## 2 Materials and Methods  
All fingernail samples were obtained from one volunteer (female, age 31) and collected within a two-week time interval. The collected samples were placed in a commercially available sealable packet and stored inside a built-in vacuum chamber (VE-ALL, AS ONE Corporation, Osaka, Japan) prior to the actual experiments. In the present study, we used a total of nine sample collections for the two experimental procedures described below (see Figure 2). In Experiments 1 and 2, the corresponding number of utilized sample collections were 4 and 5, respectively. In each collection, the cut samples weighed a total of \(\sim\)90 mg, with each aliquot's length and width in the range 7-10 mm and 1.5-2 mm, respectively. We divided the cut samples from each collection into four subsets: three for AA treatment and one for water treatment (hereafter \"WT\") as (background) control. Each subset was composed of 5-10 aliquots weighing 20-21 mg. The synthesis of the treatment solution was made by dissolving 1.06 g of AA (FUJI-FILM Wako Pure Chemical Corporation, Osaka, Japan) in a 60 mL Milli-QTM ultrapure water (resistivity = 18.2 M \(\Omega\) cm) using a digital magnetic stirrer (MS-H-PROT, DLAB Scientific Inc., California, United States) at 200 rpm for 5 min. The resulting treatment solution is equivalent to 0.1 M concentration of the AA. In procedures with AA, each of the three subsets was treated with 500 \(\mu\)L AA solution, whereas the remaining one subset was only treated with 500 \(\mu\)L of water (for control). The treatment time for all four subsets was set to 5 min. After the corresponding treatment, all treated samples were dried for 1 h inside a vacuum container (ZWLLING J.A. HENCKELS, Solingen, Germany) with the lid sealed using the designated vacuum pump (ZWLLING J.A. HENCKELS, Solingen, Germany). An illustration of the treatment procedure followed by drying is shown in Figure 1.  
Figure 1: Example illustrations of the treatment procedure of the samples followed by drying inside a vacuum container (**left**) and the storage procedure in two different mediums: vacuum and non-vacuum (**right**). Figure 2: Flow diagram for the specific steps used for Experiments 1 (**above**) and 2 (**below**). The notations UV, AAT, and IR represent ultraviolet, L(+)-ascorbic acid treatment, and irradiation, respectively. The irradiation (hereafter \"IR\") of the samples was conducted using a 160 kVp X-ray beam (6.3 mA) (Cabinet X-ray System Model 43855F with CP160 Option, Faxitron X-ray LLC, Illinois, United States). The system consists of an X-ray tube (MXR-160/22, Comet AG, Flamatt, Switzerland) with 0.8 mm Be inherent filtration. The source-to-surface distance was set to 23 cm. The dose rate was 2.71 Gy/min. Each of the subset samples was exposed to a fixed total dose of 20 Gy. The samples were positioned between the 0.5 cm (\(\varphi\) 8 \(\times\) 8 cm) and 0.1 cm (\(\varphi\) 10 \(\times\) 10 cm) thick solid water-equivalent phantom plates (PH-40 Tough Water Phantom(r), Kyoto Kagaku Co. Ltd., Kyoto, Japan). The irradiation setup is illustrated in Figure 3.  
Between UV exposure, treatment, irradiation, or measurement procedures, the samples were stored in two different mediums (see also Figure 1): (a) vacuum storage, inside an easy-zip vacuum bag (ZWLLING J.A. HENCKELS, Sollingen, Germany) with the valve sealed using the designated vacuum pump (ZWLLING J.A. HENCKELS, Sollingen, Germany) and (b) non-vacuum storage. Both vacuum-stored and non-vacuum-stored samples were placed in various location conditions (for example, 20 \({}^{\circ}\)C or \(-\)20 \({}^{\circ}\)C temperature; with or without exposure to ambient light). Specific details of the corresponding storage and location conditions for the two experimental procedures are shown in Table 1. These various storage and location conditions allowed us to observe the changes in the ESR signals of the AA-treated samples.