ISSN:0033-7579    

DOI:10.1080/00337578208222983

出版商:Taylor & Francis Group    

年代:1982

数据来源: Taylor

摘要:

Studies of radiation damage in crystalline insulators usually originate from problems connected with heavy ion impact during ion bombardment, from neutron irradiation with and without fission in nuclear reactors, or from α-decay with the resulting damage due to recoil daughter atoms of the decaying nuclei of actinide compounds. The materials involved cover a broad range of compounds, e.g. from BeO to CmO2for oxides, for which most work has been done. The damage studied ranges from production of isolated Frenkel pairs to complete amorphization of the crystalline compound (metamictization). The available knowledge is discussed. Emphasis is put on simple binary oxides and on ceramic nuclear fuel materials, i.e. oxides, carbides and nitrides of U and Pu. Recent work on irradiated glasses is also briefly discussed since these glasses are considered as promising media for safe storage of radioactive waste for long periods of time.

ISSN:0033-7579    

DOI:10.1080/00337578208222984

出版商:Taylor & Francis Group    

年代:1982

数据来源: Taylor

摘要:

This overview outlines some basic properties of defects in insulators and describes experimental techniques by which these defects may be characterized. Then the production of defects by various types of particle irradiation is discussed. Finally some discussion is presented of the potential device application of defective crystals, including tunable colour centre lasers, miniaturised optical circuitry and neutron dosimetry.

ISSN:0033-7579    

DOI:10.1080/00337578208222985

出版商:Taylor & Francis Group    

年代:1982

数据来源: Taylor

摘要:

The special situation of insulators necessitates a discussion whether the classical collision theory which had mainly been developed with view to metallic systems still holds in ionic or covalently bound solids. Special problems which should be treated are: (a) changes in the interaction potentials due to the localization of electrons of the target atoms, (b) the influence of charge states of the projectiles, which due to slower neutralization processes, might be higher on the average in insulators than in metals, (c) the effect of specific structure, multielemental composition on the trajectories (“focussing”), (d) the importance of covalent binding of the molecular units, and (e) the energy loss via chemical interactions by formation of stable bonds at low kinetic energies and transient ones at higher energies (“chemical stopping”). These problems will be treated by a comparison of experimental results with theory and computer simulation of collision cascades.

ISSN:0033-7579    

DOI:10.1080/00337578208222986

出版商:Taylor & Francis Group    

年代:1982

数据来源: Taylor

摘要:

In ionic crystals or other insulators with partial ionic binding, the ion-target interaction differs from that of neutral atoms due to different electronic distributions and overall electrical charges Consequently, the nuclear stopping power and defect production by recoiling atoms will deviate from standard values, obtained from e g Moliere-potentials In the present paper, realistic potentials between projectile ion and target ion are determined by the free electron gas model of overlapping Hartree-Fock-Slater or Lenz-Jensen ions (and neutral atoms for comparison) With the new potentials, the transferred energies T and the range of interaction is determined for either damage production (T>Ed) and for nuclear stopping (T>hω> for bound ions) In addition the excitation of optical phonons is taken into account which are excited by the transient electrical field of the charged projectile

ISSN:0033-7579    

DOI:10.1080/00337578208222987

出版商:Taylor & Francis Group    

年代:1982

数据来源: Taylor

摘要:

The computer code MARLOWE was used to evaluate energy vs. range curves in the binary collision approximation. The program was extended to the high-energy regime taking into account the stripping of electrons from the projectile and the concomitant changes in the interaction potentials especially for the inelastic part of the collisions. A complementary computer code LATTIC was developed for the parameterization of the lattice description. This code enables the application of MARLOWE to target materials with complicated crystallographic structure, such as, e.g. ThF4, KClO4and NH4Cl.

ISSN:0033-7579    

DOI:10.1080/00337578208222988

出版商:Taylor & Francis Group    

年代:1982

数据来源: Taylor

摘要:

Experimental and theoretical efforts during recent years have led to structural models for several of the fundamental point defects in oxide materials. In many of these cases it has become apparent that asymmetric relaxations of neighboring atoms play a key role in the detailed structure and properties of these defects. Such effects are not as common in prototype insulators such as alkali halides (a notable exception being the Type II FA center). Strong relaxations in a number of oxides have been treated theoretically by Schirmer using a “bound small polaron model”. Similar ideas had been incorporated into a treatment of the E1′center in SiO2by Yip and Fowler, and more recently by Griscom and Fowler. Extensive computations on the E1′, E4′, and peroxy-radical centers using MNDO and MINDO/3 quantum-chemistry approaches indicate that the defect geometries and properties are strongly related to atomic relaxations about them. It is likely that these effects are important in other insulating materials as well.

ISSN:0033-7579    

DOI:10.1080/00337578208222989

出版商:Taylor & Francis Group    

年代:1982

数据来源: Taylor

摘要:

The threshold voltage at which visible defects could be produced in MgO by electron irradiation in a HVEM has been determined over a wide range of temperature for the ⟨001⟩, ⟨011⟩ and ⟨111⟩ directions. In a separate experiment, the growth of oxygen vacancy centres produced by electron irradiation along ⟨001⟩ at 90 K was followed by optical absorption measurements for several energies between 325 kV and 500 kV. The oxygen vacancy growth measurements gave an upper limit to the threshold voltage of 300 kV which is equivalent to a value of the threshold displacement energy Eα for oxygen of 53 eV. The HVEM measurements over the temperature range 125-1070 K gave an effective threshold voltage which varied considerably with temperature. The effective threshold at room temperature and below occurred at 460 kV which is considerably higher than the optically determined oxygen threshold and must therefore correspond to the magnesium threshold. This gives a value of Edfor magnesium of 60 ± 3 eV for all three directions. At higher temperatures the effective threshold voltage fell to give minima at ∼700 K for each of the three orientations. Comparison with the optical measurements suggests that the minima are the thresholds for oxygen displacement. It is proposed that the decrease in the effective threshold is a defect mobility effect which allows dislocation loops to be formed from the displaced oxygen ions only. The minima at 700 K would therefore correspond to values of Eα for oxygen of 44±3 eV for ⟨001⟩, 64 ± 3 eV for ⟨011⟩ and 46 ± 3 eV for ⟨111⟩. At higher temperatures for ⟨001⟩ and ⟨011⟩ the effective thresholds rose to a peak at ∼900 K of 340 kV and 410 kV respectively.

ISSN:0033-7579    

DOI:10.1080/00337578208222990

出版商:Taylor & Francis Group    

年代:1982

数据来源: Taylor

摘要:

Matrix isolation of impurity atoms in a frozen rare-gas lattice is a well-known technique for the study of atoms in quasi-free states. The usual procedure consists of co-evaporating these atoms together with the matrix atoms and depositing them simultaneously on a cold substrate. Subsequently, the frozen samples can be studied by optical spectroscopy or, as has been proved more recently, by Mössbauer spectroscopy.

ISSN:0033-7579    

DOI:10.1080/00337578208222991

出版商:Taylor & Francis Group    

年代:1982

数据来源: Taylor

摘要:

Sputtering can be defined as the process whereby particles leave the surface as a direct consequence of the presence of incident radiation. When particles leave the surface as a result of receiving momentum from the collision cascade induced by the incident radiation, the process is called “physical sputtering”. If the incoming radiation (ions, electrons, or photons) induces a chemical reaction which leads to the subsequent desorp-tion of particles, the process could be classified as “chemical sputtering”. There are a number of molecules such as CH4, CF4, CF3H, CF3CI, etc., whose binding energy to a large variety of surfaces is believed to be only a few kcal/mole. Therefore, these molecules will not remain absorbed at room temperature. Consequently, if they are generated from surface atoms by radiation-induced processes, they will almost immediately desorb into the gas phase. This process is one type of chemical sputtering. Recent data obtained in plasma environments suggest that this type of reaction is a widely occurring phenomena: however, few systematic quantitative investigations of the subject have been completed. In this paper we will review the evidence for chemical sputtering and discuss mechanisms based on experimental information obtained for the chemical sputtering of silicon and SiO2under argon ion bombardment in the presence of a molecular beam of XeF2. Under these conditions, 25 or more silicon atoms can leave the surface per incident argon ion. About 75% of the silicon is emitted as SiF4(gas) and the rest leaves as silicon atoms or SiFx radicals. The total yield (silicon plus fluorine) is greater than 100 atoms/ion. The measured yields are a strong function of XeF2flux and a much weaker function of ion energy in the range 500-5000 eV. The chemical-sputtering yield for SiO2is smaller than that of silicon by about an order of magnitude, but it is still larger than the physical-sputtering yield. Moreover, SiO2is also sputtered by electrons. These results indicate that the incident radiation induces a chemical reaction between silicon and adsorbed fluorine which produces SiF4, and the SiF4 is subsequently desorbed into the gas phase. We define this process as chemical sputtering. The large yields are probably a consequence of weak binding between the surface and the SiF4molecule.

ISSN:0033-7579    

DOI:10.1080/00337578208222992

出版商:Taylor & Francis Group    

年代:1982

数据来源: Taylor