with the exponent $\ensuremath{\alpha}\ensuremath{\approx}2.3.$ The effective mass anisotropy $\ensuremath{\gamma}{=(m}_{c}{/m}_{\mathrm{ab}}{)}^{1/2},$ as determined from torque measurements, decreases after neutron irradiation. The shielding current density as a function of temperature up to 60 K is well approximated by the exponential dependence ${j}_{s}{(T)=j}_{s}(0)\mathrm{exp}(\ensuremath{-}{T/T}_{0}).$ Irradiation increases the characteristic temperature ${T}_{0}$ from about 5.9 K (in the as-prepared crystal) to ${T}_{0}=9.4\mathrm{K},$ clearly reflecting a slower decay of ${j}_{s}$ with temperature. Neutron-generated defects significantly increase ${j}_{s}$ and suppress the ``fishtail effect'' (an increase of ${j}_{s}$ with magnetic field), which was present for the unirradiated crystal.

Thermal annealing and optical darkening effects in CsI(Tl) crystals

The effects of thermal annealing and optical darkening on Φ2.54×2.54 cm 2 CsI(Tl) crystals damaged by exposure to 60 Co gamma radiation and on unirradiated crystals have been studied. Thermal annealing can restore radiation-damaged crystals to their unirradiated state and can even improve the scintillation performance of unirradiated crystals. Optical darkening of crystals with an intense UV source leads to the production of a number of bands in the absorption spectra of crystals, which may be removed by isochronal annealing.

Effect of Doping and Irradiation on Optical Parameters of Triglycine Sulphate Single Crystals

Crystals of triglycine sulphate (TGS) doped with orthonitroaniline (ONA) irradiated with different doses of gamma-radiation were used to investigate gamma-radiation effect on electrical and optical parameters. The absorption coefficient and the reflectance were measured and hence the extinction coefficient, the optical refractive index and the dielectric constants (er′, er″) of unirradiated and irradiated TGS crystals were calculated. Values of the allowed indirect optical energy gap Egopt. of TGS doped with ONA were calculated as a function of gamma-dose. Value of Egopt. decreases from 4.72 eV to 4.25 eV with increasing gamma-doses from 0 up to 3 Mrad. The values of the static dielectric constant er(0) and the effective electrical conductivity sigma0 at room temperature and their dependence on gamma-dose were also calculated.

Decomposition of γ-irradiated strontium oxialate

Isothermal decomposition of γ-irradiated (1.0–4.0 MGy) strontium oxalate has been investigated at different temperatures between 693–733 K by the gas evolution method. Decomposition isotherms of both unirradiated and irradiated crystals are characterized by (i) rapid initial gas evolution, (ii) acceleratory, and (iii) decay stages. Irradiation enhances the rate of reaction without altering the mechanism of the process, the effect being higher at higher doses. Analysis of the data reveal that a two-dimensional phase boundary and the Avrami-Erofeyev mechanism give the best fit of the results. Irradiation increases the energy of activation of the decay stage without affecting the same in the acceleratory period.

Pinning effect on fluctuation conductivity in a superconducting untwinnedYBa2Cu3O7−δsingle crystal with columnar defects

The effect of columnar defects on the critical dynamics of a superconducting untwinned ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (YBCO) single crystal has been investigated. The columnar defects are produced by Sn ion irradiation of which dose corresponds to a matching field of 1 T. The in-plane longitudinal resistivity of the irradiated YBCO crystal has been measured as a function of magnetic field $H$ and temperature $T.$ The extracted fluctuation conductivities are enhanced by the strong pinnings and do not exhibit the same three-dimensional (3D) $\mathrm{XY}$ scaling behavior as for the unirradiated YBCO single crystal; particularly at the magnetic field values near the matching field of 1 T, the fluctuation conductivities show a clear deviation from the critical dynamics. At higher magnetic fields, however, the signature of the 3D-$\mathrm{XY}$ scaling appears.

Effect of columnar defects on the vortex dynamics in a twinned YBCO 123 crystal

Vortex dynamics is investigated in a high quality twinned YBCO crystal irradiated to create columnar defects along the crystallographic c-axis with a 0.5T matching field. The dynamic creep rate Q has been studied as a function of field up to 8T between 10K and 91K. Due to competing correlated disorder from the twins and columns and also underlying point defect pinning, the dynamic behaviour is complicated at low fields. At fields above 2T, the dynamics resembles that of unirradiated twinned YBCO crystals at all temperatures.

Magnetoplastic effect in irradiated NaCl and LiF crystals

The effect of low x-ray irradiation doses (≈102 rad) on the magnetoplastic effect — the detachment of dislocations from paramagnetic centers under the action of an external magnetic field B — in alkali-halide crystals has been investigated. The measurements were performed on LiF crystals and three types of NaCl crystals, differing in impurity content. The dependence of the mean free path l of the dislocations on the rotational frequency ν of a sample in a magnetic field was especially sensitive to low irradiation doses. In unirradiated crystals this dependence is a single-step dependence and is characterized by a critical frequency νc ∝B2 above which the magnetoplastic effect is not observed. The frequency νc depends only on the type of paramagnetic centers, and not on their density. Even the lowest irradiation dose employed (<100 rad) leads to a sharp restructuring of the dependence l(ν), converting it into a two-step dependence (for edge dislocations) with an additional critical frequency νc2, that is insensitive to the irradiation dose, and that corresponds to the appearance of magnetically sensitive stoppers of a new type under irradiation. The initial critical frequency νc1, as a rule, also varies with the dose, reflecting the change in state of the impurity complexes (Ca in NaCl and Mg in LiF). Specifically, it is shown for NaCl(Ca) crystals that as the irradiation dose increases, the frequency νc1 increases, gradually approaching the value νc2, so that by the time the dose is ≈300 rad, the dependence l(ν) once again becomes a single-step dependence, dropping sharply only for ν⩾νc2. It is shown that the addition of a small number of Ni atoms to a NaCl crystal makes the Ca complexes radiation resistant, and the critical frequency νc1 corresponding to them initially equals νc2 for crystals with no Ni. The recombination kinetics of radiation defects in the case in which the samples are irradiated under a tungsten lamp was investigated. A possible physical model of the observed dependences is discussed.

Magnetic relaxation of highly textured Bi 2Sr 2CaCu 2O 8+δ polycrystalline fibres

The magnetic relaxation of Bi 2Sr 2CaCu 2O 8+δ superconducting highly textured polycrystalline materials (fibres) grown by laser induced floating zone melting methods have been studied using ac susceptibility and dc magnetisation techniques. Effective pinning energies, U eff, have been derived for a wide range of current densities, J, and magnetic fields both parallel and perpendicular to the fibre axis. In agreement with the microstructure U eff, ∥( J) > U eff,⊥ ( J) and approximately follows for both orientations the phenomenology observed on single crystals for c-axis parallel fields. Near the irreversibility line U eff is J independent, expressed as: U eff( T, B) = U eff(0 K, 1 T) B −0.5 (1 − T/ T c). Moreover, both U eff,∥( J) and U eff,⊥ ( J) values are higher than analogous U eff, c values on single crystals. Accordingly, for a given temperature the irreversibility line appears at larger fields, about ten times for fields perpendicular to the fibre axis and even higher in the parallel case. A careful analysis of the pinning energy distributions in fibres has been undertaken showing the presence of a high energy shoulder, absent in unirradiated single crystals.

Structure and mechanical properties of irradiated magnesium aluminate spinel

The relationship between structure and mechanical properties of MgAl 2O 4 spinel single crystals following 400 keV Xe 2+ irradiation at 100 K to doses up to 1 × 10 20 ions/m 2 was examined. The structural changes in the irradiated layer were studied using electron diffraction from cross-sectional samples. The nano-indentation technique was used to determine the mechanical properties. At low doses, the material transformed into a metastable crystalline phase characterized by the rearrangement of cations. At the onset of this transformation, the Young's modulus and hardness rose to values about 10 and 15% higher, respectively, than those of the unirradiated crystal. Upon further irradiation, the metastable crystal became amorphous. The Young's modulus and hardness of the amorphous state were about 30 and 60% less, respectively, than the corresponding values of unirradiated spinel. These results, in conjunction with the findings of a recent computer simulation study, provide important insights into the exceptional radiation resistance of magnesium aluminate spinel.

Luminescence Studies on γ-Irradiated BaF2:Pb Single Crystals

Optical absorption (OA), thermally stimulated emission (TL), and fluorescence studies were conducted on lead-doped (0.1 wt%) BaF 2 single crystals. The OA spectrum of unirradiated crystals exhibited the A band of Pb 2+ ions at 6.05 eV. γ-irradiation of the sample produced Pb 3+ as well as Pb + ions. The strong OA bands observed around 4.28 and 5.7 eV after γ-irradiation can be attributed to Pb 3+ . Photoexcitation of the crystal in the A band region yielded an emission around 4.8 eV. In the excitation spectrum taken after irradiation for the 4.8 eV emission, two bands, one at 3.54 eV and the other at 5.9 eV were observed. TL exhibited three glow peaks at 396, 616, and 736 K. TL emission under each of the glow peaks exhibited a single band around 4.7 eV which is the characteristic Pb 2+ emission. The OA bands observed at 1.71 and 2.58 eV are attributable to Pb + (1) centres. The growth and decay of the OA band at 1.71 eV with increasing temperature indicated the active involvement of these centres in the TL process. This centre seems to be stable up to around 610 K and the decay of the same during a TL run can be correlated with the glow peak at 736 K. The probable mechanism for the TL emission is suggested.

Correlation of flux lines in single-crystal Bi2Sr2CaCu2O8 with columnar defects.

A detailed study has been made of the irreversibility line in ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$, both for unirradiated crystals and also for samples containing irradiation-induced columnar defects aligned at either 0\ifmmode^\circ\else\textdegree\fi{} or 45\ifmmode^\circ\else\textdegree\fi{} to the c axis. For the unirradiated reference sample the proposed scaling rules for the case of isotropic pinning provide an excellent description of the data. In contrast, for the case of the irradiated samples the breakdown of the scaling due to the presence of anisotropic pinning provides a powerful method with which to trace the development of anisotropic pinning due to correlated disorder. For fields below the matching field ${\mathit{B}}_{\mathrm{\ensuremath{\Phi}}}$ the existence of strong anisotropic pinning is demonstrated, while for B\ensuremath{\gg}${\mathit{B}}_{\mathrm{\ensuremath{\Phi}}}$ pinning is shown to be isotropic. A characteristic temperature ${\mathit{T}}_{1}$ is identified, above which the effect of anisotropic pinning is strongly reduced due to increased thermally induced delocalization of the vortices away from the columnar defects.

Neutron powder diffraction refinement of boron carbides nature of intericosahedral chains

Our Rietveld structure refinements of neutron powder diffraction data of boron carbides with 10, 13, 16, and 20 at.% carbon indicate 15–25% vacancies at the central position of the intericosahedral chains. In addition, we find a comparable number of interstitials residing off the intericosahedral chains. However, X-ray diffraction studies on unirradiated boron carbide single crystals do not generally find these defects. This discrepancy may indicate that the observed defects are the remnants of defect cascades created by the energetic fission products of 10B during the neutron diffraction measurements. The tendency to form vacancies in the center of the intericosahedral chain is consistent with the weak and soft binding of chain-center atoms that we previously inferred from other measurements.

Effect of Neutron Irradiation on Knoop Microhardness Anisotropy in MgO·3Al2O3 Single Crystals

MgO·3Al2O3 single crystals were irradiated with neutron fluences of 8.3 × 1022 n/m2 at 100°C and 2.4 × 1024 n/m2 at 470°C (E &gt; 1.0 MeV) in the Japan Materials Testing Reactor. The Knoop microhardness of several orientations on the (100) plane of both the irradiated and unirradiated crystals were measured with different indentation loads. The change in hardness profile of the crystals was almost the same after the two irradiation conditions. The hardness increased by 4–15% because of the irradiations depending on the crystallographic orientation, the larger change being observed at orientations between the (001) and (011) directions. While both the {111} and {110} slip systems are simultaneously active in the unirradiated MgO·3Al2O3, the {111} system may be the dominant slip system in the neutron‐irradiated crystals. It is concluded that the restriction of the {110} slip system is caused by irradiation‐induced interstitial ions.

Initial oxidation kinetics of FeNi single crystals before and after ion bombardment

Auger electron spectroscopy has been used to study the oxygen chemisorption kinetics on the (110), (100), and (111) faces of Fe + 36 at% Ni single crystal before and after bombardment with 5 keV argon ions to doses of 10 15–10 17 cm −2. The samples were exposed in O 2 in a range up to 5000 L at temperatures of 20–300°C. At room temperature the chemisorption kinetics for the unirradiated crystal depends on its orientation, but it becomes almost isotropic after high-dose ion bombardment. At 300°C, the anisotropy in O 2 uptake on the irradiated crystals is restored. The rates of Ni segregation and Ar thermal desorption for the irradiated alloy are found to depend on crystal orientation. Ion bombardment enhances the oxidation of the crystal.

Electrical and thermoluminescence properties of γ-irradiated La2CuO4 crystals

Abstract Measurements of the electrical properties of unirradiated as well as γ-irradiated La2CuO4 crystals were carried out at different temperatures in the frequency range of 0.1-100 kHz. Thermoluminescence (TL) studies were also performed on such crystals in the temperature range of 300-600K. The conductivity of the unirradiated La2CuO4 crystals were found to obey the power law frequency dependence at each measured temperature below the transition temperature (Tc = 450K). The activation energies for conduction and dielectric relaxation time have been calculated. The TL response and the dc resistance were found to increase with γ-irradiation dose up to 9-10 kGy. The results showed that the ferroelastic domain walls of La2CuO4 crystal as well as its TL traps are sensitive to γ-raditaion. This material can be used in radiation measurements in the range 225 Gy-10 kGy.

Optical and thermally stimulated emission studies on γ-irradiated SrF2: Pb single crystals

Optical absorption (OA), fluorescence, and thermally stimulated emission (TL) studies are made on a Pb-doped (0.3 wt%) SrF2 crystal. Unirradiated crystals show a prominent OA band around 6.05 eV. This band increases after γ-irradiation and also a broad band around 4.77 eV appears which can be attributed to Pb3+ ions. Fluorescence emission is observed at 5.35 eV when the crystal is excited in the Pb2+ absorption region which is around 6.05 eV. Thermoluminescence glow shows a very intense broad peak at 653 K. TL emission shows only the characteristic Pb2+ emission at 5.35 eV. Also the OA of heavily irradiated crystals shows the presence of Pb centres. The probable TL mechanism is that the trapped electron from a Pb centre when thermally released can recombine with Pb3+ ions to give the characteristic Pb2+ emission. All these results are discussed.

Effect of gamma-irradiation on the thermal decomposition of barium oxalate hemihydrate

The effect of γ-irradiation (1.0–4.0 MGy) on the thermal decomposition of barium oxalate hemihydrate has been studied at 723K by a gas evolution method. Decomposition isotherms of unirradiated and irradiated crystals are characterized by (i) rapid initial gas evolution, (ii) acceleratory and (iii) decay stages. Irradiation enhances the rate of decomposition without altering the mechanism of the process, the effect being higher at higher irradiation doses. The analysis of the data reveal that the two-dimensional phase boundary reaction model gives the best fit to the results.

Optical Absorption, Fluorescence, and Thermally Stimulated Emission on CaF2 : Dy: Pb Single Crystals

AbstractThe optical absorption (OA) spectra of unirradiated crystals of CaF2 doped with 0.1% Dy (by mol%) and 0.1% Pb, hereafter called crystal I, and doped with 0.2% Dy and 0.1% Pb, called crystal II, indicate the presence of a small amount of dysprosium in their divalent state (Dy2+). There is an increasing absorption towards 6.5 eV. On y‐irradiation the concentration of Dy2+ ions is found to increase. Fluorescence of these crystals shows characteristic Dy3+ emission at 1.62, 1.85, 2.16, and 2.85 eV (4F9/2→6H9/2), 6H11/2, 6H13/2, 6H15/2. In thermoluminescence (TL) glow after γ‐irradiation at room temperature crystal I shows six peaks in the temperature range (300 to 700 K) studied. Dy3+ is found to be the major luminescent centre under all these glow peaks. In addition to the Dy3+ emission bands there are two bands in the UV region at 4.94 and 5.93 eV in TL emission. For higher concentrations of lead the results are somewhat different. Crystal III (with 0.01% Dy and 0.5% Pb) and crystal IV (with 0.02% Dy and 0.5% Pb) show a strong absorption band around 6.4 eV. On irradiation prominent absorption bands are observed around 2.04 and 3.21 eV. These are attributed to an F centre associated with the rare‐earth ion in its divalent state (Dy2+ –F centre complex), akin to the FA centre in alkali halides. It is also found that the lead ions (Pb2+) when excited, transfer their energy to nearby Dy3+ ions, leading to the characteristic emission of Dy3+ ions.

Thermally Stimulated Charge and Light Emission from Unirradiated LiNbO3 Single Crystals

Thermally Stimulated Charge and Light Emission from Unirradiated LiNbO3 Single Crystals

Thermally Stimulated Charge and Light Emission from Unirradiated LiNbO3 Single Crystals

Thermally Stimulated Charge and Light Emission from Unirradiated LiNbO3 Single Crystals