Close Frenkel Pairs Research Articles

Point defects in alkaline-earth fluorides after thermal neutron irradiation

Single crystals of MgF2, CaF2, SrF2 and BaF2 have been irradiated with polarized, thermal neutrons at temperatures between 10 K and 300 K. The neutron capture process $$^{19} F(\vec n, \gamma )^{20} \vec F$$ creates polarized, β-active20F(τ β =16s) probe nuclei, which are displaced by the γ recoil and thus produce some nearby point defects. These defects have been studied by their influence on20F NMR signals being monitored via the asymmetric β-decay radiation of the polarized20F nuclei. From our results we conclude that in the fluorites CaF2, SrF2 and BaF2 thermal neutron irradiation leads to the formation of anion Frenkel pairs, the predominant intrinsic defect in such crystals. Correlated recombination of these close Frenkel pairs with activation enthalpies of about 0.2 eV was observed at temperatures around 80 K. A second annealing stage occurs near 220 K corresponding to an activation enthalpy of about 0.6 eV. Possible annealing mechanisms responsible for this stage are discussed. In tetragonal MgF2 two annealing stages were found at 13 K and 60 K corresponding to 0.037 eV and 0.17 eV, respectively. The stage at 60 K possibly reflects migration of the H entre whereas the interpretation of the 13 K stage remains an open question.

Irradiation induced intrinsic excitonic luminescence in cadmium telluride

Four new, sharp excitonic lines (B 1 to B 4 ) dominate the luminescence of CdTe, after low temperature electron irradiation. We show that they are of intrinsic nature. Two thresholds (195 keV, 280 keV) are attributed to Cd displacement and to formation of close Frenkel pairs with different configurations. Another higher threshold (480 keV) is associated with the formation of a more complex defect. All the lines disappear completely in three annealing stages at 50 K, 70 K, 140 K. The dominant lines B 1 , B 2 are tentatively related to excitons bound to isoelectronic Cd vacancy/Cd interstitial close pairs.

System for continuous monitoring of point defect concentrations during irradiations and anneals

An integrated system, cryostat, electronics, and computer used for detailed resistivity measurements during low-temperature irradiations and anneals of metals is described. The thermal response time and rate of heat removal from the specimens are optimized. The system has excellent stability for extended isothermal anneals; measurements of deviations from Mathiessen's rule are discussed to illustrate this feature. Examples are given of the use of the system to determine dense sets of thermal activation energies during anneals through state I. These determinations are made by linear heating at different rates during recombinations of close Frenkel pairs, and by abruptly and repeatedly changing the annealing temperature during the recovery of more widely separated pairs.

Anisotropy of defect creation in electron-irradiated iron crystals

Single crystals of \ensuremath{\alpha}-iron were irradiated perpendicularly to the (100), (110), and (111) planes with electrons in the range 0.35-1.7 MeV and their electrical resistivity change rates were measured. A geometrical model of the threshold-energy surface for atomic displacement in a bcc lattice produces a fit to the experimental data leading to the following values for the threshold energies in the principal crystal directions: ${T}_{d}^{〈100〉}=17\ifmmode\pm\else\textpm\fi{}1$ eV, ${T}_{d}^{〈111〉}=20\ifmmode\pm\else\textpm\fi{}1.5$ eV, and ${T}_{d}^{〈110〉}\ensuremath{\gtrsim}30$ eV. The specific resistivity of a Frenkel pair is deduced to $\ensuremath{\rho}_{F}^{\mathrm{Fe}}=(30\ifmmode\pm\else\textpm\fi{}5)$ \ensuremath{\mu}\ensuremath{\Omega}cm/at.%. From the obtained ${T}_{d}'\mathrm{s}$ we derived an interatomic potential of the Born-Mayer type, valid in the range $1.2\ensuremath{\le}r\ensuremath{\le}2.5$ \AA{}. We propose as a good choice: $V(r)=8900{e}^{\ensuremath{-}4.5r}$ eV. The recovery due to isochronal annealing during stage I, after irradiation at different electron energies, was measured and related to specific recovery mechanisms. Thus, the first important substage, ${I}_{B}$ (\ensuremath{\sim} 66 K), is due to the recovery of close Frenkel pairs created in the $〈100〉$ direction, while a comparison of calculated cross sections suggests that ${I}_{C}$ (\ensuremath{\sim} 87 K) possibly stems from $〈111〉$ close pairs. Substage ${I}_{D}$ (90-110 K) is complex; its first part, below 100 K, originates mostly from defects produced in the $〈100〉$ direction and the second part, above 100 K, together with ${I}_{E}$, principally originates from defects produced in the $〈111〉$ direction.

The magnetic after‐effect spectrum in cobalt between 20 and 350 K after electron irradiation at 4.2 K

AbstractAfter electron irradiation of cobalt single crystals a spectrum of magnetic after‐effect processes is observed where nearly all the relaxation peaks anneal out rapidly during the measurement. For the discussion of the experimental results a survey is given of the configurations of interstitial atoms and close Frenkel pairs which can be expected for the h.c.p. lattice. Taking into account resistivity annealing experiments the relaxation process at 325 K can be attributed to a diffusion after‐effect which is due to a freely migrating defect.

The Initial Susceptibility of Cobalt after Electron Irradiation at 4.2 K

AbstractAfter electron irradiation at 4.2 K the initial ferromagnetic susceptibility of cobalt shows a marked decrease (“susceptibility dip”) of about 50% when the specimen is annealed between 20 and 30 K and regains its initial value in recovery stage Ic of the electrical resistivity between 40 and 50 K. Similar, yet smaller “susceptibility dips” are observed at 5, 55, 70, and 150 K. The 30 K decrease of the initial susceptibility is tentatively ascribed to the pinning of domain walls by interstitial atoms of hexagonal symmetry in the [0001]‐dumbbell configuration which, however, may be stable only under the influence of close Frenkel pairs in their neighbourhood. This model is in good agreement with simultaneous electrical resistivity and magnetic after‐effect measurements reported elsewhere which indicate that the anisotropy axis of the freely migrating interstitial atom has a component parallel to the basal plane.

Damage recovery in vanadium between 3.8-6.0 K

Abstract The isochronal recovery of thermal neutron damage in V was recently studied1 from 6–320 K by means of resistivity changes. One interesting result was the observation of continuous recovery from 6–40 K in contrast to the spectra of discrete recovery processes seen at low temperatures for thermal neutron damage in nearly all other metals studied. The absence of recovery rate peaks in this low temperature range led to the speculation that the onset of recovery in V may occur by means of long-range interstitial migration rather than the collapse of close Frenkel pairs which is believed to initiate the recovery in most other metals. Another possibility is that close pair recovery in V occurs below 6 K. This speculation is not unreasonable, since recovery below 6 K has been observed in Au, Cd, In, and Zn.2 In the case of Cd, a large recovery rate peak occurs at 5.2 K with a subpeak at 3.6 K.3 In this report, we give results of an investigation which extends the recovery study of thermal neutron damage in V to 3.8 K.

EPR Observation of Close Frenkel Pairs in Irradiated ZnSe

Four distinct EPR spectra observed in ZnSe irradiated by 1.5-MeV electrons at 20.4\ifmmode^\circ\else\textdegree\fi{}K are identified as simple zinc-vacancy, zinc-interstitial close pairs of different microscopic configuration. Correlation of defect alignment with incident-electron-beam direction confirms the identification. Isolated zinc vacancies are also produced but with zero initial production rate, suggesting that their production is a two-step process, the first being the formation of close pairs.

Low Temperature Electron-Irradiation Damage and Recovery in Pyrolytic Graphite

The electrical resistivity changes by electron irradiation at 5 K and the isochronal annealing of resistivities in the temperature range 5-85 K (Stage I) in pyrolytic graphite are studied in order to get information on point defects. Both the a -axis and the c -axis resistivity changes by irradiation are explained by the ordinary transport theory with defect scattering. Small recoveries in Stage I are shown to be composed of four substages: I A (5-15 K); I B (15-45 K); I C (45-65 K); and I D (65-85 K). Effects of the irradiating electron energy, the radiation-doping and the graphitization temperature of samples on these substages are also studied. Substages I A and I B may be caused by the correlated rearrangement of close Frenkel pairs, where interstitial atoms form loose coupling with their own vacancies. The activation energy for I A is roughly estimated to be 0.027±0.004 eV. In I C the long-range free migration of interstitial atoms seems to occur.

Stability Problems, Low-Energy-Recoil Events, and Vibrational Behavior of Point Defects in Metals

In a copper crystallite containing many hundreds of atoms the static and dynamic behavior of point defects is simulated by means of an electronic computer. This follows the lines of an earlier work by Gibson et al. We have extended their investigation to collisions with subthreshold energies using a new computer program particularly suited for low-energy processes. The atomic interaction is approximated by a purely repulsive Born-Mayer potential together with surface forces applied to the boundary of the crystallite. The mechanical equilibrium of vacancy, interstitial, and many Frenkel-pair configurations has been determined. There are 74 lattice sites around a (split) interstitial which are unstable for a vacancy. The formation energy (2.79 eV) of a stable Frenkel pair turns out to be practically independent of the distance between vacancy and interstitial. The effect of subthreshold collisions on point defects has been studied. At low-energy transfers (\ensuremath{\lesssim} 0.3 eV) only vibrations can be excited. The (split) interstitial exhibits several localized modes: (i) an axial vibration, (ii) a twofold degenerate librational mode, and (iii) a twofold degenerate c. m. oscillation perpendicular to the axis. The energy storage in the axial mode is about 20% of the initial energy transfer. Jumps of point defects are induced by energy transfers of \ensuremath{\gtrsim} 0.3 eV for interstitials and \ensuremath{\gtrsim} 0.6 eV for vacancies. The directional dependence of these energy thresholds has been studied. The cross section for interstitial jumps due to MeV electrons is roughly ${10}^{+4}$ b. Effects of subthreshold energy transfers to close Frenkel pairs have also been investigated.

Elektronenbestrahlungsexperimente an Tantal

Abstract In high purity Ta-foils the damage production during electron irradiation at 4.5 °K has been studied using electrical resistivity measurements. The observed dose curves show a strong saturation effect, i. e., the effective damage rates decrease with increasing defect densities. Two mechanisms contribute to this radiation annealing effect: 1) Spontaneous recombination of newly produced interstitials or vacancies with vacancies or interstitials already present in the crystal (direct re­ combinations) and 2) spontaneous recombination of close Frenkel pairs under the influence of nearby low energy recoil events (subthreshold events). The contribution of both direct recombin­ations and subthreshold events can be separated by subthreshold test irradiations as well as by radiation doping experiments. Both methods yield mutually consistent results. From these one can deduce a value of the spontaneous recombination volume of about 50 atomic volumes and a value for the cross section for the spontaneous recombination of a close Frenkel pair by subthreshold energy transfers from 1.2 MeV electrons of about 90,000 barns.

The annealing of graphite irradiated with electrons at 80°K

Abstract After fast electron irradiation at 80°K graphite exhibits, in its electrical resistivitiy during isochronal annealing, a reverse stage followed by normal stages. These constitute the Austerman and Hove peak. The reverse stage has previously been attributed to the separation of close Frenkel pairs of defects. In the present work the height of the peak has been measured as a function of bombarding electron energy between 0·4 and 2·0 Mev. The peak has been studied by measuring changes of electrical resistance of reactor grade A polycrystalline graphite. The measured peak height is approximately constant for electron energies from 2·0 Mev to just below 1·0 with a tendency to decrease as the electron energy decreases below 1·0 to 0·4 Mev. A method of analysis of the peak height in terms of the fraction (fiv) of close Frenkel pairs is given and from estimates of the energy dependence of fiv based on the theory of radiation damage it has not proved possible to account for the observed results. Other models are considered and analysed in a similar way. While it is not possible to provide a unique explanation it is concluded that the reverse stage must arise from the enhancement, rather than the diminution, of the number of close Frenkel pairs. The most likely model is one in which the free interstitial is mobile at ca. 90°K (energy of motion about 0.3 ev).

Point-Defect Calculations for an fcc Lattice

Formation and migration energies and atomic configurations have been calculated for interstitials and vacancies in an fcc lattice, and the interaction of close Frenkel pairs has been studied. A mathematical model was used in which the atoms in a spherical crystallite of about 530 atoms are treated as individual particles, each with three degrees of freedom, while the remainder of the crystal is treated an an elastic continuum with atoms imbedded in it. A two-body central force previously used in calculations for $\ensuremath{\alpha}$-iron (a bcc metal), was used to simulate the interaction of atoms in the fcc lattice. Configurations were found using a digital computer by choosing a starting configuration roughly approximating the situation under consideration and successively adjusting the value of each variable occurring in the energy equation so that the magnitude of the generalized force associated with it was zero until equilibrium was reached. The object of this calculation was to investigate the consequences of using an interatomic interaction for an fcc metal with a form similar to that used for previous bcc calculations. The interaction used should be reasonably appropriate for both $\ensuremath{\gamma}$-iron and nickel and the results are compared with experimental values for both metals. The stable interstitial was a "split" configuration in which two atoms were symmetrically split in a $〈100〉$ direction about a vacant normal lattice site. The formation energy was 4.08 eV and the activation energy for motion was 0.15 eV. A number of metastable configurations were found, the most important of which was the $〈111〉$ "split" interstitial, which was metastable by 0.16 eV and had a migration energy of 0.13 eV. The vacancy formation energy and activation energy for migration were 1.49 and 1.32 eV, respectively. The interaction between a vacancy and an interstitial was very complex and short in range. Configurations were found which were bound, repulsive, and trapped, and there were 32 unstable lattice sites.

Concentration Dependence of Stage I Annealing in Irradiated Copper

An analysis is given of the radiation-damage annealing to be expected under conditions of constant warmup rate. It is found that third-order kinetics are to be expected for defects moving freely in one dimension. A discussion is given of the Wigner catastrophic annealing effect. Using the analysis, a detailed discussion is given of the annealing curves found for stored energy released on annealing deuteron-irradiated copper in stage I. Most of the conclusions reached previously by Magnuson, Palmer, and Koehler and by Corbett, Smith, and Walker using resistance measurements are confirmed. Evidence for the existence of clusters of damage in deuteron-irradiated specimens is found. Peak ${\mathrm{I}}_{E}$ is found to follow second-order kinetics, providing evidence against the possibility of freely migrating interstitials in one dimension. It is found that peak ${\mathrm{I}}_{D}$ cannot be interpreted with a unique activation energy and is interpreted instead as a superposition of additional close Frenkel pairs. Using a combination of deuteron and electron damage measurements, a value of the energy-to-resistivity ratio for Frenkel pairs is found for peak ${\mathrm{I}}_{E}$ which agrees within experimental error with that found for stage I using deuteron results alone.

The effect of quenched-in vacancies of stage I recovery of neutron-irradiated gold and aluminium

An attempt was made to clarify the anomalously low stage I recovery in irradiated gold. The first part of stage I for any metal is thought to be due to the recombination of close Frenkel pairs and the latter part due to the migration of interstitials or crowdions. Aluminum was chosen as a metal possessing normal stage I recovery. (R.E.1J.)