Answer to Question #6701 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Surveys and Measurements (SM)

The following question was answered by an expert in the appropriate field:

I was asked to discard a bench that was used during radioactive research. How do I know the bench is not contaminated? According to my boss, the count rate for eight wipe samples collected from the bench are between 10.5 to 20.5 cpm (counts per minute). The two blank samples read 17.7 and 18.7 cpm. What does it mean? Can the field blanks have readings that are higher than wipe samples collected from the "contaminated" surface? Is there a safety standard for cpms? Is there any special handling procedure required?

In order to evaluate whether the bench is contaminated you (or someone else) must first make measurements with an appropriate detector to evaluate the presence of radioactivity. You describe data from wipe samples, and such samples are valid in many cases for assessing removable contamination. It is also necessary to evaluate fixed contamination that may not be removable via wipes. If the radionuclides that had been used on the bench are radionuclides that can be detected with portable instrumentation, then such instrumentation should be used to evaluate the presence of such radionuclides. If tritium (3H) was a radionuclide of concern, then it might even be necessary to take some representative samples of the bench surface and to subject them to analysis. Such analysis could be as simple as allowing the sample to equilibrate in a liquid scintillation cocktail mix and counting in a liquid scintillation counter.

Once you have obtained data for fixed and removable contamination, you can assess the figures to try to determine whether contamination is present. If your facility is going through an official decontamination and decommissioning process, you will likely have to abide by specific release criteria that are predicated upon dose limitations when the property is disposed of. It does not sound as if this is your situation, but rather as if you are simply trying to get rid of an unwanted item. The simplest way to evaluate the possible contamination levels is to use instrumentation that is adequate to measure the radiations of possible concern and to compare the readings obtained to the background count rates for the same instruments.

This appears to be where you are at with respect to the wipe samples. Regarding such, it is indeed possible for the background (blank) count rate to exceed the sample count rate even if the sample has no contamination on it. This is a matter of statistics. If we use Poisson statistics, the standard deviation in a single count, σC, is the square root of the count, and the standard deviation, σR, in a count rate, R, (obtained from a count, C, over a time interval, t) is the square root of the count divided by the time, C1/2/t.

For example, if 30 counts were obtained in a three-minute background count, the standard deviation in the count would be 301/2 = 5.48, and the standard deviation in the count rate (R = 10 cpm) would be 5.48/3 = 1.83 cpm. There is approximately a 95 percent probability that any three-minute background count that we took (so long as the background did not change) would fall within ± 2 σC of the mean count. If 30 counts is a reasonable estimation of the mean count, and 5.48 cpm is its standard deviation, this means that 95 percent of the repetitive counts would expectedly fall between 19 counts and 41 counts. Thus, if one were counting samples that had no contamination, one would expect to see counts both above and below the mean blank count on a regular basis.

The question then becomes: when is an observed count indicative of the presence of net activity (i.e., contamination)? The question is perhaps most cogent when the observed gross sample count is greater than the blank count. (For purposes here, since you have specified count rates, we will refer to count rates rather than counts.) A generally acceptable approach to answering this question is to determine what is called the critical level, LC, which represents a net count rate such that any observed net sample count rate greater than LC will be assumed to be indicative of the presence of real activity. The critical level, as defined originally by Currie (Currie LA. Limits for qualitative detection and quantification determination. Analytical Chemistry 40(3):587-593; 1968), is given by

LC = k(Rb/Ts + Rb/Tb)1/2,

where k is the number of standard deviations above the mean expected net count rate of zero when no activity is present at which the critical level is specified to lie;

Rb is the background (blank) count rate;

Tb is the background counting time, and

Ts is the sample counting time.

In most health physics applications the value of k is specified as 1.645; for a normal distribution this would imply that the fraction 0.05 (5 percent) of the net counts would fall above the critical level even when no net activity was present in the sample. This level is often specified by what is called an alpha value; thus alpha, α, would be equal to 0.05. By accepting such a value, we are saying that we are willing to accept false positive results 5 percent of the time when evaluating samples that have no net activity. If we do then observe a net count rate above the critical level we would conclude that net activity is present. This can be used as a simple tool to decide, on a reasonable statistical basis, whether activity is present so that we can proceed with whatever action is appropriate.

For the wipe data you have presented we don't have all the information to do exact calculations, but we can do a representative example. Let us assume that the blank count rate was 18.2 cpm and it was obtained by counting the blank for five minutes. Let us also assume that the gross sample count rate was the high value of 20.5 cpm that you gave and let us assume that it was based on a two-minute count. We can estimate the critical level as

LC = 1.645(18.2/2 + 18.2/5)1/2 = 5.87 cpm.

The observed sample gross count rate of 20.5 cpm yields a net count rate of 20.5 - 18.2 = 2.3 cpm. Since this value is less than the critical level we would conclude that the wipe sample contained no net activity.

It is important to note that the critical level can be decreased by increasing the counting time for the sample and/or background. For example, if we had increased the counting times for both the sample and the background to 20 minutes we would have obtained a critical level of 2.22 cpm. The net count rate of 2.3 cpm would have just exceeded this value, leading us to conclude that the sample did contain net activity. The net count rate can be converted to activity by dividing the count rate by the appropriate counting efficiency. The determination that net activity may exist does not necessarily mean that a given item cannot be disposed of, but such disposal must be consistent with stipulations and requirements of your regulating group. A proper determination that no activity exists is justification for disposing of the item as uncontaminated. Naturally, when the net sample count rate is less than the critical level, we would assume that no net activity is present in the sample.

The description of the critical level used above is applicable to samples that have been analyzed in a laboratory counting system in which a count is obtained over a measured time interval. The count rate is obtained by dividing the count by the counting time. If a ratemeter is used to obtain count rate directly, as might be the case when making measurements of fixed contamination with some portable instruments, the approach to calculating the critical is somewhat different. We will not discuss that here, but if that becomes an issue for you, please post another question.

There are a number of other statistical methods and criteria that can be used in making judgments about the presence of activity in contamination samples. For example, the concepts of the lower limit of detection and/or minimum detectable activity (MDA) are two related quantities that are commonly used in decision making. These quantities are calculated from the critical level and are larger than the critical level. While they are often used in making decisions about the presence of activity, I believe it is more appropriate to use the critical level for such judgments.

You can obtain more information about these and other decision tools by reviewing some of the questions that have been answered previously on the Health Physics Society Ask the Experts Web site. If you go to the Experts' Answers page and type "statistics" in the Search box, you will get a list of questions and answers; questions 5958, 2018, 6175, 3644, and 2888, and perhaps others, include discussions and references that you might find helpful. Good luck.

George Chabot, PhD, CHP

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