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MDC = 3 + 4.65 * Square Root of CB * Count Time/Instrument Efficiency * Surface Efficiency Factor * Count Time * Proba Area In cm2/100cm2
CB = Background counts
TB = Background counting time (min)
ei = the instrument efficiency (count per particle)
es = the contaminated surface efficiency (particle per disintegration)
WA = the area of the detector window (cm2)
A: I believe your question refers to "calculating MDC" rather than instrument efficiency. The general equation for minimum detectable concentration (MDC) is presented on p. 3-5 of NUREG-1507 (PDF File). The term C_B refers to the background count in time T. That is, C_B is the total number of counts obtained during the counting interval T. For example, let's assume that the number of background counts obtained using a gas proportional detector for one (1) minute count is 300 counts. Then C_B is 300 counts and T is 1 (one minute). Now let's assume you wanted to reduce the MDC by counting background for two minutes. In this case C_B would be 600 counts (there is 600 counts collected over the two-minute count time) and T is 2 (two minutes).
There appears to be an error in the equation you provide in your question. The square root term in the numerator should either be C_B (as discussed above) or R_B * T, where R_B is the background count rate (in cpm). In either case the square root term must be unitless (just "counts").
Hope this clarifies the issue.
Q: I am designing a VSAP (FSS) for surface soil. Since Pu-238 is the major COC, I'm having difficulty developing an acceptable scanning protocol since our BAT is a FIDLER/count rate meter set up. Appendix H leads one to believe that the sensitivity is in the 10 to 20 pCi/g range. Empirical studies done on site have shown that it is closer to 250pCi/g and scan MDCs are over 400pCi/g. My DCGLemc is 165 pCi/g based on RBGV. Can you provide information or cite the correlation study used for Appendix H or provide some guidance on how to reasonably scan for Pu-238? Using area factors, I'm approaching 100 samples per 2000m2. This seems a bit excessive.
A: Pu-238 emits low energy x-ray radiation (13.6 keV), which makes it very difficult to detect, even with a FIDLER. One thought is to identify a surrogate radiation to measure, such as the Am-241 59 keV. However, this is only an option if you have other radionuclides that can serve as surrogates.
As you indicate in your question, Appendix H in MARSSIM states that the sensitivity of a FIDLER for Pu-238 is about 20 pCi/g. This does seem low, even for the specific conditions stated (i.e., contamination is limited to the top 1 mm of soil). Given that the background for the FIDLER is 400 cpm (as indicated in MARSSIM Appendix H, p. H-32), and assuming a 2-second observation interval and d' of 2.32 (25% false positives and 95% detection); the minimum detectable count rate of the surveyor is 360 cpm (from MARSSIM Ch. 6). This means that an additional 360 cpm (above background) would be needed in order for Pu-238 to be detected with 95% confidence. The more difficult question is how much Pu-238, for a specific geometry, is needed to produce a net count rate of 360 cpm. To answer this question one needs both the detector efficiency of the FIDLER for 13.6 keV x-rays, as well as modeling information (e.g., from MicroShield) that provides the x-ray fluence rate at the detector from a specified geometry (areal extent and depth) of Pu-238 in soil.
To use the data provided in Appendix H, the FIDLER sensitivity is stated to range from 500 to 700 cpm/microCi/m2. Assuming that the FIDLER responds at the high end of this range, the scan MDC can be calculated for a range of soil geometries. For example, if the areal extent of contamination 1 m x 1 m, then the scan MDC can be determined as a function of depth of Pu-238. The results indicated that an optimal scan MDC of 740 pCi/g was achieved for soil contaminated to thicknesses of 0.5 to 2 cm. Contaminated soil depths greater than a few centimeters resulted in an increased scan MDC (880 pCi/g at 5 cm, and 4700 pCi/g at 15 cm) because the increased activity concentration of Pu-238 with depth did little to add to the detectability of these low energy x-rays. Conversely, a contaminated soil thickness of only 1 mm resulted in a scan MDC of 840 pCi/g. These results agree with your empirical data that scan MDCs for Pu-238 in soil are greater than 400 pCi/g. It may well be that empirical studies provide the best estimate of the scan MDC.
Independent of the scan MDC for Pu-238, you might consider compositing as a way to reduce the number of soil samples needed to satisfy hot spot concerns. Remember that the additional measurements are used to show that there are no “hot spots” that exceed the DCGLemc. The additional measurements are not needed for the statistical tests. As an example, lets say that 18 soil samples are needed to meet the requirements of the Sign test, but that a high scan MDC for Pu-238 meant 90 samples were needed. Obviously these 72 extra analyses for Pu-238 would be costly (a bit of an understatement). Fortunately, it is only necessary to show that the levels of contamination in these 72 additional samples are not above the DCGLemc (165 pCi/g in your case).
Here's how it works. Take the 90 soil samples - and let's assume you plan to composite three samples at a time. Each of the 90 samples is split in thirds (ensure that you have collected sufficient material for analysis requirements); the remaining soil from each sample is set aside as a backup and might not need to be analyzed. Groups of three adjacent soil samples (keeping track of soil sample location for each grouping) are composited with one other (note: it is important that appropriate compositing procedures be followed). We now have 30 composites. These 30 are analyzed and the statistical test is performed on the data. Let's assume that we pass the statistical test. The next step is to ensure that none of the original 90 samples exceeded the DCGLemc of 165 pCi/g. This can be shown if the 30 composites had less than 55 pCi/g each, i.e., the DCGLemc divided by the number of samples in each composite. If one of the 30 composites exceeded this value, say one sample has 80 pCi/g, the three backups from the original samples used to produce the composite would then be analyzed to determine whether one of them actually exceeded the DCGLemc, and if so, which one(s). The net effect of this compositing example is to reduce the number of samples analyzed from 90 to 30.
A: The ISO-7503 ["Evaluation of surface contamination - Part 1: Beta-emitters (maximum beta energy greater than 0.15 MeV) and alpha emitters"] does indeed address the issue of smear counting. The standard provides an equation where the net count rate from the smear count is divided by three factors: instrument efficiency, surface efficiency and removal factor. The values of surface efficiency are the same as for direct measurements of surface activity in the field (i.e., 0.25 for alpha emitters, and 0.25 and 0.5 for beta emitters, depending on the beta energy). The removal factor should either be experimentally determined, or a default value of 0.1 used.
A widely cited reference for reporting
environmental (and D&D) data is the U.S. Environmental Protection Agency,
Upgrading Environmental Radiation Data, Health Physics Society Committee
Report HPSR-1 (1980), EPA 520/1-80-012, August 1980. Among other things, the
report states "the uncertainty should be reported to no more than two
significant figures, and the value itself should be stated to the last place
affected by the qualification given by the uncertainty term." This guidance
is most relevant to laboratory measurements-- given that laboratory data
usually have uncertainties reported along with the value.
A: The background level used for calculating the minimum detectable concentration (MDC) for a survey instrument is usually based on actual (empirical) measurements. There are no standard numbers that should be used in lieu of actual measurements of background. For example, the background level for a concrete surface should be determined by a number of static measurements on a concrete surface in a non-impacted area. The average background value is then used in the calculation of the instrument's MDC.
Given that the background level can vary as a function of surface material construction, instrument MDCs can be calculated and reported for a number of surface types. According to the MARSSIM (p. 6-35), "MDC values should be calculated for each type of area, but it may be more economical to simply select a background value from the highest distribution expected and use this for all calculations."
A: Draft NUREG-1506 "Measurement Methods for Radiological Surveys in Support of New Decommissioning Criteria" (1995) has not been superseded, nor finalized. This NUREG report, as stated in the abstract, "contains a description of proposed methodologies for measuring low-level radiation and radioactivity that could be used in conducting surveys associated with decommissioning of licensed NRC facilities." Perhaps most noteworthy about this report is its coverage of in situ gamma spectrometry that can be used for both indoor and outdoor survey applications. Draft NUREG-1506 will soon be electronically available on the DDSC Guidance on Selection and Use of Survey Instruments page.
A: Surface activity measurements performed on corrugated steel pose a challenge due to the variable source-to-detector distances. Having to measure alpha contamination only complicates the matter. NUREG-1507 might be able to offer a reasonable solution.
First, the total efficiency using the ISO-7503 approach is calculated by multiplying the instrument efficiency by the surface efficiency (0.25 for alpha). The instrument efficiency should be determined using a NIST-traceable source. However, the resultant instrument efficiency must be corrected for the source-to-detector distance. [The instrument efficiency is typically determined at contact with the source, yet the corrugated steel measurements present varying source-to-detector distances]. This is where NUREG-1507, Section 4 may help.
Table 4.6 provides the reduction in gas proportional detector response (normalized) for Th-230 (alpha emitter) as a function of distance:
Let's assume that the corrugated steel is known to have a peak-to-valley distance of 2 cm. The detector response from alpha contamination in the valley would only be 10% of that at the top, in contact with the detector. The idea is to determine the average source-to-detector distance offered by the corrugated steel. For simplicity, let's assume that the midpoint of the peak-to-valley distance represents the average distance (1 cm in this case). So from Table 4.6 in NUREG-1507 the normalized response at this distance is 0.58. Therefore, the total efficiency for the alpha measurements would be given by (0.58)* (instrument efficiency)* (surface efficiency).
Incidentally, the other approaches mentioned in the submitted question may also work well, particularly the application of Th-230 NIST-traceable material to the corrugated steel and empirically determining the appropriate instrument efficiency.
Your proposed solution sounds reasonable. In
fact, while ISO-7503 states that "the dimensions of the calibration source
should be sufficient to cover the window of the instrument detector", it
goes on to state that in rare circumstances "sequential measurements with
smaller distributed sources of at least 100 cm2 active area shall be carried
out. These measurements should cover the whole window area or at least
representative fractions of it and shall result in an average value of
instrument efficiency." The only deviation is that you plan to use a 2-inch
diameter source to accomplish the averaging, rather than a source of at
least 100 cm2.
A: Many D&D professionals are using the ISO-7503 approach for calculating total efficiencies for portable survey instruments, and it is a technical issue evaluated by the NRC. The appropriate section of MARSSIM that specifies use of ISO-7503 is section 6.5.4 Instrument Calibration. In NUREG-1727 (Sept 2000), Sec 14 on Radiation Surveys states (under Evaluation Criteria), "The final status survey design is adequate if it meets criteria in" and in the 6th bullet, "MARSSIM Sections 6.5.3 and 6.5.4 for selection of acceptable survey instruments, calibration..." The NRC's consolidated decommissioning guidance, NUREG-1757, vol. 2 (draft) contains the same guidance.
On the other hand, not everyone is using ISO-7503. Those sites that have NUREG/CR-5849 as the basis of their D&D plan are typically not using it. In general, the larger D&D sites are, such as the power reactor D&D sites. My recommendation is to get explicit approval from the regulator if you plan to deviate from the MARSSIM guidance.
The context of your question deals with the calculation of scan MDC, and
specifically the impact that the depth of contamination has on the scan MDC
value. Note that contamination depth refers to the contamination thickness
as measured from the surface to the stated depth. The depth of contamination
was varied from 12 cm to 18 cm (15 cm is the nominal value used) in this
evaluation. Page 208 of DHP reports the results for the low-energy gamma
radiation from Am-241: scan MDC equals 44.8 pCi/g for a depth of
contamination of 12 cm, and 44.7 pCi/g for a depth of 18 cm. These results
can be interpreted to mean that when the contamination is present at a
greater depth (at the same concentration) the scan MDC for Am-241 is
marginally better (44.7 vs. 44.8 pCi/g).
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