MDA is one of the important parameters to represent the performance of the radiation measurement system. The MDA is determined by the counting efficiency, counting time, emission yield, and net count level of blank sample in the region of interest (ROI). In routine radiobioassay program, typical and achievable MDAs were provided from IDEAS guideline (General guidelines for the estimation of committed effective dose from incorporation monitoring data) reported by the EURADOS [2]. In this study, we selected the concerned radionucldies which can be released to the environments and cause internal exposure of general public as well as radiation workers in case of radiation accidents. ¹³⁴Cs, ¹³⁷Cs, and ¹³¹I were determined as the concerned radionuclides in the present work. American National Standard Institute (ANSI) also suggested the MDAs for direct radiobioassay, also known as in-vivo measurement, to be applied for routine monitoring program of internal exposure [3]. The MDAs selected from ANSI are relatively higher than those provided by EURADOS. The MDAs of these radionuclides for the in-vivo measurement are summarized in Table 1.

MDDs for ¹³⁴Cs, ¹³⁷Cs, and ¹³¹I were calculated adjusting MDAs from 50 to 1,000 Bq to find out the optimal in-vivo monitoring condition. In general, committed effective dose (CED) can be calculated as following simple equation:

where, DCC is dose conversion coefficient [Sv·Bq⁻¹], M is measurement quantities of in-vivo monitoring [Bq], and m(t) is bioassay function representing retention fraction as a function of time t after intake [Bq·Bq-intake⁻¹]. Then MDD can be derived as below:

In this study, we used DCAL software [4], which was developed at the Oak Ridge National Laboratory, to obtain the bioassay function of ¹³⁴Cs and ¹³⁷Cs in the whole body and ¹³¹I in the thyroid. Elapsed time after initial intake was considered for 365 d when we calculated the retention fraction of each radionuclide. DCCs of the members of public were used from ICRP 72 [5] in which age-dependent dose coefficients were provided from intake of radionuclides. Only inhalation was employed as a pathway of intake of the concerned radionuclides. And 10-yr old child and adult were used as age group of the public in this calculation. Table 2 shows the biokinetic parameters and DCCs used in this study. In order to find out the optimal MDA level for emergency response, MDDs were calculated adjusting MDAs from 50 to 1,000 Bq.

We used portable 3×3 inch NaI gamma spectrometer (RIIDEYE-M-G3, Thermo, Waltham, MA) to quantify the MDA of ¹³⁴Cs, ¹³⁷Cs and ¹³¹I. Counting geometries shown in Figure 1 were applied to measure radioactivity retained in whole body and thyroid, respectively. And we used BOMAB (Bottle Manikin Absorption) phantom [6] and IAEA/ANSI neck phantom [7] including mixed gamma-emitting radionuclides as a certified reference material (CRM) for the calibration of two counting geometries, whole body counting and thyroid monitoring, respectively. Counting time was employed to be 60, 120, 240 s for each counting geometry. We repeatedly measured the activity of the blank sample 5 times for each counting time and evaluated the averaged MDA of ¹³⁴Cs, ¹³⁷Cs, and ¹³¹I depending on the counting time. After that, we compared the evaluated MDA with the suggested optimal MDAs to test the acceptance of the optimal MDA.

Bioassay functions, m(t), as the retention fraction for 365 day after initial intake were calculated using the DCAL software. Figure 2 shows bioassay functions of 10-yr old child and adult for ¹³⁴Cs, ¹³⁷Cs, and ¹³¹I. Bioassay functions during the elapsed period showed the similar pattern between ¹³⁴Cs and ¹³⁷Cs since the biokinetic model and chemical properties are almost same for Cs isotopes. From the calculation results of Cs isotopes, we verified that retention fraction of 10-yr old child decreased more rapidly than adult. On the other hand, bioassay function in the thyroid for ¹³¹I showed the different pattern with Cs isotopes. Retention fraction in the thyroid did not have a significant difference between 10-yr old child and adult. The faction in the thyroid showed the high peak around 1.2 d after intake and very sharply decreased to 10⁻⁵ within 100 d.

Figures 3 to 5 show the MDDs depending on the age and the MDAs for ¹³⁴Cs, ¹³⁷Cs, and ¹³¹I. MDDs of ¹³⁴Cs were slightly higher than those of ¹³⁷Cs for each age group. Even though their bioassay functions showed the similar pattern, MDDs of ¹³⁴Cs were 1.48 times higher than those of ¹³⁷Cs since the DCCs of ¹³⁴Cs are higher than those of ¹³⁷Cs.

We found that MDDs increased not only when the elapsed time passed after the initial intake of the concerned radionuclides but also while the MDAs increased from 50 to 1,000 Bq. In case of ¹³⁷Cs, MDDs for 10-yr old child increased more rapidly than adult because the retention fraction for 10-yr old child showed to be decreased more rapidly after the intake. In case of ¹³¹I, MDDs for 10-yr old child were 2.73 times higher than adult since the DCCs of ¹³¹I for child are higher than adult.

ICRP 103 recommended that reference levels for the highest planned residual doses in emergency situations are typically in the 20 mSv to 100 mSv band of projected doses [8]. However, these levels are relatively higher than dose limit (=1 mSv y⁻¹) of the public in planned exposure situations. In addition, Occupational Intake of Radionuclides (OIR) report issued by ICRP suggested that common practice of individual monitoring for workers whose internal dose from annual intakes could exceed 1 mSv should be done [9]. In this study, in order to provide rapid screening of internal exposure in case of radiation emergencies, a minimum detectable level was determined to set committed effective dose of 0.1 mSv which is 10 times lower than dose limit of the public corresponding with dose level in which practice of individual monitoring for workers is required.

We suggested the optimal MDAs and monitoring period summarized in Table 3 to perform the individual monitoring. 1,000 Bq for ¹³⁴Cs and ¹³⁷Cs, and 100 Bq for ¹³¹I were suggested as the optimal MDAs in case of radiation emergencies. And the optimal monitoring period of ¹³⁴Cs and ¹³⁷Cs was within 100 d and 130 d, respectively, after the event causing internal exposure, and the monitoring period of ¹³¹I was within around 20 d after the event to detect internal exposure below 0.1 mSv. These optimal levels were derived on the basis of dose levels for 10-yr old child because these levels were more conservative than those derived from adult. Moreover, we can extend the monitoring period to provide in-vivo monitoring services for adult since the MDDs for adult were lower than those for 10-yr old child.

Figure 6 shows MDAs of ¹³⁴Cs, ¹³⁷Cs, and ¹³¹I measured using portable NaI gamma spectrometer. The optimal MDAs were achievable when counting time was 120 s for ¹³⁷Cs and 60 s for ¹³⁴Cs and ¹³¹I. However, the optimal MDAs were determined based on the committed effective dose of 0.1 mSv in this work. The optimal MDAs could be adjusted by dose level which we applied as a minimum detectable level in case of radiation emergencies. When we determine higher MDD as a detectable level for emergency response, MDAs could be increased more than the suggested optimal MDAs. Furthermore, we need to consider the increased background level which can cause an increase of MDAs of the concerned radionuclides in case of radiation emergencies.

The optimal monitoring period is also dependent on the MDD, the age group of monitoring subjects, and the type of radionuclide. For ¹³⁴Cs, ¹³⁷Cs, and ¹³¹I, we found that the optimal monitoring period will be extended when the higher MDDs were applied for in-vivo monitoring. And adult age-group showed longer monitoring period than 10-yr old child for the concerned radionuclides in this work.

In addition, only 10-yr old child and adult were considered to represent the member of public in this work. Taking account of other age-groups would be necessary to apply in-vivo monitoring program for the member of public covering all age-groups. And this study only covers in-vivo monitoring for gamma-emitting radionuclides such as Cs and I isotopes. On the other hand, in case of suspected internal exposure for alpha- or beta-emitting radionuclides, the other monitoring method such as in-vitro radiobioassay should be applied to identify and quantify these radionuclides. And the other portable gamma spectrometers can be used instead of NaI detector even though NaI detectors are widely used as a portable gamma spectrometer.