Answer to Question #10217 Submitted to "Ask the Experts"

Do some materials "reflect" or "absorb" background radiation differently (much like materials reflect or absorb heat or light radiation differently)? If you are using a common survey instrument such as a pancake GM (Geiger Mueller) detector, can the background radiation that you measure be affected by 15 to 20 cpm just by moving from one type of material to another?

The short answer to your question is "Yes." There are a number of factors that influence the magnitude of the background radiation dose rate and the types and amounts of some materials are among the influencing factors.

In some situations the presence of materials, especially materials with relatively high mass density or larger thicknesses of less dense materials above the detector (between the detector and the heavens) or below the detector (between the detector and the earth) may cause a reduction in the measured background radiation because of attenuation of the radiation coming from the cosmos or the earth, respectively. For example relatively modest thicknesses of lead are able noticeably to reduce the background dose rate to a detector. Also, larger thicknesses of less dense material may make a noticeable difference. This is often evident if one makes measurements of the typical background radiation in the land area around a pond or lake and then makes similar measurements from a boat well out, far removed from the shoreline, into the body of water where the water depth is at least several feet. The water is quite effective at shielding gamma radiation coming from the earth, and the measured dose rate will often be markedly lower than that measured on land. The effects of radiation shielding by low-density material are also evidenced by the fact that cosmic radiation levels increase appreciably with increasing altitude above about 1,000 meters because of shielding provided by the atmosphere. Below about 1,000 meters the radiation level may actually decrease with altitude because of attenuation of the gamma radiation from the earth by the lower atmosphere.

The typical pancake-style GM detector has a facial area of about 15 to 20 square centimeters, and the typical unshielded background count rate at sea level is often about 20 to 70 cpm, depending on location and detector configuration. Increased shielding of the detector, as in instances noted above, could account for measurable changes in count rate as high as the 15–20 cpm that you mention. The gamma radiation coming from the earth can be scattered by various materials with some resulting changes in direction, although scattering in the forward direction is favored, and it is unlikely that scattering effects would be detectable at the levels to which you refer. The cosmic radiation is dominated by mu mesons, and additions of up to about 10 cm of lead (equivalent) material above the detector will cause a reduction in count rate, but beyond that amount there will be little attenuation of the remaining very high energy mu mesons and, in some cases, added thicknesses may increase the count rate as a consequence of interactions producing additional radiation in the shield that may have a higher probability of interaction in the detector.

In addition to the possible change in count rate associated with shielding effects, the count rate may change as a result of encountering materials that have larger than normal amounts of natural radioactivity. A well-known example of this is when measurements are made in the immediate vicinity of a granite building. Granite often has significant amounts of uranium and thorium, and the decay of these leads to many radioactive progeny within the granite. Several of these emit significant gamma radiation that, along with the common presence of significant potassium, which contains natural radioactive potassium-40, may enhance the background readings in the area. Increases in count rate by a factor of two are not unusual in such cases. Granite is also a popular covering material for kitchen counters, and measurements made directly above such a countertop will often show elevated readings compared to usual background. Materials that contain appreciable potassium may also show increased count rates. This can be demonstrated when a reading is taken of a package of typical salt substitute (potassium chloride). The potassium always contains a small amount of naturally occurring radioactive potassium-40, which emits beta and gamma radiation. If the detector window is not covered and some of the salt substitute is spread out on a surface and the probe held close to the surface above the salt, the detector reading may be greatly increased because of the increased response to the beta radiation that would be measurable in this geometry.

There have also been cases of enhanced readings that have resulted from contamination of building materials with man-made radionuclides. For example, there have been several cases in which cobalt-60 has been accidentally incorporated into steel at the time of the steel production, and items made from the steel have shown readings very much elevated above the expected background.

There are numerous other instances of elevations in background readings from naturally occurring radioactive materials (NORM), technologically enhanced naturally occurring radioactive materials (TENORM), and man-made radionuclides that have found their way into our living environment. The above are just a few examples.

In conclusion, there are many instances when background levels may change—some being reductions as a result of radiation attenuation (what you are referring to as radiation being absorbed) in materials and some being increases as a consequence of enhanced radioactivity in the materials being measured. The process of ionizing radiation background levels increasing from scatter effects (what you refer to as reflection) does occur, but you would not likely be able to measure the small increase with the GM detector because such increases would be buried among the usual statistical fluctuations that typify background measurements. We often find surprises and/or reasons to question our results if we make background measurements in a variety of locations and situations, and the process is frequently educational. I wish you well in your measurements.

George Chabot, PhD, CHP