Neutron-Irradiated LiF Research Articles

Plasticization of neutron-irradiated LiF crystals upon thermal stimulation of a single-system dislocation glide

It has been shown that post-radiation annealing of LiF crystals irradiated by high neutron fluences (1015–1018 neutrons/cm2) at comparative low temperatures (300–400°C) creates optimal conditions for single-system dislocation glide, which favors a complete recovery of the plasticity with conservation of a significant fraction of radiation hardening.

Photoluminescence of Neutron-Irradiated LiF Single Crystals

Photoluminescence of Neutron-Irradiated LiF Single CrystalsIn 1980, a single LiF crystal was irradiated by thermal neutrons up to the exposition dose 1019n/cm2at the Salaspils nuclear reactor. The crystal has been kept at room temperature for 29 years. Samples in size of 5×4×1 mm3were cut from the crystal and some of them annealed at 920 K for 30 min. To study photoluminescence, measurements with a confocal laser scanning microscope Leica TCS SP5 were performed, during which 3D-defects of size of ~1μm3were revealed. Photoluminescence was observed in the red spectral region with the peak of ~ 655 nm when excited with an argon laser (Δ = 458 nm).

Li colloids created by electron-irradiation of LiF: A great wealth of properties

Like in lithium oxide, it is easy to nucleate very pure metallic lithium precipitates by electron-irradiation of lithium fluoride crystals. Irradiations performed near room temperature give metallic colloids which are characterized by electron spin resonance (ESR). The metallic character of the precipitates is demonstrated by the temperature behaviour of the ESR line intensity, which follows the temperature-independent Pauli law. A typical value for the metal concentration is about a few percent. Varying the irradiation parameters, i.e. the total fluence and/or the instantaneous flux, affects considerably the ESR properties of the colloids: first, the ESR line width changes by almost two orders of magnitude, indicating that the sizes of the precipitates strongly depend on these irradiation conditions. Second, when measuring the variation of the ESR intensity, proportional to the metal content, two very different behaviours are observed during recovery (dynamic or isochronal). When the flux is high, an intense and narrow metallic line is obtained, which disappears rapidly with annealing (300 °C). For lower flux, the colloids have a broader ESR signal, i.e. a much smaller size, and are not destroyed by annealing until the LiF crystal melts (870 °C). 7 Li NMR measurements extending earlier NMR experiments on neutron-irradiated LiF indicate that the Knight-shifted signal is split into two components with different widths. The position of the narrower component seems to correspond to big lithium particles having the features of bulk metallic lithium. The second component is likely to regroup particles of different sizes and geometry with different hyperfine interaction strength.

Photoluminescence of F-aggregate centers in thermal neutron irradiated LiF TLD-100 single crystals

In LiF, the photoluminescence emission ofF2 andF3+ centres are produced on irradiation peaks in the red (660 nm) and green (520 nm) region respectively. In this work, their relative sensitivities for thermal neutrons and gamma rays are compared.

A New Hole Centre in Neutron Irradiated LiF Crystals

AbstractA new colour center is observed by ESR in low temperature γ‐irradiated and then heat treated LiF crystals which are heavily neutron irradiated previously. From the analysis of the ESR spectra this center is assigned to an asymmetric interstitial fluorine atom associated with a lattice fluoride ion. This center lies in one of the (110) planes and its molecular axis makes an angle of 23° with the [110] axis. The ESR parameters are determined by fitting the calculated resonance fields to the experimental ones.

Evidence for a phase-transition-induced change in the surface spin-flip probability of conduction electrons from CESR on n-irradiated, LIF; its application as an intensity reference

In view of its application as an intensity marker in electron spin resonance, the conduction electron spin resonance signal of Li particles in neutron-irradiated LiF has been analysed regarding its lineshape, linewidth, intensity and spectral composition in the temperature range 4.2<or=T<or=300 K at the X and K bands. While the lineshape is Lorentzian at 300 K, apparently, strong deviations from this shape develop at lower temperatures, without any noticeable change in the g-value or the peak-to-peak linewidth Delta Bpp approximately=0.065 mT; the linewidth, as expected, is found to be dominated by electron-surface scattering. Double numerical integration of the absorption-derivative dPmu a/dB spectra shows the number of spins to be T-independent, as expected from a Pauli-paramagnetic behaviour. Data analysis reveals that, as T decreases, the single Lorentzian line observed at 300 K gradually evolves more and more to a sum of Lorentz lines (likely 2) of all identical g but of different widths 0.065<or= Delta Bpp<or=0.218 mT. This finding has important implications for the use of LiF:Li as an intensity marker, both as regards the Delta Bpp2Y'm method (Y'm being the signal amplitude) of the double integration method to the signal intensity from the dPmu a/dB spectra. The effect arises from a change upon cooling of the spin-flip probability epsilon Li for scattering of conduction electrons at a non-magnetic interface, i.e., Li metal-surrounding; this is tentatively ascribed to the martensitic phase transition in Li.

Tritium Centers in Neutron-Irradiated LiF Studied by ESR Spectroscopy and Dissolution in Deuterated Nitric Acid

Article Tritium Centers in Neutron-Irradiated LiF Studied by ESR Spectroscopy and Dissolution in Deuterated Nitric Acid was published on August 1, 1984 in the journal Radiochimica Acta (volume 37, issue 2).

Autoradiograph of tritium in neutron-irradiated LiF

The distribution of tritium on the surface layer of neutron-irradiated LiF was studied by means of autoradiography. The autoradiographs were compared with microphotographs of the surfaces which were chemically etched after the autoradiographs had been taken. Temperature-programmed desorption of tritium and u.v. absorption spectra were studied on the same sample. Many sharp lines appeared in the autoradiograph. Some of these lines are observed in the microphotograph, but some of them appeared only when the surface was etched. It was concluded from these results that tritium accumulates preferentially on step-edges and dislocations of the surface.

Positron Angular Correlation in Uncoloured and Neutron Irradiated LiF and LiF:OH

physica status solidi (b)Volume 118, Issue 2 p. K129-K132 Short Note Positron Angular Correlation in Uncoloured and Neutron Irradiated LiF and LiF:OH D. N. Popov, D. N. Popov Department of Physics, Technical University “Angel Kunchev”, Russe Search for more papers by this authorN. M. Nancheva, N. M. Nancheva Department of Physics, Technical University “Angel Kunchev”, Russe Search for more papers by this authorB. N. Zlatov, B. N. Zlatov Department of Physics, Technical University “Angel Kunchev”, Russe Search for more papers by this authorG. P. Georgiev, G. P. Georgiev Department of Physics, Technical University “Angel Kunchev”, Russe Search for more papers by this author D. N. Popov, D. N. Popov Department of Physics, Technical University “Angel Kunchev”, Russe Search for more papers by this authorN. M. Nancheva, N. M. Nancheva Department of Physics, Technical University “Angel Kunchev”, Russe Search for more papers by this authorB. N. Zlatov, B. N. Zlatov Department of Physics, Technical University “Angel Kunchev”, Russe Search for more papers by this authorG. P. Georgiev, G. P. Georgiev Department of Physics, Technical University “Angel Kunchev”, Russe Search for more papers by this author First published: 1 August 1983 https://doi.org/10.1002/pssb.2221180256 Russe 7004, Bulgaria. AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Volume118, Issue21 August 1983Pages K129-K132 RelatedInformation

X-ray diffraction profiles fron neutron-irradiated LiF single crystals

The detailed measurement of the Bragg line profiles of several X-ray reflections from six single-crystal samples of LiF irradiated under different and well defined conditions in the core of a swimming-pool type nuclear reactor are reported. The crystals were irradiated with three different spectra: (a) total neutron spectrum plus the γ background radiation, (b) same as (a) without thermal neutrons, the crystal being wrapped in a cadmium foil, and (c) the background γ radiation alone. The experimental results presented display the changes in the profiles due to the different radiation spectra used and to the change in irradiation temperature within the interval from 113 to about 173°C. That peak intensities decrease while integrated intensities increase with irradiation is a well known result amply confirmed here. Integrated intensities in these experiments increase from 4 to 14 times with respect to the unirradiated crystal. The strongest reflections (111, 200 and 220) experienced the highest increases. The influence of the temperature of irradiation has been verified through measurements of reflections 111, 200 and 220 from crystals irradiated at temperatures of 113, 148 and 173°C. As the irradiation temperature rises the peak intensities clearly tend to those of the unirradiated crystal while integrated intensities attain a maximum at about 150°C. The results also indicate that at least under the conditions prevailing in the present experiments the highest amount of damage is due to thermal neutrons (E ≤ 0.45 eV).

E.S.R. and optical study of the interstitial h centres in heavily neutron irradiated Lif

The paramagnetic spectra of the hole-centres created in LiF by a heavy neutron irradiation at 78°K have been studied by E.S.R. and optical techniques. Samples which have been irradiated at a dose at least equal to 10 19 solneutrons cm 2 exhibit , after they have been warmed at 200°K, several spectra related to 〈111〉-oriented asymmetric centres. These centres are stable up to 500°K. One of them, designated the H N1 centre, has been studied in detail. The analysis of the paramagnetic spectrum shows that this defect is an interstitial fluorine atom which forms an asymmetric ( F - −1 3 - F - −2 3 ) molecule ion with a lattice fluorine ion. Optical experiments have shown that the excited Σ level is at 4.1 eV from the ground Σ level. Two excited π levels have been found at 2.5 and 2.1 eV. This asymmetric H centre is formed from a primary symmetric H centre, designated the H N2 centre. The H N2 centre is 〈110〉-oriented. The transformation of the symmetric H N2 centre into the asymmetric H N1 one is associated with important changes in the values of the isotropic hyperfine interaction and of the g ⊥ shift.

E.S.R. and optical study of the interstitial H centres in heavily neutron irradiated LiF

E.S.R. and optical study of the interstitial H centres in heavily neutron irradiated LiF

Motional effects on the epr spectrum of a perturbed H centre in neutron irradiated LiF

The electron paramagnetic resonance (EPR) spectrum of an asymmetric interstitial H centre in LiF has been studied as a function of temperature. The shifts with temperature of the EPR lines have been measured from 7 to 150 K. They are interpreted by a libration process of the molecule ion around its mean orientation. Near 180 K, the EPR lines broaden. The thermally activated broadening arises out from reorientational motions of the molecule ion. The reorientation rates show that there are two distinct jumping motions between 180 and 230 K; one of them implies rotations of the molecule ion around the average 〈111〉 orientation and is assumed to be induced by the same librations modes as those which produce the line-shifts. The other one is the pyramidal jumping motion between the four equivalent 〈111〉 orientations.

Motional effects on the EPR spectrum of a perturbed H centre in neutron irradiated LiF

The gravitational force exerted by a central planet deforms its orbiting moons (tides) and periodically changes their rotation (librations). Tides and librations of large icy satellites rotating synchronously with their orbital motion are usually studied separately but we here show that tides strongly reduce the libration amplitude of large icy satellites with a subsurface ocean. Moreover, libration also slightly changes the tides. Periodic tides affect libration in three ways: first the periodic tidal bulges modify the gravitational torque exerted by the central planet on the satellite, secondly torques between the periodic tidal bulges of a solid layer and the static shape of another solid layer couple the libration of the ice shell and solid interior, and thirdly, the zonal tides periodically change the polar moment of the satellite, which acts as the inertia for rotational motion. We develop a formalism to include these elastic effects in the study of librations and calculate the amplitudes of the longitudinal librations at orbital period for the largest icy satellites in the Solar System: Europa, Ganymede, Callisto, and Titan. Without a subsurface ocean, elastic effects on libration are of the order of a few percent. However, if a subsurface ocean exists as indicated by several observations for the satellites considered, elastic tides have a crucial effect. If the solid layers of the satellites are considered rigid, the libration amplitude is at least one order of magnitude larger than for an entirely solid satellite. Periodic elastic tidal deformations of the layers, in particular of the ice shell, counterbalance this large increase and keep the libration amplitude at the level of the libration of the satellite without subsurface ocean. Besides gravitationally forced librations, seasonal variations in the atmosphere of Titan also cause the rotation of Titan to change on a seasonal timescale. We show that tidal elastic effects can strongly increase the amplitude of these long-term variations in the rotation rate by a factor of about four with respect to a rigid shell. We predict that these variations can have an amplitude above the detection limit of the Cassini radar.

ESR Study of Neutron-Irradiated LiF Crystals

The paramagnetic centers produced in LiF crystals by neutron-irradiation at different temperatures have been studied by ESR. The F-like centers, including F-centers, clustered F-centers and electrons trapped in vacancy clusters, are observed in the crystal irradiated at about 0°C. By irradiation at 80 K, however, the concentration of F-like centers is small, and the molecular ion centers predominate. The two types of interstitial centers, stable even at room temperature, are also observed in this irradiation; models are proposed for the two types. Neutral hydrogen (H°) and tritium (T°) atoms are observed in the irradiations at both room temperature and 80 K. These atoms are trapped in vacancy clusters in the crystals. It is suggested from the results of irradiations between 0°C and 80 K that vacancies evaporated from the zone play an important role in formation of the F-centers. The effects of fast neutrons at liquid helium temperature are described.

Etude par resonance electronique du tritium atomique forme au sein de cristaux de fluorure de lithium irradie aux neutrons

An EPR investigation of neutron irradiated LiF single crystals shows that atomic tritium may be found in several sites of the lattice. One of them has been identified: it is the 1 4 , 1 4 , 1 4 interstitial site. Quantitative interpretation of this EPR spectrum needs the relaxation of the lattice. The nearest neighbour ions have to be pushed away from the interstitial site by 20 per cent of their distance.

Delayed Neutrons from Neutron-Irradiated LiF Containing Color Centers

It is shown that the neutron-electron interaction is orders of magnitude too small to allow neutrons to be bound to electron-excess color centers in alkali-halide crystals, as was indicated experimentally by Grant and Cobble.

Many-quantum transitions in conduction electron spin resonance

Resonance absorption spectra have been observed which may be interpreted as many-quantum transitions where the quantization is along the effective field in the rotating frame. The spin system used was that of the conduction electrons in Li metal in a crystal of neutron irradiated LiF. An analysis of the experimental results using a modified Bloch equation under the assumptions that [Formula: see text] and that the spin system relaxes towards the instantaneous field is presented. The agreement between theory and experiment is excellent.

NMR quadrupolar effects in neutron irradiated LiF

Quadrupolar effects have been observed in the NMR spectrum of neutron irradiated LiF crystals. This is related to the presence of Li atom precipitates which are of two types - small platelets and bigger isotropic clusters.

Résonances magnétiques de petites particules de lithium

A number of X-ray diffraction studies have shown that, in heavily neutron-irradiated LiF crystals, very small metallic particles are formed. These particles of an exceptionally high degree of purity assume a great variety of shapes and dimensions. With a simple theoretical model many characteristic effects can be predicted on the magnetic resonances of some of these particles: a narrow Knight-shift-less NMR spectrum, a Curie-type paramagnetism, and extremely narrow EPR lines. All these effects have been verified by carefully comparing the structure of the samples as given by X-ray diagrams and the EPR and NMR spectra. Moreover, electronic paramagnetic relaxation inside very pure metallic particles was found to be caused by a surface mechanism different from the one proposed in previous works.