DCAS-RPT-008 Rev 00

Supersedes: None 

DOE Review Release: 05/08/2020

Evaluation of the Pb-212 Detection Limit for a Lung Count 

at the Y-12 Facility from 1992 -1995

Lara Hughes 

Document Owner, DCAS

James W. Neton

Subject Matter Expert, NIOSH Contractor

April 2020

Approval: Signature on file

T. Taulbee, Associate Director for Science

Date: 04/27/2020

National Institute for Occupational Safety and Health (NIOSH) 

Division of Compensation Analysis and Support (DCAS) 

1090 Tusculum Avenue 

Cincinnati, Ohio 45226
Document No. DCAS-RPT-008 Revision No. 00 Effective Date: 04/27/2020 Page 2 of 26

PUBLICATION RECORD

EFFECTIVE 

DATE

REVISION 

NUMBER DESCRIPTION

04/27/2020 00 New document initiated to evaluate the detection limit for Pb-212 for 

a lung measurement taken at the Y-12 whole body counting facility 

from 1992 to 1995. Incorporates internal review comments.
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TABLE OF CONTENTS

SECTION TITLE PAGE

1.0 Introduction .............................................................................................................................. 4

2.0 Discussion of the Y-12 In vivo counting methodology 1992-1995............................................. 5

3.0 Evaluation of Detection Efficiency for Lung Counts 1992-1995 ................................................ 6

3.1 Efficiency for U-238 using the Th-234 63.29 and 92.38 keV emissions......................... 6

3.2 Efficiency for U-235 using the 185.71 keV emission ..................................................... 9

3.4 Evaluation of Detection Efficiency for Pb-212 at 238.65 keV ...................................... 12

4.0 EVALUATION OF BACKGROUND IN THE 238 keV REGION............................................... 14

5.0 EVALUATION OF MINIMUM DETECTABLE ACTIVITY......................................................... 16

6.0 Summary and conclusions ..................................................................................................... 18

References ............................................................................................................................ 18
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1.0 INTRODUCTION

When evaluating lung count results for thorium at the Y-12 facility, the records were found to fit into 

three different categories. The first category (years available: 1958-1982) consists of thorium lung 

counts that list the thorium lung burden in form of milligram units. Those results cannot be used for 

dose reconstruction, because the associated calibration information is not available. The second 

category of lung counts (years available: 1979-1991) consists of records that report the activity data 

for gamma emitting thorium chain radionuclides Ac-228 and Pb-212. These records can be 

interpreted using available NIOSH methods. The third category (years available: 1992-1994) consists 

of lung count results that only contain the Ac-228 activity value, but not the Pb-212 value. These 

records are addressed in this report. It is expected that any of the two latter approaches can be used 

to evaluate thorium lung burden data past the 1994 cut-off for this analysis, which was precipitated by 

the Y-12 Special Exposure Cohort evaluation (SEC-00250). 

The purpose of this report is to establish a Minimum Detectable Activity (MDA) for Pb-212 for an in 

vivo examination of Y-12 workers who were potentially exposed to Th-232. Th-232 does not emit 

photons that can be used for in vivo detection. Using the methodology prescribed in ORAUT-OTIB0076 [ORAUT 2014], however, the MDA for Pb-212 can be used to evaluate a missed dose for Th232 based on an in vivo examination. To establish the MDA for Pb-212, this evaluation uses a 

compilation of lung counting reports collected from the Y-12 site as the basis for this analysis [Y-12 

1989-1995]. Other documents relevant to this subject were reviewed and incorporated into the 

analysis as appropriate.

Y-12 [1989-1995] is a large PDF file that contains 23,970 pages of in vivo examination reports 

associated with lung counts taken at the Y-12 facility between 1989 and 19951. A review of this PDF 

file indicated that there were several types of report formats, each one associated with the specific 

type of lung count that was performed during the reported period. The earliest format covered the time 

period through 1991 (see example provided in Attachment 1). As can be seen, eight individual values 

were reported on the hand-written cover sheet, including values for uranium, potassium and lead. In 

later years, the hand-written report shown in Attachment 1 also included a computer printout of the 

results that included detector configuration and name (see Attachment 2). From January through June 

of 1992, lung counts for Y-12 personnel were made at the K-25 in vivo exam center using low-energy 

germanium detectors. An example report is provided in Attachment 3. In addition, the second page of 

the report provided a spectral plot of the count data (Attachment 4). As can be seen in Attachment 3, 

an MDA value is reported for Th-232, but it is not known which Th-232 progeny was used to estimate 

this value. 

1 As discussed later, from January to June of 1992, in vivo exams of Y-12 workers were made at the K-25 in vivo exam center. In vivo 

exams made at this facility were not included in this analysis.

Starting around June of 1992, the Y-12 in vivo exam center began counting workers using an array of 

low-energy germanium detectors. An example of the report format for this type of count is provided in 

Attachments 5 and 6. For all the counts during this time period, measured activity values (or MDA

values) were reported for U-235, U-238, and Ac-228. Additionally, a spectral plot was included.

Occasionally, values were also reported for Np-237, but the number of reported Np-237 values was 

judged to be insufficient for use in this evaluation. None of the reports provided an activity value or 

MDA for Pb-212. 

Because uncensored Pb-212 values were reported for any Y-12 counts performed before 1992, these 

values can be used directly in the estimate of an intake or the calculation of a missed dose for Th-232 
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using the methodology described in ORAU-OTIB-0076 [ORAUT 2014]. For counts performed on Y-12 

workers in the 1992 time-frame at the K-25 in vivo counter (Attachments 3 and 4), there are 

insufficient data available at this time to estimate Pb-212 lung burdens. While spectral plots are 

provided on the K-25 in vivo exam reports, they did not provide any net or background count 

information for positively identified peaks. In theory, the background in the Pb-212 region could be 

approximated by examining the counts displayed in these plots. The plots, however, have many 

channels compressed into a small space, making it difficult to evaluate the background with any 

degree of accuracy. ORAU is currently working on obtaining additional information on the counts 

made at K-25 during this time period. The information required to estimate MDAs for Pb-212 for the 

K-25 counter includes raw spectral data (i.e., channel by channel printouts) and the efficiency 

calibration curves that were used to estimate Th-232 values. Interviews with subject matter experts 

and additional data captures might more clearly define the parameters associated with these 

measurements. 

For counts made at the Y-12 facility between 1992 and 1995, peak search reports provide MDAs, 

measured activity results (including net counts in the peak), and background count data. If sufficient 

positive peak information is available, this should allow for a reasonable estimate to be made of the 

MDA for Pb-212. The remainder of this report provides the approach used to develop the parameters 

needed to estimate the MDA for counts made at Y-12 between 1992 and 1995. These parameters are 

the detection efficiency for the Pb-212 photopeak at 238 keV and the subject background counts in 

that region.

2.0 DISCUSSION OF THE Y-12 IN VIVO COUNTING METHODOLOGY 1992-1995

Starting in June of 1992, the in vivo measurement methodology at Y-12 most likely used an array of 

four low-energy germanium detectors positioned over the lungs. While no specific document that 

characterized the system could be found, the use of germanium detectors is supported by the spectral 

plots that are included with each lung count. As can be seen in Attachment 6, the plotted spectrum 

clearly exhibits the characteristics of a high-resolution germanium system. In addition, a writeup that 

was attached to some of the early lung counts during this period stated that the algorithm used to 

identify photopeaks was the “peak/singlet” routine and that two of the detectors were new (see 

Attachment 7)2

. 

2 A copy that can be expanded and read more clearly can be viewed at SRDB #176611, pdf page 165.

The peak/singlet routine was specifically developed for the evaluation of peaks in low-energy lung 

counting applications. The identification of photopeaks was accomplished using a methodology that 

was described by Spitz et al. [1985]. It consists of applying a variance stabilizing transformation to the 

raw spectral data. A background spectrum is then synthesized using a median smoothing technique. 

Peaks are identified whenever the gross spectral data exceeds the calculated background by a 

predetermined statistical confidence level. Thus, whenever a photopeak is positively identified, the 

number of background counts under the net photopeak area is also reported.

In addition to providing the net and background peak area, the in vivo exam report also included a 

value for the subject’s chest wall thickness. This indicates that the efficiency of detection (as well as 

the reported activity or MDA) was not only dependent on energy but on the subject’s chest wall 

thickness. Because of this, the detection efficiency associated with an individual subject count is 

represented by a three-dimensional function. An example that depicts this relationship for the Y-12 

HPGe detection system in 2008 is provided in Figure 1 below [Gose and Veinot 2006, pdf page 16] 
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Figure 1. Detection Efficiency for a Lung Count at Y-12 in 2008 for a 4 detector Array

3.0 EVALUATION OF DETECTION EFFICIENCY FOR LUNG COUNTS 1992-1995

If the detection efficiencies for the nuclides that were reported (i.e., U-235, U-238, and Ac-228) can be 

determined, it would be possible to interpolate a detection efficiency for Pb-212. Table 1 provides the 

energies and abundances of the photon emissions that were used by the in vivo software to quantify 

activity. All values are from Kocher [1981]. 

Table 1. Energies and Abundances of Photons Evaluated

Nuclide Energy (keV) Abundance (%)3

3 Abundance in this context refers to the fractional photon yield that is emitted during the radioactive decay process.

U-238 (Th-234) 63.29 3.8

U-238 (Th-234) 92.38 2.72

U-238 (Th-234) 92.80 2.69

U-235 185.71 54.0

Pb-212 238.65 44.6

Ac-228 338.32 11.4

3.1 EFFICIENCY FOR U-238 USING THE TH-234 63.29 AND 92.38 KEV EMISSIONS

As shown in Table 1, there are three useful energies associated with the decay of Th-234, the 

daughter of U-238. The first at 63.3 keV has an abundance of 3.8%, while there are two higher energy 

emissions at 92.38 and 92.80 keV, each with an abundance of 2.72% and 2.69%, respectively. The 

attachment that was placed with the initial lung counts in 1992, stated that the 92.38 keV photopeak 
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was used for the quantification of U-238 (see Attachment 7). Thus, the 2.72% abundance will be used 

in this analysis.

It should be noted that the discussion provided in Attachment 7 indicates that the positive peaks in the 

63 and 92 keV regions were most likely due to contamination in two newly installed detectors. The 

positive peak search results and reported activities, however, can still be used to evaluate the 

detection efficiency at these energies. This is because the software assumes that any positive Th-234 

peaks identified in the spectrum are due to internal contamination.

To identify reports that had positive Th-234 peaks, Y-12 [1989-1995] was searched for all reports that 

contained the character strings “63.” And “92.”4. To facilitate this search, a version of Y-12 [1989-

1995] was created that enabled Optical Character Recognition (OCR). This new version resulted in 

the original file being split into 24 files, with files 1-23 containing 1,000 pages and the last file 

containing 970 pages. After reviewing the results of this search, there were six lung count reports in 

the 1992 – 1995 time period that had a positively identified photopeak at 63 keV and a reported 

activity for U-238. In addition, there were eight in vivo exam reports that had a positively identified 

photopeak at 92 keV and a reported activity for U-238. These reports also provided net counts, 

background counts, and chest wall thickness (CWT). Using these data, it was possible to calculate the 

detection efficiency for each reported count using the following equation:

Where: Y = the photon yield in gammas per disintegration. 

4 While this search included in vivo exam reports made at K-25, none of these counts were included in this analysis. This is because, no

quantitative peak information (i.e., net and background counts) was provided in these reports. This is true for all subsequent 

character searches described in this report.

Tables 2 and 3 provide a summary of the values extracted from the reports, as well as the calculated 

efficiency, using these reported values.

Table 2. Summary of Data Extracted from Y-12 [1989-1995] for 63 keV photopeak

Report 

page

Net

Counts

CWT

(cm)

Activity

(nCi)

Efficiency5

(counts/gamma)

5 The values reported in this column were calculated using the net counts and activity in this table and the equation provided in this 

section.

691 24 5.83 5.1 0.00188

1255 12 4.37 1.4 0.00342

1475 30 3.13 2.2 0.00544

1988 15 5.44 2.7 0.00222

2899 43 2.64 2.6 0.00659

3450 17 2.53 0.95 0.00714
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Table 3. Summary of Data Extracted from Y-12 [1989-1995] for 92.8 keV photopeak

Report 

page

Net

Counts

CWT

(cm)

Activity

(nCi)

Efficiency5

(counts/gamma)

160 23 2.03 0.8 0.01601

11767 20 1.97 0.67 0.01663

12240 30 1.38 0.81 0.02063

13286 15 4.00 1.1 0.00760

14260 7 2.58 0.29 0.01345

15104 60 4.19 4.6 0.00727

15901 28 5.92 4.1 0.00380

17573 3 3.61 0.18 0.00928

As previously shown in Figure 1, the detection efficiency for a low-energy lung count is a function of 

both energy and chest wall thickness (CWT). Thus, the detection efficiencies at the 63 and 92 keV

regions of interest were evaluated as a function of CWT using the CWT values contained in Tables 2 

and 3. As shown in Figures 2 and 3, the data fit an exponential function with R2 values of 0.9981 for 

the 62.38 keV peak and 0.9991 for the 92.8 keV peak.

Figure 2. Detection Efficiency at 63.29 keV
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Figure 3. Detection Efficiency at 92.8 keV

3.2 EFFICIENCY FOR U-235 USING THE 185.71 KEV EMISSION

For a Y-12 lung count, U-235 was quantified using the 185.71 keV emission which has an abundance 

of 54%. In a similar manner to that used for the U-238 analysis, the 24 OCR files that were created 

from Y-12 [1989-1995] were searched for the character string “185.” and “186.”. This search resulted 

in the identification of 8 in vivo exam reports that had a positive 185 or 186 keV photopeak and also a 

reported U-235 activity. Table 4 provides the data that were extracted. As with the U-238 analysis, the 

efficiency value in the table was calculated using equation provided in Section 3.1. 
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Table 4. Summary of Data Extracted from Y-12 [1989-1995] for 185.7 keV photopeak

Report 

page

Net

Counts

CWT

(cm)

Activity

(nCi)

Efficiency6

(counts/gamma)

6 The values reported in this column were calculated using the net counts and activity in this table and the equation provided in this 

section.

2511 29 3.75 0.21 0.003875

5078 17 4.43 0.14 0.003407

5280 40 2.64 0.19 0.005907

5851 40 2.70 0.20 0.005612

5854 33 2.70 0.16 0.005787

10020 335 1.48 1.1 0.008545

11331 31 4.51 0.27 0.003222

18059 3 3.30 0.0193 0.004361

As with the U-238 analysis, the CWT and efficiency data in Table 4 were fit to an exponential function

with an R2 of 0.9863. The results of this fit are provided in Figure 4. 

Figure 4. Detection Efficiency at 185.7 keV

3.3 EFFICIENCY FOR AC-228 USING THE 338.32 KEV EMISSION

For a Y-12 lung count, Ac-228 was quantified using the 338.32 keV emission which has an 

abundance of 11.4%. In a similar manner to that used for the U-238 and U-235 analysis, the 24 OCR 

files that were created from Y-12 [1989-1995] were searched for the character string “338.”. This

search resulted in the identification of 5 in vivo exam reports that had a positive 338 keV photopeak
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and also reported an Ac-228 activity. Table 5 provides the data that were extracted. As with the U-238 

and U-235 analyses, the efficiency value in the table was calculated using the equation provided in 

Section 3.1. 

Table 5. Summary of Data Extracted from Y-12 [1989-1995] for the 338.32 keV photopeak

Report 

page

Net

Counts

CWT

(cm)

Activity

(nCi)

Efficiency7

(counts/gamma)

7 The values reported in this column were calculated using the net counts and activity in this table and the equation provided in this 

section.

1531 9 3.37 0.42 0.002848

11503 6 4.54 0.44 0.0018124

13618 16 3.16 0.79 0.0026918

17715 16 3.72 0.93 0.0022866

18678 11 3.67 0.63 0.0023206

As with the U-238 and U-235 analyses, the CWT and efficiency data in Table 5 were fit to an 

exponential function with an R2 value of 0.9204. The results of this fit are provided in Figure 5. 

Figure 5. Detection Efficiency at 338.32 keV
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3.4 EVALUATION OF DETECTION EFFICIENCY FOR PB-212 AT 238.65 KEV

Using the exponential functions generated for the 63.29, 92.80, 185.7, and 338.32 keV peaks, the 

detection efficiency at a CWT of 1.5 cm was calculated8. The results of these calculations are 

provided in Figure 6.

8 It was decided to calculate the efficiency at 1.5 cm, because that was the thickness (CWT) of the thinnest chest overlay provided by the 

Y-12 calibration phantom.

Figure 6. Detection Efficiency at a CWT of 1.5 cm at the Y-12 Lung Counter 1992-1995

Although the shape of the actual efficiency curve is represented by a smooth function, there are 

insufficient data points in this example to flesh out the details of the actual shape. It can be seen, 

however, that the efficiency increases from 63 keV up to a peak at 93 keV9 and then declines in a 

roughly exponential fashion down to 338 keV. This is characteristic of low-energy photon 

measurements, including those made with intrinsic germanium detectors [ICRU 2003]. To interpolate 

the detection efficiency at 238 keV, an exponential function was fit between the efficiencies at 185 and 

338 keV. The results of this fit are provided in Figure 7.

9 The peak (or knee) of this efficiency curve is probably somewhat higher than 93 keV.
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Figure 7. Exponential fit between 185 and 338 keV, 1.5 cm CWT

Using the relationship shown in Figure 7, the detection efficiency at the Pb-212 photopeak energy of 

238 keV at a CWT of 1.5 cm was calculated to be 0.006368 counts/gamma. The detection efficiency 

at 238 keV is a function of CWT. The reduction in detection efficiency at higher energies, however, is

less dependent on photon scattering than it is at lower energies like 63 and 93 keV. This is because 

the attenuation coefficient for photon scattering at higher energies is reduced. Thus, at higher

energies, the reduction in efficiency due to the change in geometry (i.e., the increased distance from 

the deposited activity) contributes more to the overall reduction than it does at lower energies. In fact, 

examination of the attenuation coefficient for the efficiency functions reveals that the reduction in 

efficiency due to chest wall thickness is the same at both 185 and 338 keV. That is, the exponential 

reduction coefficients are both 0.321 per cm, whereas the reduction coefficients at 63 and 93 are 

0.398 and 0.373 per cm, respectively. Thus, it is reasonable to infer that the reduction in efficiency as 

a function of chest wall thickness at 238 keV can be assumed to be 0.321 per cm as well. Using this 

reduction coefficient and the calculated efficiency of 0.006368 counts / gamma at 1.5 cm CWT10, the 

relationship for efficiency as a function of chest wall thickness can be calculated as follows:

10 Although a 1.5 cm chest wall thickness (CWT) was used for this calculation, any thickness could be used to derive this relationship. 

For example, using a 3.5 cm CWT in the calculation (which is the average CWT for all the positive lung counts) produces the same 

MDA function that appears in Figure 10. 

EEEEEE238 = 0.010307 ∗ exp(−0.321 ∗ CCCCCC)
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As can be calculated, the efficiency at 238 keV can vary by a factor of 2.6, going from 0.006368 at 1.5 

cm CWT to 0.002431 at 4.5 cm. As will be discussed in Section 5, this change in efficiency will directly 

affect the MDA calculation for a lung count.

4.0 EVALUATION OF BACKGROUND IN THE 238 keV REGION

The other value needed to estimate an MDA for Pb-212 is the number of background counts in the 

238 keV region of interest. Originally, it was thought that this could be estimated by examining the 

spectral printout that was included with each in vivo exam report. An example of the spectral printout 

is provided in Figure 8 below. 

Figure 8. Example Spectral Printout

If the full-width-at- half-maximum (FWHM) were known, one could estimate the number of counts in 

the 238 keV region by adding up the counts displayed in the printout. As can be seen in Figure 8, the 

printout covers the region of 0-400 keV with an energy calibration of slightly less than 0.1 keV per 

channel. Thus, the spectrum constitutes a printout of approximately 4,000 (actually 4096) data 

channels. Given that the compressed nature of the spectrum made it difficult to estimate background 

accurately, it was decided to investigate other methods of establishing background in the 238 keV 

region. As previously discussed, the peak search algorithm provides the calculated background 

counts under each peak that is positively identified. An examination of the in vivo reports contained in 

Y-12 [1989-1995] for a character string of “238.” indicated that were at least 24 reports that contained 

a positive peak for the 238 keV emission of Pb-212. A summary of the peak search results extracted 

from these reports is provided in Table 6. The average background in the Pb-212 region was 

calculated to be 34.4 counts with a standard deviation of 9.7 counts.
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Table 6. Summary of Peak Search Results for Peaks Identified at 238 keV

Pdf Page11

[ORAUT 2014]

11 These were independent measurements of 24 different workers.

Net 

Counts

Background

Counts

571 15 38

617 55 46

1034 21 33

1122 21 39

1536 23 30

2777 13 45

3758 13 36

6071 2 34

6692 26 27

6871 15 21

7404 13 38

7549 14 30

8629 18 22

10477 27 23

10822 13 29

10874 12 49

12191 29 20

12206 10 29

14682 22 37

15922 13 48

17605 30 25

17955 19 52

18809 20 50

20636 19 25

Mean NA 34.42

Std. Deviation NA 9.70

For comparison purposes, the background counting statistics at other energies associated with radon 

daughter activity was also evaluated. For this comparison, Y-12 [1989-1995] was searched for the 

character strings “295.” and “352.”, which are peak energies associated with the decay of Pb-214.

Twenty-nine peaks were identified at 295 keV and twenty-four peaks were identified at 352 keV. The 

average background areas under these peaks were 32.7 and 29.7 for the 295 and 353 keV peaks, 

respectively. Figure 9 provides a chart of the background areas in each region that was evaluated. 
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Figure 9. Background Counts Collected During a 30 Minute Measurement

As can be seen, the decrease in the average background counts as the energy increases is 

consistent with the trend that can be seen in the example spectrum provided in Figure 8. 

5.0 EVALUATION OF MINIMUM DETECTABLE ACTIVITY

The formula for calculation of an MDA is based on the statistical concept derived by Currie [1968]. For 

an in vivo exam this is expressed as: 

MMMMMM (nn nn) = 4.66 ∗ σσbb

ee ∗ yy ∗ tt ∗ 2200

Where:

σb = the standard deviation of a background count;

e = the detection efficiency in counts/gamma;

y = the photon yield in gamma/disintegration = 0.446 for Pb-212; 

t = the count-time in minutes = 30; and,

2200 = the number of disintegrations per minute per nCi of activity.

Normally, the standard deviation of the background count is approximated by taking the square root of 

the number of counts in the background region. In this case the square root of the mean background 

count of 34.42 is 5.87. The background for a lung count, however, is subject to more variability than 

what would be expected by the Poisson distribution. This is because other parameters, such as 

subject size and K-40 content, can have significant effects on the background. Because of this, the 

standard deviation of the background calculated from the subject population of 9.70 will be used in the 

MDA calculation. 

As discussed in Section 3.4, the detection efficiency is a function of CWT. Thus, the complete MDA 

formula for a 30-minute lung count at 238 keV is:
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MMMMMM (nn nn) = 4.66 ∗ σσbb

(0.010307 ∗ exp(−0.321 ∗ CCCCCC)) ∗ 0.446 ∗ 30 ∗ 2200

Using the above formula, the MDA was calculated for CTWs of 1.5., 2.5, 3.5, 4.5, and 5.5 cm. The 

results of these calculations are provided in Figure 10. 

Figure 10. MDA at various CWTs for a 30 minute count.

As can be seen in Figure 10, the MDA ranges from 0.24 nCi at 1.5 cm to 0.86 nCi at 5.5 cm CWT.

This is the a priori minimum detectable activity. That is, this is the a priori amount of activity that needs 

to be present for the lung count to positively detect a critical level of activity with 95% certainty. Once 

a lung count is performed, however, one can use the critical level to determine if a lung count has 

detected positive activity after the count has been made (i.e., a posteriori). In this case, the critical 

level, as discussed by Currie [1968] is expressed as:

LLcc (nn nn) = 2.33 ∗ √BB

ee ∗ yy ∗ tt ∗ 2200

Where:

B = the number of background counts in the 238 keV region.

To evaluate the critical level on a count-by-count basis would require the exact background counts in 

the 238 keV region of interest. As discussed above, the resolution of the spectral plots was judged to 

be insufficient to develop this estimate on a case-by-case basis. In lieu of calculating the critical level 

for each count, it is recommended that the MDA be used as the upper end of a triangular distribution
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with a lower bound of zero and a central estimate of one-half the MDA. This is consistent with the 

current NIOSH practice for estimating missed dose from bioassay data with non-detectable results.

6.0 SUMMARY AND CONCLUSIONS

This report provides an evaluation of the MDA for a lung count at the Y-12 facility between 1989 and 

1995. Based on a review of data contained in Y-12 [1985-1995], it was decided to limit the evaluation 

to the time period 1992 – 1995 when germanium detectors were used to perform lung counts at Y-12.

The time period prior to 1992 provided values and MDA’s for Pb-212, so no evaluation was 

necessary. There is a period of time from January to June of 1992, when the K-25 facility was used to 

perform lung counts on Y-12 personnel. Additional information is needed to evaluate the MDA for Pb212 for these counts. 

Examination of the Y-12 in vivo exam reports for the 1992-1995 time period revealed that they 

contained sufficient data to estimate the MDA for Pb-212. Using the positive peak search results in Y12 [1989-1995] (which included background counts) and the reported activities for U-238, U-235, and 

Ac-228, efficiency curves were generated as a function of chest wall thickness at 62.39, 92.38, 

185.71, and 338.32 keV. An interpolation was then done to estimate the detection efficiency for Pb212 at 238.65 keV.

To establish background for Pb-212, 24 positive peak search results for 238 keV were extracted from 

Y-12 [1989-1995]. These results had a mean count of 34.4 with a standard deviation of 9.7.

Comparison of these counts with those in other energy regions were consistent with the pattern 

observed in the spectral plots. That is, there were a diminishing number of counts in the spectrum as 

energy increased.

Using the efficiency and background data developed above, an equation for the Pb-212 MDA as a 

function of chest wall thickness was established. Given that it was not possible to estimate the 

background levels on a count per count basis, the critical level for an individual count was not made.

In light of this, it is recommended that the MDA be used in the calculation of missed dose for a Th-232 

intake. This calculation should use the MDA as the upper bound of a triangular distribution, with a 

lower bound of zero and a central estimate of one-half the MDA. This is consistent with the current 

NIOSH practice for estimating missed dose from bioassay data with non-detectable results.

REFERENCES

Currie, L.A., 1968, Limits for Quantitative Detection Quantitative Determination Application to 

Radiochemistry, 

Gose, B. T., Veinot, K. G., 2006, Verification and Validation of the Y-12 Lung Counting System, Y-12 

National Security Complex, Oak Ridge, TN. [SRDB Ref. ID: 048609] 

ICRU (International Commission on Radiation Units and Measurements), 2003, Direct Determination 

of Body Contents of Radionuclides, Report 69, Volume 3, No. 1, Bethesda, Ashford England, pp. 76-

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Attachment 1 
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Attachment 2
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Attachment 3
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Attachment 4
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Attachment 5
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Attachment 6
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Attachment 7