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REVIEW PAPER

Use of Commercially Available Optically Stimulated Luminescence Dosimeter As Extremity Dose Estimator

Georgeson, David L.; Christiansen, Byron H.1

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doi: 10.1097/HP.0000000000001260
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Abstract

INTRODUCTION

The purpose of this study is to determine the effectiveness of the Landauer (Landauer, Inc., Glenwood, IL) nanoDot dosimeter, which was analyzed by using the MicroStar reader (Landauer, Inc.), in providing timely and accurate extremity dosimeter estimates. This proposed process will provide a field estimate of extremity dose for use in tracking extremity exposure between official dosimeter analyses. The ability to achieve realistic extremity-dose estimates in the field shortly following work where significant exposure is expected is a much sought-after capability, as the application of time-motion dose analysis using direct radiation measurements has proven to be inaccurate and operationally inefficient.

Operational organizations at Idaho National Laboratory (INL) perform a large amount of hands-on work that requires extremity monitoring. Several incidents where the actual received extremity dose was significantly larger than what had been expected has led to the program being administratively tightened (INL 2015; USDOE 2008, 2017). Administrative actions included lowering the extremity dose threshold at which the extremity dosimeter is returned for early processing and, subsequently, the use of an individualized extremity dose-tracking card that is used to document the estimated extremity dose between the nominal extremity dosimeter monitoring periods. Additionally, upon reaching the extremity dose threshold, the individual is restricted from performing work requiring extremity monitoring until the dose of record is reported.

Typically, extremity dose estimates for a given work evolution were based on the simple product of the measured dose rate and the time spent in the work area. Several years’ worth of operational information comparing the estimated dose to the actual dose of record revealed that the existing methods used to estimate extremity dose were overly conservative and caused significant operational downtime. In fact, often the estimated extremity dose was not even close to the actual (e.g., an estimated extremity dose of 5 mSv resulted in an actual dose of record of only 0 mSv).

These issues inspired INL’s radiological control organization to explore methods to perform real-time extremity monitoring in an effort to improve the dose estimating process. The desired outcome was to optimize several operational factors, including reduced downtime, simplicity, providing a monitor that exhibits similar operational characteristics to the reference dosimeter, and being reasonably accurate (i.e., −20% to +50% of the actual dose of record) for the majority of the radiological environments (primarily beta/photon) encountered at INL. The early success encountered with active extremity monitors was proven to be limited and did not meet all of the stated goals. This led to the pursuit of a unique solution based on the application of optically stimulated luminescent (OSL) dosimeter technology (nanoDot) developed by Landauer, Inc.

In 2009, INL transitioned to an external dosimetry program using OSL. This process led to the introduction of the nanoDot dosimeter and MicroStar reader to INL’s radiological control program. It was recognized at the time that a small, self-contained, single-chip OSL dosimeter that can be read easily in the field would have many potential applications for the radiological control program (Georgeson and Christiansen 2018).

The use of a device capable of estimating extremity dose that bears a close resemblance to the official dosimeter comes with some potential issues, including:

  • Comparing the estimated employee nanoDot extremity results to official extremity results, when the resultant estimates might differ from the official results by as much as 50%;
  • Difficulty in maintaining a comparable geometry during use; and
  • Potentially complex logistics associated with reading the nanoDot in the field.

These issues were tracked and evaluated as a part of this study.

The Landauer nanoDot is a small, uniquely identifiable barcoded single OSL chip dosimeter. The nanoDot dosimeter has the ability to be read using the MicroStar field dosimeter reader. The Landauer nanoDot can also be re-read without significant loss of information, if necessary (Perks et al. 2019; Jursinic 2007). These capabilities allow for the use of a simple wear design to be developed and add increased flexibility to this new process.

The overarching goal of this work was to provide a simple process that could be administered reasonably by the field radiological control organization and understood by those wearing the estimators. It should be noted that it is well understood that extremity monitoring is subject to multiple technical issues, such as geometry, and it was not the purpose of this study to discuss or address those. INL employs the Landauer Saturn finger ring for extremity monitoring. These are worn in pairs—one on each hand. It was hypothesized that a reasonable estimate of the extremity dose could be achieved by positioning the extremity dose estimator in a geometry similar to the extremity dosimeter used for the dose of record. Workers were directed to mount the nanoDot extremity dose estimator on the finger adjacent to and facing the same direction as the accredited extremity dosimeter used for the dose of record.

MATERIALS AND METHODS

The nanoDot OSL consists of a single 7-mm-diameter, 0.2-mm-thick plastic disk, infused with aluminum oxide doped with carbon (Al2O3:C). The disks are encased in a 24 × 12 × 2 mm3 light-tight plastic holder. The nanoDot can be read in the field using the Landauer MicroStar dosimeter reader. Further, an inherent feature of OSL technology is the ability to be re-read as needed. A signal loss of less than 0.05% per re-read was reported (Jursinic 2007).

For this study, general purpose (GP) nanoDots were used. GP nanoDots have their sensitivity assigned based on a measured average sensitivity of the OSL roll, from which nanoDots are extruded; all nanoDots originating from the same roll are assigned the same sensitivity value ±10% (Landauer 2019a).

Landauer’s MicroStar dosimeter reader, when connected to a laptop computer, is a portable single-element reader that uses an array of green light-emitting diode (LED) lights (532 nm) rather than heated gas to stimulate the dosimeter light output. The nanoDot light output spectra are filtered to allow only blue light (420 nm) to impinge on the photomultiplier tubes (PMTs) (Perks et al. 2007; Smetana et al. 2008).

To evaluate the adequacy of the Landauer nanoDot as a supplemental extremity monitor, the following tests were performed:

  • The dose response of the nanoDot was compared to the dose measured by the Landauer Type U finger rings, evaluated for 137Cs, 90Sr, depleted uranium (DU), M60 x rays, and M150 x rays, respectively. Exposures of approximately 5 and 15 mSv for each radiation type were chosen to represent the occupational extremity exposures currently received by INL workers. The typical isotopic mix encountered at INL is mixed fission and activation products; therefore, the associated energies became the basis for testing.

It must be noted that Landauer transitioned from the Type U to the Saturn Type finger ring during the field testing phase of this study. Landauer’s Type U and Saturn thermoluminescent dosimetry (TLD) ring dosimeters possess identical Harshaw TLD-100 chips with slightly different holder designs. The TLD in the Type U ring dosimeter is contained in a polystyrene cap with a nominal structural absorber of about 40 mg cm−2 (Salasky 2008). The TLD in the Saturn ring dosimeter is contained in a polyethylene cap with a nominal structural absorber of about 27.9 mg cm−2 (Salasky 2014). The difference in dose response between the ring dosimeters is negligible (Bean 2017); therefore, no distinction is made between the two ring dosimeter types, and the results of either extremity dosimeter are considered valid. Ergonomics as it relates to wearer comfort, dexterity, durability, and readability were also evaluated.

The intent of these tests was to determine a single calibration factor for field use in converting the reader output to a dose value that is a reasonable estimate of what a US Department of Energy Laboratory Accreditation Program (DOELAP)-accredited extremity dosimeter would determine for most occupational exposure scenarios encountered at INL. To obtain the estimated extremity dose, the subsequent correction factor is multiplied by the nanoDot corrected-light output. A reasonable estimate for this project was defined as a reading that falls between −20% and +50% of the known value for most measurements. The estimated-dose value will be used to track estimated extremity dose between reads of the accredited dosimeter. It was understood that, in situations where a particular isotope is singularly present or in situations where the energy ranges encountered vary significantly from those tested, the correction factor may not yield a reasonable estimate. Further, for most low-energy beta and gamma applications, contact measurements can vary greatly with only the smallest variation in distance and orientation. Subsequently, the single calibration factor chosen will be designed to overestimate those results. Thus, the anticipated worst case scenario would be the premature triggering of the dosimeter’s extremity in early processing.

DOSE RESPONSE DEVELOPMENT

All exposures were performed with the nanoDot and Type U dosimeters mounted on the American National Standards Institute (ANSI) finger phantom, as shown in Fig. 1. Ten exposure conditions were evaluated with five nanoDot/finger ring dosimeter pairs for each exposure. Cesium-137 and x-ray calibration exposures were performed in the gamma-beam irradiator (GBI) and the x-ray beam room at INL’s Health Physics Instruments Laboratory (HPIL), respectively. Beta-calibration exposures were performed using a 90Sr beta irradiator and a uranium-slab irradiator at the Radiological and Environmental Sciences Laboratory (RESL) operated by the US Department of Energy (DOE). The radiation fields at HPIL and RESL are traceable to the National Institute of Standards and Technology (NIST) and are used in the performance of the DOELAP testing, with RESL being the primary testing facility for all laboratories operated by DOE.

Fig. 1
Fig. 1:
Extremity dosimeters exposure configuration.

The average and uncertainty for each of the five nanoDot/finger ring dosimeter pairs from each of the results from the 10 exposure conditions were determined and summarized in Table 1. A nanoDot calibration factor was determined for each exposure condition in units of dose per unit corrected light output, as follows:

nanoDot Calibration Factor=Reported TypeURing DosenanoDot Corrected Light Output,
Table 1 - Summary of average ring dose vs. average light output measured by MicroStar.
Exposure type Type U ring average dose (mSv) Standard
deviation
Relative uncertainty nanoDot average corrected light output Standard
deviation
Relative uncertainty nanoDot
calibration factor
(10 −3 mSv/light output)
Calibration uncertainty
137Cs High 25.1 2.7 0.11 11647 278 0.02 2.2 0.11
137Cs Low 8.4 0.9 0.10 3853 111 0.03 2.2 0.11
M60 High 42.4 1.7 0.04 52348 442 0.01 0.8 0.04
M60 Low 14.4 1.5 0.10 17464 181 0.01 0.8 0.10
M150 High 41.6 4.5 0.11 34701 509 0.01 1.2 0.11
M150 Low 11.7 1.1 0.10 9485 338 0.04 1.2 0.10
90Sr High 26.2 1.6 0.06 11463 261 0.02 2.3 0.07
90Sr Low 9.3 0.8 0.09 3723 65 0.02 2.5 0.09
DU High 13.0 1.7 0.13 4534 547 0.12 2.9 0.18
DU Low 5.6 0.4 0.07 2273 173 0.08 2.4 0.11

where:

Reported Type U Ring Dose=the average Reported Type U Ring Dose, which incorporates a processor correction factor of 1.25 for all INL ring measurements.

nanoDot Corrected Light Output = the average nanoDot Corrected Light Output, which represents the direct reading of the nanoDot divided by a manufacturer-provided normalizing factor specific to that dosimeter.

The uncertainty for each calibration factor was determined as the relative uncertainty for the finger ring results and the relative uncertainty for the nanoDot results added in quadrature:

Calibration Factor UncertaintyUC=UFR2+Un2,

where:

UFR = Finger Ring Uncertainty;

Un = Average nanoDot Uncertainty; and

UC = Calibration Factor Uncertainty.

FIELD TESTING

Exposure conditions found in the field possess variables not captured by the laboratory dose-response testing including unknown mixed beta and gamma fields, varying attenuation due to multiple layers of protective gloves, and the positioning of the nanoDot in relationship to the finger ring. Therefore, several field measurements were made with workers wearing the nanoDot in typical field applications where extremity monitoring with the Type U dosimeter is performed.

The nanoDot was worn by taping it to the finger adjacent to the worker’s Type U finger-ring dosimeter, as shown in Figs. 2 and 3. Measurements of the nanoDots were compared with the results of the adjacent Type U finger ring dosimeters provided by Landauer. The relative uncertainty for the Type U finger ring measurement was taken as 0.18 for a photon dose at 10 mSv (Salasky 2008). The relative uncertainty for the Saturn Type finger ring measurement was taken as 0.07 for a photon dose at 10 mSv (Salasky 2014).

Fig. 2
Fig. 2:
nanDots and tape used in field tests.
Fig. 3
Fig. 3:
Extremity dosimeter adjacent to nanoDot estimator.

Based on the field measurements, the weighted average of the ratio of extremity ring dose to the supplemental nanoDot monitor was observed to be 1.6 × 10−3 mSv per unit corrected light output, as shown in Table 2. The weighted average was determined as follows:

X¯=i=1nUiRii=1nUi,
Table 2 - Finger-ring and nanoDot results for field exposures.
Type U Ring serial number Dose
(mSv)
Rounding uncertainty nanoDot
serial number
Net corrected light output Calculated ratio
(10 −3 mSv/light output)
Relative
uncertainty
Ratio x uncertainty
9660519R 5.0 0.010 DN084667979 3639 1.4 0.21 0.30
9660518R 5.7 0.009 DN08466158N 3942 1.4 0.21 0.30
9743761R 2.7 0.019 DN08467057Q 1888 1.4 0.21 0.30
9743760R 2.9 0.017 DN08467036U 1786 1.6 0.21 0.34
9743723R 4.3 0.012 DN08466825I 3383 1.3 0.21 0.27
9743722R 5.4 0.009 DN08466871L 5047 1.1 0.21 0.23
9743724R 4.4 0.011 DN08466813N 3003 1.5 0.21 0.32
9743725R 3.9 0.013 DN08466109Q 2397 1.6 0.21 0.34
9743639R 0.6 0.083 DB08842627B 508 1.2 0.23 0.27
9743638R 0.7 0.071 DB08840292O 477 1.5 0.22 0.33
9743672R 0.4 0.125 DB08842640N 233 1.7 0.25 0.42
9743753R 2.0 0.025 DB08840316M 1372 1.5 0.21 0.32
9743752R 1.8 0.028 DB08841944A 1054 1.7 0.21 0.36
9743756R 2.3 0.022 DB088419391 1270 1.8 0.21 0.38
9743757R 2.0 0.025 DB08840226N 981 2.0 0.21 0.42
9743868R 5.1 0.010 DN084668894 2734 1.9 0.21 0.40
9743867R 5.0 0.010 DN08466812P 3147 1.6 0.21 0.34
9955568R 1.0 0.050 DB08842572I 544 1.8 0.22 0.39
9955569R 1.1 0.045 DB08841612N 570 1.9 0.22 0.41
9954907R 4.4 0.011 DB08842137K 2297 1.9 0.21 0.40
9954908R 4.2 0.012 DB08840265L 2128 2.0 0.21 0.42
9887192R 0.4 0.125 DB08842048J 256 1.6 0.25 0.39
9887193R 0.4 0.125 DB08842115Q 269 1.5 0.25 0.37
9954892R 4.9 0.010 DB08842050Y 2221 2.2 0.21 0.46
9954891R 5.5 0.009 DB08841825C 2824 1.9 0.21 0.40
9954948R 15.9 0.003 DB08842120Z 7015 2.3 0.21 0.49
9954947R 17.4 0.003 DB08842922F 7959 2.2 0.21 0.46
9954917R 4.3 0.012 DB088425869 1835 2.3 0.21 0.49
9954918R 4.2 0.012 DB08840496C 1811 2.3 0.21 0.49
1011344R 3.1 0.016 DB08834341R 4593 0.7 0.21 0.15
1011345R 3.1 0.016 DB08834346H 4214 0.7 0.21 0.15
101342R 5.0 0.010 DN08466850P 7638 0.7 0.21 0.15
1011343R 6.5 0.008 DN08467058O 9339 0.7 0.21 0.15
1094217R 2.6 0.019 DB088426792 1634 1.6 0.21 0.34
1094217R 3.0 0.017 DB08841997Z 2137 1.4 0.21 0.30
1011103R 1.6 0.031 DB08835274J 829 1.9 0.21 0.41
1011149R 1.6 0.031 DB08834339C 789 2.0 0.21 0.43
Sum 8.0 12.9
Weighted average calculated ratio
(10−3 mSv/light output)
1.6

where:

X− = average calculated ratio weighted by uncertainty;

R = calculated ratio; and

U = relative rncertainty in the calculated ratio.

The uncertainty for each test field measurement was determined as the relative uncertainty for the Type U finger ring (0.18), the relative error due to rounding (based on 0.05 mSv), and the calibration uncertainty for the nanoDot 137Cs exposures (0.11) added in quadrature:

Relative uncertainty in the calculated ratioU=UFR2+UR2+Un2,

where:

UFR = finger ring total uncertainty;

UR = finger ring rounding uncertainty;

Un = nanodot calibration uncertainty; and

U = relative uncertainty in the calculated ratio.

Based on the evaluation of dose contribution for processes requiring extremity monitoring in INL technical evaluation TEV-3185, “Evaluation of Source Specific Correction Factors for Facilities and Processes Requiring Extremity Monitoring” (INL 2017), the majority of field conditions can be represented by exposure to high-energy photons and betas, which are created for purposes of this experiment by 137Cs and 90Sr samples, respectively. The results of field testing indicate that the average ratio observed from field measurements is slightly less than what was determined by the dose response testing results for 137Cs (i.e., 2.2 × 10−3) and 90Sr (2.5 × 10−3), respectively.

In addition to these tests, ergonomics as they relate to wearer comfort, dexterity, durability, and readability were also evaluated through user feedback. The nanoDot estimator was designed to use a commonly available attachment mechanism (i.e., medical tape), which provided the additional benefit of being soft and flexible, thus greatly improving user comfort. Feedback from radiological workers in the field who wore the nanoDots was positive. There were no complaints registered regarding the nanoDot interfering with the workers' ability to perform their assigned tasks or discomfort of any kind while wearing the estimator. Durability and readability were assessed upon receipt of the nanoDot to the laboratory where it was read. None of the nanoDots used during the field tests were found to be damaged or unreadable. The manufacturer-applied labels were still readable via the barcode reader attached to the MicroStar.

CALIBRATION FACTOR DETERMINATION AND VALIDATION

It is not unreasonable to expect the nanoDot to provide an estimated dose within 30% of the true value under laboratory conditions. However, due to the variables that were encountered during the field tests while evaluating the nanoDots as supplemental extremity monitors, achieving an estimated dose within 30% of the known value for all measurements was difficult.

Because of the wide range of ratios observed during the tests, the average ratio observed by the initial field testing was used as more of a qualitative, rather than a quantitative, measure of an ideal calibration factor. The purpose of a supplemental extremity dosimeter is to ensure workers’ extremity doses are below INL annual administrative control levels of 100 mSv and to provide a basis for decisions to analyze finger rings prior to the end of their normal wear periods of 1 mo. As such, the recommended correction factor was established to provide an estimated dose that will fall between −20% and +50% of the known value for most measurements. Therefore, it was determined that the application of an in-field calibration factor of 2.0 × 10−3 mSv per unit corrected light output would provide a reasonably conservative estimate with respect to personnel protection for most extremity-monitoring scenarios. The most likely situation where a non-conservative estimate is determined is when the geometry of the nanoDot does not match that of the finger ring (i.e., the nanoDot is not perpendicular to the radiation source). To validate the efficacy of the field calibration factor and to demonstrate that the supplemental extremity monitor using the nanoDot would provide an accurate estimate in most conditions, the calibration factor of 2.0 × 10−3 mSv per unit corrected light output was applied to the light output values of the field-test nanoDot dosimeters to obtain the estimated dose, as shown in Fig. 4.

Fig. 4
Fig. 4:
Plot of the percent difference of type U finger ring dose of record and nanoDot supplemental monitor dose estimate field test pre-implementation.

Of the initial 37 nanoDot ring comparisons presented in this study, none produced a bias less than 20%, and only nine produced a bias greater than 50%. These results were considered operationally acceptable; subsequently, the supplemental monitor was implemented. Since the implementation of the supplemental monitor, additional data have been collected to further support the effectiveness of the supplemental monitoring process, as shown in Fig. 5. To further add another layer of conservatism, background subtraction of the nanoDot results was no longer performed when calculating the dose estimate. As of this writing, 92 sets of rings have been assigned and paired with the supplemental monitor. Eleven (6%) of the 184 nanoDot ring comparisons presented here produced a bias of less than 20%. Forty (22%) of the 184 nanoDot ring comparisons produced a bias of greater than 50%. Twenty-six of the 40 results greater than +50% were from ring doses less than 1 mSv; this discrepancy is likely due to not employing background subtraction from the nanoDot results. All results were considered operationally acceptable and have noticeably reduced the number of early processing requests, which in turn has reduced operational downtime.

Fig. 5
Fig. 5:
Plot of the percent difference of Saturn finger ring dose of record and nanoDot supplemental monitor dose estimate field test post-implementation.

In an effort to evaluate non-typical INL source terms, two alternate test environments were explored. Three nanoDot/Saturn ring pairs were exposed to a glass vial containing pure 252Cf in a laboratory fume hood. Note that the nanoDot is only sensitive to beta and gamma radiation (Landauer 2019b). A pure 252Cf environment will produce gamma radiation through fission and resulting fission products. The results presented in Table 3 indicate that the nanoDot supplemental monitor can provide an operationally acceptable estimate. Two nanoDot/Saturn ring pairs were set up in a 241Am environment. Americium-241 produces low energy gamma radiation, which was best represented by the testing performed with the M60 x-ray field. As presented in Table 1, the calibration factor determined for the M60 x ray was 0.8×10−3 mSv per unit light output; therefore, the two sets exposed to 241Am were evaluated using a calibration factor of 1.0 × 10−3, rather than the 2.0 × 10−3 mSv per unit corrected light output, to better represent the 241Am environment. As shown in Table 4, when applying a more appropriate correction factor based on facility and operational knowledge, the resulting confidence in the estimate of extremity dose is greatly enhanced.

Table 3 - Beta/gamma dose estimates from 252Cf field test environment.
Saturn Ring
serial number
Dose
(mSv)
nanoDot
serial number
Estimated dose
(mSv)
Percent
Difference
3193943SB 2.11 DB088534878 2.0 −4
3193959S3 2.32 DB088526669 2.6 11
3193960SA 3.60 DB088496458 2.7 −25

Table 4 - Dose estimates from 241Am field test environment.
Saturn Ring
serial number
Dose
(mSv)
nanoDot
serial number
Estimated dose (mSv) Percent
Difference
3193926SC 30.89 DB08853427E 35.6 15
3193944SA 29.85 DB088525976 32.3 8

CONCLUSION

The ability to provide acceptable extremity-dose estimates using a fast-turnaround in-field process has greatly enhanced the ability of the radiological control organization at INL to track and manage extremity dose, and subsequently, the nanoDot is deemed suitable for use as a supplemental extremity monitor for beta/gamma radiation when worn in a geometry similar to the finger ring. For INL, the nanoDot provides a reasonable estimate of extremity dose when a calibration factor of 2.0 × 10−3 mSv per unit corrected light output is applied to the corrected light output. The MicroStar reader with the calibration factor embedded in its software can be used for the analysis of nanoDots when they are used as supplemental extremity monitors (Yahnke 2019). The large differences noted between the nanoDot and the Saturn Ring for some fields requires the use of alternate correction factors based on the specific radiation environments, such as those presented in Table 1. Although the use of the nanoDot supplemental monitor is considered operationally acceptable, inaccuracies in the estimate arise when the orientation of the nanoDot is not consistent with the orientation of the Saturn Ring with respect to the radiation source or when the radiological source term is vastly different from the assumed source term.

FUTURE WORK

As the application of the nanoDot as a supplemental extremity monitor matures, refinements on the correction factors and use of the MicroStar reader will be applied. Further, development of an improved nanoDot mount will enhance the consistency of the geometry between the nanoDot and Saturn ring and reduce dependence on proper application by the user of the extremity monitor. Ultimately, the goal of this application will be to pursue and acquire DOELAP accreditation for the nanoDot monitoring system.

Acknowledgments

This information was prepared as an account of work sponsored by an agency of the US Government. Neither the US Government nor any agency thereof, nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness, of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. References herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the U.S. Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the US Government or any agency thereof.

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Keywords:

accidents, nuclear; dose assessment; dosimetry, external; dosimetry, thermoluminescent

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