Abstract: Background, aim and scope Electron Spin Resonance (ESR) as a microwave technology, is an analytical method to detect and research the paramagnetic substance with unpaired electrons. ESR dating is an application of this technique to date sediments since their last burial. Compared to other dating methods, ESR dating has advantages in large time span, different kinds of samples, repeated measurements and the short measurement period. Since 1980s ESR dating has been applied to sediments, however, there exist some problems in sample preparation and ESR signal measurement. The purpose of the paper is to introduce the experimental procedures for those who are interested in ESR dating of glacial till.Materials and methods Sample collection: the quartz was one of the best minerals for detecting ESR signals. In the field, we selected the moraine profiles that had rich in quartz and were not retrofitted. It was better to collect silt and fine sand samples through subglacial grinding or being exposed to the sun. The samples must avoid to the direct sunshine from the field to the laboratory. We gathered samples 2—3 kg and made detailed records and environment descriptions. And we also collect sample using sealed aluminum boxes for water content. If there was not gamma spectrometer for measuring environmental dose rate in situ, the contents of uranium, thorium, potassium from the samples were measured using neutron activation analysis (NAA). The rigorous sampling procedure and detailed field records were of benefit to analysis and comparison of the data. Extraction and purification of quartz from the sediment: we sieved out 0.063—0.25 mm of purified quartz grains from samples for ESR dating. At first, we removed carbonate with hydrochloric acid and organic matter with hydrogen peroxide. Next, we eliminated magnetic mineral with permanent magnet. Then, we separated minerals of different specific gravities with the heavy liquid. Heavy mineral, quartz, feldspar and mica were separated and pure quartz was obtained. Lastly, the residual feldspar and quartz surface damaged by alpha-ray was etched by hydrofluoric acid. After that, we removed fluoride with 10% hydrochloric acid, cleaned and dried the samples in an oven at 40°C. Through hydrofluoric acid etching, the purity of quartz, which is checked under the binocular microscope, should reach at least 95%, and 99% or higher would be better. The purification of quartz could exclude the effects of other minerals for accurate and repeatable measurements of ESR signals. Additional irradiation of purified quartz samples: we divided the quartz into aliquots with 250 mg or 300 mg. One aliquot was not irradiated and others were irradiated at different doses using 60Co gamma-ray source. There were two methods to erasure short life signal. One way is to heat samples at a certain temperature. Another is to let samples sit for 7—10 days or more. In the experiment, the corresponding doses were set according to the response of sample’s ESR signal to additional irradiation. Measurement of ESR signal: the ESR signals for dating glacial tills could be chosen germanium (Ge) center measured at room temperature or aluminum (Al) center and titanium (Ti) center at low temperature. The area under the absorption curve represents the peak intensity of ESR signals. The peak to peak height of the signal differential could be instead with peaks of the same line and width. To prevent the difference among the sample separations, the position of samples and the centre of the resonant cavity should be coincide.Results The results showed that: (1) The grain-size fraction of 0.125—0.25 mm should be selected for quartz purification. (2) The process of heavy liquid separation could improve sample purity effectively and reduce signal interference from other minerals. (3) The choice of different ESR signal centers could result in the dating difference.Discussion The grain size of samples had an impact on ESR signals. Fine grains would be better for dating glacial tills because ESR signal of quartz in them is more likely reset. However, it is difficult to obtain enough purified quartz from the grain-size fraction of 0.063—0.125 mm extracted from glacial tills. Therefore, we suggest that the grain-size fraction of 0.125—0.25 mm is selected for quartz purification. The sample purity also affects ESR signals. For poorly sorted glacial tills, the process of heavy liquid separation could improve sample purity effectively and reduce signal interference from other minerals. The choice of different ESR signal centers could result in the dating difference. The spectral line widths might increase at room temperature and the resolution might reduce. The small power was needed at low temperature. Conclusions The accurate and stable signals required standard experimental procedures and better signal selection.Recommendations and perspectives Establishment of standard procedures in sampling, sieving, extraction of quartz, quartz purification, additional irradiation and measurement of ESR signal can make the data be duplicated in different laboratories so as to make sure that the dating data can be compared with each other.
Keywords: the technique of ESR dating; experimental procedures; quartz; glacial tills