Radiation damage and sample heating

Radiation damage and sample heating arise from the absorption of X-rays in the sample. The sensitivity of the specimen to these effects depends markedly on the temperature of the specimen. Initially comments will be restricted to room (or near room) temperature where protein crystals maintain their liquid-like nature in the solvent channels going through the crystal. 

Ionisation processes occur resulting in free radicals, which are potent agents for chemically damaging biological macromolecules. These free radicals have a natural diffusion rate and, hence, radiation damage is dose-rate-dependent, i.e. a larger total dose can be tolerated by a sample if that dose is delivered at a higher rate. 

There are data available on dose rate and dose tolerances. Doses of 109rad destroy the structure of organic molecules and 106rad kill living cells (Bordas and Mandelkow 1983). These authors discuss the case of microtubule polymerisation levels of 30 000-50 000 rad delivered at rates of 2000 rad min-1 with medical X-ray sources impair this function whereas on DORIS in solution scattering experiments 1.5x 106rad delivered at a rate of 60 000 rad min-1 are needed, i.e. an increase of 30 in 

Sample heating in SR beams as a consideration was introduced by Stuhrmann (1978) his example is cited in detail in section 6.5.4.1. Hel-liwell and Fourme (1983) considered radiation damage and sample heating in evaluating the usefulness of the prospective fluxes at the specimen that might be anticipated using the ESRF. Helliwell and Fourme (1983) and Helliwell (1984, pp. 1470-3) discuss the need to go to shorter X-ray wavelengths (e.g. 0.5 A), to reduce the fraction of absorbed photons, and to use cryotemperatures, with frozen crystals mounted on a copper fibre, to limit the temperature rise experienced by the sample. In this way, frozen microcrystals of protein of size lO/zm should be successfully studied on the ESRF. This application of the ESRF was further discussed in Helliwell (1989). 

A formulation is now given to describe the effect of the interaction of X-rays with protein crystals. 

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Energetic particles of the galactic cosmic radiation (GCR) have a mean penetration depth in rock of about 50 cm, comparable to the typical size of a meteorite. GCR-induced effects therefore provide a means to study the history of meteorites as small objects in space or in the top few meters of their parent body. These effects include cosmic ray tracks, i.e., the radiation damage trails in a crystal lattice produced by heavy ions in the GCR (Fleischer et al. 1975), and thermoluminescence, i.e., the light emitted by a heated sample which had been irradiated by energetic particles (Benoit and Sears 1997). By far most important, however, are cosmogenic nuclides, produced by interactions of primary and secondary cosmic ray particles with target atoms. 

Some time ago. Sales and co-workers observed the ESR spectra of ancient bones before and after heating (24). All the ancient bone samples that they studied were at least 7500 years old, and most were older than 300,000 years. In these, aside from resonances that were due to transition metal ions in the bone, the spectra were dominated by the defect signal that is produced by radiation damage to the mineral component of the bone, and which is the signal that is used to date bone and tooth material. This signal is not observable in bone or tooth that is less than about 30,000 years old (24). The temperature dependence of the spectra from the samples that Sales and co-workers observed presumably represents the temperature dependence of the spectrum of this defect. 

A major appUcation of dynamic microscopy is the study of pol)nner structure and its development as a function of temperature in a hot stage. This type of work is rare in electron microscopes. Heating a polymer sample in an electron microscope requires caution, as not only will heating increase the rate of radiation damage, but also the polymer may outgas or degrade and evaporate, contaminating the microscope vacuum system and associated x-ray detectors. 

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