Answer to Question #7241 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:

Q

I work at an institution where we have been checking for contamination using a Ludlum 2241 with a 44-9 probe. In one specific case, 131I is the isotope of interest. As one of my license conditions, I need to confirm that all surface contamination levels are <16.7 Bq (disintegrations per second) per 100 cm2 over a 1 square meter area.

It appears that everyone in industry is using a pancake probe similar to our 44-9 for these types of surveys, but I can't seem to justify an efficiency value high enough to prove compliance.
 

The 131I decays through beta emission 100 percent of the time; then there are miscellaneous gammas. I have measured known quantities of 131I in approximately the same geometry to my compliance surveys (some beta attenuation is occurring), and I only come up with 2 percent efficiency at best (measured efficiency, as close to 2π as I can get). With that measured efficiency I can't prove much of anything.

I asked Ludlum about the gamma efficiency for the probe (the 44-9 manual only shows response relative to 137Cs), and they explained I should assume about 0.01 percent!

The meter is good for finding contamination, but after cleaning, how in the world can we justify the area/surface clean with such horrible efficiencies?
 

A

This question raises a number of issues and concerns. I will try to comment on some of these and, where there is opportunity to possibly resolve some questions, I will attempt to do so.

In order to meet your requirement that all surfaces are not contaminated beyond a level of 16.7 Bq per 100 cm2, you must survey the surfaces with the probe close enough to the respective surfaces so that you can identify, within rather narrow boundaries, where any detected contamination might be, and you must have sufficient sensitivity to be able to measure the requisite levels. This usually requires holding the probe close to the contaminated surfaces to increase the geometry factor and to minimize beta attenuation in air.

When measuring beta emitters, most available detectors—whether they be gas detectors (typified by GM and proportional types), scintillation detectors (such as plastic scintillators), or semiconductor detectors (such as silicon)—have an intrinsic efficiency of close to 1.0 for beta radiation as long as the detector window is thin and the beta energies are not very low. Since the detector is used in conjunction with associated electronic components, it is necessary to establish a pulse threshold to reject unwanted noise, and this will decrease the efficiency somewhat. The fact is, however, that the pancake probe you are using has about as good a beta-detection efficiency as is available on many commercial detectors available for survey work, particularly when viewing spot contamination. It has a window that is about 2 mg cm-2 thick, and you might be able to improve somewhat on this with some alternative detectors that offer somewhat thinner windows, but the slight improvement is not likely to help much in assessing your 131I contamination.

Where you might do better, in terms of improving sensitivity for area-distributed activity, is by using a larger active area detector than the 15 cm2 44-9 probe. Large-area gas-flow proportional detectors, with window areas of 100 cm2 or more, are available from various manufacturers. These have the advantage of being able to view more surface area at one time, thus allowing measurement of more radioactivity when such activity is distributed over a relatively large area. Some require a portable supply of counting gas, thus making them not as convenient to use as the GM.

You have stated that you have made measurements that implied about a 2 percent counting efficiency for the 131I under near 2π geometry with the 44-9 probe. This efficiency is much less than what one might expect for this probe in such a geometry, the expected efficiency likely being around 20 to 25 percent for uniform 131I contamination on a smooth surface that offers no significant self-attenuation. The implication is that, if your 2 percent value is correct, there is a considerable degree of beta particle attenuation occurring, as you have concluded; this could be associated with penetration of 131I into the surface material and/or appreciable amounts of dirt, dust, or other foreign material deposited along with or over the 131I surface contamination. 

For 131I spread uniformly over a given one-square meter area at a surface concentration of 16.7 Bq per 100 cm2, the expected beta activity directly below the 44-9 probe (held close to the surface), whose active facial area is 15 cm2, would be only about 2.5 Bq and, assuming your 2 percent counting efficiency, this is not sufficient activity to produce a count rate that would be distinguishable from the usual background (the background count rate for the 44-9 probe is probably in the vicinity of 0.58 cps, and the net count rate from the 2.5 Bq would be only about 0.05 cps). On the other hand, a single spot of contamination in an otherwise clean one-square meter area, could contain as much 1.67 × 103 Bq, and even with only a 2 percent counting efficiency, such activity could be readily detected if the probe were directly over the activity.

We would have to conclude, as you have, that 2 percent detection efficiency, while adequate for some activity distributions, would not be suitable for your purposes to allow detection when activity is dispersed significantly over a given one-square meter area of interest when the probe is held in contact with, or very close to, the surface. At this point we should note that when beta detection efficiencies are evaluated for a probe such as the Ludlum 44-9, the sources that are used are often either point sources or area distributions with active areas often less than or equal to the active area of the detector. Disregarding attenuation effects, such efficiencies apply reasonably well when the detector is placed in contact with the contaminated surface in the field.

In actuality, most surveyors prefer not to hold the probe in contact with the surface to be measured so as to avoid contamination of the probe, and when a thin-window detector that has thick sidewalls (as does the 44-9 probe) is withdrawn somewhat away from a surface that is contaminated over a relatively large area, beta particles from outside the window area may be able to reach the detector window, although they may have been precluded from doing so when the detector was at the surface. This effect can actually increase the counting efficiency beyond that quoted by the manufacturer or beyond what the user may determine using small area sources. We can demonstrate this in a calculation using an assumed uniform large area distribution of 131I surface contamination. Beta particle attenuation shall be considered only at the time we apply the counting efficiency number.

If we assume that the probe window is 1 cm above the surface, we can estimate the number of beta particles that might be incident on the window, accounting for beta radiation originating from outside the probe window area as well as from within the area directly below the window. For estimation purposes we shall assume that the restriction on source area contributing to detector response is fixed by the range in air of the average-energy beta particles from 131I decay, and we shall evaluate the number of beta particles arriving per unit time per unit area—i.e., the beta fluence rate—at a point at the center of the detector window (we shall further assume that the beta particle fluence rates at other points of the detector window are the same as that at the detector center; neglecting edge effects, this assumption is reasonable as long as the contaminated area of interest is considerably larger than the detector window area).

For an unattenuated beta source, it is easy to show that the fluence rate, N, of beta particles at a point at height, H, above the center of a disc-shaped area of radius, R, is given by

φ = (SA/4) ln(1 + R2/H2),

where SA is the number of beta particles emitted per unit surface area per unit time. For the limit of 16.7 Bq per 100 cm2 that you cite, SA = 0.167 cm-2 s-1; for a height of 1 cm and a value of R = 36 cm (this is based on the range of the average-energy beta particles from 131I [0.18 MeV] in air at 22 oC).

We then calculate:

φ = (0.167 cm-2 s-1/4)(ln(1 + 362/12)) = 0.151 cm-2 s-1.

For the 15 cm2 window area of the probe, the rate at which beta radiation would then impinge on the window is (15 cm2)(0.151 cm-2 s-1) = 2.26 s-1. I shall assume that the efficiency of 2 percent that you estimated applied to the total beta emission rate from the source; since the 2.26 s-1 that we have calculated applies only to beta particles incident on the detector, we should multiply your value by 2 to obtain an efficiency of 0.04 counts per disintegration. If we use this efficiency of 0.04, then we would predict a net count rate of about 0.090 cps. This rate is about 15 percent of the usual background rate and is sufficient for detection only for reasonably extended counting times. If we used the digital output from the Model 2241 scaler and the background rate, based on only a one-minute count, was 0.58 cps, and if the contaminated surface was also counted for one minute, we could calculate a critical level net count rate of 0.23 cps and a limit of detection of 0.50 cps. Longer counting times would naturally yield smaller values for the critical level and the limit of detection. (The limits here are based on the common use of probabilities of 0.05 for both false positive and false negative acceptance and the methods of Currie [Currie LA, Limits for qualitative detection and quantification determination. Analytical Chemistry 40(3):587-593; 1968]). About a 25 minute counting time of both the background and the contaminated surface would be necessary to achieve a limit of detection about equal to the predicted 0.09 cps.

You have noted that the gamma detection efficiency for the GM probe is very low, thus precluding its use for surface contamination assessment at the levels of concern. (The gamma detection efficiency for the 44-9 probe is low, but I don’t believe it is as low as the 0.01 percent that you have cited. Using the manufacturer’s gamma response to 137Cs photons of 6.3 x 103 cps per mSv-h-1, I have calculated a photon detection efficiency of about 0.5 percent for photons incident approximately normally on the 15 cm2 detector face. For a source in a 2π geometry the efficiency would be about 0.25 percent, or possibly somewhat larger for the angular incidence.) While there are alternative gamma detector types—such as solid plastic scintillators or NaI(Tl)—that, for a given volume, have much higher photon detection efficiencies than the GM detector, they also would have difficulty attempting to resolve the signal from area-dispersed 131I at the level you require. This can be ascertained by considering 131I dispersed uniformly at an area concentration of 16.7 Bq per 100 cm2 over a 1 square meter disc-shaped area. Such a distribution would produce a photon air kerma rate at 1 cm above the center of the contaminated area of about 6.1 x 10-13 J kg-1 s-1, or 6.1 × 10-7 µGy s-1. Considering that typical background air kerma rates are around 2.4 × 10-5 µGy s-1, we would not likely be successful in resolving the small contribution from the dispersed contamination from the normal statistical fluctuations that we would expect in the background signal. If the activity were again concentrated in a small spot, the signal at 1 cm from such a source would be much stronger, the anticipated kerma rate at 1 cm being about 2.4 × 10-4 µGy s-1, a value which could be easily measured.

As you have inferred, the cited measurement inadequacies do not leave you in a very satisfactory position. If you can show that the 131I contamination of interest is generally localized in few small spots per square meter of surface you may be able to demonstrate that your beta survey methods have sufficient sensitivity even if the detection efficiency is only 2 percent. If you cannot rule out the likelihood of area dispersed activity, then you may be forced to do some reevaluation of measurement techniques and/or allowable surface contamination levels.

I don’t know how extensive a study you did to estimate the 2 percent efficiency figure, but it may well be worthwhile to do a fairly detailed study, if you have not already, to evaluate the efficiency for a number of different contaminated areas so that you have a good handle on what value you should use. Such evaluations would include the direct in-situ measurements with the probe(s) of interest along with an independent evaluation of the activity being viewed. This may require removing part of the surface material to perform laboratory analyses of the 131I content. If you have confirmed that activity may be dispersed over relatively large areas, you should make measurements to evaluate the detector performance when in contact with a large area source vs. when the detector is moved away from the surface to a fixed distance, perhaps 1 cm, so that you can judge whether any increased sensitivity is adequate to meet your measurement needs. Keep in mind the possible advantage of using a larger-area beta detector to enhance sensitivity.

If your analyses show that instrumental methods are not sensitive enough to perform in-place surface-contamination surveys, you may be able to assess removable contamination through measurements of surface wipes. Such analyses could involve beta-counting of wipes in a laboratory low background system, such as a gas-flow proportional counter, or possibly gamma measurements in a NaI(Tl) well counter. You would then have to do independent assessments of the ratio of removable to fixed surface contamination so that, on the basis of a wipe analysis, you could estimate the total surface-contamination concentration. Such ratio analyses may require multiple samples of contaminated surface material to be removed and subjected to laboratory analyses to measure fixed activity to demonstrate what is a proper ratio to apply. If you have large areas to survey, wipe sampling is not a very satisfactory method to use because of the much greater expenditure of manpower, time, and money required to do thorough surveys compared to in-place instrumental measurements of contamination.

Finally, if you demonstrate that meaningful measurements cannot be made, you and/or your management may want to consider whether the 16.7 Bq per 100 cm2 level that you are using can be adjusted upwards. Such action would likely have to be based on an analysis that demonstrates that the dose impact of raising the surface-contamination limit would be acceptable to you and to regulators and possibly to other interested or involved individuals or groups.

Good luck as you attempt to work through your measurement problems.

George Chabot, PhD, CHP

 

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