Answer to Question #12396 Submitted to "Ask the Experts"

I have a problem for which I'm not able to find an answer. In the region where I live we import firewood from eastern Europe and there is no way to find timber from other places more secure from a radiological point of view. What I'm concerned about is that cesium-137 (¹³⁷Cs) contained in firewood is 100–150 times higher in the produced ashes. Now I know from my research on the internet that 3,000 Bq kg^-1 in the ashes is good from a health point of view, that 6,000 Bq kg^-1 is less good . . . and that more than 10,000 Bq/kg is not good at all.

My question is this: What values, with the Geiger counter at contact with the ashes, expressed in microSeivert per hour (µSv -hr^-1), should my Geiger counter display (more or less) for each of the above-mentioned concentration activities? If you don't know exactly the answer could you give me an indicative reference value, based on your experience, in order to enable me to make the conversion from Bq kg^-1 to µSv -hr^-1, with the Geiger at contact?

Addendum:  Can you also discuss the possible impact on measurements of radioactive potassium in the ash?

While your question appears fairly simple, the answer is not necessarily, so I shall briefly explain why this is the case and try to provide some information that will allow you to make at least some reasonably simple measurements that might be helpful.

Three reasons for the complexities include (1) the specific characteristics of the Geiger-Mueller (GM) radiation detector that you are using, (2) the quantity of ashes that you are measuring, and (3) the geometry of the mass of ashes that you are measuring. Geiger detectors come in a variety of shapes, mostly long and short cylindrical tubes. Some have thin windows, usually mounted on one end of the detector tube, that will allow detection of relatively low penetrating radiation such as beta particles and, sometimes, alpha particles. Among the most common of these types capable of measuring low-penetration radiation is a thin window detector often referred to as a pancake detector because the detector has a rather squat profile, being a cylinder with a small length and relatively large diameter. Since I do not know which type of detector you have, in some of the discussion that follows I will assume one of the most common of these pancake type detectors with an active window area of about 15 square centimeters (cm²).

I should first point out that most GM detectors that are purchased have been calibrated to respond to gamma radiation, usually with the 662 kiloelectton volt (keV) gamma radiation from ¹³⁷Cs decay being the preferred type. The calibration results yield a gamma dose rate conversion factor, often in units of count rate per unit dose rate—e.g., 330 counts per minute (cpm) per µSv -hr^-1 is typical for the detector noted above. The detectors are commonly calibrated to yield equivalent dose rates (e.g., in µSv -hr^-1) in gamma radiation fields. If such a thin window detector is exposed to significant low penetrating radiation, such as beta radiation, it will produce a reading on the detector that the user will not readily be able to interpret unless the detector has been calibrated for the particular radiation, energy, and geometry that is experienced in the field. As a consequence, if one is using such a detector and intends to measure only the gamma radiation in a radiation environment that includes gamma radiation along with beta and/or alpha radiation, one should cover the window face of the detector with a sufficient thickness of material (usually plastic or some other low atomic number material—even paper sheets stacked together will do) to stop the low penetrating radiation; about 3 mm of typical plastic (such as polymethyl methacrylate, also known as Lucite, Perspex, Crylux, and Plexiglas) or about 40 sheets of common copy paper are adequate to block essentially all beta radiation from ¹³⁷Cs.

At the concentrations that you have noted, relatively small-sized ash samples of even the highest cited concentration of 10,000 Bq kg^-1 would not produce a sufficient net reading to be useful under commonly acceptable measurement geometries. I should note at this point that, while the reference concentration values you are using are acceptable for your own purposes, they are somewhat arbitrary. I am not aware of any published results that have shown that a concentration of 10,000 Bq kg^-1 of ¹³⁷Cs in the ash represents a significant health concern. Here is a link to a 2015 paper by M. Calabrese et al. that shows that calculated doses for a wood ash concentration of 16,500 Bq kg^-1 were quite small. The calculated external dose from irradiation by the ¹³⁷Cs gamma radiation for a resident of a single home in Northern Italy was 0.055 µSv in one year; the dose from inhalation of airborne ash was higher at 0.193 µSv in one year.

The method I am recommending depends on making measurements close to the surface of a relatively large amount of ash maintained in a fixed geometry. This method, using the gamma radiation only, would require a large batch of ash greater than about 2 meters in all dimensions to obtain optimal results (I'll discuss smaller volumes below). Naturally, one would require some kind of container to keep the ash in a reasonable geometry. For the largest response, a very large volume would be desirable—e.g., a cylindrical drum 2 m in diameter by 2 m deep or a box 2 m x 2 m x 2 m. This is a lot of ash, and may not be practical, depending on your situation. Some wood-burning individuals do pile up their ash and save it for possible future use and may possibly have enough ash to use this procedure. Assuming so, we would place the beta-shielded radiation detector at the center of one surface close to contact with the exposed ash, usually the top surface, of ash (I have used a distance of 1 cm from the ash surface to the detector). A plastic bag around the detector will prevent ash from contaminating and /or damaging the detector. In this situation we can invoke a principle known as energy spatial equilibrium in which the gamma energy emitted per unit mass of ash would be about the same as the gamma-derived energy absorbed per unit mass of ash. This allows a direct calculation of the dose rate in the ash. The dose rate at the surface of the ash is half of that we calculate, since the calculation assumes ash completely surrounding the detector in an infinite volume.

I have gone through this process for the large ash volume, using an assumed ¹³⁷Cs ash concentration of 10,000 Bq kg^-1, and obtained an expected net gamma dose equivalent rate at the surface of the ash of about 1.42 µSv -hr^-1, a value that should be easily distinguishable above background levels. Naturally, the dose rate is proportional to the activity concentration so that the 3,000 Bq kg^-1 ash would yield an expected equivalent dose rate of 0.424 µSv -hr^-1, and the 6,000 Bq kg^-1 ash would yield 0.848 µSv -hr^-1, all values being easily resolvable from the typical background rate of 0.1 to 0.2 µSv -hr^-1. This procedure can be carried out with smaller volumes of ash, but the calculations are considerably more complex. I have used a commercially available computer code called Microshield (produced by Grove Software) to obtain expected results for the larger volume and for smaller, more practical volumes of ash held in a cylindrical geometry. The dose point is centered above the flat surface of the ash at a height of 1 cm. I have used a value of 0.83 g cm^-3 for the density of the ash, a value consistent with published results. Keep in mind that this density value could vary depending on how the ash is handled and stored, and changes could have some effect on the results. Results are shown below.

Results of dose rate assessments based on cylindrical volumes of ash, contaminated at a level of 10,000 Bq kg^-1

As can be seen from the above results, the dose rates for the smaller volumes become progressively lower; the 10 cm diameter cylindrical volume would produce a net dose rate perhaps 1.6 times higher than a background of about 0.1 µSv -hr^-1, yielding a gross dose rate of 0.26 µSv -hr^-1 for the 10,000 Bq kg^-1 ash, markedly different from the background. The 10 cm x 10 cm cylindrical volume may also be adequate for lower activity concentrations, perhaps 5,000 Bq kg^-1 or so, if care is taken in the measurement, and the background is not especially high. The 20 cm diameter cylinder dose rate would be easier to discern, and would be likely high enough even with the 3,000 Bq kg^-1 ash for which we would expect a net reading of about 0.11 µSv -hr^-1, implying a gross reading of 0.21 µSv -hr^-1 for a 0.1  µSv -hr^-1 background. For low activity concentrations the larger volumes are desirable. The larger volume that you can use, the more accurate and more precise will be your results, and the greater range of concentrations you will be able to assess. Note that I have shown three significant digits in the above results; this has been done only for the sake of easy comparisons among the results. This is not intended to imply a commensurate level of accuracy associated with a given measured value since the measured values are subject to various uncertainties, including the size and placement of the detector, the accuracy of its calibration, possible variations in the directional response of the detector, variations in packing and possible geometry of the ashes, and other factors.

The above results were generated for cylindrical geometries, but the results would not differ by more than about 10 to 15 % from the above if a cylinder with a given diameter and height (equal to each other) were replaced by a cubical volume with each side of the volume equal to the diameter of the cylinder. This geometry might be easier for some people since cubical boxes are fairly easily obtained compared to some cylindrical containers of the required size. For dimensions between any of those shown above, you should be able to interpolate within values reasonably easily to estimate expected results. Based on what you have described, I assume that your detector has been calibrated to provide the appropriate dose rate from ¹³⁷Cs gamma radiation and has sufficient sensitivity to measure the expected dose rates.

Methods involving measurements of the beta radiation from ¹³⁷Cs decay are also possible at high concentrations, but the uncertainties may be extreme even at your cited value of 10,000 Bq kg^-1, and I am not recommending you attempt such measurements as a means to interpret activity concentrations.

Regarding potassium in wood, a tiny fraction of all potassium is naturally occurring radioactive ⁴⁰K, and this does contribute to the external dose rate from wood ash. Potassium-40 is also a beta and gamma emitter. The beta radiation from ⁴⁰K is more energetic than that from ¹³⁷Cs and would require about twice as much absorbing material (e.g., plastic) to stop the beta radiation. The gamma radiation from ⁴⁰K is also more energetic than that from ¹³⁷Cs (1.46 MeV compared to 0.662 MeV). However, the number of gamma rays emitted per decay of the ⁴⁰K is about eight times less than those from ¹³⁷Cs.

The potassium concentration in different woods can vary. I have done some representative calculations for the ⁴⁰K in wood ash using an assumed ash concentration of 4,859 Bq kg^-1 (based on a wood average value of

43 Bq kg^-1 from wood chip data from the above-cited Calabrese reference and a reasonably conservative wood-to-ash concentration factor of 113, based on an average of several published values). Again, I assumed cylindrical geometry.

You may observe that these values are about 3 to 4 times less than what we would have obtained from the same geometries with ¹³⁷Cs at the same activity concentration (4,859 Bq/kg). They are lower primarily because of the lower gamma yields from the K-40 compared to the ¹³⁷Cs, as noted above. These results are simply meant to be representative for more-or-less typical situations. Naturally, variations in any of the assumed input values will affect results.

Keep in mind that all of the above results have been generated based on assumed input conditions, as described. Actual measurements may incur unknown or inadvertent deviations from what is intended, and these may affect measured results so that uncertainties of unevaluated extent may result. If you are consistent in the way you make your measurements, you may experience some errors, but the results may still be useful for making relative judgments about the levels of ¹³⁷Cs in the wood ash. I wish you well.

George Chabot, PhD, CHP