## 1 Introduction  
It is recognized that some medical workers involved in interventional radiology/cardiology, diagnostic/therapeutic nuclear medicine, positron emission tomography, brachytherapy, among others, could receive notably high radiation doses to their extremities. Their hand doses tend to be higher than the personal doses routinely measured with commonly used personal dosimeters; sometimes they may exceed the skin dose limit (500 mSv per year) even though the values from their personal dosimeters are insignificant [1; 2; 3]. In addition, it is still difficult to properly and routinely monitor local hand exposure as dose distributions change to a large extent, depending on the type of handling radiation or radionuclide, and the geometric situation between the hands and radiation source. Recent studies also reported that their fingertip doses could be significantly higher than the average hand dose measured with a ring-type dosimeter [3; 4]. We thus need to develop a novel and practical measure to solve this issue. For this aim, a combination of electron spin resonance (ESR) measurement and fingernails is worth to be investigated. The application of \"fingerail ESR dosimetry\" has a good potential for quantifying the number of radiation-induced free radicals that could stay long after exposure. For many decades, ESR (or electron paramagnetic resonance, EPR) of biomaterials, such as tooth enamel and bone, has been used successfully for retrospective dosimetry in atomic bomb survivors, radiological accident victims, residents around nuclear testing sites, and radiotherapy patients [5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20]. However, in the case of emergency dosimetry, which requires an urgent assessment of absorbed doses to affected individuals, the number of potential applications using ESR is relatively small and the present guidelines are yet to be developed. Indeed, in both short-term and long-term aftereffects of any radiation accident, a systematic protocol for feasible dose assessment, using the available biomaterials is an utmost need. Additionally, in the search for developing potential tools for retrospective dosimetry, most of the attention has been paid to alternative materials that are carried close to the human body. Fingernails are no doubt one of the most promising biomaterials in this context. Its potential use as a dosimetry tool was first suggested in the late 1980s [21]. Fingernails are mainly composed of \(\alpha\)-keratin, a protein which consists of three long right-handed \(\alpha\)-helical peptide chains twisted into a left-handed coil strengthened by disulfide bridges (S-S) formed from adjacent cysteine groups [22]. Its exposure to ionizing radiation could generate stable free radicals that are the main sources of ESR signals. Subsequent studies on the ESR signals from fingernails revealed that the spectra are composed of three main components: radiation-induced signal (RIS), which is assumed to be correlated with absorbed dose; mechanic-induced signal (MIS), which is attributable to \(\alpha\)-radicals formed along the shear edge of the nail through the breakage of S-S bonds caused by cutting/clipping and other mechanical stresses [21; 22; 23]; pre-existing native signal (PES), whose origin remains unknown [24]. Despite such complexities in the signal processing, the easier sampling of fingernails compared to tooth enamel and bone has attracted some researchers who paved the way for notable developments in fingernail ESR dosimetry techniques in the 21 st century [25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40]. Comprehensive reviews about the free radical mechanism and characteristics of fingernail ESR signals can also be found in the literature [41; 42; 43]. In those studies, many authors noted that one of the most problematic issues regarding fingernail ESR dosimetry is how to identify and quantify the true component of RIS alone, separated from other background signals (BKG).