Answer to Question #13630 Submitted to "Ask the Experts"

I live in a home with quite a bit of black natural rock (I suspect granite) around the house, and I am planning to have kids soon and am very worried about the effects of the radiation from granite/natural rock. I've gone through many of your Q&As stretching back to 2008. I have several questions:

1. What radiation does granite/natural rock give off? I've read in some of your answers there's radon, and there's gamma. But what exactly am I exposed to? Am I exposed to all of alpha, beta, and gamma radiation? Of these three forms of radiation, what should I be most concerned about from granite/natural rocks in the house?
2. I've also read from previous experts that apparently the "scintillator" is the best instrument to measure radiation from granite/natural rock. Can you kindly confirm? If that is so, I'm quite confused because there are apparently different kinds of scintillators—there's plastic, there's NaI, there's cesium I think, and there's also germanium. So, I'm a bit confused—which one should I use? Apparently germanium is the best at detecting and measuring gamma, but the ones I can find in the market from Thermo-Fisher are mainly plastic and NaI.
3. I've also looked at your answers in the past and can't find one. Suppose I purchase a scintillator. Do I need to calibrate it myself? Or does the manufacturer already calibrate it?
4. Finally, is there a level of radiation above which is considered unsafe? Expressed in µrem h^-1, rem h^-1, µSv h^-1, or mSv h^-1? I'm just thinking that the scintillator will give me a reading, but how should I interpret the reading? How would I know if that radiation level is high enough to be concerned?

I'll attempt to address each part of your question by number:

1. We should first distinguish between radioactivity and radiation. Radioactivity refers to the actual radioactive atoms, such as radon (e.g., ²²²Rn), radium (e.g., ²²⁶Ra), uranium (e.g., ²³⁸U), thorium (e.g., ²³²Th), some potassium (⁴⁰K), and many other possible radionuclides produced by radioactive decay in the uranium and thorium decay chains. Radon has received some emphasis because it is a gas, and small amounts of it may effuse from granite and other rocks.
   
   The radioactive decay processes are associated with the emission of specific ionizing radiations such as alpha particles, beta particles, and gamma rays. Alpha radiation has very little penetrating ability and, depending on how the surface of the granite has been treated, may be present in very small intensities or not measurable at all. It is not an external health hazard because any alpha particles incident on the skin surface are typically stopped by the dead skin layer. Beta radiation is more penetrating than alpha radiation and can contribute to a detector response near the counter surface if the detector has a sufficiently thin window. It is stopped by less than one centimeter of soft tissue and is primarily a concern to the skin if the beta intensity is sufficiently high. Gamma radiation is much more penetrating and can be detected with a variety of detector types; it is often considered  to be the radiation of most concern from granite countertop radiation because of its potential to deliver dose to deeper tissues in the body. Other natural rock in and around the home may also contain small amounts of similar radionuclides that one would expect in granite, although concentrations would likely be less in most other nongranitic rocks.
2. The scintillator detectors that have generally been used for these types of measurements have been primarily sensitive to gamma radiation, usually having thick enough encapsulation to eliminate alpha and beta response. They have an expected higher sensitivity for gamma radiation than do Geiger-Mueller (GM) detectors, the latter being probably the most commonly used detector type.
   
   It is my personal opinion that, while a plastic scintillator detector (often described as a microrem meter), will yield the most accurate reading for gamma dose, if you are intending to purchase a single detector for making measurements, I would recommend a thin-window GM detector. This detector has the advantage that it is sensitive to beta and alpha radiation and is a good detector to use to look for radiation and to compare relative values from possible emitters around your home or in other locations. Because these detectors are not generally calibrated to allow beta (or alpha) dose measurements, the readings of the thin-window detector cannot be easily interpreted in terms of dose significance when the detector is close to the surface being assessed. To measure the gamma dose rate close to the surface, I recommend using about a 1 to 2 cm-thick slab of plastic placed between the surface and the detector face (if you don't have plastic, using solid wood, perhaps 1.5 to 3 cm in thickness, would be adequate). This reading with the plastic shield in place represents your estimate of the gamma dose rate.
   
   The detector is typically calibrated by the manufacturer to yield a dose rate to gamma radiation from a specific radionuclide, ¹³⁷Cs. This calibration is not exact but will suffice to give you an acceptable estimate of the gamma dose rate near the surface. You can purchase a decent GM detector for perhaps one-third to one-half the likely cost of a scintillator detector.
   
   If you still decide to buy a scintillator detector, my preference is for the plastic scintillator because it has a dose response very similar to soft tissue. The NaI detector and CsI detector may have somewhat greater sensitivities, but they are very different from tissue in atomic composition and usually have strong energy-dependent responses.
   
   The germanium detector that you mention is referred to as a semiconductor detector and is almost exclusively used for gamma energy measurements and is not particularly suited to your needs. Its cost, including the associated required electronics, would also be much greater than the other detectors we are discussing.
3. If you buy an instrument new from a manufacturer/vendor you can expect that it will have been calibrated to yield an acceptable measure of gamma radiation dose rate. Usually the dose quantity used in calibration is a reasonable estimator (such as ambient dose equivalent) of a quantity used in radiation protection called effective dose. It is generally recommended that instruments in use be calibrated at least annually. It is important to keep in mind that when the instrument is calibrated to allow an estimate of dose to a person, the calibration assumes a uniform radiation field over the incident surface of radiation (at least the whole major trunk of the body). When using such a detector to measure gamma dose rate near the surface of a countertop, or the surface of some other earth-based material, the only way the number would have any realistic approximation to the true dose to an individual would be if the individual lay stretched out face down on the countertop or other material surface. The actual dose that people might receive would be better estimated by making measurements at distances from the countertop or other sources where people are spending time. The dose rate expectedly will drop off markedly with increasing distance from the surface.
4. Regarding interpretation of the significance of dose measurements, we should emphasize here that, based on a fairly lengthy history of use of granite countertops in homes, I foresee no likelihood that any of the dose rates you measure or any of the potential doses to home occupants will be high enough to represent any kind of health or safety concern. Based on various measurements that have been made, we might expect typical granite to yield a reading approximately 0.1 µSv h^-1 above the normal background reading (usually between 0.1 and 0.2 µSv h^-1). This can vary with different granites, however, and also with the thicknesses of the granite pieces.
   
   A pertinent number that applies to the limitation of public exposure from all non-background sources of radiation is 1 mSv (1000 µSv) annually, on average, attributable to a particular source. This number has been recommended by the National Council on Radiation Protection and Measurements (NCRP)in its Report No. 116 and has been used by other groups and agencies, both domestic and foreign. While the 1 mSv annual limit was primarily intended in regard to incidental public exposure associated with the use and/or storage of sources of radiation for other applications, it is a useful and appropriate number to consider when assessing exposure from other sources such as decorative stone and tiles used in homes and natural stone that may be part of the home environment. It is a value well below what would represent any health concern, but it is sufficiently high, as a single source of extraneous public exposure, to raise a question as to whether the use of the offending product is justified or whether reduced time should be spent in certain areas. I foresee no likelihood that the natural stone or other decorative elements in and around your home would produce annual doses approaching the 1 mSv value.
   
   As a brief example, for an individual who spent four hours daily in proximity to a typical granite piece, with a dose rate above background of 0.1 µSv h^-1, the accrued annual excess dose would be about 145 µSv, an insignificant amount in terms of any negative health consequences and about 15% of the 1,000 µSv reference value that we cited. Normally, the elevated dose rate would exist very close to the granite surface, and dose rates at locations where any significant part of an individual's body would be irradiated would be considerably less. For perspective, keep in mind that the typical annual dose to a person in the United States from natural background radiation sources, including radon, is about 3,000 µSv, and the typical dose from medical procedures is about the same.

Kids can have a lot of fun climbing on rocks. I hope that is something that both they and you enjoy when you do have children.

George Chabot, CHP, PhD