Answer to Question #10133 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

We have been producing refractory ceramics special shapes out of zircon silicate (40 percent ZrO2) for the last 30 years; recently we needed to ship by air an order but were informed that wooden box with our shapes inside was detected with high radioactive levels (>30 cps) by the airport cargo scanner and then revised by the courier company by using a Radeye PRD detector and measurements were confirmed (55 cps).

Another service company was contacted and they used a NDS Products Gamma detector and its findings were 1.6 mR h-1.

We decided to acquire a SE Intl Radiation Alert Monitor 4EC for our testing and found that the reading on the same wooden pallet was 150 cpm on the upper meter scale (equals 2.5 cps) and 0.15 mR h-1 on the lower meter scale.

We also have checked our raw materials, equipments and accessories and there is nothing with a level higher than 0.2 mR h-1 by using our instrument.

We are now in a deadlock as the airfreight company is reluctant to take our materials and our readings show our shapes are safe.

As you may deduce we are not experts in this subject. Is there something that is being read incorrectly? Are the units for one detector and ours different? I have read from your excellent Q&A site that granite (and maybe zircon materials) have to be measured in a different way.

A

What you have described is a not uncommon situation in which multiple individuals, all using different instruments, make measurements of the same radioactive source(s), and all obtain substantially different results. Such discrepancies usually occur when (1) the instrument response characteristics among the instruments used are significantly different (i.e., the count rates per unit dose rate are different), (2) the instruments differ in physical characteristics such that significant geometry differences arise when the instruments are placed close to the source of interest (e.g., detector dimensions are different and when detectors are close to a source they are irradiated nonuniformly to varying extents), and/or (3) the operator(s) misuse the instrument(s) or misinterpret the readings.

The Radeye PDR instrument, which you cite as having been used by the courier company, uses a sodium iodide scintillation detector. This solid crystalline detector has a much higher efficiency for gamma rays than do many other detector types, such as the very common Geiger-Mueller (GM) detector, exemplified by the SE International Radiation Alert Monitor 4EC that you purchased. The manufacturer-quoted response of the Radeye PDR instrument is 1.5 cps/µR-h-1 for 137Cs photons (662 keV) and 30 cps/ µR-h-1 for 241Am photons (60 keV). These would translate to 90,000 cpm/mR-h-1 and 1.8 x 106 cpm/mR-h-1, respectively. The manufacturer’s response specification for the SE 4EC instrument is 1,000 cpm/mR-h-1 for 137Cs photons; this is 90 times less than the response of the Radeye instrument. It is not surprising then that your reading with your SE 4EC GM detector was more than 20 times less than the reading obtained by the courier company using the Radeye scintillation detector. The reading obtained by the company using the NDS Products gamma detector was about 10 times greater than the reading you obtained with the SE 4EC. You do not specify which model NDS instrument was used, and they make a fairly large number of portable gamma measuring instruments, many of them being GM types. Depending on which instrument it was, part or all of the difference in readings could be associated with differences in response.

Another point to note is that the Radeye instrument is extremely energy-dependent in its response, showing a response 20 times as great to lower energy 241Am photons than to the 137Cs photons. The SE 4EC GM detector is energy compensated so that it presents a more uniform response with photon energy. This difference is another cause of confusion and misinterpretation in many measurement situations.

In the case of measuring zircon-based ceramics the major expected gamma-emitting radionuclides are the many radioactive progeny that result from the decay of 238U and 232Th. A relatively wide range of gamma ray energies are associated with the decay of these species, and this can contribute to increased variability and uncertainty in the radiation levels measured, especially when a detector such as the Radeye instrument is used. The sodium iodide detector has a great advantage in sensitivity, a fact that makes it very useful in detecting small amounts of gamma-emitting radioactivity. Unfortunately, this large sensitivity, without consideration of the energy dependent characteristics and the proper translation of count rates to dose rates, can lead to improper conclusion about the significance of the detected radioactivity.

Regarding the second factor that I mentioned – the effects of differences among physical characteristics – these can be important and were almost certainly responsible for part of the differences observed among the readings you cite. When detectors are placed close to a radioactive source the detector may not be irradiated uniformly. The shape and size of the detector are important factors that affect the field uniformity, generally with larger volume detectors resulting in poorer uniformity and lower readings compared to smaller volume detectors, assuming that no other influencing factors are at play. You can read more about this type of problem as addressed in the answer to a previously asked question, #8263, on the HPS Ask the Experts web site. Some detectors, such as solid sodium iodide detectors, may also produce nonuniformity of irradiation through photon attenuation in the detector that is more significant than might be experienced for other detector types, such as gas-filled GM detectors.

Finally, discrepancies in readings and interpretations may result from errors or misjudgments made by the involved individuals. At times instruments may not have been properly calibrated for the type(s) of measurements and radiations encountered. In other cases, readings on one instrument may be obtained and erroneously compared against readings obtained with a different type of instrument. When instrument readings are being used to judge the appropriateness of a package for shipment, it is important to know the response characteristics of the instrument(s) being used and how a reading, such as a count rate, can be converted to a perhaps more useful quantity such as a dose rate. For some detectors, such as the sodium iodide detector we mentioned, such conversions are not easy to make. As a last point, we should note that when comparative measurements of a package are made, it is important that the measurements be taken at the same location on the package.

All things considered, the results that you have found are not surprising. The discrepancies in measurements may lead to poor decisions on the parts of individuals in authority, and this can make it difficult for a seemingly simple shipping task to be completed without excessive strain.

I hope the above discussion is helpful. I wish you well in resolving your problems.

George Chabot, PhD

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