## 4 Conclusions  
In the present work, we confirmed that the BKG (non-RIS) of fingernails can be significantly reduced by applying the DTT reducing reagent. After the DTT treatment, the fingernail samples provided good signal stability for both background and RIS signals with less intra-individual variations. Additionally, we demonstrated that the freezer storage would be more effective to maintain the stability of RIS after \(\gamma\)-ray irradiation than vacuum storage; it is thus desirable that collected fingernail samples be stored below the freezing point for performing more accurate dosimetry.  
These findings are preferably to be applied in the monitoring of hand exposure of medical workers. More concretely, workers who might face high hand dose exposure from radiation sources are proposed to take the following steps to carry out precise dose assessments: (1) dip or wipe one's fingers with a reducing reagent before the relevant work; (2) get a small portion of the fingernail (e.g., from the little finger) as a control sample and store it in a freezer (\(<\)0 \({}^{\circ}\)C); (3) perform the work; (4) cut the fingernails soon after completing the work in the same way as with the control sample; (5) store the collected samples in the freezer; (6) measure the ESR signals of all the samples, including the control at the same time; (7) evaluate the finger dose by subtracting the MIS measured with the control sample from the ESR signal of the respective sample; (8) assess the three-dimensional dose distribution of the hand.  
While the efficacy of DTT for chemical treatment was particularly good for BKG reduction and RIS stabilization in the fingernail ESR dosimetry, this procedure has some weaknesses. One is that the DTT treatment substantially reduced the peak-to-peak intensities of both BKG and RIS. This fact implies that the chemical treatment would reduce the capacity of trapping free radicals, which could induce an unfavorable saturation response at a lower dose level. Further, the samples used in this study were  
Figure 7: Changes in the background ESR signal (peak-to-peak amplitude, App) in unirradiated fingernail samples stored in the (**a**) vacuum chamber and (**b**) freezer measured before and after (soon, 1, 2, 3, and 7 days) the DTT treatment.  
obtained from the fingernails of only one individual and that possible inter-individual variations have not been identified. It must be critical for practical application of fingernail ESR dosimetry to verify that the findings of the current study can be universally applied through comprehensive analyses of the fingernail samples taken from different individuals. Another technical drawback is the toxicity of DTT, which allows only trained professionals to use hazardous chemicals. Considering these issues, we are currently focusing on research to find a better antioxidant material that is safer and easier to handle for any workers posing occupational risks of hand exposure from radiation sources.  
Conceptualization, H.Y. and C.A.B.G.; validation, C.A.B.G. and J.E.T.; investigation, C.A.B.G., H.Y. and J.E.T.; resources, H.Y.; data curation, C.A.B.G. and J.E.T.; writing--original draft preparation, C.A.B.G.; writing--review and editing, H.Y.; supervision, H.Y.; funding acquisition, H.Y. All authors have read and agreed to the published version of the manuscript.  
This research was partially funded by JSPS KAKENHI Grant Number 18 KK0147 and the Program of the Network-type joint Usage/Research Center for Radiation Disaster Medical Science funded by the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan and Hiroshima University.  
This study was approved by the Ethics Review Committee of Hiroshima University (Number E-1495).  
The authors declare no conflict of interest.