Investigating below the MDA

INTRODUCTION

The Atomic Energy Control Board is in the process of promulgating new Canadian regulations for workers that are exposed to radioactive materials. This change will result in a dose limit of 100 mSv in a five-year period with a maximum of 50 mSv in any one year. For planning purposes an average of 20 mSv per year is often used. These new, and lower, limits will mean that lung counting for Canadian workers exposed to separated insoluble (type M or S) natural uranium will no longer detect an intake until after the individual has exceeded the dose limit.

As previously shown (Kramer et al. 1999), the counting efficiency of a lung counter is dependent on the chest wall thickness of the subject. The average chest wall thickness of the Cameco workforce was found to be approximately 3.8 cm. The minimum detectable activity expected for a subject with this chest wall thickness was shown to be about 8 mg for a lung counter consisting of either a four phoswich detector array or a four germanium detector array. Depending on the intake time, this lung burden corresponds to an exposure that will result in a dose equal to or twice the impending dose limit if the average of 20 mSv is assumed.

The role of the lung counter for monitoring workers exposed to separated natural uranium is now in question and its value in routine monitoring appears to have been degraded. To assess how lung counting may fit into Cameco’s radiation protection program a joint measurement program between Health Canada’s Human Monitoring Laboratory (HML), which operates the National Calibration Reference Centre for In Vivo Monitoring (Kramer and Limson-Zamora 1994), and Cameco was initiated. The two facilities collaborated to make lung counting measurements on seven employees as described below. It was expected that five of the employees may have had small measurable lung burdens as they worked with insoluble UO_2.

Although the investigation described in this paper uses both the 63 keV and 185 keV photons from natural uranium, Cameco (and similar facilities) can only use the 185 keV photons for lung counting as the company manufactures separated natural uranium. The daughter products are removed during the process and cannot be assumed to be in equilibrium.

METHODS AND MATERIALS

The seven subjects were lung counted first at the HML using a germanium detector lung counter and then again the following day at Cameco using a phoswich detector lung counter. Four more subjects were counted at the HML at a subsequent time. The counting time was lengthened from the usual 1,800 seconds to 3,600 seconds to improve sensitivity at both sites. Any observable activity was assumed to be due to Type S UO₂ as the Cameco plant had been shut down for the month immediately prior to the lung count being performed. The lung counting systems at the HML and Cameco have been described in detail elsewhere (Kramer et al. 1998)

Analysis of the results using the HML’s software did not detect any significant levels of activity in any of the seven subjects so a more in-depth analysis was performed manually. The spectra were exported to text files and imported into Microsoft Excel. The spectra for each individual were summed and plotted. Two regions were selected: 62 - 65 keV and 184.5 - 187 keV. The counting data was copied to Jandel’s TableCurve and a Gaussian curve imposed onto the data.

The analysis of the counting data at the Cameco site was completely different. The analysis software, WBC-ELD version 1.0-512 (whole body count - Eldorado), supplied by Radiation Management Corporation. RMC was absorbed by Canberra Industries in 1995. The software uses the spectral shapes of phoswich spectra obtained from ²³⁴ Th, ²³⁵U, ¹³⁷Cs, ⁴⁰K, and ²²⁶Ra standards measured in the LLNL phantom (with and without overlay plates) to obtain a theoretical fit to the observed spectrum using a non-linear least squares fit (Householder’s technique).

A different approach to analysing this data was taken by collecting a standard spectrum using the LLNL torso phantom with the B4 overlay plate and a lung set containing 701 mg of natural uranium. This spectrum was also exported to Microsoft Excel. The counts were normalised to 1 mg. Multiples of this spectrum were then added to each of the seven subjects’ spectra to simulate an extra 5, 10, 15, 20, 25, 30, 35, and 40 mg. These spectra were analysed as above. Linear regression was performed on the results with the intercept (at added amount equal to zero) was taken as the lung burden of the subject.

To confirm the validity of this approach two other experiments were performed. The first was using the LLNL torso phantom containing small amount of natural uranium in the form of pellets. The pellets were placed in a blank lung set that contained holes to attempt to simulate a homogeneous distribution. The activities used were in the range of 0.96 to 4.11 mg natural uranium. The B4 overlay was used to give an average chest wall thickness of 3.83 cm to better match the chest wall thickness of the Cameco workforce. The same analysis procedure as above was applied to the counting data.

Unfortunately, the results are confounded by the small, but significant, contamination of the HML’s LLNL phantom with low enriched uranium. This limitation was mitigated by counting the phantom for a long period and estimating the equivalent amount of U-nat present. This was then applied as a correction to the results obtained from the standard addition technique.

As the addition of multiples of the normalised standard spectrum to either a subject’s spectrum or the pellet spectrum resulted in an excellent straight line so it was not possible to get an estimate of the uncertainty of the intercept that expressed a meaningful uncertainty on the lung burden. This issue was solved by using a Monte Carlo simulation contained in Decisioneering’s Crystal Ball version 4.0c. A spreadsheet was created that simulated the photopeak count rate corresponding to each of the spectra that had multiples of the normalised standard added. The count rate corresponding to zero mg added was assigned to a Poisson distribution and the forecast was assigned to the intercept of the linear regression. The range of intercept values that fell in the 95% confidence interval were taken to represent the mean " 2F.

Another technique of spectral addition was applied to the eleven spectra obtained from the Cameco employees. This addition of the spectra was possible as the HML’s Ge detectors are very stable. The energy calibration remains constant over long periods of time.

RESULTS AND DISCUSSION

Normal Analysis: The results of counting the seven subjects at the HML and Cameco sites is shown in Table 1. A typical spectrum is shown in Figs. 1 - 2. Fig. 2 has markers that indicate the expected position of photopeaks from ²³⁴ Th and ²³⁵U. It also shows two prominent peaks at around 75 keV. These are Pb fluorescence x rays emitted from the walls of the HML’s low background counting chamber and not from the subject.

The spectra collected at the HML were analysed for both the 63 keV and the 185 keV photons emitted from natural uranium. In most cases the subjects were below both the decision level and the minimum detectable activity. Only GJ and PC exceed the decision level for natural uranium measured from the 63 keV and 185 keV photons, respectively. All the other individuals would be declared “clean”. The results from the HML and Cameco using this technique are in general agreement with those obtained using the normal method of analysis.

At the Cameco site, GJ, PC, RR, and LN exceed the minimum detectable activity for natural uranium measured from the 63 keV photons, but none of the subjects exceed the minimum detectable activity for natural uranium measured from the 185 keV photons. However, GJ, RR, and LN do exceed the decision level. The measurements made using the 185 keV photons are the ones that must be used by Cameco as the ²³⁴ Th daughters cannot be assumed to be in equilibrium with the parent nuclide.

Interestingly, the results at the Cameco site are all higher than those measured at the HML. This may be due to the very different nature of the analysis methods between the two facilities. The HML normally analyses the spectrum assuming that nothing is present unless clearly seen i.e., a peak search technique. In this case however, analysis was forced assuming that the counts in the 63 keV and 185 keV regions of interest were elevated and contained a contribution above a flat background from a small amount of natural uranium in the lungs. Cameco analyses a spectrum assuming that natural uranium, ⁴⁰K, ¹³⁷Cs, and background are responsible for the spectrum’s shape and then fits the shape of the subject’s spectrum to a combination of stored standard spectra. The agreement between the two sites, where data exist, is between a factor of 1.02 and 2.45.

Standard Addition: In an effort to better determine the amount of natural uranium that might be in the lung a standard addition technique was developed. Portions of a natural uranium spectrum were added to each subject’s spectrum. Analysis of these simulated spectra gave simulated lung burdens. The measured lung burden was plotted against the amount added and the plot extrapolated back to zero added. This was the value taken to be the lung burden present in the subject. All of these plots yield very straight lines that have a correlation coefficient greater than 0.99. This is simply due to a methodology that adds multiples of a constant amount of signal to the subject’s spectrum. It means however, that the errors on the intercept obtained from the linear regression of this data are not meaningful and do not represent the uncertainty on this value.

The results from the addition of multiples of the normalised standard spectrum indicated that this was not a useful technique and further work was halted.

Spectral Addition: The composite spectrum of a Cameco employee who underwent 10 sequential lung counts for a total of 10 hours is shown in Fig. 3. Comparing Figs. 2 and 3 one can see that the ²³⁵U photopeak (186 keV) has become clearly visible above background. Analysing Fig.3 using a chest wall thickness representative of the group of employees one obtains a lung burden of 4.00 mg. This is well above the MDA of 2.5 mg for this count.

Increasing the counting time from 30 minutes will give increased sensitivity. For example, the expected decreases in MDA as the counting time is increased from 30 minutes by factors of 2, 4, 8, 16, and 20 are factors of 1.4, 2, 2.8, 4, and 4.5 respectively.

This finding opens up two possibilities: Group monitoring and summed individual monitoring.

In the former case, a group of employees can be counted for one hour each. Their spectra can be summed and analysed for ²³⁵U. Failure to detect an²³⁵U lung burden in the composite spectrum will mean that none of the employees has a significant lung burden above their individual decision levels. This method suffers from the fact that one or two persons may have a small burden that could be diluted by the other components in the spectrum. This method will work well if all the workers are assumed to be equally exposed and all are expected to have similar lung burdens. If a measurable amount of ²³⁵U is found in the composite spectrum it can be analysed using an chest wall thickness that is the average of the group and an equal amount assigned to each person.

A more powerful technique would be to measure the same individual sequentially over a period of days and sum those spectra. Naturally, it is assumed that during this monitoring period the employee is not potentially exposed. This technique has the advantages of not averaging activity over a group of workers, using the person specific calibration data to assess lung burden, and dramatically reducing the MDA for that person

CONCLUSIONS

Investigating below the MDA is imprecise as the statistics of a random process cannot be avoided. However, the use of the standard addition seems to have merit in distinguishing between uncontaminated and slightly contaminated individuals. Unfortunately, the technique does not provide uncertainty values of the activity estimate. Further work was halted as results suggested that the technique was unreliable.

The spectral addition technique offers more advantages. It can be applied to a group of workers or to an individual who is repeatedly monitored over a period of a few days. In the former case it is most useful if no ²³⁵U is detected and in the latter case if greatly increases measurement sensitivity. The spectral addition techniques can reduce the MDA by about a factor of about 4.5 from the normal 30 minute counting time currently used at Cameco

REFERENCES

Brodsky, A. Accuracy and detection limits for bioassay measurements in radiation protection. Washington D.C.: US Nuclear Regulatory Commission. Report NUREG-1156; 1986.

Kramer, G.H.; Limson Zamora, M. The Canadian National Calibration Reference Centre for Bioassay and In-Vivo Monitoring: A Program Summary. Health Phys. 67(2): 192-196; 1994.

Kramer G.H.; Hauck, B.M.; Allen, S.A. Comparison of the LLNL and JAERI torso phantoms using Ge detectors and phoswich detectors. Health Phys. 74(5):594-601; 1998.

Kramer, G.H.; Hauck, B.M; Allen, S.A. Chest Wall Thickness measurements and the dosimetric implications for workers in the Uranium Industry. Health Phys. (Submitted 1999).

Palmer, H.E.; Rieksts, G.A; Jefferies, S.J.; Gunston, K.J. Improved counting efficiencies for measuring ²³⁹Pu in the lung in the sitting position. Health Phys. 57(5): 747-752; 1989.

Summerling, T.J.; Quant, S.P. Measurements of the human anterior chest wall by ultrasound and estimates of chest wall thickness for use in determination of transuranic nuclides in the lung. Rad. Prot. Dosim. 3(4): 203-210; 1982.

Vickers, L.R. The gender specific chest wall thickness prediction equations for routine measurements of ²³⁹Pu and ²⁴¹Am within the lungs using HPGe detectors. Health Phys. 70(3): 346-357; 1996.

Table 1: Measured lung burdens of the subjects and the decision level, L_C , and minimum detectable activities, MDA, for those subjects using the 63 and 185 keV photons at the HML and Cameco sites. N/A denotes no analysis possible.

	HML			Cameco
Subject	L_C (mg)	MDA (mg)	Found (mg)	L_C (mg)	MDA (mg)	Found (mg)

63 keV
SA	4.9	9.9	N/A	1.9	5.3	N/A
GJ	3.7	7.3	3.9	1.9	3.7	4.0
RF	5.1	10.3	2.0	2.8	5.5	4.9
PC	3.8	7.5	3.1	1.9	3.9	6.0
RR	4.0	8.0	2.5	2.1	4.2	6.0
LN	3.9	7.8	3.3	2.0	4.0	6.4
SP	3.1	6.2	N/A	1.5	3.0	1.3
TB	4.5	9.0	4.3	0.8	1.5
LT	3.4	6.8	0.9	0.7	1.4
CW	5.7	11.5	2.3	0.9	1.7
SM	3.5	7.0	1.4	0.7	1.4

185 keV
SA	5.6	11.2	N/A	4.1	8.3	N/A
GJ	4.4	8.7	2.2	2.9	5.9	3.5
RF	5.8	11.6	N/A	4.3	8.6	2.2
PC	4.5	8.9	5.0	3.0	6.0	N/A
RR	4.7	9.4	4.1	3.3	6.6	4.5
LN	4.6	9.2	N/A	3.2	6.3	4.5
SP	3.7	7.4	N/A	2.4	4.7	N/A
TB	5.2	10.3	3.9	2.9	5.7
LT	4.1	8.2	1.7	2.4	4.8
CW	6.3	12.6	3.6	3.7	6.7
SM	4.2	8.4	2.3	2.5	5.0

1 Subject PC measured in Cameco’s Phoswich lung counter

2 Subject PC measured in HML’s Germanium lung counter.

3: Summed spectra of a Cameco employee simulating an 10 hour count.