High-dose Electron Research Articles

High-temperature and ionization-induced effects in lithium-doped MgO single crystals

Thermal quenching of Li-doped MgO crystals from temperatures in excess of 1300 K results in the formation of ${[\mathrm{Li}]}^{0}$ defects, which are substitutional lithium ions each with a bound hole at a neighboring oxygen site. Unlike ${[\mathrm{Li}]}^{0}$ defects generated at low temperatures by a low dose of ionizing radiation, those formed by quenching are stable against hole release well above room temperature. This stability is much higher than for all previously reported hole defects in alkaline-earth oxides. These stable ${[\mathrm{Li}]}^{0}$ defects have also been produced by high-dose electron irradiations at both 290 and 85 K, with formation cross sections of ${\mathrm{\ensuremath{\sim}}10}^{5}$ and ${10}^{3}$ barns, respectively. The magnitude and temperature dependence of the cross section suggests a radiation-induced diffusion, rather than an elastic collision, process as the production mechanism. These defects anneal at 450 K in the electron-irradiated crystals and 830 K in the quenched ones. A model involving localized regions of high lithium concentration is proposed in order to explain the stability of the bound hole at the ${[\mathrm{Li}]}^{0}$ defect.

V- and V0 centers in CaO single crystals

The stable, intrinsic V - center in CaO single crystals has been produced at room temperature by high-dose electron irradiation. This center, which consists of a hole trapped at an oxygen ion adjacent to a calcium vacancy, exhibits an EPR spectrum at 77 K which has 〈100〉 axial symmetry and is described by S = 1 2 , g ‖ = 2.0021(2) and g ⊥ = 2.0697(2). At room temperature, the spectrum consists of a single thermally-averaged isotropic line at g = 2.047(1). Upon γ-irradiation at 77 K, the isolated V - concentration is reduced and V 0 centers (pairs of trapped holes at single vacancies) are produced. A room-temperature thermal anneal reverses the effect of the low-temperature γ-irradiation.

Der Einfluß tiefer Temperaturen auf die Strahlenschädigung von organischen Kristallen durch 100 keV-Elektronen / The Influence of Very Low Temperature on the Radiation Damage of Organic Crystals Irradiated by 100 keV-Electrons

Abstract The radiation damage of electron irradiated organic compounds (paraffin, tetracene) was investigated in the temperature range 300 - 4 K. The radiation damage was measured by the decrease of diffraction intensities or the contrast of extinction contours. The damage of the crystals at 4 K required considerable higher electron doses than at 300 K. This result is explained as follows: The total radiation damage proceeds by two steps: 1) By primary radiation damage - excitation of energy levels with subsequent dissociation - which is almost temperature independent, 2) by secondary radiation damage - diffusion of molecular fragments - which decreases strongly with decreasing temperature.

Changes in the Electron Spin Resonance Spectrum of Glycine with Increasing Doses of Radiation

IT has been reported1 that paramagnetic centres are produced in glycine by equal doses of γ-rays, fast neutrons and α-particles in the proportions 1 : 0.7 : 0.1. As the lower efficiency of the α-particles might well be due to the high linear energy transfer along their tracks, an investigation of the effects of very high doses of electrons on the electron spin resonance spectrum of glycine and related substances seemed worth while.

Imaging an optical disc by the combined use of scanning tunnelling microscopy and scanning electron microscopy

We present the data obtained by scanning tunnelling microscopy combined with scanning electron microscopy of the digitally encoded structure on a stamper used to fabricate optical discs. The combination allows us to focus the STM tip on a preselected spot with a precision of ≅0·3 μm. The data show the superiority of STM for a more detailed characterization of shape, width, length, height and fine structure appearing on the sample. We also show the influence of tip shape on STM resolution. Simultaneous use of both microscopes is possible but high electron doses produce an insulating layer of contaminants thick enough to make STM operation impossible.