ESR Dating and Dosimetry: Principles and Procedures

Abstract

Principles and procedures of ESR dating and dosimetry are described together with the events which set the clock time of radiation effects to zero in ESR dating. A brief history of ESR dating and dosimetry is also described.-67 -New Applications of Electron Spin Resonance Downloaded from www.worldscientific.com by SOUTHERN TAIWAN UNIVERSITY OF SCIENCE AND TECHNOLOGY on 12/14/14. For personal use only. ESR Dating and Dosimetry Principles of ESR Dating and DosimetryNatural radiation from radionuclides including uranium (^HJ and 23S U), thorium (^3 2r Th) and their daughter radionuclides, and potassium ( 40 K) in the environment or inside an archaeological or geological material produces electronic and atomic defects in the material. The radiation produces free (unpaired) electrons, some of which may be trapped by impurities as well as by inherent defects. They are often quite stable and accumulate with time. These unpaired electrons may be in either the organic or inorganic portion and can be detected by ESR. The ESR signal intensity is proportional to the spin concentration and so to the total dose, i.e., the radiation dose rate and the time elapsed after their formation or an event, such as heating, which affects the spin concentration.Thermoluminescence (TL) is observed when the trapped electrons (or holes) are released thermally and recombine with holes (or electrons) at an elevated temperature (see Figure 1.9). On the other hand, ESR detects unpaired electrons created by a-, /?-and y-rays, as is again shown in Figure 3.1. The direction of the unpaired electron spin (self-rotation), represented by a dancing girl and boy is changed by microwave absorption under a magnetic field as shown in (c) and (d). The concentration of the unpaired electrons can be determined directly by microwave absorption as the ESR signal intensity. These unpaired electrons trapped by an impurity or left at a hole are simply called "defects" in this book. The difference between TL and ESR dating is only in the detection method of defects from which "total dose of natural radiation (TD)" is obtained. (c) Figure 3.1 The principle of ESR dating, (a) Paired electrons are spinning in opposite directions, (b) ionization by a-, /3-or y-rays knocks off one of the paired electrons (girl) leaving the other (boy) in a hole, (c) the unpaired electron (girl) is trapped by an impurity (gangster), and (d) the spin direction of unpaired electrons is changed by microwave absorption. The concentration of unpaired electrons in proportion to the TD or ED is determined with ESR.